Surface Oxidation Processes in Compound Semiconductors Studied by Profile Imaging
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SURFACE OXIDATION PROCESSES IN COMPOUND SEMICONDUCTORS STUDIED BY PROFILE IMAGING PING LU AND DAVID J. SMITH Dept. of Physics and Center for Solid State Science, University, Tempe, AZ 85287
Arizona State
ABSTRACT The profile imaging technique has been used to study the oxidation of ZnTe and InP surfaces induced by in situ reaction due to the electron beam of the microscope and by ex situ heating-in air. For both materials, in situ reaction with the electron beam resulted in desorption of the anion SecTes and the formation of the metal oxide. The observation of In metal
particles,
and
the
fact
that
the
rate
of
formation
of
In 20 3
was
substantially reduced by an improvement of the vacuum near the specimen region, suggested that the presence of oxygen is not involved in the desorption process. The ex situ heating of ZnTe up to 260*C in air resulted in crystals of ZnO and Te metT, generally in a layered surface region with the sequence of ZnTe/Te/ZnO. The large Te crystals usually had an epitaxial relationship with the bulk ZnTe but the small ZnO crystals had random orientations. The ex situ heating of InP to 380*C in air only gave rise to small crystals of Wn203 i-nrandom orientations. INTRODUCTION
A knowledge of the atomic geometries of semiconductor
surfaces and
their interaction with gaseous oxygen is very important for understanding the optical and electrical behavior of semiconductors. Traditional surface characterization methods such as low energy electron diffraction(LEED) and Auger electron spectroscopy only provide information averaged over a relatively large surface area. On the other hand, the profile imaging technique utilizes the atomic resolution capabilities of the high-resolution electron microscope(HREM) to provide real-space local information about surface structures and dynamic events[l]. In this paper, we report studies of surface oxidation processes of several compound semiconductors using the profile imaging method. The oxidation of InP and ZnTe surfaces caused by in situ reaction due to the electron beam under normal imaging conditions h-as Fe-en compared with the results of ex situ heating in air, and the consequences of improved vacuum condl-tion-s(-lO-° torr) have also been briefly investigated. Further details of the ZnTe studies can be found elsewhere[21. EXPERIMENTAL Small amounts of ZnTe crystals were crushed under purified methanol to form a fine powder which was then deposited on holey carbon support films. In order to avoid, or at least minimize, surface contamination, those specimens of ZnTe for in situ oxidation were immediately transferred to the microscope airlock where they were dried out during the pump-down process. Specimens of ZnTe for ex situ oxidation were heated in air at 260'C for various times before they were transferred into the microscope for observation. For InP specimens, crystals in orientation were first cut into 3mm discs, then mechanically polished to a thickness of - 20 microns, and finally ion milled to perforation by the use of 5keV I' ions.
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