Monolithic multi-color LED device

Abstract

A monolithic multi-color semiconductor LED device (10), comprises: a p-doped GaN layer (42), an n-doped GaN layer (44), a first multiple quantum well structure (21) interposed between the p-doped GaN layer (42) and the n-doped GaN layer (14), the first multiple quantum well structure (21) being configured to emit light in at least one waveband (λ1, λ2) when a forward voltage (V) is applied between the p-doped and the n-doped GaN-layer (42, 44), a waveband being a wavelength range of no more than 50 nm, preferably of no more than 20 nm, wherein the de- vice (10) furthermore comprises a second multiple quantum well structure (22) provided in such a manner that one of the p-doped and the n-doped GaN layer (42, 44) is interposed between the first multiple quantum well structure (21) and the second multiple quantum well structure (22), the second multiple quantum well structure (22) being configured to convert a part of the light emitted by the first multiple quantum well structure (21) to at least a longer wavelength, so that device (21) is configured to emit light in several different wavebands (λ1, λ2,..., λ6) when the forward voltage (V) is applied, and wherein both the first and the second multi- ple quantum well structure (21, 22) comprise a plurality of stacked quantum well structures (31), each quantum well structure comprising an InxGa1-xN-film (31.1) interposed between two GaN-films (31.2), and all InxGa1-xN-films and GaN-films of the first and second multiple quantum well structure have the same orientation.

Description

[0001] Monolithic multi-color LED device

[0002] Description

[0003] Today, semiconductor light emitting devices, in particular light emitting diodes (LEDs) are widely used as they have considerable advantages over traditional incandescent light sources, including lower power consumption, longer lifetime, improved physical robustness and smaller size.

[0004] As the wavelength of the emitted light corresponds to the band gap of the semiconductors used, the spectrum of simple LED lights is rather narrow.

[0005] In many applications however, it is desired for a light source to cover a broader spectrum, e.g. for daylight lamps or greenhouse lights. In particular for vertical farm applications, white lights are required that closely mimic the sun spectrum or at least parts thereof in order to successfully grow lettuce and other vegetables.

[0006] In the prior art, several techniques are known for generating white light using LEDs.

[0007] On the one hand, a blue LED can be used in combination with a coating made from a fluorescent material such as phosphor absorbing the blue light from the LED and emitting a broad spectrum of colors, resulting in white light.

[0008] However, the spectra of current broad band monolithic LEDs using this technique exhibit some mismatch with the sun spectrum. Furthermore, the energy efficiency of these LEDs is rather poor which translates to lower photosynthetic efficiency in e.g. vertical farm applications and other lighting applications such as indoor lighting, human centric lighting etc.

[0009] As an alternative, a plurality of LEDs for different colors, e.g. red, blue and green LEDs, can be combined in a light emitting device in order to produce white light, and the intensity of the individual LEDs can be adjusted to mimic the sun spectrum. However, this will considerably increase the system cost and the complexity of the LED driving circuitry in addition to higher power consumption.

[0010] Against this background, the object of the invention is to provide an alternative monolithic multi-color semiconductor LED device reducing at least some the disadvantages of the known devices mentioned above.

[0011] This object is solved by a light emitting device with the features of claim 1 . Particular embodiments of the invention are described in the dependent claims.

[0012] The invention proposes a monolithic multi-color semiconductor LED device, comprising a p-doped GaN layer, an n-doped GaN layer and a first multiple quantum well structure interposed between the p-doped and the n-doped GaN layer, wherein the first multiple quantum well structure is configured to emit light in at least one and preferably no more than two different wavebands when a forward voltage is applied between the p-doped GaN layer and the n-doped GaN layer, and wherein the device furthermore comprises a second multiple quantum well structure provided in such a manner that one of the p-doped GaN layer and the n- doped GaN layer is interposed between the first multiple quantum well structure and the second multiple quantum well structure, the second multiple quantum well structure being configured to convert a part of the light emitted by the first multiple quantum well to at least a longer wavelength, so that device is configured to emit light in several different wavebands when the forward voltage is applied between the p-doped GaN layer and the n-doped GaN layer.

[0013] Each multiple quantum well structure comprises a plurality of stacked quantum well structures, each quantum well structure comprising an lnxGai-xN-film interposed between two GaN-films, and the GaN-films and lnxGa-i.xN-films of the first and second multiple quantum well structure all have the same orientation, e.g. they are all Ga-face films. Immediately adjacent quantum well structures can share a GaN-film, i.e. the multiple quantum well structure can in particular comprise or consist of a given number n of lnxGa-i.xN-films and n+1 GaN-films that are stacked in an alternating manner, the multiple quantum well structure starting and ending with an GaN-film.

[0014] In particular, if the first multiple quantum well structure is configured to emit light only in a single waveband (monoband LED), the device is configured to emit light in two or more different wavebands, and when first multiple quantum well structure is configured to emit light in exactly two different wavebands (dual band LED), the device is configured to emit light in three or more different wavebands.

