This PDF file contains the front matter associated with SPIE Proceedings Volume 8262, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

Throughput requirement of the epitaxial process of GaN on Si is described. The impact of the growth rate of AlGaN for the buffer layer of GaN on Si is highlighted. In the attempt of growing GaN on Si, we have tested a production scale high flow speed MOVPE reactor (TAIYO NIPPON SANSO UR25k) for 6 inch X 7 wafers. Al_0.58Ga_0.42N was grown with the growth rate of 1.85μm/hr at 30 kPa. AlN was grown with the growth rate of 1.4μm/hr at 13kPa. AlN/GaN SLS (5nm/20nm) was also grown at the growth rate of 1.4μm/hr. An excellent uniformity of aluminum concentration of less than 0.5% was also obtained for Al_0.58Ga_0.42N. The challenge which we are facing to further increase of the throughput is summarized.

High resolution transmission electron microscopy and aberration-corrected scanning transmission electron microscopy (STEM) reveal a new void defect in GaN, Si-doped GaN, and InGaN. The voids are pyramid shaped with symmetric hexagonal {0001} base facets and {10^-11} side facets. The pyramid void has a closed or open core dislocation at the peak of the pyramid, which continues up along the [0001] growth direction. The closed dislocations have a 1/3 11^-20 edge dislocation Burgers vector component, consistent with known threading dislocations. The open core dislocations are hexagonal shaped with pure screw character, {10^-10} side facets, varying lateral widths, and varying degrees of hexagonal symmetry. STEM electron energy loss spectroscopy spectrum imaging revealed a larger C concentration inside the void and below the void than above the void. We propose that carbon deposition during metal organic chemical vapor deposition growth acts as a mask, stopping the GaN deposition locally. Subsequent layers of GaN deposited around the C covered region create the overhanging {10^-11} facets, and the meeting of the six {10^-11} facets at the pyramid's peak is not perfect, resulting in a dislocation.

GaN c-plane substrates were patterned to obtain 30-70 μm wide differently angled regions. The patterning technique was based on multilevel pattern photolithography and ion etching and was similar to the one used for the diffraction optics elements fabrication. The region angles were between 0.2 and 3.4 degrees with respect to the c-plane. It is shown that photoluminescence and cathodoluminescence wavelengths of InGaN/GaN quantum wells grown by metalorganic vapor phase epitaxy depend on each region's angle. Laser diodes grown on freestanding patterned GaN are also demonstrated. The lasing wavelength of chips grown in differently angled substrate regions are different. We attribute those differences to indium content differences in each of the angled regions.

In this paper, promising experimental results for the p-type electrical properties of carbon-doped (C-doped) AlGaN are discussed. P-type conductivity was experimentally achieved in C-doped (0001) plane AlGaN layers with from a small amount to 55% solid Al composition, but not in (0001) plane GaN. The maximum free hole density (determined by van der Pauw geometry-Hall effect measurement) achieved for an AlGaN layer with 10% solid Al composition was p= 3.2 x 10¹⁸ cm^-3. The maximum net ionized acceptor densities (NIAD = (N_A ^--N_D ⁺)), which were determined by capacitance-voltage measurement, for AlGaN with 6, 10, 27, and 55% solid Al compositions, were all in the range of (3-7) x 10¹⁸ cm^-3. Moreover, the electrical activity of the carbon acceptors was estimated to be 55-71% from the NIAD and secondary-ion microprobe mass spectrometry analysis data on the carbon concentration. Activation energy of carbon acceptors was estimated to be 22-30 meV from this electrical activity. On the other hand, optical property of C-doped AlGaN was compared with undoped AlGaN. Then we found new emission, which related to carbon acceptors, at smaller energy side by 29-35 meV from band edge-emission of the AlGaN. A p-n junction was also fabricated using the C-doped p-type AlGaN.

GaN epilayers were implanted with Eu to fluences of 1×10¹³ Eu/cm² and 1×10¹⁵ Eu/cm². Post-implant thermal annealing was performed in ultra-high nitrogen pressures at temperatures up to 1450 ºC. For the lower fluence effective structural recovery of the crystal was observed for annealing at 1000 ºC while optical activation could be further improved at higher annealing temperatures. The higher fluence samples also reveal good optical activation; however, some residual implantation damage remains even for annealing at 1450 ºC which leads to a reduced incorporation of Eu on substitutional sites, a broadening of the Eu luminescence lines and to a strongly reduced fraction of optically active Eu ions. Possibilities for further optimization of implantation and annealing conditions are discussed.

