This PDF file contains the front matter associated with SPIE Proceedings Volume 8986 including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

The economic and energy efficiency promise of solid state lighting drive a continuous need to reduce the cost and improve the efficiency of visible LEDs. Current III-N LED manufacturing is dominated by non-native substrate approaches which result in strained epitaxial films, high dislocation density in the buffer and device active regions, and large epiwafer bow, which together present major limitations in terms of LED device yield, performance, and cost. Large area sapphire substrates (6”, 8”, and beyond) are becoming available and present cost reduction potential yet they are even more vulnerable to strain and bow related issues. We present a novel approach to realizing a zero-bow relativelythick GaN on sapphire template that can be made at essentially any diameter and which bodes well for improved device yield, performance, and cost. The elimination of bow is achieved through a simple cost-effective stress balancing technique involving backside deposition. The result is a large area GaN template which is flat at all temperatures and which has 10× lower dislocation density than current GaN buffers. We report on the properties of these templates and provide device data for LED structures grown thereon.

With current technologies, dislocation-free bulk shape GaN crystals with centimeter-size can be grown from small seed crystals. This technique can be realized by putting a sapphire plate with a small hole (0.5~1.5 mm in diameter) on a GaN plate seed. Centimeter-sized bulk GaN single crystals with large dislocation-free areas could be fabricated by this technique. Cathodoluminescence measurement at the interface between the seed and the grown crystal has revealed that almost all dislocations propagated from the GaN seed were bent and terminated at the initial growth stage. To enlarge the diameter of bulk shape GaN crystals, we have developed the coalescence of GaN crystals from many isolated small seeds. As a first step, we grew two GaN point seeds and coalesced them. Two GaN point seeds were established by mounting a sapphire plate with two small holes. We have found the two GaN crystals grown from two separate seed area coalesced without generating dislocations at a coalescence boundary. The grown GaN crystal can remove from substrate easily during the growth. This phenomenon is effective to reduce the stress in the grown GaN crystal. 2-inch GaN crystals by the coalescence technique. Some of the crystals have very large curvature radius (~100 m).

We fabricated the homoepitaxial AlN layers and AlGaN/AlN multi-quantum wells (MQWs) grown on freestanding AlN (0001) substrates by metalorganic vapor phase epitaxy. Crystallographic and optical characteristics of these AlN layers and AlGaN/AlN MQWs were investigated by X-ray diffraction (XRD), SEM, AFM, PL, and so on. These characteristics of MQWs are strongly dependent on growth condition of homoepitaxial AlN layer. In case that AlN layers are grown with high temperature and low V/III ratio, there are many hillocks on the surface and no PL signal from MQW. In contrast, strong PL emission cannot be obtained from AlGaN/AlN MQW on AlN layer grown with lower temperature and higher V/III ratio, because this MQW has large surface roughness from results of XRD and AFM measurement. We optimized the growth condition of AlN layers and obtained the high quality AlGaN/AlN MQW with smooth surface, strong PL emission.

We present our results of (20-21) GaN growth on (22-43) patterned sapphire substrates. The substrates are patterned by etching trenches with c-plane-like side-facets. On these facets, the metalorganic vapor phase epitaxy (MOVPE) GaN growth starts in c-direction and forms triangularly shaped stripes eventually coalescing to a (20-21) oriented layer. X-ray rocking curves measured parallel to the stripes of the symmetric (20-21) reflection show a full width at half maximum of 675 arcsec. Well known from epitaxy of c-plane GaN, an in-situ-deposited SiN mask could help to reduce the defect density further. Systematic investigations of the deposition time and position of the SiN interlayer resulted in a significant improvement of crystal quality confirmed by X-ray and low temperature photoluminescence measurements. Additionally, MOVPE GaN templates with and without SiN mask were overgrown by hydride vapor phase epitaxy. Also now, the SiN interlayer improves the crystal and especially the surface quality of the HVPE layer.

As the GaN material research is reaching maturity with the phenomenal success in LED industry, there is now need to look beyond conventional epitaxy. In this paper we will summarize a few novel directions that we are pursuing. In the first part of this paper, we highlighted our effort to grow single crystal GaN on amorphous substrate. With the successive applications of a phenomenon called evolutionary selection along two perpendicular axes, we remove the degree of freedom in grain orientations from 3 to 0 and successfully prepared single-crystalline GaN on amorphous oxide template. We dedicated the second part of this paper to our recent findings in GaN nanomembrane. Via conductivity selective electrochemical etching, we have fabricated GaN nanomembrane as thin as 90 nm. The thin and “soft” GaN nanomembrane is proven to maintain its as-grown crystal quality. We have also demonstrated a 300 nm thick InGaN/GaN nanomembrane LED.

