Display device and method of manufacture

Abstract

A display device comprises a variable-wavelength light emitting diode (LED), and a Band Stop filter positioned between the variable-wavelength LED and a light-emitting side of the display device. The variable-wavelength LED comprises an n-doped portion, a p-doped portion, and a light-emitting region located between the n-doped portion and the p- doped portion, the light-emitting region being configured to emit light at a peak emission wavelength under electrical bias thereacross, and the variable-wavelength LED is configured so that the peak emission wavelength of the LED is variable over an emission wavelength range of at least 40 nm by varying a driving condition provided to the LED. The Band Stop filter is configured to attenuate emitted light with a wavelength falling inside a stopband wavelength range and the Band Stop filter is configured so that the stopband wavelength range is inside the emission wavelength range of the variable-wavelength LED. A method of manufacturing such a display device is also provided.

Description

[0001] Display Device and Method of Manufacture

[0002] The present invention relates to a display device comprising a variable-wavelength LED, and a method of manufacturing such a display device. In particular, the invention relates to a display device comprising a variable-wavelength LED and a Band Stop filter.

[0003] Background lll-V semiconductor materials are of particular interest for semiconductor device design, in particular the family of Ill-nitride semiconductor materials.

[0004] “Ill-V” semiconductors include binary, ternary and quaternary alloys of Group III elements, such as Ga, Al and In, with Group V elements, such as N, P, As and Sb, and are of great interest for a number of applications, including electronics and optoelectronics.

[0005] Of particular interest is the class of semiconductor materials known as “Ill-nitride” materials, which includes gallium nitride (GaN), indium nitride (InN) and aluminium nitride (AIN), along with their ternary and quaternary alloys. (AI,ln)GaN is a term encompassing AIGaN, InGaN and GaN. Ill-nitride materials have not only achieved commercial success in solid-state lighting and power electronics, but also exhibit particular advantages for quantum light sources and light-matter interaction.

[0006] While a variety of Ill-nitride materials are commercially interesting, Gallium nitride (GaN) is widely regarded as one of the most important new semiconductor materials, and is of particular interest for a number of applications.

[0007] The present invention will be described primarily by reference to GaN and InGaN, but may advantageously be applicable to alternative Ill-nitride material combinations.

[0008] It is known that the introduction of pores into bulk Ill-nitrides, such as GaN can profoundly affect its material properties (optical, mechanical, electrical, and thermal, etc.). The possibility of tuning a wide range of material properties of GaN and Ill-nitride semiconductors by altering its porosity therefore makes porous GaN of great interest for optoelectronic applications.

[0009] In a significant improvement over conventional three-colour LED displays, Poro Technologies Ltd, the present applicant, has developed variable-wavelength LEDs formed from Ill-nitride semiconductor materials grown over a porous region of Ill-nitride material. Instead of a conventional LED which emits at a single colour, these variable-wavelength LEDs can emit a broad spectrum of different emission wavelengths in response to varying the driving conditions provided to the variable-wavelength LED. These variable-wavelength LEDs, and their method of manufacture, are disclosed in international patent application no. PCT / GB2022 / 051997, published as W02023 / 007174.

[0010] As the peak emission-wavelength of such variable-wavelength LEDs can be tuned by varying the driving conditions, a single variable-wavelength LED can take the place of multiple “single-colour” subpixels. In particularly preferred embodiments, the same variablewavelength LED can be tuned to emit any one of red, green or blue light by tuning the driving conditions provided to that LED. This means that a colour display can be formed from an array of variable-wavelength LEDs, with each variable-wavelength LED acting as a colour-variable pixel, or alternatively as a colour-variable subpixel in a subpixellated display.

[0011] In Poro Technologies Ltd’s dynamic-pixel-tuning (DPT®) variable-wavelength LEDs, the peak emission wavelength of variable-wavelength LEDs is strongly dependent on the magnitude of the electrical driving signal provided to a given LED, and longer-wavelength emission colours require lower-magnitude driving currents / voltages which inherently produce a lower luminance. The full width at half maximum (FWHM) spectral width of the emission peaks emitted by such variable-wavelength LEDs is also typically broader than the emission peaks of conventional “single-colour” LEDs. This presents a challenge, as the emission spectra emitted by the variable-wavelength LED in different operating modes can have a region of overlap, which reduces the purity of the desired emission colour, shifts the dominant “observed” wavelength away from the intended spectral peak wavelength and reduces the size of the colour gamut displayable by the display device.

[0012] It is well known in the art that to improve the colour purity of a monochrome LED, a spectral element can be added, such as a bandpass filter or a distributed Bragg reflector (DBR) centred on the single emission colour of the LED to narrow the FWHM of the LED’s emission peak. Another solution is to use colour filters to block emitted colours other than the colour of interest. Both of these solutions are only effective, however, in arrangements where spatially separated sub-pixels are used to emit discrete primary emission wavelengths. These approaches would be unsuitable for displays in which the same LED emits multiple different primary emission wavelengths, as a spectral element which narrows the emission peak of one primary emission wavelength would block the emission of other primary emission wavelengths from the same LED. In addition to being unsuitable for displays in which multiple colours are emitted by the same pixel or subpixel, existing solutions have various disadvantages. Different colour filters are required above each pixel or subpixel, which introduces additional processing steps and increases complexity and cost. In addition, spectral filters made of quantum dot or other phosphor materials usually show lower efficiency for conversion, poor material stability and short lifetime. These materials also have response times which inherently limit frame rates and can cause blur in moving systems, making them undesirable for high- resolution and high-frame-rate display devices.

[0013] Summary of Invention

[0014] The present application relates to a display device comprising a variable-wavelength light emitting diode (LED), and a method of manufacturing such a display device. The display device may preferably comprise a plurality of such variable-wavelength LEDs, for example an array of variable-wavelength microLEDs.

[0015] The invention is defined in the independent claims, to which reference should now be made. Preferred or advantageous features of the invention are defined in the appended sub-claims.

[0016] Display Device

[0017] In a first aspect of the present invention there is provided a display device comprising: a variable-wavelength light emitting diode (LED); and a Band Stop filter positioned between the variable-wavelength LED and a light-emitting side of the display device.

[0018] The variable-wavelength LED comprises an n-doped portion, a p-doped portion, and a light-emitting region located between the n-doped portion and the p-doped portion. The light-emitting region is configured to emit light at a peak emission wavelength under electrical bias thereacross. The variable-wavelength LED is configured so that the peak emission wavelength of the LED is variable over an emission wavelength range of at least 40 nm by varying a driving condition provided to the LED by a power supply.

[0019] The Band Stop filter is configured to attenuate emitted light with a wavelength falling inside a stopband wavelength range; and the Band Stop filter is configured so that the stopband wavelength range is inside the emission wavelength range of the variable-wavelength LED. A Band Stop filter, otherwise known as a band rejection filter, is an optical filter which allows the transmission of most wavelengths of light unaltered, but which attenuates wavelengths within the filter’s stopband range to very low levels. The Band Stop filter is an optical Band Stop filter, which may optionally attenuate wavelengths within the stopband using scattering or destructive interference.

[0020] The Band Stop filter is positioned between the variable-wavelength LED and the lightemitting side of the display device, such that light emitted by the variable-wavelength LED passes through the Band Stop filter before passing out of the light-emitting side of the display device. The light-emitting side of the display device may comprise an outlet through which light is emitted out of the display device, such that Band Stop filter is positioned between the variable-wavelength LED and the outlet of the display device.

[0021] As the stopband of the Band Stop filter is inside the emission wavelength range of the variable-wavelength LED, the Band Stop filter is configured to attenuate a portion of the wavelengths which are emittable by the variable-wavelength LED. By selecting a Band Stop filter with a stopband covering a particular wavelength range, the Band Stop filter may be used to attenuate, or block, a portion of one or more emission peaks emitted by the variable-wavelength LED.

[0022] The light-emitting region is configured to emit light at a peak emission wavelength under electrical bias thereacross. Due to the inherent emission characteristics of the variablewavelength LED, however, the variable-wavelength LED emits an emission spectrum with a spectral width which centres on the peak emission wavelength and also contains a range of wavelengths both above and below the peak emission wavelength. The spectral width of any emission peak can be defined by its FWHM.

[0023] Compared to single-colour conventional LEDs, variable-wavelength LEDs used in the present invention may emit peak emission wavelengths with a broader FWHM. By incorporating a Band Stop filter into the display device, and selecting a Band Stop filter with a stopband that covers a portion of the emission wavelength range of the variablewavelength range of the variable-wavelength LED, the emission wavelengths emittable out of the display device can be narrowed. The use of a stopband to block a portion of the wavelengths emitted by the LED can effectively narrow the width of the emission peaks that are transmitted out of the display device, improving the colour purity achievable with a variable-wavelength LED. In the prior art, band pass filters have been used to artificially narrow the emission peaks of separate LED subpixels. Band pass filters allow the transmission of a narrow range of wavelengths in a pass band, and block all other wavelengths. In the present invention, rather than using band pass filters, Band Stop filters are used. Band Stop filters work in the opposite way from Band Pass filters, and instead block the transmission of a relatively narrow range of wavelengths in a stopband, while allowing the transmission of all wavelengths outside the stopband.

[0024] The display device is preferably configured to operate the variable-wavelength LED in a plurality of discrete emission modes by providing a plurality of discrete driving conditions to the LED. The variable-wavelength LED emits a different peak emission wavelength in each discrete emission mode, and each emission mode generates its own emission mode spectrum which contains wavelengths surrounding the peak emission wavelength.

[0025] The variable-wavelength LED is preferably driveable to emit light at a first peak emission wavelength in response to a first electrical driving condition provided to the variablewavelength LED. The variable-wavelength LED is preferably also driveable to emit light at a second peak emission wavelength in response to a second electrical driving condition. The first and second peak emission wavelengths are both within the emission wavelength range of the variable-wavelength LED.

[0026] The Band Stop filter may preferably be configured to attenuate emitted wavelengths in a stopband which is positioned between the first peak emission wavelength and the second peak emission wavelength. The stopband wavelength range is preferably positioned between the first peak emission wavelength and the second peak emission wavelength to attenuate overlapping emission wavelengths which are emitted in both the first and second modes. By attenuating emitted wavelengths between the first and second peak emission wavelengths, the Band Stop filter may advantageously eliminate colour-overlap between the two emission peaks, increasing the purity of the colour emitted in both modes. This effectively reduces the FWHM of both the first and second peak emission wavelengths, and expands the colour gamut displayable by the display device.

[0027] The display device preferably comprises one or more Band Stop filters positioned between the variable-wavelength LED and the light-emitting side of the device, each Band Stop filter being configured to block the transmission of emitted wavelengths within a respective stopband. The use of one or more Band Stop filters can advantageously block the transmission of certain stopband wavelengths out of the display device to narrow the FWHM of the emission peaks, while still allowing transmission of all other wavelengths emitted by the variable-wavelength LED. This Band Stop filter is particularly suitable for use with a variable-wavelength LED, when the same variable-wavelength LED is driven to emit multiple discrete emission peaks.

[0028] An example of variable-wavelength LEDs usable in the present invention are known as dynamic-pixel-tuning (DPT®) variable-wavelength LEDs from Poro Technologies Ltd, and are disclosed in international patent application no. PCT / GB2022 / 051997, published as W02023 / 007174.

[0029] Although a continuum of different emission wavelengths can be emitted directly from a variable-wavelength LED by varying the driving conditions supplied to that LED, additional chromaticities may be displayed by driving the LED to emit a plurality of “primary” wavelengths, such that the overall output signal observed by a viewer is a temporal combination of the emitted primaries.

[0030] In a subpixellated display, different primary emission wavelengths can be emitted by separate subpixels. The subpixels may all be variable-wavelength LEDs, or some subpixels may be non-variable-wavelength LEDs.

[0031] In one preferred embodiment, the display device is a subpixellated display, in which the variable-wavelength LED is a first subpixel which is driveable to emit light at a first peak emission wavelength in response to a first driving condition being provided to the first subpixel, and in which the display comprises a second LED subpixel which is driveable to emit light at a second peak emission wavelength in response to a second driving condition being provided to the second subpixel. The second LED subpixel is preferably a second variable-wavelength LED subpixel. In this embodiment the first and second subpixels are subpixels of a display pixel, and the first and second subpixels are driveable independently of one another.

[0032] In subpixellated embodiments of the display device, due to the proximity of the subpixels to one another, the light emitted by the subpixels at the first and second peak emission wavelengths combines to form an output signal with an output chromaticity and output luminance Lo. A viewer observing the display device will perceive only a single output signal from the subpixellated pixel within the display frame, and that perceived output signal will have a single output chromaticity and luminance which are the temporal average of the emission wavelengths emitted by the variable-wavelength LED subpixels.

[0033] In another preferred embodiment, the variable-wavelength LED is a pixel of the device, and the display device is a field-sequential display. In this embodiment, the same variablewavelength LED pixel is driven to emit a plurality of discrete peak emission wavelengths by driving the LED pixel with a plurality of different driving conditions. The variable-wavelength LED is driveable to emit light at a first peak emission wavelength in response to a first electrical driving condition provided to the variable-wavelength LED, and the variablewavelength LED is driveable to emit light at a second peak emission wavelength in response to a second electrical driving condition provided to the variable-wavelength LED.

[0034] In a sequential-field display, the same variable-wavelength LED may be controlled to emit multiple discrete peak emission wavelengths, by supplying different driving conditions to the LED in sequential subframes of a display frame. In this way, the same variablewavelength LED may be a pixel that emits a plurality of selected “primary emission wavelengths” one after another within the duration of a single display frame.

