Raman Shift and Broadening in Stress-Minimized Ge Nanocrystals in Silicon Oxide Matrix
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Kent Ridge, Singapore 119260, SINGAPORE. ABSTRACT: Ge nanocrystals (nc-Ge) embedded in silicon oxide films were synthesized using RF magnetron sputtering and post-annealing procedure. To minimize the stress effect and inhomogeneity, we intentionally lower the cooling rates and reduce the temperature gradient during annealing. Significant Raman shifts ranging from 2.0 to 5.8 cm 1 have been observed from samples annealed at different temperatures. The size-dependent shift and broadening is found to be in good agreement with the phonon confinement mode together with the Gaussian weighting function, and the isotropic T02 phonon dispersion relation introduced by Sasaki et al. The Raman spectra can also be well-fitted using peaks calculated from the phonon confinement model. The inhomogeneous Raman peak broadening from our samples annealed at lower temperatures are attributed to the non-Gaussian size distribution of Ge nanocrystals. INTRODUCTION The strong visible photoluminescence at room temperature from indirect band gap nanocrystals (especially group IV elements, such as Si, Ge etc) has attracted much attention recently due to their possible optoelectronic applications. Many studies on nc-Ge embedded in silicon oxide films have been carried out since strong light emission was observed from Ge nanocrystals embedded in silicon oxide films through quantum confinement effects. This material is of interest also because the sample preparation method is compatible with the conventional integrated circuit fabrication process As a fast, convenient and non-destructive method, Raman spectroscopy [- ] has been widely [12]
used in the characterization of the nc-Ge systems to determine nc-Ge size - , crystallinity , etc. Due to phonon confinement effects, Raman spectra of nc-Ge show a downshift and broadening when the nanocrystal size is smaller than 30 m [13, 14]. In order to interpret the Raman spectrum quantitatively, a phenomenological phonon confinement model has been developed by Richter et al to account for the peak position shift, broadening, and asymmetry of the bands observed in nanocrystals [151. The essential parts of this model are the phonon confinement weighting function and the dispersion function. Campbell and Fauchet compared three kinds of weighting functions and suggested that the Gaussian weighting function c(0, q)1 or exp(-q 2 .L' /1667') could give the best agreement with Si and GaAs nanocrystal data. In the selection of dispersion function, the situation is controversial. A dispersion relation from the linear-chain model (LCM) was reported in 1987 [16]. A dispersion from the experimental neutron data of TO phonon scattering along the [100] direction (TO[l100]) was also used [9]. Sasaki et al analyzed the Raman spectra from gas evaporated Ge small particles using a series of isotropic dispersion relations obtained through fitting neutron experimental data along [100], [110] and [111] directions, forcing a15,6] good fit near the F point [14]. Other dispersion functions were also reported in the literature 6
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