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Deep levels were observed using capacitance deep level optical spectroscopy (DLOS) in an AlGaN/GaN heterostructure equivalent to that of a heterojunction field effect transistor. Band gap states were assigned to either the AlGaN or GaN regions by comparing the DLOS spectra in accumulation and pinch-off modes, where the former reflects both AlGaN- and GaN-related defects, and the latter emphasizes defects residing in the GaN. A band gap state at E-c-3.85 eV was unambiguously identified with the AlGaN region, and deep levels at E-c-2.64 eV and E-c-3.30 eV were associated with the GaN layers. Both the AlGaN and GaN layers exhibited additional deep levels with large lattice relaxation. The influence of deep levels on the two-dimensional electron gas sheet charge was estimated using a lighted capacitance-voltage method. (c) 2006 American Institute of Physics.

The authors have investigated the adsorption and subsequent desorption of Ga on AlN (0001) with line-of-sight quadrupole mass spectrometry (QMS). The authors present desorption data consistent with a continuous Ga-flux dependent accumulation of a laterally contracted Ga bilayer on AlN (0001) from 0 to 2.7 +/- 0.3 ML GaN equivalent coverage, and further Ga accumulation in macroscopic Ga droplets. The temperature dependence of Ga-adsorbate QMS desorption transients was investigated and the authors determined that the desorption activation energies for individual monolayers of the Ga adsorbate on AlN (0001) were similar to Ga desorption from GaN (0001). For the (first) pseudomorphic Ga-adsorbate monolayer on AlN, the authors measured a maximum Ga coverage of 1.0 +/- 0.1 ML and desorption activation energy of 6.2 +/- 0.3 eV. For the (second) laterally contracted Ga monolayer (1.7 +/- 0.3 ML) the desorption activation energy was 3.8 +/- 0.1 eV. (c) 2006 American Vacuum Society.

The authors report on the excitation wavelength dependence of terahertz emission from n-InN and bulk p-InAs pumped with femtosecond pulses tunable from 800 to 1500 nm. The terahertz amplitude, normalized to pump and probe power, from both narrow band gap semiconductors remains relatively constant over the excitation wavelength range. In addition, terahertz radiation from In- and N-face InN samples with bulk carrier concentrations ranging from 10(17) to 10(19) cm(-3) is also investigated, showing a strong dependence of terahertz emission on bulk carrier concentration. The experimental results agree well with calculations based on drift-diffusion equations incorporating momentum conservation and relaxation. (c) 2006 American Institute of Physics.

Plasma-assisted molecular beam epitaxial growth of Mg-doped, p-type and Si-doped, n-type m-plane GaN on 6H m-plane SiC is demonstrated. Phase-pure, m-plane GaN films exhibiting a large anisotropy in film mosaic (similar to 0.2 degrees full width at half maximum, x-ray rocking curve scan taken parallel to [112 ($) over bar0] versus similar to 2 degrees parallel to [0001]) were grown on m-plane SiC substrates. Maximum hole concentrations of similar to 7x10(18) cm(-3) were achieved with p-type conductivities as high as similar to 5 Omega(-1) cm(-1) without the presence of Mg-rich inclusions or inversion domains as viewed by cross-section transmission electron microscopy. Temperature dependent Hall effect measurements indicate that the Mg-related acceptor state in m-plane GaN is the same as that exhibited in c-plane GaN. Free electron concentrations as high as similar to 4x10(18) cm(-3) were measured in the Si-doped m-plane GaN with corresponding mobilities of similar to 500 cm(2)/V s measured parallel to the [112 ($) over bar0] direction. (c) 2006 American Institute of Physics.

The authors demonstrate the impact of growth kinetics on the surface and structural properties of N-face InN grown by molecular beam epitaxy. Superior surface morphology with step-flow growth features is achieved consistently under In-rich conditions in a low-temperature region of 500-540 degrees C. Remarkably, off-axis x-ray rocking curve (omega scans) widths are found to be independent of the growth conditions. The band gap determined from optical absorption measurements of optimized InN is 0.651 eV, while photoluminescence peak emission occurs at even lower energies of similar to 0.626 eV. Hall measurements show room temperature peak electron mobilities as high as 2370 cm(2)/V s at a carrier concentration in the low 10(17) cm(-3) region. Analysis of the thickness dependence of the carrier concentration demonstrates a n-type surface accumulation layer with a sheet carrier concentration of similar to 3x10(13) cm(-2). (c) 2006 American Institute of Physics.

Prospective equivalent internal quantum efficiency (eta(int)) of approximately 34% at 300 K was demonstrated for the blue emission peak of nonpolar m-plane (1 (1) over bar 00) InxGa1-xN/GaN multiple quantum well light emitting diodes (LEDs) fabricated on freestanding m-plane GaN substrates. Although the eta(int) value is yet lower than that of conventional c-plane blue LEDs (> 70%), the results encourage one to realize high performance green, amber, and red LEDs by reducing the concentration of nonradiative defects, according to the absence of the quantum-confined Stark effects due to the polarization fields parallel to the quantum well normal. The electric field component of the blue surface emission was polarized perpendicular to the c axis with the in-plane polarization ratio of 0.58 at 300 K. (c) 2006 American Institute of Physics.

We study the effect of different deposition conditions on the properties of In-polar InN grown by plasma-assisted molecular beam epitaxy. GaN buffer layers grown in the Ga-droplet regime prior to the InN deposition significantly improved the surface morphology of InN films grown with excess In flux. Using this approach, In-polar InN films have been realized with room temperature electron mobilities as high as 2250 cm(2)/V s. We correlate electron concentrations in our InN films with the unintentionally incorporated impurities, oxygen and hydrogen. A surface electron accumulation layer of 5.11x10(13) cm(-2) is measured for In-polar InN. Analysis of optical absorption data provides a band gap energy of similar to 0.65 eV for the thickest InN films. (c) 2006 American Institute of Physics.