The growth of hexagonal GaN-on-Si(100) sample prepared by pulsed laser deposition (PLD) was employed in the development of GaN-on-Si technology. In contrast to common GaN-on-sapphire and GaN-on-Si(111) technologies, the use of the GaN film on Si(100) by PLD provides low-cost and large-area single crystalline GaN template for GaN applications, via a single growth process without any interlayer or interruption layer. The evolution of GaN growth mechanism on Si(100) substrate with various growth times is established by SEM and TEM data, which indicated that the growth mode of the GaN films gradually changes from island growth to layer growth when the growth time increases up to 2hrs. Moreover, no significant GaN meltback was found on the GaN sample surface due to the high-temperature operation of PLD. The GaN sample was subjected to MOCVD treatment to regrow a GaN layer. The results of X-ray diffraction analysis and photoluminescence measurement show the reliability of the PLD-GaN sample and are promising for the development of the GaN-on-Si technology using PLD technique.

The optical properties of GaN/Al_0.15Ga_0.85N multiple quantum wells grown on m-plane oriented substrate are studied in 8K–300K temperature range. The optical spectra reveal strong in-plane optical anisotropies as predicted by group theory. Polarized time resolved temperature-dependent photoluminescence experiments are performed providing access to the relative contributions of the non-radiative and radiative recombination processes. We deduce the variation of the radiative decay time with temperature in the two polarizations.

The gain saturation is a crucial factor limiting achievable output power of superluminescent diodes (SLD), as it exponentially depends on optical gain value. Contrary to laser diodes, in SLDs gain is increasing with the increasing current even much above the transparency conditions. Therefore, SLDs provide us with an unique possibility to examine gain under high current densities (high carrier injection). In our work we examined SLDs fabricated in a “j-shape” ridge-waveguide geometry having chips of the length of 700 μm and 1000 μm, emitting in the blue-violet region. By comparing the amplified spontaneous emission measured along the device waveguide with true spontaneous emission measured in perpendicular direction, we are able to extract optical gain as a function of injected current. We show, that in our devices spontaneous emission exhibits a square-root-like dependence on current which is commonly associated with the presence of “droop” in case of nitride light emitting diodes. However, along the waveguide axis, fast processes of stimulated recombination dominate which eliminates the efficiency reduction. Calculated optical gain shows a substantial saturation for current densities above 8 kA/cm².

A point defect control scheme is demonstrated, to control point defects during the growth of doped wide bandgap semiconductors. First the theoretical description of this new concept is presented, second GaN:Mg is used as a model system and as an experimental example to show its feasibility. It can be shown that above bandgap UV-light illumination during the growth can reduce the passivation and compensation of Mg acceptors in GaN:Mg. The amount of hydrogen impurities, that usually passivates Mg at doping concentrations around Mg:2x10¹⁹ cm^-3, is significantly reduced by UVillumination. The resistivity of samples grown with UV is similar to the resistivity of post-growth annealed samples. No post growth annealing was needed. In contrast samples that are doped below Mg:<1x10¹⁸ cm^-3 become n-type conductive when the samples are grown with UV illumination. This observation suggests a reduced incorporation of Mg acceptors due to the UV light. At low Mg doping concentrations the native donor incorporation by O donors dominates the conductivity over Mg acceptors. UV-illumination therefore reduces compensation of donors by Mg acceptors. Thus, these observations support the concept of UV illumination as a way to control the Fermi level of different charged point defects to control compensation in doped semiconductors.

GaN nanowires form spontaneously on a wide variety of substrates without suffering from extended defects. However, their quasi-one-dimensional nature causes these structures to have an extended free surface, resulting in a surface-to-volume ratio orders of magnitude larger than that of a planar layer. Additionally, the high nucleation density of spontaneously formed GaN nanowire ensembles results in an unintentional, but inevitable coalescence between individual nanowires. In this work, we investigate the impact of both the surface and the coalescence of nanowires on the recombination dynamics of excitons in GaN nanowire ensembles. Using simple models to simulate the change in recombination dynamics of bound excitons in GaN NWs with varying diameter and coalescence degree, we show that the comparatively short decay times at low temperatures are not generally caused by either of these mechanisms. Furthermore, we demonstrate that the biexponential decay for the donor-bound exciton is also not related to a coexistence of nonradiative and radiative recombination channels, but originates from a coupling of the donor- and acceptor-bound exciton states in the GaN NWs.