[0035] In use, the variable-wavelength LED of the display device is preferably driven to emit light at both the first peak emission wavelength and the second peak emission wavelength in turn, by varying the driving condition provided to the variable-wavelength LED.

[0036] The display device is preferably driven with field-sequential control, so that the first and second peak emission wavelengths are emitted by the same variable-wavelength LED during discrete display subframes within a single display frame. In other words, the same LED emits the first and second peak emission wavelengths one after another, during a short space of time (a display frame). The light emitted at the first and second peak emission wavelengths combines to form an output signal with an output chromaticity and output luminance Lo. A viewer observing the display device will perceive only a single output signal from the variable-wavelength LED within the display frame, and that perceived output signal will have a single output chromaticity and luminance which are the temporal average of the emission wavelengths emitted field-sequentially by the variablewavelength LED.

[0037] The Band Stop filter is preferably configured to attenuate or block all emitted light with wavelengths falling within the stopband of the Band Stop filter. The Band Stop filter is designed to block wavelengths within the stopband either by absorption or transmission.

[0038] The stopband wavelength range is narrower than the emission wavelength range of the variable-wavelength LED, so that emitted wavelengths in a portion of the emission wavelength range which is outside the stopband are not attenuated by the Band Stop filter. The stopband is thus configured to cover only a portion of the emission wavelength range of the variable-wavelength LED.

[0039] The Band Stop filter is preferably configured so that the stopband wavelength range attenuates a portion of the emission wavelength range of the variable-wavelength LED, but transmits shorter emitted wavelengths which are below the stopband wavelength range and longer emitted wavelengths which are above the stopband wavelength range.

[0040] Discrete Emission Modes

[0041] In a preferred embodiment, the display device is configured to operate the variablewavelength LED in a plurality of discrete emission modes by providing a plurality of discrete driving conditions to the LED, in which the variable-wavelength LED emits a different peak emission wavelength in each discrete emission mode, and in which each emission mode generates an emission mode spectrum which contains wavelengths surrounding the peak emission wavelength.

[0042] The Band Stop filter may be configured so that the stopband is positioned between the peak emission wavelengths of the discrete emission modes, so that the Band Stop filter attenuates wavelengths between said peak emission wavelengths. The Band Stop filter may preferably be configured to attenuate emitted wavelengths in a stopband which is positioned between the peak emission wavelengths of different emission modes.

[0043] The Band Stop filter may preferably be configured to attenuate wavelengths of emitted light in an overlap range which is emitted in the emission mode spectra of multiple emission modes. This is distinctly different from approaches taken in the prior art, which typically employ transmission bands centred at the peak colour positions. Instead of targeting single emission peaks with transmission bands, the present invention may instead block spectral regions between the emission peaks of separate emission modes.

[0044] By attenuating emitted wavelengths which fall in an overlap between discrete emission peaks of discrete emission modes, the Band Stop filter may advantageously eliminate colour-overlap between the two emission peaks, increasing the purity of the colour emitted in both modes. This effectively reduces the FWHM of the emission peaks on either side of the stopband, and expands the colour gamut displayable by the display device.

[0045] The effect of the Band Stop filter may thus be to reduce spectral power on one side of a primary emission peak (a peak emission wavelength emitted by an emission mode of the display device).

[0046] The device may preferably be controlled using field-sequential control, so that the variablewavelength LED is controlled to operate in different emission modes in different display subframes.

[0047] In a preferred embodiment for example, the variable-wavelength LED is driveable in a first mode to emit light at a first peak emission wavelength in response to a first electrical driving condition provided to the variable-wavelength LED, and driveable in a second mode to emit light at a second peak emission wavelength in response to a second electrical driving condition. The first and second peak emission wavelengths are both within the emission wavelength range of the variable-wavelength LED.

[0048] When the variable-wavelength LED is operated in the first mode, the LED emits a first emission mode spectrum centred on the first peak emission wavelength and covering a range of emitted wavelengths on either side of the first peak emission wavelength. When the variable-wavelength LED is operated in the second mode, the LED emits a second emission mode spectrum centred on the second peak emission wavelength and covering a range of emitted wavelengths on either side of the second peak emission wavelength. The first emission mode spectrum and the second emission mode spectrum overlap with one another in an overlap range, which is a range of wavelengths in which both the first and second emission mode spectra have an intensity greater than zero.

[0049] In a preferred embodiment, the Band Stop filter may be configured to block emitted wavelengths in a stopband wavelength range between the first peak emission wavelength and the second peak emission wavelength. The stopband wavelength range is preferably positioned between the first peak emission wavelength and the second peak emission wavelength to attenuate overlapping emission wavelengths which are emitted by the variable-wavelength LED in both the first and second modes. The Band Stop filter is preferably configured to attenuate emitted wavelengths which are emitted in a spectral overlap range in both the first and second modes.

[0050] The Band Stop filter may be configured so that the stopband does not contain any of the peak emission wavelengths generated by the discrete emission modes.

[0051] Alternatively, one or more of the peak emission wavelengths generated by the discrete emission modes may be positioned within the stopband, while another portion of the emission mode spectrum generated in that emission mode is not inside the stopband.

[0052] In a preferred embodiment, the first peak emission wavelength is below 570 nm and the second peak emission wavelength is above 610 nm, such that the LED emits green light in response to the first driving condition and red light in response to the second driving condition, and the stopband of the Band Stop filter is configured to attenuate wavelengths between the first and second peak emission wavelengths. By positioning the stopband between the first and second peak emission wavelengths, the green and red emission peaks are effectively narrowed, and spectral overlap between the green and red emission peaks is blocked.

[0053] The Band Stop filter may be configured to attenuate wavelengths in a plurality of stopband wavelength ranges inside the emission wavelength range of the variable-wavelength LED. The Band Stop filter being configured to have multiple stopbands within the emission wavelength range of the variable-wavelength LED.

[0054] Alternatively the display device may comprise a plurality of Band Stop filters with discrete stopband wavelength ranges inside the emission wavelength range of the variablewavelength LED. The plurality of Band Stop filters may be configured to attenuate emitted wavelengths within respective stopbands in the emission wavelength range of the variablewavelength LED. The plurality of Band Stop filters are preferably positioned between the variable-wavelength LED and the light-emitting side of the display device, so that light emitted by the variable-wavelength LED must be transmitted through the plurality of Band Stop filters prior to being emitted out of the device. The plurality of Band Stop filters may be stacked between the variable-wavelength LED and the light-emitting side of the display device, so that light emitted by the variable-wavelength LED must be transmitted through all of the Band Stop filters prior to being emitted out of the device. By using a Band Stop filter which attenuates wavelengths in a plurality of stopbands, or using a plurality of Band Stop filters which have a plurality of respective stopbands, it is possible to narrow a plurality of emission peaks of the variable-wavelength LED.

[0055] Three Emission Modes

[0056] In a preferred embodiment, the variable-wavelength LED is controllable to emit three discrete peak emission wavelengths, by varying the driving conditions provided to the LED between three discrete emission modes.

[0057] The variable-wavelength LED is preferably controllable in a first mode to emit at a first peak emission wavelength in response to a first driving condition provided by the power supply, in a second mode to emit at a second peak emission wavelength in response to a second driving condition provided by the power supply, and in a third mode to emit at a third peak emission wavelength in response to a third driving condition provided by the power supply.

[0058] The Band Stop filter is preferably configured to attenuate emitted wavelengths in a first stopband between the first and second peak emission wavelengths, and in a second stopband between the second and third peak emission wavelengths.

[0059] Particularly preferably, the Band Stop filter is configured so that the stopband attenuates overlapping wavelengths which are emitted in the first and second modes, and the device preferably comprises a Band Stop filter with a second stopband configured to attenuate overlapping wavelengths which are emitted in the second and third modes. The display device may either comprise a single filter with two discrete stopbands, or two Band Stop filters each with a respective stopband.

[0060] In a particularly preferred embodiment, the LED is controllable to emit a blue peak emission wavelength in response to a first driving condition provided by the power supply, to emit a green peak emission wavelength in response to a second driving condition provided by the power supply, and to emit a red peak emission wavelength in response to a third driving condition provided by the power supply. For example the LED may be controllable to emit a first peak emission wavelength in the range 400-500 nm in response to a first driving condition provided by the power supply, to emit a second peak emission wavelength in the range 500-560 nm in response to a second driving condition provided by the power supply, and to emit a third peak emission wavelength greater than 600 nm in response to a third driving condition provided by the power supply. The effect of the Band Stop filter is to reduce spectral power on one side of a selected “primary” peak emission wavelength. For example, the Band Stop filter may have a stopband selected to reduce spectral power on the green side of a blue emission peak or on the blue side of a green emission peak. Unlike band pass filters in the prior art, the stopband of the Band Stop filter is targeting the spectral region between primary emission peaks as opposed to targeting the primary emission peaks themselves. A benefit of this arrangement over others is that it is effective whatever colour (whichever emission mode) the variable-wavelength LED is emitting - a Band Stop filter with a stopband in blue-green does not impact red emission, and a Band Stop filter with a stopband in green-red does not impact blue emission.

[0061] The LED may be controllable to emit a first peak emission wavelength in the range 430-470 nm in response to a first driving condition provided by the power supply, to emit a second peak emission wavelength in the range 510-560 nm in response to a second driving condition provided by the power supply, and to emit a third peak emission wavelength in the range 600-660 nm in response to a third driving condition provided by the power supply.

[0062] The first, second and third driving conditions are preferably first, second and third driving currents of different magnitudes, corresponding to first, second and third current densities. The first, second and third driving conditions may alternatively be first, second and third driving voltages of different magnitudes.

[0063] Band Stop Filter

[0064] The Band Stop filter is an optical Band Stop filter, which may optionally attenuate wavelengths within the stopband using scattering or destructive interference. A variety of optical Band Stop filters are known in the art of optics, and a variety of such filters are usable in the present invention.

[0065] The Band Stop filter may be an optical notch filter, which is a Band Stop filter having a narrow stopband and a high Q factor.

[0066] The Band Stop filter may be selected to provide high attenuation (low transmission) within the stopband, combined with high transmission outside of the stopband. With conventional dielectric Band Stop filters, for example, it transmission >90% outside the stopband and transmission <10% within the stopband should be achievable. The Band Stop filter may have a stopband with a width of at least 10 nm, or at least 20 nm, or at least 25 nm, or at least 30 nm. The Band Stop filter may have a stopband with a width of less than 75 nm, or less than 60 nm, or less than 50 nm.

[0067] The Band Stop filter may be constructed from crystalline semiconductor or insulating material. For example the Band Stop filter may be formed from conventional dielectric materials.

[0068] The Band Stop filter may comprise one or more layers of porous Ill-nitride semiconductor material. The Band Stop filter may for example comprise a layered structure in which a plurality of first layers having a first refractive index alternate with a plurality of second layers having a second refractive index different from the first refractive index. The Band Stop filter may comprise a layered structure in which a plurality of layers of porous Ill-nitride semiconductor material alternate with a plurality of non-porous layers of Ill-nitride material, or in which a plurality of layers of porous Ill-nitride material having a first porosity and a first refractive index alternate with a plurality of porous layers of Ill-nitride material having a second porosity and a second refractive index different from the first refractive index. The Band Stop filter may comprise a distributed Bragg reflector (DBR) configured to reflect, and thus block the transmission of, wavelengths within the stopband. The thicknesses of the layers in the Band Stop filter are preferably configured to attenuate wavelengths in the stopband by destructive interference.

[0069] The Band Stop filter may comprise a rugate filter, also known as an index gradient filter. Instead of alternating layers with two fixed refractive indices, in a rugate filter the refractive index is slowly varied through the structure. The rugate filter may comprise one or more porous layers of Ill-nitride semiconductor material, in which the porosity of the porous layer varies through the thickness of the layer, such that the refractive index of the porous layer varies through the thickness of the layer.

[0070] The Band Stop filter may optionally be a bistimulus Band Stop filter configured to attenuate emitted wavelengths in two separate stopbands. For example the Band Stop filter may be configured to attenuate transmitted wavelengths in a first stopband between a blue peak emission wavelength and a green peak emission wavelength, and in a second stopband between the green peak emission wavelength and a red peak emission wavelength.

[0071] Array of LEDs The display device preferably comprises a plurality of variable-wavelength LEDs each configured to receive its own power supply, for example an array of variable-wavelength LEDs. Each of the plurality of LEDs are preferably controllable so that the peak emission wavelength of each LED is controllable by varying a driving condition provided to that LED.

[0072] The device is preferably configured so that light emitted by each variable-wavelength LED passes through a Band Stop filter before being emitted out of the device. The device may be configured so that light emitted by the plurality of variable-wavelength LEDs passes through the same Band Stop filter(s) before passing out of the light-emitting side of the display device. For example, the Band Stop filter or filters may be arranged to extend over a plurality of, or all of, the variable-wavelength LEDs in the display, so that light emitted by a plurality of variable-wavelength LEDs passes through the same Band Stop filter(s) before being transmitted out of the light-emitting side of the display device.

[0073] The display device preferably comprises control electronics configured to modulate the driving current provided to the plurality of variable-wavelength LEDs.

