WO2014124486A1

WO2014124486A1 - Light emitting device using super-luminescence in semiconductor layer grown with moss-burstein effect

Info

Publication number: WO2014124486A1
Application number: PCT/AU2014/000113
Authority: WO
Inventors: Kenneth Scott Alexander Butcher; Dimiter Georgiav ALEXANDROV
Original assignee: Meaglow Ltd
Priority date: 2013-02-13
Filing date: 2014-02-12
Publication date: 2014-08-21

Abstract

A device and/or method for reducing, avoiding or removing band to band self absorption mechanisms in light emitting devices, such as Light Emitting Diodes, to reduce the possibility of non-radiative loss mechanisms and improve efficiency. A light emitting semiconductor layer in a LED is sufficiently doped to induce a Moss-Burstein effect in the semiconductor so that the carrier concentration in the semiconductor is so high that electrons fill the conduction band and the Fermi level is above the conduction band minima, that is the semiconductor becomes degenerate. An example device structure is presented with an active layer of degenerate n-doped InGaN. Super-luminosity is achieved for what would normally be considered inefficient high indium content InGaN light emitting devices, for example when the high carrier concentration InGaN layer is fabricated under nitrogen rich film growth conditions.

Description

Light Emitting Device Using Super-luminescence

In Semiconductor Layer Grown With Moss-Burstein Effect

Technical Field

[001] The present invention generally relates to light emitting devices including, made of, or at least partially made of, the material indium gallium nitride, and more particularly, for example, to Light Emitting Diodes (LEDs), that in operation make use of the Moss- Burstein effect.

Background

[002] Light Emitting Diodes (LEDs) generally have a semiconductor light generation region situated on or near a substrate and are widely used as light sources. In order to meet the demands for light output and of various applications, it is important that the light output efficiency of LEDs continues to be improved or maximized. FIG. 1 illustrates a basic structure of a conventional LED 100 which comprises a substrate 110, a first ohmic contact 114 to the substrate 110, a first type semiconducting material 111, such as n- AlGalnP, formed on top of the substrate 110, a second type semiconducting material 112, such as p-AlGalnP, formed on the first type semiconducting material 111, and a second ohmic contact 113 to the second type semiconducting material 112. A double hetero- structure of a conventional LED, which is similar to that of the LED 100 of FIG. 1, is also known in which an active material layer is formed between the second type semiconducting material 112 and the first type semiconducting material 111. The energy gap of the active material layer is smaller than those of the first and second type semiconducting materials 111, 112 so as to provide a carrier confinement effect to confine electrons and holes in the active material layer. Thus, the electrons and holes recombine in the active material layer to emit light.

[003] The external quantum efficiency of these types of known LEDs is not high because of various factors which can include, a current crowding effect, incident light critical angle and re-absorption of light inside the LED. [004] Self re-absorption of light in LEDs is a prominent problem. The light that is emitted by a conventional non-degenerate semiconductor during band to band emission processes can also excite electrons from near the valence band and thence be reabsorbed by non- radiative processes. One way that is commonly used to overcome this problem is to use thin light emitting layers, in particular quantum wells of a few nanometers thickness are commonly used. For high brightness LEDs quantum wells are used as the light emitting element, these are of lower band gap than the rest of the LED structure so that once light escapes the well the light only should be reabsorbed by defects in the rest of the structure. Self re-absorption in the quantum well is limited because the well is so thin that the statistical chances of light re-absorption are limited.

[005] For thicker radiating semiconductor layers this is not the case. Re-absorption of light and light loss from non-radiative processes is a more prominent problem for thicker semiconductor layers.

[006] Photon absorption in a semiconductor layer generally occurs by the photoelectric effect. For a semiconductor with a low carrier concentration, a common process is for the photon to be absorbed by the excitation of an electron from the valence band to a higher energy level, normally the conduction band. The electron may be swept away in the conduction band under the influence of an electric field so that the light is lost. This process commonly occurs for solar cells. Alternately, light can be re-emitted by the electron falling from the level at which it was captured to a lower energy level. The photon produced by this process (the photoelectric effect) might be of the same energy (or wavelength) as the original photon, or it might be of a slightly lesser energy. Photons may be absorbed and re-emitted several times before escaping from the semiconductor device in which they are generated, however each time the photon is absorbed there is a chance that a non-radiative absorption process may occur, or that the electron might be swept away, so that the light is lost. To overcome the problem of non-radiative losses, often related to defects in a material, exceptionally good quality material is generally required.

