Self-assembled Ge quantum dots on SiC substrates grown by UHV-CVD
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Self-assembled Ge quantum dots on SiC substrates grown by UHV-CVD C. Calmes, V. Le Thanh1, D. Bouchier, S. E. Saddow2, V. Yam, D. Débarre, R. Laval, and C. Clerc3 Institut d'Electronique Fondamentale, UMR-CNRS 8622, bât. 220, Université Paris-Sud, 91405 ORSAY Cedex (France) 1 CRMC2-CNRS, Campus de Luminy, Case 913, 13009 Marseille (France) 2 Electrical Engineering Department, University of South Florida, Tampa, FL 33618 (USA) 3 Centre de Spectroscopie Nucléaire et de Spectroscopie de Masse, bât. 108, Université ParisSud, 91405 ORSAY Cedex (France)
Abstract We report our first results using a ultra high vacuum chemical vapor deposition (UHV-CVD) system to form Ge quantum dots on off-axis SiC substrates. Pure SiH4 and hydrogen-diluted GeH4 were used as gas precursors. The SiC substrates were chemically cleaned using the modified RCA process and the SiO2 layer was removed in-situ under a low SiH4 flow rate at a temperature between 1030°C and 1080°C. The Ge quantum dots were grown at a temperature of 750°C. In-situ reflection high-energy electron diffraction (RHEED) was used to monitor the surface cleaning and the Ge quantum dot growth. Ex-situ scanning electron microscope and atomic force microscopy were used to confirm the presence of Ge dots. The observed dots are smaller (350 Å width and 100 Å height) than similar Ge dots grown on Si. Introduction The formation of self-assembled nanocrystals (i.e., quantum dots) has been an area of intense interest with significant work being reported in the SiGe/Si heterostructure system [1,2]. This work has been extended to include stacked layers of quantum dots using Si spacers for optoelectronic applications [3]. In all of this work the resulting quantum dot material consisted of an alloy of Si/Ge with the quantum dot size on the order of 100 nm in diameter with a height of 14-15 nm [3]. Quantum dots on SiC have already been reported using molecular beam Epitaxy (MBE) to form Si [4], and Si / Ge nanocrystals [5] on SiC(0001). This work was based on the Stranski-Krastanov (SK) growth mode via the use of a Si wetting layer to serve as the nucleation surface on the SiC substrate. In this paper we report the synthesis of self-assembled Ge quantum dots on 6H-SiC(0001) using an ultra-high vacuum chemical-vapor deposition (UHV-CVD) system. Experimental set-up The ultra high vacuum chemical vapor deposition (UHV-CVD) system offers the possibility of growing epilayers in a wide range of hydride pressures, from a background pressure of 10-10 Torr up to 10-1 Torr, with silane and germane as precursors [2]. The SiC substrate was fixed on a Tantalum plate and heated using a graphite/boron nitride heating element. The growth temperature TG is monitored using an infra-red pyrometer probing the active surface of the wafers. The gas flow rates are monitored with mass-flow meters, while the growth pressure was adjusted with a manual throttle valve. Since this valve was kept open during all the steps of the growth, the growth pressure was always below 4.5×10-4 Torr. A Reflection High Ene
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