[0074] Variable-wavelength LED

[0075] The variable-wavelength LED is configured to emit a variable peak emission wavelength in response to variations in the electrical driving condition (variations in the driving current or driving voltage) provided to the LED. The peak emission wavelength of the LED is preferably continuously controllable over an emission wavelength range of at least 40 nm by varying the driving current to the LED. The peak emission wavelength may preferably be variable over an emission wavelength range of at least 50 nm, or at least 60 nm, or at least 70 nm, or at least 80 nm by varying the driving current, preferably over a range of up to 100 nm or 110 nm or 120 nm or 140 nm, or 160 nm, or 180 nm, or 200 nm, or 400 nm, or 450 nm.

[0076] In a particularly preferred embodiment, the display device comprises a plurality of variablewavelength LEDs, and each variable-wavelength LED in the display has the same the variable-wavelength LED device epitaxial structure, such that each variable-wavelength LED in the display device is a separate variable-wavelength LED which is configured to emit a variable peak emission wavelength in response to variations in the driving conditions provided to that LED. By driving the variable-wavelength LED pixels separately through a driver circuit it is thus advantageously possible to tune the emission wavelength of each pixel individually by providing different driving currents to the variable-wavelength LED pixels.

[0077] The display is preferably connected to a driver circuit which is configured to supply a variable-magnitude driving current or a variable-magnitude driving voltage, to each variable-wavelength LED pixel, to vary the peak emission wavelength of the variablewavelength LED.

[0078] A variable-wavelength light emitting diode (LED) usable in the display device preferably comprises: an n-doped portion; a p-doped portion; a light-emitting region located between the n-doped portion and a p-doped portion, the light-emitting region comprising a light-emitting layer which emits light at a peak emission wavelength under electrical bias thereacross; wherein the LED is configured to receive a power supply, in which the peak emission wavelength of the LED is continuously controllable over an emission wavelength range by varying, or controlling, a driving condition provided to the variable-wavelength LED. The peak emission wavelength of the variable-wavelength LED is preferably continuously controllable, or continuously variable, over an emission wavelength range of at least 40 nm by varying, or controlling, the power supply.

[0079] As the peak emission wavelength of the variable-wavelength LED is controllable, or variable, over an emission wavelength range the LED can be described as a variablewavelength LED.

[0080] The variable-wavelength emission behaviour of the LED structure is enabled by the fact that the LED structure (the n-doped portion, the light-emitting region and the p-doped portion) are grown over a template containing a porous region. The present inventors have found that the presence of a porous region of Ill-nitride material in the template structure prior to overgrowth of the LED structure leads to higher quality crystal growth and thus significant benefits including the possibility of varying the emission wavelength of the LED light-emitting region. The mechanism by which the porous region enables the variable wavelength emission of the LED is the subject of ongoing study. Benefits provided to the LED by the porous region include strain relaxation, lattice parameter enlargement, wafer bow reduction, and mechanical and thermal influence during the light-emitting region being grown at high temperatures. The variable-wavelength LED is configured to receive a supply of power, or a drive current, from a power supply or LED driver. The term “driving condition” is used herein to refer to the power, or current, or voltage, supplied to drive an LED during use.

[0081] The peak emission wavelength of the LED may preferably be continuously controllable, or continuously variable, over an emission wavelength range by varying, or controlling, the magnitude of a drive current provided to the variable-wavelength LED.

[0082] In traditional LED devices, changes to the driving current provided to the LED produces a very small shift in emission wavelengths, but the present inventors have found that the wavelength shift can be broadened and controlled to a greater extent than traditional LED materials. Rather than the few nm emission range of prior art devices, the LED of the present invention is controllable to emit over a far broader emission range, for example a range of at least 40 nm. As the present LED is tunable to emit over a broad wavelength range, it may be referred to as a variable-wavelength LED.

[0083] The LED may be a dynamic colour-tunable LED, in which the peak emission wavelength of the LED is tunable by varying the driving conditions provided to the LED. The driving conditions are preferably supplied to the or each variable-wavelength LED in the display device by a driving circuit which is connected to a power supply.

[0084] The LED is preferably driveable to emit at a single peak emission wavelength in response to a stable driving condition, but to emit at different peak emission wavelengths in response to variations in the power supply. Thus the LED may be used to emit a particular colour for a prolonged period, or alternatively the LED may be driven to emit a variety of different wavelengths by providing varying driving conditions.

[0085] Preferably, the n-doped portion, the p-doped portion and the light-emitting region all comprise or consist of Ill-nitride material, preferably GaN, InGaN, AIGaN or AllnGaN

[0086] The variable-wavelength LED preferably contains a single epitaxially-grown diode structure containing the n-doped portion, the p-doped portion and the light-emitting region. Thus the variable peak emission wavelengths of the LED are all emitted by the same LED diode structure and composition.

[0087] The LED preferably comprises a porous region of Ill-nitride material. The light-emitting region of the LED is preferably formed over a porous region of Ill-nitride material. In some embodiments, one of the n-doped portion or the p-doped portion may contain the porous region of Ill-nitride material. In other embodiments, the n-doped portion; the p-doped portion; and the light-emitting region are provided on a substrate which comprises the porous region of Ill-nitride material. During epitaxial growth of the LED, the light-emitting region is preferably overgrown after the porous region has been formed.

[0088] The present inventors have found that a porous region of Ill-nitride material enables the same LED to emit at a range of peak emission wavelengths, rather than at one specific wavelength. The peak emission wavelength of the LED may be varied across an emission wavelength range by varying the power supply provided to the LED. The present invention therefore provides a variable-wavelength LED, which may be controlled to emit at any wavelength across a continuous emission wavelength range. By varying the driving conditions provided to the LED by the power supply, the LED is capable of emitting at any wavelength within the emission wavelength range of said LED, and not simply at discrete peak emission wavelengths.

[0089] The present inventors have found that the ability of the LED to emit at tuneable wavelengths across a broad emission range may be imparted by either incorporating a porous region of Ill-nitride semiconductor material into the LED structure, or forming the LED diode structure over a porous region of Ill-nitride semiconductor material. Benefits provided to the LED by the porous region include strain relaxation, lattice parameter enlargement, wafer bow reduction, and beneficial mechanical and thermal influences during the growth of the light-emitting region at high temperatures.

[0090] The light-emitting region of the LED is preferably formed over a porous region of Ill-nitride material during manufacture, so that the porous region influences the structure and mechanical properties of layers of semiconductor that are epitaxially deposited over the porous region. Layers of semiconductor material that are deposited over the porous region during growth experience benefits such as strain reduction, lattice parameter enlargement, and wafer bow reduction, which are imparted to the LED light-emitting region and affect its structure and its light-emitting behaviour.

[0091] Once the LED light-emitting (active) region has been epitaxially grown over the porous region, and the quality of the active region has been enhanced by the influence of the porous region, the beneficial effects of the porous region on the emission properties are permanently imparted to the LED active region. Thus the LED diode structure may be retained on the porous region, in which case the variable-wavelength LED comprises a porous region of Ill-nitride material, or alternatively, during the processing of LEDs into devices after epitaxial growth, the porous region may be removed from the LED structure.

[0092] The width of the emission wavelength range may vary depending on the structure and composition of the LED structure (the n-doped portion, light-emitting region and p-doped portion), and on the structure and porosity of the porous region. The width of the emission wavelength range may also vary depending on the size and shape of the LED (the pixel size and shape).

[0093] In preferred embodiments, the peak emission wavelength is controllable over an emission wavelength range of at least 40 nm, or at least 50 nm, or at least 60 nm, or at least 70 nm, or at least 80 nm by varying the power supply. Preferably the peak emission wavelength is controllable over an emission wavelength range of up to 100 nm, or 110 nm, or 120 nm, or 130nm, or 140nm, or 150nm, or 160nm, or 170nm, or 180nm, or 190nm, or 200 nm, or 400 nm, or 450 nm. The size of the emission wavelength range obtainable by the present LED is thus far greater than the emission ranges achievable with LEDs of the prior art.

[0094] The variable-wavelength LED is advantageously controllable to emit at any peak emission wavelength within its emission wavelength range. By varying the characteristics of the power supply and LED pixel size and shape, the variable-wavelength LED may therefore be controlled to emit light at any selected peak emission wavelength within this range.

[0095] The emission wavelength of the variable-wavelength LED is preferably continuously variable across its emission wavelength range in response to driving conditions provided by a power source being varied continuously across a range of driving conditions.

[0096] The position of the emission wavelength range in the electromagnetic spectrum may also vary depending on the design of the variable-wavelength LED structure (the n-doped portion, light-emitting region and p-doped portion). For example the wavelengths contained in the emission wavelength range may depend on the number and composition of light emitting layers in the variable-wavelength LED. A large variety of LED active regions are known in the art for emitting at different wavelengths in the visible spectrum, so by lightemitting region forming the LEDs of the present invention with different light-emitting regions, emission wavelength ranges covering different portions of the spectrum may be obtained. The variable-wavelength LED emission wavelength range may be between 400 nm and 850 nm, or between 400 nm and 800 nm, or between 400 nm and 690 nm, or between 400 nm and 675 nm. The emission wavelength range may be a sub-range within the range of 400 nm to 750 nm. The emission wavelength range may be tuned to cover any part of this range by selecting different LED active regions and controlling the size and shape of the LED pixels.

[0097] Preferably the emission wavelength range of the variable-wavelength LED extends from a lower end below 410 nm, or 430 nm, or 450 nm, or 470 nm, or 500 nm, or 520 nm, or 540 nm, or 560 nm, to an upper end above 570 nm, or 580 nm, or 600 nm, or 610 nm, or 630 nm, or 650 nm, or 675 nm. The first and second ends of the emission wavelength range may be tuned depending on the selection of LED structure and LED shape and size, as described above.

[0098] For example in preferred embodiments the lower end of the emission wavelength may be between 400 nm and 450 nm (violet) or between 450 nm and 500 nm (blue) or between 500 nm and 570 nm (green), and the upper end of the emission wavelength may be between 570 nm and 590 nm (yellow), or between 590 nm and 610 nm (orange), or between 610 and 700 nm (red).

[0099] In a preferred embodiment, the variable-wavelength LED emission wavelength range may extend from a lower end that is below 500 nm to a higher end that is above 610 nm, so that the peak emission wavelength of the LED may be varied to emit at any wavelength from blue (below 500 nm) to red (above 610 nm) by varying the driving condition provided to the variable-wavelength LED. Providing a single LED design that can be controlled to emit at blue wavelengths (450-500 nm), green (500-570 nm) and also at yellow (570-590 nm), orange (590-610 nm) and red (610-760 nm) is highly advantageous, and could provide significant advantages for LED displays.

[0100] In other preferred embodiments, the variable-wavelength LED emission wavelength range may extend between 520 nm and 660 nm, or between 550 nm and 650 nm, by varying the driving condition provided to the variable-wavelength LED.

[0101] In a particularly preferred embodiment, the peak emission wavelength is controllable between 540 nm and 680 nm, or between 560 nm and 675 nm, by varying the driving condition provided to the variable-wavelength LED. Thus, the same LED may be controllable to emit at a peak emission wavelength anywhere between 540 nm in the green and 680 nm in the red. Green and red LEDs have historically been more difficult to manufacture than shorter wavelength blue LEDs due to issues such as the difficulty of incorporating the required indium content into the light-emitting region. Providing a single LED design that can be controlled to emit at green wavelengths (500-570 nm) and also at yellow (570-590 nm), orange (590-610 nm) and red (610-760 nm) is therefore highly advantageous, and could provide significant advantages for LED displays.

[0102] In another preferred embodiment, the peak emission wavelength is controllable between 520 nm and 675 nm, or between 550 nm and 650 nm, by varying the driving condition provided to the variable-wavelength LED.

[0103] Although the variable-wavelength LED can emit across a continuous emission wavelength range, in some embodiments it may be desirable to control the LED to function in a plurality of discrete emission modes, for example in response to a power supply having a plurality of driving modes. For example by driving the LED in a plurality of different modes corresponding to discrete emission colours, a simplified colour display may be provided, in which discrete emission colours are mixed in known methods to give a desired visual effect.

[0104] The variable-wavelength LED is preferably controllable to emit at least two discrete peak emission wavelengths by varying the driving conditions provided by the power supply between two discrete driving conditions (such as two discrete magnitudes of drive current). The LED may be controllable to emit at a first peak emission wavelength in response to a first driving condition provided by the power supply (which may be a drive current having a first magnitude), at a second peak emission wavelength in response to a second driving condition provided by the power supply (which may be a drive current having a second magnitude different from the first magnitude).

[0105] The variable-wavelength LED is preferably controllable to emit at least three discrete peak emission wavelengths by varying the driving conditions provided by the power supply. The peak emission wavelength of the variable-wavelength LED may thus be variable over at least three “colours” in the EM spectrum.

[0106] The variable-wavelength LED may be controllable to emit at a first peak emission wavelength in response to a first driving condition provided by the power supply, at a second peak emission wavelength in response to a second driving condition provided by the power supply, and at a third peak emission wavelength in response to a third driving condition provided by the power supply.

[0107] The variable-wavelength LED may preferably be controllable to emit a blue peak emission wavelength in response to a first driving condition provided by the power supply, to emit a green peak emission wavelength in response to a second driving condition provided by the power supply, and to emit a red peak emission wavelength in response to a third driving condition provided by the power supply.

[0108] The variable-wavelength LED may be controllable to emit a first peak emission wavelength in the range 400-500 nm in response to a first driving condition provided by the power supply, to emit a second peak emission wavelength in the range 500-550 nm in response to a second driving condition provided by the power supply, and to emit a third peak emission wavelength greater than 600 nm in response to a third driving condition provided by the power supply.