(Al,Ga)N light emitting diodes (LEDs), emitting over a large spectral range from 360 nm (GaN) down to 210 nm (AlN), have been successfully fabricated over the last decade. Clear advantages compared to the traditional mercury lamp technology (e.g. compactness, low-power operation, lifetime) have been demonstrated. However, LED efficiencies still need to be improved. The main problems are related to the structural quality and the p-type doping efficiency of (Al,Ga)N. Among the current approaches, GaN nanostructures, which confine carriers along both the growth direction and the growth plane, are seen as a solution for improving the radiative recombination efficiency by strongly reducing the impact of surrounding defects. Our approach, based on a 2D - 3D growth mode transition in molecular beam epitaxy, can lead to the spontaneous formation of GaN nanostructures on (Al,Ga)N over a broad range of Al compositions. Furthermore, the versatility of the process makes it possible to fabricate nanostructures on both (0001) oriented “polar” and (11 2 2) oriented “semipolar” materials. We show that the change in the crystal orientation has a strong impact on the morphological and optical properties of the nanostructures. The influence of growth conditions are also investigated by combining microscopy (SEM, TEM) and photoluminescence techniques. Finally, their potential as UV emitters will be discussed and the performances of GaN / (Al,Ga)N nanostructure-based LED demonstrators are presented.

We report on the properties and growth kinetics of defect-free, photoluminescence (PL) efficient mushroom-like nanowires (MNWs) in the form of ~30nm thick hexagonal-shaped InGaN-nanodisk on GaN nanowires, coexisting with the conventional rod-like InGaN-on-GaN nanowires (RNWs) on (111)-silicon-substrate. When characterized using confocal microscopy (CFM) with 458nm laser excitation, while measuring spontaneous-emission at fixed detection wavelengths, the spatial intensity map evolved from having uniform pixelated emission, to having only an emission ring, and then a round emission spot. This corresponds to the PL emission with increasing indium composition; starting from emission mainly from the RNW, and then the 540 nm emission from one MNWs ensemble, followed by the 590 nm emission from a different MNW ensemble, respectively. These hexagonal-shaped InGaN-nano-disks ensembles were obtained during molecular-beam-epitaxy (MBE) growth. On the other hand, the regular rod-like InGaN-on-GaN nanowires (RNWs) were emitting at a shorter peak wavelength of 490 nm. While the formation of InGaN rod-like nanowire is well-understood, the formation of the hexagonal-shaped InGaN-nanodisk-on-GaN-nanowire requires further investigation. It was postulated to arise from the highly sensitive growth kinetics during plasma-assisted MBE of InGaN at low temperature, i.e. when the substrate temperature was reduced from 800 °C (GaN growth) to <600 °C (InGaN growth), during which sparsely populated metal-droplet formation prevails and further accumulated more indium adatoms due to a higher cohesive bond between metallic molecules.

In this paper, we demonstrate that p-type InN nanowires can be realized by direct magnesium (Mg) doping. X-ray photoelectron spectroscopy experiments on the sidewalls of Mg-doped InN nanowires indicate no surface electron accumulation with the background electron concentration below 1×10¹⁶ cm^-3. The presence of Mg-acceptors is clearly observed by the low-temperature photoluminescence (PL) spectroscopy, which shows a band-to-band recombination PL peak and a Mg-acceptor energy level related PL peak. The peak energy separation is about 60 meV, which is consistent with the Mg activation energy in InN. Single nanowire field-effect transistors are further fabricated and investigated, which clearly exhibits a p-type transistor behavior, representing the first direct evidence for p-type conduction in any InN structures. The potential device applications based on InN nanowires are also discussed.

We propose a plausible and realistic idea for realizing high-efficiency III-nitride semiconductor tandem solar cells which utilize nearly entire AM-1.5 solar spectrum conversion under a subcell photocurrent matching rule. For the sake of drastic improvement/suppression of pn-junction leakage current, each subcell in the proposed tandem solar cells is composed of superstructure InN/GaN magic alloys, i.e. coherently grown (InN)_m/(GaN)_n short-period superlattices with simple integer pairs of (m, n) ≤ 4 in monolayers, which solve lattice-mismatch and immiscible problems in a conventional InGaN ternary alloy system. The InN/GaN magic alloys are further applicable to band engineering that provides potential wells for a thermal/photo sensitization effect and graded-bandgap structures for efficient carrier collection under the same (m, n) ratio alloys or keeping the coherent structure. Theoretical maximum conversion efficiency is 51% (58% under 250-suns concentration) for a 4-tandem cell configuration.