We report on the use of in-situ SiNx nanomask for defect reduction in nonpolar a-plane GaN films, grown by metal-organic chemical vapor deposition. High-resolution x-ray diffraction analysis revealed that there was a monotonic reduction in the full width at half maximum, both on-axis and off-axis, with the increase in the SiNx thickness. Atomic force microscopy images revealed a significant decrease in the root-mean-square roughness and the density of submicron pits. Cross-section and plan-view transmission electron microscopy on the samples showed that the stacking fault density decreased from 8x10(5) to 3x10(5) cm(-1) and threading dislocation density decreased from 8x10(10) to 9x10(9) cm(-2). Room temperature photoluminescence measurement revealed that the band-edge emission intensity increased with the insertion of the SiNx layer, which suggests reduction in the nonradiative recombination centers. (c) 2006 American Institute of Physics.

The impact of threading dislocation density on Ni/n-GaN Schottky barrier diode characteristics is investigated using forward biased current-voltage-temperature (I-V-T) and internal photoemission (IPE) measurements. Nominally, identical metal-organic chemical vapor deposition grown GaN layers were grown on two types of GaN templates on sapphire substrates to controllably vary threading dislocation density (TDD) from 3x10(7) to 7x10(8) cm(-2). I-V-T measurements revealed thermionic emission to be the dominant transport mechanism with ideality factors near 1.01 at room temperature for both sample types. The Schottky barrier heights showed a similar invariance with TDD, with measured values of 1.12-1.13 eV obtained from fitting the I-V-T results to a thermionic emission-diffusion model. The I-V-T results were verified by IPE measurements made on the same diodes, confirming that the Ni/n-GaN barrier heights do not show a measurable TDD dependence for the TDD range measured here. In apparent contrast to this result is that the measured forward bias I-V characteristics indicate a shift in the observed forward bias turn-on voltage such that at the higher TDD value investigated here, a larger turn-on voltage (lower current) is observed. This difference is attributed to localized current blocking by high potential barrier regions surrounding threading dislocations that intersect the Ni/GaN interface. A simple model is presented that reconciles both the observed voltage shift and variations in the extracted Richardson constant as a function of threading dislocation density. With this model, an average local barrier surrounding dislocation of similar to 0.2 V is obtained, which diverts current flow across the forward biased Schottky interface to nondislocated regions. (c) 2006 American Institute of Physics.

This paper presents growth orientation dependence of the piezoelectric polarization of InxGa1-xN and AlyGa1-yN layers lattice matched to GaN. This topic has become relevant with the advent of growing nitride based devices on semipolar planes [A. Chakraborty , Jpn. J. Appl. Phys., Part 2 44, L945 (2005)]. The calculations demonstrate that for strained InxGa1-xN and AlyGa1-yN layers lattice matched to GaN, the piezoelectric polarization becomes zero for nonpolar orientations and also at another point approximate to 45 degrees tilted from the c plane. The zero crossover has only a very small dependence on the In or Al content of the ternary alloy layer. With the addition of spontaneous polarization, the angle at which the total polarization equals zero increases slightly for InxGa1-xN, but the exact value depends on the In content. For AlyGa1-yN mismatched layers the effect of spontaneous polarization becomes important by increasing the crossover point to similar to 70 degrees from c-axis oriented films. These calculations were performed using the most recent and convincing values for the piezoelectric and elasticity constants, and applying Vegard's law to estimate the constants in the ternary InxGa1-xN and AlyGa1-yN layers.

We have investigated the Ga-adlayer mediated growth of GaN quantum dots at 707 degrees C on AlN (0001) by simultaneous use of in situ reflection high-energy electron diffraction and line-of-sight quadrupole mass spectrometry during rf-plasma assisted molecular beam epitaxy (PA-MBE). We have found that the Ga-adsorbate coverage of 1.0 ML (monolayer) is critical in the mediation of a fundamental change in the GaN Stranski-Krastanov (SK) growth mode on AlN. When the Ga-adsorbate coverage was less than 1.0 ML, the SK transition occurred during GaN growth. For larger Ga-adsorbate coverages, the SK transition occurred after the desorption of the Ga coverage in excess of 1.0 ML. We performed variable GaN coverage growth experiments followed by desorption of the Ga adsorbate and subsequent GaN thermal decomposition to determine that the critical GaN coverage for the SK transition was less than 2.0 ML under Ga-droplet PA-MBE growth conditions.

The spatial distribution of strain, dislocations, and crystallographic orientation in uncoalesced and coalesced GaN layers grown on striped Si or SiC substrates was studied by polychromatic X-ray microdiffraction and high resolution monochromatic X-ray diffraction. Tilt boundaries formed at the column/wing interface depending on the growth conditions and geometry of the striped substrate. The measurements revealed that the free-hanging wings are tilted upward at room temperature. A misorientation between the GaN(0001) and the Si(111) or SiC(0001) surface normal is observed. Distinct structural properties of the pendeo and cantilever epitaxially grown samples are discussed.

We use photoreflectance (PR) spectroscopy to study the electronic band structure modification of GaN films grown along different nonpolar orientations due to biaxial, anisotropic in-plane strain. The exciton transition energies of an unstrained, high-quality C-plane GaN film are used to accurately determine the crystal-field and spin-orbit splitting energies. For films with a nonpolar orientation, the resonant features observed in the PR spectra exhibit a strong in-plane polarization anisotropy and different transition energies from the ones measured in the C-plane GaN film. The deformation potential D-5 is accurately determined from four GaN films with a nonpolar orientation using the measured energies together with the polarization properties and out-of-plane strain. (c) 2006 American Institute of Physics.

