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Definitions

• the present invention relates to light emitting diodes (LEDs) and to methods of producing LED arrays. It has particular application in arrays of LEDs on the micrometer scale.
• Ill-nitride light emitting diodes LEDs
• pLEDs micro-sized LEDs
• micro-LEDs micro-LEDs
• Ill-nitride pLEDs exhibit a number of unique features for display applications compared with organic light- emitting diodes (OLEDs) and liquid crystal displays (LCDs).
• OLEDs organic light- emitting diodes
• LCDs liquid crystal displays
• Ill-nitride microdisplays where pLEDs are the major components, are self-emissive. Monochromatic displays using pLEDs exhibit high resolution, high efficiency, and high contrast ratio.
• OLEDs are typically operated at a current density which is several orders of magnitude lower than semiconductor LEDs in order to maintain a reasonable lifetime.
• the luminance of OLEDs is relatively low, typically 3000 cd/m 2 for a full colour display, while Ill-nitride pLEDs exhibit high luminance of above l0 5 cd/m 2 .
• Ill-nitride pLEDs intrinsically exhibit long operation lifetime and chemical robustness in comparison with OLEDs. Therefore, it is expected that Ill-nitride pLEDs could potentially replace LCD and OLEDs for high resolution and high brightness display in a wide range of applications in the near future, such as smart phones.
• pLEDs exhibit significantly reduced junction capacitance as a result of reduced dimension compared with broad-area LEDs, and thus potentially lead to high-speed transmission with a GHz modulation bandwidth in VLC applications.
• Ill-nitride pLEDs are exclusively fabricated by means of combining a standard photolithography technique and subsequent dry etching process on a standard Ill-nitride LED wafer, which is similar to the fabrication of conventional broad-area LEDs with a typical device area of 300pm x 300pm or even larger dimension (Z. Y. Fan, J. Y. Lin and H. X. Jiang, J Phys. D: Appl. Phys. 41, 094001(2008); H. X. Jiang and J. Y. Lin, Optical Express 21, A476 (2013)).
• the only major difference in device fabrication between broad-area LEDs and pLEDs is the device dimension. Typically, the diameter of a pLED ranges from 50pm down to several micrometres.
• the present invention provides a method of producing a light emitting diode (LED) array, the method comprising: forming a semiconductor layer of group III nitride material; forming a dielectric mask layer over the semiconductor layer, the dielectric mask layer having an array of holes through it each exposing an area of the semiconductor layer; and growing an LED structure in each of the holes.
• LED light emitting diode
• the LED structures may be grown on the exposed areas of the semiconductor layer.
• the growth will generally be in the upward direction, as growth from the dielectric sidewalls of the holes will not occur.
• the upward growth of the LED structures within the holes may therefore result in a layered LED structure with each of the layers being generally flat or planar, and of substantially constant thickness.
• the semiconductor layer may be formed on a substrate, for example of group III nitride, such as GaN, or of sapphire, silicon (Si) silcon carbide (SiC), or of glass.
• the step of growing an LED structure in each of the holes may comprise growing an n- type layer, at least one active layer, and a p-type layer in each of the holes.
• the at least one active layer may be between the n-type and p-type layers.
• the at least one active layer may comprise at least one quantum well layer, and may comprise multiple quantum well layers. These may be formed, for example, of InGaN or another suitable group III nitride material.
• the n-type and p-type layers may also be of group III nitride material, such as GaN, InGaN or AlGaN.
• the at least one active layer may have an upper surface which is below the top of the dielectric layer. Where there is only one quantum well layer, the upper surface is the upper surface of that quantum well layer. Where there are a plurality of quantum well layers, the upper surface is the upper surface of the uppermost quantum well layer.
• the upward direction may be defined as the direction of growth of the semiconductor layer and/or of the LED structures.
• the step of forming the dielectric mask layer may comprise growing a layer of dielectric material, forming a mask over the dielectric mask layer, for example using photolithography, and etching the array of holes into the layer of dielectric material using the mask.
• the dielectric layer may be grown around the areas which then form the holes, for example using a mask, formed by photolithography with subsequent growth and/or etching, during growth of the dielectric layer.
• the method may further comprise etching each of the exposed areas of the semiconductor layer before growing the LED structure in each of the holes.
• the semiconductor layer may provide a common contact to all of the LED structures.
• the semiconductor layer may be doped.
• it may comprise a single layer of n-type or p-type group III nitride material.
• the semiconductor layer may comprise first and second sub-layers with a hetero-interface between them arranged to form a two dimensional charge carrier gas at the hetero-interface.
• the sub-layers may form a buffer layer and a barrier layer.
• the two dimensional charge carrier gas may, for example, be a two dimensional electron gas (2DEG).
• a two dimensional hole gas (2DHG) could also be used, but typically these have lower charge carrier density and/or mobility.
• a hetero-structure comprising, for example, a layer of GaN and a layer of AlGaN or InGaN, or more generally two layers of AlGaN with different Al contents or two layers of InGaN with different In contents, can form a 2DEG at the interface between the two layers, with the electron density in the 2DEG varying with a number of factors including the Al content of the AlGaN layer or the In content of the InGaN layer.
