This PDF file contains the front matter associated with SPIE Proceedings Volume 9363, including the Title Page, Copyright information, Table of Contents, Authors, and Conference Committee listing.

SCAAT^TM has been developed as a novel ammonothermal method which enables to obtain strain free, high quality and large size bulk gallium nitride (GaN) crystals under high pressure and high temperature super-critical ammonia. One of the unique features of this technique is relatively high growth rate of more than a few hundred micrometers per day toward polar and non-polar axis with excellent crystalline quality. The morphology, X-ray rocking curve, etch pit density and electric properties are presented.

In this work, the Al_xGa_1-xN-on-sapphire template was successfully prepared by halide vapor phase epitaxy (HVPE) at 1100oC. The flow ratio (R) of the HCl flow rate through Al metal to the total HCl flow rates through Al and Ga metals was adjusted to control the Al content. The Al_xGa_1-xN films without phase separation can be obtained as the R value exceeds 0.67. The low R value strongly enhances the formation probability of GaN instead of Al_xGa_1-xN. This indicates that a sufficient concentration of the Al precursor is certainly vital for the formation of Al_xGa_1-xN compounds. The Al_xGa_1-xN prepared at the R value of 0.80 can achieve best crystallinity and the lowest surface roughness. Furthermore, the optical transmittance of the Al_xGa_1-xN under R=0.80 is above 70% between the wavelength of 225 nm and 400 nm. As a result, the Al_xGa_1-xN under R=0.80 can be regarded as a suitable growth template for the ultraviolet light-emitting diodes. The internal quantum efficiency of the 370 nm-quantum wells on the Al_xGa_1-xN/sapphire prepared under R=0.80 shows 127% improvement as compared with that on the undoped-GaN/sapphire.

In this article homoepitaxial HVPE-GaN growth in directions other than [0001] is described. Three crystallization runs on (11-20), (10-10), (20-21), and (20-2-1) seeds were performed. In each experiment a different carrier gas was used: N₂, H₂, and a 50% mixture of N₂ and H₂. Other conditions remained constant. An influence of the growth direction and carrier gas on growth rate and properties (morphology, structural quality, and free carrier concentration determined by Raman spectroscopy) of obtained crystals was investigated and discussed in details. For all crystallographic directions a lower growth rate was determined with hydrogen used as the carrier gas. Also, the highest level of dopants was observed for crystals grown under hydrogen. A possibility to obtain highly conductive GaN layers of high quality without an intentional doping is demonstrated.

Spectroscopic ellipsometry is applied for determining the ordinary and extraordinary dielectric functions of wurtzite GaN for different free-electron concentrations. The analysis of the dielectric functions in the infrared spectral range yields the frequencies of the coupled longitudinal plasmonphonon modes. From the related plasma frequencies the anisotropy of the effective electron mass is determined. The data in the ultraviolet range indicate unambiguously that the L-valley of the conduction band for undoped material is located at least ≈ 2 eV above the conduction band minimum at the Γ-point. In contrast to the behavior at the Γ-point, the band structure at the L-valley does not undergo a renormalization with increasing free-electron density.

We studied the temperature dependence and the recombination dynamics of the photoluminescence of (1-101)-oriented semi-polar Al_0.2Ga_0.8N/GaN multiple quantum wells (MQW). The polarized low-temperature PL measurements reveal that radiative recombination exhibit an anisotropic behavior. The PL intensity at room temperature is reduced by one order of magnitude with respect to low temperature. The radiative decay time exhibits a mixed behavior: it is roughly constant between 8K to ranging near 140-150K and then rapidly increases with a slope of 10 ps.K^-1. This behavior is indicative of coexistence of localized excitons and free excitons which relative proportion are statistically computed.

We present an investigation of optical properties of yellow light emitting (Ga,In) N-based devices grown by metalorganic vapor phase epitaxy (MOVPE) on c-plane sapphire with different designs: well width, indium composition up to 23 percent in the well layer. Using time-resolved photoluminescence measurements collected in range of 8-300K, temperature – dependent photoluminescence decays are determined, they exhibit similar behaviors for all samples. These quantum devices always display a two-mode exponential decay with a long decay time and a short one in a ratio about four to five. The photoluminescence intensities measured from low temperature to room temperature give large values of activation temperature that indicate the increasing of the non-radiative recombination rate when the temperature increases. The average decay times are found by a procedure using fitting sigmoidal functions. These decay times increase exponentially with the indium concentration and the well width due to influence of quantum confined Stark effect on these devices. Finally, in order to estimate the performances of our samples, we plotted the decay times obtained versus product of the indium content and the well width together with others published decay times.