[0109] Preferably, the variable-wavelength LED is controllable to emit a first peak emission wavelength in the range 430-460 nm in response to a first driving condition provided by the power supply, to emit a second peak emission wavelength in the range 510-560 nm in response to a second driving condition provided by the power supply, and to emit a third peak emission wavelength in the range 600-660 nm in response to a third driving condition provided by the power supply.

[0110] The first, second and third driving conditions may be first, second and third current densities, or the first, second and third driving conditions may be first, second and third power densities.

[0111] The morphology of quantum wells (QWs) in the active light-emitting region may be varied. For example the light-emitting region may contain uniform QWs with well-defined interfaces or fragmented QWs with less well-defined interfaces, fragmentation, or QW well width / composition fluctuation or quantum dots like localisation centres. This control of QW morphology can determine the range of the variable emission wavelength to be controlled and manipulated.

[0112] The light-emitting region preferably comprises a plurality of quantum wells (QWs). The quantum wells may be continuous. The quantum wells may be fragmented, or discontinuous. The variable-wavelength LED may comprise a current constraining layer, or a current limiting layer, which is a dielectric layer configured to confine the lateral area of the LED through which current is conducted. The use of a current constraining layer may advantageously allow further control of the current density, in order to better control the peak emission wavelength of the LED.

[0113] The current constraining layer may advantageously enable the manipulation of the power density provided to the variable-wavelength LED, in order to control the peak emission wavelength.

[0114] The current constraining layer is preferably a layer of dielectric material. For example, the current constraining layer may be any dielectric, for example SiC>2, SiN or SiNx.

[0115] The current constraining layer may be positioned in a variety of positions in the variablewavelength LED, as long as it confines the lateral area of the LED through which current is conducted. The current constraining layer may be positioned in the LED between an electrical n-contact and an electrical p-contact.

[0116] The current constraining layer may be positioned adjacent to either the n-doped portion or the p-doped portion of the LED. For example the current constraining layer may be positioned between the n-doped portion and the light-emitting region. Alternatively the current constraining layer may be positioned between the light-emitting region and the p- doped portion. The current constraining layer may be positioned between an electrical contact and the LED structure (n-doped portion, p-doped portion and light-emitting region).

[0117] The current constraining layer preferably comprises an aperture extending through the current constraining layer, or one or more apertures extending through the current constraining layer. The aperture may preferably be positioned in the centre of the current constraining layer. For example the current constraining layer may comprise a circular opening in the centre of the LED structure.

[0118] The variable-wavelength LED may be configured so that an electrical contact is in contact with the LED structure via the aperture in the current constraining layer, so that the area of the aperture defines a contact area over which the contact and the LED structure are touching.

[0119] The lateral dimensions of the or each aperture is preferably much smaller than the lateral dimensions of the LED. By providing an aperture through the dielectric current constraining layer, high local current density may be achieved, which may advantageously enable improved control of the power through the LED.

[0120] For example the lateral width (or diameter) of the aperture may be equal to or less than 50% of the lateral width of the LED structure (the LED mesa). The width of the aperture may be equal to or less than 45%, or 40%, or 35%, or 30%, or 25%, or 20% of the width of the LED structure.

[0121] The relative area of the aperture compared to the overall area of the current constraining layer (the blocked region) may be varied to modify the local current density.

[0122] The light emitting region preferably comprises a multiple quantum well (MQW) containing a plurality of quantum wells (QWs), or quantum dots, quantum wires, or other quantum nanostructures.

[0123] In some embodiments, the light-emitting region comprises a plurality of quantum wells (QWs), and the quantum wells are continuous.

[0124] The present inventors have found that non-uniformities in the light-emitting region have a significant effect in broadening the emission wavelength range across which a light-emitting region can emit light in response to variations in the power supplied to the LED. In the prior art, non-uniformities in the light-emitting region are typically considered problematic flaws, which are unwanted and should be avoided in any way possible because the goal is typically a high-quality, low-flaw semiconductor wafer. The present inventors have eschewed this prejudice in the art, and found that intentionally creating non-uniformities in the light emitting region may advantageously broaden the emission wavelength range and result in a variable-wavelength LED which can emit over a far broader wavelength range than has ever been possible in the prior art.

[0125] In alternative embodiments of the present invention, the light-emitting region is non- uniform, fragmented, or discontinuous. The light-emitting region may be deliberately introduced to achieve the effect of carrier localisation centres in InGaN quantum wells, such as multiple types of QW region with different Indium composition and well width and quantum barriers, non-uniform, or fragmented, or broken, or gappy, or discontinuous quantum wells which would result in fluctuation in the well width, InGaN quantum dots or nanostructures, quantum wells formed on polar, semi-polar or non-polar facets. In a preferred embodiment, the light-emitting region comprises a plurality of quantum wells (QWs), and the quantum wells are non-uniform, fragmented, or discontinuous.

[0126] The plurality of QWs may comprise fluctuations in well-width. For example the well width of the QWs may fluctuate by at least 2%, 5%, 10%, 20%, 25%, or 50%, or 75%. The well width fluctuations can be variations between quantum wells (vertical direction) as well as within one quantum well (lateral direction).

[0127] The plurality of QWs may comprise fluctuations in alloy composition. For example the indium composition of the QWs may vary by at least 2%, 5%, 10%, 20%, 25% or 50% or 75% across the light-emitting region.

[0128] The inventors have found that fluctuations in well-width and / or alloy composition may induce carrier localisation centres, either in the upper interface or lower interface of the QWs. Any carrier localisation centres would induce the variable wavelength in the variablewavelength LED of the present invention. The larger the density of those carrier localisation centres, the larger the variable wavelength range can be achieved.

[0129] The variable-wavelength LED may comprise a v-shaped pit which extends, or propagates, through the light emitting active region. Preferably the LED comprises a plurality of v- shaped pits which extend through the light-emitting region.

[0130] Preferably the variable-wavelength LED may comprise a density of v-shaped pits (measured looking down onto the LED structure from above) of at least 1 x 107 / cm2, for example at least 5 x 107 / cm2or at least 1 x 108 / cm2, for example a density of v-shaped pits of 1 x 107 / cm2to 5 x 109 / cm2.

[0131] The variable-wavelength LED may comprise a density of v-shaped pits of less than 5 x 109 / cm2, for example a density of v-shaped pits of less than 1 x 109 / cm2or less than 5 x 108 / cm2.

[0132] V-shaped pits are a phenomenon known in the art of epitaxial semiconductor growth, and methods of growing v-shaped pits in semiconductor structures are known in the art. For example, v-shaped pits and their growth are described in the prior art in The effect of nanometre-scale V-pits on electronic and optical properties and efficiency droop of GaN- based green light-emitting Diodes’, Zhou et al; Scientific Reports | (2018) 8:11053 | DOI : 10.1038 / S41598-018-29440-4. These v-shaped pits are v-shaped when viewed in cross-section, but in reality form as conical or funnel-shaped voids in semiconductor structures that are grown from the bottom up using conventional epitaxial growth methods. While the cross-section of the pits is v- shaped, the pits are typically hexagonal when viewed from above. The point of the v- shaped pits are always directed downwards towards earlier-deposited layers of semiconductor structure, as the pits widen as subsequent layers of epitaxial growth are deposited on top of the structure.

[0133] Although v-shaped pits are known in the art, they are typically considered a problematic flaw in semiconductor structures, which are unwanted because the goal is typically a high- quality, low-flaw semiconductor wafer.

[0134] In the unusual situations where v-shaped pits have been incorporated into semiconductor structures in the past, the v-shaped pits have been used as a screening mechanism to create higher band gap regions which prevent current carriers going down threading dislocations as a leakage path.

[0135] In some preferred embodiments of the present invention, however, v-shaped pits are intentionally incorporated into the variable-wavelength LED structure. The v-shaped pits extend far enough down into the semiconductor structure that they terminate in a layer below the active light-emitting region. This means that the v-shaped pits must extend through the thickness of the active light-emitting region.

[0136] The present inventors have found that v-shaped pits extending through the light-emitting region of the LED structure may advantageously broaden the emission wavelength range over which a variable-wavelength LED can emit.

[0137] As the v-shaped pits extend through the active region of the LED, during epitaxial growth from the bottom up, quantum well (QW) layers that are planar across the rest of the structure are grown on the sloping side walls of the v-shaped pits. The QWs deposited on the pit sidewalls are distorted and stretched around the sides of the pits, so end up being of different thickness and composition compared to the planar QWs across the bulk of the structure.

[0138] Around the v-shaped pits, QW layers of semiconductor material are grown as flat planar layers. The active light-emitting region is thus planar around the v-shaped pit. In the location of the v-shaped pits, however, the active layers are distorted and stretched downwards along the sidewalls into the v-shaped pit. This stretching effect changes the thickness of the QWs on the sidewalls of the pit, so that they are different in thickness compared to the planar QW layers formed over the rest of the LED structure.

[0139] The inventors have found that v-shaped pits can create local strain relaxation, and MQWs deposited on the sidewall of these v-pits will have different thickness and composition compared to the rest of the MQW, hence the MQW in the region of the v-shaped pits will produce a different emission wavelength.

[0140] The quantum wells grown on the side walls of the v-shaped pit are thinner than the bulk planar QWs elsewhere in the structure, which may affect the QW bandgap and allow the QWs in this region to emit at wavelengths different from those emitted by the planar QWs elsewhere in the structure. In addition to this, the QWs on the pit sidewalls may end up having a higher indium (In) content than the surrounding planar QWs, because the sidewalls expose a semipolar facet of the QWs - this facet incorporates more indium during epitaxial growth, so the QWs in the region of the v-shaped pits may be higher in indium than the planar QWs around the pits. Higher indium incorporation typically leads to longer peak emission wavelengths. Both the QW thickness and the indium content affect the emission wavelengths produced by the light-emitting region. The presence of v-shaped pits in the LED structure may thus advantageously modify the composition and thickness of QWs in the light-emitting region in a way that expands the emission wavelength range over which the LED can be driven to emit light.

[0141] V-shaped pits typically grow from threading dislocations in the semiconductor structure. The threading dislocations are perpetuated upwards through the structure as additional layers are grown over layers containing a threading dislocation, and at a certain point the dislocation widens into a v-shaped pit. Typically the skilled person aims to keep threading dislocation concentrations low in order to produce a “high quality” low-flaw wafer.

[0142] V-shaped pits can alternatively be grown using 3-dimensional epitaxial growth modes. 3D epitaxial deposition techniques are known in the art and are typically used to grow “islands” or “pyramids” of semiconductor material on a template. By controlling deposition of the LED structure using 3D epitaxial deposition techniques, v-shaped pits can be artificially grown in desired locations, with no need for a threading dislocation to be present to “seed” the formation of the v-shaped pit. By using this deposition control, the bottom (nadir) of the pit may be created at a desired location in the structure - both a desired lateral position and a desired height in the structure, for example in a particular layer of the semiconductor structure below the active light-emitting region.

[0143] The bottom of the v-shaped pit may be located in the connecting layer of the semiconductor structure. The connecting layer may be positioned between the porous region and the n-doped portion.

[0144] The bottom of the v-shaped pit may be located in a pre-strain layer of the semiconductor structure. The pre-strain layer may be positioned above the n-doped portion and below the light-emitting region.

[0145] Preferably the variable-wavelength LED comprises a plurality of v-shaped pits which extend through the active light-emitting region.

[0146] Preferably the variable-wavelength LED comprises a density of v-shaped pits (measured looking down onto the LED structure from above) of at least 1 x 107 / cm2, for example at least 5 x 107 / cm2or at least 1 x 108 / cm2. The LED may comprise a density of v-shaped pits of less than 5 x 109 / cm2, for example a density of v-shaped pits of less than 1 x 109 / cm2or less than 5 x 108 / cm2.

[0147] For example the variable-wavelength LED may comprise a density of v-shaped pits of 1 x 107 / cm2to 5 x 109 / cm2, or 5 x 107 / cm2to 5 x 109 / cm2, or 1 x 108 / cm2to 5 x 108 / cm2.

[0148] The variable-wavelength LED may comprise more than 0.1 v-shaped pit per square micrometre, or more than 1 v-shaped pits per square micrometre, or more than 2 v-shaped pits per square micrometre.

[0149] The concentration of v-shaped pits in the variable-wavelength LED is preferably controlled, as too many v-shaped pits may negatively affect the light emission of the LED by disrupting radiative recombination. For example the LED may comprise fewer than 10 v-shaped pits per square micrometre, or fewer than 8 v-shaped pits per square micrometre, or fewer than 6 v-shaped pits per square micrometre.

[0150] In a preferred embodiment the LED structure may comprise no greater than 109threading dislocations per square centimetre. Preferably the semiconductor structure below the active light-emitting region (typically a substrate, the porous region and a connecting layer) comprise no more than 109threading dislocations per square centimetre. The threading dislocation density is preferably limited to this level so that further epitaxial growth does not create too many v-shaped pits in the light-emitting region.

[0151] Both the density and size (the depth) of the v-shaped pits may be controlled. The size of the V-pits can be controlled by the position and the growth conditions of the pre-strain layer and the low-temperature nGaN layer where the pits started.

[0152] The morphology of quantum wells (QWs) in the active light-emitting region may be varied. For example the light-emitting region may contain uniform QWs with well-defined interfaces or fragmented QWs with less well-defined interfaces, fragmentation, or QW well width / composition fluctuation or quantum dot like localisation centres. This control of QW morphology can determine the range of the variable emission wavelength to be controlled and manipulated.