[007] There is a need for new or improved light emitting devices, particularly such as LEDs, which address or at least ameliorate one or more problems inherent in the prior art. For example, for LEDs made of InGaN, quantum efficiency drops significantly with higher indium content, in part due to non-radiative absorption processes.

[008] The reference in this specification to any prior publication (or information derived from the prior publication), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that the prior publication (or information derived from the prior publication) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

Summary

[009] This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Preferred Embodiments. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

[010] The Applicant has identified that the Moss-Burstein effect can be used to create a condition of "semiconductor transparency" so as to achieve superluminescence. In one example aspect, there is also the potential of two-dimensional holes at the interface of semiconductor layers (for example at a GaN-InGaN interface). In another example aspect, an n-doped semiconductor layer should be thick enough that the layer can act as a waveguide to achieve or enhance superluminescence, for example in the case of InGaN a thickness of greater than or equal to about 100 nm. [011] According to one example aspect, there is provided a light emitting device, comprising: a substrate layer; a p-doped semiconductor layer; and, a degenerate n-doped semiconductor layer. Preferably, the degenerate n-doped semiconductor layer is made of or includes, or is at least partially made of, indium gallium nitride (InGaN). [012] According to another example aspect, there is provided a method of manufacturing a light emitting device, including the steps of: depositing a p-doped semiconductor layer on a substrate; and, depositing a degenerate n-doped InGaN semiconductor layer on at least part of the p-doped semiconductor layer. [013] High indium content InGaN suffers from lower quantum efficiency with increasing indium content. Hence, to-date is has been unexpected that so-called super-luminosity can be achieved in such material. Lower quantum efficiency often indicates an increased concentration of non-radiative recombination centres, which would normally inhibit super- luminosity. Thus, in another aspect, the degenerate n-doped semiconductor layer is made of or includes, or is at least partially made of, high indium content InGaN.

[014] Preferably, electrical contacts are attached to the p-doped semiconductor layer and the n-doped semiconductor layer, which are used to apply an electrical bias across an interface of the p-doped semiconductor layer and the n-doped semiconductor layer to produce light. In one example aspect, the emitted light is broader in wavelength than light from an equivalent device including a non-degenerate semiconductor layer of the same material. In another example aspect, the position of the Fermi level is above the bottom of the conduction band in the n-type semiconductor or below the top of the valence band in the p-type semiconductor. In another example aspect, the degenerate n-doped or p-doped semiconductor layer is doped to induce a Moss-Burstein effect. In another example aspect, the degenerate n-doped or p-doped semiconductor layer is greater than several nanometers thickness.

[015] In other particular example aspects there is provided a method of and/or device for reducing, avoiding, inhibiting or removing band to band self absorption mechanisms in LED devices or structures, thereby limiting or reducing the possibility of non-radiative loss mechanisms and greatly improving the external quantum efficiency of an LED device.

[016] Various embodiments work particularly well for InGaN LED devices or structures with relatively thick junction layers that might otherwise cause re-absorption of emitted light (such as for example LED structures with light emitting layers greater than a few or several nanometers thickness), but also work for quantum well devices having thin light emitting layers of a few nanometers thickness or less. [017] In another example aspect, the concepts of the invention can be applied to provide low-loss channeling of photons in an optical channel or material to enhance the operation of an InGaN based laser diode. [018] It should be noted that although the present invention does not necessarily eliminate the possibility of light loss from defects in a material, the possibility of light re- absorption is inhibited or reduced. Furthermore, the path taken by light exiting a light emitting device, such as an LED or laser diode, is more direct (unless the light is internally reflected) since fewer randomly oriented re-absorption and re-emission processes occur. This can lessen the chance of light being absorbed due to a defect in a material, which can also lessen the chance of an electron being produced by photo-absorption and thus potentially being lost by being swept away in an electric field.

Brief Description Of Figures

[019] Example embodiments should become apparent from the following description, which is given by way of example only, of at least one preferred but non-limiting embodiment, described in connection with the accompanying figures.

[020] FIG. 1 (prior art) illustrates a cross-sectional view of a conventional light emitting diode (LED).

