This PDF file contains the front matter associated with SPIE Proceedings Volume 7939, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

In this paper, we reported the recent advances in the growth of GaN crystals on GaN templates and spontaneously nucleated GaN seeds by Na flux method. In the growth on GaN templates, it was clarified that the growth mode could be controlled by changing the flux composition. Based on the changes in the growth mode under different flux compositions, a growth sequence that is effective for the growth of thick GaN substrates with a low dislocation density was proposed. In the growth on pyramidal GaN seeds, we investigated the dependence of the growth rate, crystallinity and the growth habit on the flux composition. Results showed that a low Ga composition was preferred to grow high-crystallinity prismatic GaN crystals with a high growth rate. When a spontaneously nucleated GaN seed was used, a bulk GaN crystal with a hexagonal pillar consisting of six m-facets, and its length and diameter were 10 mm and 8 mm, respectively, was obtained. Furthermore, we found that the addition of Ca and Li to Ga-Na melt improved transparency of GaN crystals grown on pyramidal GaN seeds.

GaN nanowires (NWs) were grown selectively on Si(111) substrate without catalyst by plasma-assisted molecular-beam epitaxy under N-rich conditions. The selective growth was obtained using regular arrays of holes patterned in a silicon oxide layer on top of a thin AlN buffer. The optical properties of the selectively grown GaN NWs have been studied using cathodoluminescence (CL) spectroscopy. Both, CL spectra measured on NW ensembles and spatially resolved monochromatic images were investigated. From a comparison of morphology and CL studies it emerges that NW coalescence is responsible for the appearance of the defect related emission.

A new growth method, named droplet elimination by radical-beam irradiation (DERI), consisting of a metal-rich growth process (MRGP) and a droplet elimination process (DEP), was proposed for the growth of high-quality InN. The DERI method was also developed to the growth of InGaN ternally alloys. A periodic InN/InGaN structure was successfully fabricated using the phenomena that Ga was preferentially captured in a growing InGaN layer and In was forced outward to the surface in the MRGP and that the swept In was transformed to epitaxial InN on the InGaN underlayer in DEP. The DERI method enables the simple and reproducible growth of both high-quality InN film and peridic InN/InGaN structure by using in situ monitoring techniques.

Metalorganic chemical vapor deposition (MOCVD) growths of linearly-shaped staggered InGaN quantum wells lightemitting diodes are performed. The use of linearly-shaped staggered InGaN QWs leads to the shift of both electron and hole wavefunction toward the center of the quantum well region with enhanced momentum matrix element, which results in the enhancement of the spontaneous radiaitve recombination rate. The power-density-dependent cathodoluminescence measurements for both conventional and linearly-shaped staggered InGaN QW show 2.5-3.5 times increase in the integrated cathodoluminescence intensity by using the novel active region.

Photoluminescence (PL) properties are reported for a set of m-plane GaN films with Mg doping varied from mid 10¹⁸cm^-3 to well above 10¹⁹ cm^-3. The samples were grown with MOCVD at reduced pressure on low defect density m-plane bulk GaN templates. The sharp line near bandgap bound exciton (BE) spectra observed below 50 K, as well as the broader donor-acceptor pair (DAP) PL bands at 2.9 eV to 3.3 eV give evidence of several Mg related acceptors, similar to the case of c-plane GaN. The dependence of the low temperature BE spectra on excitation intensity as well as the transient decay behavior demonstrate acoustic phonon assisted transfer between the acceptor BE states. The lower energy donor-acceptor pair spectra suggest the presence of deep acceptors, in addition to the two main shallower ones at about 0.23 eV. Similar spectra from Mg-doped GaN nanowires (NWs) grown by MOCVD are also demonstrated and briefly discussed.

We have studied the surface photovoltage (SPV) for band-to-band illumination on a variety of p-type (Mg-doped) GaN samples. In particular, differences in the steady-state and transient SPV have been investigated in air and vacuum for samples grown by hydride vapor phase epitaxy (HVPE) or metal-organic chemical vapor deposition (MOCVD). The SPV spectra for both samples behave in a similar manner, but larger SPVs are generated for HVPE vs. MOCVD samples under identical illumination conditions. Interestingly, we have found that p-type GaN can be sensitive to the illumination geometry, where illumination of the electrical contacts results in an anomalous "offset" of the SPV signal. Regardless of illumination geometry, such offsets always appear in the case of MOCVD samples, whereas they do not appear for HVPE samples when the contacts are not illuminated. Since we have never observed such behavior for n-type samples, it appears that the stability of p-type samples under illumination may be an issue.

Using high resolution X-ray diffraction and Hall effect measurements, we found that the tensile strain caused by dislocation inclination in Si doped GaN became immeasurable when carbon co-doping was used to compensate the free carriers. This result suggested that the tensile strain is related to free carrier concentration instead of Si concentration. Such an effect could be explained by the Fermi level effect on the dislocation climb at the growth surface mediated by Ga vacancies, whose concentration is strongly influenced by the Fermi level position. At a high electron carrier concentration, the formation energy of Ga vacancies is low and Ga vacancy concentration is high. Therefore, the dislocation climb-rate is enhanced, which results in a higher tensile strain. This phenomenon is similar to the wellknown Fermi level effect on Ga vacancy governed diffusion in the GaAs system.

