Micro-emitters with stimulated emission

Abstract

A method of producing an array of stimulated emission light emitting devices comprises: forming a plurality of layers of material; forming a dielectric mask layer over the plurality of layers, the dielectric mask layer having an array of holes through it each exposing an area of one of the layers of semiconductor material, and growing a light emitting structure in each of the holes arranged to emit light of at least one wavelength by stimulated emission. At least one of the plurality layers forms an upward reflector arranged to reflect light of said at least one wavelength.

Description

[0001] Micro-emitters with Stimulated Emission

[0002] Field of the Invention

[0003] The present invention relates to light emitting devices and to methods of producing light emitting devices. In particular, but not exclusively, it relates to micro laser diodes and superluminescent diodes.

[0004] Background to the Invention

[0005] Surface-emitting laser diodes (LDs), such as Vertical Cavity Surface Emitting Lasers (VCSELs), exhibit many major advantages compared with edge-emitting LDs, for instance, ultralow threshold, naturally circular field pattern, single mode emission, and very small beam divergence due to stimulated emission. As a result, a surface emitting LD is a highly favourable lighting source, in particular for the applications of high-density information storage and high-resolution laser printing. If the dimension of a surface emitting LD can be reduced down to a few micrometers, the above superior properties make such a surface emitting LD one of the best candidates for manufacturing microdisplays which have wide ranging applications in smart watches, smart phones, smart bands, and augmented reality & virtual reality (AR & VR) devices.

[0006] LDs are different from LEDs which are operated based on a spontaneous emission mechanism. Therefore, unlike lasing emission, the emission from a LED is not coherent and thus exhibits random directions (i.e., a very large beam divergence). Current microdisplays, such as AR & VR micro-displays, employ microLEDs (pLEDs) with a small dimension (^ 10 pm), for example, Ill-nitride based pLEDs for blue, green and red. The light emitted from a pLED array is collimated by lenses (leading to an increase in size and weight) and then coupled into a waveguide for delivering images to an output pupil. It is well known that the in-coupling efficiency between a LED and a waveguide is typically low due to the large beam divergence of the LED as a result of spontaneous emission. This becomes even worse for pLEDs due to reduced dimensions. Consequently, only ~ 1% of the electricity (after converting into optical power due to the low in-coupling efficiency and the low quantum efficiency of a pLED) can reach an output pupil, leading to ~ 99% of energy waste. This poses a fundamental challenge in achieving high performance AR & VR micro-display systems from the perspective of a system level.

[0007] If surface-emitting LDs on a microscale are used instead of pLEDs, the situation is different. The stimulated emission from micro surface emitting LDs provides four major advantages in comparison to pLEDs: (1) due to stimulated emission, the in-coupling efficiency between a surface emitting pLD and a waveguide can be increased to 80% from 2-10% in the case of using pLEDs; (2) the carrier recombination lifetime can be reduced from nanoseconds for Ill-nitride semiconductor based LEDs down to picoseconds for a micro surface emitting laser. As a result, the significantly reduced recombination lifetime will help suppress not only surface recombination processes but also efficiency droop which is the intrinsic limitation of Ill-nitride semiconductor pLEDs; (3) the spectral linewidth can be reduced down to 5 nm from >30 nm for Ill-nitride semiconductor pLEDs.; (4) It will make AR / VR micro-display smaller and lighter.

[0008] Visible light communication (VLC) is an emerging wireless communication technology which transmits data by modulating visible emitters, which are mainly Ill-nitride semiconductor LEDs. In principle, the modulation bandwidth is ultimately determined by the carrier recombination lifetime of Ill-nitride LEDs assuming that the dimension of a LED is small enough. Unfortunately, the naturally long carrier recombination lifetime of Ill-nitride LEDs significantly limits the modulation bandwidth due to spontaneous emission. In remarkable contrast, the carrier recombination lifetime of a laser diode as a result of stimulated emission is three orders of magnitude shorter than that of a LED, leading to a massive increase in modulation bandwidth by a factor of at least three orders of magnitude in comparison with a LED.