[021] FIG. 2 illustrates a band diagram of a semiconductor with a significant or large Moss-Burstein effect present. [022] FIG. 3 illustrates a band diagram of a semiconductor with a significant or large Moss-Burstein effect, as per FIG. 2, but with a hole (h) injected into the top of the valence band (VB).

[023] FIG. 4 illustrates a cross-sectional view of an example LED structure with a degenerate n-doped semiconductor layer.

[024] FIG. 5 illustrates a cross-sectional view of a more specific example InGaN LED structure. [025] FIG. 6 shows a graph of absorption coefficient squared vs. photon energy, calculated from optical transmission measurements, for the specific example InGaN LED structure of FIG. 5.

[025 A] FIG. 7 illustrates a band diagram corresponding to FIG. 6.

[026] FIG. 8 shows light emission spectra from the specific example InGaN LED structure of FIG. 5.

[027] FIG. 9 shows the light output of an example InGaN LED (open circles represent 3 x 10 cm^" carrier concentration) with a Moss-Burstein effect in the initial unbiased n- InGaN layer, and shows a super-linear dependence of light output on applied current, compared to a sample (solid circles represent 3 x 10 cm^" carrier concentration) with lower doping and usual sub-linear dependence of light output.

[028] FIG. 10 illustrates an example method for manufacturing an InGaN based LED structure with a degenerate n-doped semiconductor layer. Preferred Embodiments

[029] The following modes, given by way of example only, are described in order to provide a more precise understanding of the subject matter of a preferred embodiment or embodiments. In the figures, incorporated to illustrate features of an example embodiment, like reference numerals are used to identify like parts throughout the figures.

[030] FIG. 2 illustrates an energy band diagram of a semiconductor with a large Moss- Burstein effect present. Because electrons 205 fill the conduction band (CB) 210 to the sum of the band gap (EG) and the Fermi level (Ep), that is EO+EF, an electron (e) 215 in the valence band (VB) 220 must absorb light with energy greater than EQ+EF to cause the electron 215 to transition from the valence band 220 to the conduction band 210.

[031] FIG. 3 illustrates an energy band diagram of a semiconductor with a large Moss- Burstein effect, as per FIG. 2, but with a hole £h) 310 injected into the top of the valence r RECTIFIED SHEET ^{J r}

(Rule 91 ) ISA/AU free electrons 205 in the conduction band 210, so the wavelength of emitted light 320 is expected to be relatively broad. However, the energy of the emitted photon 320 will be less than EG+EF SO that the possibility of re-absorption is significantly reduced. [032] In an example embodiment, a light emitting semiconductor layer in a LED is sufficiently doped, which may require relatively heavy or a high level of doping, to induce a Moss-Burstein effect in the semiconductor, or between semiconductors, in use or in operation. For the Moss-Burstein effect, the carrier concentration of the semiconductor is so high that electrons 205 fill the conduction band 210 so that the Fermi level (EF) is above the conduction band minima, i.e. the semiconductor becomes degenerate with a substantial, high or very high carrier concentration. The Moss-Burstein effect is a process by which the apparent band gap of a semiconductor is increased as the absorption edge of the conduction band is pushed to higher energy as a result of states close to the bottom of the conduction band, but within the conduction band being populated. The Moss-Burstein effect is observed for a degenerate semiconductor, which is a semiconductor with such a substantial or high level of doping that the material starts to act more like a metal than as a semiconductor.

[033] This means that electrons must absorb photons with energies above the Fermi level, and well above the band gap, in order to occupy a vacant site in the conduction band. The advantage of this arrangement is that the absorption of photons by an electron transitioning from near the valence band to near the bottom of the conduction band cannot occur. The transition is blocked by electrons that already occupy the bottom of the conduction band. Hence, when holes 310 are injected from a p-type layer into a degenerate n-type layer (see FIG. 3) photons 320 are created by electrons 315 falling from the conduction band 210 below the Fermi level to the valence band 220 to recombine with the holes 310. This creates a wider spectrum of light 320 than for a non-degenerate layer. However, the light 320 created by this process cannot be reabsorbed, for example because of an electron moving from say the valence band 220 to the bottom of the conduction band 210, since the previously unoccupied state in the conduction band 210 is immediately filled with another free electron 205 from higher in the conduction band 210. When this happens there will be no available position for an excited electron to transit to from the valence band 220 by absorption of the light 320, except to above the Fermi level, however the photon 320 must then have a higher energy than it was created with, so the photon 320 cannot be reabsorbed. The photon 320 therefore travels, on average, a longer distance in the semiconductor layer without the possibility of re-absorption due to band to band transitions.