[0015] LEDs do not generate truly monochromatic light but light within a narrow wavelength range. Therefore, in this application, a waveband is to be understood as referring to a wavelength range of no more than 50 nm, preferably of no more than 20 nm and more preferable of no more than 8 nm. In particular, it can refer to light with an emission spectrum having a full width at half maximum of not more than 8 nm to 20 nm centered around a particular wavelength.

[0016] Different wavebands can be completely disjoint or they can partially overlap.

[0017] The n-doped GaN layer and / or the p-doped GaN layer can be in direct contact with the first multiple quantum well structure, but it is not excluded to provide additional layers interposed between first multiple quantum well structure and the n-doped / p- doped layer, so that the respective layer is not in direct contact with the first multiple quantum well structure.

[0018] While the first multiple quantum well structure is electrically pumped by the voltage applied to the device to generate photons in at least one but preferably no more than two different wavebands (within the definition given above), e.g. blue and violet photons, the second multiple quantum well structure is optically pumped by these photons in order to generate photons with longer wavelengths, e.g. yellow, green and / or red, a process which is referred to herein as down conversion.

[0019] In particular, the second multiple quantum well structure can be configured to convert a part of the light emitted by the first multiple quantum well structure to several longer wavelengths, preferably so that the light emitted by the LED devices covers the entire visible spectrum and / or mimics the sun spectrum or any other desired spectrum.

[0020] According to the invention, as the second multiple quantum well structure is not electrically pumped, no complex driving circuitry is needed in order to generate light with a broad spectral range. This also decreases the power consumption of the device.

[0021] Furthermore, for greenhouse lights and in particular vertical farms, the invention will offer a better light coverage to the plant (i.e. single point source emission) to increase the growth yield and plant properties.

[0022] The photon down conversion can occur step-wise, i.e. the second multiple quantum well structure can comprise at least two groups of quantum well structures, the first of those groups being configured to convert a part of the light emitted by the first multiple quantum well structure to light within a first waveband and the second of those groups being configured to convert a part of the light emitted by the first group and / or by the first multiple quantum well structure to light within a second waveband, wherein the first waveband comprises wavelengths larger than those of the waveband(s) emitted by the first multiple quantum well structure and the second waveband comprises wavelengths larger than those of the first waveband. This can of course be generalized to any desired number of groups of quantum well structures, each group being optically pumped by the photons emitted by the preceding group or groups.

[0023] The amount of light emitted by the first multiple quantum well structure is, up to a certain maximum current, generally proportional to the current injection. Above a given maximum current, the LED starts to heat up and the light generation decreases. Based on the type of quantum wells, the quantum yield, i.e. the ratio of conversion of light can be in general from 30% to 99%. The entire device can be manufactured in a monolithic process as a monolithic device based on a stack of semiconducting layers grown by conventional techniques using e.g. MOVPE (metal organic vapor phase epitaxy) and / or MBE (molecular beam epitaxy) reactors.

[0024] The n-doped GaN layer can be e.g. GaN:Si or GaN:Ge, wherein the concentration of Si or / and Ge doping or co-doping into the GaN matrix can vary for example from 5x1017at.crrr3to 1x1021at.cnr3

[0025] The p-doped GaN layer can be e.g. Gan:Mg or GaN:Fe with the same or similar dopant concentrations as described above for the n-doped layer.

[0026] In this context, it is noted that in this application, the term “layer”, unless explicitly noted otherwise, can of course refer to a single uniform layer but can also refer to a structure comprising several stacked sub-layers that are different from each other in structure and / or composition.

[0027] In order to maximize the light emitting surface, the device can be a flip chip device, i.e. two electrodes, one of which is in contact with the n-doped GaN layer and the other one of which is in contact with the p-doped GaN layer, can be both provided on that side of the device opposed to the side through which the light is emitted from the device. In this application, this side through with light is emitted from the device is referred to as the “upper” side or top side of the device. Therefore, in this application, terms such as “above”, “below” or similar are to be understood in this sense and do not necessarily relate to the orientation of the device in space.

[0028] In this case, a layer of the device below the two multiple quantum well structures, in particular the bottom-most layer, is preferably reflective for photons or is provided with a reflective coating, so that light emitted from the first and / or second multiple quantum well structure towards the bottom side of the device can be reflected and then extracted through the top side of the device. For the anode, e.g. a pure silver layer with a thickness of about 10Onm or a layer stack of the type Ni(5nm) / Au(100nm) can be used, wherein the figures in brackets refer to the thickness of the respective sub-layers of the stack.

[0029] Preferable examples for the cathode are layer stacks of the type Ti(10nm) / Au(100nm) or Ti(10nm) / AI(100nm) / Ni(25nm) / Au(100nm). The Ti / Au type is preferred due to the low temperature annealing process.