[0009] The lasing threshold for a surface emitting LD depends on the size of the device which allows for injection current. A standard fabrication procedure requires dry-etching techniques to form a device mesa without any exception, where a typical dry-etching approach mainly based on ion bombardment, either inductively coupled plasma (ICP) or reactive-ion etching (RIE) techniques, must be used to etch through the emitting region. Once the size of a surface emitting LD goes down to 10 pm, the dry etching unavoidably leads to severe sidewall damage (enhancing non-radiative recombination), heavy optical losses and an increase in lasing threshold. This has been well-known from the perspective of fabricating pLEDs, where the external quantum efficiency (EQE) of a blue LED with a diameter of < 5pm is limited to a few percent, whilst a blue LED in a macro dimension can reach up to 80% EQE.

[0010] In addition to surface emitting LDs on a microscale, the invention also relates to other kinds of micro emitter, such as a microscale surface superluminescent diode (pSSLD) which is operated based on a stimulated emission mechanism like a laser diode (LD) but does not require an optical feedback mechanism such as a cavity with high reflectivity facets to achieve lasing. Therefore, the structure and the device fabrication of a pSSLD are simpler than those of a LD. However, like micro surface emitting LDs, the stimulated emission from pSSLDs also provides the four major advantages in comparison to pLEDs, as described above, relying on a spontaneous emission mechanism. Moreover, pSSLDs are expected to remove the issue of speckle which can cause problems for a laser-based high-definition display system. The fabrication of pSSLD also faces the same issue as that for micro surface LDs due to the requirements of the utilization of a dry-etching approach, as stated above.

[0011] Therefore, it is crucial to develop a different approach to the growth and then the fabrication of microscale surface emitting LD arrays and pSSLD arrays to address these fundamental issues.

[0012] Summary of the Invention

[0013] The invention provides a method of producing an array of stimulated emission light emitting devices, the method comprising: forming a plurality of layers of material; forming a dielectric mask layer over the plurality of layers, the dielectric mask layer having an array of holes through it each exposing an area of one of the layers of material, and growing a light emitting structure in each of the holes arranged to emit light of at least one wavelength by stimulated emission, wherein at least one of the plurality layers form an upward reflector arranged to reflect light of said at least one wavelength. Each of the devices may be a super-luminescent diode (SLD).

[0014] Each of the devices may be a laser diode. The method may further comprise forming a downward reflector over the light emitting structures, thereby to form a resonator cavity between the downward reflector and the upward reflector.

[0015] The upward reflector and the downward reflector may form a resonance cavity having resonance at at least one resonance wavelength.

[0016] Each of the light emitting structures may comprise an active layer of gain material. The diameter of the holes and the refractive index of the gain material may be selected to produce whispering gallery mode resonance at at least one resonance wavelength.

[0017] There may be a plurality of said resonance wavelengths in the visible spectrum. The plurality of resonance wavelengths may include at least one which is a resonance wavelength of the resonance cavity. Alternatively or in addition the plurality of resonance wavelengths may include at least one wavelength which is a WGM resonance wavelength.

[0018] The method may further comprise forming an electrical contact layer over the light emitting structures.

[0019] At least one of said plurality of layers may form an electrical contact connecting together at least some of the light emitting devices. The electrical contact may be formed between the upward reflector and the dielectric layer.

[0020] The invention further provides an array of stimulated emission light emitting devices comprising a plurality of layers of material, a dielectric layer extending over the plurality of layers and having an array of stimulated emission light emitting structures extending through it and arranged to emit light having at least one peak wavelength, wherein at least one of the plurality layers forms an upward reflector arranged to reflect light of at least said peak wavelength. Polarized light plays a crucial role in waveguide optics, a key component in AR / VR / MR systems. Light from a microdisplay is coupled into the waveguide through input gratings and then directed toward the eye via output gratings. These gratings are typically polarization sensitive. Using polarized light maximizes diffraction efficiency and minimizes optical losses. Effective polarization control reduces ghosting, double images, and unwanted reflections, whereas unpolarized light can result in poor contrast, color artifacts, and blurry overlays.

[0021] However, LEDs inherently emit unpolarized light due to their spontaneous emission mechanism. Current AR / VR / MR systems which are mainly based on microLEDs address this by using polarizer filters to convert the unpolarized light from microLEDs into polarized light, an approach that results in the loss of at least 50% of the light intensity. These polarizers also add bulk and weight to the system.

