This PDF file contains the front matter associated with SPIE Proceedings volume 7602, including the Title Page, Copyright information, Table of Contents, Introduction, and the Conference Committee listing.

Large-sized and high-quality free standing GaN are required with the development of GaN-based devices. We have developed new techniques to reduce the price of GaN substrates. In this paper, we introduce a simple fabrication way of freestanding GaN substrate using hydride vapor phase epitaxy (HVPE). An evaporable buffer layer was applied for the fabrication of 2inch freestanding GaN to separate from a sapphire substrate, in other words, a freestanding GaN was fabricated only by HVPE (one-stop process) without any process.

Semipolar nitrogen terminated (1-101)GaN film was grown on a patterned (001)Si substrate using selective area metal organic vapor phase epitaxy. By using a high temperature grown AlN buffer layer upon oblique (111) Si facets on the substrate, we achieved a GaN crystal film with low dislocation density. Because the growth of GaN crystal was selforganized on the facets, we achieved two dimensional growth mode automatically, and the surface roughness was as small as 0.2nm. Incorporation of magnesium (Mg) and carbon (C) in the (1-101) GaN was studied extensively. It was found that the Mg doping efficiency is superior on (1-101) face to that on (0001) face. In case of C doping, a shallow acceptor level was generated in (1-101) face and p-type conduction was realized.

We report on the molecular beam epitaxial growth, fabrication, and characterization of InN/Si nanowire heterojunction solar cells. Vertically aligned p-doped-intrinsic-n-doped (p-i-n) InN nanowires spontaneously formed on n-type Si(111) substrates as well as n-i InN nanowires spontaneously formed on p-Si(111) were demonstrated. With the use of an in situ deposited In seeding layer, such InN nanowires exhibit non-tapered morphology. InN nanowire solar cells display a rectifying ratio of larger than 1,000 under dark, which provides a strong evidence of successful p-doping of InN nanowires. We measured a short-circuit current density of ~ 85 mA/cm² and a power conversion efficiency of ~ 1.62% under AM 1.5G illumination at approximately 100 mW/cm². Further improvement in the device performance is being investigated by optimizing the growth and fabrication process.

We demonstrated very thick (~400 nm) AlInGaN quaternary alloy grown on GaN epilayer by MOCVD. The Optical, electronic, crystalline quality and surface morphology of AlInGaN/GaN hetero structures with various TMI molar flow rate were be extensively discussed. With Al0.89In0.02GaN/GaN fully lattice matched structure, less and small V-defect pits and good crystal quality comparing with the other lattice mismatched AlInGaN/GaN hetero-structures were discovered. Finally, the difference AlInGaN quaternary epilayers could be directly applied to high power LED structure in the future by the same barrier growth conditions.

In this paper some physical and chemical basis of ammonothermal method of bulk gallium nitride synthesis in ammonobasic route. Excellent structural and wide spectrum of electrical parameters of truly bulk GaN crystals obtained this way are revealed. In considered crystals a low dislocation density (5 x 10³ - 5 x 10⁴ cm^-2) is attainable. At the same time the crystal lattice is extremely flat and the rocking curve is very narrow (FWHM=16 arcsec). Both polar and nonpolar ammonothermal GaN substrates enabled to grow high optical quality, strain-free homoepitaxial layers

The non-polar m-plane GaN film and polarized InGaN/GaN light-emitting diode (LED) grown by metal-organic chemical vapor deposition (MOCVD) on LiAlO₂(100) substrates were investigated. Firstly, the simulation of excitonic transition energies and polarization effects on band structure of non-polar plane GaN was studied using the k· p Hamiltonian approach. Due to small lattice mismatch between GaN and LiAlO₂ substrate, X-ray diffraction (XRD) revealed that the obtained m-plane GaN film has only [11-00] orientation with single-crystalline quality. In addition, anisotropic crystallographic properties and strain were found, which originates from the broken hexagonal symmetry. The anisotropic strain further separates the energy levels of top valence band at Γ point. The energy splitting as 37meV as well as in-plane polarization anisotropy for transitions are found by the polarized photoluminescence spectra at room temperature, which is consistent with our simulation. The fabricated InGaN/GaN LED on LiAlO₂(100) emits green polarized light at room temperature. And the polarization degree of the emission reaches up to 60% at the wavelength of 520nm.