[034] It has been noted by Green et al. (M. A. Green, J. Zhao, A. Wang, P. J. Reece and M. Gal, "Efficient silicon light-emitting diodes", Nature 412 (2001) 805) that the same device structure that allows for the production of a good solar cell can be used as a good LED structure. Prof. Martin Green, whose group at the University of New South Wales, Australia, is believed to currently hold a number of records for solar cell collection efficiency for silicon devices, also is believed to currently hold the record for achieving the most efficient light emission from silicon being almost 100 times higher than that achieved previously (at close to 1% efficiency). In Green et al. it is stated that the improvement is due to a reduction of parasitic non-radiative recombination (i.e. through the use of high quality material) while "taking advantage of the reciprocity between light absorption and emission by maximizing absorption at relevant sub-bandgap wavelengths." In other words, Green et al. teaches that the device components that worked well for silicon solar cells (light absorbers) could be advantageous for light emitters. Significantly, Green et al. also suggests that further efficiency improvement could be obtained by reducing the free carrier concentration.

[035] However, in contrast to Green et al, the Applicant has surprising found that, in respect of light emitting devices of the present invention, advantageous features or suggestions identified in Green et al. are incorrect. In particular, the assertion in Green et al. that improved efficiency could be achieved by reducing the free carrier concentration is not correct for light emitting devices according to the present invention. In embodiments of the present invention, efficiency is increased because there is reduced or inhibited re- absorption of photons due to a higher carrier concentration. Also, there is no reciprocity between light emission and light absorption in example embodiments. Quite the opposite, the reliance on a Moss-Burstein effect to disallow, inhibit or reduce light re-absorption means that various embodiments, while working well as a light emitting device such as a LED, would not work well as a solar cell since normal band gap transitions are inhibited. [036] Additionally, laser diodes are made so that carrier population inversion layers occur. A condition called semiconductor transparency arises when inversion is achieved. Conventionally, in that case the carrier concentration of the layers is much lower, and inversion is electrically pumped. However, in example embodiments of the present invention, the Moss-Burstein effect achieves the condition of semiconductor transparency without a need to electrically pump the laser diode. A high carrier concentration enables the condition of semiconductor transparency which is a pre-requisite for super- luminescence caused by stimulated emission. The presence of a Moss-Burstein effect artificially induces the condition of semiconductor transparency. It should also be noted that past a certain current threshold stimulated emission may occur if there are few non- radiative recombination centers in the forbidden region of the bandgap, though this may not be coherent emission. This effect is observed as a super-linear dependence on device current, i.e. super-luminescent properties. This effect is generally not expected for an InGaN layer of high indium content, as the quantum efficiency decreases with increasing indium content for standard InGaN based LEDs. These devices were therefore not expected to show any super-luminescent properties.

[037] FIG. 4 illustrates a cross-sectional view of an example LED structure 400. A high, heavily or substantially n-doped semiconductor layer 410 is deposited on at least part of a p-type semiconductor layer 420 (i.e. a p-doped semiconductor layer 420), which is deposited on a substrate layer 430 (i.e. substrate 430). This arrangement results in a Moss- Burstein effect for the heavily or substantially n-doped semiconductor layer 410. The n- doped semiconductor layer 410 is degenerate. In this example, a high, heavily or substantially n-doped semiconductor layer 410 is taken to have a carrier concentration of greater than or equal to about 10 cm^" .

[038] Using electrical contacts 440, 450 an electrical bias is applied across the LED structure 400 (i.e. across the interface of n-doped semiconductor layer 410 and p-doped semiconductor layer 420) resulting in holes (h) 460 being injected into the n-doped semiconductor layer 410 from the p-type semiconductor layer 420. Preferably, the band gap of the p-type semiconductor layer 420 is larger than the band gap of the n-doped semiconductor layer 410 so that injection of electrons from the n-doped semiconductor layer 410 into the p-type semiconductor layer 420 is inhibited. Application of a suitable electrical bias results in emission of light 470. Electrical contact layers of a good conducting material can be applied to at least part of the surface of layers 410 and/or 420.