[0022] In contrast, micro-lasers and microscale surface superluminescent diodes operate via stimulated emission and inherently emit polarized light. This intrinsic polarization offers the advantages outlined above: significantly improved imaging resolution, substantial energy savings, reduced physical dimensions of the microdisplay, and a simplified design and manufacturing process in comparison with the microLED-based AR / VR / MR devices.

[0023] The method or the LD or the SLD array may further comprise, in any workable combination, any one or more features of the preferred embodiments of the invention as will now be described with reference to the accompanying drawings.

[0024] Brief Description of the Drawings

[0025] Figure la shows an as-grown DBR and part of a cavity layer formed in a process according to a first embodiment of the invention;

[0026] Figure lb shows the structure of Figure la with a mask layer formed over the lower cavity layer; Figure 1c shows the structure of Figure lb with a masking pattern formed in the mask layer;

[0027] Figure Id shows the structure of Figure 1c with micro-lasers grown in holes in the mask layer to form a micro-laser array;

[0028] Figure le shows the micro-laser array of Figure Id with a current spreading layer formed over the micro-laser array;

[0029] Figure If shows the micro-laser array of Figure le with a top DBR formed over the current spreading layer;

[0030] Figure 2 is a plane-view SEM image of part of a micro-laser array wafer produced using the method of Figures la to If;

[0031] Figure 3 is an emission image of the device of Figure 2, with a size of 80 x 80pm containing an array of micro-LDs each with a diameter of 3.6 pm;

[0032] Figure 4 is an intensity / voltage characteristic of the device of Figure 2;

[0033] Figure 5 shows lasing spectra of the micro-laser device of Figure 2 as a function of injection current;

[0034] Figure 6 shows the lasing spectrum of the micro-laser device of Figure 2 obtained under a 9mA injection current, showing the spectral linewidth of less than 1 nm by ignoring the background emission;

[0035] Figure 7 shows lasing spectra from a single micro-laser recorded as a function of optical pumping power density at room temperature, showing multi -wavelength lasing at 443nm, 492 nm and 522 nm; Figures 8a to 8c show integrated emission intensity and FWHM as a function of optical pumping power density for the three lasing peaks at 443 nm, 492 nm and 522 nm, respectively;

[0036] Figure 9a shows an as-grown DBR and part of a cavity layer formed in a process of forming a SLD according to a further embodiment of the invention;

[0037] Figure 9b shows the structure of Figure 9a with a mask layer formed over the lower cavity layer;

[0038] Figure 9c shows the structure of Figure 9b with a masking pattern formed in the mask layer;

[0039] Figure 9d shows the structure of Figure 9c with micro-SLD grown in holes in the mask layer to form a micro-SLD array; and

[0040] Figure 9e shows the micro-SLD array of Figure 9d with a current spreading layer formed over the micro-SLD array.

[0041] Detailed Description

[0042] Referring to Figure la, a lower semiconductor layer 100 of group III nitride or other suitable semiconductor, for example a standard un-doped GaN (u-GaN) layer, is initially grown on a substrate 102. The substrate 102 may be a GaN substrate, or may be any foreign substrate such as sapphire, silicon (Si), silicon carbide (SiC) or even glass. The lower semiconductor layer 100 may be grown by means of any standard GaN growth method using either metal-organic vapour phase epitaxy (MOVPE) or molecular beam epitaxy (MBE), or any other suitable growth technique. An upward reflector 101 is grown over the lower layer 100. The upward reflector 101 may be formed of a single layer of reflective material such as a metal, or it may be formed of a plurality of further layers which are arranged to form a lower distributed Bragg reflector (DBR) in which there are alternating layers 101a, 101b of two different materials, arranged to reflect light upwards as will be described in more detail below. A microcavity layer 103 is grown over the lower DBR layers 101. This layer 103 forms part of an array of microcavities as will be described below, and may be arranged to form an electrical contact layer for the LD devices. The layer 103 may be a semiconductor material, for example be of n-type GaN (n-GaN). The layer may have a thickness from 50nm to 10pm.

[0043] Referring to Figure lb, a dielectric layer 104, such as silicon dioxide (SiCh) or silicon nitride (SiN), or any other suitable dielectric material, is deposited on the layer 103 by PECDV or any other suitable deposition technique. The thickness of the dielectric layer may be in the range from 20nm to 500pm.