GaN based heterojunction devices, especially in the form of AlGaN/GaN high electron mobility transistors (HEMTs), are capable of delivering high-frequency power amplifiers and power switches with performances far superior than those offered by the mainstream silicon technology and other advanced semiconductor technologies. Rapid progress has been made in material growth and device processing technologies during the last decade. Meanwhile, there remain technical challenges to overcome. For example, unlike the silicon MOSFET technology, in which the devices' threshold voltage can be locally adjusted in the processing stage by ion implantation, the threshold voltage of conventional Cplane Ga-face AlGaN/GaN high electron mobility transistors (HEMTs) is mainly determined in the material growth stage, and exhibit large negative values due to the strong spontaneous and piezoelectric polarization effect, even without any intentional doping. A robust approach to modulating the local potential and local carrier concentration in AlGaN/GaN HEMTs was developed based on the fluorine plasma ion implantation technology. The most significant development based on this technology is the demonstration of self-aligned enhancement-mode (or normally-off) AlGaN/GaN HEMTs with low on-resistance. The stability of fluorine ions in III-nitride is of great importance to applications since it will ultimately determine the E-mode HEMTs' reliability. This paper attempts to address the stability of fluorine ions in III-nitride materials and devices based on a series of experimental and theoretical studies.

Bulk GaN substrates promise to bring the full potential of nitride-based devices to bear since they offer a low thermal and lattice mismatched alternative to foreign substrates for epitaxial growth. However, due to the high cost and low availability of bulk GaN substrates, effects such as surface misorientation (offcut), surface polishing, and preparation of such substrates on subsequent epitaxy are still not well understood. As such, AlGaN/GaN heterostructures with nominal Al compositions of 25% were grown by MOCVD on semi-insulating bulk GaN substrates with offcuts ranging from 0.05 to 1.95° in the m-direction (10 10) to attempt to determine the optimal offcut for bulk GaN substrates for AlGaN-based HFET devices. X-ray diffraction (XRD) studies indicate that the Al composition does not vary with offcut, however reciprocal space mapping shows evidence of strain relaxation of the AlGaN in samples grown on substrates with offcut >1.1°. Additionally, we observed a minimum in sheet resistance of the 2DEGs for substrates with offcuts near 0.5°, arising from higher mobilities in these samples. Evidence of an optimal substrate misorientation is important for AlGaN-based devices grown on bulk GaN substrates.

We have explored the practical applicability of our recently found stability criterion for HFETs with spontaneous polarization of the gate insulation, to determine in which conditions the criterion is satisfied. In essence, the criterion requires a faster decrease of the spontaneous gate polarization than of the resistivity when temperature increases locally. The instability will be stronger in certain temperature intervals, and will result in degradation and 1/f noise increase. The implications for engineering design for increased device reliability at large positive gate bias and high power dissipation are analyzed. Our criterion and optimization principle also includes the effect of the plasmon-optical phonon resonance effects in the channel, which could even reverse the channel resistance decrease when temperature is raised in the stationary regime.

Positron annihilation is a non-destructive technique for investigating vacancy-type defects in condensed matter. When a positron is implanted into a sample, it annihilates with an electron and emits two 511-keV γ quanta. From measurements of Doppler broadening spectra of the annihilation radiation and positron lifetimes, one can detect point defects such as monovacancies, vacancy clusters, and vacancy-impurity complexes. The regions sampled can range from the surface to a depth on the order of microns. In the present study, we have used the positron annihilation technique to study relationship between the impurity doping and vacancies in GaN. Defects in ion-implanted GaN and their annealing properties were studied. The defects introduced by the implantation were identified as divacancies, and the defect reaction during isochronal annealing were found to depend on ion spices. A relationship between intra-4f transitions of Er and vacancies was studied. A correlation between the defect concentration and the PL intensity was observed. We will demonstrate that the positron annihilation technique is sensitive to vacancy-type defects in GaN, and it can contribute to the development of optical and electronic devices based such materials.

Large bowings of the band gap and its pressure coefficient in In-containing nitride semiconductor alloys are observed. Photoluminescence measurements for In_xGa_1-xN and In_xAl_1-xN combined with other experimental data show large scatter of the results. A comparison with ab-initio calculations suggests that this scatter can be ascribed to the formation of In clusters during the sample preparation. The explanation of the observed anomalies taking into account chemical and size effects indicates a specific nature of InN, different from other nitrides and other In-based binary semiconductors.

Biexciton binding in Al_xGa_1-xN ternary alloys as a function of alloy composition is reviewed on the basis of our recent experimental observations. The biexciton binding energy in GaN and AlN was evaluated to be 5.6 and 19.3 meV, respectively. The biexciton binding energy in Ga-rich Al_xGa_1-xN ternary alloys (x=0.019~0.092) and Al-rich Al_xGa_1-xN ternary alloys (x=0.81 and 0.89) was also evaluated on the basis of two-photon absorption of biexcitons. The biexciton binding energy in Ga-rich ternary alloys increased linearly with aluminum composition and reached to 16.6 meV for x=0.092. This value was three times as large as the biexciton binding energy in GaN. Similarly, the biexciton binding energy in Al-rich ternary alloys increased with decreasing aluminum composition and reached to 56 meV for x=0.81. A strong enhancement of the biexciton binding was observed for both Ga-rich and Al-rich ternary alloys. The enhancement was attributed to the effect of localization due to alloy disorder. The results indicated that a linear interpolation between GaN and AlN did not apply to the biexciton binding energy in the ternary alloys. A large bowing existed in the biexciton binding energy in the ternary alloys.