Behavior of carbon (C) doping in a (1^-101)AlGaN has been investigated by grazing incidence FTIR analyses at room temperature. The sample was grown by MOVPE on (1^-101)facets of GaN triangular stripes made on (111)Si substrate. Intentional C doping was performed by introducing C₂H₂ into the reactor during the growth. In the FTIR spectra, a C related LVM mode was found out at 950 cm^-1 which was associated with A₁(LO) mode of AlN at 890cm-1. The behavior was similar to the results found in an un-intentionally Al doped GaN sample. Linear chain model with an effective mass gives the LVM energy of Al-C bond at 930 cm^-1, a little lower than the experimental observation. The C doping on the N site might be performed forming a complex with additional elements.

The development and application of nitride-based light-emitting diodes (LEDs) is hindered by the low hole conductivity of Mg-doped layers. As an alternative, polarization-induced hole doping of graded p-AlGaN layers was recently demonstrated. Using previously manufactured 440nm LEDs as device examples, this paper evaluates the effect of polarization doping by advanced numerical device simulation, both for Ga-face and N-face growth. Recently published material parameters are employed in the simulation, including new data for the Auger coefficients. The simulations reveal that Auger recombination is the main carrier loss mechanism in these devices, electron leakage seems to exert a much smaller influence on the internal quantum efficiency. The importance of internal physical mechanism is studied in detail, including the Poole-Frenkel field ionization of Mg acceptors, which is commonly held responsible for polarization doping effects. Surprisingly, we find that the field ionization inside the graded p-AlGaN layers is not stronger than in conventional electron blocking layers.

Optically-injected carrier dynamics were investigated in bulk polar and nonpolar GaN in 10¹⁵-to-10²⁰ cm^-3 carrier density range, exploring single- and two-photon photoexcitation conditions. The excitation decay and recombination rates were monitored by time-resolved photoluminescence and free-carrier absorption techniques, while diffusivity was investigated by light-diffraction on transient grating technique. Carrier dynamics in c- and m-plane thick freestanding HVPE GaN revealed nearly linear increase of carrier lifetime with temperature in the 80 - 800 K range whereas the bipolar carrier diffusivity decreased with temperature. This feature suggests that the measured long lifetime values of 40-50 ns at RT result from diffusion-governed carrier flow to interface defects at GaN hexagons, which act as centers of nonradiative recombination. The fast PL transients under carrier injection to submicrometer thick layer were fitted by using the determined diffusivity and lifetime values and revealed a strong impact of vertical carrier diffusion, surface recombination, and reabsorption processes. Radiative and nonradiative emission rates were analyzed by various optical techniques to discriminate contribution of excitons and free carriers at various temperatures and injected carrier densities.

Optical polarization properties of nonpolar quantum wells and their efficiency droop at high charge carrier densities are discussed. Therefore, a photoluminescence experiment connecting both characteristics is presented. The additional property of polarization resolution provides information about the two lowest interband transitions and the occupation of holes in the two highest valence subbands. The ratio of occupation in the two subbands is a direct projection of the Fermi-Dirac statistics. Because of the carrier dependency of the Auger losses, the quantum well internal efficiency drops in the high charge carrier regime. Here, we observe that the peak of the internal quantum efficiency of the individual subband occurs at different excitation densities as a direct consequence of the Fermi-Dirac statistics.

It is well known that hydrogen passivation of Mg in Mg-doped GaN reduces free hole concentrations. While there are numerous studies of passivation of Mg in GaN, little work has been reported concerning passivation rates in AlGaN alloys. We investigated the hydrogen interaction with Mg in nitrides by measuring the intensity of the electron paramagnetic resonance (EPR) signal associated with the acceptor. The samples were isothermally annealed in sequential steps ranging from 5 min - 6.6 h between 300 and 700 ^oC in H₂:N₂ (7%: 92%) or pure N₂. The signal intensity decreased during the H₂N₂ anneal and was revived by the N₂ anneal as expected; however, the rate at which the intensity changed was shown to depend on Al concentration. In addition, while all signals were quenched at 700 ^oC in H₂:N₂, a 750 ^oC N₂ anneal reactivated only about 30% of the Mg in the alloys and 80% of the intensity in the GaN film. These data suggest that the rate of passivation and activation of Mg by hydrogen is dependent on the concentration of Al in the Al_xGa-_1xN layer. The EPR annealing data could prove to be beneficial in improving p-type optimization in AlGaN alloys.

We characterize the transport properties of [11^-20] GaN/Ga₂O₃ nanowire (NW)-MOSFET epitaxially grown on (0001) sapphire substrates. When passivated with 10nm-thick Ga₂O₃ on the {1^-10^-1 }_GaN triangular facets, the 50 nm-dia. Ga₂O₃/GaN NW-MOSFET with 50nm gate length exhibits a saturation current of 130 μA, transconductance of 64 μS, current on/off ratio of 10⁴, subthreshold swing of 100mV/dec, and unity current (power) gain bandwidth f_T (f_MAX) at 150 (180)GHz. Using a 3D diffusion and drift model analysis, we found that the short channel effect in a L_g=50nm Ga₂O₃/GaN NW-MOSFET at an aspect ratio of 5 was suppressed due to contribution from polarization-induced negative space charge of -2.8×10¹² cm² at the abrupt crystalline interface between GaN NW and sapphire. The superior DC transport properties and good RF response can be ascribed the to polarization-induced 2D electron gas (2DEG) density of 7× 10¹² cm² with mobility of 1000cm²/V-sec confined at the semi-polar {1^-10^-1} GaN/Ga₂O₃ interfaces.