[0044] Referring to Figure 1c, an array of holes 106 is then formed in the dielectric layer 104. The holes 106 are typically on the micrometer scale and therefore referred to as microholes. This may be done by means a photolithography technique and then etching processes (which can be dry-etching or wet-etching). In forming the micro-holes 106, the dielectric layer 104 is etched through its entire thickness down to the upper surface of the layer 103. If the holes 106 are round, they may have diameters from 1pm to 500pm, and the pitch distance, i.e. the distance between the centres of adjacent micro-holes, may be, for example, from 1 pm to 500 pm. Further etching, of the layer 103, only within the micro-hole areas, may be performed using the remained dielectric layer 104 as a mask. The n-GaN etching depth can be from zero (meaning there is no GaN etching) to 10pm, depending on the n-GaN layer thickness. Typically the optimum etching method or conditions will be different for the layer 103 than for the dielectric layer 104. For example, SFe etchant can be used to etch the dielectric layer 104, but will not etch the n-GaN layer 100. Therefore etching all of the way through the dielectric layer 104 and stopping at the top surface of the layer 103 is simple to achieve. This also has advantages for the quality of the LD structures grown in the holes 106.

[0045] The holes 106 are of a round cross section in the embodiment shown, but other cross sections may be used, for example oval or square. The diameter, the individual location, the shape, and the inter-pitch of the micro-arrayed LDs are fully determined by the dielectric microhole masks. It means that the dimension, the individual location, the shape and the inter-pitch of micro-LDs are fully controlled.

[0046] Next, referring to Figure Id, a LD structure is grown on the exposed areas of the layer 103. However, because only discrete areas of the layer 103 are exposed by the microholes 106 in the dielectric layer or mask, the LD structures are formed as an array of discrete LDs 108, separated by the remaining parts of the dielectric layer 104 between the micro-holes 106. The LD structures 108 are grown by either MOVPE or MBE techniques, or any other suitable growth technique. The growth occurs upwards from the exposed areas of the GaN (or other semiconductor) of the upper layer 103, and not from the side walls of the holes 106. Therefore the layered LD structure can be built up inside each of the holes 106 with each of the layers being substantially flat or planar. The LD structures may comprise an n-GaN layer 110, an InGaN prelayer (not shown), an active region 112, a thin p-type Al GaN layer as a blocking layer (not shown), and then a final p-doped GaN layer 114. The active region 112 may comprise InGaN based multiple quantum wells (MQWs). The prelayer can be, for example, either an InGaN layer with low indium content and a typical thickness of <100 nm or an InGaN / GaN superlattice with low indium content (the total thickness of the superlattice is typically below 300 nm). As mentioned above, due to the dielectric mask 104, the LD structures 108 can be grown only within the micro-holes 106, as shown in Figure 1c, forming a micro-LD (pLD) array.

[0047] It is important that the uppermost layer of the InGaN MQWs 112 should not extend above the upper surface of the dielectric layer 104, which could result in a short-circuit effect after the template is fabricated into a final pLD array. It is also important that the overgrown n-GaN 110 within each of the micro-hole areas directly contact the layer 103 within the un-etched parts of the template below the dielectric mask 104 so that all the individual pLDs are electrically connected to each other through the layer 103 of the unetched parts below the dielectric mask 104.

[0048] Referring to Figure le, once the separate LD array structure is completed, further device fabrication is carried out, including the formation of electrical contacts for the array. For example an upper contact layer (or current spreading layer) 116 may be formed over the dielectric mask layer 104 and over the upper p-GaN layer of the individual micro-LD devices 108. The upper contact layer 116 therefore forms a p-contact for all of the LD devices 108. This may be a common p-contact layer for all of the LD devices 108, or may be formed as a plurality of separate areas, each contacting a respective group of one or more of the LD devices, and having a separate contact formed on it. This allows the LD devices 108 to be switched in groups therefore forming an addressable array. The upper contact layer 116 may be formed of ITO or Ni / Au or Pt / Au alloys.

[0049] Finally, referring to Figure If, a downward reflector 118, which may be an upper DBR reflector, may be formed over the upper contact layer. As with the upward reflector 101, the downward reflector 118 may be formed of a plurality of further layers which are arranged to form a lower distributed Bragg reflector (DBR) in which there are alternating layers of two different materials, as will be described in more detail below. Alternatively the downward 118 reflector may be formed of a single layer of reflective material, such as a metal.