We investigate the effect of carrier distribution characteristics in InGaN multiple-quantum-well (MQW) structures on the efficiency droop of light-emitting diodes (LEDs). Here, three kinds of inhomogeneous carrier distributions are studied; inhomogeneous carrier distribution in the vertical direction between QWs, that in the horizontal direction of a QW plane due to the current crowding in the LED chip, and that inside QW materials by carrier localization in the Inrich areas. It is found, by numerical simulation, that the built-in polarization field in InGaN MQWs makes the hole distribution between QWs more inhomogeneous, which enhances the efficiency droop. In addition, nonuniform current spreading is also found to have a significant influence on the efficiency droop by the inhomogenous carrier distribution in the plane of a QW. When the carrier distribution characteristic is investigated in a microscopic scale, the localization of carriers in the In-rich areas is expected to reduce the effective active volume where carriers are able to recombine, and enhances the efficiency droop due to the large increase in the carrier density at inhomogeneously distributed In-rich regions.

Thermoelectric properties of AlInN alloys, grown by metalorganic vapor phase epitaxy (MOVPE), with In-contents (x) from 11 % up to 21.34% were characterized and analyzed at room temperature. The thermoelectric figure of merit (Z*T) values of the n-Al_1-xIn_xN alloys were measured as high as 0.391 up to 0.532 at T = 300 K. The use of high In-content (x = 21.34%) AlInN alloys leads to significant reduction in thermal conductivity [κ = 1.62 W/(mK)] due to the increased alloy scattering, however, the optimized thermoelectric material was obtained for AlInN alloy with In-content of 17% attributed to its large power factor.

Although it is known that GaN tend to decompose in hydrogen environments, there has been few investigations in hydrogen etch of GaN. This study performs a systematic research on hydrogen etch of GaN under various pressures. It is observed that hydrogen atoms initially etch into GaN to form pinholes. Dislocations are usually the preferred places for initial hydrogen etch, but not all etched holes result from dislocations. When etched at low pressure and high temperature, deep vertical holes extending several microns can be formed by the hydrogen etch. However, when etch is performed at high pressure, apparent lateral etch are observed under the initial holes, leading to bollard-like GaN posts. From this systematic study, a model has been proposed to explain the vertical and the lateral etching mechanisms. With the established model, a sequential etch of GaN in hydrogen under varying pressure has been designed to successfully maintain a smooth GaN front surface, but to etch the underlying GaN to form a porous cave structure. Thick GaN films are then overgrown on such GaN layers with the hydride vapor phase epitaxy technology. It is demonstrated that the overgrown GaN thick films can self-separate from the underlying Al₂O₃ substrates.

This paper reports on two plasmonic systems based on III-Nitrides: (i) InGaN - Au nanoparticles; (ii) InN with spontaneously formed buried In nanoparticles. By using NSOM and μ-PL studies we established necessary conditions for the efficient interaction of localized plasmons excited in a gold nanoparticle with localized excitons situated near the InGaN surface. The tens-fold PL intensity enhancement was observed for ~100 nm Au nanoparticles. The narrow lines of single excitons have been registered at low temperature. Due to specific thermodynamic properties of InN, its optical properties are strongly influenced by plasmons in spontaneously formed In nanoparticles. The μ-CL study of MBE grown InN/In structures with intentional periodic 0-48 ML In insertions has shown that the bright CL spots (peaked spectrally at ~0.7eV) coincide always with the agglomerates of In nanoparticles. The intensity increase by the factor of ~10² is in good agreement with calculations of the average enhancement factor in InN/In nanocomposites comprising the nanoparticles of an arbitrary shape and orientation. Time-resolved PL studies have demonstrated pronounced Purcell effect, i.e. significant shortening of spontaneous recombination rate, in both systems. Terahertz emission (~3 THz) observed in InN epilayers under electrical pumping is ascribed to surface plasmon polariton waves coupled to electromagnetic field at quasi-periodical structural imperfections.

We present a review of nitride laser diodes lasing at short ultraviolet wavelengths. The room temperature operations of the AlGaN based laser diodes under pulsed current mode are presented. The optical and temperature characteristics as well as the carrier recombination of the devices are also investigated.

With this paper we describe recent results on the physical mechanisms responsible for the gradual degradation of GaN-based laser diodes and Light-Emitting Diodes (LEDs). The results described in the following were obtained by means of an extensive electrical and optical characterization of laser diodes and LEDs submitted to accelerated stress conditions. The experimental evidence described within this paper demonstrate that: (i) during stress, the threshold current of laser diodes can significantly increase, possibly due to a diffusion-related process; (ii) slope efficiency of laser diodes does not significantly change as a consequence of stress; (iii) LED samples - with the same epitaxial structure of laser diodes - show a significant decrease in optical power during stress time; degradation is more prominent at low measuring current levels, suggesting that it is due to the increase in non-radiative recombination; (iv) the worsening of the optical characteristics of LEDs and laser diodes is significantly correlated to the increase in the defect-related current components. Results described within this paper strongly support the hypothesis that the degradation of laser diodes and LEDs submitted to stress at high current densities (>4 kA/cm²) is due to the increase in the concentration of defects within the active layer of the devices, activated by the high flux of accelerated carriers through the quantum-well region.

We present state-of-the-art performance from green and blue InGaN-based laser diodes fabricated on nonpolar/semipolar substrates. Using these novel crystal orientations, we demonstrate high-power, high-efficiency, and long-lifetime continuous-wave laser operation. For green wavelengths at 520 nm, we report on continuous wave single mode lasing with over 65 mW of output power and wall plug efficiencies over 2%. In the blue regime we describe single-mode lasers operating with over 23% wall-plug-efficiency and with output powers greater than 750 mW. To the best of the author's knowledge, this efficiency represents the highest reported to date for a single-mode blue laser. These InGaNbased devices offer dramatic improvement in performance, size, weight, and cost over conventional gas and solid state lasers for use in defense, biomedicine, and consumer projection displays.

