Evidence is presented that nitrogen plasma sources utilizing a pyrolytic boron nitride liner may be a significant source of B contamination during growth and processing. Auger electron spectroscopy analysis performed during nitridation of sapphire indicate the resulting layers contain a significant amount of BN. The formation of Al1−xBxN would explain the observation of a lattice constant several percent smaller than AlN as measured by reflection high-energy electron diffraction. The presence of cubic inclusions in layers grown on such a surface may be related to the segregation of BN during the nitridation into its cubic phase.

Commercial light emitting devices (LEDs) containing InGaN layers offer unrivalled performance in the violet (∼400 nm), blue (∼450 nm) and green (∼520 nm) spectral regions. Nichia Chemicals Company has also produced amber InGaN LEDs with peak output near 590 nm. Here, we predict, on purely theoretical grounds, a surprisingly high limiting value of 1020 nm (peak) for InGaN intrinsic emission. We partly confirm this prediction by spectroscopic measurements of samples with photoluminescence (PL) peaks between 370 nm and 980 nm. In addition, we have measured the indium content of a range of light-emitting layers, using Rutherford Backscattering Spectrometry (RBS), Extended X-Ray Absorption Fine Structure (EXAFS) and Energy Dispersive X-Ray Analysis (EDX). The PL peak energy is found to depend linearly on the indium fraction: violet-emitting layers have an indium content of ∼8%, blue layers ∼16% and green layers ∼25%. A linear extrapolation to the limit set by the Stokes' shift prediction, mentioned earlier, yields a limiting indium concentration of only ∼52%. The profound impact of these results on future extensions of nitride technology and current theoretical models of InGaN is briefly discussed.

Recent advances in the processing of complex-oxide materials has allowed us to monolithically grow ferroelectrics of lead lanthanum zirconate titanate (PLZT) and barium strontium titanate (BST) systems on a GaN/sapphire structure. High quality films of PLZT and BST were grown on GaN/c-Al2O3 in a thickness range of 0.3-5 [.proportional]m by a solgel technique. Field-induced birefringence, as large as 0.02, was measured from a PLZT layer grown on a buffered GaN/sapphire structure. UV illumination was found to result in more symmetrical electrooptic hysteresis loop. BST films on GaN demonstrated a low frequency dielectric constant of up to 800 with leakage current density as low as 5.5.10−8 A/cm2.

We report on the fabrication of device-quality AlN heterostructures grown on SiC for high-temperature electronic devices. The AlN films were grown by pulsed laser deposition (PLD) at substrate temperatures ranging from 25 °C (room temperature) to 1000 °C. The as-grown films were investigated using x-ray diffraction, Rutherford backscattering specttroscopy, ion channeling, atomic force microscopy, and transmission electron microscopy. The AlN films grown above 700 °C were highly c-axis oriented with rocking curve FWHM of 5 to 6 arc-min. The ion channeling minimum yields near the surface region for the AlN films were ∼2 to 4%, indicating their high degree of crystallinity. TEM studies indicated that AlN films were epitaxial and single crystalline in nature with a large number of stacking faults as a results of lattice mismatch and growth induced defects. The surface roughness for the films was about 0.5 nm, which is close to the unit cell height of the AlN. Epitaxial TiN ohmic contacts were also developed on SiC, GaN, and AlN by in-situ PLD. Epitaxial TiN/AlN/SiC MIS capacitors with gate areas of 4 * 10-4 cm2 were fabricated, and high-temperature current-voltage (I-V) characteristics were studied up to 450 °C. We have measured leakage current densities of low 10-8 A/cm2 at room temperature, and have mid 10-3 A/cm2 at 450°C under a field of 2 MV/cm.

Transmission electron microscopy has been used to study defects formed in Mg-doped GaN crystals. Three types of crystals have been studied: bulk crystals grown by a high pressure and high temperature process with Mg added to the Ga solution and two types of crystals grown by metal-organic chemical vapor deposition (MOCVD) where Mg was either delta-doped or continuously doped. Spontaneous ordering was observed in bulk crystals. The ordering consists of Mg rich planar defects on basal planes separated by 10.4 nm and occurs only for growth in the N to Ga polar direction (000 1N polarity). These planar defects exhibit the characteristics of stacking faults with a shift vector of a 1/3 [1100] +c/2 but some other features identify these defects as inversion domains. Different type of defects were formed on the opposite site of the crystal (Ga to N polar direction), where the growth rate is also an order of magnitude faster compared to the growth with N-polarity. These defects are three-dimensional: pyramidal and rectangular, empty inside with Mg segregation on internal surfaces. The same types of defects seen for the two growth polarities in the bulk crystals were also observed in the MOCVD grown GaN samples with Mg delta doping, but were not observed in the crystals where Mg was added continuously.