Electrically active defects in AlGaN/GaN high electron mobility transistors (HEMTs) are the source of intense study due to their linkage to the mechanisms for GaN HEMT degradation upon a variety of stress conditions. The ability to directly characterize traps and identify their sources in GaN HEMTs is challenging, however, and this is due to a combination of the large bandgap of these materials and the complex electrostatics present in these device structures. Furthermore, the targeted extreme operating conditions intended for GaN HEMTs, whether designed for RF or power applications, greatly exacerbate the ability to identify those defects that are most relevant to device degradation under actual operation. Over the past few years, however, we have developed several electronic defect characterization methods based on deep level optical spectroscopy (DLOS) and thermally-based deep level transient spectroscopy (DLTS), which have been adapted from basic studies of defects in GaN and AlGaN to become applicable to working HEMTs. These so-called constant drain current (CI_D) DLOS/DLTS methods are able to directly provide individual trap concentrations and energy levels for traps that may exist throughout the AlGaN/GaN HEMT bandgap, and can discern between traps under the gate or in the transistor access regions. This talk will first review the CI_D-DLOS/DLTS methodology and then will focus on the application of these methods to stressed and un-stressed AlGaN/GaN HEMTs. Direct correlations of the formation of several specific traps are made with a variety of HEMT degradation mechanisms induced by stresses that include RF, DC and also proton irradiation

Advances in frequency performance of heterostructure field-effect transistors (HFETs) are discussed in terms of dissipative processes. The conditions for fastest dissipation coincide reasonably well with those for fastest operation and slowest device degradation. The correlation has its genesis in dissipation of the hot-phonon heat accumulated by non-equilibrium optical phonons launched by hot electrons. The hot-phonon heat causes defect formation and additional electron scattering in a different manner as compared with the effects due to conventional heat accumulated by acoustic phonons. The desirable ultrafast conversion of hot phonons into acoustic phonons is assisted by plasmons as demonstrated through measurement of hot-phonon lifetime. Signatures of plasmons have been also resolved in hot-electron transport, transistor frequency performance, phase noise, and device reliability. The plasmon-assisted ultrafast dissipation of hot-phonon heat explains the known necessity for application a stronger negative gate bias to a channel with higher as-grown electron density.

AlGaN-GaN high electron mobility transistors (HEMTs) are most suitable for commercial and military applications requiring high voltage, high power, and high efficiency operation. In recent years, leading AlGaN HEMT manufacturers have reported encouraging reliability of these devices, but their long-term reliability especially in the space environment still remains a major concern. In addition, degradation mechanisms in AlGaN HEMT devices are still not well understood, and a large number of traps and defects present both in the bulk and at the surface lead to undesirable characteristics. Study of reliability and radiation effects of AlGaN-GaN HEMTs is therefore necessary before GaN HEMT technology is successfully employed in satellite communication systems. For the present study, we investigated electrical characteristics of AlGaN-GaN HEMTs and AlGaN Schottky diodes irradiated with protons. We studied two types of MOCVD-grown AlGaN HEMTs on semi-insulating SiC substrates (HEMT-1 and HEMT-2) as well as MOCVD-grown Al_0.27Ga _0.73N Schottky diodes on conducting SiC substrates. Our HEMT-1 structure consisted of a GaN cap, AlGaN/AlN barrier, and 2 μm GaN buffer layers. Our HEMT-2 structure consisting of undoped AlGaN barrier and GaN buffer layers grown on an AlN nucleation layer showed a charge sheet density of ~10¹³/cm² and a Hall mobility of ~1500 cm² /V^.sec. Our HEMT-1 devices had a Pt-Au Schottky gate length of 0.2 μm, a total gate width of 200-400 μm periphery, and SiN_x passivation. Electrical characteristics of AlGaN-GaN HEMTs and AlGaN Schottky diodes were compared before and after they were proton irradiated with different energies and fluences. Current-mode deep level transient spectroscopy (DLTS) and capacitance-mode DLTS were employed to study pre-proton irradiation trap characteristics in the AlGaN-GaN HEMTs and AlGaN Schottky diodes, respectively. Focused ion beam (FIB) was employed to prepare both cross-sectional and plan view TEM samples for defect analysis using a high resolution TEM. In addition, electrical characteristics of GaAs MESFETs used as reference devices were compared before and after they were proton irradiated.

We discuss the design and demonstration ultra-low resistance III-nitride tunnel junctions, and how tunnel junctions could solve the long-standing problem of efficiency droop in solid state lighting. We have used nanoscale band engineering based on polarization and mid-gap states to reduce tunneling resistance by four orders of magnitude. We will discuss experimental demonstration of highly efficient tunnel junctions (resistivity ~ 0.1 mOhm-cm²) in PN junctions, p-contact free LEDs, and multiple junction structures. Finally we will show how tunneling based carrier regeneration in multiple active region cascade LEDs could help to enable low current, high voltage operation to overcome the efficiency droop problem.