The monolithic alternating current light emitting diode (ACLED) has been revealed for several years and was regarded as a potential device for solid state lighting. In this study, we will discuss the characteristics, development status, future challenges, and ITRI's development strategy about ACLED, especially focusing on the development progress of the monolithic GaN-based Schottky barrier diodes integrated ACLED (SBD-ACLED). The SBD-ACLED design can not only improve the chip area utilization ratio but also provide much higher reverse breakdown voltage by integrating four SBDs with the micro-LEDs array in a single chip, which was regarded as a good on-chip ACLED design. According to the experimental results, higher chip efficiency can be reached through SBD-ACLED design since the chip area utilization ratio was increased. Since the principle and the operation condition of ACLED is quite different from those of the typical DCLED, critical issues for ACLED like the current droops, the flicker phenomenon, the safety regulations, the measurement standards and the power fluctuation have been studied for getting a practical and reliable ACLED design. Besides, the "AC LED application and research alliance" (AARA) lead by ITRI in Taiwan for the commercialization works of ACLED has also been introduced.

GaN based micro emitter optoelectronic device array has been proved to be the core component for wide variety of applications such as microdisplay, biosensor, projection etc. Etching is one of the key steps to form the GaN micro emitter array device, including inductively coupled plasma (ICP) dry etch and alkaline solution wet etch. This paper reports the recent progress made by Ostendo Technologies Inc in fabricating the ultra-high density, large aspect-ratio etching formed monolithic GaN micro emitter optoelectronic device array. The unit density reaches 1M per cm², with good uniformity across the whole wafer. Perpendicular etching sidewall was achieved, with smooth surface roughness which is significance feature used for laser diodes (LDs) device.

Nitride-based light-emitting diodes (LEDs) suffer from a reduction (droop) of the internal quantum efficiency with increasing injection current. This droop phenomenon is currently the subject of intense research worldwide, as it delays general lighting applications of GaN-based LEDs. Several proposals have been forwarded to explain the efficiency droop. Among the suggested droop mechanisms are defect-related recombination, Auger recombination, and electron leakage. However, different sample preparation and measurement conditions as well as the application of different models lead to a confusing and sometimes contradicting variety of efficiency droop observations and explanations. This paper combines different droop models in a simple yet unified framework and it helps to bring more clarity to the ongoing droop discussion.

InGaN light emitting diodes (LEDs), which have become key components of the lighting technology owing to their improved power conversion efficiencies and brightness, still suffer from efficiency degradation at high injection levels. Experiments showing sizeable impact of the barrier height provided by an electron blocking layer (EBL) or the electron cooling layer prior to electron injection into the active region strongly suggest that the electron overflow resulting from ballistic and quasi-ballistic transport is the major cause of efficiency loss with increasing injection. Our previous report using a first order simple overflow model based on hot electrons and constant LO phonon scattering rates describes well the experimental observations of electron spillover and the associated efficiency degradation in both nonpolar m-plane and polar c-plane LEDs with different barrier height EBLs and electron injection layers. LEDs without EBLs show three to five times lower efficiencies than those with Al_0.15Ga_0.85N EBLs due to significant electron overflow to the p-type region in the former. For effective means of thermalization in the active region within their residence time and possibly longitudinal optical phonon lifetime, the electrons were cooled prior to their injection via a staircase electron injector, i.e. an InGaN staircase structure with step-wise increased In composition. The investigated m-plane and c-plane LEDs with incorporation of staircase electron injector show comparable electroluminescence performance regardless of the status of EBL. This paper discusses hot electron effects on efficiency loss, means to cool the electrons prior to injection.

We describe recent work on InGaN lasers and AlGaN UV LEDs at the Palo Alto Research Center (PARC). The presentation includes results from InGaN laser diodes in which the usual epitaxial upper cladding layer is replaced with an evaporated or sputtered non-epitaxial material, such as indium tin oxide, silver, or a silver-palladium-copper alloy [1, 2]. Non-epitaxial cladding layers offer several advantages to long wavelength InGaN laser diodes, such as eliminating the need to expose vulnerable InGaN active layers to the high temperatures required for growing conventional p-AlGaN cladding layers subsequent to the active layer growth. The presentation also discusses our recent results on AlGaN UV LEDs. UV LEDs with 300 micron square geometries operating at λ = 325 nm exhibit output powers of 13 mW with differential quantum efficiencies of 0.054 W/A measured under wafer-level, unpackaged condition with no heat sink. LEDs operating at λ = 290 nm under similar test conditions display output powers of 1.6 mW for large-area 300 μm X 1 mm devices.

Doped ZnO is one of the materials currently being considered in industrial applications as a possible replacement for ITO as a transparent conducting oxide. The properties of doped ZnO anodes prepared at Arkema Inc. are analyzed in 3D using high-throughput mapping tools. The 2D resistivities of the coatings measured by 4-point probe compare well with the resistivity values calculated from the spectroscopic ellipsometer measurements. It was found that the dependence of effective mass of doped ZnO on Hall-electron concentration influences optically-calculated mobilities and electron concentrations. To study the variation of the film properties along z-axis, the films are polished using mechanical planarization technique. The electrical and crystallographic depth profiles for these films are studied by differential Hall-effect and grazing-angle x-ray spectroscopy. The electron mobility increases continuously from the glass-film interface (12 cm²/Vs) to the ZnO film surface (19 cm²/Vs). The electron concentration depth profile has bell-like dependence with a maximum at 1.55 x 10²¹ cm^-3. In addition to the increasing grain size, the texture coefficients for the (002) reflection decrease and (103) reflection increase towards the air-film interface. Examples of the applications of the doped ZnO anodes in the OLED structures suggest improvement of external quantum efficiency with introduction of an Al₂O₃ undercoat.