We present a comprehensive study of the optical characteristics of (Al, In)GaN epilayers measured by photoluminescence (PL), integrated PL intensity, and time-resolved PL spectroscopy. For not only InGaN, but also AlGaN epilayers with large Al content, we observed an anomalous PL temperature dependence: (i) an “S-shaped” PL peak energy shift (decrease-increase-decrease) and (ii) an “inverted S-shaped” full width at half maximum (FWHM) change (increase-decrease-increase) with increasing temperature. Based on time-resolved PL, the S shape (inverted S shape) of the PL peak position (FWHM) as a function of temperature, and the much smaller PL intensity decrease in the temperature range showing the anomalous emission behavior, we conclude that strong localization of carriers occurs in InGaN and even in AlGaN with rather high Al content. We observed that the following increase with increasing Al content in AlGaN epilayers: (i) a Stokes shift between the PL peak energy and the absorption edge, (ii) a redshift of the emission with decay time, (iii) the deviations of the PL peak energy, FWHM, and PL intensity from their typical temperature dependence, and (iv) the corresponding temperature range of the anomalous emission behavior. This indicates that the band-gap fluctuation responsible for these characteristics is due to energy tail states caused by non-random inhomogeneous alloy potential variations enhanced with increasing Al content.

We have investigated the high-pressure high-temperature annealing of Mg/P-implanted GaN films using visible and ultraviolet (UV) micro-Raman spectroscopy. The results illustrate the use of Raman spectroscopy to monitor processing of GaN where fast feedback is required. The structural quality and the stress in ion-implanted GaN films was monitored in a 40nm-thin surface layer of the sample as well as averaged over the sample layer thickness. We find the nearly full recovery of the crystalline quality of ion-implanted GaN films after annealing at 1400-1500°C under nitrogen overpressures of 1.5GPa. No significant degradation effects occurred in the GaN surface layer during the annealing. The high nitrogen overpressures proved very effective in preventing the nitrogen out-diffusion from the GaN surface. Stress introduced during the annealing was monitored. Raman spectra of ion-implanted GaN films were investigated at different temperatures and excitation wavelengths to study the GaN phonon density of states.

When the stoichiometric TiN was deposited directly on GaN, we obtained columnar TiN grains of 5-20 nm section which cross the whole film thickness and are rotated mostly around the [111] axis. The conventional epitaxial relationship is obtained and no amorphous patches are observed at the interface. The deposition of TiN on Si doped GaN layers lead to the formation of an ohmic contact, whereas we obtain a rectifying contact on p type layers.

GaN films have been grown atop Si-terminated 3C-SiC intermediate layer on Si(111) substrates using low pressure metalorganic chemical vapor deposition (LP-MOCVD). The SiC intermediate layer was grown by chemical vapor deposition (CVD) using tetramethylsilane (TMS) as the single source precursor. The Si terminated SiC surface was obtained by immediately flow of SiH4 gas after growth of SiC film. LP-MOCVD growth of GaN on 3C-SiC/Si(111) was carried out with trimethylgallium (TMG) and NH3. Single crystalline hexagonal GaN layers can be grown on Si terminated SiC intermediate layer using an AlN or GaN buffer layer. Compared with GaN layers grown using a GaN buffer layer, the crystal qualities of GaN films with AlN buffer layers are extremely improved. The GaN films were characterized by x-ray diffraction (XRD), photoluminescence (PL) and scanning electron microscopy (SEM). Full width at half maximum (FWHM) of double crystal x-ray diffraction (DCXD) rocking curve for GaN (0002) on 3C-SiC/Si(111) was 890 arcsec. PL near band edge emission peak position and FWHM at room temperature are 3.38 eV and 79.35 meV, respectively.

