Search

The thermal conductivity of n- and p-type doped gallium nitride (GaN) epilayers having thicknesses of 3-4 µm was investigated using time domain thermoreflectance. Despite possessing carrier concentrations ranging across 3 decades (1015-1018 cm-3), n-type layers exhibit a nearly constant thermal conductivity of 180W/mK. The thermal conductivity of p-type epilayers, in contrast, reduces from 160 to 110W/mK with increased doping. These trends--and their overall reduction relative to bulk--are explained leveraging established scattering models where it is shown that, while the decrease in p-type layers is partly due to the increased impurity levels evolving from its doping, size effects play a primary role in limiting the thermal conductivity of GaN layers tens of microns thick. Device layers, even of pristine quality, will therefore exhibit thermal conductivities less than the bulk value of 240 W/mK owing to their finite thickness. [ABSTRACT FROM AUTHOR]

Smooth, commensurate alloys of 〈111〉-oriented Mg0.52Ca0.48O (MCO) thin films are demonstrated on Ga-polar, c+ [0001]-oriented GaN by surfactant-assisted molecular beam epitaxy and pulsed laser deposition. These are unique examples of coherent cubic oxide|nitride interfaces with structural and morphological perfection. Metal-insulator-semiconductor capacitor structures were fabricated on n-type GaN. A comparison of leakage current density for conventional and surfactant-assisted growth reveals a nearly 100x reduction in leakage current density for the surfactant-assisted samples. HAADF-STEM images of the MCO|GaN interface show commensurate alignment of atomic planes with minimal defects due to lattice mismatch. STEM and DFT calculations show that GaN c/2 steps create incoherent boundaries in MCO over layers which manifest as two in-plane rotations and determine consequently the density of structural defects in otherwise coherent MCO. This new understanding of interfacial steps between HCP and FCC crystals identifies the steps needed to create globally defect-free heterostructures. [ABSTRACT FROM AUTHOR]

We demonstrate the use of anomalous x-ray scattering of constituent cations at their absorption edge, in a conventional Bragg-Brentano diffractometer, to measure absolutely and quantitatively the polar orientation and polarity fraction of unipolar and mixed polar wurtzitic crystals. In one set of experiments, the gradual transition between c+ and c- polarity of epitaxial ZnO films on sapphire as a function of MgO buffer layer thickness is monitored quantitatively, while in a second experiment, we map the polarity of a lateral polar homojunction in GaN. The dispersion measurements are compared with piezoforce microscopy images, and we demonstrate how x-ray dispersion and scanning probe methods can provide complementary information that can discriminate between polarity fractions at a material surface and polarity fractions averaged over the film bulk. [ABSTRACT FROM AUTHOR]

A two-step homoepitaxial growth process producing step-free surfaces on low dislocation density, Ga-polar ammonothermal GaN single crystals is described. Growth is conducted under very low supersaturation conditions where adatom incorporation occurs predominantly at step edges, and lateral growth is strongly preferred. The achievable step-free area is limited by the substrate dislocation density. For ammonothermal crystals with an average dislocation density of ∼1 × 104 cm−2, step-free mesas up to 200 × 200 μm2 in size are achieved. These remarkable surfaces create a unique opportunity to study the effect of steps on the properties and performance of semiconductor heterostructures. [ABSTRACT FROM AUTHOR]

The synthesis of ytterbium monoxide (YbO) by molecular beam deposition (MBD) is investigated for developing a lattice-matched oxide to gallium nitride. Because Yb2O3 is the thermodynamically stable oxide at atmospheric pressure, YbO has previously only been prepared by high-pressure bulk synthesis reactions. This article demonstrates that through low growth temperature and the kinetic control of molecular fluxes allowed by MBD, the polycrystalline monoxide phase can be stabilized in thin film form on silicon substrates. Once deposited, the YbO rocksalt structure is found to be stable at room temperature and atmospheric pressure. Elemental analysis indicates that films are primarily composed of Yb and O. Elemental concentrations of H, C, and N, which are known to form phases with Yb of similar structure and lattice parameter to YbO, are shown to be below the 1 at. % level. [ABSTRACT FROM AUTHOR]

Deposition of lanthanide oxides on GaN is investigated as a means to produce stable dielectric with good interface quality. Epitaxial growth of cubic <111>-oriented Sc2O3 is observed on GaN [0001] with a lattice mismatch of 7.2%, whereas La2O3 growth, with a lattice mismatch of ∼21%, results in a mixed phase epitaxial film comprised of the bixbyite and hexagonal allotropes. Substantial roughening accompanies this mixed phase film. By inserting a thin Sc2O3 interfacial layer between La2O3 and GaN and a Sc2O3 capping layer, the crystal quality of the predominantly La2O3 layer is improved and the stack is stabilized against hydroxide formation under ambient conditions. [ABSTRACT FROM AUTHOR]

GaN surface and near-surface chemistry influence on band offsets of oxide overlayers is demonstrated through X-ray photoelectron spectroscopy measurements using epitaxial (111)-oriented MgO films on (0001)-oriented Ga-polar GaN as a case study. For identical cleaning and MgO growth conditions, GaN subsurface oxygen impurities influence the GaN bare surface band bending and the ultimate band offset to MgO heterolayers. As the GaN surface oxygen concentration increases from an atomic concentration of 0.9% to 3.4%, the valence band offset to MgO decreases from 1.68 eV to 1.29 eV, respectively. This study highlights the sensitivity of the oxide/nitride interface electronic structure to GaN epilayer preparation and growth conditions. [ABSTRACT FROM AUTHOR]

Abstract: Molecular beam deposition systems allow for unparalleled control of film composition and structure. This article addresses the capacity for controlling metal and oxidant fluxes in the Yb/O2 system to access the metastable phase ytterbium monoxide (YbO). Experiments exploring the growth of polycrystalline YbO x films by molecular beam deposition demonstrate that a 2:1 molar ratio of Yb:O2 fluxes is necessary to achieve preferential growth of the divalent oxide. Applying similar deposition conditions to a (001) GaN surface leads to the growth of epitaxial (111) YbO films. Similar to other rocksalt oxides grown on GaN surfaces, YbO films display a 3D growth mechanism that leads to a grainy morphology with crystallites of 50nm lateral dimensions. Rocking curves in ω and φ have full-width half-maximum values of 1.77° and 4.1°, respectively; further improvements in crystal quality appear to be limited by the thermal stability of the YbO phase. [Copyright &y& Elsevier]