We have proposed an efficiency droop model which can comprehensively explain experimental IQE droop phenomena occurring at different temperatures, materials, and active structures. In our model, carriers are located and recombined both radiatively and nonradiatively inside randomly distributed In-rich areas of InGaN-based QWs and the IQE droop originates from the saturated radiative recombination rate and the monotonically increasing nonradiative recombination rate there. Due to small effective active volume and small density of states of In-rich areas, carrier density is rapidly increased even at low current density and the radiative recombination rate is easily saturated by different distributions of electrons and holes in the momentum k-space. A measurement method that can separately estimate the radiative and nonradiative carrier lifetimes just at room temperature is theoretically developed by analyzing the time-resolved photoluminescence (TRPL) response. The method is applied to a blue InGaN/GaN QW LED. The experimental results show that the radiative carrier lifetime increases and the nonradiative carrier lifetime saturates with increasing TRPL laser power, which is one of direct evidences validating our IQE droop model.

Recent progress in III-nitride LED modeling is reviewed with the focus on physical models that provide a better understanding of such hot issues, as factors limiting the internal quantum efficiency of light emission and high-current efficiency droop, polarization doping in graded-composition III-nitride alloys and its utilization in LEDs, current crowding in LED dice and its impact on the light extraction efficiency, and optimal light conversion in white LED lamps. Specific features of III-nitride materials, their impact on the LED operation, and models accounting for these features are considered. Insufficient understanding of transport mechanisms of non-equilibrium electrons and holes and their localization in InGaN inhomogeneous active regions are discussed along with other still unsolved problems. Influence of technological factors on LED heterostructures and their operation is argued in the context of further model developments.

We present a review of the recent developments to enhance the external quantum efficiency (EQE) in GaN based vertical light-emitting diodes (V-LEDs). The combined use of quasi-photonic crystal and photochemical etching significantly improved the light extraction efficiency (LEE) of V-LEDs by a factor of 5. The enhancement of light output power by the nanotexturing of n-face n-GaN was remarkably influenced by reflectance of the p-contact. The enhanced LEE was also demonstrated by depositing a spontaneously formed MgO nano-pyramids and ZnO refractive-index modulation layer on the surface of V-LEDs, resulting in the increase of output power by 49 %, comparing with the V-LEDs with a flat n-GaN surface. The thermal stability of Ag-based p-type ohmic contact was siginficantly enhanced by addition of Cu, In, and Mg atoms to Ag layer, leading to high light reflectance and low contact resisitivity. Finally, we present a method of increasing light output power and suppressing efficiency droop in V-LEDs without modifying the epitaxial layers. These improvements are achieved by reducing the quantum-confined Stark effect by reducing piezoelectric polarization that results from compressive stress in GaN epilayer. This compressive stress is relaxed due to the external stress induced by an electro-plated Ni metal substrate.

We have found composition variations along the growth direction within regions of nominally constant indium composition in InGaN light emitting diode structures grown by metal-organic chemical vapor deposition using atomic resolution Z-contrast imaging in a scanning transmission electron microscope (STEM). Within 60 nm thick nominally In_0.01Ga_0.99N layers, we found periodic enhancements in the indium concentration into 4 bands separated by 11 nm. Energy dispersive spectroscopy spectrum imaging confirmed that the higher intensity in the high angle annular dark field (HAADF) Z-contrast STEM images was in fact caused by locally higher indium concentration. We observed no lateral indium composition fluctuations.

The optical polarization of the in-plane emission of c-plane oriented (In)(Al)GaN multiple quantum well light emitting diodes in the spectral range from 288 nm to 386 nm has been investigated by electroluminescence measurements. The intensity of transverse-electric polarized light relative to the transverse-magnetic polarized light decreases with decreasing emission wavelength. This effect is attributed to the different electronic band structures in the active region of the light emitting diodes. A changing aluminum and indium mole fraction in the (In)(Al)GaN quantum wells results in a rearrangement of the valence bands at the Γ-point of the Brillouin zone. For shorter wavelengths the crystal-field splitoff hole band moves closer to the conduction band relative to the heavy and light hole bands and as a consequence the transverse-magnetic polarized emission increases. Moreover, the in-plane polarization is shown to depend on the injection current. The correlation between the in-plane polarization and the injection current has been found to be different for light emitting diodes with InGaN and (In)AlGaN multiple quantum wells. The results highlight that polarization effects need to be considered when optimizing the light extraction from ultraviolet light emitting diodes in the (In)AlGaN materials system.

In this study, hetero-structure p-i-n type epitaxy wafers were deposited on the GaN/sapphire templates with different buffer layers by the MOVPE system. The absorption layers sandwiched in top p-GaN and bottom n⁺-GaN layers were designed into different short-period InGaN/GaN superlattice structures with specific pair numbers to maintain a total absorption thickness of 200 nm. As the buffer layer was properly adjusted, the V_OC and J_SC were enhanced by 35% and 95%, respectively. In addition to material qualities, the thickness of GaN buffer layers and piezoelectric-induced stain in the InGaN film itself also influenced the PV device performance.

The electron transport properties of AlGaN/GaN heterojunction field effect transistors (HFETs) were studied by two-dimensional drift-diffusion (DD) modeling method. The model performs self-consistent numerical computation on the Poisson equation, carrier statistics and current and continuity equations. The spontaneous and piezoelectircal polarization charges uniquely for GaN material was taken into account, which influences significantly the electron distribution and current density. Sliced 1D Schrödinger equation along c direction was solved to obtain electron distribution. The electrical field at the 2DEG channel was found to have a peak position locates at the gate edge towards the drain side, reaching 106 V/cm at high bias condition. The surface potential boundary conditions also were found to have profound influence to the simulation results.

