We have studied the structural and optical properties of a series of (In,Ga)/GaN multiple quantum wells with identical thicknesses but varied In content grown by plasma-assisted molecular beam epitaxy. Careful choice of the growth parameters returns samples with smooth and abrupt interfaces. The shift of the photoluminescence transition energy with externally applied biaxial tension was investigated. We observed a red-shift for small In contents while a blue-shift was detected for higher In contents. This result is in qualitative agreement with band profile calculations taking into account both the band gap deformation potentials and the piezoelectric polarization in these structures. However, the magnitude of the shift is well in excess of the calculated one. We attribute this finding to a substantial deviation of the piezoelectric constants from those calculated for unstrained material. Finally, we estimate the piezoelectric polarization of InGaN/GaN for linear and non-linear terms in strain.

The lateral growth of high quality InN on the stripe-pattern GaN/sapphire substrate with an OMVPE system was studied. The surface morphology and structural properties were investigated. Epitaxial films were achieved due to the greatly reduced strain by lateral growth. Two kinds of growth mode were observed due to different growth conditions of V/III ratios. X-ray rocking curve with FWHM of 700 arcsec shows the good quality of the film and E2 mode of Raman spectrum with FWHM of 3.5 cm-1 is among the best results ever reported in the literature.

The optical properties of Carbon doped cubic GaN epilayers have been investigated by temperature and intensity dependent photoluminescence measurements. RF-plasma assisted molecular beam epitaxy equipped with an e-beam-evaporation source for carbon doping is used to grow the cubic GaN layers on GaAs (001) substrates. With increasing Carbon flux a new photoluminescence line at 3.08 eV appeared at 2K. This line is attributed to a donor acceptor transistion, which involves the shallow CN acceptor. From the spectral position the binding energy of the C acceptor is estimated to be about EC = 0.215 eV. Our experiments demonstrate that C indeed introduces a shallow acceptor in cubic GaN with an acceptor binding energy, which is about 15 meV lower than that observed for the Mg acceptor in cubic GaN. However, at high C fluxes a deep red luminescence band appeared at 2.1 eV, indicating compensation effects.

We report on the non-invasive measurement of temperature, i.e., self-heating effects, in active AlGaN/GaN heterostructure field effect transistors (HFETs). Micro-Raman spectroscopy was used to produce temperature maps with ≈1 μm spatial resolution and a temperature accuracy of better than 10°C. Significant self-heating effects in the source-drain opening of AlGaN/GaN HFETs were measured. Devices grown on sapphire and SiC substrates were compared. Three-dimensional finite-difference heat dissipation calculations were performed as function of device geometry.

Mg-doped quaternary InAlGaN is very attractive for use as p-side layers of 300-nm band ultraviolet (UV) light-emitting diodes (LEDs) or laser diodes (LDs), because high hole conductivity is expected to obtain for wide bandgap (~4 eV) InAlGaN with Mg-doping. We fabricated p-n junction diode consisting of Mg-doped In0.02Al0.28Ga0.70N and Si-doped Al0.25Ga0.75N, and demonstrated intense UV emission under CW current injection at room temperature. The rising voltage in I-V curve was around 3.8 V and the breakdown voltage was as high as 10 V. Single peaked intense emission was observed at 340 nm from around InAlGaN/AlGaN p-n junction area without any deep level emission. Also we found that Ni/Au electrode directly fabricated on Mg-doped InAlGaN is useful. From these results, Mg-doped InAlGaN is considered to be very attractive for use as p-side layer of UV-LEDs or LDs.

This paper addresses issues with the manufacturability of AlGaN/GaN FET structures. A robust Metalorganic Chemical Vapor Deposition growth process has been developed that will now allow reliability measurements to be obtained on the resulting devices. During a small scale production run mobilities in excess of 1600 cm2/V.s, sheet charge (Ns) between 0.8x1013 and 1.2x1013 cm-2, and Rs<400 Ω/square with less than 2% variation across the wafer and less than 0.5% variation from wafer to wafer were obtained. Issues for producing a manufacturable process on sapphire and semi-insulating SiC substrates using in-situ monitoring will be addressed. The equivalence of growth on sapphire and SiC substrates for process optimization will be shown.

Gallium nitride films are grown by plasma-assisted molecular beam epitaxy (MBE) on vicinal 6H-SiC(0001) substrates with [1 1 00] and [11 2 0] miscut directions. The hydrogen-etched substrates display straight, or chevron shaped steps respectively, and the same morphology is observed on the GaN films. X-ray rocking curves display substantially reduced width for films on the vicinal substrates compared to singular substrates, for the same Ga/N flux ratio used during growth.

We show that the strong bowing of the bandgap of GaInN, which is primarily due to bowing of the valence band edge, translates into a strongly composition dependent ratio of the conduction band offset to the valence band offset with respect to GaN. For common In mole fractions of 0-20 % this leads to a reversal of the band offset ratio and to very weak electron con nement. This theoretical picture is veri ed by comparing results of time-resolved spectroscopy on asymmetric AlGaN/GaInN/GaN and AlGaN/GaN/AlGaN quantum wells. Since electron con nement is much stronger for GaN/AlGaN wells than for GaInN/GaN wells, the effect of asymmetry is very weak for the former and fairly strong for the latter.