This letter reports AlGaN/GaN high-electron mobility transistors with capless activation annealing of implanted Si for nonalloyed ohmic contacts. Source and drain areas were implanted with an Si dose of I x 10(16) cm(-2) and were activated at similar to 1260 degrees C in a metal-organic chemical vapor deposition system in ammonia and nitrogen at atmospheric pressure. Nonalloyed ohmic contacts to ion-implanted devices showed a contact resistance of 0.96 Omega (.) mm to the channel. An output power density of 5 W/mm was measured at 4 GHz, with 58% power-added efficiency and a gain of 11.7 dB at a drain bias of 30 V.

In this letter, unpassivated high power deeply recessed GaN-based high electron mobility transistors (HEMTs) are reported. The introduction of a thick graded AlGaN cap layer and a novel fluorine-plasma surface treatment reduced the gate-leakage current and increased breakdown voltage significantly, enabling the application of much higher drain biases. Due to excellent dispersion suppression achieved at an epitaxial level, an output power density of more than 17 W/mm with an associated power added efficiency (PAE) of 50% was measured at 4 GHz and V-DS = 80 V without SiNx passivation. These results demonstrate the great potential of this novel epitaxial approach for passivation-free GaN-based HEMTs for high-power applications.

We have investigated the adsorption and subsequent desorption of Ga on (0001) GaN using simultaneous line-of-sight quadrupole mass spectrometry (QMS) and reflection high-energy electron diffraction (RHEED). The in situ QMS and RHEED desorption transient measurements demonstrate the Ga flux dependent accumulation of the theoretically predicted laterally contracted Ga bilayer [J. E. Northrup , Phys. Rev. B 61, 9932 (2000)] under conditions similar to those used during GaN growth by rf-plasma molecular beam epitaxy. We correlated bioscillatory RHEED desorption transients [C. Adelmann , J. Appl. Phys. 91, 9638 (2002)] to QMS-measured Ga-adsorbate coverage and found both to be consistent with layer-by-layer desorption of the Ga-adsorbate bilayer. The QMS-measured steady-state Ga-adlayer coverage exhibited a continuous increase from 0 to 2.4 ML (monolayer) with respect to impinging Ga flux at substrate temperatures of 640-700 degrees C. We observed an exponential dependence of the Ga flux corresponding to 1.0 ML Ga-adsorbate coverage on substrate temperature and we measured an apparent activation energy of 2.43 +/- 0.11 eV and an attempt prefactor of 6.77x10(12) nm/min (4.36x10(11) Hz) for this transition. (c) 2006 American Institute of Physics.

We investigated the suitability of ZrO2 as a high-k dielectric for GaN material systems. Thin Zr films (4 nm) were deposited by electron-beam evaporation at room temperature oil n-type GaN and Al(0.22)Gao(0.78)N (29 nm)/GaN high electron mobility transistor (HEMT) structures. The Zr-coated samples were subsequently oxidized at temperatures in the range of 200-400 degrees C in an ozone environment. Atomic force microscopy studies after oxidation show that the ZrO2 forms a conformal layer on the underlying GaN template. Cross-section transmission electron microscopy studies showed little intermixing of the ZrO2 with the AlGaN/GaN. The relative dielectric constant of the ZrO2 was determined to be 23. In comparison with HEMTs with bare gates (no dielectric between the gate metal and AlGaN), the HEMTs with ZrO2 showed two to three order of magnitude reduction in gate leakage current. Optimization of the HEMT process on sapphire substrates with ZrO2 under the gates yielded devices with powers of 3.8 W/mm and 58% power-added efficiency at 4 GHz. (c) 2006 American Vacuum Society.

Recombination dynamics of the 268 nm photoluminescence (PL) peak in a quaternary Al0.53In0.11Ga0.36N/Al0.58In0.02Ga0.40N multiple quantum well (MQW) grown on relaxed AlGaN templates were studied. Although the polarization field in the compressively strained Al0.53In0.11Ga0.36N wells was as high as 1.6 MV/cm, the value of integrated PL intensity at 300 K divided by that at 8 K (eta(int)) was as high as 1.2%. The value was similar to that obtained for the 285 nm PL peak in an Al0.30Ga0.70N/Al0.70Ga0.30N MQW (1.3%), though the A1N molar fraction in the wells was higher by a factor of 1.7. According to these results and the fact that time-resolved PL signal exhibited a stretched exponential decay shape, the improved eta(int) of the AlInGaN wells was attributed to a beneficial effect of the exciton localization as is the case with InGaN alloys; doping or alloying with InN was confirmed to work also on AlGaN in improving eta(int) to realize deep UV optoelectronic devices.

We demonstrate a technique to reduce the extended defect densities in a-plane GaN deposited on r-plane sapphire. The SiO2 lateral epitaxial overgrowth mask consisted of (GaN) stripes. Both the mask and GaN were etched through the mask openings and the lateral growth was initiated from the etched c-plane GaN sidewalls, and the material was grown over the mask regions until a smooth coalesced film was achieved. Threading dislocation densities in the range of 10(6)-10(7) cm(-2) were realized throughout the film surface. The on-axis and off-axis full width at half maximum value and surface roughness were 0.082 degrees, 0.114 degrees, and 0.622 nm, respectively. (c) 2006 American Institute of Physics.

The electrical conductivity and deep level spectrum of GaN grown by molecular beam epitaxy and codoped with carbon and silicon were investigated for substrate temperatures T-s of 650 and 720 degrees C as a function relative carbon and silicon doping levels. With sufficiently high carbon doping, semi-insulating behavior was observed for films grown at both temperatures, and growth at T-s=720 degrees C enhanced the carbon compensation ratio. Similar carbon-related band gap states were observed via deep level optical spectroscopy for films grown at both substrate temperatures. Due to the semi-insulating nature of the films, a lighted capacitance-voltage technique was required to determine individual deep level concentrations. Carbon-related band gap states underwent substantial redistribution between deep level and shallow acceptor configurations with change in T-s. In light of a T-s dependence for the preferential site of carbon incorporation, a model of semi-insulating behavior in terms of carbon impurity state incorporation mediated by substrate temperature is proposed. (c) 2006 American Institute of Physics.