[0030] The n-doped GaN layer, the p-doped GaN layer and the first multiple quantum well structure are part of a semiconductor light emitting diode (LED), which can preferably be a flip chip LED.

[0031] By varying the film thickness and the In content x of the lnxGai-xN-films of the first and second multiple quantum well structure as well as the number of quantum well structures configured to emit light in a given waveband, the emission spectrum of the light emitting device can be configured in the desired manner, e.g. in order to mimic the sun spectrum.

[0032] All GaN-films and lnxGa-i-xN-films of the first and second multiple quantum well structure have the same orientation. In particular, the GaN-films and lnxGa-i.xN- films of the first and second multiple quantum well structure can all be Ga-face films.

[0033] Generally, Ga-face GaN growth is thermodynamically more stable than N-face growth, and commercial GaN-based LEDs are usually Ga-face GaN-LEDs.

[0034] However, it should not be excluded that the GaN-films and lnxGa-i.xN-films of the first and second multiple quantum well structure can be all N-face films.

[0035] The thickness of the GaN-films of the quantum well structures can be from 0.5 nm to 50 nm, in particular from 2 nm to 20 nm and more in particular about 5 nm, and / or the thickness of the lnxGa-i.xN-films of the quantum well structures can be from 0.5 nm to 50 nm, in particular from 0.5 nm to 10 nm and more in particular from 2 nm to 3.5 nm.

[0036] According to a preferred embodiment of the invention, the quantum well structures of the first multiple quantum well structure have the following set of characteristics:

[0037] • the thickness of the GaN-films is 5 nm,

[0038] • the In content of the lnxGai-xN-films is 16 %, and

[0039] • the thickness of the lnxGa-i-xN-films is 2 nm.

[0040] In this manner, the LED comprising the first multiple quantum well structure can emit blue light with a wavelength of about 450nm.

[0041] Furthermore, the quantum well structures of the second multiple quantum well structure can comprise at least one of the following types or groups of quantum well structure, preferably several and most preferably all of the following types or groups of quantum well structure :

[0042] • a quantum well structure with a thickness of the GaN-films of 5nm, a thickness (t2) of the lnxGa-i.xN-film of 3 nm and an In content (x) of the lnxGa-i.xN- film of 17 %,

[0043] • a quantum well structure with a thickness (t1 ) of the GaN-films of 5nm, a thickness (t2) of the lnxGa-i.xN -film of 2.5 nm and an In content (x) of the lnxGai-xN -film of 20 %,

[0044] • a quantum well structure with a thickness (t1 ) of the GaN-films of 5nm, a thickness (t2) of the lnxGa-i.xN -film of 3.5 nm and an In content (x) of the lnxGai-xN -film of 21 .5 %,

[0045] • a quantum well structure with a thickness (t1 ) of the GaN-films of 5nm, a thickness (t2) of the lnxGa-i.xN -film of 3.5 nm and an In content (x) of the lnxGai-xN -film of 25 %.

[0046] Photon down conversion in these types of quantum well structures gives rise to emitted photons of the device with a wavelength of about 475nm, 500nm, 550nm and 600nm respectively, resulting in a broad emission spectrum resembling natural light. In order to further manipulate the emitted light, e.g. to shape the light emission and / or the increase the light extraction or reflection, the light emitting device can further comprise a photonic crystal provided on top of the second multiple quantum well structure or on the bottom side of the device, in particular a two-dimensional photonic crystal made from an AlxOyNx-y cover layer, preferably with 0.1 < x < 1 and 0 < y < 1 . The photonic crystal can provide an artificial roughness and / or influence the emitting angle of the light.

[0047] AlxOyNx.y based crystal photonics, in particular N-face AlxOyNx.ybased crystal photonics, can advantageously be used to improve the light extraction because of the lower refractive index of AlxOyNx.yas compared to conventional GaN-based crystal photonics.

[0048] Moreover, in comparison to conventional Al-face AIN-based crystal photonics, advantages of the N-face AlxOyNx.y-based crystal photonics as used in a particular embodiment of this invention are lower cost processing (e.g. due to wet etching) and greater flexibility as the refractive index can be adjusted by varying both x and y in AlxOyNx.y. Furthermore, AlxOyNx.ycan act as moisture barrier which improves the reliability of the device.

[0049] The second multiple quantum well structure or at least a part thereof, in particular in combination with a cover layer, can be formed into a regular arrangement of e.g. pillars or mesas forming microwires, each microwire preferably being covered by a passivation layer. These microwires can act as waveguides influencing the spatial distribution of the emitted light.

[0050] The passivation layer can be e.g. made from SiC>2, SiN or AI2O3 and can have a thickness of about 100nm.

[0051] According to a particular embodiment of the invention, the device can comprise an additional structure for photon down conversion that is provided above the second multiple quantum well structure, in particular a quantum dot structure or a phosphor coating layer. In this manner, in particular the far red part of the desired spectrum can be more easily generated while the middle part is generated by the second multiple quantum well structure.