[0050] In the finished structure as shown in Figure If, each of the LD devices 108, together with the lower and upper reflectors 101, 118, forms a vertical cavity surface emitting laser (VCSEL). A driving voltage is applied between the n-contact on top of layer 103 and the current spreading layer 116. Application of that voltage across each of the LD devices 108, if sufficiently high, produces a population inversion in the active layers 112. The volume containing each of the LD devices 108, and extending between the lower and upper reflectors 101, 118, including part of the cavity layer 103, forms a resonating microcavity in which light is reflected repeatedly through the active layers 112 causing lasing. The light forms a standing wave pattern, and the emitting active layers 112 are located so as to be at the antinodes of the optical field so as to maximise the coupling between the photons and the carriers. Some of the light emitted from the active layers 112 is transmitted through the upper reflector 118 and is therefore emitted from the top surface of the device.

[0051] Figure 2 is a plane-view SEM image of the micro-LD array wafer produced as described above, showing that the diameter of each micro-LD 108 is 3.6 pm. Figure 3 is an emission image of a device with a size of 80 x 80 gm2containing an array of micro-LDs each with a diameter of 3.6 gm. Figure 4 is a current-voltage (I-V) characteristic of the devices of Figure 2. Figure 5 is a plot of the lasing spectra of the micro-LD array of Figure 2 as a function of injection current. Figure 6 is a plot of the lasing spectrum of the array of Figure 2 obtained under a 9mA driving current showing the spectral linewidth of less than 1 nm when ignoring the background emission.

[0052] In an alternative configuration, the laser devices may each be a different type of surface emitting laser, namely a Tamm plasmon laser. A Tamm plasmon laser is similar to a VCSEL, but it substitutes the upper DBRs 118 of a VCSEL by using Tamm plasmon confinement. The only difference between a Tamm plasmon laser and a VCSEL is that the upper reflector 118 comprises a thin metal layer (the surface plasmon frequency of the metal needs to match the required lasing wavelength) with an optimised thickness, instead of the top dielectric DBR. Using Ill-nitride lasers as an example, two key parameters are required to achieve lasing at a required wavelength. These are metal (silver for blue lasers, Au for green lasers and longer wavelength lasers) thickness, and the total thickness of microcavity. As the metal reflector needs to be deposited on top of the current spreading layer 116 (e.g. ITO as p-contact), the overgrown layer 104, the ITO 116, and the n-GaN layer 103 (i.e., all the layers above the bottom DBR) will form a microcavity. The reduced modal volume associated with the LD will further reduce the lasing thresholds. Since the lasing is due to Tamm plasmon confinement, the carrier recombination lifetime will be further reduced, leading to even larger modulation bandwidth which is particularly useful for visible light communication applications.

[0053] Monolithically integrated multi-colour micro-lasers

[0054] In principle, the micro-lasers 108 described above can support a number of resonances at different wavelengths. In some cases it is possible to achieve more than one resonance in the vertical direction in the microcavity between the upward and downward reflectors. This is achieved by tuning the vertical depth of the resonance cavity. However a further option for producing multiple resonances is due to the formation of one or more whispering gallery mode (WGM) resonances. These can be used in addition to, or as an alternative to, the vertical resonance. WGM resonances can emit light with a peak in intensity at a number of different resonance wavelengths, where both the mode resonance wavelengths and the separation between them can be precisely tuned by controlling the diameter of a micro-laser. The wavelength separation of WGMs is given by the equation described below: X= XA2 / 27tRn where X is the wavelength, R the radius of the micro-laser and n is the refractive index of the gain medium, in this case the active layers 112. Consequently, for a Ill-nitride based micro-laser, WGMs with a large spacing can be generated due to its small diameter, leading to a large separation in the visible spectral regime covering a wide range from blue to green or even yellow / red spectral region. This is very difficult to achieve by any conventional lasers. The unique property makes a micro-laser an excellent candidate for simultaneously generating lasing with a number of wavelengths, potentially forming white lasing with a low threshold. In order to test the idea, a single micro-laser with a diameter of approximately 1pm and a microcavity of approximately 750nm thick was grown and then fabricated. Optical pumping was carried out on the single micro-laser at room temperature, aiming to achieving lasing with multiple wavelengths.