We investigated the doping characteristics of Mg doped, Mg-Si co-doped, and Mg-Zn codoped GaN films grown by metalorganic chemical vapor deposition. We have grown p-GaN film with a resistivity of 1.26 Ωm and a hole density of 4.3 × 1017 cm−3 by means of Mg-Si co-doping technique. The Mg-Si co-doping characteristic was also explained effectively by taking advantage of the concept of competitive adsorption between Mg and Si during the growth. For Mg-Zn co-doping, p-GaN showing a low electrical resistivity (0.7 Ωm) and a high hole concentration (8.5 × 1017 cm−3) was successfully grown without the degradation of structural quality of the film. Besides, the measured specific contact resistance for Mg-Zn co-doped GaN film is 5.0 × 10−4 cm2, which is lower value by one order of magnitude than that for only Mg doped GaN film (1.9 × 10−3 Ωm2).

AlxGa1−xN {x=30% (doped and undoped), 45% (doped)} thin films were grown by MOCVD on ∼2 µm thick GaN layer using Al2O3 substrate. These films were designed to be the active parts of HFETs with nsí product of about 1016(Vs)−1. The layers were then studied by means of transmission electron microscopy (TEM) techniques. In this paper, it is shown that the AlxGa1−xN layer thickness was non-uniform due to the presence of Vshaped defects within the AlxGa1−xN films. The nucleation of these V-shaped defects has taken place about 20 nm above the AlxGa1−xN/aN interface. Many of these Vshaped defects were associated with the presence of the threading dislocations propagating from the GaN/Al2O3 interface. We show that the density of these V-shaped defects increases with the doping level and also with the Al mole fraction in the films. The formation mechanism of the V-shaped defects seems to be related to the concentration of dopants or other impurities at the ledges of the growing film. This suggestion is supported by high resolution TEM analysis. The growth front between the V-shaped defects in the lower Al concentration thin films was planar as compared with F99W3.77 the three-dimensional growth in the doped, higher Al concentration film. This interpretation of the origin of the V-shaped defects is consistent with the observed lowering of the Schottky barrier height in n-doped AlGaN/Ni Schottky diodes.

Yellow luminescence (YL) has been studied in GaN:Mg doped with Mg concentrations ranging from 1019 to 1021 cm−3 by spectral CL (T=5K) and TEM and explained by suggesting that a different mechanism could be responsible for the YL in p-type GaN with respect to that acting in n-type GaN.

Transitions at 2.2, 2.8, 3.27, 3.21, and 3.44 eV were found. In addition to the wurtzite phase, TEM showed a different amount of the cubic phase in the samples. Nano tubes with a density of 3×109 cm−2 were also observed by approaching the layer/substrate interface. Besides this, coherent inclusions were found with a diameter in the nm range and a volume fraction of about 1%.

The 2.8 eV transition was correlated to a deep level at 600 meV below the conduction band (CB) due to MgGa-VN complexes. The 3.27 eV emission was ascribed to a shallow acceptor at about 170-190 meV above the valence band (VB) due to MgGa.

The 2.2 eV yellow band, not present in low doped samples, increased by increasing the Mg concentration. It was ascribed to a transition between a deep donor level at 0.8-1.1 eV below the CB edge due to NGa and the shallow acceptor due to MgGa. This assumption was checked by studying the role of C in Mg compensation. CL spectra from a sample with high C content showed transitions between a C-related 200 meV shallow donor and a deep donor level at about 0.9- 1.1 eV below the CB due to a NGa-VN complex. In our hypothesis this should induce a decrease of the integrated intensity in both the 2.2 and 2.8 eV bands, as actually shown by CL investigations.

Deliberate oxygen doping of GaN grown by MOVPE has been studied. The electron concentration increased as the square root of the oxygen partial pressure. Oxygen is a shallow donor with a thermal ionization energy of 27 ±2 meV. A compensation ratio of Θ = 0.3-0.4 was determined from Hall effect measurements. The formation energy of ON of EF = 1.3 eV, determined from the experimental data, is lower than the theoretically predicted value.

Photoelectrochemical (PEC) etching technique has been proven to be an effective method to etch GaN. Despite its success, investigations on etching-induced damage are still scare. In this work, the damage induced by PEC etching of GaN in KOH electrolyte was studied. Photoluminescence (PL) spectroscopy was used to explore the origin of etching-induced damaged layer. From the variable temperature PL measurements, the origin of etching-induced damage was attributed to be the defect complex of VGa-ON (gallium vacancy bonds to oxygeon on nitrogen antisite). With determination of the defect origin, the electronic transition in the etch damage-related yellow luminescence (YL) band was suggested to be deep donor-like state to shallow-acceptor transition. In addition, a post-treatment method with boiled KOH chemical etching was developed to remove the thin damaged layer. In this method, crystallographic etching characteristics of boiled KOH was observed to assist in the formation of smooth sidewall facets. As revealed by the reduction of yellow luminescence, we propose this novel technique as a near damage-free etching method.

