X-Ray Diffraction Studies of Annealed SiGe/Si SLS
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X-RAY DIFFRACTION STUDIES OF ANNEALED SiGe/Si SLS WALTER P. LOWE, R. A. MACHARRIE AND J.C. BEAN AT&T Bell Laboratories, 600 Mountain Ave., Murray Hill, NJ 07974
ABSTRACT We have done x-ray diffraction to study the affect of annealing on a SiGe/Si strained-layer superlattice (SLS). The post anneal spatial distribution of the Ge and Si atoms within the alloy layer is of particular interest. X-ray data taken from an annealed sample along the superlattice direction (100) show two sharp and well separated superlattice envelope functions. In contrast to the unannealed sample, and what would be a typical scattering spectra for a superlattice, the envelopes bisect the position of the unannealed sample's envelope function. The position of the envelope function is related to the tetragonal distortion ET in the SiGe alloy layers. The two separate envelopes (a and 0) show that the SiGe alloy (100) planes have divided themselves into two groups with different interplanar spacings after the anneal. Each group with a tetragonal distortion such that c;< er, > e and = -J where eT is the tetragonal distortion in the unannealed sample. This is strong evidence for coherent phase separation perpendicular to the growth direction, along the SiGe alloy layer. In another investigation a search of reciprocal space for the 2 X 2 (... SiSiGeGe... ) (111) chemically ordered state showed no evidence for the existence of such an ordered state.
INTRODUCTION The strain-layer superlattice because of its highly strained state is studied with respect to how the strain changes in response to external stimuli. One such important and interesting stimulus is prolonged elevated temperatures. The dependence of the systems behavior on temperature is paramount in that device fabrication requires that the superlattice undergo anneals. Since the electronic energy bands are very sensitive to strain, any slight change in the strain away from the intended profile could prove harmful to device characteristics. The relationship between strain, energy gap and band offset will not be discussed here, but rather this discussion will be limited to the subtle changes in strain that can occur during annealing. Several theories have been presented recently on what happens when an SLS relaxes from its highly strained state [1-41. The theories that address the issue structurally allow the interface to become incommensurate resulting in a non-pseudomorphic structure [4]. Some of them describe a phase separated state [3,41 which lends support to our interpretation of x-ray data from this experiment.
MatLRes. Soc. Symp. Proc. Vol. 130. 019
Matls r RAesearch Society
206
DATA AND ANALYSIS This particular sample of Si.5 Ge.5 /Si was previously reported to transform to a chemically ordered state after being annealed [6]. The ordered state was described as consecutive pairing of pure Si and pure Ge planes along the (111) direction in the fashion {... SiSiGeGe...}. Half integral reflections at 1/2(111), 1/2(311), 1/2(331), etc., consistent with this ordering scheme were observ
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