[0055] Figure 7 shows the emission spectra of the single micro-laser under optical pumping with different excitation power densities between 0.4 and 120kW / cm2. A few weak emission peaks corresponding to different WGMs were observed under low excitation power densities. With increasing excitation power density to be above a certain level, three very sharp and strong emission peaks appeared at 443nm, 492nm and 522nm, respectively. This covers a wide spectral range from blue through deep blue to green. In each case, the emission intensity rises dramatically along with a significant reduction in full width at half maximum (FWHM) when excitation power density is further increased, indicating a lasing action. It is worth highlighting that two-colour lasing (one in the blue spectral region and another in the green spectral region) can be achieved by properly tunning the diameter of a micro-laser and the thickness of the microcavity of the micro-laser.

[0056] Further evidence to confirm the lasing behaviours of the three emissions at 443 nm, 492 nm and 522 nm is provided by measuring light-light (L-L) characteristics, as shown in Figure 8a to 8c. In each case, the L-L curve plotted in a log-log scale exhibits a “s” shaped behaviour, the typical fingerprint of lasing action demonstrating the three-step process towards lasing, namely, spontaneous emission, then amplified spontaneous emission, and final lasing oscillation. From the L-L plots, the thresholds can be estimated to be 27kW / cm2, 15kW / cm2 and 5kW / cm2 for the lasing peaks at 443nm, 492nm and 522nm, respectively. Figure 8 a to c also provide the FWHMs of the three emission peaks as function of excitation power density, indicating that in each case the FWHM undergoes a dramatic reduction when the excitation power density exceeds the threshold, further confirming the lasing behaviours.

[0057] Monolithic on-chip multi-colour micro-lasers are expected to be very good devices to achieve a full colour micro-display with the best resolution for AR / VR applications. Fabrication into a micro-laser is an effective approach to achieving multi-colour lasing with a low threshold.

[0058] Surface Super-luminescent Diodes

[0059] Referring to Figures 9a to 9e, the light emitting device may be a micro surface super- luminescent diode (pSSLD). pSSLDs are operated based on a stimulated emission mechanism like a laser diode (LD) but do not require an optical feedback mechanism such as a cavity with high reflectivity facets to achieve lasing. Therefore, the structure and the device fabrication of a pSSLD are simpler than those of a LD. To achieve stimulated emission (not lasing), an enhanced optical confinement along the vertical direction is required and can be achieved by adding a distributed Bragg reflector (DBR) below the emitting region of a pLED. For a surface emitting LD such as VCSEL, a top DBR is required which leads to substantial extra steps in device fabrication (in particular the p- contact). These extra steps are not required for a pSSLD. This means that the fabrication and the packaging of pSSLDs are as simple as those of pLEDs.

[0060] Referring to Figure 9a - 9e the procedure for producing a pSSLD array using Ill-nitride semiconductors is very similar to the process of Figure la - If but does not require the last step, i.e., the step of Figure If . A standard un-doped GaN layer (not shown) is initially grown on a GaN substrate (not shown) or any foreign substrate such as sapphire, Si, SiC or even glass using either metal-organic vapour phase epitaxy (MOVPE) or molecular beam epitaxy (MBE) technique or any other growth techniques, followed by the preparation of a distributed Bragg reflector (DBR) structure 201. The DBR structure 201 can be Al(Ga)N / GaN system, which contains a number of pairs of alternating Al(Ga)N and GaN layers 201a, 201b grown by MOVPE or MBE or any other growth techniques.

[0061] Alternatively, the DBR structure 201 of the pSSLD array, or indeed that of the LD array described above, can be a number of pairs of alternating GaN and nano-porous GaN layers, where a significant refractive index contrast between GaN and nano-porous GaN can be obtained in each pair. A standard electrochemical (EC) etching can be used to convert heavily doped GaN into nano-porous GaN. Under a positive anodic bias, the injection current will flow through the n-doped GaN part which is conductive leading to the oxidation of n-doped GaN, and the oxidized layer is then chemically dissolved in an acidic electrolyte, converting the n-doped GaN into nano-porous GaN. Therefore, EC etching can be performed on n-type GaN with good conductivity only, while un-doped GaN which is not conductive remains un-etched.

[0062] On the top of the DBR structure 201, a further n-doped GaN layer 203 is then grown, which can form a contact layer. The thickness of the n-GaN layer 203 can be from 50 nm to 10 pm.