Low threshold electron emission from planar AlN/Silicon heterostructures is reported. The surface emitting ballistic electron structure consisted of an undoped AlN layer grown on Silicon by Molecular Beam Epitaxy, a Ti/Au Ohmic contact, and a thin Pt Schottky contact fabricated by e-beam deposition. Tunnel-transparent Pt Schottky contact was deposited on a 1 μm thick Silicon Dioxide (SiO₂) layer and covered a 4 x 4 matrix of 50 μm diameter via produced in the SiO₂ layer using photolithography The measurements were performed in vacuum (~10^-8 Torr) using a metal grid separated from the structure by a 60 micron thick Kapton^® polyimide film having an opening aligned with the via. Bias voltages in the range of 0-130 V were applied across the Schottky diode, while currents were recorded across the structure for grid voltages ranging from 0 to 50 V. The field emission nature of the measured currents was confirmed by plotting the Fowler-Nordheim dependence. Current density of at least 2.5x10^-4A/cm² was achieved for a grid voltage of 50 V and a bias of 130 V. Degradation of the structure performance was observed at bias voltages exceeding 90 V as a result of Schottky barrier modification under the elevated temperature and high electric field operation. The solid-state electron emitting structure indicated a threshold field as low as 0.2 V/μm under applied grid voltage of 12 V.

GaN nanowires are grown on Si(111) as templates for pendeoepitaxial coalescence overgrowth under different V/III ratios by molecular beam epitaxy. The degree of coalescence in the nanowire template increases with decreasing V/III ratio or doping with Mg. The morphology of the GaN nanowire template strongly influences that of the pendeoepitaxial layer after coalescence as well as its optical quality.

We demonstrate completely integrated tunable coupled cavity InGaN/GaN lasers with emission wavelength centered on 409 nm. Threshold currents are 650 mA per cavity for 8.7 um wide laser ridges. Experimental tuning map is explained with estimation of refractive index change due to free carrier injection and the Vernier effect. Multimode laser emission with the average full width half maximum of 0.3 nm, electronic tuning range of 1.6 nm and thermal tuning range of 2.4nm is observed.

We report the observation of lasing action from optically pumped gallium nitride nanorod arrays in a quasicrystal pattern. The nanorods were fabricated from a GaN substrate by nanoimprint patterned etching, followed by epitaxial regrowth to form crystalline facets. The imprint was a 12-fold symmetric quasicrystal pattern. The regrowth grew a multiple quantum well core-shell structure on nanorods. The cathodoluminescent emission of quantum wells red shifts from the bottom to top region of nanorod. Under optical pumping, multiple lasing peaks were observed. The lasing modes formed by 12-fold symmetric photonic quasicrystal nanorod arrays are discussed.

We investigated the concentrating properties of nitride based solar cells at light intensities of up to 200 suns at room temperature. The devices were GaInN-based solar cells with a GaInN/GaInN superlattice active layer on a freestanding GaN substrate. The conversion efficiency of these solar cells increased with increasing of concentration up to 200 suns. We obtained the solar cells with a pit-free structure and up to 3.4% conversion efficiency by irradiating concentrated sunlight with intensities of up to 200 suns. The short-circuit current density, open-circuit voltage, fill factor, and conversion efficiency were 510 mA/cm², 1.9 V, 70%, and 3.4%, respectively, under an air mass 1.5G at 200 suns and room temperature. We also discuss the relationship between crystal quality and solar cell performance.

A new device structure was investigated numerically to improve the conversion efficiency of a single junction p- InGaN/n-InGaN solar cell, where the energy band gap of the p-type top layer was increased gradually by varying the In composition during the growth. The gradual increase of the band gap generates a built in electric filed in the p-type top layer which accelerates drift motion of photo-excited electrons. Numerical results showed that more than one order of magnitude enhancement of the photo-current is achieved by the built in electric field as high as 100V/cm.

N-type GaN exhibits upward, near-surface band bending that can be decreased by generating a surface photovoltage (SPV). Fitting SPV measurements with a thermionic model based on the emission of charge carriers over the nearsurface barrier provides information about the band bending in dark. We have studied the temperature dependent SPV behavior from a Si-doped, n-type GaN sample grown by hydride vapor phase epitaxy in order to determine how the magnitude of band bending changes at higher temperatures. We have measured the effect of temperature and oxygen on the steady-state SPV behavior, where oxygen is photo-adsorbed on the surface under band-to-band illumination in an air/oxygen ambient more efficiently at higher temperatures. As predicted, the intensity-dependent SPV measurements performed at temperatures between 295 and 500 K exhibit a decrease in the maximum SPV with increasing temperature. When illumination ceases, the band bending then begins to restore to its dark value with a rate proportional to the sample temperature, which also fits a thermionic model.