[0153] The light-emitting region preferably comprises a plurality of quantum wells (QWs). The quantum wells may be continuous. The quantum wells may be fragmented, or discontinuous.

[0154] If QWs are continuous and very uniform in thickness and composition, recombination of charge carriers can only happen in regular well defined ways. On the other hand, if QWs are fragmented or discontinuous, this creates lots of nanostructures, which in turn creates different band gaps that result in emission of different colours.

[0155] The light-emitting region, and / or the LED, may have lateral dimensions (width and length) of greater than 100 pm and less than 300 pm. In this case, the LED may be termed a “miniLED”. In preferred embodiments, the mini-LED may be square or circular or square with circular corners and have dimensions such as 300 pm x 300 pm, 200 pm x 200 pm, 100 pm x 100 pm.

[0156] The light-emitting region, and / or the LED, may alternatively have lateral dimensions (width and length) of less than 100 pm. In this case, the LED may be termed a “micro-LED”. The micro-LED may preferably have lateral dimensions of less than 80 pm, or 70 pm, or 60 pm, or 50 pm or 30 pm, or 25 pm, or 20 pm, or 15 pm or 10 pm, or 5 pm or 3 pm or 2 pm.

[0157] In preferred embodiments, the micro-LED may be square or circular or square with circular corners and have dimensions such as 75 pm x 75 pm, 50 pm x 50 pm, 40 pm x 40 pm, 30 pm x 30 pm, 25 pm x 25 pm, 20 pm x 20 pm or 10 pm x 10 pm, or 5 pm x 5 pm, or 2 pm x 2 pm, or 1 pm x 1 pm, or 500 nm x 500 nm or smaller. The light-emitting region, and / or the LED, may alternatively have lateral dimensions (width and length) of less than 1 pm. In this case, the LED may be termed a “nano-LED”. The nano-LED may preferably have lateral dimensions of less than or 500nm, or 200nm, or 100nm, or 50nm.

[0158] The LEDs may be circular, triangle, rectangular, square, oval, diamond, hexagonal, pentagonal, and any combination of these shapes. In the case of irregular-shapes of pixel design, at least one dimension should fall within the dimensions defined above in order for the LEDs to be classed as mini- or micro-LEDs. For example the width or diameter of the LEDs are preferably less than 100 pm so that the LEDs are classed as micro-LEDs.

[0159] Porous Regions

[0160] The variable-wavelength LEDs preferably comprise one or more porous regions of Ill- nitride material.

[0161] The n-type region, the light-emitting region and the p-type region (which may be called the LED structure, or LED diode structure) are preferably grown over a semiconductor template which contains the porous region. The semiconductor template may also contain a number of layers of semiconductor material arranged to provide a suitable substrate for the overgrowth of the LED structure.

[0162] The porous region may be a porous layer, such that the light emitting diode comprises a porous layer of Ill-nitride material. Preferably the porous region may be a porous layer that is continuously porous, for example formed from a continuous layer of porous Ill-nitride material.

[0163] The porous region may comprise a plurality of porous layers, and optionally a plurality of non-porous layers. In preferred embodiments of the invention, the porous region is a stack of alternating porous and non-porous layers, with the top surface of the stack defining the top of the porous region, and the bottom surface of the stack defining the bottom of the porous region. The light-emitting region may be formed over a porous region comprising a stack of porous layers of Ill-nitride material.

[0164] Each variable-wavelength LED may comprise its own porous region of Ill-nitride material, or alternatively a shared porous region may be epitaxially connected to a plurality of the variable-wavelength LEDs. Method of manufacture

[0165] According to a second aspect there may be provided a method of manufacturing a display device. The display device may preferably be a display device according to the first aspect described above.

[0166] The method of manufacture, comprising the step of forming a semiconductor structure on a substrate, the semiconductor structure comprising: a Band Stop filter and a variablewavelength LED.

[0167] The variable-wavelength LED may comprise: an n-doped portion; a p-doped portion; and a light-emitting region located between the n-doped portion and a p-doped portion, the light-emitting region comprising a light-emitting layer which emits light at a peak emission wavelength under electrical bias thereacross in which the variable-wavelength LED is configured so that the peak emission wavelength of the LED is variable over an emission wavelength range of at least 40 nm.

[0168] The Band Stop filter is configured to attenuate emitted light with a wavelength falling inside a stopband wavelength range, and the Band Stop filter is configured so that the stopband wavelength range is inside the emission wavelength range of the variable-wavelength LED.

[0169] The method may comprise the step of forming the variable-wavelength LED over the substrate. The method may comprise the further step of forming the Band Stop filter over the variable-wavelength LED, and processing the semiconductor structure so that the Band Stop filter is positioned between the variable-wavelength LED and a light-emitting side of the display device.

[0170] The method may comprise the step of forming the Band Stop filter on the substrate, and forming the variable-wavelength LED over the Band Stop filter.

[0171] The method may further comprise the step of removing the substrate from a first surface of the semiconductor structure, and processing the semiconductor structure so that the first surface forms the light-emitting side of the display device, with the Band Stop filter positioned between the variable-wavelength LED and the light-emitting side of the display device. The method may comprise the step of overgrowing the variable-wavelength LED over a porous region of Ill-nitride material.

[0172] The method may optionally comprise the step of porosifying an n-doped region of Ill-nitride material to form a porous region of Ill-nitride material.

[0173] The Band Stop filter may comprise one or more layers of porous Ill-nitride semiconductor material.

[0174] The method may comprise the step of forming the Band Stop filter by porosifying a plurality of layers of porous Ill-nitride semiconductor material alternating between a plurality of non- porous layers of Ill-nitride material, or forming a first plurality of layers of porous Ill-nitride material having a first porosity alternating with a second plurality of porous layers of Ill- nitride material having a second porosity. The thicknesses and refractive indices of the layers in the Band Stop filter may be configured to attenuate wavelengths in the stopband by destructive interference.

[0175] The method may alternatively comprise the step of forming a rugate Band Stop filter by porosifying a plurality of layers of Ill-nitride semiconductor material with a porosity which varies through each layer.

[0176] The method may comprise the step of forming a porous region of Ill-nitride material in at least one of the n-doped portion or the p-doped portion, and forming the light-emitting region over a porous region of Ill-nitride material.

[0177] The method may comprise the step of forming the light emitting active region with carrier localisation centres in quantum wells, such as multiple types of QW region with different Indium composition and well width and quantum barriers, non-uniform, or fragmented, or broken, or gappy, or discontinuous quantum wells which would result in fluctuation in the well width, quantum dots or nanostructures, quantum wells formed on polar, semi-polar or non-polar facets.

[0178] The light-emitting region may comprise a plurality of quantum wells (QWs), and in which the quantum wells are non-uniform, fragmented, or discontinuous. The plurality of QWs may comprise fluctuations in indium composition, and / or well width fluctuations.

[0179] The method may comprise the step of forming one or more v-shaped pits in the LED structure, so that the v-shaped pit extends through the light-emitting region, preferably forming a plurality of v-shaped pits which extend through the light-emitting region. The method may comprise the step of forming at least 0.1 v-shaped pits per square micrometre, or at least 1 v-shaped pits per square micrometre, or at least 2 v-shaped pits per square micrometre. The method may comprise the step of forming a density of v-shaped pits in the light-emitting region of at least 1 x 107 / cm2, for example at least 5 x 107 / cm2or at least 1 x 108 / cm2, for example a density of v-shaped pits of 1 x 107 / cm2to 5 x 109 / cm2. The method may comprise the step of forming a density of v-shaped pits in the light-emitting region of less than 5 x 109 / cm2, for example a density of v-shaped pits of less than 1 x 109 / cm2or less than 5 x 108 / cm2.

[0180] Brief Description of the Drawings

[0181] Embodiments of the invention will now be described with reference to the figures, in which:

[0182] Figure 1 is a schematic illustration of a display device incorporating colour filters to convert white light into red, green and blue emitted light;

[0183] Figure 2 is a schematic illustration of a display device incorporating colour converters to convert blue light into red and green emitted light;

[0184] Figure 3 illustrates typical emission mode spectra for a variable-wavelength LED emitting in blue, green and red emission modes;

[0185] Figure 4 is a schematic illustration of a display device according to a preferred embodiment of the present invention;

[0186] Figure 5 is a schematic illustration of a display device according to a preferred embodiment of the present invention;

[0187] Figure 6 is a schematic illustration of a display device according to a preferred embodiment of the present invention;

[0188] Figure 7a is a schematic illustration of the typical emission mode spectra for a variablewavelength LED emitting in blue, green and red emission modes, overlaid with the stopband of a Band Stop filter; Figure 7b is a comparative illustration of colour gamuts emittable by an sRGB display, and the variable-wavelength LED display device of Figure 9a with and without the Band Stop filter;

[0189] Figure 8a is a schematic illustration of the typical emission mode spectra for a variablewavelength LED emitting in blue, green and red emission modes, overlaid with the stopband of a first exemplary Band Stop filter;

[0190] Figure 8b is a comparative illustration of colour gamuts emittable by an sRGB display, and the variable-wavelength LED display device of Figure 9a with and without the Band Stop filter;

[0191] Figure 9a is a schematic illustration of the typical emission mode spectra for a variablewavelength LED emitting in blue, green and red emission modes, overlaid with the stopband of a second exemplary Band Stop filter;

[0192] Figure 9b is a comparative illustration of colour gamuts emittable by an sRGB display, and the variable-wavelength LED display device of Figure 10a with and without the Band Stop filter;

[0193] Figure 10a is a schematic illustration of exemplary emission mode spectra for a variablewavelength LED emitting in blue, green and red emission modes, overlaid with the two stopbands of idealised Band Stop filters;

[0194] Figure 10b is a schematic illustration of the resulting emission spectrum emittable from the display device of Figure 10a;

[0195] Figure 10c is a comparative illustration of colour gamuts emittable by an sRGB display and the variable-wavelength LED display device of Figures 10a and 10b with and without the two-stopband Band Stop filter;

[0196] Figure 11a is a schematic illustration of exemplary emission mode spectra for a variablewavelength LED emitting in blue, green and red emission modes, overlaid with the stopband of an idealised Band Stop filter;

[0197] Figure 11b is a comparative illustration of colour gamuts displayable by an sRGB display and the variable-wavelength LED display device of Figure 11b with and without the Band Stop filter; Figures 12a, 12b and 12c illustrate experimentally-measured transmission spectra for three commercially-available Band Stop filters;

[0198] Figure 12d illustrates an experimentally-measured transmission spectrum for a combination of the three Band Stop filters of Figures 12a, 12b and 12c;

[0199] Figure 12e illustrates experimentally measured colour gamuts displayable using variablewavelength LEDs and the Band Stop filters of Figure 12d;

[0200] Figure 13a is a schematic illustration of emission mode spectra for a variable-wavelength LED emitting in blue, green and red emission modes, overlaid with the two idealised stopbands of a first preferred Band Stop filter;

[0201] Figure 13b is a schematic illustration of the resulting emission spectrum emittable from the display device of Figure 13a;

[0202] Figure 13c is a comparative illustration of colour gamuts displayable by an sRGB display and the variable-wavelength LED display device of Figures 13a and 13b with and without the two-stopband Band Stop filter;

[0203] Figure 14a is a schematic illustration of emission mode spectra for a variable-wavelength LED emitting in blue, green and red emission modes, overlaid with the two idealised stopbands of a second preferred Band Stop filter;

[0204] Figure 14b is a schematic illustration of the resulting emission spectrum emittable from the display device of Figure 14a;

[0205] Figure 14c is a comparative illustration of colour gamuts displayable by an sRGB display and the variable-wavelength LED display device of Figures 14a and 14b with and without the two-stopband Band Stop filter;

[0206] Figure 15a is a schematic illustration of emission mode spectra for a variable-wavelength LED emitting in blue, green and red emission modes, overlaid with the two idealised stopbands of a third preferred Band Stop filter;

[0207] Figure 15b is a schematic illustration of the resulting emission spectrum emittable from the display device of Figure 15a; Figure 15c is a comparative illustration of colour gamuts displayable by an sRGB display and the variable-wavelength LED display device of Figures 15a and 15b with and without the two-stopband Band Stop filter;

[0208] Figure 16a is a schematic illustration of emission mode spectra for a variable-wavelength LED emitting in blue, green and red emission modes, overlaid with the two idealised stopbands of a fourth preferred Band Stop filter;

[0209] Figure 16b is a schematic illustration of the resulting emission spectrum emittable from the display device of Figure 16a;

[0210] Figure 16c is a comparative illustration of colour gamuts dislayable by an sRGB display and the variable-wavelength LED display device of Figures 16a and 16b with and without the two-stopband Band Stop filter;

[0211] Figure 17a-c are graphs illustrating the experimentally measured WLD and transmission characteristics of the four preferred Band Stop filters of Figures 13a to 16c;

[0212] Figure 18 is a comparative illustration of colour gamuts emittable by an sRGB display and the variable-wavelength LED display device of Figures 13a to 17, with nine variations of two-stopband Band Stop filter;

[0213] Figure 19a is a schematic illustration of emission mode spectra for a variable-wavelength LED emitting in blue, green and red emission modes, comparing the original unfiltered emission spectra with those of three alternative Band Stop filters, Notch_1, Notch_2, and Notch_3;

[0214] Figure 19b is a graph of the brightness ratio of blue, green and red emission modes for the four examples of Figure 19a;

[0215] Figure 19c is a comparative illustration of colour gamuts emittable by the four examples of Figures 19a and 19b;

[0216] Figure 20a is a schematic illustration of a display device according to a preferred embodiment of the present invention; and

[0217] Figure 20b is a schematic illustration of a display device according to another preferred embodiment of the present invention. In order to obtain multi-coloured light emission from a monochrome display, it is known in the prior art that spectral elements such as colour filters and colour-converters can be used.