Commercially available inorganic white light emitting diodes (LEDs) are essentially based on the combination of a blue InGaN based LED chip covered by a long wavelength emitting (yellow, red) phosphor. We propose to avoid this step of phosphor deposition by taking advantage of the fact that yellow to red emission can be achieved using InGaN alloys. By stacking an InGaN/GaN multiple quantum well (QW) emitting in the yellow, acting as a light converter, and a short wavelength blue-violet pump LED grown on top, white light emission can be obtained. Furthermore, if we extend the emission spectrum of the light converter into the red, a warm white light color is demonstrated when a pump LED is grown on top. However, the high In content InGaN QWs of the light converter have a low thermal stability and the QW efficiency tends to degrade during the growth of the pump LED. Three different solutions are explored to avoid the thermal degradation of the light converter. The monolithic LED structures were grown by molecular beam epitaxy (MBE), by a combination of both MBE and metal-organic chemical vapor phase epitaxy (MOCVD), or by a low temperature full-MOCVD process. The best results are obtained using a complete MOCVD growth process. The structure and the MOCVD growth conditions are specifically adapted in order to avoid the thermal degradation of the large In composition InGaN QWs emitting at long wavelength during the growth of the subsequent layers.

We have grown blue light-emitting diodes (LEDs) with low threading dislocation densities (TDDs) by using SiN interlayers on Si (111) substrates. Our growth technique using SiN layers makes it possible to decrease twist components (edge-type threading dislocation components). The edge-type TDDs are almost the same values as those of LEDs grown on Al₂O₃ (0001) substrates. EQE of LEDs grown on Si (111) substrates increases with decreasing edge-type dislocation in the low-current-density region, and the EQE of the sample with low TDD is almost as high as that of the LED grown on an Al₂O₃ (0001) substrate at room temperature. It is found that the hot/cold factors (HCFs) of LEDs grown on Si (111) substrates increase with decreasing edge-type dislocations in the low-current-density region, but are less than those of an LED grown on an Al₂O₃ (0001) substrate. Time-resolved photoluminescence (TRPL) shows that the dominant origin of the thermal quenching is edge-type dislocations in our samples, but other defects such as screw-type dislocations also contribute to it. We also found the fluctuated emission patterns consisting of bright and dark areas originated from the difference of Shockley–Read–Hall (SRH) type defect densities in the multi-quantum wells (MQWs) grown on Si (111) substrates. The bright areas spread, and the configurations of the bright areas change into ring-like patterns with reducing edge-type TDDs. We suggest that the internal quantum efficiency (IQE) of dark areas should be promoted to improve the performance of the MQWs grown on Si (111) substrates.

In this paper, we discussed the influence of the indium fluctuation to the efficiency droop in LEDs. Both the real and randomly generated indium fluctuation are used in the 3D simulation and compared to the uniform indium distribution quantum wells. We found that the electrical and optical properties in LEDs such as the carrier transport, radiative and Auger recombination, and the droop effect, are strongly affected by these nanoscale indium fluctuations.

We report on design and lasing characteristics of GaN vertical-cavity surface-emitting lasers (VCSELs) with an elongated cavity for use in uniform elements of a two-dimensional (2D) laser array. Calculations of VCSELs with the elongated cavity taking into account the wavelength dispersion of the refractive index show that the transverse mode spacing can be significantly narrower than the gain spectrum with a small tradeoff of the differential quantum efficiency. The result clearly shows that the elongated cavity is robust against the thermally induced peakshift of the gain spectrum, and thus preferable for use in elements of density packed laser array for which uniform operation of each element is crucial. The VCSEL with the elongated cavity fabricated by the wafer thinning technique operates under current injection by using highly reflective distributed Bragg reflectors (DBRs) made of transparent ZrO₂ and SiO₂ film stacks. Together with high reflectivity and wide stop band of the DBR, the elongated cavity of 6 μm (36λ) allows multimode lasing oscillation with a mode spacing of 2.9 nm, which is one order of magnitude narrower than the gain spectrum. In addition, we demonstrate a 5x5 GaN VCSEL array.