Electronic Raman scattering experiments have been carried out on both MBE and MOCVD-grown Mg-doped wurtzite GaN samples. Aside from the expected Raman lines, a broad structure (FWHM ≅ 15cm−1) observed for the first time at around 841cm−1 isattributed to the electronic Raman scattering from neutral Mg impurities in Mg-dopedGaN. From the analysis of the temperature-dependence of this electronic Ramanscattering signal binding energy of the Mg impurities in wurtzite GaN has been found to be Eb ≅ 172 ± 20meV. These experimental results demonstrate that the energy between the ground and first excited states of Mg impurities in wurtzite GaN is about 3/5 of its binding energy.

Erbium-doped films were grown on sapphire and silicon substrates by reactive sputtering, with different Er concentrations in the film. GaN films deposited at 800 K were determined to be polycrystalline by x-ray diffraction analysis, and retained their polycrystalline structure after annealing in nitrogen at 1250 K. The Er-doped films showed optical transmission beginning at about 360 nm, and the Er dose and film purity were determined with Rutherford backscattering spectroscopy. Photoluminescence and cathodoluminescence spectroscopy showed sharp emission lines corresponding to Er 3+ intra 4fn shell transitions over the range from 9 – 300 K. At above-bandgap optical and electron excitation, the 4S3/2 and 4F9/2 transition dominate, and are superposed on the “yellow band” emission. The infrared emission line at 1543 nm, corresponding to the Er 4I⅓ to 4I3/2 transition is also observed.

GaAsN epilayers and quantum wells with a good structural quality and surface morphology were grown by low pressure metal organic vapor phase epitaxy using tertiarybutylhydrazine as a novel nitrogen source. The dependence of nitrogen incorporation on growth temperature was studied for epitaxy with arsine and tertiarybutylarsine precursors. A nitrogen content of 6.7 % was achieved using tertiarybutylhydrazine and tertiarybutylarsine at a low growth temperature of 530 °C. The observed room temperature luminescence shows an increasing redshift with increasing nitrogen contents of the wells.

The formation of the {1210} stacking fault, which has two atomic configurations in wurtzite (Ga,Al,In)N, has been investigated by high resolution electron microscopy and energetic calculations. It originates from steps at the SiC surface and it can form on a flat (0001) sapphire surface. A modified Stillinger-Weber potential was used in order to investigate the relative stability of the two atomic configurations. They have comparable energy in AlN, whereas the 1/2<10 11>{1 210} configuration is more stable in GaN and InN. In GaN layers, only the 1/2<10 11>{1 210} configuration was observed. The 1/6<20 2 3> configuration was found in small areas inside the AlN buffer layer where it folded rapidly to the basal plane, and when back into the prismatic plane, it took the 1/2<10 11>{1 210} atomic configuration.

GaN layers contain large densities (1010 cm−2) of threading dislocations, nanopipes, (0001) and { 1120 } stacking faults, and { 1010 } inversion domains. Three configurations have been found for pure edge dislocations, mainly inside high angle grain boundaries where the 4 atom ring cores can be stabilized. Two atomic configurations, related by a 1/6 < 1010 > stair rod dislocation, have been observed for the { 1120 } stacking fault in (Ga-Al)N layers. For the {1010} inversion domain boundaries, a configuration corresponding to the Holt model was observed, as well as another with no N-N or Ga-Ga bonds.

The surface morphology and the room temperature 1.54 µm photoluminescence (PL) intensity from GaN:Er grown by gas source molecular beam epitaxy have been investigated as a function of C concentration as introduced by CBr4. Similar to previous results with increasing Er level, increasing the C concentration initially improved the surface smoothness as measured by atomic force microscopy (AFM) and scanning electron microscopy (SEM), with RMS roughness improving by a factor of seven over undoped GaN. The PL also improved dramatically. However, the highest amounts of C investigated produced a decrease in the PL as well as a roughening of the film surface. These effects indicate that the GaN:Er had reached its C solubility limit, producing an increased amount of defect induced nonradiative recombination.