Recent studies demonstrated that degradation of InGaN-based laser diodes is due to an increase in non-radiative recombination rate within the active layer of the devices, due to the generation of defects. The aim of this paper is to show - by DLTS - that the degradation of InGaN-based laser diodes is strongly correlated to the increase in the concentration of a deep level located within the active region. The activation energy of the detected deep level is 0.35-0.45 eV. Hypothesis on the nature of this deep level are presented in the paper.

Highly n-doped GaN is a material of a reduced refractive index which may substitute AlGaN as a cladding layer in InGaN laser diodes. In this study we focus on the determination of the optical absorption and the refractive index of GaN:O having the electron concentration between 1·10¹⁸ - 8·10¹⁹ cm^-3. Though the measured absorption coefficient for the highest doped GaN are rather high (200 cm^-1) we show, using an optical mode simulation, that you can design a InGaN laser diode operating in blue/green region with decent properties and low optical losses. We propose to use relatively thin AlGaN interlayer to separate plasmonic GaN from the waveguide and thus to dramatically reduce the optical losses.

We studied light-current characteristics in InGaN laser diode subjected to aging process. We observed anomalous behavior consisting in apparent increase of bimolecular recombination constant B. We proposed that the existence of a carrier escape mechanism proportional to N² can fully account for this paradox. We show that it is possible to observe cathodoluminescence contrast in the degraded laser diodes. This contrast has uniform character through all the area of device. Our laser diodes are also characterized by deep defect center lying 0.82 eV below the conduction band minimum although we don't have yet a direct evidence of the existence of this level with device degradation.

The resonator orientation of InGaN-based lasers on semipolar planes influences the optical polarization and the gain. We present gain measurements of semipolar (11-22) laser structures with differently oriented resonators and for various polarization states. The optical polarization state and the thresholds for lasers on different semipolar and nonpolar orientations are compared. The experimental results are accompanied by numerical calculations of the material gain as well as investigation of the surface morphology and resulting waveguide losses in dependence of the crystal orientation.

The laser threshold and lateral mode confinement of blue (440 nm) InGaN multiple quantum well (MQW) laser diodes have been investigated. Ridge-waveguide (RW) laser diodes with different ridge etch depth ranging from 25 nm above the active region (deep-ridge waveguide) to 200 nm above the active region (shallow-ridge waveguide) have been fabricated. The comparison of devices with the same resonator length shows that the threshold current densities are significantly lower for deep-ridge waveguide laser diodes. The difference in lasing threshold becomes more eminent for narrow ridges, which are required for single mode operation. For shallow-ridge devices the threshold current density increases by more than a factor of three when the ridge width is decreased from 20μm to 1.5μm. For the deep-ridge waveguide devices instead, the lasing threshold is almost independent of the ridge waveguide width. The effect has been analyzed by 2D self-consistent electro-optical simulations. For deep-ridge devices, the simulated thresholds and far-field patterns are in good agreement with the simulations. For shallow-ridge devices, however, questionable theoretical assumptions are needed. Two possible causes are discussed: extremely large current spreading and strong index anti-guiding.

This work shows the process of computing coupling coefficients of first-order distributed feedback (DFB) metalsemiconductor quantum-well lasers which emit the violet light for data storage of information technology. The optical waveguide structure for such a laser has semiconductor layers and a built-in metal grating layer. The interface between the metal layer and its neighboring semiconductor layer has a sinusoidal corrugation functioning as the grating. To compute the coupling coefficient of the metal-grating waveguide, a model is constructed by Floquet-Bloch formalism (FB). Ray optics technique (RO) is also used to calculate the coupling coefficients. These two methods have close results.

We have grown LED structures on top of a robust n-type GaN template on 8-inch diameter silicon substrates achieving both a low dislocation density and a 7 um-thick template without crack even at a sufficient Si doping condition. Such high crystalline quality of n-GaN templates on Si were obtained by optimizing combination of stress compensation layers and dislocation reduction layers. Wafer bowing of LED structures were well controlled and measured below 20 μm and the warpage of LED on Si substrate was found to strongly depend on initial bowing of 8-inch Si substrates. The full-width at half-maximum (FWHM) values of GaN (0002) and (10-12) ω-rocking curves of LED samples grown on 8-inch Si substrates were 220 and 320 arcsec. The difference between minimum and maximum of FWHM GaN (0002) was 40 arcsec. The dislocation densities were measured about 2~3×10⁸/cm² by atomic force microscopy (AFM) after in-situ SiH4 and NH₃ treatment. The measured quasi internal quantum efficiency of 8-inch InGaN/GaN LED was ~ 90 % with excitation power and temperature-dependent photoluminescence method. Under the un-encapsulated measurement condition of vertical InGaN/GaN LED grown on 8-inch Si substrate, the overall output power of the 1.4×1.4 mm² chips representing a median performance exceeded 484 mW with the forward voltage of 3.2 V at the driving current of 350 mA.