Scanning probe techniques including scanning capacitance microscopy, scanning capacitance spectroscopy, scanning Kelvin probe force microscopy, and atomic force microscopy have been used to assess structure and local electronic properties of Ga-face and N-face p-type GaN and of inversion domain boundaries in p-type GaN. Epitaxial layers of p-type GaN were grown by molecular-beam epitaxy, and by adjustment of the Ga:N flux ratio samples containing both Ga-face and N-face material were obtained. Under identical growth conditions, net incorporation of electrically active Mg acceptors was found to be more efficient for material with Ga-face polarity. Only a very small dependence of surface potential on polarity was observed, in contrast to results reported for n-type GaN, in which a substantial dependence of Schottky barrier height on polarity has been found. In addition, elevated net concentrations of ionized Mg acceptors were observed in Ga-face regions in the immediate vicinity of some, but not all, inversion domain boundaries, consistent with theoretical suggestions that incorporation of high concentrations of Mg within an inversion domain boundary can lead to increased concentrations of Mg acceptors near the inversion domain boundary. (c) 2006 American Vacuum Society.

We demonstrate the growth and fabrication of a semipolar (10 (13) over bar) InGaN/GaN green (similar to 525 nm) light emitting diode (LED). The fabricated devices demonstrated a low turn-on voltage of 3.2 V and a series resistance of 14.3 Omega. Electroluminescence measurements on the semipolar LED yielded a reduced blueshifting of the peak emission wavelength with increasing drive current, compared to a reference commercial c-plane LED. On-wafer measurements yielded an approximately linear increase in output power with drive current, with measured values of 19.3 and 264 muW at drive currents of 20 and 250 mA,respectively. The external quantum efficiency did not decrease appreciably at high currents. Polarization anisotropy was also observed in the electroluminescence from the semipolar green LED, with the strongest emission intensity parallel to the [1 (2) over bar 10] direction. A polarization ratio of 0.32 was obtained at a drive current of 20 mA. (c) 2005 American Institute of Physics.

We report on the fabrication and high-frequency characterization of AlGaN/GaN high-electron mobility transistors (HEMTs) grown by molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD). In devices with a gate length of 160 nm, a record power density of 10.5 W/mm with 34% power added efficiency (PAE) has been measured at 40 GHz in MOCVD-grown HEMTs biased at V-DS = 30 V. Under similar bias conditions, more than 8.6 W/mm, with 32% PAE, were obtained on the MBE-grown sample. The dependence of output power, gain, and PAE on gate and drain voltages, and frequency have also been analyzed.

We report on the plasma-assisted molecular-beam epitaxy of Mg-doped (10 (1) over bar0) GaN on (10 (1) over bar0) 6H-SiC. Secondary ion mass spectroscopy measurements show the incorporation of Mg into the GaN films with an enhanced Mg incorporation under N-rich conditions relative to Ga-rich growth. Transport measurements of Mg-doped layers grown under Ga-rich conditions show hole concentrations in the range of p=1x10(18) to p=7x10(18) cm(-3) and a dependence between hole concentration and Mg beam equivalent pressure. An anisotropy in in-plane hole mobilities was observed, with the hole mobility parallel to [11 (2) over bar0] being higher than that parallel to [0001] for the same hole concentration. Mobilities parallel to [11 (2) over bar0] were as high as similar to 11.5 cm(2)/Vs (at p similar to 1.8 x 10(18) cm(-3)). (c) 2005 American Institute of Physics.

(0001)-oriented epitaxial wurtzite III-nitride layers grown on mismatched substrates have no resolved shear stress on the natural basal and prismatic slip planes; however, strained Ill-nitride layers may gradually relax. We report on the stress relaxation of Al0.49Ga0.51N layers grown on nominally relaxed Al0.62Ga0.38N buffer layers on sapphire. The reduction in elastic strain of the Al0.49Ga0.51N was enhanced by Si doping which caused an increased surface roughness. Despite the Si doping, the films always sustained step-flow growth. The extent of relaxation of the Al0.49Ga0.51N layer was determined by on-axis omega-2 theta scans of (0001) peaks and reciprocal space maps of inclined (off-axis) peaks. Cross-section and plan-view transmission electron microscopy studies showed that the threading dislocations in the Al(0.49)Gao(0.5)N layer inclined from the [0001] direction towards (1 (1) over bar 00) directions by similar to 15-25 degrees, perpendicular to their Burgers vector (1/3 ) These inclined threading dislocations have a misfit dislocation component and thus provide stress relief. The contribution of the dislocation inclination to the degree of relaxation has been formulated and the energy release has been determined for dislocation inclination in mismatched stressed layers. (c) 2005 American Institute of Physics.

Titanium dioxide (TiO2, with the rutile structure) was grown on (0001) oriented GaN and (0001) Al0.33Ga0.67N/GaN heterostructure field effect transistor (HFET) structures by molecular beam epitaxy. X-ray diffraction showed (100)TiO2 parallel to (0001)(GaN(AlGaN)) and [001](TiO2) parallel to (GaN(AlGaN)) with three rotational variants of the TiO2. Transmission electron microscopy of 50 nm thick TiO2 films on GaN and AlGaN/GaN showed sharp interfaces with no intermixing or reaction between the oxide and semiconductor. The TiO2 exhibited a columnar film microstructure with a lateral domain size of a few nanometers parallel to (10 (1) over bar)(TiO2) and a few tens of nanometers parallel to 00160 2 2 Metal-oxide HFIAs with 50 nm thick TiO2 dielectric layers under the gate were processed and compared to HFETs without the TiO2 dielectric layer. The transconductance of the HFETs with TiO2 was 140 MS/mm, approximately 20% less than HFETs with no dielecric, and the pinchoff voltages of the two stuctures were comparable. The dielectric constant of the TiO2 was similar to 70. The gate leakage current of the HFETs with ,TiO2, similar to 4 X 10(-6) mA/mm at. 50 V, was approximately 4 orders of magnitude lower than that of the. HFETs with no dielectric. Band offset measurements were performed using x-ray photoelectron spectroscopy, and the valence band of the rutile TiO2 and the GaN nearly line up. (c) 2005 American Vacuum Society.

