Studies of compositional variations in germanium quantum dots grown on silicon
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Studies of compositional variations in germanium quantum dots grown on silicon. Alan D.F.Dunbar, Matthew P.Halsall, Uschi Bangert, Alan Harvey,Philip Dawson, Bruce A. Joyce1, Jing Zhang1 and Isabel Berbezier2. Department of Physics, UMIST, Manchester M60 1QD. United Kingdom. 1 Department of Physics, The Blackett Laboratory, Imperial College, London SW7 2BZ, United Kingdom. 2 CRMC2, Campus de Luminy, Case 913, F13288 Marseille Cedex 9, France.
ABSTRACT We report optical and scanning transmission electron microscopy studies of germanium dots grown on silicon. In an attempt to control the self-organized growth process and promote dot size uniformity the dot layers were grown on a 4.5nm Si0.6Ge0.4 alloy template layer. Photoluminescence results indicate the formation of carrier confining Ge rich islands, whilst Raman scattering results indicate the presence of an alloy throughout the structures formed. The samples were studied in the UK high resolution scanning transmission electron microscopy facility at Liverpool, UK. Energy dispersive analysis of individual line scans through the sample show that the structures are composed of an alloy throughout with an asymmetric distribution of Germanium in the dots and in the wetting layer close to the dots. We discuss the results in the light of the proposed growth mode for these dots and conclude that attempts to manipulate the composition of these dots during growth may be problematic due to the self-organized nature of their formation. INTRODUCTION. The formation of semiconductor quantum dots in highly mismatched systems by the Stranski-Krastanov growth mode is an area of much current research. For applications in optoelectronic devices most researchers have concentrated on the direct gap InAs/GaAs system where quantum dot based lasers have already been demonstrated1. However, the technologically important germanium on silicon system is, in many ways, even more interesting as it offers the possibility of creating an optically active pseudo-direct gap within the dots2. A Si based near infra-red laser would have a multitude of applications, especially in the are of optical/electronic interconnects for fibre optic communications. Several groups have described the formation of Ge dots during the growth of Ge on Si over the last five years3,4. The dots formed typically have a very low aspect ratio compared to the InAs/GaAs system with heights/width ratios of the order of 10. They also are known from AFM studies to have a wide size distribution5. They do, however, exhibit strong broadband photoluminescence centered on an energy of approximately 0.8eV. The width of the band is thought to be due to the large dot size distribution. In addition it has recently become apparent that substantial intermixing of silicon and germanium occurs during growth. Moreover this intermixing is believed to be strain driven with the observation of intermixing even at a growth temperature of 400oC6. If this dot system is to realize it’s potential then the issue of size and composition uniformity will n
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