[039] In an alternate form, layer 410 could be deposited on substrate layer 430 and layer 420 deposited on at least part of layer 410, with layer thicknesses appropriately varied. Various physical vapor deposition (PVD) or chemical vapor deposition (CVD) film growth techniques can be utilised to form the required layers, and a range of layer thicknesses can be utilised depending on specific selected materials. For example, thicker light emitting layers, that is thicker than thin layers providing quantum wells, can be used of greater than a few, or greater than several, nanometers thickness.

[040] A range of different semiconductor materials, other materials, structures, geometries and/or substrates, can be used to produce a LED, for example depending on the wavelength of light desired to be produced. One or more additional layers could be used for a variety of purposes, for example an active material layer positioned between semiconductor layers 410, 420, and/or electrical contact layers. Example semiconductor materials that could be used, or could be used in one or more different combinations, include Gallium arsenide (GaAs), Aluminium gallium arsenide (AlGaAs), Gallium arsenide phosphide (GaAsP), Aluminium gallium indium phosphide (AlGalnP), Gallium(III) phosphide (GaP), Indium gallium nitride (InGaN), Gallium(III) nitride (GaN), Indium nitride (InN), Indium tin oxide (ln₂0₃ and Sn0₂), Zinc oxide (ZnO), Aluminium gallium phosphide (AlGaP), Zinc selenide (ZnSe), Silicon carbide (SiC) as substrate, Silicon (Si) as substrate, Diamond (C) as substrate, Boron nitride (BN), Aluminium nitride (A1N), Aluminium gallium nitride (AlGaN), and/or Aluminium gallium indium nitride (AlGalnN).

[041] FIG. 5 illustrates a cross-sectional view of a more specific example InGaN LED structure 500. A semiconductor layer 510 of high, heavily or substantially n-doped InGaN

(which is herein taken to be a carrier concentration of greater than or equal to about 10 20 cm^"3) is deposited on part of a semiconductor layer 520 of p-type (i.e. p-doped) GaN, being a template deposited on a sapphire substrate 530. The InGaN has a high, or relatively high, indium content, for example compared to a standard InGaN LEDs. Various film growth techniques can be used such as metal organic chemical vapor deposition (MOCVD). In one example, the high indium content can be achieved by growing InGaN under nitrogen rich conditions. Example layer thicknesses for the InGaN layer are preferably equal to or greater than about 100 nm (or in a specific example for the results presented below about 170 nm) for semiconductor layer 510 and about 3 um for semiconductor layer 520, although it should be appreciated that a broad range of layer thicknesses can be used. It is believed that a sufficient thickness for the InGaN layer is important so that the material can act as a waveguide.

[042] In a specific but non-limiting example, the light emitting device can be prepared as follows. Ino.30Gao.70N layers can be grown directly on p-type GaN templates and c-plane sapphire substrates using the Migration Enhanced Afterglow (MEAglow) system and technique, described in US Patent No. 8,580,670 (Butcher) which is incorporated herein by cross-reference. [043] InGaN with relatively high indium content must be grown at temperatures substantially lower than the optimal temperatures for GaN film growth. For commercial LED structures, p-GaN and p-AlGaN layers often cap the underlying InGaN layers used for light emission, the temperatures used for the growth of these p-type layers can severely affect underlying InGaN of high indium content. The Applicant therefore used the reciprocal structure to study the quality of InGaN layers grown without a p-type cap. Electroluminescence data was collected from the structures, though a high series resistance was present through the p-type GaN. Unexpectedly, in a peculiar physical situation the Applicant observed super-luminescent behaviour for some samples, the results of which are presented here.

[044] For the film growth, sapphire substrates were prepared by heating in air at 1050 °C for four hours to remove polishing damage and surface absorbates. MOCVD grown p-GaN templates were prepared by etching with HC FkO solution for 10 minutes prior to loading into the MEAglow film growth system. The growth system had a base pressure of < 10^' Torr. Samples were introduced to the growth chamber through a load lock. Film growth started with 2 hours of heating under nitrogen gas flow at 750 mTorr. Thereafter the substrates were lightly nitrided for 1 minute with the plasma source set to 100 Watts (RF plasma at 13.56 MHz). Film growth commenced with pulsed metalorganic introduction and with a continuous RF nitrogen plasma (600 W). The growth mode is somewhat similar to modulated metal epitaxy and/or DERI methods of Molecular Beam Epitaxy (MBE) based film growth, though in this case metalorganics are used and the plasma source is a wider area hollow cathode plasma source specifically developed for the MEAglow system. The film growth also occurs at a much higher pressure, which helps eliminate more damaging plasma species through gas collision.