[0218] Figure 1 , for example, schematically illustrates a display device in which an array of white- light-emitters emit white light, and red, green and blue colour filters are positioned between the white-light-emitters and the light-emitting side of the device. White light emitted by the white-light-emitters passes through the colour filters, which converts the transmitted white light into red, green and blue emitted light respectively. In this arrangement it is necessary for each colour filter to have its own dedicated white-light-emitter, each of which can be turned on and off so that the device can be controlled to emit red, green and blue light selectively. The red, green and blue colour filters and their corresponding white-light- emitters can thus be controlled as spatially-separated red, green and blue pixels or subpixels.

[0219] Figure 2 illustrates an alternative situation in which an array of blue-light-emitters emit blue light, and red and green colour-converters are positioned between some of the blue-light- emitters and the light-emitting side of the device. At least one blue-light-emitter is not covered by a colour-converter, so that blue light emitted by the uncovered blue-light-emitter can pass directly out of the light-emitting side of the display device.

[0220] Colour-converters can be conventionally made from quantum dots or phosphor materials, and function by absorbing the blue light which is emitted by the light-emitters below the colour-converters, and re-emitting the absorbed light with a new red or green wavelength.

[0221] In this arrangement it is necessary for each red colour-converter to have its own dedicated blue-light-emitter, and for each green colour-converter to have its own dedicated blue-light- emitter, and that each blue-light-emitter can be turned on and off so that the device can be controlled to emit red, green and blue light selectively. The red and green colour-converters and their corresponding blue-light-emitters can thus be controlled as spatially-separated red and green pixels or subpixels, while the uncovered blue-light-emitter can be controlled as a spatially-separate blue pixel or subpixel.

[0222] Both of these solutions of colour-filtering and colour-conversion are only usable in display devices where spatially separated sub-pixels are used to emit discrete primary wavelengths. Another known solution is to use spectral filters, such as bandpass filter with a pass band centred on a primary emission peak to narrow the FWHM of that emission peak.

[0223] As described above, in the present invention a variable-wavelength LED is used to emit more than one peak emission wavelength. This means that the discrete colours emitted by the variable-wavelength LED are not spatially-separated, so the conventional colour-filter and colour-converter approaches of Figures 1 and 2 cannot be used.

[0224] As described above, an optoelectronic display device containing a variable-wavelength LED can be operated to display any chromaticity within a displayable colour gamut, by controlling the LED to emit light in a plurality of emission modes. Each emission mode defines a primary emission wavelength of the display device, and emitting those primary wavelengths in combination provides a combined output signal with the desired output chromaticity and output luminance.

[0225] The size of the colour gamut which is achievable depends on the wavelengths of the primary emission modes that are selected, and also on the breadth of the emission spectrum which is emitted by the variable-wavelength LED in each emission mode.

[0226] In the present invention a variable-wavelength LED is preferably driven in a field-sequential mode, so that the same variable-wavelength LED emits a plurality of primary emission wavelengths by operating the variable-wavelength LED in a plurality of discrete emission modes.

[0227] Variable-wavelength LEDs usable in the present invention are known as dynamic-pixel- tuning (DPT®) variable-wavelength LEDs from Poro Technologies Ltd, and are disclosed in international patent application no. PCT / GB2022 / 051997, published as W02023 / 007174.

[0228] In preferred embodiments, the same variable-wavelength pixel is used to emit all of the selected primary emission wavelengths using field-sequential emission. In this case, all of the primary emission wavelengths are selected to be within the emission wavelength range of the variable-wavelength LED.

[0229] As described above, instead of driving variable-wavelength LEDs to emit a continuum of emission wavelengths by supplying a continuously varying driving current, such LEDs can be conveniently driven by selecting two, or three, or more than three “primary” emission wavelengths at which the variable-wavelength LED is operated. Output signals having desired output chromaticities and output luminances can then be generated from temporal combinations of pulses of light at the primary emission wavelengths, similarly to known field-sequential displays.

[0230] For the sake of simplicity, the following examples reference a preferred embodiment in which three emission modes are used to drive a variable-wavelength LED using field- sequential driving. In order to obtain a large colour gamut triangle of displayable output chromaticities, in the preferred embodiment the LED is driven in first, second and third emission modes to emit first, second and third primary emission wavelengths which are red (R), green (G), and blue (B) respectively.

[0231] Figure 3 illustrates typical emission mode spectra for a variable-wavelength LED emitting light in a blue emission mode 402, green emission mode 404 and red emission mode 406. One drawback of using variable-wavelength LEDs rather than single-wavelength LEDs is that the emission peaks of the variable-wavelength LEDs have broader emission spectra (wider FWHM) than conventional single-wavelength LEDs.

[0232] In the case of the central green emission peak 404, the broad emission spectrum emitted by the variable-wavelength LED contains wavelength overlap ranges 408, 410 either side of the green peak emission wavelength. A green-blue overlap range 408 contains wavelengths of light which are emitted in green emission mode 404 and also in blue emission mode 402, while a green-red overlap range 410 contains wavelengths of light which are emitted in green emission mode 404 and also in red emission mode 406. The presence of these overlap ranges means that when the variable-wavelength LED is driven to emit in green emission mode 404, the wavelength overlap with the red and blue peaks makes the green mode spectrum appear less pure green and more “white” to an observer. These broad emission spectra and overlap ranges thus mean that the purity of the emitted green light is lower, although the observed “average” wavelength may still be the same.

[0233] As wavelengths in the green-blue overlap range 408 are emitted when the variablewavelength LED is driven to operate in the blue emission mode 402, the emitted spectrum contains a significant range of green wavelengths. This moves the observed wavelength towards the green, although the perceived purity of the blue emission mode is affected less than for the green case above.

[0234] Likewise for red emission, as wavelengths in the green-red overlap range 410 are emitted when the variable-wavelength LED is driven to operate in the red emission mode 406, the emitted spectrum contains a significant range of green wavelengths. This moves the observed wavelength away from red and towards the green, although the perceived purity of the red emission mode is affected less than for the green case above.

[0235] Table 1 illustrates exemplary FWHM values for a dynamic-pixel-tuning (DPT®) variablewavelength LEDs from Poro Technologies Ltd operating at three peak emission wavelengths.

[0236] Table 1.

[0237] As shown in Table 1 , typical emission spectra for a variable-wavelength LED with a broad emission range have much wider FWHM than conventional “single-colour” LEDs. This means that the emission peaks produced by a variable-wavelength LED contain significant ranges of overlap with adjacent emission peaks intended to produce different primary emission colours. This reduces the purity of single emission colours, shifts the dominant wavelength away from the spectral peak wavelength, and reduces the size of the displayable colour gamut.

[0238] Figure 4 is a schematic illustration of a display device 500 according to a preferred embodiment of the present invention. As shown in Figure 4, the display device 500 comprises a substrate 510, a notch filter 520 and a variable-wavelength LED 530. The notch filter is a high-Q-factor Band Stop filter, and both the notch filter 520 and the variablewavelength LED 530 are epitaxially grown on the substrate 510 from Ill-nitride materials using known methods, for example those set out in international patent application no. PCT / GB2022 / 051997, published as W02023 / 007174.

[0239] The exposed outer surface of the substrate 510 forms the light-emitting-side of the display device, as indicated by the arrow in Figure 4, which shows light emitted by the variablewavelength LED being transmitted through the notch filter 520 and the substrate 510 before exiting the display device. As the notch filter is positioned between the variable-wavelength LED and the light-emitting side of the device, light emitted by the LED is always filtered by the notch filter 520 before it exits the device.

[0240] Figures 5 and 6 are schematic illustrations of alternative display devices according to embodiments of the present invention.

[0241] In the display device 600 shown in Figure 5, a variable-wavelength LED 630 containing a p-type region, an n-type region and an active light-emitting region containing a multiple quantum well (MQW) is positioned on a substrate 610. A Band Stop filter 620 is positioned over the variable-wavelength LED 630. The Band Stop filter 620 is formed from a multilayered stack of porous and non-porous GaN, with the layer thicknesses selected to generate destructive interference which prevents the transmittance of wavelengths of light within two stopband wavelength ranges - one stopband being positioned below a predetermined primary emission wavelength of the LED, and the other stopband being positioned above the predetermined primary emission wavelength of the LED. When the variable-wavelength LED 630 is driven to emit the primary emission wavelength, the wavelengths close to the peak emission wavelength are transmitted through the Band Stop filter 620 at close to 100% intensity, while emitted wavelengths further away from the peak emission wavelength are inside the stopbands of the Band Stop filter, and are therefore attenuated by destructive interference and prevented from passing out of the light-emitting side of the device.

[0242] In the display device 700 of Figure 6, three micro-LEDs 730, each of which is a variablewavelength LED according to W02023 / 007174, are connected to a Band Stop filter 720. Although the LEDs and the Band Stop filter were been grown on a substrate, the substrate has been removed from the display device 700. As in Figure 5, the Band Stop filter 720 is formed from a multi-layered stack of porous and non-porous GaN, with the layer thicknesses selected to generate destructive interference which prevents the transmittance of wavelengths of light within two stopband wavelength ranges. As described further below, the two stopbands are spectrally positioned to fall between three predetermined primary emission wavelengths of the LED, so that the Band Stop filter 720 attenuates emitted wavelengths which fall in overlap ranges common to more than one primary emission peak.

[0243] Figure 7a is a schematic illustration of the typical emission mode spectra for a variablewavelength LED emitting in blue emission mode 802, green emission mode 804 and red emission mode 806, overlaid with the transmission spectrum of a Band Stop filter which features an idealised stopband 820.

[0244] As described in relation to Figure 3, a green-blue overlap range 808 contains wavelengths of light which are emitted in green emission mode 804 and also in blue emission mode 802, while a green-red overlap range 810 contains wavelengths of light which are emitted in green emission mode 804 and also in red emission mode 806.

[0245] The stopband 820 of the Band Stop filter is positioned to attenuate a wavelength range of around 570 nm - 620 nm, which covers the majority of the green-red overlap range 810. In the idealised stopband 820, emitted wavelengths falling within this stopband wavelength range are attenuated by 80%, to around 20% of their original intensity.

[0246] In use, this means that when the variable-wavelength LED is driven to emit its green primary emission peak 804, wavelengths of 570-620 nm which are emitted as part of the LED’s inherent green peak width are filtered by the stopband 820 and reduced in intensity. This effectively narrows the width of the emission peak emitted by the LED in green emission mode, creating a purer “green” emission. Likewise when the variable-wavelength LED is driven to emit its red primary emission peak 806, wavelengths of 570-620 nm which are emitted as part of the LED’s inherent red peak width are filtered by the stopband 820 and reduced in intensity. This effectively narrows the width of the emission peak emitted by the LED in red emission mode, creating a purer “red” emission

[0247] As shown in Figure 7b, the narrowing of the emission peaks by the Band Stop filter increases the size of the displayable colour gamut by 16.6%, as the narrower emission peaks and purer emission colours shift the positions of the filtered primary emission wavelengths in the CIE xy colour space.

[0248] Figure 7b illustrates the relative colour gamuts displayable using a conventional sRGB display with three primaries formed by three discrete red, green and blue subpixels, and compares this with a variable-wavelength LED which is driven in field-sequential mode using the three primary emission wavelengths 802, 804, 806 in Figures 3 and 7a, and with the same variable-wavelength LED plus the Band Stop filter of Figure 7a. Of the three colour gamuts displayable with these three options, the largest by 16.6% area is the Band Stop filtered display of Figure 7a. The effect of the Band Stop filter is to reduce spectral power on one side of a selected “primary” peak emission wavelength. For example, the Band Stop filter may have a stopband selected to reduce spectral power on the green side of a blue emission peak or on the blue side of a green emission peak. Unlike band pass filters in the prior art, the stopband of the Band Stop filter is targeting the spectral region between primary emission peaks as opposed to targeting the primary emission peaks themselves. A benefit of this arrangement over others is that it is effective whatever colour (whichever emission mode) the variable-wavelength LED is emitting - a Band Stop filter with a stopband in blue-green does not impact red emission, and a Band Stop filter with a stopband in green-red does not impact blue emission.

[0249] Similarly to Figure 7a, Figure 8a illustrates three typical primary emission mode peaks for a variable-wavelength LED emitting in blue, green and red emission modes, overlaid with the stopband 920 of a first exemplary Band Stop filter.

[0250] As described above, Band Stop filters can be formed from periodic porous / non-porous GaN structures, and a variety of Band Stop filters can be used in the present invention.

[0251] The exemplary Band Stop filter of the stopband illustrated in Figure 8a comprises 15 periods of alternating: 54.9 nm-thick non-porous GaN layer (refractive index n=2.45), and 76.25 nm-thick porous GaN layer (n=2.15).

[0252] As shown in Figure 8b, the peak-narrowing effect of the stopband increases the Gamut area displayable by the display device by 22.2% relative to the same variable-wavelength LED without the Band Stop filter.

[0253] Similarly to Figures 7a and 8a, Figure 9a illustrates three typical primary emission mode peaks for a variable-wavelength LED emitting in blue, green and red emission modes, overlaid with the stopband 1020 of a second exemplary Band Stop filter. Unlike the Band Stop filter modelled in Figure 8a, the stopband 1020 in Figure 9a is positioned in the bluegreen overlap range of overlapping wavelengths which are emitted by both the blue and green primary emission peaks.