Infrared (IR) spectroscopy and ultraviolet near-resonance enhanced Raman scattering have been used as alternative techniques to characterize the AlN films grown on Si(111). Epitaxial AlN films have been prepared by plasma source molecular beam epitaxy (PSMBE) at growth temperatures of 400 and 650 oC. The AlN/Si grown at 650°C shows distinct reflection high-energy electron diffraction (RHEED) patterns with the following epitaxial relations: AlN [0001] ‖ Si [111], and AlN < 011 0 > ‖ Si < 112 >. This is in sharp contrast with the observation of a spotty RHEED pattern for AlN/Si sample grown at 400°C, which appeared the same when viewed along any Si azimuth. Furthermore, the X-ray diffraction (XRD) rocking curve width showed a significant reduction from 5.7ofor sample grown at 400°C to 2° for the one grown at 650°C. The observation of Raman active A1(LO) and E2, and IR active E1(TO) zone center phonons, in both the films, is in accordance with the c-axis orientation of the films. However, the observed phonon mode frequencies and line widths indicate that the AlN samples grown at 400°C have less strain but more disorder compared to the ones grown at 650°C in agreement with RHEED and XRD data.

We have investigated Coplanar Waveguide (CPW) elements on AlN for use in future AlGaN/GaN based power amplifiers. This technology becomes crucial if a via-hole technology is not available. Lines, discontinuities, metal-insulator-metal (MIM) capacitors and resistors were measured and modelled. These elements are embedded between two adaptors for RF probing. A technique was developed to de-embed the adaptors from the overall measurement and hence correctly determine the properties of the element itself. Measurements on elements containing multiple ports with right angles can best be carried out using standard calibration techniques followed by carefully reorienting the probes. It is shown that for accurate design of matching networks operating at 10 GHz each element has to be carefully modelled. The method presented in this paper can be a useful contribution tackling some of the problems related to the design of these networks.

GaN metal oxide semiconductor diodes were demonstrated utilizing Sc2O3 as the gate oxide. Sc2O3 was grown at 100°C on MOCVD grown n-GaN layers in a molecular beam epitaxy (MBE) system, using a scandium elemental source and an Electron Cyclotron Resonance (ECR) oxygen plasma. Ar/Cl2 based discharges was used to remove Sc2O3, in order to expose the underlying n-GaN for ohmic metal deposition in an Inductively Coupled Plasma system. Electron beam deposited Ti/Al/Pt/Au and Pt/Au were utilized as ohmic and gate metallizations, respectively. An interface trap density of 5 × 1011 eV-1cm-2was obtained with the Terman method. Conductance-voltage measurements were also used to estimate the interface trap density and a slightly higher number was obtained as compared to the Terman method. Results of capacitance measurements at elevated temperature (up to 300°C) indicated the presence of deep states near the interface.

High-power AlGaInN-based laser diodes (LDs) operating with high reliability in the 400-nm band have been successfully fabricated using a high-productivity process. Epitaxial lateral overgrowth (ELO) over a 10-m m region was employed to obtain a broad growth area with low dislocation density, and the thickness of the ELO-GaN layer was limited to approximately 5 m in order to minimize wafer bending. These techniques allow for the easy and reproducible alignment of the laser stripe on the region of low dislocation density. The insertion of a GaInN interlayer between the active layer and the AlGaN electron blocking layer was effective for reducing the strain between these two layers, resulting in homogeneous luminescence from the active layer and lower operating current. A mean time to failure of 15000 h under 30-mW continuous-wave operation at 60°C was realized as a direct result of the lower operating current. Productivity was remarkably improved by performing epitaxial growth on a 3-inch substrate. Highly uniform laser wafers were successfully fabricated by achieving minimal temperature variation (1000 ±7°C) over the 3-inch substrate. The resultant laser structures varied in thickness by only ±5%, and the photoluminescence wavelength was consistent within ±2.5 nm over the entire 3-inch substrate. The average threshold current of 550 LDs selected from a fourth wafer was 32.7 mA, with small standard deviation of 3.2 mA.

Non-uniformity in GaN thin films deposited on 6H-SiC can make determining the effects of growth variables difficult. Results presented in this work show the effects of the SiC substrates on the GaN films, and how to correct for these effects to obtain meaningful data about the properties of the thin film rather than the substrate underneath. Rocking curve values of GaN thin films are found to track almost 1:1 with the values of the underlying SiC. Plotting rocking curves with respect to the substrate, as well as a variable of importance can therefore yield more meaningful and reliable comparisons instead of plotting the data for the variable alone. This procedure is used to demonstrate the effects of thickness and AlN and AlGaN buffer layers on GaN thin films.