[045] Two sets of film growth were undertaken, films grown at 540 °C were of high carrier concentration with poor X-ray diffraction suggesting a large native defect concentration. Samples grown at a lower temperature of 450 °C had substantially better X- ray diffraction, and lower carrier concentration. For growth on the p-GaN templates a section of the template was shadow masked with pieces of sapphire to allow access to the p-GaN layer for subsequent electroluminescence testing. [046] The alloy composition was determined from the position of the main peak in X-ray diffraction spectra. TEM images showed the interface with the p-GaN to be quite rough, however this is consistent with AFM measurement observations of those templates prior to growth. The commercial p-GaN templates were much rougher than typical undoped or n- doped templates, AFM measurements indicate RMS surface roughness values of 1.9 to 7.6 nm dependent on supplier, whereas n-GaN templates have values more typically around 0.3 nm. Although the interface between p-GaN and Ino.3₀Gao_.70N is relatively rough, it still appeared to be a relatively clean interface, however the lattice mismatch leads to a large number of extended defects (including threading dislocations) which would be sites of native defects. In this case the lattice mismatch is less than that for samples grown on sapphire, so the material quality is somewhat better, however the large number of defects appear to lead to a high carrier concentration. This was also evidenced by optical transmission measurements.

[047] Hall effect measurements using the samples grown on sapphire indicate high carrier concentrations of greater than or equal to about 1 x 10 cm , for instance 3 x 10 cm^" , for the samples grown at high temperatures, this is consistent with the presence of a Moss- Burstein effect. In contrast, the highest carrier concentration of the lower temperature for the samples grown at high temperatures, this is consistent with the presence of a Moss- Burstein effect. In contrast, the highest carrier concentration of the lower temperature samples was only 3 x 10¹⁹ cm^"3 and no Moss-Burstein effect was evident for the higher quality samples grown at the lower temperatures.

[048] Referring again to FIG. 5, using electrical contacts 540, 550 a forward bias is placed across the LED structure 500 resulting in holes (h) 560 being injected from the p- type layer 520 into the degenerate n-doped InGaN layer 510. The band gap of the GaN is so much larger than that of the InGaN that injection of electrons from the n-doped InGaN layer 510 into the p-type GaN layer 520 is inhibited.

[049] A strong broad yellow light emission 570 is observed. There is a slight blue emission (not illustrated) from the contact region of electrical contact 540 which is from electron injection into the p-GaN layer 520 from a surface oxide near the electrical contact 540. A blue emission is typical of a normal GaN p-n junction and provides a point of reference for the brightness of the strong yellow light emission 570 from the InGaN material. Yellow light emission from InGaN is usually very hard to achieve as the indium content of the material has to be high. In this example however the yellow light emission is very strong and easily identified.

[050] FIG. 6 shows a graph of absorption coefficient squared vs. photon energy, calculated from optical transmission measurements and normally used for determining the band gap of a direct band gap material. This technique usually provides a measure of the band gap of a material, however in the case where a large or significant Moss-Burstein effect is present EF+EQ, the value of the Fermi level (EF) above the band gap (EG) is found instead (see FIG. 7). In this case, for the example of FIG. 5, because of the Moss-Burstein effect, the band gap cannot be determined, but only the position of the Fermi level above the band gap, which is 2.67 eV or equivalent to 465 nm. Some free electron absorption seen at lower energies indicates that there is a large population of free electrons present in the conduction band.

[051 ] FIG. 8 shows light emission spectra from the InGaN LED structure 500 of FIG. 5.

Note that the shortest wavelength corresponds to the energy EF+EG found from the

RECTIFIED sHEET ^J

(Rule 91 ) ISA/AU measurements shown in FIG. 6. A relatively high applied voltage may be required because of the large series resistance in the p-type layer 520.

[052] The Fermi level value seen in FIG. 6 and FIG. 7 corresponds to the low wavelength cut-off of the electroluminescent emission spectra seen in FIG. 8. For a normal non- degenerate semiconductor device, the band gap found from an optical absorption coefficient squared vs. energy plot would normally correspond to the peak of this emission rather than the low wavelength cut-off. This clearly demonstrates the difference between devices 400, 500 and traditional non-degenerate semiconductor devices.

