Electrode Reaction of GaAs Metal Semiconductor Field‐Effect Transistors in Deionized Water

Abstract

The degradation of GaAs metal semiconductor field-effect transistor (MESFET) performance after rinsing for a short time in deionized water is investigated. It is caused by the oxidation of the GaAs surface especially in the periphery of metal electrodes. The mechanism of the rapid surface oxidation is contributed by the electrode reaction between GaAs and electrode metal. Oxygen dissolved in deionized water has a significant effect on the reaction.The GaAs integrated-circuit (IC) process is substantially different from the Si IC process because there is no high qualitynative oxide of GaAs. In Si IC processing thermal oxidation of the Si surface is performed first, and the surface of Si is covered by SiO2 except when ohmic contacts are to be formed. In GaAs IC processing, however, the surface of GaAs often is exposed to ambience because the native oxide of GaAs is unstable. Chemical vapor deposited SiQ (CVD SiO2) is used often to cover the GaAs surface. But the CVD SiO2 is removed repeatedly and redeposited in processing steps such as annealing, ohmic metallization, and Schottky gate formation. The GaAs surface of Schottky or ohmic contact areas, and peripheral regions of these electrodes are exposed to atmospheric ambience in some processing steps such as thermal-treatments, chemical treatments, and deionized water rinses. The exposure of the GaAs surface to thermal-treatments causes the evaporation of As from the surface and the exposure to water rinses causes rapid oxidation of the surface which results in the formation of troughs around the electrodes. 1 Rapid oxidation seriously damages GaAs MESFETs. So far there have been few discussions about the rapid oxidation, while there have been many discussions about As evaporation from the GaAs surface. We must clarify the surface oxidation mechanism and find a way of suppressing it immediately because more mass production of GaAs ICs is going to start for application to new microwave systems such as mobile telephone systems and satellite broadcasting systems.This paper investigates the mechanism of GaAs surface oxidation rapidly progressing in the peripheral region of metal electrodes in deionized water. It is proved experimentally that the oxidation rate depends on the dissolved oxygen in deionized water. Ways to suppress rapid surface oxidation are proposed. Oxidation of the GaAs Surface in the Periphery of ElectrodesIt is well known that the oxidation of GaAs naturally progresses in water or in atmosphere resulting in the growth of an oxide film several nanometers thick. 2 When such a thin oxide film uniformly covers the GaAs surface, various treatments such as dry etching and chemical etching can be performed without any problem after removing the thin oxide film. We can design processing steps which neglect the effect of the oxidation. After metal electrodes are formed on the GaAs surface, however, the oxidation rate is accelerated locally as high as several hundreds of nanometers per hour, and marked degradation of GaAs MESFET performance occurs. The increase...

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