We have investigated point defects in GaN grown by HVPE by using steady-state and time-resolved photoluminescence (PL). Among the most common PL bands in this material are the red luminescence band with a maximum at 1.8 eV and a zero-phonon line (ZPL) at 2.36 eV (attributed to an unknown acceptor having an energy level 1.130 eV above the valence band), the blue luminescence band with a maximum at 2.9 eV (attributed to Zn_Ga), and the ultraviolet luminescence band with the main peak at 3.27 eV (related to an unknown shallow acceptor). In GaN with the highest quality, the dominant defect-related PL band at high excitation intensity is the green luminescence band with a maximum at about 2.4 eV. We attribute this band to transitions of electrons from the conduction band to the 0/+ level of the isolated C_N defect. The yellow luminescence (YL) band, related to transitions via the −/0 level of the same defect, has a maximum at 2.1 eV. Another yellow luminescence band, which has similar shape but peaks at about 2.2 eV, is observed in less pure GaN samples and is attributed to the C_NO_N complex. In semi-insulating GaN, the GL2 band with a maximum at 2.35 eV (attributed to V_N) and the BL2 band with a maximum at 3.0 eV and the ZPL at 3.33 eV (attributed to a defect complex involving hydrogen) are observed. We also conclude that the gallium vacancy-related defects act as centers of nonradiative recombination.

In this study, the effects of chemical treatment on the properties of MOS capacitors and metal-oxide-semiconductor high electron mobility transistor (MOS-HEMT) were studied. The structure consist of Al₂O₃/u-GaN/AlN buffer/ Si substrate and Al₂O₃ (10 nm)/u-AlGaN (25 nm)/u-GaN (2μm)/AlN buffer/Si substrate for MOS capacitor and MOS-HEMT device, respectively. There are four chemical treatment recipes, which consist of organic solvents, oxygen plasma, BCl₃ plasma, dilute acidic solvent, hydrofluoric acid and RCA-like clean process to remove the metal ions, organic contamination and native oxide. Four different chemical treatment recipes treated the surface of u-GaN before Al₂O₃ was grown on the treated surface to reduce the interface state trap densities (D_it). The Dit value was calculated from measurement of C-V curve with 1M Hz frequency. The formation of interface state trap of u-GaN surface is modified by different chemical solution of varied chemical treatment recipe, which further influence the breakdown voltage (V_bk), on-resistance (R_on), threshold voltage (V_th) and drain current (I_d) of MOS-HEMT. The V_th of MOS-HEMT with organic solvents clean treatment is -11.00V. The MOS-HEMT after BCl₃ plasma and organic solvents clean treatment shows the lowest V_th of -9.55V. The electronic characteristics of MOS HEMT device with four different chemical treatment recipes were investigated in this article.

Simple, easy-to-use and physically meaningful analytical models have been developed for the assessment of the combined effect of the lattice and thermal mismatch on the induced stresses in an elongated bi-material assembly, as well as on the thermal mismatch on the thermal stresses in a tri-material assembly, in which the lattice mismatched stresses are eliminated in one way or another. This could be done, e.g., by using a polished or an etched substrate. The analysis is carried out in application to Gallium Nitride (GaN)-Silicon Carbide (SiC) and GaN-diamond (C) filmsubstrate assemblies. The calculated data are obtained, assuming that no annealing or other stress reduction means is applied. The data agree reasonably well with the reported (available) in-situ measurements. The most important conclusion from the computed data is that even if a reasonably good lattice match takes place (as, e.g., in the case of a GaN film fabricated on a SiC substrate, when the mismatch strain is only about 3%) and, in addition, the temperature change (from the fabrication/growth temperature to the operation temperature) is significant (as high as 1000 ̊C), the thermal stresses are still considerably lower than the lattice-mismatch stresses. Although there are structural and technological means for further reduction of the lattice-mismatch stresses (e.g., by high temperature annealing or by providing one or more buffering layers, or by using patterned or porous substrates), there is still a strong incentive to eliminate completely the lattice mismatch stresses. This seems to be indeed possible, if polished or otherwise flattened (e.g., chemically etched) substrates and sputter deposited GaN film is employed. In such a case only thermal stresses remain, but even these could be reduced, if necessary, by using compliant buffering layers, including layers of variable compliance, or by introducing variable compliance into the properly engineered substrate. In any event, it is expected that strong adhesion could be achieved by using an appropriate fabrication technology, so that no GaN film cracking would be possible, if the film is in tension, or delamination buckling could occur if the film is in compression. The developed models can be used to assess the possibilities and opportunities associated with GaN materials technology.

A photocathode electron source using p-type GaN and p-type InGaN semiconductors with a negative electron affinity (NEA) surface has been studied for its ability to maintain an extended NEA state. The key technology of NEA photocathodes is the formation of electric dipoles by atoms on the surface, which makes it possible for photo excited electrons in the conduction band minimum to escape into the vacuum. This means that in order to keep the electron energy spread as small as possible, the excitation photon energy should be tuned to the band gap energy. However, the NEA surface is damaged by the adsorption of residual gas and the back-bombardment of ionized residual gas by photoelectrons. The p-type GaN and InGaN semiconductors were measured a lifetime of quantum yield of excitation energy corresponding to the band gap energy in comparison to the p-type GaAs as the conventional NEA photocathode. Lifetime of NEA-photocathodes using the GaN and InGaN were 21 times and 7.7 times longer respectively than that using the GaAs.