We report results of an investigation of electric field induced heating at low temperature in GaN 3-dimensional electron gas films grown on sapphire substrates. The excess noise of the electron gas in a patterned GaN film, while the substrate is held at low temperature, is used to determine the electron temperature. We calculate the rate of power dissipation and compare our results with a calculation of acoustic deformation potential scattering processes in GaN. We discuss the existence of a thermal boundary resistance between the GaN film and the sapphire substrate.

Two-electron transition occurs when the exciton bound to a neutral donor (DBE) recombines and leaves the donor in an excited state. The two-electron energy is therefore lower than that of the DBE peak by the difference in ground and excited state of the neutral donor. In a magnetic field the two-electron satellite splits into several components. These intra-donor excitations have been studied in homoepitaxial GaN up to magnetic fields reaching 23T. For Faraday (B‖c) configuration the two-electron transition splits mainly into 2s, 2p0, 2p+ and 2p- components. The total splitting between 2p+ and 2p- is equal to Landau energy. For Voigt (B???c) configuration in addition to transition to 2s, 2p0, 2p- and 2p+ there are additional lines which origin is discussed. It has been found that for two configurations of magnetic field the separation between 2p+ and 2p- is not exactly equal, what indicates the anisotropy of the electron effective mass. It has been found that m| = 0.205m0 and m??? = 0.225m0.

It has been predicted that amorphous GaN has a low density of states in the gap, and therefore has potential as a useful opto-electronic material in the blue-green spectral region. We have synthesised amorphous GaN films on various substrates by ion assisted deposition and investigated the effects of sample preparation conditions on the conducting and optical properties. The room temperature resistivity ρ0 of stoichiometric (Ga:N of 1:1) films is above 105 Ω cm, and these films exhibit a complex form for the temperature dependence of the resistivity. Films having an excess of Ga show a much lower ρ0. The optical absorption shows ar 0 band-gap of 3 eV, with the gap falling below that value when the amorphous network incorporates homopolar (Ga-Ga) bonds. The best films are thus transparent across the visible region with a low density of gap states, undetectable in optical absorption. The photoluminescence spectra obtained from these a-GaN films consist of a broad green light emission peaking at 528 nm. Preliminary photoconductivity measurements show sensitivity in the UV.

We studied photoluminescence (PL) and PL excitation (PLE) spectra in a large number of undoped GaN layers grown on sapphire by molecular beam epitaxy (MBE), metal-organic chemical vapor deposition (MOCVD) and hydride vapor phase epitaxy (HVPE). The HVPE-grown GaN layers with thickness of ~200 m m were separated from the sapphire substrate by laser lift-off and represented bulk freestanding templates of very high quality. Identical position and shape of the YL band were reproduced in many samples grown by MBE and MOCVD: maximum at ~2.23 eV and full width at half maximum (FWHM) of about 460 meV at room temperature. However, in some samples the band maximum was observed at about 2.0 eV. The freestanding templates reveal a broad band (FWHM=530-680 meV) whose position depends on excitation energy and intensity, varying from 2.22 eV to 2.47 eV. PLE spectra taken from various samples represented a broad band with apparent maximum at about 3.3 eV. For below-gap excitation, the intensity of the YL band was independent of temperature except for the one in the freestanding template. The latter was temperature independent above 60 K, however at lower temperatures the PL intensity decreased by 5 times. An activation energy of 15 meV has been determined that is related to a barrier in the adiabatic potential in the excited state of the defect.

Characterization of Schottky barrier UV detectors with a comb-shaped electrode and a transparent electrode for VUV region using synchrotron radiation was carried out. These Schottky type detectors are effective to operate in UV and VUV light (50<l >360 nm). In particular, the latter realizes high responsivity (0.01 A/W for VUV light) and no photoemission of Au and GaN. The penetration depth for VUV light was estimated to 0.01 m. So the VUV light was absorbed in near the surface of i-GaN layer or the interface of Au and i-GaN layer.

Recent interest in intersubband (IS) transitions in semiconductor heterostructures with large band offset has been fueled by attempts to extend the wavelength range of IS-based optical devices to the fiber-optics wavelength range around ~ 1.55 μm. GaN/AlGaN-based heterostructures are of particular interest due to their large effective electron mass and large longitudinal optical phonon energy. Both are essential to achieve ultrafast electron relaxation at large transition energies. IS absorption in GaN/AlGaN single and coupled double quantum wells (DQWs) has been measured. The samples were grown by molecular beam epitaxy on sapphire substrate and with a large (0.65 or 0.9) AlN-mole fraction in the barriers. Peak absorption wavelengths as short as 1.35 μm and 1.52 μm were measured for a symmetric DQW of 12 Å wide wells coupled by a 10 Å wide barrier, which also showed evidence of excited-state anti-crossing. As expected, asymmetric DQWs displayed no such anti-crossing, and the ground-state anti-crossing energies were found to be much smaller – as a result of the comparatively large effective electron mass – than the energy broadening of individual transitions. The asymmetric DQWs displayed peak absorption wavelengths between 1.5 and 2.9 μm. The electron relaxation time, attributed to longitudinal optical phonon scattering has been measured by pump-probe technique as 240 fs for a coupled DQW sample.