An analysis is presented on the effect of the strain field originating from a subsurface stressor (point source of dilatation or a dilatating ellipsoidal inclusion) on the electronic properties of nitride semiconductors. With good accuracy, real quantum dots can be modeled as such stressors. We consider the following material structure design: a uniform semi-infinite GaN matrix with a buried stressor or a GaN matrix with a single (In,Ga)N quantum well, which is grown pseuodomorphically between the stressor and the free surface. We utilize isotropic elasticity to determine the strain field in the structures under investigation. We then apply a k.p perturbation theory approach to examine the shifts of the conduction and valence band edges caused by the stressor. We find lateral confinement for electrons and holes, which can be proposed for the realization of strain-induced quantum dots in the quantum well. (C) 2005 American Institute of Physics.

We present further developments and understanding of the commonly observed crosshatch surface morphology in strain-relaxed heteroepitaxial films. We have previously proposed that the crosshatch morphology is directly related with strain relaxation via threading dislocation glide which results in both surface step and misfit dislocation (MD) formation [see Andrews , J. Appl. Phys. 91, 1933 (2002)-now referred to as Part I]. In this article, we have used solutions for the stress fields and displacement fields for periodic MD arrays which include the effects of the free surface. These solutions avoid truncation errors associated with finite dislocation arrays that were used in Part I. We have calculated the surface height profile for relaxed films where the misfit dislocations were introduced randomly or the misfit dislocations were placed in groups with alternating sign of the normal component of their Burgers vector. We have calculated the surface height profiles where the slip step remains at the surface ["slip step only" (SSO)] and where the slip step is eliminated ["slip step eliminated" (SSE)] due to annihilation of opposite sense steps, such as could happen during growth or lateral mass transport. For relaxed films, we find that the surface height undulations, characteristic of crosshatch, increase with increasing film thickness for the SSO case, whereas the surface becomes flatter for the SSE case. Experiments on relaxed In0.25Ga0.75As films on (001) GaAs show that the surface height undulations in the [110] direction increase with increasing film thickness. Thus, we conclude that with increasing film thickness the crosshatch in the slow diffusion [110] direction is best described by the SSO case. (C) 2004 American Institute of Physics.

Carbon tetrabromide (CBr4) was studied as an intentional dopant during rf plasma molecular beam epitaxy of GaN. Secondary ion mass spectroscopy was used to quantify incorporation behavior. Carbon was found to readily incorporate under Ga-rich and N-rich growth conditions with no detectable bromine incorporation. The carbon incorporation [C] was found to be linearly related to the incident CBr4 flux. Reflection high-energy electron diffraction, atomic force microscopy and x-ray diffraction were used to characterize the structural quality of the film's postgrowth. No deterioration of structural quality was observed for [C] from mid 10(17) to similar to10(19) cm(-3). The growth rate was also unaffected by carbon doping with CBr4. The electrical and optical behavior of carbon doping was studied by co-doping carbon with silicon. Carbon was found to compensate the silicon although an exact compensation factor was difficult to extract from the data. Photoluminescence was performed to examine the optical performance of the films. Carbon doping was seen to monotonically decrease the band edge emission. Properties of carbon-doped GaN are interpreted to be consistent with recent theoretical work describing incorporation of carbon as function of Fermi level conditions during growth. (C) 2004 American Institute of Physics.

We report AlGaN-GaN high electron mobility transistors (HEMTs) grown by molecular beam epitaxy (MBE) on SiC substrates with excellent microwave power and efficiency performance. The GaN buffers in these samples were doped with carbon to make them insulating. To reduce gate leakage, a thin silicon nitride film was deposited on the AlGaN surface by chemical vapor deposition. At 4 GHz, an output power density of 6.6 W/mm was obtained with 57% power-added efficiency (PAE) and a gain of 10, dB at a drain bias of 35 V. This is the highest PAE reported until now at 4 GHz in AlGaN-GaN HEMTs grown by MBE. At 10 GHz, we measured an output power density of 7.3 W/mm with a PAE of 36% and gain of 7.6 dB at 40-V drain bias.

Polarity and dislocation dependence study of photoelectrochemical wet etching on GaN was carried out on lateral epitaxial overgrown nonpolar (11 (2) over bar0)a-GaN/(1 (1) over bar 02)r-plane sapphire substrate. This LEO nonpolar GaN sample has low dislocation density Ga- and N-faces exposed horizontally in opposite directions, which can be exposed to identical etching conditions for both polarity and dislocation dependence study. It is observed that N-face GaN is essentially much chemically active than Ga-face GaN, which shows the hexagonal pyramids with {10 (1) over bar(1) over bar} facets on the etched N face. No obvious etching was observed on Ga face in the same etch condition. As for dislocation dependence, the "wing" (low dislocation density) region was etched faster than the "window" (high dislocation density) region. Smooth etched surfaces were formed with the ((1) over bar(1) over bar2 (2) over bar) facet as an etch stop plane both on Ga and N-wing region. (C) 2004 American Institute of Physics.

