Strain Engineering and Luminescence in Si/SiGe Three Dimensional Nanostructures
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Strain Engineering and Luminescence in Si/SiGe Three Dimensional Nanostructures Nikhil Modi1, Leonid Tsybeskov1, David J. Lockwood2, Xiao Z. Wu2, and Jean Marc Baribeau2 1 Department of Electrical and Computer Engineering, New Jersey Institute of Technology, 106 Warren St, Newark, NJ 07102, U.S.A. 2 Institute for Microstructural Sciences, National Research Council, Ottawa, ON, Canada ABSTRACT Strain engineering in composition-controlled Si-Si/Ge nanocluster multilayers with high germanium content (~ 50%) is achieved by varying thicknesses of Si/SiGe layers and studied by low temperature photoluminescence (PL) measurements. The PL spectra show reduction in strained silicon energy bandgap and a splitting presumably associated with partial removal of heavy hole-light hole degeneracy in SiGe valence band. Time-resolved PL measurements performed under different excitation wavelengths show dramatically different PL lifetimes, ranging from ~ 2 µs to 10 ns and an unusually high PL quantum efficiency. The results are explained by using the Si/SiGe interface recombination model, which is supported by ultra-high resolution transmission and analytical electron microscopy measurements. INTRODUCTION The long-standing goal of monolithically integrated silicon optoelectronics suffers from the unavailability of efficient, CMOS-compatible light emitters at technologically important wavelengths (1.3 – 1.6 µm). Being indirect bandgap semiconductors, group IV bulk materials show poor internal quantum efficiency of radiative recombination. While enhanced emission efficiency has been demonstrated by nanostructures of these materials (e.g., porous silicon, Si nanocrystals, etc.), luminescence is peaked at energies above the fundamental bandgap of bulk Si, farther from the low-loss wavelength region for optical communications, and is still quite slow. Also, the integrated circuit industry necessitates any engineered light emitting device to be compatible with the modern manufacturing process. Induced lattice strain in crystalline semiconductors such as Si and Ge causes a rearrangement of constant energy surfaces and consequently, a shift in the energy minima of the band structure. A multilayer structure consisting of silicon/silicon-germanium nanocrystals (NCs) embedded in thin films of strained Si, further offers the possibility of enhancing the spatial localization of carriers and consequently, the efficiency of electron-hole recombination. This paper reports an investigation of the effect of strain on the recombination of localized electronhole pairs in Si/SiGe three-dimensional nanostructures. The variation of local strain in Si layers is accomplished by selectively changing thicknesses of Si and SiGe NC layers during growth, and a detailed photoluminescence study provides key data in the development of comprehensive carrier recombination model for the fabricated nanostructures.
EXPERIMENT A multilayer structure with nine pairs of Si0.4Ge0.6 NCs embedded in thin films of Si was grown using Molecular Beam Epitaxy (MBE) in a VG Semicon
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