The electric fields present in a heterostructure field effect transistor (HFET) give rise to large densities of hot electrons and hot phonons not in equilibrium with the lattice (thus the term "hot"). Hot electrons give rise to an accumulation of hot phonons, which play a deleterious role in device performance and reliability, particularly as 2DEG densities increase. Thus, it is important that hot phonon effects be mitigated. The hot phonon effect arises due to the fact that LO phonon (longitudinal optical phonon) scattering is the primary scattering mechanism at high fields in GaN, and that LO phonons tend to accumulate in the channel due to long lifetimes and low group velocity; minimizing the existence in time of hot phonons in the channel is the overall goal. That said, the lifetime of the LO phonons decaying into propagating LA modes is not constant, and it is through the exploitation of this fact that one gains the ability to enhance the performance and reliability of HFETs. This has been evidenced by operating HFETs at particular bias conditions which tend to lend themselves to relatively short LO phonon lifetimes. Alternatively, minimization of hot phonon lifetimes may be achieved through novel device designs. In this work, we outline designs for minimizing hot phonon effects in HFET devices with sheet densities >2 x 10¹³ cm^-2. We propose "camel-back" structures with wavefunction "spreading layers" to maximize 2DEG charge while mitigating hot phonon effects arising from high density 2DEGs.

The physics and the engineering formulas of 1/f noise in ErAs-based all-epitaxial, GaN-based, and other types of Schottky diodes are presented in a way related to the general pn junction quantum 1/f noise formulas developed by the author and van der Ziel earlier, but with inclusion of the image force contribution of an electron at the metal-semiconductor interface. On this base the phase noise introduced by mixers constructed with the ErAs Schottky diodes was also studied and can now be calculated analytically with the Quantum 1/f effect formulas.

We report the fabrication of GaN nanopillars and their laser action characteristics under optical pumping measurement. The nanopillars were fabricated from a GaN epitaxial wafer by self-assembled Ni nanomasked etching, followed by epitaxial regrowth to form crystalline facets on the etched nanopillars. The regrowth process is intended to reduce surface defects created during ICP-RIE etching. The density of etched GaN nanopillars is about 8.5x10⁸/cm² and the diameter and height of GaN nanopillars are about 250 nm and 650 nm, respectively. The as grown GaN nanopillars exhibit a random distribution with hexagonal pillar geometry. The sample is optically excited by frequency tripled Nd:YAG pulsed laser. The Gaussian waist of pumping spot is 1.8 um. At low pumping intensity, the emission has a broad spontaneous emission spectrum with maximum at 363 nm. As pump intensity increases, a narrow peak at 363 nm emerges quickly from the broad spontaneous emission back ground. The lasing action occurs at threshold pump power density of 122 MW/cm². The emission linewidth decreases with pumping power across threshold and reaches a lowest value of about 0.38 nm above threshold. The excitation-power-dependent spectra show that the lasing wavelength has a slight blue shift as pump power increases. We remark that this is due the band filling of the increasing excited carrier density.

To realize high-efficiency light-emitting diodes (LEDs), it is essential to increase light extraction efficiency. The moth-eye structure, consisting of periodic cones with a submicron-scale pitch on a surface/interface, is known to enhance the light extraction efficiency of light-emitting diodes. Previously, nitride-based flip-chip blue LEDs with the moth-eye structure on the backside of the substrate were demonstrated to exhibit high light extraction efficiency. In this report, face-up blue LEDs with a double moth-eye structure are described. One moth-eye structure was formed on the sapphire substrate and the other structure was formed on the ITO contact, where the pitch of the cones in both structures was 500 nm. The patterning of cones with such a small pitch was carried out by a low-energy electron-beam lithography technique. The output powers of mounted LEDs without resin encapsulation were measured using an integrated sphere. For reference, face-up LEDs without a moth-eye structure and with a single moth-eye structure on an ITO contact were also examined. The single moth-eye and double moth-eye LEDs have 1.2 and 1.4 times higher output power than the non-moth-eye LED, receptively.

Non-polar (1-100 ) and semipolar (1-101)GaN layers were grown on (112) and (001) Si substrates, respectively, by metalorganic chemical vapor deposition. In both cases, grooves aligned parallel to the <110> Si direction were formed by anisotropic wet etching to expose vertical {111}Si facets for growth initiation. The effect of growth conditions (substrate temperature, chamber pressure, ammonia and trimethylgallium flow rates) on the growth habits of GaN was studied. It was found that low pressure and low ammonia flow rate are beneficial for m-facet formation, while high ammonia flow rate promotes formation of (1-101) facets. Steady-state and time-resolved photoluminescence measurements revealed that the optical quality of (1-101) oriented GaN is comparable to that of c-plane GaN film grown on sapphire. The nonpolar (1-100 ) GaN shows only weak emission and fast non-radiative recombination rate. The poor optical quality of the mplane GaN can be explained by carbon incorporation during the growth under low pressure. Although further optimization of the growth conditions for better optical quality is required, preliminary results obtained for semipolar (1-101) -oriented GaN are encouraging.

We present a study of semi-polar (1-101) InGaN-based light emitting diodes (LEDs) grown on patterned (001) Si substrates by atmospheric-pressure metal organic chemical vapor deposition. A transmission electron microscopy image of the semi-polar template shows that the threading dislocation density was decreased significantly. From electroluminescence measurement, semi-polar LEDs exhibit little blue-shift and low efficiency droop at a high injection current because the reduction of the polarization field not only made the band diagram smoother but also restricted electron overflow to the p-GaN layer as shown in simulations. These results indicate that semi-polar InGaN-based LEDs can possess a high radiative recombination rate and low efficiency droop at a high injection current.