The MOCVD growths and device characteristics of 500-nm emitting InGaN quantum well (QW) light-emitting diodes (LEDs) with the insertion of thin (~1 nm) AlInN barrier layers were investigated for efficiency droop suppression. Preliminary device characteristics of InGaN QW LEDs with thin AlInN barrier layers were also presented.

The effect of active layer design on the efficiency of InGaN light emitting diodes (LEDs) with the light emission in blue (~420 nm) has been studied. Correlation between the internal quantum efficiency (IQE) and relative external quantum efficiency (EQE) and salient features of structures on c-plane InGaN LEDs which contain multiple quantum wells (MQWs) of different barrier height (either In_0.01Ga_0.99N or In_0.06Ga_0.94N barriers) and thickness (3 nm and 12 nm) as well as different double heterostructure (DH) designs (3 nm, dual 3 nm, 6 nm, dual 6 nm, 9 nm and 11 nm) with inserted 3 nm In_0.06Ga_0.94N barrier. Pulsed electroluminescence (EL) and optical excitation power-dependent photoluminescence (PL) measurements indicated that the thinner and lower In_0.06Ga_0.94N barriers bode well for high EQE and IQE. Furthermore, increase of the effective active region thickness by multiple InGaN DH structures (dual, quad and hex) separated by 3 nm In_0.06Ga_0.94N barriers is promising at high injection levels. Although increasing the single DH thickness from 3 to 6 nm improves the peak relative EQE by nearly 3.6 times due to increased density of states and increased emitting volume, the IQE suffers a nearly 30% loss. Further increase in the DH thickness to 9 and 11 nm results in a significantly slower rate of increase of EQE with current injection and lower peak EQE values presumably due to degradation of the InGaN layer. Increasing the number of 3 nm DH active regions with 3 nm In_0.06Ga_0.94N barriers improves EQE, while still maintaining high IQE (above 95% at a carrier concentration of 10¹⁸ cm^-3) and showing negligible EQE degradation up to 550 A/cm² due to increased emitting volume and high radiative recombination coefficients and high IQE.

The characteristics of high voltage LED consisted of an array of 64 micro-cells GaN LEDs was investigated through using different substrate. In this study, two kinds of high voltage LEDs are presented; one is grown on sapphire substrate and the other one is on mirror/Si substrate. The output power of high voltage LEDs with sapphire and mirror/Si substrate is 170 and 216 mW at an injection current of 24 mA, respectively. The LEDs on mirror/Si substrate leads to the superior performance in output power as compared with one on sapphire substrate is attributed to the improvement of thermal dissipation and light extraction.

In this paper, we introduced the advantages of Vertical Light emitting diode (VLED) on copper alloy with Si-wafer level packaging technologies. The silicon-based packaging substrate starts with a <100> dou-ble-side polished p-type silicon wafer, then anisotropic wet etching technology is done to construct the re-flector depression and micro through-holes on the silicon substrate. The operating voltage, at a typical cur-rent of 350 milli-ampere (mA), is 3.2V. The operation voltage is less than 3.7V under higher current driving conditions of 1A. The VLED chip on Si package has excellent heat dissipation and can be operated at high currents up to 1A without efficiency degradation. The typical spatial radiation pattern emits a uniform light lambertian distribution from -65° to 65° which can be easily fit for secondary optics. The correlated color temperature (CCT) has only 5% variation for daylight and less than 2% variation for warm white, when the junction temperature is increased from 25°C to 110°C, suggesting a stable CCT during operation for general lighting application. Coupled with aspheric lens and micro lens array in a wafer level process, it has almost the same light distribution intensity for special secondary optics lighting applications. In addition, the ul-tra-violet (UV) VLED, featuring a silicon substrate and hard glass cover, manufactured by wafer level pack-aging emits high power UV wavelengths appropriate for curing, currency, document verification, tanning, medical, and sterilization applications.

