Formation of Ge Nanocrystals Passivated with Si by Gas Evaporation of Si and Ge
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Formation of Ge Nanocrystals Passivated with Si by Gas Evaporation of Si and Ge Junjie Si, H. Ono, K. Uchida, S. Nozaki, and H. Morisaki Department of Electronic Engineering, University of Electro-Communications, Chofu-shi, Tokyo 182-8585, JAPAN ABSTRACT Two methods are employed in the gas evaporation technique to form Ge nanocrystals with the Si-passivated surface. One uses one boat with a SiGe alloy as a source, and the other uses two boats each with Si and Ge. As a result of characterization by the x-ray diffraction (XRD) measurement, Raman scattering and x-ray photoelectron spectroscopy (XPS), it is found that Ge nanocrystals with the Si-passivated surface were formed by coevaporation of Si and Ge from two boats, while SiGe alloy nanocrystals were formed by evaporation of the Si- Ge alloy source from one boat. INTRODUCTION There are various techniques to fabricate semiconductor nanocrystals. However, most of the techniques do not have good control over the surface of semiconductor nanocrystals, and the surface often becomes unintentionally oxidized. Uncontrolled oxidation of the surfaces makes it difficult to experimentally observe physical effects from a zero-dimensional structure such as the quantum confinement. Among the techniques reported for fabrication of Ge quantum dots, the epitaxial growth with the self-organization process, which is based on the Stransky-Krastanov growth mode of Ge on Si to form Ge quantum dots, may be the best in forming the well-defined Ge surface[1-3]. However, since the shape of a Ge quantum dot formed by the epitaxial growth is not spherical, its electronic structure is not well predicted from the established theory. Therefore, a simpler technique to form a spherical quantum dot with a well-defined surface is preferred to the epitaxial growth. This paper presents two methods based on gas-evaporation of Si and Ge to form such Ge nanocrystals. One uses two boats each with Si and Ge, ant the other uses one boat with a SiGe alloy source. EXPERIMENT A conventional gas-evaporation technique with a boron-nitride (BN) boat was modified in two ways to form Ge nanocrystals with the Si- passivated surface. In one method referred to as the double-boat method, a boat with a Si source was placed 1.5 cm above a boat with a Ge source so that the Ge nanocrystals formed by condensation of the Ge vapor could travel through the Si vapor for surface passivation. Substrates were placed 4 cm above the upper boat. The temperatures of the Si and Ge boats were set at 1450 and 1600°C, respectively, during the
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evaporation. The boat temperature for Ge was made higher than for Si so that Ge nanocrystals could be made below the Si boat to avoid forming a SiGe alloy. In the other method referred to as the single-boat method, and Si0.5Ge0.5 alloy source in one boat was heated at 1500°C. The position of the boat is the same as the Ge boat in the double-boat method, and substrates were placed 5.5 cm above the boat. In both methods, argon gas with a pressure of 5 torr was used as an ambie nt in the gas e
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