Homoepitaxial GaN Films Research Articles

(Invited) Shallow Donor Impurities in GaN

Significant advances have been achieved during the last three decades in heteroepitaxial, homoepitaxial, and bulk growth of nitride semiconductors. Despite this remarkable progress, the full realization of the functionality of this material system is often limited by the presence of pervasive residual impurities inherent to the high-temperature growth process, and/or by the presence of lattice point and extended defects. A number of important fundamental aspects of these properties must be understood and controlled in order to realize optimum device performance.We review here on a systematic investigation, employing a combination of defect-sensitive techniques, carried out on freestanding HVPE-GaN films to identify the chemical nature of the shallow donors. High-resolution, variable temperature PL experiments were performed in the spectral region associated with recombination processes involving the ground and excited states of the neutral donor bound excitons and their two-electron-satellites in GaN substrates and homoepitaxial films. High-sensitivity SIMS and high-resolution FTIR measurements unambiguously identify Si and O shallow donors with ground state binding energies of 30.18 meV and 33.20 meV, respectively. These binding energies are in excellent agreement with values obtained by the analyses of the two-electron-satellite PL spectra considering the participation of ground and excited state donor bound excitons. High-resolution PL studies of homoepitaxial GaN films confirm the SIMS and the FTIR impurity identifications. These results were conveniently employed to verify the incorporation and activation of shallow donors in high crystalline quality GaN substrates.This work supported by the Office of Naval Research

Characterization of Homoepitaxial GaN Films on Commercial GaN Substrates

The realization of next-generation medium to high power vertical GaN devices relies heavily on the epitaxial growth of GaN on commercially available, high-quality GaN substrates. Recent advances in commercially available GaN substrates have enabled the design and fabrication of vertical GaN devices that demonstrate promise and feasibility, but are limited by materials challenges of homoepitaxial GaN growth. Although homoepitaxial GaN is expected to provide more control over growth characteristics (e.g. crystallinity and doping), its reliability and reproducibility suffer at the hand of the native GaN substrates available to date. The variations in commercial GaN substrates include defect density, surface roughness, wafer bow, and photoluminescence properties. Thus, elucidating the role of GaN substrate properties on the growth and characteristics of resulting homoepitaxial GaN films has emerged as a new challenge towards next-generation GaN power devices. In this presentation, a systematic study of the morphological, electrical, and optical properties of GaN films on GaN substrates will be presented to identify substrate and epitaxial film metrics that administer high mobility, low leakage current, low on-resistance, and high breakdown voltage.

Influence of active nitrogen species on surface and optical properties of epitaxial GaN films

Influence of active nitrogen species on surface and optical properties of homoepitaxial GaN films grown on GaN epilayers has been investigated. The epitaxial GaN films were grown at varying plasma powers (350–500 W) under identical growth conditions. High resolution X-Ray diffraction, Field Emission Scanning Electron Microscopy, Atomic Force Microscopy and Photoluminescence measurements were employed to characterize the structural, morphological and optical properties of the grown GaN films. High plasma power (500 W) lead to an increment in active nitrogen radicals and yielded high crystalline quality with reduced dislocations compared to low plasma power (350, 400 W) which divulge the presence of metallic gallium on the surface and low roughness. The valence band maximum position, electron affinity and ionization energy of the films increased with increment in plasma power. PL measurements revealed narrow and intense band to band edge emission with negligible defect related yellow band peak for the sample grown at 500 W. The analysis conveyed that higher amount of active nitrogen species encouraged good optical properties with insignificant defect states which can be employed for the fabrication of high performance optoelectronic & photovoltaic devices.

Site-specific comparisons of V-defects and threading dislocations in InGaN/GaN multi-quantum-wells grown on SiC and GaN substrates

Systematic site-specific comparisons of the densities of threading dislocations (TDs) and associated V-defects in six-period InxGa1−xN/GaN multi-quantum wells (MQWs) with In mole fractions of x=0.1 and 0.2 have been conducted using several electron microscopy techniques including electron channeling contrast imaging. The MQWs were grown on GaN epitaxial films deposited on either (0001) GaN substrates or on (0001)AlN/6H-SiC heterostructures. The densities of TDs and V-defects in the surfaces of the homoepitaxial GaN films and the In0.1Ga0.9N/GaN MQWs were both 5.5×106cm−2. By contrast, the V-defect densities in the In0.2Ga0.8N/GaN MQWs exceeded by two and three times the densities of TDs measured in the surfaces of the underlying GaN films grown on GaN and AlN/SiC substrates, respectively. Similar observations of V-defects not associated with threading dislocations have been previously reported to occur in InGaN MQWs grown on GaN template films on sapphire substrates and having either an initially high In mole fraction or grown at the end of a series of MQWs with an increasing In mole fraction. From a synthesis of the results of this previous work using GaN/sapphire substrates with the present results for bulk GaN substrates, it is hypothesized that increases in stress generated by the larger mismatches in lattice parameters between the GaN film and the In0.2Ga0.8N/GaN MQWs and the additional stresses generated between the GaN barriers and the wells within these MQWs generated new TDs within the MQWs, which then acted as nucleation centers for new V-defects.