[0052] If a regular arrangement of microwires is provided as described above, the phosphor or similar coating layer can also be provided in the spaces between adjacent microwires.

[0053] The device can comprise at least one etch stop layer.

[0054] In particular if the second multiple quantum well structure comprises several groups of quantum well structures, the quantum well structures of different groups being configured to emit light in different wavebands, etch stop layers can be provided in between adjacent groups in order to better control the fabrication of microwires.

[0055] An enhancement of the efficiency of multiple quantum well structures can be achieved by doping the InGaN layer (well) and / or the GaN layers (barrier) by Si and / or Ge doping in the range of 1018at / cm3to 9x1021at / cm3This enhancement is due the screen of electric field present in the well, i.e. a reduction of the Stark effect in the quantum well.

[0056] Also doping by Si or Mg could be done in the AlxGai-xN layer used for electron blocking (using Mg) or for hole blocking (using Si) with a doping range of from about 5x1017at / cm3to 5x1020at / cm3both for hole and electron doping.

[0057] The LED device according to the invention can be advantageously used for indoor lighting, in particular human centric lighting or for indoor farming, more in particular for vertical farm applications, and the applicant reserves the right to claim protection for corresponding lighting equipment for the purposes mentioned above, the lighting equipment comprising a LED device or an arrangement of LED devices according to the invention as described in this application. According to a further aspect, the invention also relates to a method for manufacturing a light emitting device according to any of the device claims as described above.

[0058] Preferably the method comprising the following steps: providing a substrate, preferably a GaN substrate; depositing a first stack of layers on a first, preferably Ga- terminated, surface of the substrate, the first stack of layers comprising the p- doped GaN layer, the n-doped GaN layer and the first multiple quantum well structure provided between the electron supplying layer and the hole supplying layer, as well as the second multiple quantum well structure, optionally flipping the substrate and the first stack of layers deposited thereon by 180°; depositing a second stack of layers on a second, preferably N-terminated, surface of the substrate that is opposite the first surface, the second stack of layers comprising a cover layer, preferably an AlxOyNx-y cover layer (55), more preferably with 0.1 < x < 1 and 0 < y < 1.

[0059] Other substrate materials are e.g. AI2O3 (sapphire) AIN, SiC, diamond and ZnO.

[0060] As an alternative, after depositing the first stack of layers, the substrate can be removed, e.g. by laser ablation, thus exposing a surface of the device on which a wet or dry etching could be performed followed to improve light extraction. Then, a second stack of layers is subsequently deposited for performing down-conversion. The second stack of layer could be phosphors, quantum dots or a combination of, embedded in an optical encapsulate with high optical transparency.

[0061] Preferably, the method comprises the additional step of microfabricating a desired structure, e.g. a regular arrangement of pillars forming microwires from the second stack of layers and / or from the second multiple quantum well structure. The step of microfabricating can in particular comprise a step of wet-etching. In the following, the present invention is explained in more detail with reference to some preferred embodiments of the invention as illustrated in the accompanying figures.

[0062] Fig. 1 is a schematic illustration of a device according to a first very simple embodiment of the invention.

[0063] Fig. 2 illustrates important parts of a device according to a second embodiment of the invention, focusing on the composition of the different layers.

[0064] Fig. 3 shows in partial figures (a), (b) and (c) different stages of forming a device according to a further embodiment of the invention.

[0065] Fig. 4 shows in partial figures (a) and (b) further stages of forming a device according to an alternative embodiment of the invention based on the embodiment shown in Fig. 3 (c).

[0066] Fig. 5 shows in partial figures (a), (b) and (c) three further embodiments of the invention.

[0067] Figs. 6 and 7 show further embodiments illustrating different manners of arranging the cathode and the anode of the device.

[0068] Fig. 8 is a flowchart illustrating a method for manufacturing a light emitting device according to an embodiment of the invention.

[0069] Corresponding features of different embodiments are denoted by the same reference signs in the figures illustrating these embodiments. Furthermore, for clarity reasons, if several identical features are present in one figure, some but not necessarily all of them are denoted by reference signs. In a similar manner, not all features are necessarily denoted by reference signs in each figure, but mainly those reference signs necessary or helpful for explaining the respective figure are included. Furthermore, it is to be noted that all figures are highly simplified and schematic illustrations that are used mainly to illustrate some principle ideas of the invention and that are in particular not drawn to scale, unless indicated otherwise. The figures are in particular not meant to give any information regarding the thickness or thickness ratios of the illustrated layers in any way. Instead, only for reasons of simplicity of drawing, all layers are illustrated as having the same “thickness”.

[0070] The device 10 shown in Fig. 1 comprises a p-doped GaN layer 42 as a hole supplying layer (HSL) 12, an n-doped GaN layer 44 as an electron supplying layer (ESL) 14 and a first multiple quantum well structure (MQW1 ) 21 interposed between the p-doped GaN layer 42 and the n-doped GaN layer 44.

