R. J. Gorman

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...

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.

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°.

Properties of Si-doped GaN films grown using multiple AlN interlayers

Electrical, optical, and structural properties of Si-doped GaN films grown on multiple AlN interlayers (IL) sandwiched between high-temperature (HT) GaN are presented. We show that as the number of AlN IL/HT GaN layers increases, the electron mobility increases in the top Si-doped GaN layer, showing a near doubling from 440 to 725 cm2 V−1 s−1. Cross-sectional transmission electron microscopy images reveal a significant reduction in the screw dislocation density for GaN films grown on the AlN IL/HT GaN layers. The symmetric and off-axis x-ray linewidths increase as the number of AlN IL/HT GaN layers increase, indicating a greater relative misalignment of the adjacent HT GaN layers. Photoluminescence spectra of undoped and Si-doped GaN films on the multiple AlN IL/HT GaN layers have small yellow-band intensity. Analysis based on a single-donor/single-acceptor model for the electrical conduction suggests that the improved electron mobility is the result of a reduced acceptor concentration in the top GaN film...

Growth of (100) GaAs by vertical zone melting

Abstract The vertical zone melt growth technique (VZM) was used to grow undoped, semi-insulating GaAs. Single crystals 34 mm in diameter were grown in the direction. A molten zone length of 23 mm resulted in a solid-liquid interface which deviated only slightly from planarity to give a convex growing interface. With a vertical temperature gradient of 9δ C cm -1 (measured in the furnace), the dislocation density was a uniform (2−5) × 10 3 cm -2 throughout the crystal. Hall measurements, carbon determinations and EL2 determinations all indicated that the GaAs had a relatively uniform impurity content in the first 80% of the crystal. VZM was also used to zone level Sn dopant to give an n-type carrier concentration varying by approximately 20% over 80% of the crystal length.

A eutectic dislocation etch for gallium arsenide

A eutectic etchant consisting of 50 mole percent KOH and 50 mole percent NaOH has been developed having a melting point of 170°C. The etchant has an etch rate on 〈100〉 GaAs of 0.2 μm per min at 325°C and is useful on 〈111〉 and 〈100〉 surfaces. The etchant reveals structures not developed with the molten KOH etchant.

A eutectic hydroxide etch for β-SiC

Abstract The NaOH-KOH eutectic etch, used at 623 K, reveals antiphase boundaries, stacking faults and dislocations in β-SiC grown on (100) Si substrates. The etch also demonstrates the dramatic reduction in number of antiphase boundaries in β-SiC grown on off-axis (100) Si substrates.

Zone melt growth of GaAs for gamma ray detector applications

Abstract Vertical Zone Melt Growth (VZM) is being investigated as a technique for preparation of bulk GaAs crystals with applications as ambient temperature gamma ray detectors/spectrometers. Zone refining (repeated passes of a molten zone across a crystal) and zone levelling (one pass of a molten zone which is gallium rich) are being investigated for improving the purity of bulk GaAs and lowering the deep level donor defect (EL2) concentration. GaAs which has been zone refined and GaAs which has been zone levelled with a gallium rich melt have been grown, characterized and processed as detectors. In this paper, we discuss details of the VZM growth of GaAs and the crystalline and electronic properties achieved by zone levelling and zone refining.

A Kinetic Model for GaN Growth

A kinetic model is presented to explain GaN growth. The model is based on established values for the N and Ga desorption kinetics and well founded assumptions on the adsorption and decomposition of the N and Ga containing precursors. When grown on similar nucleation layers, it is shown that high quality GaN films are achieved when the V/III ratio is chosen to be slightly larger than the Ga and N desorption rates. The model is verified by comparing the structural, optical, and electrical properties of the GaN to the growth temperature and V/III ratio. The model explains several features of GaN growth including, growth conditions for smooth surface morphology, growth conditions for highly resistive GaN, and a possible explanation for the origin of Ga and N vacancies in GaN. Based on the growth model, ordering of the GaN during growth is achieved via an adsorption/desorption cycle where Ga and N containing species are exchanged between the gas phase boundary layer and the solid surface. Consequences of the model on establishing growth conditions and run-to-run reproducability are also discussed.

Enhanced GaN Decomposition at Movpe Pressures

GaN decomposition was studied above 800°C in flowing H 2 and N 2 for pressures ranging from 10 to 700 torr. From careful weighings of the GaN film on sapphire before and after annealing, the rates for GaN decomposition, Ga surface accumulation, and Ga desorption were obtained. An enhancement in the GaN decomposition rate was observed in H 2 pressures greater than 100 torr. Even with this enhanced GaN decomposition, the Ga desorption rate is nearly constant at higher pressures. As a result, Ga droplets accumulate on the surface. For N 2 pressures ranging from 76 to 400 torr no net enhancement in the GaN decomposition rate is observed and the GaN decomposition rate is reduced compared to identical annealing conditions in H 2 . This suggests that HI- is acting chemically to reduce the barrier for GaN decomposition. This may occur through a surface mediated dissociation of H 2 followed by the formation of more mobile and volatile hydrogenated N and Ga species. The significance of this study for GaN growth is that by increasing the GaN decomposition, the Ga atoms diffuse farther and subsequently re-incorporate into the growing lattice, increasing the GaN crystal quality. Connections between the enhanced GaN decomposition rate and the coalescing of nucleation layer during the ramp to high temperature and the consequences for the high temperature growth are discussed.