Influence of Ga/N ratio on morphology, vacancies, and electrical transport in GaN grown by molecular beam epitaxy at high temperature

The effect of Ga/N flux ratio on surface morphology, incorporation of point defects and electrical transport properties of GaN films grown by plasma-assisted molecular beam epitaxy in a recently developed high-temperature growth regime was investigated. The homoepitaxial (0001) GaN films grown at ∼780–790 °C showed smoothest morphologies near the cross-over between N-rich and Ga-rich growth (0.75<Ga/N<1.1) contrasting previous observations for low-temperature growth. The higher-quality growth near Ga/N∼1 resulted from lower thermal decomposition rates and was corroborated by slightly lower Ga vacancy concentrations [VGa], lower unintentional oxygen incorporation, and improved electron mobilities. The consistently low [VGa], i.e., ∼1016 cm−3 for all films attribute further to the significant benefits of the high-temperature growth regime.

Surface-roughness correlations in homoepitaxial growth of GaN(0001) films by NH3 supersonic jet epitaxy

Homoepitaxial GaN films were grown on GaN(0001)∕6H-SiC substrates by NH3 supersonic jet epitaxy at 750°C using a constant Ga flux of 2.9×1014cm−2s−1 and varying the NH3 flux and average kinetic energy. Atomic force microscopy (AFM), scanning electron microscopy, and in situ reflection high-energy electron diffraction evidence an abrupt transition from quasi-two-dimensional basal-plane growth to three-dimensional faceted growth at approximately 1∕2 of the maximum Ga-limited growth rate, irrespective of NH3 kinetic energy. Topographical scaling analysis of the AFM images reveals that the smooth and rough GaN(0001) films have static scaling exponents (α) of 0.88±0.05 and 1.10±0.06, respectively. The dynamic scaling exponent (β) for rough films is approximately 0.25. A comparison of these scaling exponents with predictions based on continuum growth models indicates that competition between surface diffusion and stochastic roughening governs the evolution of surface morphology during GaN growth.

Diffusion length of carriers and excitons in GaN—influence of epilayer microstructure

We demonstrate a correlation between the microstructure of epilayers and the diffusion length of free carriers and excitons in heteroepitaxial and homoepitaxial GaN films. We show that the diffusion length is shorter than predicted for a given density of dislocations. With improving structural quality of the films and lower dislocation density the diffusion lengths of free carriers and excitons increase, which, in turn, increases the rate of nonradiative recombination at dislocations. This process may explain a surprisingly small change of light emission efficiency observed for GaN epilayers with varying densities of dislocations.

Acid etching for accurate determination of dislocation density in GaN

Hot phosphoric-acid etching and atomic force microscopy (AFM) were used to etch and characterize various GaN materials, including freestanding GaN grown by hydride vapor-phase epitaxy (HVPE), metal-organic chemical-vapor deposition (MOCVD) GaN films on sapphire and silicon carbide, and homoepitaxial GaN films on polished freestanding-GaN wafers. It was found that etching at optimal conditions can accurately reveal the dislocations in GaN; however, the optimal etch conditions were different for samples grown by different techniques. The as-grown HVPE samples were most easily etched, while the MOCVD homoepitaxial films were most difficult to etch. Etch-pit density (EPD) ranging from 4 × 106 cm2 to 5 × 109 cm-2 was measured in close agreement with the respective dislocation density determined from transmission electron microscopy (TEM).

Optical investigation of shallow acceptor states in GaN grown by hydride vapor-phase epitaxy

The evolution of low-temperature photoluminescence (PL) spectra with the thickness of the layer (3–400 μm) is investigated on high-quality GaN grown by hydride vapor-phase epitaxy. With increasing layer thickness, three acceptor bound exciton peaks are found to reduce in intensity, although the impurity concentrations, measured by secondary ion mass spectrometry, do not depend on the sample thickness. The observed acceptor transitions are attributed to intrinsic defects, originating from the substrate/layer interface and decreasing in density with the thickness of the layer. The optical properties, studied by reflectance, temperature and excitation power dependent PL, are compared to those of homoepitaxial GaN films grown by metalorganic chemical vapor deposition.

