Characterization of structrual and optical properties of Si1 -xGex nanocrystal produced by laser-induced pyrolysis and by ion implantation
Final Report Abstract
During the first part of the present project, we have synthesized mixed Si/Ge nanocrystals (Si1-xGex NCs) in the form of thin layers on quartz windows using CO2 laser-induced pyrolysis of a SiH4/GeH4 gas mixture in a flow reactor in conjunction with the cluster beam deposition technique. The structural analysis by high-resolution electron microscopy (HRTEM) reveals nanoparticles (NPs) with a fee crystalline core surrounded by an amorphous oxide layer. The typical diameter is between 3 and 4 nm. Rutherford backscattering (RBS) measurements show that the Ge content in the NCs scales linearly with the concentration of GeH4 in the precursor gas mixture ([GeJNc = 3.5 [GeH4]gas). This finding is confirmed by energy-dispersive X-ray spectroscopy (EDX). We did not find any indication of a phase separation of Ge and Si; instead the HRTEM and EDX results point to Si/Ge alloy NPs with homogeneous crystalline core. Photoluminescence (PL) studies on a sample of Si0.92Ge0.08 NCs deposited on a quartz window reveals optical properties consistent with the radiative recombination of excitons in a quantum-confined NP. The reduction of the fluorescence lifetime by a factor of two (compared to pure Si NCs) is in agreement with theory. The improved PL efficiency that should be associated with the reduced lifetime, however, has not yet been demonstrated. Varying the Ge concentration in the NPs, we could observe visible photoluminescence up to a concentration of ≈25 %. Beyond that value, no PL was detected. This is particularly true for pure Ge NCs. We ascribe this finding to the difficulty to passivate Ge dangling bonds with oxygen. Germanium-doped SiO2 NPs show strong defect luminescence in the visible. Spectral and time-resolved analysis reveals two oxygen-related defect centers (oxygen-deficient as well as non-bridging oxygen defect centers) pointing to an inhomogeneous oxidation. Upon annealing, we observe the formation of Ge nanoclusters (giving rise to a new defect band in the violet spectral range) and, at higher temperature, the formation of Si NCs, which show their characteristic quantum-confined PL in the red spectral range. The identification of Ge and Si nanocrystals proves that the oxide was sub-stoichiometric in oxygen prior to the annealing. During the second part of this project, we employed Ge ion implantation and annealing to produce Ge NCs in fused silica windows. The successful formation of nanocrystals was demonstrated by HRTEM and the observation of the violet 3.0 eV PL band, which can be associated with oxygen-related defects at the interface between Ge NCs and the surrounding SiO2 matrix. Mixed Ge/Si NCs were produced by co-implanting Si+ and Ge+ with subsequent annealing. At higher Ge concentration, the samples showed the same defect PL bands as pure Ge NCs in SiO2. It was only when the Ge concentration was smaller than 10 % that we could observe quantum-confined PL. This PL band, appearing at 850 nm for pure NCs in SiO2, slightly shifted to red when the Ge content was increased. These observations indicate that we could successfully synthesize light-emitting Si1-xGex alloy NCs for x ≤ 0.1 by ion implantation and subsequent annealing.
Publications
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Evolution of photoluminescence properties of SixGe1-x nanocrystals synthesized by laser-induced pyrolysis. Appl. Phys. Lett. 95, 013115-1-3(2009)
L. B. Ma, T. Schmidt, O. Guillois, and F. Huisken
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Evolution of multiple peak photoluminescence of Ge-doped silicon oxide nanoparticles upon thermal annealing. Phys. Rev. B 82, 165411 (2010)
L. B. Ma, T. Schmidt, C. Jäger, and F. Huisken
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Photoluminescence studies of Ge-doped silicon nanocrystals and silicon oxide nanoparticles. In: Proceedings of the BONSAI Symposium Breakthroughs in Nanoparticles for Bio-Imaging, ed. by E. Borsella, AIP Conference Proceedings 1275, 71-74 (2010)
T. Schmidt, L. B. Ma, and F. Huisken