The realization of reliable single photon emitters operating at high temperature and located at predetermined positions still presents a major challenge for the development of solid-state systems for quantum light applications. We demonstrate single-photon emission from two-dimensional ordered arrays of GaN nanowires containing InGaN nanodisks. The structures were fabricated by molecular beam epitaxy on (0001) GaN-on-sapphire templates patterned with nanohole masks prepared by colloidal lithography. Low-temperature cathodoluminescence measurements reveal the spatial distribution of light emitted from a single nanowire heterostructure. The emission originating from the topmost part of the InGaN regions covers the blue-to-green spectral range and shows intense and narrow quantum dot-like photoluminescence lines. These lines exhibit an average linear polarization ratio of 92%. Photon correlation measurements show photon antibunching with a g⁽²⁾(0) values well below the 0.5 threshold for single photon emission. The antibunching rate increases linearly with the optical excitation power, extrapolating to the exciton decay rate of ~1 ns^-1 at vanishing pump power. This value is comparable with the exciton lifetime measured by time-resolved photoluminescence. Fast and efficient single photon emitters with controlled spatial position and strong linear polarization are an important step towards high-speed on-chip quantum information management.

GaN/Al_0.5Ga_0.5N quantum dots deposited on the (11-22) plane have been grown by combining Molecular Beam Epitaxy (MBE) and Metal Organic Vapor Phase Epitaxy (MOVPE). The (11-22) GaN oriented template was realized by MOVPE starting from a M-plane oriented sapphire substrate. The average dot sizes are the following: between 15 and 20 nm in the <-1-123> and <1-100> directions and a height ranging between 0.8 and 1.4 nm. Their density is ranging between 2 and 8x10¹⁰cm^-2. The crystal field splitting is measured in Al_0.5Ga_0.5N via polarized microphotoluminescence. We study the photoluminescence properties of small quantum dots which present innovative optical properties among which are the evolution of the polarization of the emitted photons at different temperatures. We also analyze the distortion of the photoluminescence at different time delays after the excitation pulse. A redshift is found that is attributed to the complex thermally-induced delocalization of the carriers through the assembly of dots from the smaller ones to the bigger ones.

This paper reviews the status quo of several GaN-based power devices. By using unique polarized heterostructure of GaN/AlGaN materials, NSJ (Natural Super Junction) diode was proposed to break through the trade-off between the blocking voltage and the on-resistance. Normally-off GaN-based device was realized by GIT (Gate Injection Transistor) structure, where p-type AlGaN hole-injector are provided as a gate reducing the on-resistance by conductivity modulation. Experimentally fabricated inverter system using GaN GIT achieved the world-highest conversion efficiency over 99.3%. Further, by combining the microwave performance and power handling capability of of GaN-based devices, we developed Drive-by-Microwave (DbM) technology as a DC-isolated gate driver for GaN power switch. This is a fusion of microwave and power devices for the first time using GaN-based materials

Gallium nitride transistors are going to dominate the power semiconductor market in the coming years. The natural form of GaN-based devices is “normally-on” or depletion mode (d-mode). Despite these type of devices can be used in power semiconductor systems by means of special drivers or in a cascode package solution, yet the market demands for normally-off or enhancement mode (e-mode) devices. In this work, we directly compare and analyze the two most common approaches to obtain GaN-based e-mode devices: recessed gate MISHEMTs and p-GaN HEMTs. Both approaches have their pro’s and con’s as well as their critical process steps.

Proton irradiation from the backside of the samples were employed to enhance off-state drain breakdown voltage of AlGaN/GaN high electron mobility transistors (HEMTs) grown on Si substrates. Via holes were fabricated directly under the active area of the HEMTs by etching through the Si substrate for subsequent backside proton irradiation. By taking the advantage of the steep drop at the end of proton energy loss profile, the defects created by the proton irradiation from the backside of the sample could be precisely placed at specific locations inside the AlGaN/GaN HEMT structure. There were no degradation of drain current nor enhancement of off-state drain voltage breakdown voltage observed for the irradiated AlGaN/GaN HEMTs with the proton energy of 225 or 275 keV, for which the defects created by the proton irradiations were intentionally placed in the GaN buffer. HEMTs with defects placed in the 2 dimensional electron gas (2DEG) channel region and AlGaN barrier using 330 or 340 keV protons not only showed degradation of drain current, but also exhibited improvement of the off-state drain breakdown voltage. FLOODS TCAD finite-element simulations were performed to confirm the hypothesis of a virtual gate formed around the 2DEG region to reduce the peak electric field around the gate edges and increase the off-state drain breakdown voltage.

