Soojeong Choi

Effects of Threading Dislocation Density on the Gate Leakage of AlGaN/GaN Heterostructures for High Electron Mobility Transistors

AlGaN/GaN heterostructures were regrown on three semi-insulating GaN templates with threading dislocation densities of ~2×1010, ~5×108, and ~5×107 cm-2. Regrowths were carried out under Ga-rich conditions by plasma-assisted molecular beam epitaxy to determine the effects of threading dislocation density on leakage through Schottky contacts on the AlGaN/GaN heterostructures. A similar AlGaN/GaN heterostructure was directly grown on 4H-SiC under Ga-rich conditions for comparison with the regrown heterostructures. High electron mobility transistors were fabricated. Decreasing the threading dislocation density from ~2×1010 to ~5×107 cm-2 yielded up to a 45-fold decrease in the average reverse Schottky diode current at -10 V bias.

Surface oxide relationships to band bending in GaN

A trend of increased near-surface valence band maximum band bending with increasing O∕Ga relative fraction was observed, extrapolating to 2.7eV±0.1eV for pristine GaN surfaces (0% O 1s peak area). This trend of apparent oxide overlayer coverage affecting the band bending linearly could lead to better understanding and characterization of oxidized GaN surfaces to control band bending for sensors or other devices.

Observation of columnar microstructure in lattice-matched InAlN/GaN grown by plasma assisted molecular beam epitaxy

Nominally lattice matched InAlN/GaN was grown by plasma-assisted molecular beam epitaxy, and the intrinsic microstructure was investigated via x-ray diffraction, transmission electron microscopy, and atom probe tomography. The InAlN showed a cellular structure, which was comprised of ∼10 nm wide Al-rich cores and ∼1 nm In-rich InAlN intercellular boundaries. Despite the strong laterally non-uniform In distribution, both vertical and lateral lattices are unperturbed by the cellular structure, as evidenced by strong thickness fringes in on-axis ω−2θ high resolution x-ray diffraction scans, coherence lengths derived from on-axis (0002) and off-axis (101¯2) ω−2θ high resolution x-ray diffraction scans, and a modified Williamson-Hall analysis for on-axis reflections.

Molecular beam epitaxy of InAlN lattice-matched to GaN with homogeneous composition using ammonia as nitrogen source

InAlN lattice-matched to GaN was grown by molecular beam epitaxy (MBE) using ammonia as the nitrogen source. The alloy composition, growth conditions, and strain coherence of the InAlN were verified by high resolution x-ray diffraction ω-2θ scans and reciprocal space maps. Scanning transmission electron microscopy and energy-dispersive x-ray spectroscopy of the InAlN revealed the absence of lateral composition modulation that was observed in the films grown by plasma-assisted MBE. InAlN/AlN/GaN high electron mobility transistors with smooth surfaces were fabricated with electron mobilities exceeding 1600 cm2/Vs and sheet resistances below 244 Ω/sq.

Growth, Structural, and Electrical Characterizations of N-Polar InAlN by Plasma-Assisted Molecular Beam Epitaxy

N-polar InxAl1-xN (0.02 560 °C. A smooth surface morphology was obtained for In0.18Al0.82N lattice-matched to GaN. Subsequently, N-polar In0.18Al0.82N was used as a charge-inducing barrier in a N-polar GaN HEMT structure and electrical characterizations including current–voltage (I–V) measurements were performed.

In situ spectroscopic ellipsometry to monitor surface plasmon resonant group-III metals deposited by molecular beam epitaxy

The evolution of the surface plasmon resonance of Al, Ga, and In deposited by molecular beam epitaxy on GaN surfaces was monitored in real-time using spectroscopic ellipsometry. The correlation between the metal plasmon resonance modes, the particle size, and the growth mode is addressed. Ga and In deposited on GaN substrates form nanoparticles while the Al is shown to form a nearly coalesced thin film. The plasmon resonance of the Ga and In nanoparticles redshift with increasing average particle size while the pseudodielectric function of Al approaches that of a Drude metal.

Electrical and optical properties of p-type InN

We have performed comprehensive studies of optical, thermoelectric and electrical properties of Mg doped InN with varying Mg doping levels and sample thicknesses. Room temperature photoluminescence spectra show a Mg acceptor related emission and the thermopower provides clear evidence for the presence of mobile holes. Although the effects of the hole transport are clearly observed in the temperature dependent electrical properties, the sign of the apparent Hall coefficient remains negative in all samples. We show that the standard model of two electrically well connected layers (n-type surface electron accumulation and p-type bulk) does not properly describe Hall effect in p-type InN.

Characteristics of InN grown on SiC under the in-rich regime by molecular beam heteroepitaxy

InN epitaxial films were grown by N2 plasma-assisted molecular beam epitaxy on 4H- and 6H-SiC substrates using low-temperature InN nucleation layers. InN films grown at various In fluxes under the In-rich regime show improved crystal quality, surface morphology, and optical properties, without sizable metallic In incorporation. Photoluminescence measurements show emission up to room temperature, band gap values as low as 0.64eV at T=10K, and carrier concentrations of the order of 8×1017cm−3.

Kinetics of gallium adlayer adsorption/desorption on polar and nonpolar GaN surfaces

Spectroscopic ellipsometry installed on a GEN-II plasma assisted molecular beam epitaxy machine has been shown to be an effective in situ real time tool for monitoring the kinetics of gallium adlayer adsorption/desorption on the GaN surface. In this work, the authors present data on the study of Ga adsorption/desorption on polar c-plane GaN (0001) and nonpolar m-plane GaN (1−100) surfaces for Ga beam equivalent pressures in the range of 8.96×10−8–1.86×10−7Torr, Ga pulses in the range of 5–360s, and for substrate temperatures between 650 and 750°C.

Study of the dielectric function of hexagonal InN: Impact of indium clusters and of native oxide

The complex dielectric function of hexagonal InN has been determined in the 0.72–6.50eV photon energy range using spectroscopic ellipsometry. The InN films have been synthesized using molecular beam epitaxy on Si-face 6H-SiC(0001) substrates. The fundamental band gap E0 and higher energy interband critical points have been identified at room temperature. The impact of indium clusters and of the InN native oxide on the dielectric function is discussed.