We report results of ab initio calculations on (GaN)n nanoclusters with n = 12, 16, 22, and 24 using generalized gradient approximation (GGA) for the exchange-correlation energy. We find cage structures of GaN clusters to be lower in energy as compared to bulk fragments which have been fully optimized. Selected cages have been doped with Gd and Nd atoms by substituting Gd and Nd on Ga sites, respectively. The atomic structures of such doped nanoclusters have been optimized using spin-polarized GGA and by including all f electrons on the rare earth atoms. We find the doped nanoclusters to be magnetic with 7 μ_B (3 μ_B) magnetic moments when 1 Gd (Nd) atom is doped. When 2 Gd (Nd) atoms are substituted on different Ga sites, there is often a ferromagnetic coupling between the rare earth atoms, but we also find zero net magnetic moment in the nanocluster depending upon the separation between the doped atoms suggesting that both ferromagnetic and antiferromagnetic couplings may occur between the rare earth dopants.

Semipolar (11-22 ) GaN layers grown by metalorganic vapor phase epitaxy (MOVPE) and molecular beam epitaxy (MBE) are studied by transmission electron microscopy (TEM). The layers exhibit numerous defects with different geometries in comparison to the growth along the c-axis, they are identified as mostly partial dislocations, basal and prismatic stacking faults. The dislocations density is in the order of 5x10⁹ cm^-2, the corresponding Burger vectors are b = 1/6 <20-23>, b= 1/3 <10-10> and a small fraction of perfect a type dislocations with b = 1/3 <11-20> has been observed. The basal stacking fault density is in the order of 1x10⁶ cm^-1. In an attempt to reduce the defect density, Si_xN_y interlayers have been used as nanomasks for epitaxial lateral overgrowth, our analysis shows that this leads to a quite small reduction of the defects as compared to the starting layer.

A bottleneck for heat dissipation is discussed in terms of plasmon-assisted decay of nonequilibrium longitudinal optical phonons launched by hot electrons. According to experiment at low and moderate electric fields, the fastest decay takes place at electron density of ~6.7×10¹² cm^-2 and ~2.7×10¹² cm^-2 for heterostructures with 2DEG channels located in GaN and GaInAs, respectively. Hot-electron temperature and gate voltage can be used to shift the optimal density. A GaN-based heterostructure field effect transistor degrades slower when dissipation of the LO-mode heat is faster.

InGaN light-emitting diodes (LEDs) utilizing ZnO layers heavily doped with Ga (GZO) as transparent p-electrodes were fabricated and their characteristics were demonstrated to be superior to those of LEDs with metal Ni/Au electrodes. Highly conductive and highly transparent GZO films were grown on p-GaN contact layers of the LED structures by plasma-assisted molecular beam epitaxy under metal-rich conditions. The c and a lattice constants of GZO were found to be close to the bulk values, indicating small lattice distortion of GZO. The as-grown GZO films showed resistivities as low as 2.2-2.9×10^-4 Ω cm. Upon rapid thermal annealing at the optimum temperature of 675 °C, the resistivity decreased reaching a value of ~1.9×10-4 Ω cm. Unlike the LEDs with Ni/Au contacts, the LEDs with GZO electrodes showed no filamentation and very uniform light emission at high current densities. The peak value of the relative external quantum efficiency for the LEDs with GZO contacts has substantial improvement compared with that for the LEDs with Ni/Au contacts. Under pulsed excitation mode, GZO-LEDs withstood current densities up to 5000 A/cm².

InAlN/GaN layers grown by metalorganic vapor phase epitaxy have been investigated by transmission electron microscopy (TEM), atomic force microscopy (AFM) and X-ray diffraction (XRD). It is shown that the surface morphology and the crystallinity of the InAlN layers critically depend on the In composition. AFM analysis points out that step flow growth may not be easily attained in this system: when the InAlN or AlN interlayer thickness is increased, the growth mode becomes three dimensional. However, the formed islands are hundred of nanometers apart, and were not observed in the transmission electron microscope. From the TEM investigation, it is pointed out that v-shaped defects are present at the surface of some layers and they may be as deep as the whole ternary alloy, they are connected to threading dislocations originated from the underlying template layers. As expected the measured residual strain is minimum when the ternary layer composition is close to 16-17% In.

The dark current of Quantum Well Inter-subband Photodetectors is known to be affected by quantum 1/f noise. This limits the detectivity. This paper compares various experimental 1/f noise measurements in QWIPs with the prediction of the conventional quantum 1/f noise expression for GaN/AlGaN QWIPs. Both the collisionless and collision-dominated cases are considered. The elementary process causing the dark current is the transfer of an electron from one well to the neighboring well. This happens under the influence of the applied electric field, and has in general both thermally activated and tunneling components. The larger the applied electric field, the larger is the squared velocity change of the carriers, and the larger is the obtained conventional quantum 1/f effect. The detectivity of the devices is calculated on this basis. The results are compared with measurements of 1/f noise in QWIPs by J. Jiang⁶, C. Jelen⁷, and M. C. Hsu⁸, and the agreement is good.