We report on the growth of GaN quantum dots and the control of their density in the Stranski-Krastanov mode on AlN (0001) by rf-plasma molecular beam epitaxy at 750 degreesC. After depositing the equivalent of 2-3 ML GaN coverage, as limited by N fluence under Ga-droplet growth conditions, excess Ga was desorbed and Stranski-Krastanov islands formed under vacuum. We present the dependence of island density as a function of GaN coverage (for two growth rates: 0.10 and 0.23 ML/s), as estimated from atomic force microscopy and cross-sectional transmission electron microscopy. With a GaN growth rate of 0.23 ML/s, the island density was found to vary from less than 3.0x10(8)-9.2x10(10) cm(-2) as the GaN coverage was varied from 2.2 (critical thickness) to 3.0 ML. For a GaN growth rate of 0.10 ML/s, the island density varied from 2.0x10(10) to 7.0x10(10) cm(-2) over a GaN coverage range of 2.0-3.0 ML. For each growth rate, the GaN islands were found to be of nearly uniform size, independent of the quantum dot density. (C) 2004 American Institute of Physics.

This letter describes the relationship between the morphological evolution of heteroepitaxial a-plane GaN films and the formation of the extended defect structure. The initial a-plane GaN growth on a-plane SiC substrates (via a high temperature AlN buffer layer) follows a Volmer-Weber growth mode. Consequently, the coalescence of three-dimensional (3D) islands generates threading dislocations which dominate the nonpolar GaN film's microstructure (3x10(10) cm(-2)). Exposed nitrogen-face surfaces, identified using x-ray diffraction measurements and convergent beam electron diffraction analysis, are present throughout the 3D growth and are the likely source of basal plane faulting (7x10(5) cm(-1)) within the film. Atomic force microscopy and scanning electron microscopy were used to image the morphological transition, which was correlated to changes in the a-GaN crystal tilt mosaic measured by x-ray rocking curves. (C) 2004 American Institute of Physics.

The polarity of AlN epitaxial layers grown on (0001) sapphire, SiC, and nitrided sapphire substrates was examined by convergent beam electron diffraction, and the morphology and microstructure were characterized by atomic force microscopy and scattering contrast transmission electron microscopy (TEM). The AlN films grown on sapphire and SiC without a buffer layer or nitridation of the substrate were flat and had Al polarity. From TEM studies in both cross section and plan view, it was found that the threading dislocation (TD) density was similar to1x10(8) and 2x10(10) cm(-2) for screw-component and pure edge component dislocations, respectively. N-face AlN was realized by pregrowth nitridation of sapphire substrates, but these films contained a small volume fraction of Al-face inversion domains which were related to hexagonal pyramids defined by {1(2) over bar 02} facets. The density of screw-component TDs was significantly reduced due to nitridation. Cross-sectional TEM showed that the film grown on nitrided sapphire was almost free of screw-component TDs and that the density of pure edge TDs was similar to4x10(9) cm(-2). (C) 2004 American Institute of Physics.

Dislocation-related conduction paths in n-type GaN grown by molecular-beam epitaxy and a mechanism for local suppression of current flow along these paths are analyzed using conductive atomic force microscopy, scanning Auger spectroscopy, and macroscopic current-voltage measurements. Application of an electric field at the GaN surface in an ambient atmospheric environment is shown to lead to local formation of gallium oxide in the immediate vicinity of the conduction paths, resulting in the strong suppression of subsequent current flow. Current-voltage measurements for Schottky diodes in which local conduction paths have been suppressed in this manner exhibit reverse-bias leakage currents reduced by two to four orders of magnitude compared to those in Schottky diodes not subjected to any surface modification process. These results demonstrate that the dislocation-related current leakage paths are the dominant source of leakage current in Schottky contacts to n-type GaN grown by molecular-beam epitaxy, and elucidate the nature of a microscopic process for their suppression. (C) 2003 American Institute of Physics.

The lateral wet oxidation of strained AlAsSb was studied. AlAs0.80Sb0.20 interlayers were grown on a GaAs substrate and capped with a lattice-matched In0.25Ga0.75As layer. The AlAsSb was oxidized between 350 and 450 degreesC. Oxidation temperatures >400 degreesC resulted in poor surface morphology and delamination. Oxidation of thicker AlAsSb interlayers (h approximate to 2000 Angstrom) resulted in metallic Sb layers forming between the AlOx and the semiconductor interfaces. The remaining Sb metal at the oxide-semiconductor interface was similar to15% oxidized. Lateral wet oxidation of thinner AlAsSb interlayers (hless than or equal to500 Angstrom) resulted in Sb inclusions at the oxide-semiconductor interface. The Sb inclusions were 1.5-2.0 mum in diameter and the inclusion thickness was approximately equal to the AlAsSb layer thickness. Methanol (CH3OH) was added to the water mixture with the intent to stabilize the otherwise unstable stibine (SbH3) such that Sb could be removed from the oxidizing structure. However, methanol addition resulted in a decreased oxidation rate and a change in the Sb precipitate morphology. The Sb inclusions observed in pure water oxidation changed to a Sb finger-like morphology with increasing methanol concentration. The Sb fingers we're, 1.0-2.0 mum wide and as long as the oxidation depth. Oxidation of AlAsSb interlayers h less than or equal to 200 Angstrom were limited by the incorporation of Ga from the substrate and capping layer into the oxidation layer. Doping the oxidation AlAsSb interlayer 1 X 10(18) cm(-3) n type (Si or Te) did not result in any improvement in Sb segregation. (C) 2003. American Vacuum, Society.

Cathodoluminescence technique combined with transmission electron microscopy (TEM-CL) has been used to characterize optical properties of dislocations in GaN epilayers. The dislocations act as nonradiative centers with different recombination rates. TEM-CL observation showed that even for the same Burgers vector of a, the dislocations show different electrical activity depending on the direction of dislocation line, i.e., the edge-type dislocation parallel to the c plane is very active, while the screw-type one is less active. The simulation of the CL images gives us the information of parameters such as carrier lifetime and diffusion length. (C) 2003 American Institute of Physics.

