Masamichi Yamada

Characterization of Oxidized GaAs (001) Surfaces Using Temperature Programed Desorption and X-Ray Photoelectron Spectroscopy

Temperature programed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) were carried out on oxidized GaAs (001) surfaces in order to obtain insight into the durability of the oxide masks used in in situ selective-area processing. The TPD spectra comprised three successive desorption peaks showing the desorption of arsenic at 390°C, Ga2O at 475°C, and both Ga2O and arsenic above 500°C. XPS revealed the disappearance of As oxide and an increase of Ga oxide during the first desorption. The coexistence of two forms of Ga oxide, i.e., Ga2O and Ga2O3, is suggested, and a mechanism of oxide desorption is proposed.

Growth Conditions of Subgrain-Free LiNbO3 Single Crystals by the Czochralski Method

Subgrain-free LiNbO3 single crystals were grown by the CZ method. It is generally accepted that thermal fluctuations are a serious drawback in growing a high-quality crystal. However, we found that subgrain-free LiNbO3 crystals could be grown only under regular thermal fluctuation of the melt. In order to analyze the effect of thermal fluctuation, we measured such properties of molten LiNbO3 as the viscosity, density, surface tension, and electrical conductivity. From these measurements, we speculate the existence of clusters made of niobium oxide molecules and lithium oxide molecules below 1280°C. The volume fraction of these clusters decreases as the temperature of molten LiNbO3 increases. A reduction of the density of these clusters prevents LiNbO3 crystals from producing subgrains. This reduction is regarded as one of the necessary conditions required for growing a subgrain-free LiNbO3 single crystal.

Effect of Atomic Hydrogen on GaAs (001) Surface Oxide Studied by Temperature-Programmed Desorption

Atomic hydrogen (H)-induced modification of Ga2O3-like oxide on GaAs (001) was studied by temperature-programmed desorption. H treatment at 300°C caused a new Ga2O desorption starting at around 400°C, whereas a sample treated with molecular hydrogen gave simultaneous desorption of Ga2O and elemental As only above 500°C, the same as observed in the untreated sample. X-ray photoelectron spectroscopy showed a reduction in O1s intensity after the H treatment. These results indicate that atomic hydrogen converts Ga2O3-like oxide into volatile Ga2O-like oxide and consequently lowers the oxide-removal temperature.

Analysis of GaAs MOMBE Reactions by Mass Spectrometry

The thermal decomposition of trimethylgallium (TMG) under metal-organic molecular beam epitaxy (MOMBE) conditions is studied by mass spectrometry. For the first time, it is observed that the amount of Ga-containing species desorbed from the GaAs surface decreases above 350°C. Measurements on CH3, CH4 and C2H6 indicate that TMG pyrolyzes by releasing methyl radicals. The amount of Ga-containing species desorbed from the SiO2 surface does not show a steep decrease up to 530°C, which indicates that no thermal decomposition occurs on SiO2. This explains the mechanism of selective epitaxy in MOMBE.

Characterization of GaAs-(001) Surface Photo-Oxide Formed by Visible-Light Irradiation

The effects of visible-light irradiation (70-280 mW/cm2) on oxidation of a GaAs (001) surface under an oxygen atmosphere (3.6×101 and 8.6×104 Pa) were investigated by temperature programmed desorption and X-ray photoelectron spectroscopy (XPS). XPS showed that irradiation at an intensity of 140 mW/cm2 increased the amount of surface oxygen by about 15% over dark oxidation of the corresponding oxygen exposure. Under 540°C isothermal conditions, the oxide desorption rate was initially low, but reached its maximum level with a time delay, showing the existence of expanding voids in the oxide layer during desorption. Light irradiation, as well as higher oxygen exposure, decreased the initial desorption rate, and increased the time delay by up to 6.4 min. This is interpreted as indicating that irradiation reduces the density of the weak parts in the oxide layer through an increase in the oxide thickness.

Real-Time Observations on the Cleaning Process of Patterned GaAs Substrates.

We report on real-time scanning microprobe reflection high-energy electron diffraction (µ-RHEED) observations on the cleaning process of mesa-etched GaAs (100) surfaces for the first time. Both the initial (100) surfaces and the (111)A sidewall have been found to be uniformly cleaned at temperatures of about 400° C using atomic hydrogen ( H•); further, RHEED intensity oscillations more than 50 periods have been observed during direct molecular-beam epitaxy (without a GaAs buffer layer) of GaAs on the cleaned (100) surface. On the other hand, an inhomogeneous desorption of the oxide layer has been observed during conventional thermal cleaning under an As4 pressure at about 600° C. The results indicate that the H• treatment of patterned substrates is useful for obtaining smooth and clean surfaces without the growth of a GaAs buffer layer.