This paper reviews our recent results on the impact of iron and carbon compensation on the dynamic performance of GaN-HEMTs; based on pulsed and transient characterization, we demonstrate that: (i) the use of Fe-doping may lead to a significant current collapse, due to the presence of a trap with activation energy Ea=0.6eV. We discuss the properties of this trap and its physical origin; (ii) high C-doping levels may favor dynamic Ron increase, due to the presence of a trap level located at Ev+0.84 eV. The effect of this trap can be significantly reduced through the use of a double heterostructure.

In this paper we report recent developments on high power blue laser chips. Reduction of internal losses as well as optimized thermal management had been essential to increase optical output power. R and D samples with average performance of 3W optical output at junction temperatures of 130°C are demonstrated. The chips are suitable for use in a novel multi chip housing: For the first time up to 20 blue laser chips have been packaged into one compact housing resulting in the first InGaN laser device with optical output > 50W. The highly integrated package offers a unique small size. The outer dimensions of the package are 25.5mm x 35mm with an emitting surface of 16mm x 16.5mm. Therefore the complexity of optical alignment is dramatically reduced and only a single sheet multi lens array is required for beam collimation. Besides the unique technical performance the multi-die package offers significantly lower assembly costs because of the reduced complexity and assembly time. The butterfly package contains 4 bars with up to 5 multimode laser chips in series connection on each bar operating at 2.3A. The typical module wavelength is 450nm +/- 10nm. At a case temperature of 50°C the R and D samples achieve efficiencies of typ. 30% and an optical output power of 50W corresponding to an electrical power consumption of ~165W. This new technology can be used for high performance light engines of high brightness projectors.

The AlGaInN material system allows for laser diodes to be fabricated over a very wide range of wavelengths from u.v., ~380nm, to the visible ~530nm, by tuning the indium content of the laser GaInN quantum well. Thus AlGaInN laser diode technology is a key enabler for the development of new disruptive system level applications in displays, telecom, defence and other industries.

Ultraviolet (UV) lasers are of paramount importance for applications in water purification, diagnosis and bio-agent detection. Here we report that, with the use of dislocation-free AlGaN nanowires formed directly on Si substrate, electrically injected UV emission in the wavelength range from 319 nm to 335 nm can be readily achieved, which is the shortest wavelength range ever reported for electrically injected semiconductor lasers. In this work, catalyst-free AlGaN nanowire arrays are grown directly on Si substrate by radio frequency plasma-assisted molecular beam epitaxy (MBE). Our detailed calculation shows that such vertically aligned randomly distributed sub-wavelength scale nanowire array can sustain random lasing action. Various lasing peaks from 319 nm to 335 nm can be measured from such AlGaN nanowire samples under electrical injection. The threshold is measured to be in the range of tens of A/cm² at cryogenic temperature, which is significantly smaller than the commonly reported GaN-based quantum well lasers. The measured linewidth is as narrow as 0.2 nm.

Vertical-cavity surface-emitting lasers (VCSELs) offer distinct advantages over conventional edge-emitting lasers, including lower power consumption, single-longitudinal-mode operation, circularly symmetric output beams, waferlevel testing, and the ability to form densely packed, two-dimensional arrays. High-performance GaN-based VCSELs are well suited for applications in high-density optical data storage, high-resolution printing, lighting, displays, projectors, miniature atomic clocks, and chemical/biological sensing. Thus far, the performance of these devices has been limited by challenges associated with the formation of high-reflectance distributed Bragg reflectors (DBRs), optical mode confinement, carrier transport, lateral current spreading, polarization-related electric fields, and cavity-length control. This manuscript discusses the state-of-the-art results for electrically injected GaN-based VCSELs and reviews approaches to overcome the key challenges currently preventing higher performance devices. The manuscript also describes the development of nonpolar GaN-based VCSELs on free-standing GaN. Nonpolar orientations exhibit anisotropic optical gain within the quantum well plane and uniquely enable VCSELs with a well-defined and stable polarization state. In addition, a detailed description of a band-gap-selective photoelectrochemical etching (BGS PECE) process for substrate removal and fine cavity length control on free-standing GaN substrates is provided.

UV light emitters in the UV-B spectral range between 280 nm and 320 nm are of great interest for applications such as phototherapy, gas sensing, plant growth lighting, and UV curing. In this paper we present high power UV-B LEDs grown by MOVPE on sapphire substrates. By optimizing the heterostructure design, growth parameters and processing technologies, significant progress was achieved with respect to internal efficiency, injection efficiency and light extraction. LED chips emitting at 310 nm with maximum output powers of up to 18 mW have been realized. Lifetime measurements show approximately 20% decrease in emission power after 1,000 operating hours at 100 mA and 5 mW output power and less than 30% after 3,500 hours of operation, thus indicating an L50 lifetime beyond 10,000 hours.