Photoluminescence study of homoepitaxial N-polar GaN grown on differently misoriented single crystal substrates

We report results of a photoluminescence (PL) study of homoepitaxial N-polar GaN films grown by metal-organic chemical vapour deposition on vicinal (0 0 0 1 ̄ ) GaN single crystal substrates. Off-angles of 2° and 4° towards the [1 1 2 ̄ 0] direction as well as 4° in the [1 0 1 ̄ 0] direction were investigated. Along with a remarkable improvement of the epilayer morphology, a significant reduction of the unintentional/intrinsic donor concentration is achieved for all considered misorientations. As a consequence, PL spectra with narrow bound and free excitonic lines were observed. The misorientation of 4° towards the [1 1 2 ̄ 0] direction results in an N-polar epilayer of the best optical quality.

Cw and time-resolved spectroscopy in homoepitaxial GaN films and GaN–GaAlN quantum wells grown by molecular beam epitaxy

We have grown GaN films and GaN–AlGaN quantum wells (QWs) on homoepitaxial substrates, by molecular beam epitaxy using ammonia. Both the GaN film and the QW are found to have superior excitonic recombination properties which are extremely promising for the development of indium free ultra-violet lasers based on nitrides.

Faceted inversion domain boundary in GaN films doped with Mg

Homoepitaxial GaN films, doped with Mg, were grown by rf-plasma molecular-beam epitaxy on Ga-polarity (0001) templates. Convergent-beam electron diffraction analysis establishes that the film polarity changes from [0001] to [0001_] when the Mg flux during growth is approximately 1 ML/s. Secondary ion mass spectrometry indicates a doping concentration of ∼1020 cm−3 in the film where the inversion occurs, and a reduced Mg incorporation in the [0001_] material. Transmission electron microscopy shows that the inversion domain boundary is faceted predominantly along the {0001} and {h,h,−2h,l} planes, with l/h approximately equal to 3. Using first-principles total energy calculations, we show that the {h,h,−2h,l} segments of the boundary are stabilized by the incorporation of Mg in threefold coordinated lattice sites.

Bulk GaN single-crystals growth

Gallium nitride powder was prepared from gallium and ammonia at temperatures of 1000–1200°C. Parameters of the crystallographic lattice as well as photoluminescence and Raman spectra were determined for the obtained powder. As a result of GaN powder sublimation, GaN single crystals of 3×2×0.2 mm were received, at temperatures 1200–1250°C. Single crystals of gallium nitride were also synthesised in a reaction of gallium vapours with ammonia. Crystals up to 1.5×1×0.1 mm were obtained at the temperature range of 850–1150°C. The obtained single crystals were studied by X-ray methods resulting in structure refining, mapping of reflexes and rocking curves. In addition, Raman spectroscopy studies and conductivity measurements were carried out. It was also demonstrated that the crystallographic structure of the crystals and powders was well pronounced. Homoepitaxial GaN films of several tens of micrometers were deposited onto selected GaN single crystals. The influence of grain size on the GaN sublimation process was studied. The 50–500 μm GaN crystalline powder sublimation resulted in the growth of single crystals whose quality was significantly better in comparison to effects of the sublimation of a few micrometer crystalline powder.

Step Flow Surface Morphology in Plasma Assisted Molecular Beam Epitaxy Grown GaN

ABSTRACTThe surface morphologies of homoepitaxial GaN films grown by molecular beam epitaxy (MBE) on metalorganic chemical vapor deposition (MOCVD) grown GaN template layers were investigated, using atomic force microscopy (AFM). Typical surface morphology of MBE-grown films on MOCVD-templates was dominated by spiral hillocks due to the high density of dislocations having a screw character and large driving force of MBE growth. Introduction of the AlN multiple interlayer (AlN -MIL) into MBE-GaN layers suppressed the formation of spiral hillocks. It was attributed to obstructing the dislocation propagation by AlN-MIL. Migration enhanced epitaxy (MEE) growth of GaN also reduced the density and tightness of spiral hillocks. This observation was attributed to that MEE growth technique decreased the driving force of growth.

Photoluminescence and reflectance spectroscopy of excitonic transitions in high-quality homoepitaxial GaN films

We report on highly resolved photoluminescence (PL) and reflectance (RF) spectra of a homoepitaxial GaN layer grown by metal-organic vapor phase epitaxy. This sample exhibits narrow linewidths of several PL emission peaks, down to $\ensuremath{\approx}100\ensuremath{\mu}\mathrm{eV}$ full width at half maximum for some bound exciton transitions. As a consequence, we have detected new PL features as, e.g., a fivefold fine structure of the donor-bound exciton line at $\ensuremath{\approx}3.471\mathrm{eV},$ and other known PL transitions could be determined with high precision. In RF measurements, the extraordinary quality of the epitaxial layer allowed observation of weakly damped excitonic ground-state transitions and of narrow excited exciton transitions with high signal-to-noise ratio.