High performance 375-nm ultraviolet (UV) InGaN/AlGaN light-emitting diodes (LEDs) was developed using a heavy Si-doping technique with metalorganic chemical vapor deposition (MOCVD). From the transmission electron microcopy (TEM) image, the dislocation density was reduced after inserting a heavily Si-doping growth mode transition layer (GMTL) between un-doped GaN layer and Si-doped Al_0.02Ga_0.98N contact layer. The internal quantum efficiency (IQE) of the sample with GMTL measured by power-dependent photoluminescence shows 39.4% improved compared with the sample without GMTL. When the vertical type LED chips (size: 1mm×1mm) driving by a 350-mA current, the output powers of the LEDs with and without GMTL were measured to be 286.7 mW and 204.2 mW, respectively. As much as 40.4% increased light output power was achieved. Therefore, using the GMTL to reduce dislocation defects would be a promising prospective for InGaN/AlGaN UV LEDs to achieve high internal quantum efficiency.

Based on an analysis on the chemical bonding energy of a luminescence center and the thermal stability of luminescence emission, we describe a design idea of high performance phosphor materials. It is to incorporate a superhard chemical bond, Si-C, into the host crystals. Two families of phosphor formulations are designed and prepared through high temperature solid state reaction. The structure determination results of the phosphors demonstrated the incorporation of the Si-C bonds in the crystal. The effect of the Si-C bond in the formulations is evident on luminescence and optical spectra. The thermal quenching profiles of the phosphors show enhanced thermal stability of the luminescence emission as the carbon content is increased.

This paper reviews the recent progress towards III-nitride intersubband devices based on quantum wells. We first present recent achievements in terms of GaN-based quantum cascade detectors operating at near-infrared wavelengths. We show that these devices are intrinsically extremely fast based on femtosecond time-resolved measurements of the photocurrent. The design of III-nitride quantum cascade detectors, which relies on the engineering of the internal electric field, is flexible enough to allow for two-color detection. We finally discuss the potential of III-nitride intersubband devices in the THz frequency domain and present the recent observation of THz absorption using low aluminium content AlGaN/GaN step quantum wells.

III-Nitride semiconductors are promising nonlinear materials for optical wavelength conversion. However second harmonic generation in bulk GaN is weak because GaN is strongly dispersive. We show that appropriate photonic crystal patterning in GaN helps to overcome dispersion and provides quasi-phase matching conditions, resulting in substantially increased conversion efficiency obtained in a flexible manner. Enhancement factors of more than five orders of magnitude can be achieved. Use of photonic crystals makes it possible to reduce the effective observation volume, thereby opening new opportunities such as the study of single-molecule dynamics, even in high concentration solutions. We have demonstrated sharp enhancement of the fluorescence of single molecules immobilized on the surface of a GaN photonic crysta,l when the molecules are excited via the resonant second harmonic generation process.

We apply a number of all-optical time-resolved techniques to study the dynamics of free carriers in InGaN quantum structures under high excitation regime. We demonstrate that carrier lifetime and diffusion coefficient both exhibit a substantial dependence on excitation energy fluence: with increasing carrier density, carrier lifetime drops and diffusivity increases; these effects become more apparent in the samples with higher indium content. We discuss these experimental facts within a model of diffusion-enhanced recombination, which is the result of strong carrier localization in InGaN. The latter model suggests that the rate of non-radiative recombination increases with excitation, which can explain the droop effect in InGaN. We use the ABC rate equation model to fit light induced transient grating (LITG) kinetics and show that that linear carrier lifetime drops with excitation (i.e. excess carrier density). We do not observe any influence of Auger recombination term, CN³, up to the maximum carrier density that is limited due to the onset of very fast stimulated recombination process. To support these conclusions, we present spectrally resolved differential transmission data revealing different recombination rates of carriers in localized and extended states.

We investigate theoretically the influence of indium surface segregation in InGaN/GaN single quantum wells on its optical properties. Obtained results show that the influence of the surface segregation on the dipole matrix element is not equal for all optical transition. This effect results from the joint action of the piezoelectric polarization and indium surface segregation which change selection rules. Quantum well structures having different indium amount are analyzed and found that the influence of the indium surface segregation on absorption spectra is more pronounced in quantum well structures with high indium amount.

We report on the electrical stress results in GaN-based heterostructure field-effect transistors (HFETs) with InAlN barriers. We monitored the DC characteristics and low-frequency phase noise behavior for the devices at pre- and poststress conditions for five different wafers with In compositions varying from 12% to 20% in the barriers of the structures. The devices were stressed under off-state conditions with a gate bias of -10V (pinch-off condition) and zero drain bias for 10hr. From the acquired data we observed that at higher In composition, HFETs became less sensitive to the stress. At lower In composition we noted up to 30 dBc/Hz higher low frequency noise for stressed devices over the entire frequency range of 1 Hz-100 kHz. The change in drain current and change in noise power due to electrical stress decrease as the In composition in the barriers of the HFETs increases. The most relevant stress effect is revealed by a drain current reduction which is consistent with higher noise level measured. It was found that the HFET degradation is minimum for nearly lattice matched condition InAlN barriers, i.e.; 17% In composition, at which the sheet electron density (channel current) is comparable with that in lower In composition (12% In). This latter result is promising for power applications in which reliability of devices functioning at higher drain current is crucial. The results may also be beneficial to decouple the effect of off-state stress from the hot electron and self heating effects.