[0063] Subsequently, a dielectric thin film 204 such as SiO2 or SiN or any other dielectric material with a thickness ranging from 20nm to 500pm is deposited on the n-GaN layer 203 by using PECVD or any other deposition techniques as shown in Figure 9b. Afterwards, a photolithography technique and then etching processes (which can be dryetching or wet-etching) is employed to etch the dielectric film 204 down to the surface of the n-GaN layer 203, forming an array of micro-holes 206, where the micro-hole diameter can be from 1pm to 500pm and the pitch distance can be from 1pm to 500pm as shown in Figure 9c. The components of a pSSLD structure 208 are then overgrown on the exposed surfaces of the GaN template 203, which are exposed by the micro-holes in the dielectric mask layer 204, by either MOVPE or MBE technique or any other growth technique. For example a further n-GaN layer 210, InGaN prelayers (not shown), InGaN based multiple quantum wells (MQWs) as an active region 212, and then a p-type Al GaN cladding layer (not shown) and then final p-doped GaN 214. Due to the dielectric mask, these overgrown layers 210, 212, 214 happen only within the micro-holes 206 as shown in Figure 9d, naturally forming a micro-array of pSSLD structures. It is noted that the final surface of the InGaN MQWs 212 should not be above the surface of the dielectric layer 204 so as to avoid a short-circuit effect after being fabricated into final pSSLD arrays. Another important point is that the overgrown n-GaN 210 within the micro-hole areas 206 directly contact the n-GaN layer 203 within the un-etched parts below the dielectric masks 204 so that all the individual pSSLDs 208 finally formed are electrically connected through the n-GaN layer 203 of the un-etched parts below the dielectric masks. This is very important for further manufacturing micro-display using such pSSLD arrays. Once all the overgrown layers of the individual diode devices are completed, a current spreading layer 214 (CSL), which can be ITO or any other metal, may be formed over the devices 208 and mask layer 204 and can be used to form p-type contact. Consequently, a regular array of pSSLDs can be naturally formed without involving any dry etching which has to be used for the formation of mesas for the fabrication of traditional emitters.

[0064] Because the superluminescent diodes rely on stimulated emission in a similar way to a laser, the WGM resonances also arise in these structures. Therefore, the diameter and depth of the micro-holes 206 and the thickness of the contact layer 103 can be tuned to produce emission peaks at two or more wavelengths in the same way as described above for the micro-lasers.

[0065] It will be appreciated that various modifications to the embodiments described above can be made. For example, in one modification the structure is inverted, with a p-GaN layer being grown on the substrate and covered by the dielectric layer, and then the p-GaN layer of the LD devices 108 being formed first, followed by the multiple quantum well layers, and then the n-GaN layer. An n-contact layer is then formed over the top of the dielectric layer in place of the p-contact layer, and the positions of the anode and cathode are reversed.

[0066] In the configuration of Figures la to If, the overgrown n-GaN 110 within the micro-holes 106 has to match the n-GaN of the un-etched parts of the n-GaN layer 103 below the dielectric mask 104 so that all the individual pLDs 108 are electrically connected to each other through the n-GaN layer 103. Instead of using the n-GaN of the un-etched n-GaN parts below the dielectric mask 104 as an electrically connected channel, in a further embodiment, a Group III nitride heterostructure with a two-dimensional electron gas (2DEG) at the heterojunction is used as the semiconductor layer, instead of the n-GaN layer. In this embodiment a standard AlGaN / GaN HEMT structure is used. The electron gas (2DEG) with a high sheet carried density and high electron mobility formed at the interface between the AlGaN barrier and the GaN buffer of a high electron mobility transistor (HEMT) structure is used as an electrically connected channel.