A new stability criterion for heterojunction field effect transistors (HFETs) is introduced and discussed in terms of our own stress-induced degradation experiments on the GaN varieties. The new criterion explains how to choose the sheet conductance of the 2 DEG electron plasma in the channel and the temperature dependence of the spontaneous polarization in the gate insulation for maximal stability against stress-induced degradation. According to this criterion, for maximal stability against thermal breakdown, the sheet conductance is chosen to be in a well defined vicinity of the resonance of the optical phonons with the plasma frequency of the electrons in the channel. The first steps toward studying this concept experimentally have been taken by monitoring the 1/f noise of the HFETs both at baseband and through phase noise, i.e., with exponential modulation, and comparing to the extent possible at this point with the results of the quantum 1/f noise calculation. Our measurements of 1/f noise on pristine devices and on stressed devices, degraded by sustained high drain bias, show that even a small 8% decrease in the maximal drain current produces some 5- 10 dB increase in the 1/f noise power. The latter is thus a much more sensitive indicator of damage. The quantum electrodynamic (QED)- and quantum lattice-dynamic (QLD) quantum 1/f noise formulas have been refined for the case of AlGaN/GaN HFETs through a better definition of the coherence parameters s and s'.

The ratio s of the coherent magnetic energy term and the incoherent mechanical kinetic energy terms of the drift motion in the hamiltonian of a current carrying system is calculated for the special cases of a HFET or FET. This ratio defines the resulting quantum 1/f noise from the coherent and conventional quantum 1/f effects. In this case of FETs and HFETs of much larger width w>>L_DS>t, the kinetic energy E_k of average motion with drift velocity v_d per unit length in the direction of the drain-source distance L_DS in the channel of thickness t, is still given by Nmv_d ²/2, but the magnetic energy E_m per unit length in the direction of L_DS is roughly proportional with the first power of w only, instead of w², and can be approximated by E_m = π[ln(w/2L_DS)]L_DS[nevS/c]²/w. Here S=wt is the cross section though which current flows this indicates field-decoherence along the large device width w. This yields a coherence ratio of s ≡ E_m/E_k ≈ πnr_otL_DSln(w/2L_DS), which shows that only an effective width w=w_eff about equal to L_DS should be used in the calculation of s in this special case; larger widths are subject to de-coherence. This favors lower, mainly conventional, quantum 1/f noise in these devices, in spite of the large values of w. It also explains for the first time why the huge widths are possible with impunity, i.e., without causing the much larger coherent quantum 1/f noise to appear. For non-uniform current distribution across t, and for piezoelectric coupling, improved forms are derived for s. Specifically, the coherence parameter, called s' for the piezo case, is given by s' = (gN'h/m^*v_s)( v_s/u)³F(u/v_s)t/12w, where F(u/v_s) = (2/3)(u/v_s) for small drift velocity u, much smaller than the sound velocity v_s in the semiconductor. Here N'=nwt.

The promise of InAlN-based HFET devices stems from the relatively large band and spontaneous polarization discontinuities and at the interface and the lack of misfit strain when grown lattice matched to GaN. However, there still exists some question as to what the true lattice matching condition of InAlN to GaN is due to discrepancies in the literature values of lattice parameters of the InN binary, and of the InAlN bowing parameters. We used the gate lag measurement as a supplementary technique to verify lattice matching to the underlying GaN, as we expect the strain in layers one source of lag, associated with piezoelectric charge at the surface. We observe very low lag for nearly lattice matched barrier and barriers under tensile strain, and a marked increase as the composition deviates from the lattice matched condition toward compressively strained layers. Additionally, FETs fabricated on a nearly lattice matched InAlN layer boasted a maximum drain current of over 1.5A/mm and ~2.0A/mm and transconductances of ~275mS/mm and ~300mS/mm at DC and in pulsed modes, respectively, and a cutoff frequency of 15.9GHz (an f_T*L_G product of 10.3) for a gate length of 0.65μm.

We review our current development on nano-ultrasonic by using piezoelectric nano-layers based on the GaN material system. Our study indicated that piezoelectric semiconductor nanostructures can serve as optical piezoelectric transducers to generate and detect nanoacoustic waves through the piezoelectric effect. The nanoacoustic waves, with a wavelength on the order of or shorter than 10 nm, can be used for high accuracy THz electron control, noninvasive subnanometer interface probing, and nano-ultrasonic imaging. Detailed design principles and the applications to the study of lattice/molecular dynamics in condense matters are discussed.