High output power deep-ultraviolet light-emitting diodes (LEDs) are reported. First, two series devices were operated at a forward current of 350 mA. We measured powers of 45.2, 93.3, and 65.8 mW for 255, 280, and 310-nm LEDs, respectively. Next, single-chip devices with a modified epitaxial structure and electrode pattern were evaluated. Peak wavelengths, output powers, forward voltages, and external quantum efficiencies were respectively 258.0 nm, 34.8 mW, 8.25 V, 2.07% for the 255-nm LED, 282.8 nm, 40.5 mW, 5.69 V, 2.64% for the 280-nm LED, and 312.3 nm, 40.1 mW, 6.36 V, 2.89% for the 310-nm LED. The 70% lifetime of the 280-nm LED was estimated to be over 3000 h at a junction temperature of 45°C.

We discuss factors affecting the external quantum efficiency, droop and reliability of AlGaN deep ultraviolet (DUV) light emitting diodes (LED) grown on sapphire substrates. Improvement of LED performance is achieved by suppression of the nonradiative recombination in epitaxial structures with dislocation density reduced to below 5x10⁸ cm^-2, transparent LED structure design and optimized UV encapsulation for enhanced light extraction. Relatively low light extraction efficiency remains to be a key factor limiting LED output power and quantum efficiency.

We present UV-C LEDs emitting around 235 nm grown by MOVPE on ELO AlN/sapphire substrates. In order to account for the low conductivity of high Al content AlGaN layers and the associated high contact resistances, we designed an optimized compact LED geometry based on electro-thermal simulations of the current spreading. Experimental data (layer and contact resistances) are collected on test structures and used as input parameters for 3-D current spreading simulations. With resistances of the layers (n and p) approaching 0.1 Ωcm, the use of a segmented p-area with broad n-contact fingers (10 μm or more) that are close to the mesa edge (5 μm) help to maximize the emission power in the center of the structure. Based on this knowledge a series of compact LEDs of size 500 μm x 500 μm is designed and simulated. We get confirmation that the segmentation of the p-area is the most critical parameter to limit the non-uniformity introduced by the high n-sheet resistances. Up to 17% in emission power can be gained when the n-contacts are designed properly. LEDs with the optimum geometry were processed and measured. We get a good confirmation of our model concerning the distribution of the emission power. Both simulations and measurements show current crowding at the edge of the n-contact, however the power loss in the middle of the chip is higher than predicted.

We invesstagated the relationship between the emission efficiency of InGaN/GaN multiple quantum wells (MQWs) and the V-shape pits (V-pits) forming along the threading dislocation (TD). The thinner InGaN/GaN MQWs on the side walls around V-pits would create higher local energy barriers, which can resist the carriers trapped into the non-radiative recombination centres within TDs. By inserting different InGaN/GaN superlattice (SLS) layers below the MQWs, sizes of V-pits could be properly controlled. It was found that the V-pit size on InGaN MQWs increased with increasing SLS layers, which could decrease energy barriers. On the contrary, the shorter distance between the TD center and V-pit boundary would increase the carrier capturing capability of TDs in smaller V-pits. By properly controlling the V-shape defect formation, the best internal quantum efficiency of about 70%f was found in the MQWs with underlying 15 periods SLS layers.

The efficiency droop in nitride LEDs is currently attributed to either carrier-density-dependent nonradiative recombination or to carrier leakage, both being discussed in terms of a single-particle picture. Our time-resolved photoluminescence results show that the radiative lifetime is independent of carrier density, while the nonradiative lifetime scales with the inverse of the carrier density. This can not be understood in a single-particle model. By means of a many-particle theory approach we obtain a consistent picture with both radiative and Auger recombination enhanced by excitonic electron-hole correlation. In the high carrier density limit single-particle radiative and Auger recombination are recovered.

Recent measurements have generated a need to better understand the physics of hot carriers in III-Nitride (III-N) lightemitting diodes (LEDs) and in particular their relation to the efficiency droop and current transport. In this article we present fully self-consistent bipolar Monte Carlo (MC) simulations of carrier transport for detailed modeling of charge transport in III-N LEDs. The simulations are performed for a prototype LED structure to study the effects of hot holes and to compare predictions given by the bipolar MC model, the previously introduced hybrid Monte Carlo–drift-diffusion (MCDD) model, and the conventional drift-diffusion (DD) model. The predictions given by the bipolar MC model and the MCDD model are observed to be almost equivalent for the studied LED. Therefore our simulations suggest that hot holes do not significantly contribute to the basic operation of multi-quantum well LEDs, at least within the presently simulated range of material parameters. With the added hole transport simulation capabilities and fully self-constistent simulations, the bipolar Monte Carlo model provides a state-of-the-art tool to study the fine details of electron and hole dynamics in realistic LED structures. Further analysis of the results for a variety of LED structures will therefore be very useful in studying and optimizing the efficiency and current transport in next-generation LEDs.