The dielectric permittivity, dielectric quality factor (inverse dielectric loss), and lattice parameter of 140 nm sputtered SrTiO3 films were dependent on the oxygen partial pressure and total chamber pressure (O-2+Ar) during film growth. Films were grown at 25 and 75 mTorr (mT) in an oxygen rich and oxygen deficient sputtering gas environment concurrently on (100) SrTiO3 and (111) Pt/(0001) Al2O3 substrates. Films were deposited on platinized sapphire for electrical characterization and the homoepitaxial films were used as a structural and chemical standard. High resolution triple axis x-ray diffraction results showed an increase in mismatch between the film and substrate (200) peak in homoepitaxial SrTiO3 films with higher total growth and lower oxygen pressures. Dielectric quality factors of the SrTiO3 films on platinized sapphire at 1 MHz for the 25 mT (50 sccm Ar/50 sccm O-2), 25 mT (90 sccm Ar/10 sccm O-2), 75 mT (50 sccm Ar/50 sccm O-2), and 75 mT (90 sccm Ar/10 sccm O-2) film growths were 320, 251, 209, and 102, respectively; likewise, the dielectric constants follow as 241, 230, 220, and 170, respectively. Improved film dielectric properties were observed for films closer to stoichiometric SrTiO3. (C) 2003 American Institute of Physics.

A recently observed mechanism of elastic stress relaxation in mismatched layers is discussed. The relaxation is achieved by the inclination of pure edge threading dislocation lines with respect to the layer surface normal. The relaxation is not assisted by dislocation glide but rather is caused by the "effective climb" of edge dislocations. The effective dislocation climb may result from the film growth and it is not necessarily related to bulk diffusion processes. The contribution of the dislocation inclination to strain relaxation has been formulated and the energy release due to the dislocation inclination in mismatched stressed layers has been determined. This mechanism explains recently observed relaxation of compressive stresses in the (0001) growth of AlxGa1-xN layers. (C) 2003 American Institute of Physics.

Scanning Kelvin probe microscopy (SKPM) and conductive atomic force microscopy (C-AFM) have been used to image surfaces of GaN grown by molecular beam epitaxy. Detailed analysis of the same area using both techniques allowed imaging and comparison of both surface potential variations arising from the presence of negatively charged threading dislocations and localized current leakage paths associated with dislocations. Correlations between the charge state of dislocations, conductivity of current leakage paths, and dislocation type were thereby established. Analysis of correlated SKPM and C-AFM images revealed a density of negatively charged features of similar to3 x 10(8) cm(-2) and a localized current leakage path density of similar to3 x 10(7) cm(-2), With approximately 25% of the leakage paths spatially correlated with negatively charged dislocation features. Based on correlated topography and previous studies quantifying the densities of edge, screw, and mixed dislocations, our results suggested that dislocations having an edge component behave as though negatively charged while pure screw dislocations are solely responsible for the observed leakage paths and are uncharged. (C) 2003 American Institute of Physics.

This letter discusses the structural and morphological characteristics of planar, nonpolar (11 (2) over bar0) a-plane GaN films grown on (1 (1) over bar 02) r-plane sapphire by hydride vapor phase epitaxy. Specular films with thicknesses over 50 mum were grown, eliminating the severely faceted surfaces that have previously been observed for hydride vapor phase epitaxy-grown a-plane films. Internal cracks and crack healing, similar to that in c-plane GaN films, were observed. Atomic force microscopy revealed nanometer-scale pitting and steps on the film surfaces, with rms roughness of similar to2 nm. X-ray diffraction confirmed the films are solely a-plane oriented with on-axis (11 (2) over bar0) and 30degrees off-axis (10 (1) over bar0) rocking curve peak widths of 1040 and 3000 arcsec, respectively. Transmission electron microscopy revealed a typical basal plane stacking fault density of 4x10(5) cm(-1). The dislocation content of the films consisted of predominately edge component (b(edge)=+/-[0001]) threading dislocations with a density of 2x10(10) cm(-2), and mixed-character Shockley partial dislocations (b=1/3(1) over bar 00>) with a density of 7x10(9) cm(-2). (C) 2003 American Institute of Physics.

Evaluation of the structural properties of 200-nm-thick Si-doped Al0.49Ga0.51N films, grown on nominally relaxed 1-mum-thick Al0.62Ga0.38N buffer layers on sapphire, revealed that increased Si doping promoted the relaxation of the compressively strained layers. The degree of strain relaxation R of the Al0.49Ga0.51N films, as determined by x-ray diffraction (XRD), increased from R=0.55 to R=0.94 with an increase in disilane injection from 1.25 nmol/min to 8.57 nmol/min. Transmission electron microscopy analysis showed that the edge threading dislocations (TDs) in the Al0.49Ga0.51N layers were inclined, such that the redirected TD lines had a misfit dislocation component. The calculated strain relaxation due to the inclined TDs was in close agreement with the values determined from XRD. We propose that the TD line redirection was promoted by the Si-induced surface roughness. (C) 2003 American Institute of Physics.

We present a simple strategy that minimizes the impact of surface segregation of In during the growth of (In,Ga) N/GaN multiple quantum wells by plasma-assisted molecular-beam epitaxy and simultaneously results in abrupt interfaces. The two ingredients of this strategy are (i) the use of a higher substrate temperature than commonly employed, that is, well above the In desorption point and (ii) the use of a modulated stoichiometry, that is, N-rich during growth of the well and Ga-stable during growth of the barrier. (C) 2003 American Institute of Physics.

