Opto-Electronic Properties of Ge and Si Related Nanostructures on Ultrathin Si Oxide Covered Si Surfaces
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Opto-Electronic Properties of Ge and Si Related Nanostructures on Ultrathin Si Oxide Covered Si Surfaces M. Ichikawa1, Y. Nakamura2, A. A. Shklyaev3 and N. Fujinoki1 1 Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan 2 Department of Systems Innovation, Osaka University, Osaka 560-8531, Japan 3 The Institute of Semiconductor Physics SB RAS, Novosibirsk 630090, Russia ABSTRACT We present a method to form semiconductor nanodots on Si substrates by using ultrathin Si oxide technology and the results on their opto-electronic properties. We can form ultra-small semiconductor nanodots with the size of ~5nm and ultra-high density of ~1012 cm-2 on Si surfaces covered with ultrathin SiO2 films of ~0.3nm thickness. We focus on the Ge and GeSn nanodots on Si substrates and those embedded in Si films. These structures exhibit quantum confinement effects and intense luminescence in the energy region of about 0.8 eV. INTRODUCTION Quantum confinement effects in semiconductor nanodots, lead to the concentration of continuous electronic density of states into discrete one, the enhancement of oscillator strengths due to the increase of overlap between electron and hole wave functions, and the relaxation of carrier momentum conservation. Many studies have been done for compound and group IV semiconductors since the above effects contribute to the enhancement of luminescence efficiency from nanodots [1]. Especially, in group IV indirect transition semiconductors such as Si or Ge, the relaxation of momentum conservation is thought to much enhance the luminescence efficiency [2]. This leads to the development of CMOS-compatible integrated optoelectronic systems. The Stranski-Krastanov (SK) growth on substrates is a well-known method to form nanoislands on substrates [3]. In this growth, nanoislands are grown on wetting layers so as to minimize the sum of surface, interface and strain energies during hetero-epitaxy. The size and areal density of nanoislands, however, are limited to several ten nanometers and ~1011 cm-2 respectively since they are determined by the surface diffusion on the wetting layers and growth rate of critical nuclei of deposited atoms. Many studies have been done to solve the problems so far [4]. We have developed a self-assembled method to form nanodots with ~5 nm size and ~1012 cm-2 areal density on Si substrates by depositing different materials on Si surfaces covered with ultrathin Si oxide films of ~0.3 nm thickness [5, 6]. In this article, we report a method to form ultra-small and ultra-high density nanodots on Si substrates, and focus on electronic properties of the Ge and GeSn nanodots on Si substrates and optical properties of the nanodots embedded in Si films. EXPERIMENTAL PROCEDURE We used solid source molecular beam epitaxy (MBE) apparatus to form Ge nanodots on Si substrates. After formation of thin Si oxides (~1 nm) by chemical treatment of floating zone
(FZ) p-type Si(001) wafer, the wafer surfaces were cleaned by decomposition of the Si oxides in the MBE chambe
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