• Other group III nitride hetero-interfaces can be used with the same effect.
• the method may further comprise forming one or more contact layer areas over the LED structures.
• the or each contact layer area may extend over at least one of the LED structures, so as to be in electrical contact with the at least one of the LED structures.
• the contact layer areas may be electrically isolated from each other.
• the holes, and hence the LED structures may be arranged in a regular array.
• the array may be a square array, or it may be a rectangular array or a hexagonal array.
• the array may have a pitch, i.e. a distance between the centres of each closest pair of holes or LEDs, of from 4pm to 500pm.
• the holes, and hence also the LED structures may have a maximum diameter of from 1 to 500pm, or from 5 to 500pm.
• the present invention further provides producing an LED display including an LED array according to the invention.
• the invention further provides an LED array comprising a semiconductor layer, a dielectric layer extending over the semiconductor layer and having an array of holes through it, and an LED device formed in each of the holes.
• the present invention further provides an LED display comprising and LED array according to the invention.
• Figure la shows an as-grown template formed in a process according to a first embodiment of the invention
• Figure lb shows the template of Figure la with a masking pattern formed in its mask layer
• Figure lc shows the template of Figure la with micro-LEDs grown in holes in the mask layer
• Figure Id shows the template of Figure lc with electrical contacts formed on it
• Figure 2a shows an as-grown template formed in a process according to a second embodiment of the invention
• Figure 2b shows the template of Figure 2a with a masking pattern formed in its mask layer
• Figure 2c shows the template of Figure 2a with micro-LEDs grown in holes in the mask layer
• Figure 2d shows the template of Figure 2c with electrical contacts formed on it
• Figure 3 is a section through an LED structure of the template of Figure 2d;
• Figure 4 is a scanning electron microscope image of an LED array according to an embodiment of the invention.
• Figure 6 shows variation of internal quantum efficiency of embodiments of the invention as a function of LED diameter.
• a semiconductor layer for example a standard n-type GaN (n-GaN) layer 100, 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 GaN 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.
• MOVPE metal-organic vapour phase epitaxy
• MBE molecular beam epitaxy
• the resulting“as-grown n-GaN template” may have a thickness of above lOpm, but typically the thickness is in the range from 500nm to lOpm.
• a dielectric layer 104 such as silicon dioxide (Si0 2 ) or silicon nitride (SiN), or any other suitable dielectric material, is deposited on the n-GaN layer 100 by using PECVD or any other suitable deposition technique.
• the thickness of the dielectric layer may be in the range from 20 nm to 500pm.
• 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 micro- holes. This may be done by means a photolithography technique and then etching processes (which can be dry-etching or wet-etching). The use of photolithography is advantageous as it allows the holes, and hence the LEDs formed in them, to be accurately formed, with the desired positions, shapes, and sizes.
• the dielectric layer 104 is etched through its entire thickness down to the upper surface of the n-GaN layer 100.
• the micro-hole diameter may be from 1 pm to 500pm, or from 3pm to 500pm, and the pitch distance, i.e.
• the distance between the centres of closest adjacent micro-holes may be, for example, from 4pm to 500pm.
• Further etching, of the n-GaN layer 100, 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 lOpm, depending on the n-GaN layer thickness.
• the optimum etching method or conditions will be different for the n-GaN layer than for the dielectric layer.
• SF 6 etching 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 semiconductor layer 100 is simple to achieve. This also has advantages for the quality of the LED structures grown in the holes 106.
• the holes 106 are of a round, specifically circular, cross section in the embodiment shown, but other cross sections may be used, for example oval or square.
• a standard Ill-nitride LED structure is grown on the exposed areas of the GaN layer 100.
• the LED structures are formed as an array of discrete LEDs 108, separated by the remaining parts of the dielectric layer 104 between the micro-holes 106.
• the LED 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) layer, and not from the side walls of the holes 106. Therefore the layered LED structure can be built up inside each of the holes 106 with each of the layers being substantially flat or planar.
• the LED structures may comprise an n-GaN layer 110, an active region 112, and then a final p-doped GaN layer 114.
• the active region 112 may comprise InGaN prelayers, InGaN based multiple quantum wells (MQWs), and a thin p-type AlGaN layer as a blocking layer (not shown).
• MQWs multiple quantum wells
• An example of an LED structure is described in more detail below with reference to Figure 3.
• the LED structures can be grown only within the micro-holes 106, as shown in Figure lc, forming a pLED array.
• 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 pLED array. It is also important that the overgrown n-GaN 110 within each of the micro-hole areas directly contact the n- GaN layer 100 within the un-etched parts of the template below the dielectric mask 104 so that all the individual pLEDs are electrically connected to each other through the n- GaN layer 100 of the un-etched parts below the dielectric mask 104.
• an upper contact layer 116 may be formed over the dielectric mask layer 104 and over the upper p-GaN layer of the individual micro-LED devices 108.
• the upper contact layer 116 therefore forms a common p-contact for all of the LED devices 108.
• the upper contact layer 116 may be formed of ITO or Ni/Au alloys.
