Epitaxial Ge Films on Si Substrates
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EPITAXIAL Ge FILMS ON Si SUBSTRATES J.S. MCCALMONT, D. ROBINSON, K.M. LAKIN, AND H.R. SHANKS Microelectronics Research Center, Iowa State University, Ames,
IA 50011
ABSTRACT Thin films of germanium have been prepared using an ultrahigh vacuum ionized-cluster beam (ICB) system. The dopant concentration of the films was varied by alloying the germanium source material with aluminum, a p-type dopant. X-ray diffraction analysis of the films has shown that an epitaxial (100) germanium film can be deposited on a (100) silicon substrate with a substrate temperature as low as 300*C. The results confirm that ICM deposition can be used to prepare epitaxial germanium films, but ionization of the clusters does not appear to affect the film growth. INTRODUCTION Epitaxial germanium films on silicon have recently been investigated as potential buffer layers between silicon substrates and gallium arsenide films. The Ge films have been deposited primarily by evaporation [1-31 and by molecular beam epitaxy [4-81. C. Yamagishi and co-workers 191, however, reported the use of ICB to grow epitaxial Ge films on (100) Si substrates. This paper reports recent efforts to use an ultrahigh vacuum ICB system to grow Ge films on Si substrates. EXPERIMENTAL PROCEDURE The UHV ICB system used in this work has been described previously [101. The system consi$ts of a .ingle ICB source in a UHV chamber capable of a base pressure of 10-lu to 10-11 Torr.
The source crucibles used for the depositions were madl of high density, high purity graphite with an internal volume of about 10 cm . The crucible nozzle nominally measured 1 mm in diameter and 1 mm in length. The deposition chamber is designed to accept a single three-inch substrate; about 32 cm2 or 70% of the substrate surface area is exposed to the source beam during the deposition. The (100) orientation substrates used were n-doped silicon with a resistivity in the range 4-9 9-cm. The source material for the depositions consisted of either 99.999% purity germanium or a mixture of germanium and 99.999% purity aluminum. There was about 1.3-1.4 atomic percent aluminum in the doped sources. The pieces were placed into the crucible so that the aluminum was not in contact with the crucible walls, but was supported by the germanium pieces, to minimize aluminum reaction with the graphite crucible as it melted. Prior to the first deposition the doped source was soaked at about 1000 0 C to allow the aluminum and germanium to alloy. The substrates were prepared for deposition by cleaning in organic solvents, followed by a weak hydrofluoric acid (5% HF) etch to remove native oxides. Immediately after etching, the substrate was placed in vacuum. A load lock arrangement was used to prevent frequent cycling of the deposition chamber vacuum. The depositions were performed semi-automatically through the use of a deposition controller. The controller was used to slowly ramp the source power up in two stages. First, the source was brought relatively quickly to an initial soak temperature of about 1000 0 C to mel
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