Indium-tin-oxide (ITO)/Ni/AgCu/Pt reflectors for high-performance GaN-based vertical light-emitting diodes (VLEDs) were investigated. The ITO layer was first annealed at 650 °C in air to make an Ohmic contact and then the Ni/AgCu/Pt reflectors were deposited and subsequently annealed at 400 °C in air to improve their reflectance and mechanical adhesion with the ITO layer. It was shown that the reflectance of the ITO/Ni/AgCu/Pt reflectors at 460nm was slightly increased from 82 to 87% after second annealing. Based on the secondary ion mass spectrometry depth profiles, this improvement was attributed to the formation of a transparent Ni-oxide and the existence of Cu atoms near ITO/AgCu/Pt interface regions suppressing the inter and out-diffusion of Ag. The VLEDs fabricated with the ITO/Ni/AgCu/Pt reflectors showed an approximately 4.4% higher output power and much better current-voltage characteristics than those with the Ag-based reflectors.

The epitaxial lateral overgrowth (ELO) gallium nitride (GaN) was grown with high fill factor using metal organic chemical vapor deposition (MOCVD). The inductively coupled plasma (ICP) etching of ELO-GaN based on Cl₂/Ar/SiCl₄ gas mixture was performed. Surface properties of ELO-GaN subjected to ICP etching have been investigated and optimized etching condition in ELO-GaN with ICP etching is presented. Radiofrequency (RF) power and the flow rate of Cl₂ gas were modified during the experiments. The window region, wing region and the edge region of ELO-GaN pattern present different etching characteristics. Different etching conditions were studied to get the minimized plasma-induced damage, relatively high etching rates, and excellent surface profiles. Etch depths of the etched ELO-GaN with smooth surface up to about 19 μm were achieved. The most suitable three-step etching condition is discussed with the assessment based on the morphology observation of the etched surface of ELO-GaN patterns.

To enhance the efficiency of photogenerated electron collection in the n-type working electrode, Indium Tin oxide (ITO) finger-type ohmic contacts were immersed in NaCl electrolyte because ITO is a well-known transparent and conductive optical film and the ITO/n-GaN contact exhibited ohmic property when the carrier concentration of n-GaN were close to 1×10¹⁹/cm³. We found that the performances of the n-GaN photoelectrochemical cells with finger-type ITO ohmic contacts in photocurrent densities and hydrogen gas generation rates were both better than the n-GaN without finger-type ITO ohmic contacts. Related analyses have been performed and will be presented in this paper to explain the possible mechanism from the point of view of electrochemical analysis. Besides, after the photoelectrochemical measurements we observed that the adhesion of ITO/n-GaN contacts was pretty good. Finally, we did the surface analysis by scanning electron microscope (SEM) before and after the photoelectrochemical measurements to conform the surface morphology of ITO almost did not change in the NaCl electrolyte. This indicates that ITO is a good candidate material for the immersed ohmic contact in water splitting system.

We present modal gain measurements in Ga(NAsP) heterostructures pseudomorphically grown on silicon substrate. Using the variable stripe length method we analyze the modal gain performance of an unprocessed single quantum well sample for different excitation densities. We obtain high modal gain values up to 55 cm^-1 at room temperature. These values are comparable to those of common high quality laser material. This demonstrates the high optical quality of the new dilute nitride material Ga(NAsP) and underlines its candidacy for electrically pumped lasing on silicon substrate.

GaN substrates are desirable for fabricating ultra-violet LEDs and LDs, and high-power and high-frequency transistors. High-quality GaN single crystals can be obtained by using Na flux method, but the growth habit of bulk crystals must be controlled. In this study, we investigated the effects of additives (Ca, Ba) on the growth habit and impurity concentration in the crystals. The aspect ratio (c/a) of the crystals was increased by increasing the amount of additives, showing that the growth habit could be changed from the pyramidal shape to the prism shape. Ba concentration was below the detection limit (1x10¹⁵ atoms/cm³).

We present our approach to measure the profile of nonuniformly bent GaN epi-wafers grown on sapphire substrates. By using a laser displacement sensor, the position of the epi-wafer is accurately measured and mapped. From the measured profile data, analysis of stress distributions over the nonuniformly bent wafer is performed by using a theoretical model. We show the result of theoretical analysis of how the stress tensors distribute over a wafer. The estimated stress tensors are related with optical properties such as photoluminescence of the wafer.

We have directly observed that InGaN quantum well layers were incoherently grown on 5-nm-thick GaN barrier layers in an InGaN/GaN multiple quantum well (MQW) system of a blue light-emitting diode by using a lattice image obtained by high-resolution transmission electron microscopy and fast Fourier transform mapping (FFTM) analysis of the lattice image. The lattice disorder was observed in the middle of the InGaN well layer by using high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM). In contrast, FFTM of the InGaN well layers with 10-nm-thick barrier layers showed the intervals of the (01-10) lattice planes were homogeneous, and the lattice disorder was not observed in the HAADF-STEM image. These results indicate that the excess stain in the InGaN/GaN MQW having thinner GaN barrier layers induces the lattice disorder in the InGaN well layers. Indium composition fluctuation in the InGaN well layer was also observed by using three-dimensional atom probe analysis. It indicates that the incorporation of indium atoms is affected by the imperfect structural properties of the MQW system with thinner GaN barrier layers. The intensity of electroluminescence from the sample with 10-nm-thick barrier layers in the MQWs was higher than that from the sample with 5-nm-thick barrier layers.