Electroluminescence of 285 and 340 nm AlGaN quantum well light emitting diodes (LEDs) has been studied by scanning near-field optical spectroscopy. In the 285 nm devices, the near-field scans revealed hexagonal cross hatch microcracks that can be related to strain relaxation. Besides, μm size areas emitting with a higher intensity and at a longer wavelength, presumably, due to lower AlN molar fraction, have been observed. Near-field scans performed during subsequent days revealed that with time, intensity from these spots increases and emission wavelength shifts to the red, indicating further change in the quantum well alloy composition. This has allowed distinguishing a novel LED aging mechanism that involves locally increased current, heating and Al atom migration. For the 340 nm emitting device with lower Al content in the active region, no such features have been observed.

The thermal conductivity of high-quality narrow-bandgap (0.77eV) InN grown on GaN on sapphire substrate by pulsed- MOVPE method was measured and analyzed. To accurately extract the thermal conductivities of GaN and InN films grown on sapphire substrate, 2D multilayer thermal diffusion model and extended 3ω slope technique are employed. The thermal conductivity of sapphire substrate measured is 41 W/(mK). The thermal conductivity of undoped GaN film is measured as 108 W/(mK). High-quality pulsed-MOVPE grown InN film exhibits thermal conductivity of 126 W/(mK), which is higher in comparison to the previously-reported value of porous InN ceramics 45 W/(mK), yet lower than the theoretical value 176 W/(mK) based on phonon scattering.

The surface charge behavior on n-type GaN was investigated as a function of UV exposure in ambient. It was first noticed that the surface photovoltage (SPV), or change in surface contact potential under UV illumination, could slowly decrease during illumination. The SPV for as-grown samples is ~0.6 eV and can drop by up to 0.3 eV over 1 h of UV exposure in air ambient. We believe that this slow decrease is due to the photo-induced chemisorption of oxygen species. Interestingly, samples exhibit a smaller drop in SPV during continuous UV illumination after many hours of accumulated UV exposure. This can be explained by the UV-induced growth of a thicker surface oxide layer which inhibits electron transfer from the bulk to the surface. The original SPV behavior can be restored by chemical etching to remove the additional surface oxide. We have also investigated the surface behavior by locally charging the surface before and after UV-induced oxide growth. Measurements of the surface contact potential using scanning Kelvin probe microscopy (SKPM) demonstrate that negatively charged regions initially spread laterally on the UV-exposed surface, but not on the clean surface. The UV-induced oxide therefore appears to be relatively conductive and enhance the lateral motion of injected electrons. The discharge of both the negatively and positively charged regions under dark conditions is similar for both the initial and UV-exposed surfaces and shows a logarithmic time dependence.

Mg-doped In_xGa_1-xN alloys were grown by metal organic chemical vapor deposition (MOCVD) on semi-insulating c-GaN/sapphire templates. Hall effect measurements showed that Mg-doped In_xGa_1-xN epilayers are p-type for x up to 0.35. Mg-acceptor levels (E_A) as a function of x, (x up to 0.35), were experimentally evaluated from the temperature dependent hole concentration. The observed E_A in Mg-doped In_0.35Ga_0.65N alloys was about 43 meV, which is roughly 4 times smaller than that of Mg doped GaN. A room temperature resistivity as low as 0.4 Ωcm (with a hole concentration ~5 10¹⁸ cm^-3 and hole mobility ~3 cm²/Vs) was obtained in Mg-doped In_0.22Ga_0.78N. It was observed that the photoluminescence (PL) intensity associated with the Mg related emission line decreases exponentially with x. The Mg energy levels in InGaN alloys obtained from PL measurements are consistent with those obtained from Hall-effect measurements.

For almost two decades, extensive research has been carried out on wurtzite nitride semiconductors due to their large direct band gap which offer the possibility to produce optoelectronic devices from the infrared (InN) to the ultraviolet range (GaN and AlN). The reduction of the defect densities and polarisation effects in nitride heterostructures are still challenges for the production of efficient light emitting diodes and lasers past the green range. In this contribution, the structure of the extended defects is reviewed, and their possible interaction with quantum wells and quantum dots is discussed. The presence of threading dislocations is shown to constitute a driving force for clustering. Inside semipolar layers, the glide planes are not available for the spreading of perfect dislocations, this results in the formation of a more complex defect system which we have investigated in the case of (11-22) layers.

