The Generation and Optical Activity of Misfit Dislocations by Very Low Level Transition Metal Contamination of SiGe/Si
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THE GENERATION LOW LEVEL TRANSITION AND OPTICAL METAL ACTIVITY CONTAMINATION OF MISFIT OF DISLOCATIONS SiGe/Si. BY VERY
Philip Kightley, Victor Higgs* and Peter J Goodhew. Department of Materials Science and Engineering, University of Liverpool, PO Box 147, Liverpool, England, L69 3BX. *Department of Physics, Kings College London, London WC2R 2LS. ABSTRACT. An initially pseudomorphic Si0.93Ge0.07 layer is forced to relax after the deposition of 0.003 monolayers of Cu contamination and a 600°C anneal. Photoluminescence shows the dislocations have similar luminescence features (D1 to D4) to dislocations generated in bulk silicon. TEM is used to study the generation of the misfit dislocations. It is shown that the half loops form at the sample surface and a mechanism that involves the action of both the metal contaminant and the surface steps is suggested to explain the geometric arrangement of the dislocation loops that are present. The sample was found to be only 22% relaxed whereas equilibrium theory predicts 92% relaxation. INTRODUCTION. Crystal dislocations in silicon are thought to provide deep states within the band gap. This is often demonstrated by the presence of D-bands in the Si photoluminescence (PL) specu'a [1]. The nature of the optical centres responsible for these transitions remains unidentified although it is usually assumed that both clean and decorated dislocations will act as recombination centres [2]. In recent publications Higgs et al. [3,4] have shown that dislocations produced in bulk silicon, using the conventional two stage axial deformation procedure, do not necessarily produce PL or show EBIC contrast. Only after contamination with low levels of Cu, Fe, Ni, Ag or Au were strong PL and EBIC contrast observed, with the intensity of the PL and the contrast of the EBIC both depending on the concentration of the contaminant. This showed that 'clean' dislocations could not be detected by both techniques and regarded as inactive optical and electrical recombination centres. In a previous publication [5] the activation of the formation of surface half loops was shown to occur only in the presence of a Cu contaminant although it was not possible to specify the exact source of the misfit dislocations. In the case of Higgs et al. [5] the defects were identified by PL, defect etching and transmission electron microscopy (TEM). In this paper the TEM results are elaborated upon and more details of the microstructure of the sample are presented. EXPERIMENTAL. This work has been conducted on a single wafer [6] that was sectioned and then either used for control experiments [4] or for deliberate contamination with transition metals. The sample used was a 1 •tm thick pseudomorphic Si0.93Ge0.07 layer on a Si substrate and was grown at 500°C. This paper is only concerned with the microstructure of a portion of the wafer contaminated with an estimated 0.003 monolayers of Cu and then annealed to produce the dislocations. The contamination procedure involves the dipping of the previously RCA cleaned wafer in a
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