Mg doping has been found in some situations to invert growth on Ga-face GaN to N-face. In this study, we clarified the role the Ga wetting layer plays in rf plasma molecular beam epitaxy of GaN when Mg doping, for [Mg] from similar to2 X 10(19) to similar to1 X 10(20) cm(-3) corresponding to the useful, accessible range of hole concentrations of p similar to 10(17)-10(18) cm(-3) Structures were grown in the N-rich and Ga-rich growth regime for single Mg doping layers and for multilayer structures with a range of Mg concentrations. Samples were characterized in situ by reflection high-energy electron diffraction and ex situ by atomic force microscopy, transmission electron microscopy, convergent beam electron diffraction, and secondary ion mass spectroscopy. Growth on "dry" surfaces (without a Ga wetting layer) in the N-rich regime completely inverted to N-face upon exposure to Mg. No reinversion to Ga-face was detected for subsequent layers. Additionally, Mg was seen to serve as a surfactant during this N-rich growth, as has been reported by others. Growth initiated in the Ga-rich regime contained inversion domains that nucleated with the initiation of Mg doping. No new inversion domains were found as the Mg concentration was increased through the useful doping levels. Thus the Ga wetting layer was found to inhibit nucleation of N-face GaN, though a complete wetting layer took time to develop. Finally, by establishing a complete Ga wetting layer on the surface prior to growth, we confirmed this finding and demonstrated Mg-doped GaN completely free from inversion domains to a doping level of [Mg] similar to2 X 10(20). (C) 2003 American Vacuum Society.

Achieving nitride-based device structures unaffected by polarization-induced electric fields can be realized with nonpolar GaN, although polarity plays a key role in the growth. (11 (2) over bar0) a-plane GaN films were grown on (1 (1) over bar 02) r-plane sapphire substrates and subsequently laterally overgrown using metalorganic chemical vapor deposition. Convergent beam electron diffraction analysis was used to determine the a-GaN polarity to explicitly define the film/substrate relationship, and subsequently to identify various growth features and surfaces observed throughout our studies of a-plane GaN. In particular, the effects of polarity on (1) lateral overgrowth from mask stripe openings aligned along [(1) over bar 100](GaN) and (2) pit formation in heteroepitaxial films grown under nonoptimized conditions were investigated. The fundamental differences between the polar surfaces are clearly observed; analysis of the lateral epitaxial overgrowth stripes revealed that (0001) surfaces grew faster than (000 (1) over bar) surfaces by approximately an order of magnitude, and these stable, slow-growing (000 (1) over bar) surfaces are a likely cause of pitting in a-GaN films. The growth features under investigation were imaged using scanning and transmission electron microscopy. (C) 2003 American Institute of Physics.

This letter reports on the reduction in extended-defect densities in a-plane (11 (2) over bar0) GaN films achieved via lateral epitaxial overgrowth (LEO) by hydride vapor phase-epitaxy. A variety of dielectric mask patterns was used to produce 8-125-mum-thick, fully coalesced nonpolar GaN films. The nanometer-scale pit densities in the overgrown regions were less than 3x10(6) cm(-2) compared to similar to10(10) cm(-2) in the direct-growth a-plane GaN. Cathodoluminescence revealed a fourfold increase in luminous intensity in the overgrown material compared to the window material. X-ray rocking curves indicate the films were free of wing tilt within the sensitivity of the measurements. Whereas non-LEO a-plane GaN exhibits basal plane stacking fault and threading dislocation densities of 10(5) cm(-1) and 10(9) cm(-2), respectively, the overgrown LEO material was essentially free of extended defects. The basal plane stacking fault and threading dislocation densities in the wing regions were below the detection limits of similar to5x10(6) cm(-2) and 3x10(3) cm(-1), respectively. (C) 2003 American Institute of Physics.

The influence of AlGaN and GaN cap layer thickness on Hall sheet carrier density and mobility was investigated for Al0.32Ga0.68N/GaN and GaN/Al0.32Ga0.68N/GaN heterostructures deposited on sapphire substrates. The sheet carrier density was found to increase and saturate with the AlGaN layer thickness, while for the GaN-capped structures it decreased and saturated with the GaN cap layer thickness. A relatively close fit was achieved between the measured data and two-dimensional electron gas densities predicted from simulations of the band diagrams. The simulations also indicated the presence of a two-dimensional hole gas at the upper interface of GaN/AlGaN/GaN structures with sufficiently thick GaN cap layers. A surface Fermi-level pinning position of 1.7 eV for AlGaN and 0.9-1.0 eV for GaN, and an interface polarization charge density of 1.6x10(13)-1.7x10(13) cm(-2), were extracted from the simulations. (C) 2003 American Institute of Physics.

Relaxed, high-quality, compositionally step-graded InAsyP1-y layers with an As composition of y=0.4, corresponding to a lattice mismatch of similar to1.3% were grown on InP substrates using solid-source molecular-beam epitaxy. Each layer was found to be nearly fully relaxed observed by triple axis x-ray diffraction, and plan-view transmission electron microscopy revealed an average threading dislocations of 4x10(6) cm(-2) within the InAs0.4P0.6 cap layer. Extremely ordered crosshatch morphology was observed with very low surface roughness (3.16 nm) compared to cation-based In0.7Al0.3As/InxAl1-xAs/InP graded buffers (10.53 nm) with similar mismatch and span of lattice constants on InP. The results show that InAsyP1-y graded buffers on InP are promising candidates as virtual substrates for infrared and high-speed metamorphic III-V devices. (C) 2003 American Institute of Physics.

Shubnikov-de-Haas oscillation is observed in a polarization-doped three-dimensional electron slab in a graded AlxGa1-xN semiconductor layer. The electron slab is generated by the technique of grading the polar semiconductor alloy with spatially changing polarization. Temperature-dependent oscillations allow us to extract an effective mass of m*=0.21m(0). The quantum scattering time measured (tau(q)=0.3 ps) is close to the transport scattering time (tau(t)=0.34 ps), indicating the dominance of short-range scattering. Alloy scattering is determined to be the dominant mechanism-limiting mobility; this enables us to extract an alloy-scattering parameter of V-0=1.8 eV for the AlxGa1-xN material system. Polarization-doping presents an exciting technique for creating electron slabs with widely tunable density and confinement for the study of dimensionality effects on charge transport and collective phenomena.