GaN-based photonic crystal surface-emitting lasers(PCSELs) with AlN/GaN distributed Bragg reflectors were fabricated and analyzed. Different lasing characteristics of GaN-based PCSEL has been determeined and demonstrated by the PC lattice constants. The laser emission behavior covered the whole PC patterns of 50 μm in diameter. Under the optical pumping at room temperature, the PCSEL with PC lattice constant of 230nm shows a threshold energy density of about 2.7 mJ/cm². Above the threshold, one dominated peak emits at 420.11 nm with a linewidth of 1.1 Å. The lasing wavelength emitted from PC lasers with different lattice constants occurs at the calculated band-edges provided by the PC patterns which further shows different polarization angles due to the light diffracted in specific directions, corresponding exactly to Γ, K, and M directions in the K-space. The PCSEL also shows a characteristic temperature of 148K and a spontaneous emission coupling efficiency β of about 5x10^-3. Besides, the coupled-wave model in the PC hexagonal-lattice is applied to distinguish the discrepancy in threshold power and the corresponding coupling coefficient. The results show the lasing actions within Γ, K, and M modes have the substantial relation between the threshold energy density and the coupling coefficient.

GaN-based vertical-cavity surface-emitting lasers (VCSELs) are expected to exhibit several advantages over their already widely used edge-emitting counterparts, including lower manufacturing costs, circular output beams, and longer lifetime. However, in contrast to the great success of GaN-based edge-emitting lasers in recent years, GaN-VCSELs still face significant challenges. Electrically pumped devices have been demonstrated only recently and they exhibit severe performance restrictions. We here analyze these recently manufactured GaN-VCSELs using advanced laser simulation software. The simulation self-consistently combines carrier transport, photon emission, and multi-mode optical wave guiding. For the quantum wells, Schrödinger and Poisson equations are solved iteratively at every bias point to account for the Quantum-Confined Stark Effect. Our analysis shows that thick quantum wells allow for the almost complete elimination of the built-in quantum well polarization field. The simulations also reveal several performance limiting effects, e.g., gain-peak offset, current crowding, and electron leakage. Design optimization options are discussed and simulated.

We present new advances in green, blue, and violet InGaN-based laser diodes fabricated on nonpolar and semipolar GaN substrates. Using these novel crystal orientations, we report high power, high efficiency, continuous-wave operation from single-lateral-mode electrically pumped laser diodes at wavelengths from 405 nm to 500 nm. Additionally, we present continuous-wave lasing demonstrations out to 523 nm, representing the longest continuous-wave green laser emission reported to date. Wall-plug efficiencies of over 25% in the violet region, 16.2% in the blue region, and 2.2% in the 500 nm range are presented. These InGaN-based devices offer dramatic improvement in size, weight, and cost over conventional gas or solid state lasers and may enable a variety of new applications in defense, biomedical, industrial, and consumer projection displays.

Blue laser diode (LD) structures with GaN waveguide layers and with In_0.03Ga_0.97N waveguide layers were grown. A comparison study showed In_0.03Ga_0.97N waveguide layers significantly enhance the LD performance. The mechanism behind this was investigated using reciprocal space mapping of X-ray diffraction and time-resolved cathodoluminescence measurements. Room-temperature lasing of laser diodes at 454.6 nm was realized for LD structure with In_0.03Ga_0.97N waveguide layers.

We present a review of the intersubband optoelectronic technology based on GaN/AlGaN quantum wells or quantum dots. Three kinds of devices are discussed: intersubband light emitters, photodetectors and optical modulators. The operation principle, device performance and future developments are exposed.

This work provides a contribution toward the development of a fitting-parameter-free model to describe the high field electron and hole transport in wurtzite GaN. We have developed a novel model for the carrier-phonon interaction in wurtzite GaN based on the rigid pseudoion approach, using the nonlocal empirical pseudopotential method to calculate the electronic structure and the linear response technique within density functional theory to determine the phonon dispersion. This approach makes it possible to eliminate the use of adjustable parameters and to compute the deformation potential interaction parameters both for electron- and hole-phonon scattering rates directly from first-principle. On the same footing the electrons and holes impact ionization rates and the polar carrier-phonon interaction have been computed using the GaN full band structure. Because of the complex GaN band structure, it is necessary to properly account for the multiband transport phenomena that significantly change the carrier dynamics at high energies. Neglecting this effect, would result in a significantly lower impact ionization coefficients and, as a consequence, smaller multiplication gains and higher breakdown voltages in APDs. To evaluate the ability of the new model to predict the performance of realistic devices, we have computed the electrons and holes multiplication gains, noise factors, breakdown voltages and bandwidth of different APD structures fabricated on different crystallographic planes and performed a detailed comparison with the available experimental data. We find that the model correctly predicts, within the experimental errors, the multiplication gains and breakdown voltages of several devices structures.