We describe the optical properties of semi/non-polar InGaN and AlGaN quantum wells. In semipolar (11¯22) InGaN QWs, spatially uniform but spectrally broad emissions are observed. This finding is interpreted with consideration of the exciton migration length shortened by the fast radiative recombination lifetime due to the reduced electric field. Non/semipolar AlGaN QWs are also fabricated. In the semipolar (1¯102) AlGaN QWs, the radiative recombination lifetimes faster than that in the (0001) QW are confirmed experimentally. As a consequence, much stronger emission is achieved from the semipolar AlGaN QWs at room temperature

Spatial variations of band potentials and properties of carrier recombination were examined in semipolar (2021) plane InGaN/GaN single quantum wells by scanning near-field photoluminescence (PL) spectroscopy. The quantum wells had In content from 0.11 to 0.36 and were emitting from violet to yellow-green. Near-field scans showed small PL peak energy and linewidth variations with standard deviations below 10 meV, which confirms small alloy composition variations in the quantum wells. The scans revealed large, ~5 to 10 μm size areas of similar PL parameter values, as opposed to 100 nm scale variations, often reported for InGaN wells. With increased excitation power, an untypical photoluminescence peak energy shift to lower energies was observed. The shift was attributed to density dependent carrier redistribution between nm-scale sites of different potentials. The experimental results show that in the (2021) plane InGaN quantum wells the localization potentials are shallow and the recombination properties are spatially rather uniform, which confirms the high potential of these QWs for photonic applications.

In this work we used vertically aligned GaN nanowires with well-defined crystal facets, i.e. the {11-20} a-plane, {10-10} m-plane, (0001) c-plane and {1-101} semi-polar planes, to investigate the impact of MOVPE reactor parameters on the characteristics of an InGaN layer. The morphology and optical characteristics of the InGaN layers grown of each facet were investigated by cathodoluminescence (CL) hyperspectral imaging and scanning electron microscopy (SEM). The influence of reactor parameters on growth rate and alloy fraction were determined and compared. The study revealed that pressure can have an important impact on the incorporation of InN on the {10-10} m-plane facets. The growth performed at 750°C and 100mbar led to a homogeneous high InN fraction of 25% on the {10-10} facets of the nanowires. This work suggests homogeneous good quality GaN/InGaN core-shell structure could be grown in the near future.

We fabricated InGaN LEDs prepared on β-Ga₂O₃ (201) single-crystal substrates. The substrates were produced by using the edge-defined film-fed growth (EFG) method. A Si-doped GaN epitaxial layer was grown on an electrically conductive β-Ga₂O₃ (201) substrate by metal organic chemical vapor deposition (MOCVD). The full-width at half maximum (FWHM) of (0002) and (101 1) X-ray rocking curves (XRCs) of the Si-doped GaN layer were 220 arcsec and 223 arcsec, respectively. The dark spot density measured by cathode luminescence (CL) was approximately 1.5×10⁸ cm^-2. The crystalline quality was equal to that of GaN layer on sapphire. We fabricated a vertical LED in the p-side down configuration. The peak wavelength was approximately 450 nm. The p-contact metal area was 300 ×300 μm². The light output power did not saturate at 1000 A/cm². This device characteristic indicates the great potential of Ga₂O₃ for use in high-power LEDs.

III-N nanowires (NWs) are an attractive alternative to conventional planar layers as the basis for light-emitting diodes (LEDs). In fact, the NW geometry enables the growth of (In,Ga)N/GaN heterostructures with high indium content and without extended defects regardless of the substrate. Despite these conceptual advantages, the NW-LEDs so far reported often exhibit higher leakage currents and higher turn-on voltages than the planar LEDs.

In this work, we investigate the mechanisms responsible for the unusually high leakage currents in (In,Ga)N/GaN LEDs based on self-induced NW ensembles grown by molecular beam epitaxy on Si substrates. The temperature-dependent current-voltage (I-V) characteristics, acquired between 83 and 403 K, reveal that temperatures higher than 240 K may activate a further conduction process, which is not present in the low temperature range. Deep level transient spectroscopy (DLTS) measurements show the presence of electron traps, which are activated in the same temperature interval. A detailed analysis of the DLTS signal reveals the presence of two distinct deep levels with apparent activation energies close to E_c-570 meV and E_c-840 meV, and capture cross sections of about 1.0x10^-15 cm² and 2x10^-14 cm², respectively. These results suggest that the leakage process might be related to trap-assisted tunneling, possibly produced by point defects located in the core and/or on the sidewalls of the NWs.