Blue and green InGaN-based R&D laser structures on c-plane GaN substrates are investigated. We analyzed carrier injection efficiencies as well as internal quantum efficiencies up to laser threshold. The injection efficiency of the blue laser structure is measured to be 78%. The internal quantum efficiency of spontaneous emission reaches 50% at 30A/cm² and 32% at laser threshold. For the green laser structure we found an injection efficiency of 71%, a maximum of internal efficiency of 36% and, at laser threshold. a value of 28%. Both, recombination on defects as well as Auger effect are identified as relevant loss processes up to the laser threshold. An improved 515nm R&D single mode laser in TO56 can is presented. The optical output power of the green single mode laser reaches 250mW in continuous wave operation underneath thermal roll-over. Wall plug efficiency is as high as 9%. In the next step we investigate high power multimode lasers. The new power green R&D laser reaches maximum power of 1.25W at thermal roll-over. The currentoutput characteristic is nearly linear up to 0.9A and 0.6W. At higher currents thermal bending is observed. We measured a maximum wall plug efficiency of the green multimode laser of 13%. The power blue R&D laser in TO90 metal can reaches 5.5W prior to roll-over having the wall plug efficiency of 32% at 3.5W.

The latest developments in AlGaInN laser diode technology are reviewed. The AlGaInN material system allows for laser diodes to be fabricated over a very wide range of wavelengths from u.v., i.e, 380nm, to the visible, i.e., 530nm, by tuning the indium content of the laser GaInN quantum well. Advantages of using Plasma assisted MBE (PAMBE) compared to more conventional MOCVD epitaxy to grow AlGaInN laser structures are highlighted. Ridge waveguide laser diode structures are fabricated to achieve single mode operation with optical powers of >100mW in the 400-420nm wavelength range that are suitable for telecom applications. Visible light communications at high frequency (up to 2.5 Gbit/s) using a directly modulated 422nm Galliumnitride (GaN) blue laser diode is reported. High power operation of AlGaInN laser diodes is demonstrated with a single chip, AlGaInN laser diode ‘mini-array’ with a common p-contact configuration at powers up to 2.5W cw at 410nm. Low defectivity and highly uniform GaN substrates allow arrays and bars of nitride lasers to be fabricated. GaN laser bars of up to 5mm with 20 emitters, mounted in a CS mount package, give optical powers up to 4W cw at ~410nm with a common contact configuration. An alternative package configuration for AlGaInN laser arrays allows for each individual laser to be individually addressable allowing complex free-space and/or fibre optic system integration within a very small form-factor.

This paper presents an extensive analysis of the degradation of InGaN-based laser diodes submitted to electrical stress. The analyzed devices, with emission in the violet spectral region, were submitted to constant current stress; the degradation process was monitored by means of electro-optical measurements, which indicated that stress induced an increase in the threshold current of the devices, ascribed to the generation of non-radiative defects. After stress, the (thick) top metallization was removed, and the optical behavior of the samples was characterized by microcathodoluminescence and micro-photoluminescence investigation. Results indicate that (i) stress induced a significant degradation of the efficiency of the devices under the ridge, i.e. in the region which is crossed by high current densities during ageing. (ii) the darkening of the ridge was detected both by micro-cathodoluminescence measurements (in which carriers are generated both in the barriers and in the quantum wells) and by micro-photoluminescence analysis with subbandgap excitation (with respect to the barriers). The experimental evidence collected within this paper demonstrates that the degradation of the laser diodes can be ascribed to an increase in the rate of non-radiative recombination within the active region of the devices, possibly due to a defect diffusion process. Hypothesis on the nature of the defects involved in the degradation process are formulated based on capacitance Deep Level Transient Spectroscopy measurements.

The efficiency of blue InGaN/GaN light emitting diodes (LEDs) for solid state lighting has been strongly increased in the past years. The decay of the internal quantum efficiency at current densities above 100 Acm⁻² remains, though. This effect is known as droop and effectively limits the maximum current density. The physical mechanisms behind the droop are not yet fully understood. Theories for the origin of the droop include the Auger recombination process and direct carrier leakage. In this context we present an analysis of the contribution of these effects to the droop by means of physics based simulation. In contrast to lumped simulation models the physical simulation model presented in this work enables the exact matching of the internal quantum efficiency (IQE) characteristics by including the geometry, doping, and mole fraction profiles. The analysis presented in this work uses the IQE data of a fabricated blue single quantum well LED. Fitting this IQE curve with the physics based simulation enables the estimation of limits for the Auger coefficients as well as the contribution of the direct carrier leakage. Including Auger assisted carrier leakage facilitates fitting the IQE with reduced Auger coefficients closing the gap to atomistic simulations.

