Investigation of Epitaxial Overgrowth on Germanium Quantum Dots
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Investigation of Epitaxial Overgrowth on Germanium Quantum Dots David W. Greve and Qian Zhao Department of Electrical and Computer Engineering Carnegie Mellon University Pittsburgh, PA, 15217, U.S.A. ABSTRACT We report on the characterization of germanium quantum dots grown on silicon (001) substrates by ultra-high vacuum chemical vapor deposition (UHV/CVD). In many applications small and uniform quantum dots are required which must be overgrown by a silicon epitaxial layer. We report here on the effect of carbon predeposition from methylsilane on the dot size and uniformity. In addition, we use reciprocal space mapping to evaluate the quality of epitaxial layers which overgrow the quantum dots. The results show some differences from previous reports on MBE-grown dots. INTRODUCTION Semiconductor quantum dots have important potential applications in devices such as quantum dot lasers, resonant tunnel diodes, and quantum cellular automata. In such applications small and uniform dot size is required and in addition it is necessary to overgrow the quantum dots with epitaxial layers. It has been reported that under some conditions carbon predeposition can decrease the dot size [1, 2]. In this paper, we investigate the influence of carbon predeposition on the dot size in CVD growth and also the overgrowth of self-organized germanium quantum dots with silicon. EXPERIMENTAL Quantum dots were grown at 600 C on (001) silicon substrates in a multiwafer UHV/CVD reactor using silane, germane (10% in hydrogen) and methysilane (2% in hydrogen) as reactants. Before initating growth, the native oxide was thermally desorbed at 800 ºC followed by growth of a silicon buffer layer for 10 minutes. After deposition of the silicon buffer layer, a growth pause of 2 min preceded growth of the quantum dots. Quantum dots were grown by exposing the wafer to 0.82 mTorr of 10% germane in hydrogen for varying times. In order to investigate the effect of carbon predeposition, the wafer was exposed to 0.23 mTorr of 2% methysilane in hydrogen, followed by a 2 min pause before growth of the quantum dots. For measurement of the dot size distribution, the wafers were cooled to room temperature immediately after completion of all growth steps (Fig. 1a). For studies of epitaxial layer overgrowth, some samples were overgrown with silicon by exposing to silane for 5 or 12.5 minutes (Fig. 1b) or by a 12.5 min silane exposure followed by growth of a Si0.85Ge0.15 epitaxial layer (Fig. 1c). By performing X-ray diffraction studies of the overgrown Si0.85Ge0.15 layer, we obtained a measure of the overall epitaxial layer crystal quality. This complements information from local techniques such as scanning tunneling microscopy [3] and atomic force microscopy [4].
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Figure 1. Structures examined in this study: (a) germanium dots formed on silicon buffer layer; (b) germanium dots overcoated with silicon; and (c) dots with additional overlayer of Si0.85Ge0.15. In some samples carbon was deposited after growth of the silicon buffer and before growth of the quan
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