[053] Referring to FIG. 9, electroluminescence peak intensity is shown for two example Ino.3oGao.7oN samples, both about 100 run thick, but of different carrier concentration (excitation area is the same). The high carrier concentration sample shows super- luminescence. FIG. 9 thus shows the light output intensity for two sample light emitting devices, one with a strong Moss-Burstein effect (open circles represent 3 x 10 20 cm^" 3 carrier concentration), and the other without (solid circles represent 3 x 10 cm^" earner concentration). For the higher carrier concentration InGaN device (open circles), super- luminescence is observed above approximately 40 mA with a strong increase in luminescent output. Whereas the lower carrier concentration device (solid circles) shows no such increase. It is to be noted that so-called quantum droop usually results in a sub- linear dependence of standard LED device luminescent output on current. The observation of super-luminescence is not expected for an InGaN layer having high indium content because of the observation of reduced quantum efficiency for normal test LEDs with such InGaN content. However, the Applicant has found that growing the InGaN material to have high indium content, for example under nitrogen rich conditions, helps to reduce non- radiative trapping centres that might otherwise appear in the forbidden band-gap of the material. Potentially, metal interstitial species may provide non-radiant recombination centres, though the presence of these species are inhibited under nitrogen rich growth conditions.

[054] Referring to FIG. 10 there is illustrated an example method 900 for manufacturing an InGaN based LED structure with a degenerate n-doped semiconductor layer 410. Method 900 includes step 910 semiconductor layer 420 onto a

(Rule 91 ) ISA/AU substrate 430. At step 920, a InGaN degenerate n-doped semiconductor layer 410 is deposited onto at least part of the p-doped semiconductor layer 420. More than one layer of each type of layer can be used if desired. Additional layers of various materials can also be applied if desired. Various physical vapor deposition (PVD) or chemical vapor deposition (CVD), such as metal organic chemical vapor deposition (MOCVD), growth techniques can be utilised to form the required layers, and a range of layer thicknesses can be utilised depending on specific selected materials. A range of different semiconductor materials, other materials, structures and/or substrates, can be manufactured. Method 900 can be modified to manufacture the LED structure 800 of FIG. 9 having a degenerate p- type material layer 820.

[055] In example light emitting devices the lack of or reduction in self absorption means that photons generated in a structure can travel for greater distances inside a semiconductor material. This can be advantageous for designing laser devices or super-luminescent devices, particularly those made of InGaN, for example grown under nitrogen rich conditions.

[056] A question considered by the Applicant is why the significantly poorer, high carrier concentration material grown at 540 °C, showed more than about 20 times the electroluminescent intensity of the markedly better quality samples grown at 450 °C when measured under the same conditions. The relatively low carrier concentration of the p-type GaN layer, which was the same for all InGaN samples grown, should limit the luminescence intensity. The electroluminescence data collected for the devices were all quite broad (full width half maximum values of about 100 nm).

[057] It is to be noted that super-luminescence due to spontaneous emission is usually achieved in group III nitrides using active layers with carrier concentrations of about 1 to 5 xlO¹⁸ cm^'3. However, in present examples presented herein super-luminescence is only seen when the n-InGaN layer has a substantially higher carrier concentration of about 1 x 10²⁰ cm^"3 and a prominent Moss-Burstein effect. Samples with carrier concentrations only an order of magnitude lower, but without a significant Moss-Burstein effect, did not show the super-luminescence effect.

RECTIFIED SHEET

(Rule 91 ) ISA/AU achieved by the Moss-Burstein effect for material of poor structural quality, but with a high native defect doping level. Additionally, the condition of inversion appears to have been met at least in part by the Moss-Burstein effect, and potentially at least in part by the possible presence of a two dimensional hole gas.

[059] In summary, some example aspects or advantages of various embodiments include:

1. The active light emitting layer, or layers in the same structure, have a high or very high carrier concentration so that there are free electrons in the conduction band, that is the active layer is degenerate.

2. The re-absorption of photons by the valence band to the conduction band minima transitions is blocked by the free electrons in the conduction band, leading to more efficient emission from light emitting devices.

3. A device structure with one or more active layers of degenerate n-type InGaN has been presented. Alternatively other materials capable of producing a Moss- Burstein effect can be used, including GaN, InN, Indium tin oxide and/or ZnO, to mention a few examples.

