Influence of H 2 Preconditioning on the Nucleation and Growth of Self-Assembled Germanium Islands on Silicon (001)
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Influence of H2 Preconditioning on the Nucleation and Growth of Self-Assembled Germanium Islands on Silicon (001) Gabriela D.M. Dilliway1, Nicholas E.B. Cowern2, Lu Xu3, Patrick J. McNally3, Chris Jeynes2, Ernest Mendoza2, Peter Ashburn1, Darren M. Bagnall1 1 School of Electronics and Computer Science, Univ. of Southampton, Highfield, Southampton SO17 1BJ, UK 2 Advanced Technology Institute, Univ. of Surrey, Guildford GU2 7XH, UK 3 Research Institute for Networks & Communications Engineering (RINCE), School of Electronic Engineering, Dublin City University, Dublin 9, Ireland ABSTRACT Understanding the effects of growth conditions on the process of self-organisation of Ge nanostructures on Si is a key requirement for their practical applications. In this study we investigate the effect of preconditioning with a high-temperature hydrogenation step on the nucleation and subsequent temporal evolution of Ge self-assembled islands on Si (001). Two sets of structures, with and without H2 preconditioning, were grown by low pressure chemical vapour deposition (LPCVD) at 650ºC. Their structural and compositional evolution was characterised by Rutherford backscattering spectrometry (RBS), atomic force microscopy (AFM) and micro-Raman (µRaman) spectroscopy. In the absence of preconditioning, we observe the known evolution of self-assembled Ge nanostructures on Si (001), from small islands with a narrow size distribution, to a bimodal size distribution, through to large islands. Surface coverage and island size increase steadily as a function of deposition time. On the H2 preconditioned surface, however, both nucleation rates and surface coverage are greatly increased during the early stages of self-assembly. After the first five seconds, the density of the islands is twice that on the unconditioned surface, and the mean island size is also larger, but the subsequent evolution is much slower than in the case of the unconditioned surface. This retardation correlates with a relatively high measured stress within the islands. Our results demonstrate that standard processes used during growth, like H2 preconditioning, can yield dramatic changes in the uniformity and distribution of Ge nanostructures self-assembled on Si. INTRODUCTION Substantial efforts have been devoted to research into quantum dot structures due to the interesting electronic and optical properties that can result from quantum confinement effects. Obtaining quantum structures through self-organisation has the advantage of enabling fabrication of electronic and optical devices of dimensions smaller than those accessible by lithography. The process of self-assembly of Ge nanoscale islands is of particular interest because of its compatibility with Si-based technology. The lattice constant of Ge is larger than that of Si by 4.2%, resulting in a Stranski-Krastanov growth mode, whereby after the coherent growth of ~3 ML of Ge (the ‘wetting layer’) the surface becomes non-planar and three-dimensional islands form, thus relieving the misfit strain. However, this
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