In this paper we propose a promising die-bonding material consisted of diamond-added AgSnCu solders for chip package, and combine it with our well-established cup-shaped copper technique for chip heat dissipation. The composite solder was prepared by mixing the commercial Sn-3wt.%Ag- 0.5wt.%Cu (SAC305) solder paste with the diamond paste [0.25(W)475-MA, Engis, USA] in a weight ratio of 10:1. Thermal resistance analysis shows that total thermal resistance of the LED packaged using the composite solder is only 6.4 K/W, which is much lower than both the LED using AgSnCu solder (9.2 K/W) and the LED using silver paste (10.4 K/W). As a result, the LED with the composite solder exhibits larger light output power and smaller junction temperature than the other two samples. The improved device performance is mainly due to enhanced heat dissipation of the die-bonding materials used. These results suggest that the composite diamond-added AgSnCu solder is promising in high-power LED application.

We have reported the growth and characterization of quaternary AlGaInN bulk layer and AlGaInN/InGaN MQWs grown by metal-organic chemical vapor deposition (MOCVD) for high efficiency ultraviolet light-emitting diodes. The inclusion of the small fraction of the indium in AlGaInN layer was found to lead a fewer structural defects and reduction of strain in the layer. From the temperature dependent photoluminescence (TDPL) and time-resolved PL (TRPL), the internal quantum efficiency at 300K was obtained as 60 % for sample grown with quaternary AlGaInN barrier in MQWs and 25 % for the sample with ternary AlGaN barrier. It was resulted that the dominant optical transition in AlGaInN/InGaN MQWs was due to localized exciton recombination and reduction of strain in the QW stack with indium incorporation in the barriers, resulting the longest decay lifetime from quaternary AlGaInN alloys. We measured the optical output power from the UV LED device grown with quaternary AlGaInN barriers.

Novel staggered InGaN quantum wells (QWs) and type-II InGaN-GaNAs QWs with improved momentum matrix element lead to improved internal quantum efficiency for green-emitting light-emitting diodes (LEDs). Approaches for enhancing internal quantum efficiency, light extraction efficiency, and efficiency-droop in nitride LEDs are discussed.

In a LED, electrons and holes are injected from the contacts into the quantum wells. The carriers diffuse laterally in the quantum wells. In this letter, we focus on the role of interface roughness, electron-electron scattering, and dislocation scattering in the quantum well on lateral transport and present results on the diffusion length in the lateral direction. The influence of quantum well with different indium compositions, carrier densities, dislocation densities, and interface roughness are studied. A Monte Carlo simulation program is used to study the lateral mobility in quantum well. The effects of alloy scattering, charged dislocation scattering, interface roughness scattering, and e-e scattering have been included in our Monte Carlo model. The results show that the nonradiative lifetime caused by the dislocation trapping is still a dominating role for higher indium composition. This limits the internal quantum efficiency. The large interface roughness caused by a quantum dot like structures and In-clustering effect may help to improve the efficiency by reducing the diffusion length. However, our results show that for the longer wavelength source, we should try to improve the overall internal quantum efficiency first before discussing the droop effect. One is to decrease the radiative lifetime by reducing the QCSE with nonpolar/semipolar structures. The other approach would be to reduce the dislocation density to less than 10⁶cm^-2.

High power near-UV LEDs, with wavelengths from 365nm to 410nm, were realized with an external quantum efficiency from 12% to 45% for 365nm and 410nm, resp. We show that the vertical LED design, coupled with the Silicon sub-mount package, gives very good reliability that is suitable for many high power epoxy and polymer curing applications. UV LEDs are proven to be a good replacement for high power mercury lamps currently used in the market.

In this study, we have successfully grown blue LED structure on ZnO template (deposited on sapphire substrate by atomic layer deposition, ALD) by atmospheric pressure metal-organic chemical vapor deposition (AP-MOCVD). Although GaN semiconductor material is very similar to ZnO in many ways, i.e. relatively small lattice mismatch ~1.8 % compared with traditional sapphire substrate~16 %, it still has a big challenge when GaN-based LEDs grow on ZnO template at usually growth temperature near 1100°C. With too high a temperature and a long deposited time, it would cause reaction at GaN/ZnO interface which is a vital reason that degrades the GaN crystalline quality. In view of this, we introduced an optimized thin AlN cover layer on ZnO template protecting the underneath ZnO layer and then obtained a real work LED structure. Meanwhile, the TEM measurement characterized the epilayer crystalline structure. The optical properties also were carried out by photoluminescence and electroluminescence analysis. Finally, with a suitable fabrication of LED processing, the ZnO template may has the potential as a sacrificial layer by chemical etching technical instead of conventional laser lifted-off.