[0254] The exemplary Band Stop filter of the stopband illustrated in Figure 9a comprises 15 periods of alternating: 45 nm-thick non-porous GaN layer (refractive index n=2.45), and 62.5 nm-thick porous GaN layer (n=2.15). As shown in Figure 9b, the peak-narrowing effect of the stopband increases the Gamut area displayable by the display device by 0.2% relative to the same variable-wavelength LED without the Band Stop filter.

[0255] Figure 10a shows exemplary emission mode spectra for a variable-wavelength LED emitting in blue, green and red emission modes, overlaid with two stopbands of idealised Band Stop filters. The two stopbands are provided by two Band Stop filters, each configured to provide its own stopband. The two Band Stop filters are stacked one-over- the-other in the path of the emitted light, so that light must pass through both filters before exiting the device. This effectively forms a bistimulus Band Stop filter.

[0256] Table 2 illustrates peak wavelengths and FWHM values for a dynamic-pixel-tuning (DPT®) variable-wavelength LEDs from Poro Technologies Ltd operating at three peak emission wavelengths.

[0257] Table 2.

[0258] These three emission peaks are illustrated in Figure 10a, and overlaid with the idealised transmission characteristics of a Band Stop filter with a blue-green stopband 1120 and a green-red stopband 1125. In these idealised stopbands, emitted wavelengths are attenuated to zero, while transmission outside the stopbands is 100%.

[0259] Figure 10b illustrates the resulting emission spectrum emittable from the display device of Figure 10a, by subtracting the attenuated wavelengths blocked by the two stopbands 1120, 1125. Three narrowed emission peaks remain in the spectrum. The blue emission peak 1102 and the red emission peak 1106 are narrowed, with the red peak emission wavelength slightly shifted. The green emission peak 1104 is significantly narrowed, as much of the original green emission peak is covered by the green-red stopband 1125, and the green peak which remains is shifted to shorter wavelengths. Figure 10c compares the colour gamuts emittable by an sRGB display and the variablewavelength LED display device of Figures 10a and 10b with and without the bistimulus Band Stop filter. As shown in Figure 10c, the colour gamut of the unfiltered display is relatively small, due to the proximity of the three primary emission peaks. The effect of the two stopbands, however, is to shift the effective primary peak wavelengths and narrow the emission peaks in a way that significantly expands the colour gamut displayable using the filtered display.

[0260] Figure 11a illustrates an embodiment similar to that of Figure 10a, but with only one Band Stop filter stopband 1125, which attenuates wavelengths in the range of 540 nm - 610 nm.

[0261] Figure 11b compares the displayable colour gamuts of an sRGB display and the variablewavelength LED display device of Figure 11b both with and without the Band Stop filter. As the stopband 1125 attenuates only a very small portion of the wavelengths emitted when the variable-wavelength LED is driven in its blue emission mode, the blue primary corner on the CIE xy colour gamut barely differs between the filtered and unfiltered gamuts. As the stopband 1125 attenuates a large proportion of the wavelengths emitted in the green and red emission modes however, the green and red primary corners on the CIE xy colour gamut differ significantly in the filtered display relative to the unfiltered display, and this change expands the colour gamut that can be displayed by driving the variable-wavelength LED using these three primary emission modes. Figures 12a to 12c illustrate experimentally-measured transmission spectra obtained for three off-the-shelf Band Stop filters. Figure 12d shows the experimentally-measured transmission spectrum for a combination of the same three filters layered one above another, so that emitted light passes through all three before the transmission is measured. The filters illustrated in Figures 12a, 12b and 12a were commercially-available notch filters having stopbands with central wavelengths of 514 nm, 561nm and 594nm respectively.

[0262] Figure 12e illustrates the experimentally-determined colour gamuts which are displayable using a variable-wavelength LED and the combined Band Stop filters of Figure 12d. Figure 12e shows that adding the filters enlarge the displayable area of the colour gamut compared to the no filter (NF) displayable gamut. These experimental results were obtained using off-the-shelf Band Stop filters - as a result the filtering is not ideal but comparing “filtered” and “NF” (no filter) shows that the invention works as expected.

[0263] Figures 13a-18 illustrate the same variable-wavelength LED, driven using the same three red, green and blue primary emission wavelengths, with different Band Stop filters. The unfiltered spectra in each case are made up of a red primary peak at 630 nm, a green primary peak at 540 nm, and a blue primary peak at 440 nm, all of which have a FWHM of 50 nm.

[0264] Although the peak emission wavelengths are the same in all cases, the use of Band Stop filters having different stopband wavelength ranges, with different centre wavelengths and different widths, means that the dominant transmitted wavelength (WLD) for the three primary peaks differ between embodiments. The transmission and the purity of the transmitted light also differs depending on the characteristics of the Band Stop filter.

[0265] Similarly to Figures 10a, 10b and 10c, Figures 13a, 13b and 13c illustrate the simulated effect of an idealised dual-stopband Band Stop filter on the RGB-primary emission spectrum and the idealised displayable colour gamut of a variable-wavelength LED. Figure 13a shows the emission mode spectrum for a variable-wavelength LED emitting in blue, green and red emission modes, overlaid with the two idealised stopbands of a first preferred Band Stop filter. Figure 13b shows the spectrum as it is transmitted out of the device after filtering.

[0266] In the first preferred Band Stop filter, a first stopband has its centre at a wavelength of 478 nm, and a stopband width of 75 nm, while a second stopband has its centre at a wavelength of 585 nm, and a stopband width of 90 nm.

[0267] As described above, the two stopbands can be created by forming the Band Stop filter from two stacked Band Stop filter portions, each structured to generate its own respective stopband. This filter could either be integrated into a variable-wavelength LED device using layers of porous (AI,ln)GaN, or formed of conventional dielectric materials (for example SiO2 / TiO2).

[0268] This first preferred Band Stop filter advantageously allows the emission spectrum to be filtered to produce very high-purity red, green and blue light in the transmitted spectrum.

[0269] For the first preferred Band Stop filter, the % transmission of the transmitted light, the dominant wavelength (WLD) and the purity of the transmitted light for each primary emission peak is set out in Table 3.

[0270] Table 3.

[0271] Figure 13c is a comparative illustration of simulated colour gamuts displayable by an sRGB display and the variable-wavelength LED display device of Figures 13a and 13b with and without the two-stopband Band Stop filter. This Figure shows that the addition of the dualstopband Band Stop filter to the variable-wavelength LED significantly expands the area of the triangle which defines the displayable gamut.

[0272] This first preferred Band Stop filter effectively narrows each primary colour emission, increases purity and expands the gamut.

[0273] Similarly to Figures 13a-13c, Figures 14a, 14b and 14c illustrate the simulated effect of an idealised second preferred dual-stopband Band Stop filter on the RGB-primary emission spectrum and idealised displayable colour gamut of a variable-wavelength LED. Figure 14a shows the emission mode spectra for a variable-wavelength LED emitting in blue, green and red emission modes, overlaid with the two stopbands of a second preferred Band Stop filter. Figure 14b shows the spectrum as it is transmitted out of the device after filtering.

[0274] The second preferred Band Stop filter has a first stopband with its centre at a wavelength of 490 nm, and a stopband width of 30 nm, while a second stopband has its centre at a wavelength of 585 nm, and a stopband width of 24 nm.

[0275] This second preferred Band Stop filter advantageously allows the emission spectrum to be filtered to produce very high-efficiency (high-transmission) red, green and blue light in the transmitted spectrum.

[0276] For the second preferred Band Stop filter, the % transmission of the transmitted light, the dominant wavelength (WLD) and the purity of the transmitted light for each primary emission peak is set out in Table 4. Table 4.

[0277] Figure 14c is a comparative illustration of simulated colour gamuts displayable by an sRGB display and the variable-wavelength LED display device of Figures 14a and 14b with and without the two-stopband Band Stop filter. This Figure shows that the addition of the dualstopband Band Stop filter to the variable-wavelength LED only slightly expands the area of the triangle which defines the displayable gamut.

[0278] Figures 15a-c illustrate a simulation of a variable-wavelength LED emitting in blue, green and red emission modes, and an idealised third preferred Band Stop filter with two stopbands.

[0279] The third preferred Band Stop filter has a first stopband with its centre at a wavelength of 500 nm, and a stopband width of 40 nm, while a second stopband has its centre at a wavelength of 582.5 nm, and a stopband width of 65 nm. This third preferred Band Stop filter advantageously allows the emission spectrum to be filtered to produce very high-purity red, green and blue light in the transmitted spectrum, such that Figure 15c shows that the filter significantly enlarges the displayable gamut.

[0280] For the third preferred Band Stop filter, the % transmission of the transmitted light, the dominant wavelength (WLD) and the purity of the transmitted light for each primary emission peak is set out in Table 5.

[0281] Table 5.

[0282] Figures 16a-c illustrate a simulation of a variable-wavelength LED emitting in blue, green and red emission modes, and an idealised fourth preferred Band Stop filter with two stopbands.

[0283] The fourth preferred Band Stop filter has a first stopband with its centre at a wavelength of 495 nm, and a stopband width of 30 nm, while a second stopband has its centre at a wavelength of 587.5 nm, and a stopband width of 55 nm. This fourth preferred Band Stop filter advantageously allows the emission spectrum to be filtered to produce very high-efficiency (high-transmission) red, green and blue light in the transmitted spectrum. Figure 16c shows that the filter significantly enlarges the displayable gamut relative to the same primaries without the Band Stop filter, though the slight reduction in purity of the transmitted Green primary peak means that the displayable gamut is slightly smaller than that of the third preferred Band Stop filter.

[0284] For the fourth preferred Band Stop filter, the % transmission of the transmitted light, the dominant wavelength (WLD) and the purity of the transmitted light for each primary emission peak is set out in Table 6.

[0285] Table 6.

[0286] The transmission measurements set out for preferred first to fourth Band Stop filters in Tables 3 to 6 above are charted in Figures 17a-c.

[0287] As shown by Figures 17a-c, by varying the stopband centre wavelength and the stopband width, it is possible to control the transmission of red, green and blue primary emission peaks over a wide range, with certain filter designs providing extremely high transmission of the wavelengths outside the stopbands.

[0288] The centre wavelengths and the stopband widths of the Band Stop filters are controllable by varying the layer thicknesses, arrangements and refractive indices of the materials in the Band Stop filters.

[0289] Figure 18 is a comparative illustration of colour gamuts emittable by an sRGB display and the variable-wavelength LED display device of Figures 13a to 17, with nine variations of two-stopband Band Stop filter.

[0290] Table 7 sets out the details of nine combinations of stopband which have been tested,, showing the dominant transmitted wavelengths of all three primary peaks after the emitted light has been filtered through the dual-stopband Band Stop filter. In all cases in Table 7, the lower-wavelength stopband has a width of 75nm, and the higher-wavelength stopband has a width of 90 nm. The filter tolerance was assumed to be ±5 nm from the centre wavelength.

[0291] Table 7.

[0292] The numbers in Table 7 are calculations which illustrate the impact of manufacturing variability on the filter performance. The reference design (in bold) is the first preferred Band Stop filter of Figure 13, and Table 7 includes eight variations with either -5nm, Onm or +5nm change made to the centre wavelength of the two stopbands. For filters made with dielectrics (such as SiO2, TiO2), for example, the tolerance on stopband centre can be easily controlled. These simulations show that the filter designs are not overly sensitive to manufacturing variability which could affect the centre wavelength of the stopbands.

[0293] Figures 19a, 19b and 19c illustrate four simulations of a variable-wavelength LED emitting in blue, green and red emission modes, comparing the emission spectra of an original unfiltered variable-wavelength LED with the same LED filtered by three different Band Stop filters (notch filters) having stopbands of different widths. The stopband widths of the three Band Stop filters (Notch_1, Notch_2 and Notch_3) are set out in Table 8 below, with the centre wavelengths of the filtered R, G and B emission peaks.

[0294] Table 8.

[0295] Figure 19b charts the brightness ratio of the blue, green and red emission modes for each of the original, Notch_1, Notch_2 and Notch_3 embodiments. As the stopband widths are increased, the proportion of emitted light that is blocked by the Band Stop filter increases, so the brightness of the emission modes decreases.

[0296] The differing stopband widths and positions also affects the color gamut which is displayable by each of the Notch_1 , Notch_2 and Notch_3 embodiments relative to the original (unfiltered) variable-wavelength LED. Although the presence of the Band Stop filters in these embodiments reduces the emitted brightness relative to the unfiltered LED, the Band Stop filters block wavelengths at which R, G and B emission peaks would otherwise overlap. This narrows the emission peaks and increases the purity of the emitted R, G and B light, so that the colour gamut displayable by the device is expanded relative to the gamut displayable by the unfiltered original LED.

[0297] Figure 19C shows the relative sizes of the colour gamuts displayable by the original unfiltered variable-wavelength LED, and by the same LED filtered by Notch_1, Notch_2 and Notch_3 Band Stop filters in the CIE xy colour space. The CIE xy measurements, the brightness ratio and the colour coverage ratio relative to the National Television Standards Committee (NTSC) colour gamut standard are shown in Table 9.

[0298] Table 9.