[0071] The first multiple quantum well structure 21 comprises n1 identical stacked quantum well structures 31 , each quantum well structure comprising an lnxGai-xN-film 31 .1 interposed between two GaN-films 31 .2 , wherein immediately adjacent quantum well structures 31 share a GaN-film 31.2. In other words, the first multiple quantum well structure 31 comprises n1 lnxGa-i.xN-films and n1 +1 GaN-films that are alternatingly stacked, the stack starting and ending with a GaN-film.

[0072] As the band gap of lnxGa-i.xN is smaller than that of GaN, this arrangement results in n1 quantum wells, the depth of which can be adjusted by adjusting the In content x of the lnxGa-i.xN-films.

[0073] On the bottom surface 12b of the hole supplying layer 12, an anode (A) 13 is provided which is made from or coated with e.g. silver in order to reflect light emitted by the first or second multiple quantum well structure toward and through the top surface 10t of the device 10.

[0074] On the bottom surface 14b of the electron supplying layer 14, a conventional cathode (C) 15 is provided, insulated from the anode 13, the hole supplying layer 12 and the first multiple quantum well structure 21 by a passivation layer 16, so that by applying a forward voltage V between the anode 13 and the cathode 15, a forward voltage is applied between the p-doped GaN layer 42 and the n-doped GaN- layer 44.

[0075] The electrodes 13, 15 can be provided on the device 10 by conventional methods for thin film deposition, metallization and wet etching.

[0076] The electrodes 13, 15, the hole supplying layer 12, the electron supplying layer 14 and the first multiple quantum well structure 21 form a conventional flip-chip LED which, according to the embodiment of Fig. 1 , emits light in a single waveband A1 , e.g. blue light, as indicated by the arrow starting in the MQW1 in Fig. 1 .

[0077] The forward voltage V results in electrons being injected from the electron supplying layer 14 into the quantum wells of the conduction band and holes being injected from the hole supplying layer into the quantum wells of the valence band. The electrons and holes in the quantum wells can recombine, emitting a photon in the particular waveband, wherein any surplus energy can be converted into lattice vibrations.

[0078] According to the invention, a second multiple quantum well structure 22 is provided on top of the electron supplying layer 14 (n-doped GaN layer 44) and is configured to convert a part of the light emitted by the first multiple quantum well structure 21 to a waveband A2 with longer wavelengths, e.g. to green or yellow light.

[0079] Both the first and the second multiple quantum well structure 21 , 22 are formed as stacks of alternating layers of GaN-films 31 .2 and lnxGai-xN-films 31 .1 , each stack both starting and ending with a GaN-film as described above.

[0080] According to the invention, all GaN-films 31 .2 and lnxGa-i.xN-films 31 .1 of the first and second multiple quantum well structure 21 , 22 have the same orientation and are preferably all Ga-face films. In particular, for the first multiple quantum well structure 21 , the GaN-films 31 .2 can have a thickness t1 of about 5 nm and the lnxGai-xN-films 31 .1 can have an Indium content x of about 5% and a thickness t2 of about 2 nm, resulting, when electrically pumped by the forward voltage applied between the electrodes 13, 15, in the emission of violet light with a wavelength of about 405 nm.

[0081] For the second multiple quantum well structure 22, the GaN-films 31 .2 can have a thickness t1 of about 5 nm and the lnxGa-i.xN-films 31 .1 can have an Indium content x of about 16% and a thickness t2 of about 2 nm, resulting, when being optically pumped by the photons emitted by MQW1 21 , in the emission of blue light with a wavelength of about 450 nm. This device can be called a monolithic dual emission LED.

[0082] Fig. 2 illustrates a device 10 according to a further embodiment which is a little bit more complex than the example of Fig. 1 . The second and the further embodiments will only be described in as far as they differ from the first embodiment. Otherwise, reference is made to the description of Fig. 1 above. For reasons of simplicity, the electrodes of the device are not illustrated in Figs. 2-5.

[0083] A first difference is that in the embodiment of Fig. 2, the first multiple quantum well structure is configured to emit light in two different wavebands, e.g. violet and blue light. For this purpose, the first multiple quantum well structure 21 comprises two groups of quantum well structures, wherein the first group 21.1 comprising n1 quantum well structures is configured to emit light in a first waveband, e.g. violet light, and the second group 21.2 comprising n2 quantum well structures is configured to emit light in a second waveband, e.g. blue light, when a forward voltage is applied between the hole supplying layer and the electron supplying layer and thus across the first multiple quantum well structure 21 .