For high-performance light-emitting diodes (LEDs), the high light extraction efficiency obtained by surface texturing is currently indispensable. We have developed a new method of moth-eye structure fabrication based on low-energy electron-beam projection lithography (LEEPL). The moth-eye structure comprises periodic corns with a pitch of optical wavelength scale, and is known to have a very low optical reflectivity. Since the LEEPL technique has very high throughput ability for carrying out such submicron patterning, it is applicable to the mass production of highperformance LEDs. In this study, we aim to increase the light extraction efficiency of various nitride-based LEDs, including those fabrication on sapphire and SiC substrates. The influence of the pitch of the moth-eye structure on the light extraction is also clarified. The light extraction efficiencies of flip-chip blue LEDs fabricated on SiC and sapphire substrates are increased by factors of 3.7 and 3.2, respectively, when the moth-eye structure is applied on the back of the substrate.

High brightness InGaN light emitting diodes (LEDs) require high quantum efficiency and its retention at high injection levels. The efficiency drop at a high injection levels in InGaN light emitting diodes (LEDs) has been attributed, e.g. to polarization field on polar c-plane InGaN and the heavy effective hole mass which impedes high hole densities and transport in the active quantum wells. In this study, we carried out a comparative investigation of the internal quantum efficiency (IQE) of InGaN active region in LED structures using resonant optical excitation for layers with polar (0001) orientation on c-plane sapphire, and nonpolar (1-100) m-plane orientation, the latter on specially patterned Si and on m-plane bulk GaN. Analysis of the resonant photoluminescence (PL) intensity as a function of the excitation power indicate that at comparable generated carrier concentrations the IQE of the m-plane InGaN on Si is approximately a factor of 2 higher than that of the highly optimized c-plane layer. At the highest laser excitation level employed (corresponding carrier concentration n ~ 1.2 x 10¹⁸ cm^-3), the m-plane LED structure on Si has an IQE value of approximately 65%. We believe that the m-plane would remain inherently advantageous, particularly at high electrical injection levels, even with respect to highly optimized c-plane varieties. The observations could be attributed to the lack of polarization induced field and the predicted increased optical matrix elements.

We report the internal quantum efficiency (IQE) and external quantum efficiency (EQE) of m-plane InGaN-based LEDs and c-plane LEDs emitting at 400 nm. According to excitation density dependent photoluminescence (PL) measurements, the IQE values for the m-plane LEDs are approximately 70% higher than that for c-plane reference LEDs with the same structure (multiple-quantum well active region with 3nm In_0.01Ga_0.99N barriers) and similar emission wavelength (400nm) albeit on sapphire substrate. Electroluminescence (EL) measurements reveal that the external quantum efficiency (EQE) from both LEDs shows negligible droop under electrical injection due to the employment of thin In_0.01Ga_0.99N barriers, i.e. coupled quantum wells, in the active regions. However, with increasing current, the EL intensity of the m-plane LEDs increases more rapidly than that of the c-plane LEDs, with the EL intensity reaching its peak value at ~240 Acm^-2 for the m-plane LEDs and ~420 Acm^-2 for the c-plane LEDs. Consistent with the IQE values obtained from the PL measurements, the m-plane LEDs exhibit ~ 60% higher EQE values than the c-plane counterparts. It is also worth to mention that increasing the emission wavelength (from 400 nm to 435 nm) for the m-plane LEDs does not affect the efficiency retention at high injection levels.

Ab initio density functional plane-wave calculations are performed to study the lattice constant, bulk modulus, and the pressure derivative of the bulk modulus of ternary zincblende AlInN. Results obtained using local density approximation (LDA) and generalized gradient approximation (GGA) for the exchange-correlation energy are investigated and compared with available binary experimental results. For AlN it is found that GGA provides a better agreement with experiment than LDA one does. For InN LDA appears to perform better. The deviation parameter of bulk modulus of zincblende Al_xIn_1-xN is 10.34 ± 9.27 GPa using LDA and 18.48 ± 2.28 GPa using GGA.

We demonstrate high efficiency blue light emitting diodes with defect passivation layers. The defect passivation layers were formed by defect selective wet etching, SiO₂ deposition, and chemical mechanical polishing process. The process does not require photolithography patterning. The threading dislocation density of grown sample was reduced down to ~4×10⁷ cm^-2. The defect passivated epi-wafer is used to grow light emitting diode (LED) and the output power of the fabricated chip is enhanced by 45% at 20 mA compared to a reference one without using defect passivation.

