Fabrication and Structural and Optical Properties of Amorphous Si/SiO 2 Superlattices on (100) Si
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Institutefor MicrostructuralSciences, NationalResearch Council Canada, Ottawa,KIA OR6, CANADA
ABSTRACT We report the growth and characterization of amorphous Si (a-Si)/SiO2 superlattices on (100) Si. The a-Si layers (thickness varying from 1 to 10 nm) were vacuum-deposited at room temperature by molecular beam epitaxy while the SiO 2 layers (1 nm) were grown by an ex-situ UV-ozone treatment. This procedure was repeated six times to produce periodic multilayer structures. The chemical modulation of these structures was confirmed by transmission electron microscopy and depth profiling using Auger electron spectroscopy. X-ray specular reflectivity showed that the structures have a well defined periodicity. The a-Si layers have a density approaching (>95 %) that of c-Si and an interfacial roughness that increases with the a-Si layer thickness. The Raman spectrum from the Si layers of all samples shows broad peaks near 150, 310 and 470 cm- 1 that are typical of a-Si. On annealing at high temperatures, the three Raman bands decrease in intensity, while the 470 cm- 1 band also shifts to higher frequency and becomes narrower. After annealing for 30 s at 1100 'C, the a-Si bands are weak and the 470 cm- 1 band is merging with the c-Si 520 cm- 1 line, indicating that partial re-crystallization of the Si layers has occurred. The room temperature light emission properties of these nanostructures in the green to red wavelength region is reported. The luminescence shifts to longer wavelength with increasing a-Si layer thickness, consistent with a quantum confinement mechanism. INTRODUCTION Since the demonstration of high quantum yield visible light emission from porous silicon, 1 there has been considerable interest in the fabrication and study of the luminescence properties2 of Si-based materials such as Si/insulator multilayers or nanometer scale Si structures. , 3 Nanometer sized silicon crystallites are particularly attractive because quantum confinement effects could be exploited to control the light emission wavelength leading to potential applications in optoelectronics. Different approaches including electrochemical etching 1 , ion implantation, 4 -6 chemical vapor deposition,7 and sputtering8 have been used to obtain three dimensionally confined Si-based systems. The origin of the photoluminescence in these structures remains unclear, but it is generally believed to result from either surface states at the interfaces 8 or to quantum size effects. 1-3 The interpretation of the results is complicated by the fact that most of the techniques mentioned above enable growth of Si nanoparticles with only limited control of the size and size distribution. The concept of one dimensional confinement in quantum wells and superlattices has been of paramount importance in the development of modem semiconductor devices. It also offers an alternative route for fabrication of nanoscale structures where the dimensions are controlled with high precision. Amorphous semiconductor/insulator superlattices, first demonstrated by Abeles and Tie
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