Electrochemical and Chemical Sulfidation of GaAs: A Surface Chemical Study
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ELECTROCHEMICAL AND CHEMICAL SULFIDATION OF GaAs: A SURFACE CHEMICAL STUDY J. YOTA AND V. A. BURROWS Department of Chemical, Bio, and Materials Engineering and Center for Solid State Electronics Research, Arizona State University, Tempe, Arizona 85287-6006 ABSTRACT Treatment of gallium arsenide with sulfur-containing media has been shown to improve GaAs surface electronic properties. However, there is still considerable controversy regarding the chemical nature of the surface film which results from the sulfidation, and of the basis of the electronic improvement and of the decay in the improved electronic properties with time. We have investigated the surface chemistry of the chemical sulfidation treatment of GaAs with Na2S.9H20 and the electrochemical sulfidation treatment of GaAs with Na2S-9H20-ethylene glycol. Using surface infrared spectroscopy (SIRS), we have studied the film formed on the surface after the treatments and its behavior with time. Results show that the film on the GaAs surface contains sulfur which is often associated with oxygen, that this film slowly reacts in air to form unexpected species, e.g. sodium carbonate and sulfur-oxygen group-containing compounds, and that sulfur and oxygen are non-uniformly distributed. I. INTRODUCTION It has long been known that the GaAs surface is characterized by high density of surface states, pinning of the Fermi level, high surface recombination velocity, and a Schottky barrier height that is difficult to control. In the past decade, much effort has been expended to improve the electronic quality of this GaAs surface. One of the most promising methods to improve these surface properties is by treatment in sulfur-containing media. These include treatment with gases, such as H2S, and with liquids, such as Na2S.9H20 and (NH4)2S solutions. These treatments have been shown to result in improvement in the GaAs surface electronic properties, including low surface recombination velocity [1], low surface state density [2,3], and Fermi level shifting or unpinning [46]. In addition, enhanced performance in heterojunction bipolar transistors [7,8], Schottky barriers [2,6], and solar cells [6,91 have all been observed. However, these Improved electronic properties last only for a short time and eventually decay to their original values. In addition to electrical studies, some chemical characterization of sulfided GaAs has also been performed. Measurements have been made using x-ray photoelectron spectroscopy (XPS) [10,11], Auger electron spectroscopy (AES) [12,131, and reflection high energy electron diffraction (RHEED) [14,15]. Results vary from one study to another. Some studies reported that any preexisting oxide will be completely removed by the sulfidation treatment [14,16,17], while others reported that oxides remain after the treatment, disappearing only upon heating of the GaAs sample [17,18]. Some authors reported that both As-S and Ga-S bonds are present after treatment [14,18,191, while others only detected As-S 110,171 or Ga-S 1201 bonds on the surface after t
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