R. L. Henry

Formation of high Tc superconducting films by organometallic chemical vapor deposition

The first growth of superconducting YBaCuO films by organometallic chemical vapor deposition is described. Metal β‐diketonates were decomposed thermally on MgO substrates in an oxygen‐rich atmosphere to produce amorphous brown films. Subsequent annealing in oxygen yielded dull gray films whose thickness corresponded to deposition rates of approximately 8 nm min−1. These films showed semiconductor‐like behavior at higher temperatures, followed by a broad resistive transition from 80 to 36 K with the resistance becoming zero at ∼20 K. Analysis of x‐ray data indicated the presence of the orthorhombic superconducting phase and various other metal oxides. Profilometer measurements yielded film thicknesses up to 950 nm, and scanning electron microscopy revealed faceted grains from 0.5 to 1.0 μm in size.

GROWTH MODEL FOR GAN WITH COMPARISON TO STRUCTURAL, OPTICAL, AND ELECTRICAL PROPERTIES

A kinetic model is presented to explain the metal organic vapor phase epitaxy (MOVPE) growth of GaN. The model is based upon measured desorption rates and assumptions on the precursor dissociation and sticking probabilities. The model shows how the growth temperature and V/III ratio are linked for the growth of high quality GaN films. From a comparison of growth conditions cited in the literature to the quality of GaN produced, optimal film growth appears to occur when the V/III ratio is chosen to be slightly larger than the N to Ga desorption ratio. The relationship between the growth temperature, V/III ratio, and GaN quality are explained in terms of how the growth parameters influence the incorporation of Ga and N atoms into the growing film. The Ga and N diffusion lengths are estimated to be 2–20 nm and <1 nm at 1050 °C, respectively, for practical MOVPE growth rates. Growth conditions for smooth (0001) surface morphology are described in terms of the growth model, as well as possible origins for defe...

Modulation spectroscopy as a tool for electronic material characterization

Modulation spectroscopy is an optical characterization tool that can be of great utility to the materials scientist. We present here numerous examples where a simple photo-reflectance and electroreflectance setup is used in our laboratory to determine such important material parameters as alloy composition and carrier concentration in a very short time. For determining alloy composition in semiconductors, contactless room temperature photoreflectance is nearly as sensitive as low temperature photoluminescence. Examples will be given on how to determine: the effects of surface preparation and implant damage; alloy composition and carrier homogeneity for large area wafers to better than 1%; the segregation coefficient of isoelectronic impurities in bulk semiconductors; the sub-band energies in quantum well structures; and the presence and homogeneity of built-in electric fields in MODFET structures. Particular emphasis will be placed on band edge and exciton effects on the photoreflectance and on the criteria used to distinguish between them. Materials studied included Si doped GaAs, AlxGa1-xAs for variousx grown by OMVPE and MBE, bulk InP doped with iso-electronic As and Sb, and MODFET structures.

Enhanced GaN decomposition in H2 near atmospheric pressures

GaN decomposition is studied at metallorganic vapor phase epitaxy pressures (i.e., 10–700 Torr) in flowing H2. For temperatures ranging from 850 to 1050 °C, the GaN decomposition rate is accelerated when the H2 pressure is increased above 100 Torr. The Ga desorption rate is found to be independent of pressure, and therefore, does not account for the enhanced GaN decomposition rate. Instead, the excess Ga from the decomposed GaN forms droplets on the surface which, for identical annealing conditions, increase in size as the pressure is increased. Possible connections between the enhanced GaN decomposition rate, the coarsening of the nucleation layer during the ramp to high temperature, and increased GaN grain size at high temperature are discussed.

Guided resonances in asymmetrical GaN photonic crystal slabs observed in the visible spectrum

We demonstrate that guided resonant modes can be readily observed in asymmetrical photonic crystal slabs on high-index substrates. In spite of the high radiative loss associated with all optical modes in these cases, the guided resonant modes are found to give rise to strong high-Q features in the transmission spectra. Since these photonic crystal structures are far more robust and easier to fabricate than the free-standing photonic crystal membranes used in previous studies of guided resonant modes, detailed studies of relevant optical phenomena and the implementation of proposed applications are greatly simplified.