4. Similarly, degenerate p-type layers could also be used with electron injection into the p-type material.

5. The light emission from a device using a degenerate material is broad compared to equivalent non-degenerate devices made from the same materials, and the position of the Fermi level above the band gap (found from optical absorption measurements) determines the low wavelength cut-off.

6. The low valence band to conduction band minima absorption of degenerate material layers means they can also be used as an optical wave guide. This is useful for higher efficiency super-luminescent diode device structures. Particularly those made from high indium content InGaN, where the InGaN material has been grown under nitrogen rich conditions.

[060] Optional embodiments of the present invention may also be said to broadly consist in the parts, elements and features referred to or indicated herein, individually or collectively, in any or all combinations of two or more of the parts, elements or features, and wherein specific integers are mentioned herein which have known equivalents in the art to which the invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.

[061] Although a preferred embodiment has been described in detail, it should be understood that many modifications, changes, substitutions or alterations will be apparent to those skilled in the art without departing from the scope of the present invention.

Claims

The claims.

1. A light emitting device, comprising:

a substrate layer;

a p-doped semiconductor layer; and,

a degenerate n-doped semiconductor layer.

2. The light emitting device of claim 1, wherein the degenerate n-doped semiconductor layer is made of, or at least partially made of, InGaN.

3. The light emitting device of claim 2, wherein the InGaN has a high indium content.

4. The light emitting device of any one of claims 1 to 3, wherein the degenerate n- doped semiconductor layer is adjacent the p-doped semiconductor layer.

5. The light emitting device of any one of claims 1 to 4, wherein electrical contacts are attached to the p-doped semiconductor layer and the degenerate n-doped semiconductor layer, and the electrical contacts can be used to apply an electrical bias across an interface of the p-doped semiconductor layer and the degenerate n-doped semiconductor layer so as to produce light.

6. The light emitting device of any one of claims 1 to 5, wherein a band gap of the p- doped semiconductor layer is larger than a band gap of degenerate n-doped semiconductor layer.

7. The light emitting device of any one of claims 1 to 6, wherein, in use, the degenerate n-doped semiconductor layer has a relatively high or very high carrier concentration so that there are free electrons in a conduction band.

8. The light emitting device of claim 7, wherein, in use, re-absorption of emitted photons is inhibited by the free electrons in the conduction band.

9. The light emitting device of any one of claims 1 to 8, wherein the degenerate n- doped semiconductor layer is InGaN and the p-doped semiconductor layer is GaN.

10. The light emitting device of any one of claims 1 to 9, wherein emitted light is broader in wavelength than light from an equivalent device including a non-degenerate n- doped semiconductor layer of the same material.

11. The light emitting device of any one of claims 1 to 10, wherein a position of a Fermi level is above a bottom of a conduction band in the n-type semiconductor, or below a top of a valence band in the p-type semiconductor.

12. The light emitting device of any one of claims 1 to 11, wherein the degenerate n- doped semiconductor layer is doped to induce a Moss-Burstein effect.

13. The light emitting device of any one of claims 1 to 12, wherein the degenerate n- doped semiconductor layer is greater than several nanometers thickness.

14. The light emitting device of any one of claims 1 to 13, wherein the light emitting device is an InGaN based Light Emitting Diode.

15. The light emitting device of any one of claims 1 to 13, wherein the light emitting device is an InGaN based laser diode.

16. The light emitting device of claim 15, wherein semiconductor transparency can be achieved in the laser diode without a need to electrically pump the laser diode.

17. The light emitting device of any one of claims 1 to 13, wherein the light emitting device is a super-luminescent device.

18. The light emitting device of any one of claims 1 to 13, wherein the degenerate n- doped semiconductor layer is part of an optical wave guide.

19. The light emitting device of any one of claims 1 to 18, wherein the degenerate n- doped semiconductor layer has a carrier concentration of greater than or equal to about 10²⁰ cm-³.

20. A method of manufacturing a light emitting device, including the steps of:

depositing a p-doped semiconductor layer on a substrate; and, depositing a degenerate n-doped semiconductor layer on at least part of the p-doped semiconductor layer.

21. The method of claim 20, wherein the degenerate n-doped semiconductor layer is made of, or at least partially made of, InGaN.

22. The method of claim 21, wherein the InGaN has high indium content and is grown under nitrogen rich conditions.