The efficiency droop in light-emitting diodes (LEDs) represents a gradual decrease of the internal quantum efficiency (IQE) with increasing current. Experimentally, the IQE droops are strong functions of material, epitaxial and chip structures, and operating temperature. Recently, we have proposed an IQE droop model as the saturation of the radiative recombination rate at low current and subsequent increase in the nonradiative recombination rates at high current. Once the radiative recombination rate begins to saturate at an active region, the carrier density as well as the nonradiative recombination rate rapidly increase there. Eventually, the IQE droop appears from the increase in the nonradiative recombination rate being much larger than that in the radiative one. A dominant nonradiative recombination process is not solely determined for each LED chip, but it could vary with current level and operating temperature. As temperature decreases, in general, the IQE droop becomes larger with the peak IQE occurring at an extremely small current level. We test the droop model by investigating the radiative and nonradiative recombination processes separately from the cryogenic to room temperature. The characterization methods include comparative efficiency study between photoluminescence (PL) and electroluminescence (EL), open-circuit voltage under resonant PL excitation, interrelations of current-voltage-light characteristics, and EL spectra of color-coded quantum wells (QWs). Although a sudden increase of the nonradiative recombination rate is an apparent cause of the IQE droop, the saturation of the radiative recombination rate is the common trigger behind the IQE droop issue.

Recent advances in mid-ultraviolet light-emitting diodes grown pseudomorphically on bulk AlN substrates have led to improved efficiencies and lifetimes. For a 266 nm device an output power of 66 mW at 300 mA has been achieved with an external quantum efficiency of 4.5%. More importantly, the lifetimes of these devices have been increased substantially. Testing of LEDs in both surface mount design (SMD) and TO-39 packages show L₅₀ lifetimes well in excess of 1,000 hours under a variety of case temperatures and currents. Package-related catastrophic failures are eliminated through encapsulation and hermetic sealing, further reducing failure rates and extending the lifetime.

GaN/AlN quantum dots (QDs) have been observed to emit in deep ultra violet (UV) regime. The emission wavelength can be tuned from 270 nm to 238 nm using GaN growth time and Ga flux. In this work, tunnel injection GaN/AlN QD UV LEDs have been fabricated utilizing polarization doped p-n junctions grown on AlN templates on sapphire. The QD EL emission is obtained at 250 nm whereas a second peak emission is observed at 290 nm from the p-type AlGaN. However, the enhanced doping and carrier injection in polarization doped structure boosts the deep-UV emission intensity by 26 times compared to non-polarization doped UV LED.

We demonstrate InGaN MQW LEDs on Si substrates have both high performance and low cost structure. The blue LED structures were grown by metal-organic chemical vapor deposition via unique buffer layers on 200 mm-diameter Si (111) substrates. The epitaxial wafers had slightly ex-situ convex bow without micro-cracks. Median and standard deviation of dominant wavelength by photoluminescence measurement were 448.9 nm and 2.0 nm within 4 mm edge exclusion, respectively. XRC FWHMs of GaN (0002) and GaN (10-12) were 341 arcsec and 388 arcsec, respectively, corresponding to be estimated to edge dislocation density of 2.0 × 10⁹ /cm². The blue LED structures were fabricated by legacy 8-inch (200-mm) Si device equipment without remodeling, because no particular thicker Si wafer was needed for our LED epitaxial growth. Encapsulated blue lamps were fabricated after dicing with 45 × 45 mil². Median light output power of 641 mW was obtained under injection current of 350 mA at 25 deg-C. The operating voltage was 2.9 V. It corresponds to wall plug efficiency, WPE of 63 %. Stable operation more than 6,000 hours under 450 mA at 85 deg-C was confirmed. These results suggest the InGaN MQW LEDs on the large-scale Si wafers are promising for the near future solid-state lighting.

Faceted three-dimensional (3D) AlGaN/AlN multiple quantum wells (MQWs) with semipolar {1￣101} and polar (0001) planes are fabricated by a regrowth technique based on metalorganic vapor phase epitaxy (MOVPE) on trench-patterned AlN templates. Similar 3D microfacet structures with different height are formed on top of and at the bottom of the AlN trench. Cathodoluminescence (CL) spectra are separately acquired at semipolar and (0001) facet QWs at room temperature (RT). The peak energies of {1￣101} facet QWs and (0001) facet QWs on higher 3D structures are 5.42 and 5.43 eV, respectively, while that of (0001) facet QWs on lower 3D structures is 5.23eV. Through structural analyses using transmission electron microscopy (TEM), the peak energy difference between the {1￣101} QWs and the lower (0001) QWs is ascribed mainly to suppressed internal electric fields in the {1￣101} facet QWs. Furthermore, Al spatial distribution causes the peak energy difference between the (0001) facet QWs.

