Ge/Si(100) Island and Wetting Layer Composition
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Ge/Si(100) Island and Wetting Layer Composition Yangting Zhang and Jeff Drucker, Arizona State University, Dept. of Physics and Astronomy, Tempe, AZ 85287-1504, U.S.A. ABSTRATCT Etching water soluble Ge-oxides was used to investigate Si interdiffusion into epitaxial Ge / Si(100) samples. The Ge coverage, θGe, was measured using Rutherford backscattering spectrometry (RBS) before and after water etching of samples grown at substrate temperatures between 400°C and 650°C. θGe was correlated with sample morphology determined using atomic force microscopy (AFM). The local Ge concentration was qualitatively assessed using energy dispersive x-ray (EDX) analysis. For samples grown at T=400°C, water completely dissolves the islands and no Ge is detected by RBS. For samples grown at T=600 and 650°C, AFM detects no change in the surface morphology and RBS indicates that θGe decreases by about 3 monolayers (ML). These results suggest that for growth at T=400°C, both the islands and wetting layer are relatively pure Ge while for growth at T≥600°C, the wetting layer is Ge rich compare to the SiGe alloy islands. EDX confirms this conclusion detecting no Ge signal between islands for etched samples grown at T≥600°C. Our results suggest that for growth at T≥600°C, Si interdiffusion into islands is through the region underneath the islands instead of from the wetting layer. INTRODUCTION Si interdiffusion into nominally pure Ge islands grown on the Si(100) surface has been intensively investigated [1-7]. Si incorporation into the islands reduces their strain energy, leading to different morphology evolution and alters their optical and electronic properties. Investigations employing several techniques including electron energy loss spectroscopy (EELS) [7], x-ray photoelectron spectroscopy (XPS) [5], photoluminescence spectroscopy [8], and transmission electron microscopy (TEM) [3], have concluded that Si diffuses into islands formed by deposition of pure Ge. However, the source of Si in the islands has not been conclusively identified. One viewpoint is that the alloyed islands form out of an alloyed wetting layer. Another is that the Si source is the strained substrate under the islands. Here, we use water etching of soluble Ge oxides combined with atomic force microscopy (AFM), Rutherford backscattering spectroscopy (RBS), and energy dispersive x-ray (EDX) analysis to investigate the composition of the planar wetting layer and islands formed by molecular beam epitaxy (MBE) of pure Ge onto Si(100) at temperatures between 400 and 650°C. In the Ge / Si(100) system, 3-D coherent islands form to relieve the lattice mismatch induced strain energy. Following growth of a planar, 3 monolayer (1 ML = 6.78×1014 atoms/cm2) thick wetting layer, the first islands formed are either rectangular-based huts or square-based pyramids bound by {510} facets. When these clusters grow, they transform into domes bound by steeper facets to further reduce the strain energy. Continued dome cluster growth leads to formation of dislocated domes. Previous
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