[0084] In a similar manner, the second multiple quantum well structure 22 comprises for example four groups of quantum well structures 22.3, 22.4, 22.5 and 22.6 comprising n3, n4, n5 and n6 quantum well structures as described above with respect to the first multiple quantum well structure. The quantum wells of the four different groups 22.3 to 22.6 are dimensioned so that they convert the light emitted by the first multiple quantum well structure 21 (or by any of the preceding groups) to four different wavebands, each waveband comprising wavelengths that are larger than those of the light emitted by the first multiple quantum well structure 21 . In the present example, the four groups of quantum well structures are configured to emit e.g. yellow, green, orange and red light. However a part of the blue and violet light emitted by the first multiple quantum well structure 21 is transmitted through and exits the device 10 without any downconversion.

[0085] By adjusting the layer thicknesses, the In content and the numbers of quantum wells of the different groups of quantum well structures 21 , 22, the spectrum of the light emitted from the device 10 can be adjusted in a desired manner, e.g. to mimic the sun spectrum.

[0086] An optional etch stop layer 25 (E-STOP) e.g. made from lnxAli-xN can be provided at a suitable position. In particular, the In content x in the ln>Ali-xN layer can be about 17% in order to lattice match with GaN, avoiding tensile or compressive stress in the structure. As explained later on, etch stop layers can in particular be used when fabricating microwires in a reproducible manner e.g. from the second multiple quantum well structure.

[0087] In order to prevent electrons injected into the first multiple quantum well structure 21 from entering the hole supplying layer 12 (HSL), i.e. the p-doped GaN layer 42, an optional electron blocking layer (EBL) 33, e.g. made from p-doped AlxGa-i.xN, can be provided between the first multiple quantum well structure 21 and the hole supplying layer 12. In a similar manner, an optional hole blocking layer (HBL) 34, e.g. made from n-doped AlxGa-i.xN, can be provided between the first multiple quantum well structure 21 and the electron supplying layer 14, i.e. the n-doped GaN-layer 42, in order to prevent holes injected in the first multiple quantum well structure 21 from entering the electron supplying layer 14. All the aforementioned layers can be deposited on a substrate 50 preferably made from GaN. Due to the flip chip geometry of the device 10 in Figure 2, the substrate 50 is arranged above the second multiple quantum well structure 22 in Fig. 2.

[0088] Using GaN as material for the substrate has the advantage that the films of the first multiple quantum well structure 21 and of the second multiple quantum well structure 22 deposited on the Ga-terminated side of the substrate 50 can be Ga- face films while e.g. an optional cover layer 55 made from AlxOyNx-ycan be N-face AlxOyNx-y allowing for cost efficient wet etching processes.

[0089] An optional GaN buffer layer 51 can be provided directly above and / or below the substrate 50 in order to compensate lattice mismatch.

[0090] In Figs. 3-4 different stages during the production of different alternative embedments are illustrated, while corresponding method steps are shown in Fig. 8.

[0091] In a first step of a method according to an embodiment of the invention, a substrate, preferably a GaN-substrate 50 is provided (step S1 in Fig. 8)

[0092] Fig. 3 (a) shows a device according to an embodiment after a first growth step (S2 in Fig. 8) in which a first stack of layers 61 is deposited on a first surface 50.1 of a GaN substrate 50 which is the Ga-terminated surface thereof, the first stack 61 in Fig. 3 (a) comprising, from bottom to top as illustrated, an optional GaN buffer layer 51 , a second multiple quantum well structure 22, an n-doped GaN-layer 44 acting as electron supplying layer 14, an optional hole blocking layer 34, a first multiple quantum well structure 21 , an optional electron blocking layer 33 and a p- doped GaN-layer 42 acting as hole supplying layer 12.

[0093] Fig. 3b) shows the subject-matter of Fig. 3a) after having been flipped by 180° (S3 in Fig. 8), so that now the N-terminated second surface 50.2 of the GaN substrate 50 is the topmost surface of the device. Fig. 3c) shows the subject matter of Fig. 3b) after a second stack of layers 62 has been deposited (S4 in Fig. 8) on the N-terminated second surface 50.2 of the GaN substrate 50, the second stack of layers 62 comprising an optional cover layer 55 made e.g. from AlxOyNx-y.

[0094] If desired, instead of depositing the second stack of layers on the second surface 50.2 of the substrate, the substrate 50 can be removed so that a surface of the underlying layer (e.g. of the GaN-buffer layer) is exposed, and then the second stack of layers can be deposited on this exposed surface (not illustrated).

[0095] After electrodes (not shown) are provided (S7 in Fig. 8) on the bottom side of the structure e.g. as described above with respect to Fig. 1 , the structure of Fig. 3c can be used as a light-emitting device 10.

[0096] However, Fig 4 illustrates some possibilities for alternative embodiments based on the structure of Fig. 3c)

[0097] Fig. 4a) shows the subject matter of Fig. 3c after the cover layer 55 has been processed by e.g. wet etching so that it forms a photonic crystal 56 (step S6 in Fig. 8).

[0098] In Fig. 4b), a phosphor coating layer 57 has been provided on top of the photonic crystal, both the photonic crystal and the phosphor layer 57 being configured to convert a part of the light emitted by the second multiple quantum well structure 22 to even longer wavelengths in order to generate the far red range of the desired spectrum for the light emitted by the device 10.