We report on the effects of substrate temperature and surface morphology of p-GaN templates on the properties of ZnO:Ga (GZO) layers grown by plasma-assisted molecular beam epitaxy. Substrate temperature varying from 200 °C to 450°C was found to have only a moderate effect on the electrical properties of GZO films but it greatly affects the surface morphology of the GZO films. The surface morphology and growth mode of GZO were also found to be considerably affected by the surface morphology of underlying p-GaN templates. On p-GaN templates with a smooth surface (RMS = 0.4 nm) featured by clear atomic steps, GZO layers grew in 2D growth mode and exhibited smooth surfaces with RMS roughness of 2 nm. In contrast, on p-GaN without clear atomic steps but having comparable surface roughness of 0.6 nm, GZO layers grew in 3D growth mode and exhibited rough surface (RMS roughness of ~17.0-20.0 nm). The results of surface roughness are consistent with those from TEM measurements. The lowest resistivity of ~2.3×10^-4 Ω·cm for as-grown GZO layers has been achieved at substrate temperature of 350°C, while the data for 2D GZO layers was affected by a parallel conduction channel from underneath GaN and require further studies. Although the differences in electrical properties and surface morphology existed, the GZO layers grown on different p-GaN templates showed optical transparency higher than 90% in the visible spectral range. The performance of 3D GZO layers as p-electrode was tested in InGaN light emitting diodes.

Generation-recombination processes in AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors (MOSHFETs) with HfAlO gate dielectric have been investigated through low-frequency phase-noise. Some devices tested exhibited noise spectra deviating from the well-known 1/ƒ^γ spectrum. These devices showed broad peaks attributed to generation-recombination (GR) effect in the noise-spectral-density (NSD) vs. frequency plots, which shifted toward higher frequencies at elevated temperatures. The unannealed and annealed MOSHFETs exhibited trap energy values as 0.22 eV and 0.11 eV at drain bias values of 6.3 V and 10 V, respectively. Then, we monitored the effect of source-drain bias on the excess GR noise. The time constant of the traps decreased from 16.7 ms to 2.1 ms as we increased the drain bias,V_DS, from 10 V to 18 V for the annealed devices. We also measured the evolution of the GR-like spectrum as a function of V_DS in HFETs to investigate the field-assisted emission. The zero-field trap energy is extracted as 0.71 eV from temperature dependence of emission using extrapolation technique to validate the potential barrier lowering (Frenkel-Poole effect) of the traps.

We report on the low-frequency noise (LFN) measurements on GaN based heterostructure field-effect transistors (HFETs) on sapphire with InAlN and AlGaN barriers to investigate the effects of electrical stress. The HFETs with InAlN barrier undergone a DC stress at bias conditions of V_DS=20V and V_G= -4.5 for up to 4 hours in aggregate. These devices exhibited an LFN in the form of 1/f^γ and a significant increase in the noise spectrum up to 15 dB for 2 hours and then the noise saturated for further stress durations. We also monitored the LFN for the HFETs with AlGaN barriers. The devices were stressed by applying 20V DC drain bias for up to 64 hours at various gate voltages. Stressing at a gate bias (V_G) of -2V showed negligible degradation. On the other hand, stressing at V_G=0V surprisingly reduced the noise power by about 4 to 15 dB in the frequency range of 1 Hz-100 kHz. Additionally, the InAlN-barrier HFETs exhibited 20-25 dB lower noise power than the ones with the AlGaN layer for the tested devices within the entire frequency range. The results suggest that the trap generation increases due to electrical stress in devices with InAlN barrier, whereas the noise power decreases as a function of stress in AlGaN/GaN HFETs due to an increase in the activation energy of the excess traps.

This paper reports a novel selective growth method for growing crack-free semi-polar (1-101) GaN on 7°-off (001) Si substrates by adding SiO₂ stripes in perpendicular to the V-grooves on Si. This method can effectively reduce the thermal stress between GaN and Si substrate so that crack-free (1-101) GaN films as thick as 1 μm is achieved after coalescence even without an AlN interlayer. Cathodoluminescence measurements show the presence of low dislocation density areas, which can be attributed to the bending of dislocations toward to the [1-100] and [11-20] directions during the facet growth.

Indium doped ZnO films have been successfully deposited on high resistivity GaN(0001) templates by plasma-assisted molecular beam epitaxy. N-type ZnO with carrier concentration of 4.35x10²⁰ cm^-3 and electron mobility of 5.7 cm/V-s, leading to resistivity of 2.6x10^-3 Ω-cm, are measured by Hall measurements. Under low indium doping flux, the carrier concentration increases accordingly with indium cell temperature until a max concentration is achieved as it might reach the In solubility. Further increase in doping flux leads to lower carrier concentration. X-ray diffraction spectra also show the degraded crystal quality as In doping concentration increases. Low temperature photoluminescence indicates that donor bound excitons dominate the emission while deep level emissions are not present in our In-doped ZnO films.

In this paper, we have analyzed and discussed the current spreading effect of the vertical LED depending on different electrode patterns. A fully 2D model by solving drift-diffusion and Poisson equations is used to investigate the current flow paths and radiative recombination region. The conventional vertical LED with and without the transparent conducting layer has been considered to figure out the physical mechanism of the device. With the examination of the separated electrode patterns, we find that the hole current spreading length is the critical factor to influence the lighting region due to its relatively low mobility. The effect of the spacing and geometry of the electrode pattern has been studied in this paper. We will present our work on modeling the different geometric LED device and study the optimized condition for these chips.