Self-Assembled Monolayers of Alkylphosphonic Acid on GaN Substrates

In this paper we describe the formation and characterization of self-assembled monolayers of octadecylphosphonic acid (ODPA) on epitaxial (0001) GaN films on sapphire. By immersing the substrate in its toluene solution, ODPA strongly adsorbed onto UV/O 3-treated GaN to give a hydrophobic surface. Spectroscopic ellipsometry verified the formation of a well-packed monolayer of ODPA on the GaN substrate. In contrast, adsorption of other primarily substituted hydrocarbons (C n H 2 n+1 X; n = 16-18; X = -COOH, -NH 2, -SH, and -OH) offered less hydrophobic surfaces, reflecting their weaker interaction with the GaN substrate surfaces. A UV/O 3-treated N-polar GaN had a high affinity to the -COOH group in addition to ODPA, possibly reflecting the basic properties of the surface. These observations suggested that the molecular adsorption was primarily based on hydrogen bond interactions between the surface oxide layer on the GaN substrate and the polar functional groups of the molecules. The as-prepared ODPA monolayers were desorbed from the GaN substrates by soaking in an aqueous solution, particularly in a basic solution. However, ODPA monolayers heated at 160 degrees C exhibited suppressed desorption in acidic and neutral aqueous solution maybe due to covalent bond formation between ODPA and the surface. X-ray photoelectron spectroscopy provided insight into the effect of the UV/O 3 treatment on the surface composition of the GaN substrate and also the ODPA monolayer formation. These results demonstrate that the surface of a GaN substrate can be tailored with organic molecules having an alkylphosphonic acid moiety for future sensor and device applications.

Controlled oxygen doping of GaN using plasma assisted molecular-beam epitaxy

High-quality (0001) and (0001)-GaN films were grown by plasma-assisted molecular-beam epitaxy to study the dependence of oxygen incorporation on polarity and oxygen partial pressure. Oxygen incorporates at a rate ten times faster on nitrogen-polar GaN than on the Ga polarity. Oxygen doping is controllable, reproducible, and produces low compensation material up to concentrations of at least 1018 cm−3 with higher levels showing significant compensation. Layers containing oxygen at levels above 1022 cm−3 exhibit severe cracking while oxygen concentrations less than 1021 cm−3 do not introduce significant strain. The oxygen incorporation rate has a weak dependence on Ga overpressure during Ga-stable growth but dramatically increases for conditions approaching N-stable growth.

Enhancement of electrical and structural properties of GaN layers grown on vicinal-cut, a-plane sapphire substrates

We report the observation of significant enhancement in the electrical and crystalline properties of GaN layers grown on vicinally cut, a-plane sapphire substrates. Room-temperature Hall mobility and x-ray rocking curve data show a nearly twofold improvement, independent of the processing conditions, for layers grown on substrates having vicinal angles of 1.5° compared to on-axis substrates. Transmission electron microscopy shows reduced edge dislocation density and better alignment of the grains in layers grown on vicinally cut substrates. Preliminary photoluminescence measurements also indicate pronounced differences in the yellow band spectra between the on-axis and off-axis cut substrates. These findings contrast the relatively modest improvements observed in layers grown on c-plane substrates with vicinal angles as high as 10°.

Ion‐implanted n‐channel InP metal semiconductor field‐effect transistor

Device‐quality n‐type layers have been produced by ion implantation in Fe‐doped semi‐insulating InP. 29Si has been used as the dopant and anneals were carried out with the aid of a multiple‐layered encapsulant consisting of plasma‐deposited Si3N4 and pyrolytic P‐doped SiO2. These layers have been used to make n‐channel MESFET’s for which gains of 13.7 and 9.8 dB were measured at 8 and 10 GHz, respectively. The gate metallization for these devices was Au. Low‐leakage currents and adequate gate breakdown characteristics were observed.

Aluminum, magnesium, and gold contacts to contamination free n-GaN surfaces

Current–voltage (I–V) characteristics of Al, Mg, and Au Schottky contacts to atomically clean n-GaN(0001)-1×1 surfaces prepared in an ultrahigh vacuum were investigated. The Al/n-GaN contact is rectifying at room temperature and becomes Ohmic after annealing at 500 °C. Coupled with previous photoemission spectroscopy data, this result demonstrates that the origin of the Ohmicity is the reaction-induced doping of the interface. For nonannealed interfaces, the Schottky barrier heights determined from I–V characteristics are in qualitative agreement with the results obtained by photoemission spectroscopy. We find that the ideality factor of the barrier is close to unity for the unreactive interface i.e. Au/GaN, but significantly higher for the reactive interfaces, i.e., Al/GaN and Mg/GaN. Our experimental results suggest that the reaction-induced defects and thermionic field emission play an important role in the electrical behavior of these interfaces.