[0099] The order of steps can of course deviate from that shown in Fig. 8. For example, the electrodes can be provided immediately after the first stack of layers has been deposited and / or steps S2 and S4 can be interchanged. Furthermore, it is possible to omit the step S3 and to grow e.g. the layers of the first stack from the bottom up as shown in Fig. 3(b) but on top of a lowermost substrate layer (not shown in Fig. 3(b)) and to grow the second stack of layers directly on top of the first stack.

[0100] Fig. 5 shows three further embodiments of the invention. In the embodiment of partial figure (a), the second multiple quantum well structure 22 comprises three groups 22.1 , 22.3 and 22.3 of quantum wells, the quantum wells of different groups being configured to emit light in different wavebands, and the different groups being separated from each other by etch-stop layers 25. A further etch stop layer 25 is provided between the n-doped GaN-film 44 and the first group 22.1 of the second multiple quantum well structure 22.

[0101] The etch-stop layers 25 can help to form microwires 58 having different lengths by wet-etching in a very controllable manner as illustrated in the embodiments shown in partial figures (b) and (c) of Fig. 5. It is of course also possible to provide microwires with different lengths on the same device by conventional mask etching techniques.

[0102] The microwires 58 can act as waveguides influencing the spatial distribution of the emitted light.

[0103] Based on the application, different spectral compositions of the light of the LED can be desired. For instance, at the beginning of the growth of a plant for a better photosynthesis, only 2 wavelengths are needed, but at a later stage, a broader bandwidth is preferred.

[0104] Depending on the length of the microwire 58, the spectral composition of the light predominantly emitted along the microwire can be adjusted. In this manner, based on the same type of waver or same layer stack, different LEDs with a different spectral composition (e.g. Fig. 5b and 5c) can be fabricated in a simple and reproducible manner. It is noted that in the devices shown in Fig. 5, the substrate was removed, e.g. by laser ablation, before depositing the cover layer 55 on the back side of the GaN buffer layer 51 .

[0105] Optionally, the cover layer 55 and / or the microwires 58 can be covered with an additional structure or nanocoating for photon down conversion, in particular a phosphor coating 57 or a structure comprising quantum dots or a combination thereof.

[0106] Figures 6 and 7 illustrate further options for arranging the cathode 15 of the device, Figure 6 showing an embodiment in which the cathode 15 contacts the side of the n-doped GaN layer 44, which can be beneficial for a thin n-GaN layer (< 30nm) which has a depletion layer (<10nm). The etch stop layer 25 can help to obtain the desired contact as, after etching, the n-doped GaN layer will have a doped layer due to n vacancies, resulting in a smaller contact resistance.

[0107] Figure 7 shows a configuration in which the cathode 15 is provided on top of the n- doped GaN layer 44. In comparison to the flip-chip configuration of e.g. Fig. 1 , this configuration results in a lower contact resistance due to a lower depletion region, the process for providing the electrodes is simpler as no mesa etching, passivation and wet etching is necessary, and as the Stark effect in the multiple quantum well structure is less pronounced due to the electric field being parallel to the internal polarization of the GaN layer, a better energy efficiency can be achieved.

[0108] Overall, the invention provides a cost-effective and efficient solution for a monolithic broad band multi-color light emitting device that does not require a complex driving circuitry, has a lower power consumption than conventional multi-color LED solutions and is easily adaptable to different spectral requirements.

Claims

Claims1 . Monolithic multi-color semiconductor LED device (10), comprising a p-doped GaN layer (42), an n-doped GaN layer (44), a first multiple quantum well structure (21 ) interposed between the p-doped GaN layer (42) and the n-doped GaN layer (44), the first multiple quantum well structure (21 ) being configured to emit light in at least one waveband (A1 , A2) when a forward voltage (V) is applied between the p-doped GaN layer (42) and the n-doped GaN layer (44), the first multiple quantum well structure (21 ) comprising a plurality of stacked quantum well structures (31 ), each quantum well structure comprising an lnxGai-xN-film (31.1 ) interposed between two GaN-films (31.2); a waveband being a wavelength range of no more than 50 nm, preferably of no more than 20 nm, characterized in that the device (10) furthermore comprises a second multiple quantum well structure (22) provided in such a manner that one of the p-doped GaN layer (42) and the n-doped GaN layer (44) is interposed between the first multiple quantum well structure (21 ) and the second multiple quantum well structure (22), the second multiple quantum well structure (22) comprising a plurality of stacked quantum well structures (31 ), each quantum well structure comprising an lnxGai-xN-film (31.1 ) interposed between two GaN-films (31.2), wherein all the GaN-films and lnxGa-i-xN-films (31.1 ) of the first and second multiple quantum well structure (21 , 22) have the same Ga-face or N-face crystalline orientation, the second multiple quantum well structure (22) being configured to convert a part of the light emitted by the first multiple quantum well structure (21 ) to at least a longer wavelength, so that the device (21 ) is configured to emit light in several different wavebands (A1 , A2, ... , A6) when the forward voltage (V) is applied between the p-doped GaN layer (42) and the n-doped GaN layer (44).