• An anode 118 may then be formed on the p-contact layer 116.
• a part of the dielectric layer 104 may be etched away and then the part of the LED structure on the etched dielectric layer section may be also etched down to the n-GaN, exposing an area 120 of the n-GaN 100, and a cathode 122 formed on that exposed area 120 of n-GaN.
• the continuous contact layer 116 may be replaced by a number of separate contact layer areas each of which covers a respective group of the LED structures 108.
• Each group may comprise just one LED structure 108 or it may comprise a plurality of LED structures, for example two or three or four.
• the contact layer areas are electrically isolated from each other, for example by being spaced apart from each other. This allows each group of LED structures to be addressable, i.e. to be switched on and off independently of the others.
• each of the contact layer areas can be connected to a respective switching device so as to form a display in which each of the LEDs or groups of LEDs forms a pixel.
• the accurate control of the location and size and shape of the LED structures provided by photolithography is important in ensuring that the contact layer areas can be aligned correctly with the LED structures to enable them to be individually addressed.
• 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 LED 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.
• 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 100 below the dielectric mask 104 so that all the individual pLEDs 108 are electrically connected to each other through the n-GaN layer 100.
• 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.
• 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 HEMT structure is used as an electrically connected channel.
• 2DEG electron gas
• a standard AlGaN/GaN HEMT structure is initially grown on GaN a substrate or any foreign substrates such as sapphire, Si, SiC or even glass by means of any standard GaN growth approach using either MOVPE or MBE technique or any other epitaxy technique.
• a GaN layer 200 forming a buffer layer is grown on the substrate 202 and then an AlGaN layer 201 forming a barrier layer is grown on the GaN layer 200.
• This structure is referred to herein as an“as-grown HEMT template”.
• a dielectric layer 204 such as Si0 2 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 dielectric layer 204 is etched down to the surface of the HEMT structure to form a micro-hole array 206 in the dielectric layer 204, where the micro-hole diameter can be from several pm to 500pm, and the pitch distance between adjacent hole centres may be in the range from lOpm to 500pm. Further etching the as- grown HEMT within the micro-hole areas can be performed using the remained regions of the dielectric layer 204 as a mask.
• the as-grown HEMT etching depth can be from zero (meaning there is no any etching) to lOpm, 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 200, 201 of the as-grown HEMT structure, so as to provide good electrical contact between each of the LED structures and the 2DEG.
• a standard Ill-nitride LED 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.
• This may, for example, include growing an n-GaN layer, In GaN prelayers, InGaN based MQWs as an active region, and then a thin p-type AlGaN as a blocking layer and then final p-doped GaN. Due to the dielectric mask, the LED structure grows only within the micro-holes 206, forming discrete micro-LED devices 208 within the micro-holes, as shown in Figure 2c.
• the upper surface of the InGaN MQWs 212 should be below the upper surface of the dielectric layer 204 so as to avoid a short-circuit effect after being fabricated into final pLED arrays.
• the LED structures in the LED arrays of Figures la to ld and 2a to 2d may have any suitable structure, but in one example they may include the n-GaN layer 310, an InGaN prelayer 316 formed over the n-GaN layer 310, a number of InGaN quantum well layers 312 formed over the prelayer 316, a p-doped blocking layer 318, for example of p-AlGaN, and then the p-GaN layer 314. It will be appreciated that this structure can be varied in a number of ways. As indicated above, it is preferable that the top of the uppermost one of the quantum well layers 312 is below the top of the dielectric layer.
• the top of the blocking layer 318 is also below the top of the dielectric layer. Another important point is that the overgrown n-GaN within the micro-hole areas directly contacts the interface between the AlGaN barrier and the GaN buffer of the initially as-grown HEMT structure of the un-etched parts below the dielectric mask 204 so that all the individual pLEDs are electrically connected through the 2DEG formed at the interface between the AlGaN barrier and the GaN buffer of the HEMT structure below the dielectric mask (i.e. the un-etched parts).
• any suitable standard device fabrication may be carried out, as with the embodiment of Figures la to ld and each device will include a number of individual pLED components as shown in Figure 2d where all the individual pLEDs 208, which are separated by the remained dielectric mask 204 in order to eliminate a short circuit in each device, share a common p-contact 216.
• a selective etching of the dielectric mask 204 may be required in order to let part of the surface of the HEMTs structure be exposed, where a cathode contact 222 will be fabricated on the surface of the exposed HEMTs as shown in Figure 2d.
• the selective etching can be dry-etching or wet-etching.
• Figure 4 shows a typical scanning microscope image of a pLED array epi-wafer, produced as described above, where the diameter of each pLED is 40pm.
• Figure 5 shows electro-luminescence spectra of a pLED with a diameter of 40pm as a function of injection current.
• Figure 6 shows the internal quantum efficiency (IQE) of pLEDs formed as described above, measured as a function of the diameter of pLEDs. This shows that the IQE of the LEDs increases with decreasing the diameter of pLED. The results are different from those of all previous pLEDs which are fabricated using conventional approaches. This suggests that the methods described above have avoided dry-etching induced sidewall damages typically generated during conventional fabrication processes.

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