[0299] All of the Band Stop filtered embodiments demonstrate a larger colour gamut (a higher color coverage ratio) than the unfiltered variable-wavelength LED, but a lower brightness ratio. Notch_3 Band Stop filter, which has the widest stopbands, achieves the highest color coverage ratio but the lowest brightness ratio, while the Notch_1 filter achieves a smaller color coverage ratio but the highest brightness ratio of the filtered samples. Different Band Stop filters may be selected as the most appropriate for different applications, as the demands for brightness and colour gamut vary significantly between different situations. In some situations, however, the properties of the Notch_2 Band Stop filter may provide an advantageous balance between a high color coverage ratio (a large displayable colour gamut) and a suitably high brightness ratio for R, G and B emission modes.

[0300] Figure 20a is a schematic illustration of a display device according to an embodiment in which a variable-wavelength LED microdisplay 2030 and a notch filter 2020 are positioned on the same wafer, with the notch filter 2020 located between the LEDs of the microdisplay 2030 and a series of micro-lenses 2010. In this embodiment, the notch filter 2020 could either be integrated into the device and formed from layers of porous (AI,ln)GaN grown on the LED wafer using conventional deposition techniques, or the filter could be formed of conventional dielectric materials (for example SiO2 / TiO2) and deposited onto the LED wafer before the micro-lenses are attached.

[0301] Figure 20b is a schematic illustration of a display device according to an alternative embodiment, in which a series of micro-lenses 2110 are positioned over a variablewavelength LED microdisplay 2130, and a notch filter 2120 is positioned between the micro-lenses and a projector lens 2140.

[0302] In both of these embodiments, the notch filters are positioned to filter the light emitted by the variable-wavelength LEDs of the microdisplay before they exit the devices to be viewed by a viewer. Both of these embodiments may provide their own advantages in different applications.

Claims

Claims1. A display device comprising: a variable-wavelength light emitting diode (LED); and a Band Stop filter positioned between the variable-wavelength LED and a lightemitting side of the display device; in which the variable-wavelength LED comprises an n-doped portion, a p-doped portion, and a light-emitting region located between the n-doped portion and the p-doped portion, the light-emitting region being configured to emit light at a peak emission wavelength under electrical bias thereacross, in which the variable-wavelength LED is configured so that the peak emission wavelength of the LED is variable over an emission wavelength range of at least40 nm by varying a driving condition provided to the LED; in which the Band Stop filter is configured to attenuate emitted light with a wavelength falling inside a stopband wavelength range; and in which the Band Stop filter is configured so that the stopband wavelength range is inside the emission wavelength range of the variable-wavelength LED.

2. A display device according to claim 1 , in which the variable-wavelength LED is a dynamic colour-tunable LED, in which the peak emission wavelength of the LED is tunable by varying the driving conditions provided to the LED .

3. A display device according to claim 1 or 2, wherein the light emitting diode comprises a porous region of Ill-nitride material.

4. A display device according to claim 3, wherein one of the n-doped portion or the p-doped portion contains the porous region of Ill-nitride material.

5. A display device according to claim 3, wherein the n-doped portion; the p-doped portion; and the light-emitting region are provided on a substrate which comprises the porous region of Ill-nitride material.

6. A display device according to any preceding claim, in which the emission wavelength of the LED is continuously variable across the emission wavelength range in response to driving conditions provided by the power source beingvaried continuously across a driving range.

7. A display device according to any preceding claim, in which the peak emission wavelength is controllable over an emission wavelength range of at least 50 nm, or at least 60 nm, or at least 70 nm, or at least 80 nm by varying the driving condition provided to the variable-wavelength LED, preferably over a range of up to 100 nm or 110 nm or 120 nm or 140 nm, or 160 nm, or 180 nm, or 200 nm, or 400 nm, or 450 nm.

8. A display device according to any preceding claim, in which the stopband wavelength range is narrower than the emission wavelength range of the variable-wavelength LED, so that emitted wavelengths in a portion of the emission wavelength range are not attenuated by the Band Stop filter.

9. A display device according to claim 8, in which the Band Stop filter is configured so that the stopband wavelength range attenuates a portion of the emission wavelength range of the variable-wavelength LED, but transmits shorter emitted wavelengths which are below the stopband wavelength range and longer emitted wavelengths which are above the stopband wavelength range.

10. A display device according to any preceding claim, in which the device is configured to operate the variable-wavelength LED in a plurality of discrete emission modes by providing a plurality of discrete driving conditions to the LED, in which the variable-wavelength LED emits a different peak emission wavelength in each discrete emission mode, and in which each emission mode generates an emission mode spectrum which contains wavelengths surrounding the peak emission wavelength.11 . A display device according to claim 10, in which the Band Stop filter is configured to attenuate wavelengths of emitted light in an overlap range which are emitted in the emission mode spectra of multiple emission modes.

12. A display device according to claim 10 or 11 in which the Band Stop filter is configured so that the stopband does not contain any of the peak emission wavelengths generated by the discrete emission modes.

13. A display device according to claim 10, 11 or 12, in which the Band Stop filter is configured so that the stopband is positioned between the peak emission wavelengths of the discrete emission modes, so that the Band Stop filter attenuates wavelengths between said peak emission wavelengths.

14. A display device according to claim 11 , in which one or more of the peak emission wavelengths generated by the discrete emission modes falls within the stopband, but in which a portion of the emission mode spectrum generated in that emission mode is not inside the stopband.

15. A display device according to any preceding claim, in which the variablewavelength LED is controllable in a first mode to emit a first peak emission wavelength within the emission wavelength range in response to a first driving condition provided by the power supply, and in a second mode to emit a second peak emission wavelength within the emission wavelength range in response to a second driving condition provided by the power supply, in which the stopband wavelength range is positioned between the first peak emission wavelength and the second peak emission wavelength to attenuate overlapping emission wavelengths which are emitted in both the first and second modes.

16. A display device according to claim 15, in which the Band Stop filter is configured to attenuate emitted wavelengths which are emitted in a spectral overlap range in both the first and second modes.

17. A display device according to claim 16, in which the first peak emission wavelength is below 570 nm and the second peak emission wavelength is above 610 nm, such that the LED emits green light in response to the first driving condition and red light in response to the second driving condition, and in which the stopband of the Band Stop filter is configured to attenuate wavelengths between the first and second peak emission wavelengths.

18. A display device according to any preceding claim, in which the Band Stop filter is configured to attenuate wavelengths in a plurality of stopband wavelength ranges inside the emission wavelength range of the variable-wavelength LED, the Band Stop filter being configured to have multiple stopbands within theemission wavelength range of the variable-wavelength LED.

19. A display device according to any of claims 1 to 17, comprising a plurality of Band Stop filters with discrete stopband wavelength ranges inside the emission wavelength range of the variable-wavelength LED, the plurality of Band Stop filters being configured to attenuate discrete emitted wavelengths within the emission wavelength range of the variable-wavelength LED.

20. A display device according to claim 19, in which a plurality of Band Stop filters are positioned between the variable-wavelength LED and the light-emitting side of the display device, so that light emitted by the variable-wavelength LED must be transmitted through the plurality of Band Stop filters prior to being emitted out of the device.

21. A display device according to any preceding claim, in which the variablewavelength LED is controllable to emit three discrete peak emission wavelengths, by varying the driving conditions provided to the LED between three discrete emission modes.

22. A display device according to any preceding claim, in which the variablewavelength LED is controllable in a first mode to emit at a first peak emission wavelength in response to a first driving condition provided by the power supply, in a second mode to emit at a second peak emission wavelength in response to a second driving condition provided by the power supply, and in a third mode to emit at a third peak emission wavelength in response to a third driving condition provided by the power supply.

23. A display device according to claim 22, in which the Band Stop filter is configured so that the stopband attenuates overlapping wavelengths which are emitted in the first and second modes, and in which the device comprises a Band Stop filter with a second stopband configured to attenuate overlapping wavelengths which are emitted in the second and third modes.

24. A display device according to any preceding claim, in which the LED is controllable to emit a blue peak emission wavelength in response to a first driving condition provided by the power supply, to emit a green peak emissionwavelength in response to a second driving condition provided by the power supply, and to emit a red peak emission wavelength in response to a third driving condition provided by the power supply.

25. A display device according to any preceding claim, in which the LED is controllable to emit a first peak emission wavelength in the range 400-500 nm in response to a first driving condition provided by the power supply, to emit a second peak emission wavelength in the range 500-560 nm in response to a second driving condition provided by the power supply, and to emit a third peak emission wavelength greater than 600 nm in response to a third driving condition provided by the power supply.

26. A display device according to any preceding claim, in which the LED is controllable to emit a first peak emission wavelength in the range 430-470 nm in response to a first driving condition provided by the power supply, to emit a second peak emission wavelength in the range 510-560 nm in response to a second driving condition provided by the power supply, and to emit a third peak emission wavelength in the range 600-660 nm in response to a third driving condition provided by the power supply.

27. A display device according to claim 25 or 26, in which the Band Stop filter is configured to attenuate emitted wavelengths in a first stopband between the first and second peak emission wavelengths, and in a second stopband between the second and third peak emission wavelengths.

28. A display device according to any of claims 22 to 27, in which the first, second and third driving conditions are first, second and third driving currents of different magnitudes, corresponding to first, second and third current densities.

29. A display device according to any preceding claim, in which the Band Stop filter is an optical Notch filter.

30. A display device according to any preceding claim, in which the Band Stop filter is a bistimulus Band Stop filter, or a two-stopband Band Stop filter, configured to attenuate emitted wavelengths in two separate stopbands.

31. A display device according to claim 30, in which the Band Stop filter is configured to attenuate transmitted wavelengths in a first stopband between a blue peak wavelength and a green peak wavelength, and in a second stopband between a green peak wavelength and a red peak wavelength.

32. A display device according to any preceding claim, in which the Band Stop filter has a stopband with a width of at least 10 nm, or 20 nm, or 25 nm, or 30 nm.

33. A display device according to any preceding claim, in which the Band Stop filter has a stopband with a width of less than 75 nm, or 60 nm, or 50 nm.

34. A display device according to any preceding claim, in which the Band Stop filter comprises one or more layers of porous Ill-nitride semiconductor material.

35. A display device according to claim 34, in which the Band Stop filter comprises a layered structure in which a plurality of first layers having a first refractive index alternate with a plurality of second layers having a second refractive index.

36. A display device according to claim 34 or 35, in which the Band Stop filter comprises a layered structure in which a plurality of layers of porous Ill-nitride semiconductor material alternate with a plurality of non-porous layers of Ill- nitride material, or in which a plurality of layers of porous Ill-nitride material having a first porosity alternate with a plurality of porous layers of Ill-nitride material having a second porosity.

37. A display device according to claim 35 or 36, in which the thicknesses of the layers in the Band Stop filter are configured to attenuate wavelengths in the stopband by destructive interference.

38. A display device according to any of claims 1 to 34, in which the Band Stop filter comprises a rugate filter.

39. A display device according to claim 38, in which the rugate filter comprises one or more porous layers of Ill-nitride semiconductor material, in which the porosity of the porous layer varies through the thickness of the layer, such that therefractive index of the porous layer varies through the thickness of the layer.

40. A display device according to any preceding claim, comprising a plurality of variable-wavelength LEDs each configured to receive its own power supply, in which each of the plurality of LEDs are controllable so that the peak emission wavelength of each LED is controllable by varying a driving condition provided to that LED, in which the device is configured so that light emitted by each LED passes through a Band Stop filter before being emitted out of the device.

41. A display device according to claim 40, in which the device is configured so that light emitted by the plurality of variable-wavelength LEDs passes through the same Band Stop filter before passing out of the light-emitting side of the display device.

42. A method of manufacturing a display device according to claim 1 , comprising the step of forming a semiconductor structure on a substrate, the semiconductor structure comprising a Band Stop filter and a variable-wavelength LED comprising: an n-doped portion; a p-doped portion; and a light-emitting region located between the n-doped portion and a p-doped portion, the light-emitting region comprising a light-emitting layer which emits light at a peak emission wavelength under electrical bias thereacross in which the variable-wavelength LED is configured so that the peak emission wavelength of the LED is variable over an emission wavelength range of at least 40 nm; in which the Band Stop filter is configured to attenuate emitted light with a wavelength falling inside a stopband wavelength range; and in which the Band Stop filter is configured so that the stopband wavelength range is inside the emission wavelength range of the variable-wavelength LED.

43. A method according to claim 42, comprising the step of forming the variablewavelength LED over the substrate.

44. A method according to claim 42 or 43, comprising the step of forming the Band Stop filter over the variable-wavelength LED, and processing the semiconductor structure so that the light-emitting side of the display device is positioned overthe Band Stop filter.

45. A method according to claim 42, comprising the step of forming the Band Stop filter on the substrate, and forming the variable-wavelength LED over the Band Stop filter.

46. A method according to claim 45, comprising the step of removing the substrate from a first surface of the semiconductor structure, and processing the semiconductor structure so that the first surface forms the light-emitting side of the display device, with the Band Stop filter positioned between the variablewavelength LED and the light-emitting side of the display device.

47. A method according to any of claims 43 to 46, comprising the step of overgrowing the variable-wavelength LED over a porous region of Ill-nitride material.

48. A method according to any of claims 42 to 47, in which the Band Stop filter comprises one or more layers of porous Ill-nitride semiconductor material.

49. A method according to any of claims 42 to 48, comprising the step of forming the Band Stop filter by porosifying a plurality of layers of porous Ill-nitride semiconductor material alternating between a plurality of non-porous layers of Ill-nitride material, or forming a first plurality of layers of porous Ill-nitride material having a first porosity alternating with a second plurality of porous layers of Ill-nitride material having a second porosity.

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