[0067] In order to produce such a device, a standard AlGaN / GaN HEMT structure is grown over the lower DBR layers 101. For example a GaN layer forming a buffer layer may be grown over the DBR layers then an AlGaN layer forming a barrier layer is grown on the GaN layer. This structure is referred to herein as an “as-grown HEMT template”. Subsequently, a dielectric layer such as SiCE or SiN or any other dielectric material, for example with a thickness in the range from 2nm to 500pm, is deposited on the as-grown HEMT template by using PECVD or any other suitable deposition technique. The resulting structure will be the same as that shown in Figure la, but with the HEMT structure in place of the layer 103. After that, by means of a photolithography technique and then etching processes (which can be dry-etching or wet-etching) the dielectric layer is etched down to the surface of the HEMT structure to form a micro-hole array in the dielectric layer, where the micro-hole diameter can be from 1 pm to 500pm, and the pitch distance between adjacent hole centres may be in the range from 1 pm to 500pm. Further etching the as- grown HEMT within the micro-hole areas can be performed using the remained regions of the dielectric layer as a mask. The as-grown HEMT etching depth can be from zero (meaning there is no any etching) to 10pm, depending on the AlGaN barrier position of the as-grown HEMT template. However, generally the etching will extend downwards at least as far as the hetero-interface between the two layers of the as-grown HEMT structure, so as to provide good electrical contact between each of the LED structures and the 2DEG. Next, a standard Ill-nitride LD structure is grown on the dielectric mask patterned HEMT template featured with micro-holes by either MOVPE or MBE technique, or any other epitaxy technique, for example as described above with reference to Figures 1c, and contacts provided, for example as described above with reference to Figure Id. As with the embodiment of Figure la to If, an important point is that the upper surface of the

[0068] InGaN MQWs 112 should be below the upper surface of the dielectric layer 104 so as to avoid a short-circuit effect after being fabricated into final pLD arrays.

Claims

CLAIMS1. A method of producing an array of stimulated emission light emitting devices, the method comprising: forming a plurality of layers of material; forming a dielectric mask layer over the plurality of layers, the dielectric mask layer having an array of holes through it each exposing an area of one of the layers of semiconductor material, and growing a light emitting structure in each of the holes arranged to emit light of at least one wavelength by stimulated emission, wherein at least one of the plurality layers forms an upward reflector arranged to reflect light of said at least one wavelength.

2. A method according to claim 1 wherein each of the devices is a super-luminescent diode.

3. A method according to claim 1 wherein each of the devices is a laser diode.

4. A method according to claim 3 further comprising forming a downward reflector over the light emitting structures, thereby to form a resonator cavity between the upward reflector and the downward reflector.

5. A method according to claim 4 wherein the upward reflector and the downward reflector form a resonance cavity having resonance at at least one resonance wavelength.

6. A method according to any preceding claim wherein each of the light emitting structures comprises an active layer of gain material, and the diameter of the holes and the refractive index of the gain material are selected to produce whispering gallery mode resonance at at least one resonance wavelength.

7. A method according to claim 5 or claim 6 wherein there are a plurality of said resonance wavelengths in the visible spectrum.

8. A method according to any preceding claim further comprising forming an electrical contact layer over the light emitting structures.

9. A method according to any preceding claim further comprising forming an electrical contact connecting together at least some of the light emitting devices.

10. A method according to claim 9 wherein said electrical contact is formed between the upward reflector and the dielectric layer.

11. A method according to claim 9 or claim 10 wherein the electrical contact is formed of a doped semiconductor material.

12. An array of stimulated emission light emitting devices comprising a plurality of layers of material, a dielectric layer extending over the plurality of layers and having an array of stimulated emission light emitting structures extending through it and arranged to emit light having at least one peak wavelength, wherein at least one of the plurality layers forms an upward reflector arranged to reflect light at said peak wavelength.

13. An array according to claim 12 wherein each of the devices is a super-luminescent diode.

14. An array according to claim 12 wherein each of the devices is a laser diode.

15. An array according to claim 14 further comprising a downward reflector extending over the light emitting structures, thereby to form a resonator cavity between the upward reflector and the downward reflector.

16. An array according to claim 15 wherein the upward reflector and the downward reflector form a resonance cavity having resonance at at least one resonance wavelength.

17. An array according to any one of claims 12 to 16 wherein each of the light emitting structures comprises an active layer of gain material, and the diameter of the light emitting structures and the refractive index of the gain material are selected to produce whispering gallery mode resonance at at least one resonance wavelength.

18. An array according to claim 16 or claim 17 wherein there are a plurality of said resonance wavelengths in the visible spectrum.

19. An array according to any one of claims 12 to 18 further comprising an electrical contact layer connecting together at least some of the light emitting structures.

20. An array according to claim 19 wherein said electrical contact layer is between the upward reflector and the dielectric layer.

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