AlGaN/GaN based FETs have great potential as sensitive and fast operating detectors because of their material advantages such as high breakdown voltage, high electron mobility, and high saturation velocity. These advantages could be exploited for resonant and non-resonant terahertz detection. We have designed, fabricated, and characterized AlGaN/GaN based FETs as single pixel terahertz detectors. This work focuses on non-resonant detection and imaging using GaN field plate FETs. To evaluate their performances as terahertz detectors, we have measured the responsivity as a function of gate voltage, the azimuthal angle between the terahertz electric field, the source-to-drain direction, and the temperature. A simple analytical model of the response is developed. It is based on plasma density perturbation in the transistor channel by the incoming terahertz radiation. The model shows how the non-resonant detection signal is related to static (dc) transistor characteristics and it fully describes the experimental results on the non-resonant sub-terahertz detection by the AlGaN/GaN based FETs. The imaging performances are evaluated by scanning objects in transmission mode and an example of application of terahertz imaging as new non-destructive technique for the quality control of materials is given. Results indicate that these FETs can be considered as promising devices for terahertz detection and imaging applications.

Low-frequency noise and current-transient measurements were applied to analyze the degradation of nearly latticematched InAlN/AlN/GaN heterostructure field-effect transistors caused by electrical stress. Almost identical devices on the same wafer were stresses 7 hr. at a fixed DC drain bias of V_DS=20 V and different gate biases. We noted up to 32 dB/Hz higher low-frequency noise for stressed devices over the entire frequency range of 1 Hz- 100 kHz. The measurements showed the minimum degradation at a gate-controlled two-dimensional electron gas density of 9.4x10¹² cm^-2. This result is in good agreement with the reported stress effect on drain-current degradation and current-gain-cutoff-frequency measurements, and consistent with the ultrafast decay of hot-phonons due to the phonon-plasmon coupling. Moreover, the current transient (gate-lag) measurements were also performed on pristine and highly degraded devices up to 5 ms pulse durations. Drain current is almost totally lost in degraded HFETs as opposed to a very small drop for pristine devices and no recovery observed for both indicating that generation of deep traps in GaN buffer.

Non-polar (a-plane) InGaN/GaN multiple quantum wells (MQWs) on the GaN nanorod epitaxially lateral overgrowth templates with different nanorod height have been fabricated. The average in-plane strain in the InGaN MQWs has been determined from 2.73×10^-2 to 2.58×10^-2 while the nanorod height in templates increases from 0 to 1.7 μm. The polarization ratio of the emission from InGaN MQWs varies from 85 % to 53 % along with the increase of the GaN nanorod height. The reduction of polarization ratio has been attributed to the partial strain relaxation within the epitaxial structures as a result of growth on the GaN nanorod templates and the micro-size air-voids observed in the nanorod templates.

A-plane free-standing GaN was grown on a-plane GaN templates by HVPE. A-plane GaN templates were grown on r-plane sapphire by MOCVD with multilayer high-low-high temperature AlN buffer layers. A regrowth method was used for growing GaN through HVPE. First, GaN was grown on a-plane GaN templates, followed by separating the a-plane GaN film from r-plane sapphire using LLO. Then, the GaN films were regrown using HVPE. The resulting free-standing GaN contained some voids, which causes to release the stress.

High performance 375 nm ultraviolet (UV) InGaN/AlGaN light-emitting diodes (LEDs) with a heavy Si-doped GaN growth mode transition layer (GMTL) were fabricated by metal-organic chemical vapor deposition (MOCVD). From transmission electron microcopy (TEM) image, the dislocation densities are reduced significantly by using the GMTL technique. The threading dislocation (TD) value of AlGaN grown on GMTL was significantly decreased from the control sample value of 8×10⁸ to 8×10⁷ cm^-2. Furthermore, the internal quantum efficiency (IQE) of the LEDs with GMTL was measured by power-dependent photoluminescence (PL) to be 40.6% higher than ones without GMTL. After vertical-type (size:1mm×1mm) LED chips were fabricated, the output power were measured by integrating sphere detector under 350 mA injection current driving. The output powers of the LEDs with and without GMTL were measured to be 286.7 and 204.2 mW, respectively. As much as 40.4% increased light output power was achieved. The GMTL leads to the superior IQE performance of the LEDs not only in decreasing the carrier consumption at nonradiative recombination centers but also in partially mitigating the efficiency droop tendency. Therefore, forming the GMTL between un-doped GaN and n-AlGaN to reduce dislocations would be a promising prospective for InGaN/AlGaN UV-LEDs to achieve high IQE.n the abstract two lines below author names and addresses.