The concept of non-polar (11bar00) m-plane GaN on Si substrates has been demonstrated by initiating growth on the vertical (1bar1bar1) sidewalls of patterned Si(112) substrates using metalorganic chemical vapor deposition. The Si(112) substrates were wet-etched to expose {111} planes using stripe-patterned SiN_x masks oriented along the [1bar10] direction. Only the vertical Si (1bar1bar1) sidewalls were allowed to participate in GaN growth by masking other Si{111} planes using SiO2, which led to m-plane GaN films. Growth initiating on the Si (1bar1bar1) planes normal to the surface was allowed to advance laterally and also vertically towards full coalescence. The full width at half maximum values for the GaN mplane x-ray diffraction rocking curves were 9 and 27 arcmin when rocked toward the GaN a-axis (parallel to stripes) and the GaN c-axis (perpendicular to stripes), respectively. Room-temperature photoluminescence showed strong band-edge emission with an intensity comparable to that of laterally overgrown c-plane GaN. InGaN double heterostructure active layers grown on these m-GaN templates on Si exhibited two times higher internal quantum efficiencies as compared to their c-plane counterparts at comparable carrier densities. These results demonstrate a promising method to obtain highquality non-polar m-GaN films on large area, inexpensive Si substrates.

Two different template structures of dot air-bridges and nanorods were used for 300 µm GaN growth by hydride vapor phase epitaxy (HVPE). The selective growth of arrays of dot air-bridges and nanorods whose sidewalls coated with SiO₂ are identified and exploited to form a compliant layer to decouple the impact due to the different thermal expansion and lattice mismatch between 300 µm overgrown GaN layer and the host sapphire substrate. As the process temperature cooling down from 1050 °C to room temperature in HVPE system, the 300 μm freestanding GaN substrates were obtained by the self-separation. The dislocation density was estimated by both the etching pit density method and cathodoluminescence (CL). The dislocation densities of the freestanding bulk GaN were lower than 5×10⁶ and 5×10⁷ cm^-2 for the template structure of dot air-bridges and nanorods structure, respectively.

We present analytical methods to study the physical mechanisms of the optical output stability of blue InGaN laser diodes in short and long-term stress experiments. Lasers of three ridge widths were stressed under constant current condition at a constant current density. The output power of a 1.8μm ridge laser decreased down to 40% within 15h mainly due to an increase in threshold current. Broader ridges showed a more stable output power. The decline in output power was related to changes in quantum efficiency and longer carrier lifetimes after stress. A simple recombination model was fitted to the measurements indicating no increase in non-radiative recombination centers. Instead the longer carrier lifetimes could be well explained with a decrease in carrier density due to an additional current spreading. These results were confirmed by changes in the sub-threshold wavelength shift before and after aging.

We fabricated UV laser diodes (LDs). A high-temperature annealing at 850 °C for a short duration was found to be very effective for reducing the series resistance of an n-type contact. By applying high-temperature annealing, the operating voltage of an UV LD was reduced from 10.7 V to 9.5 V at a forward current of 300 mA.

The internal quantum efficiency (IQE) and relative external quantum efficiency (EQE) in InGaN LEDs emitting at 400 nm with and without electron blocking layers (EBLs) on c-plane GaN and m-plane GaN were investigated in order to shed some light on any effect of polarization charge induced field on efficiency killer carrier spillover. Without an EBL the EQE values suffered considerably (by 80 %) for both orientations, which is clearly attributable to carrier spillover. Substantial carrier spillover in both polarities, therefore, suggests that the polarization charge is not the major factor in efficiency degradation observed, particularly at high injection levels. Furthermore, the m-plane variety with EBL did not show any discernable efficiency degradation up to a maximum current density of 2250 Acm^-2 employed while that on cplane showed a reduction by ~ 40 %. In addition, IQE of m-plane LED structure determined from excitation power dependent photoluminescence was ~80 % compared to 50 % in c-plane LEDs under resonant and moderate excitation condition. This too is indicative of the superiority of m-plane LED structures, most probably due to relatively larger optical matrix elements for m-plane orientation.

Performance, advantages and drawbacks of GaN- and SiC-based ultraviolet (UV) photodetectors are analyzed and compared. This includes metal-semiconductor-metal photodetectors, p-i-n photodiodes and avalanche photodiodes. Design, process and characterization of these devices are described. Fabricated p-i-n InGaN/GaN quantum well photodetectors are shown to exhibit a quantum efficiency of about 50% at 365 nm with a peak ultraviolet to visible rejection ratio more than 3 orders of magnitude. A novel SiC avalanche photodiode design is shown to produce stable avalanche breakdown characteristics for devices up to 2mm in diameter. A significant increase of its photo responsivity is also demonstrated when the avalanche voltage is applied.