Nonpolar m-plane GaN layers were grown on patterned Si (112) substrates by metal-organic chemical vapor deposition (MOCVD). A two-step growth procedure involving a low-pressure (30 Torr) first step to ensure formation of the m-plane facet and a high-pressure step (200 Torr) for improvement of optical quality was employed. The layers grown in two steps show improvement of the optical quality: the near-bandedge photoluminescence (PL) intensity is about 3 times higher than that for the layers grown at low pressure, and deep emission is considerably weaker. However, emission intensity from m-GaN is still lower than that of polar and semipolar (1 100 ) reference samples grown under the same conditions. To shed light on this problem, spatial distribution of optical emission over the c⁺ and c⁻ wings of the nonpolar GaN/Si was studied by spatially resolved cathodoluminescence and near-field scanning optical microscopy.

We reported the defects and optical characterizations of the ultraviolet light-emitting diodes grown on free-standing GaN substrate (FS-GaN) and sapphire. Cross-sectional transmission electron microscopy (TEM) images showed that the total defect densities of grown UV LEDs on FS-GaN and sapphire including edge, screw and mixed type were 3.6×10⁶ cm^-2 and 5.5×10⁸ cm^-2. When substrate of UV LEDs was changed from sapphire to FS-GaN, it can be clearly found that the crystallography of GaN epilayers was drastically different from that GaN epilayers on sapphire. Besides, the microstructures or indium clustering can be not observed at UV LEDs on FS-GaN from TEM measurement. The internal quantum efficiency of UVLEDs on FS-GaN and sapphire were 34.8 % and 39.4 % respectively, which attributed to indium clustering in multi-layers quantum wells (MQWs) of UV LEDs on sapphire. The relationship between indiumclustering and efficiency droop were investigated by temperature-dependent electroluminescence (TDEL) measurements.

Internal Quantum Efficiency (IQE) of two-colour monolithic white light emitting diode (LED) was measured by temperature dependant electro-luminescence (TDEL) and analysed with modified rate equation based on ABC model. External, internal and injection efficiencies of blue and green quantum wells were analysed separately. Monolithic white LED contained one green InGaN QW and two blue QWs being separated by GaN barrier. This paper reports also the tunable behaviour of correlated colour temperature (CCT) in pulsed operation mode and effect of self-heating on device performance.

The optical quality of semipolar (1 101)GaN layers was explored by time- and polarization-resolved photoluminescence spectroscopy. High intensity bandedge emission was observed in ⁺c-wing regions of the stripes as a result of better structural quality, while ^-c-wing regions were found to be of poorer optical quality due to basal plane and prismatic stacking faults (BSFs and PSFs) in addition to a high density of TDs. The high optical quality region formed on the ⁺cwings was evidenced also from the much slower biexponential PL decays (0.22 ns and 1.70 ns) and an order of magnitude smaller amplitude ratio of the fast decay (nonradiative origin) to the slow decay component (radiative origin) compared to the ^-c-wing regions. In regard to defect-related emission, decay times for the BSF and PSF emission lines at 25 K (~ 0.80 ns and ~ 3.5 ns, respectively) were independent of the excitation density within the range employed (5 – 420 W/cm²), and much longer than that for the donor bound excitons (0.13 ns at 5 W/cm² and 0.22 ns at 420 W/cm²). It was also found that the emission from BSFs had lower polarization degree (0.22) than that from donor bound excitons (0.35). The diminution of the polarization degree when photogenerated carriers recombine within the BSFs is another indication of the negative effects of stacking faults on the optical quality of the semipolar (1101)GaN. In addition, spatial distribution of defects in semipolar (1101)-oriented InGaN active region layers grown on stripe patterned Si substrates was investigated using near-field scanning optical microscopy. The optical quality of ^-c- wing regions was found to be worse compared to ⁺c-wing regions due to the presence of higher density of stacking faults and threading dislocations. The emission from the ⁺c-wings was very bright and relatively uniform across the sample, which is indicative of a homogeneous In distribution.

Diffusion lengths of photo-excited carriers along the c-direction were determined from photoluminescence (PL) measurements in p- and n-type GaN epitaxial layers grown on c-plane sapphire by metal-organic chemical vapor deposition. The investigated samples incorporate a 6 nm thick In0.15Ga0.85N active layer capped with either 500 nm p- GaN or 1300 nm n-GaN. The top GaN layers were etched in steps and PL from the InGaN active region and the underlying layers was monitored as a function of the top GaN thickness upon photogeneration near the surface region by above bandgap excitation. Taking into consideration the absorption in the active and underlying layers, the diffusion lengths at 295 K and at 15 K were measured to be about 92 ± 7 nm and 68 ± 7 nm for Mg-doped p-type GaN and 432 ± 30 nm and 316 ± 30 nm for unintentionally doped n-type GaN, respectively. Cross-sectional cathodoluminescence line-scan measurement was performed on a separate sample and the diffusion length in n-type GaN was measured to be 280 nm.