Strong polarisation-induced electric fields in C-plane oriented nitrides semiconductor layers reduce the performance of devices. Eliminating the polarization fields can be achieved by growing nitrides along non polar direction. We have grown non polar A-plane oriented InN on R-plane (1‾102) nitridated sapphire substrate by MOCVD. We have studied the structural anisotropy observed in these layers by analyzing High Resolution XRay Diffraction rocking curve (RC) experiments as a function of the in-plane beam orientation. A-plane InN epilayer have a unique epitaxial relationship on R-Plane sapphire and show a strong structural anisotropy. Full width at half maximum (FWHM) of the InN(11‾20) XRD RC values are contained between 44 and 81 Arcmin. FWHM is smaller when the diffraction occurs along the [0001] and the largest FWHM values, of the (11‾20) RC, are obtained when the diffraction occurs along the [1‾100] in-plane direction. Atomic Force Microscopy imaging revealed morphologies with well organized crystallites. The grains are structured along a unique crystallographic orientation of InN, leading to larger domains in this direction. This structural anisotropy can be, in first approximation, attributed to the difference in the domain sizes observed. XRD reciprocal space mappings (RSM) were performed in asymmetrical configuration on (13‾40) and (2‾202) diffraction plane. RSM are measured with a beam orientation corresponding to a maximal and a minimal width of the (11‾20) Rocking curves, respectively. A simple theoretical model is exposed to interpret the RSM. We concluded that the dominant contribution to the anisotropy is due to the scattering coherence length anisotropy present in our samples.

Due to their potential to improve the performance of light-emitting diodes (LEDs), novel device structures based on nanowires, surface plasmons, and large-area high-power devices have received increasing amount of interest. These structures are almost exclusively based on the double hetero junction (DHJ) structure, that has remained essentially unchanged for decades. In this work we study a III-nitride diffusion injected light-emitting diode (DILED), in which the active region is located outside the pn-junction and the excitation of the active region is based on bipolar diffusion of charge carriers. This unorthodox approach removes the need of placing the active region in the conventional current path and thus enabling carrier injection in device structures, which would be challenging to realize with the conventional DHJ design. The structure studied in this work is has 3 indium gallium nitride / gallium nitride (InGaN/GaN) quantum wells (QWs) under a GaN pn-junction. The QWs are grown at diferent growth temperatures for obtaining distinctive luminescence peaks. This allows to obtain knowledge on the carrier diffusion in the structure. When the device is biased, all QWs emit light indicating a significant diffusion current into the QW stack.

For 365-nm ultraviolet light-emitting diodes (UV LEDs), an electron blocking layer (EBL) is usually utilized to mitigate electron overflow. However, using EBL might obstruct holes from injecting into the active region. Moreover, the large polarization field in conventional EBL might also pull down the effective barrier height for electrons, and thus the electrons could easily overflow to the p-side region. To solve the above drawbacks, in this study, the Al content and p-doping concentration of the EBL in typical 365-nm UV LEDs are investigated systematically. Specifically, designs of AlGaN/GaN superlattice EBL and Al-content-graded EBL are explored in detail.

In InGaN/GaN blue light-emitting diodes (LEDs) widely utilized for general lighting, there exist various material issues that lead to the unwanted nonradiative recombination. In this paper, we utilize various characterization techniques to investigate the nonradiative recombination mechanisms in LED devices. With the characterization techniques such as temperature-dependent external quantum efficiency, current-voltage, and electroluminescence spectra, we show that different nonradiative recombination processes such as the Shockley-Read-Hall recombination and the defect-assisted tunneling can play roles in the LED devices. Information on the dominant nonradiative recombination obtained by these analyses can be used for further improving the quantum efficiency of the device.

Enhancement of optical and structural quality of semipolar (11‾22) GaN grown by metal-organic chemical vapor deposition on planar m-sapphire substrates was achieved by using an in-situ epitaxial lateral overgrowth (ELO) technique with nanoporous SiNx layers employed as masks. In order to optimize the procedure, the effect of SiNx deposition time was studied by steady-state photoluminescence (PL), and X-ray diffraction. The intensity of room temperature PL for the (11‾22) GaN layers grown under optimized conditions was about three times higher compared to those for the reference samples having the same thickness but no SiNx interlayers. This finding is attributed to the blockage of extended defect propagation toward the surface by the SiNx interlayers as evidenced from the suppression of emissions associated with basal-plane and prismatic stacking faults with regard to the intensity of donor bound excitons (D⁰X) in lowtemperature PL spectra. In agreement with the optical data, full width at half maximum values of (11‾22) X-ray rocking curves measured for two different in-plane rotational orientations of [1‾100] and [11‾23] reduced from 0.33º and 0.26º for the reference samples to 0.2º and 0.16º for the nano-ELO structures grown under optimized conditions, respectively.

Enhancement of coherent zone folded longitudinal acoustic phonon (ZFLAP) oscillations at terahertz frequencies was demonstrated in InGaN multiple quantum wells (MQWs) by using wavelength degenerate time resolved differential transmission spectroscopy. Screening of the piezoelectric field in InGaN MQWs by photogenerated carriers upon femtosecond pulse excitation gave rise to terahertz ZFLAPs, which were monitored at the Brillouin zone center in the transmission geometry. MQWs composed of 10 pairs In_xGa_1-xN wells and In_0.03Ga_0.97N barriers provided coherent phonon frequencies of 0.69-0.80 THz depending on the period of MQWs. Dependences of ZFLAP amplitude on excitation density and wavelength were also investigated. Possibility of achieving phonon cavity, incorporating a MQW placed between two AlN/GaN phonon mirrors designed to exhibit large acoustic gaps at the zone center, was also explored.