2. LED device (10) according to claim 1 , wherein the GaN-films (31 .2) and lnxGai-xN-films (31.1 ) of the first multiple quantum well structure (21 ) and of the second multiple quantum well structure (22) are all Ga-face films.

3. LED device (10) according to any of the preceding claims, wherein the LED is a flip-chip LED.

4. LED device (10) according to any of the preceding claims, wherein the thickness of the GaN-films (31 .2) of the quantum well structures (21 , 22) is from 0.5 nm to 50 nm, in particular from 2 nm to 20 nm and more in particular about 5 nm, and / or wherein the thickness of the lnxGai-xN-films (31.1 ) of the quantum well structures is from 0.5 nm to 50 nm, in particular from 0.5 nm to 10 nm and more in particular from 2 nm to 3.5 nm.

5. LED device (10) according to any of the preceding claims, wherein the quantum well structures (31 ) of the first multiple quantum well structure (21 ) have the following set of characteristics:- the thickness (t1 ) of the GaN-films (31 .2) is 5 nm,- the In content (x) of the lnxGa-i-xN-films (31 .1 ) is 16 %, and- the thickness (t2) of the lnxGa-i-xN-films (31 .1 ) is 2 nm, and wherein the quantum well structures (31 ) of the second multiple quantum well structure (22) comprise at least one of the following types of quantum well structure, preferably several and most preferably all of the following types of quantum well structure :- a quantum well structure (31 ) with a thickness (t1 ) of the GaN-films(31 .2) of 5nm, a thickness (t2) of the lnxGa-i-xN-film (31 .1 ) of 3 nm and an In content (x) of the lnxGa-i-xN-film (31.1 ) of 17 %,- a quantum well structure (31 ) with a thickness (t1 ) of the GaN-films(31 .2) of 5nm, a thickness (t2) of the lnxGa-i-xN-film (31.1 ) of 2.5 nm and an In content (x) of the lnxGa-i-xN-film (31.1 ) of 20 %,- a quantum well structure (31 ) with a thickness (t1 ) of the GaN-films(31 .2) of 5nm, a thickness (t2) of the lnxGai-xN-film (31.1 ) of 3.5 nm and an In content (x) of the lnxGa-i-xN-film (31.1 ) of 21.5 %,- a quantum well structure (31 ) with a thickness (t1 ) of the GaN-films(31 .2) of 5nm, a thickness (t2) of the lnxGa-i.xN-film (31.1 ) of 3.5 nm and an In content (x) of the lnxGa-i.xN-film (31.1 ) of 25 %.

6. LED device (10) according to any of the preceding claims, further comprising a photonic crystal (56) provided on top of the second multiple quantum well structure (22) or on the bottom side of the device (10), in particular a photonic crystal (56) made from an AlxOyNx.ycover layer (55), preferably with 0.1 < x < 1 and 0 < y < 1 .

7. LED device (10) according to any of the preceding claims, furthermore comprising an additional structure for photon down conversion that is provided above the second multiple quantum well structure, in particular a quantum dot structure or a phosphor coating layer (57).

8. LED device (10) according to any of the preceding claims, wherein the second multiple quantum well structure (22) or at least a part thereof is formed into a regular arrangement of pillars forming microwires (58), preferably covered by a passivation layer (16).

9. LED device (10) according to claim 8, further comprising at least one etch stop layer (25), preferably several etch-stop layers (25) which are more preferably provided in-between different groups (22.1 , 22.2, 22.3) of quantum well structures (31 ) of the second multiple quantum well structure (22), the quantum well structures (31 ) of different groups (22.1 , 22.2, 22.3) being configured to emit light in different wavebands.

10. Method for manufacturing a LED device (10) according to any of the device claims.11 . Method according to claim 10, the method comprising the following steps: providing (S1 ) a substrate (50), preferably a GaN substrate, depositing (S2) a first stack of layers (61 ) on a first surface (50.1 ) of the substrate (50), the first stack of layers (61 ) comprising the p-doped GaN layer (42), the n-doped GaN layer (44) and the first multiple quantum well structure (21 ) provided between the p-doped GaN layer (44) and the n- doped GaN layer (42), as well as the second multiple quantum well structure (22), depositing (S4) a second stack of layers (62) on a second surface (50.2) of the substrate (50) that is opposite the first surface (50.1), the second stack of layers (62) comprising a cover layer, preferably an AlxOyNx-y cover layer (55), more preferably with 0.1 < x < 1 and 0 < y < 1 .

12. Method according to claim 11 , furthermore comprising the following step; microfabricating (S5) a desired structure from the second stack of layers (62), in particular comprising a step of wet-etching.

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