In this study, we fabricated and compared the performance of LEDs of InGaN-based UV MQWs active region with ternary AlGaN and quaternary InAlGaN barrier layers. HRXRD and TEM measurements show the two barriers are consistent with the lattice, and smooth morphology of quaternary InAlGaN layer can be observed in AFM. The electroluminescence results indicate that the light performance of the InGaN-based UV LEDs can be enhanced effectively when the conventional LT AlGaN barrier layers are replaced by the InAlGaN barrier layers. Furthermore, simulation results show that InGaN-based UV LEDs with quaternary InAlGaN barrier exhibit higher radiative recombination rate about 62% and low efficiency droop about 13% at a high injection current. We attribute this change to a drastic improvement from increasing of carrier concentration and redistribution of carriers, because of reduction of scatterings due to better morphology in the transverse carrier transport through the InGaN/InAlGaN MQWs.

Semipolar (1^-101) GaN layers were grown by metal-organic chemical vapor deposition on patterned (001) Si substrates. The effects of reactor pressure and substrate temperature on optical properties of (1^-101) GaN were studied by steadystate and time-resolved photoluminescence. The optical measurements revealed that the optical quality of (1^-101)- oriented GaN is comparable to that of c-plane GaN film grown on sapphire. Slow decay time constants, representative of the radiative recombination, for semipolar (1-101)GaN grown at 200 Torr are found to be very long (~1.8 ns), comparable to those for the state-of-art c-plane GaN templates grown using in situ epitaxial lateral overgrowth through silicon nitride nano-network. Defect distribution in the GaN stripes was studied by spatially resolved cathodeluminescence measurements. The c⁺-wing regions of the GaN stripes were found to be dominated by a (D⁰,X) emission. Only a thin slice of emission around 3.42 eV related to basal stacking faults was revealed in c^--wing regions.

Degradation of InAlN/GaN based HFETs under stress for four bias conditions, namely, on-state high field stress (hot phonon, hot electron and self heating effect), off-state high field stress (hot electron effect), onstate low field stress (self heating effect), and reverse gate bias stress (inverse piezoelectric effect) has been examined. The degradation is characterized by monitoring electrical properties, such as, drain current reduction, gate lag, and low frequency noise. On-state high field stress has shown more than 50% reduction in the drain current and approximately 25-30 dBc/Hz increase in low frequency noise after 25 hours of stress, while other stress conditions led to much lesser degradation. It is demonstrated that the major degradation mechanism in InAlN/GaN HFETs is the hot-phonon and hot-electron effect in the realm of short term effects.

We report on the effects of metal organic epitaxy grown GaN templates with different surface morphologies, achieved under different chamber pressures of 200 and 400 Torr, on the electrical properties of GZO. For as-grown GZO layers with electron concentration above 10²⁰ cm^-3 grown on either 200-Torr p-GaN or 400-Torr p-GaN templates, the electron concentration is temperature-dependent as opposed to temperature-independence for GZO/a-sapphires, which demonstrates that the underlying GaN layers affect the GZO electrical properties measured by Hall method. By annealing in nitrogen environment or by inserting a thick ZnO buffer layer, the effects of the underlying GaN layers on GZO electrical properties can be eliminated paving the way for accurate determination of electrical properties. All three annealed GZO layers grown on 200-Torr p-GaN, 400-Torr p-GaN, and a-sapphire, exhibited comparable electron mobilities (~50 cm²/V·s at 15 K and ~41 cm²/V·s at 300 K) and similar temperature dependences while their electron concentrations are different (5.1×10²⁰, 7.1×10²⁰, and 9.2×10²⁰ cm^-3) due to the substrate-caused differences in GZO growth mode, structure, etc. By means of simulations, ionized impurity scattering was found to be the dominant scattering mechanism in the range of 15-330 K for GZO when electron concentration is higher than 5×10²⁰ cm^-3. Although other scattering events caused by defects and structures are weaker than the ionized impurity scattering, the electrical properties could be still slightly improved by finding more optimized growth conditions to eliminate defects and/or to improve crystal quality.

Polarization-induced electric fields in AlGaN quantum wells have important effects on avalanche breakdown of AlGaN quantum-well photodiodes. When the polarization-induced fields within the AlGaN well layers have the same direction as applied electric field, they can help enhance impact ionization rate and decrease threshold voltage of avalanche breakdown of AlGaN avalanche photodiodes. However, according to previous research on avalanche breakdown of AlGaN photodiodes, no distinct breakdown threshold was observed from current-voltage curve. Instead, a soft avalanche breakdown was observed across applied voltage ranging from zero to a few volts while electroluminescence spectra show a threshold of about 10 V for avalanche breakdown. In this work, by considering impact ionization of defect levels and carrier screening effect, impact ionization coefficients are calculated as functions of applied voltage and the soft breakdown is well explained. It is also found that strong carrier screening effect will decrease impact ionization rate in a certain range of voltage thus affecting device performance.