The effects of stacking faults (SFs) on optical processes in epitaxially grown semipolar (1122) GaN on m-sapphire substrate have been investigated in detail using steady-state photoluminescence (PL) and time- and polarization-resolved PL. We demonstrate that the carrier recombination dynamics are substantially influenced due to strong carrier localization in the stacking faults. In addition to nonradiative recombination, carrier trapping/detrapping and carrier transfer between the stacking faults and donors are also found to be among the mechanisms affecting the recombination dynamics at different temperatures. PL decay times of both I₁-type BSF and 3.31 eV SF (E-type BSF or prismatic stacking fault) do not show temperature dependence up to 80 K while 3.31 eV SF exhibits longer PL decay times (~3 ns) at low temperatures as compared to I₁-type BSF (~1 ns), indicative of lower efficiency for radiative recombination. After 80 K, PL decay times decreased by power of ~-1 and ~-2 for 3.31 eV SF and I₁-type BSF, respectively. It is obtained from radiative decay times with respect to temperature that the carrier localization becomes higher in I₁-type BSF compared to 3.31 eV SF increasing the temperature. I₁-type BSF also shows higher PL intensity, which is attributed to larger density, and therefore, larger contribution to recombination dynamics as compared to other type of stacking faults. Polarization-resolved PL measurements also revealed that the degree of polarization for the I₁-type BSF (0.30) was twice that for the 3.31 eV SF.

A 5λ-thick hybrid semiconductor/dielectric GaN-based microcavity grown by metal-organic chemical vapor deposition on a c-plane bulk GaN substrate was investigated using angle-resolved photoluminescence and angle-resolved cathodoluminescence techniques at room and low temperature (5.8 K), respectively. The cavity structure consisted of an InGaN multiple quantum well active region emitting at 400 nm and sandwiched between 29.5 pair bottom semiconductor AlN/GaN and 13.5 pair top dielectric SiO₂/SiN_x distributed Bragg reflectors. The cavity supported strong exciton-photon coupling with a record 75 meV vacuum Rabi splitting energy at 5.8 K. The measured room temperature Rabi splitting energy of 45 meV is still close to the highest Rabi splitting energies reported in literature confirming that the strong coupling regime still persists at room temperature.

Reduced electric field in semipolar (1122) GaN/InGaN heterostructures makes this orientation attractive for high efficiency light emitting diodes. In this work, we investigated indium incorporation in semipolar (1122) GaN grown by metal-organic chemical vapor deposition on planar m-plane sapphire substrates. Indium content in the semipolar material was compared with that in polar c-plane samples grown under the same conditions simultaneously side by side on the same holder. The investigated samples incorporated dual GaN/InGaN/GaN double heterostructures with 3nm wide wells. In order to improve optical quality, both polar and semipolar templates were grown using an in-situ epitaxial lateral overgrowth (ELO) technique. Indium incorporation efficiency was derived from the comparison of PL spectra measured on the semipolar and polar structures at the highest excitation density, which allowed us to minimize the effect of quantum confined Stark effect on the emission wavelength. Our data suggests increased indium content in the semipolar material by up to 3.0%, from 15% In in c- GaN to 18% In in (1122) GaN.

Temperature dependent recombination dynamics in c-plane InGaN light emitting diodes (LEDs) with different well thicknesses, 1.5, 2, and 3 nm, were investigated to determine the active region dimensionality and its effect on the internal quantum efficiencies. It was confirmed for all LEDs that the photoluminescence (PL) transients are governed by radiative recombination at low temperatures while nonradiative recombination dominates at room temperature. At photoexcited carrier densities of 3 – 4.5 x 10¹⁶ cm^-3 , the room-temperature Shockley-Read-Hall (A) and the bimolecular (B) recombination coefficients (A, B) were deduced to be (9.2x10⁷ s^-1, 8.8x10^-10 cm³s^-1), (8.5x10⁷ s^-1, 6.6x10^-10 cm³s^-1), and (6.5x10⁷ s^-1, 1.4x10^-10 cm³s^-1) for the six period 1.5, 2, and 3 nm well-width LEDs, respectively. From the temperature dependence of the radiative lifetimes, τ_rad α T^n/2, the dimensionality n of the active region was found to decrease consistently with decreasing well width. The 3 nm wide wells exhibited ~T^1.5 dependence, suggesting a three-dimensional nature, whereas the 1.5 nm wells were confirmed to be two-dimensional (~T¹) and the 2 nm wells close to being two-dimensional. We demonstrate that a combination of temperature dependent PL and time-resolved PL techniques can be used to evaluate the dimensionality as well as the quantum efficiencies of the LED active regions for a better understanding of the relationship between active-region design and the efficiency limiting processes in InGaN LEDs.