Stability of Sulfide Passivated Gallium Arsenide Surfaces
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J. S. SOLOMON, L. PETRY, AND S. R. SMITH University of Dayton Research Institute, Dayton, OH 45469-0167
ABSTRACT The chemical state of (NH 4)2 S treated (100)GaAs surfaces exposed to ambient conditions for several days was correlated with barrier height measurements. Surface chemistry was characterized by Auger electron spectroscopy and barrier heights were calculated from C-V measurements obtained with a Hg probe. Results show that surfaces are chemically and electrically unstable for several hours following the sulfide treatment. The chemical and electrical
states continually changed during ambient exposure up to 300 hours. Although strictly speaking, the surfaces were not passivated, the presence of sulfur did inhibit the formation of Ga and As
oxides and the incorporation of carbon. In addition, stable, low barrier heights were observed after ambient exposure for several hours. Barrier heights from C-V measurements using deposited Au and Al contacts were compared to the barrier heights obtained with a Hg probe. INTRODUCTION The primary limitation to the application of GaAs to device technology has been the native oxide that results from preprocessing wet chemical treatments. In the past few years there has been considerable hope that the problems associated with a surface oxide could be eliminated with sulfur passivation. Treating GaAs with (NH4)2S, for example, was reported to produce a surface
with low density of surface states and lower surface recombination velocities [ 1-2]. As this treatment was investigated there emerged several models to describe the passivation process and the surface chemistry that produced this effect. Some have claimed that sulfur forms a chemical bond with arsenic [3] while others have reported Ga - S bonding [4-6]. There are also conflicting reports concerning the effectiveness of (NH 4)2S in removing the native oxide [7]. Some doubt has been raised about the stability of sulfide passivated GaAs because the initially improved electronic properties have been reported to give way to the prepassivated state after exposure to ambient conditions [8-9]. If passivation is suppose to render a semiconductor surface chemically inert, prevent interfacial state formation, and impose a sufficient barrier to electron migration the question is - does (NH 4 )2 S do this? The electrical properties of sulfide passivated GaAs are not normally obtained directly from a passivated surface, but rather from a metal-semiconductor structure where pre-existing surface states are convoluted with metal- semiconductor interactions. Thus, measured electrical properties reflect the interface and not the surface states of the passivated GaAs. One could argue that the bottom line is the electrical behavior of the interface, but, in order to determine the effect of variations to the passivation process or environmental effects, the passivated surface must be characterized independent of a permanent metal overlayer. In this paper we report the chemical and electrical state of (NH 4)2S treated (100)GaAs after pro
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