LINE SHAPE IN RESONANCE RAMAN SCATTERING FROM SILICON QUANTUM DOTS R.K. SONI, L.F. FONSECA, 0. RESTO, S.Z. WEISZ AND S. TRIPATHY * Department of Physics, University of Puerto Rico, San Juan, PR-00931, USA *Department of Physics, Indian Institute of Technology, New Delhi-i 10016, INDIA ABSTRACT We have carried out a resonance Raman study of line-shape in silicon quantum dots synthesized on a quartz substrate by co-sputtering bulk Si and Si0 2 . Optical transmission measurements are used to evaluate dot size distribution. The size distribution shows peaks around 1.0 and 1.4 nm. The Si dots exhibit photoluminescence in the visible region, which shifts to higher energy with decreasing size. The size dependence of Raman scattering shows phonon softening and increasing asymmetrical broadening for small dots (< 2nm). The observed spectra are compared with calculations considering electron-hole interactions at a quasi-direct gap of a spherical quantum dot. INTRODUCTION Nanometer size Si crystals show marked improvement in photoluminescence (PL) quantum efficiency compared to bulk silicon and are, therefore, attractive for silicon based optoelectronic applications [1]. The PL observed from Si nanocrystals is attributed to quantum size effects. The effect of carrier confinement and band gap upshift in Si nanocrystals has been discussed intensively in the literature [1]. The photoluminescence energy for Si nanocrystals have been calculated by using the empirical psudopotentials [2], the third nearest neighbor tight binding [3] and time dependent tight binding techniques [4]. Despite the apparent disagreement in predicting the peak PL energy, these calculations demonstrate that the PL energy increases as the nanocrystal size decreases and the dominant contribution to the visible light emission comes from nanocrystals smaller than 2 nm. The resonantly excited PL spectrum has shown long radiative lifetimes in Si nanocrystals and provides important evidence that phonons are directly involved in the radiative recombination process [5]. Clearly, this suggests that the Si nanocrystal energy band remains indirect type. Theoretical calculations, however, indicate that a quasi-direct gap in nanocrystal Si is possible for sizes 1.0-1.5 nm in diameter [3]. Raman scattering techniques are commonly used for semiquantitative determination of size effects on vibrational modes, such as confined optical phonon, surface phonon and confined acoustic phonon, in nanocrystal or quantum dots (QD) [6-11]. An adequate knowledge of the bulk phonon dispersion is a prerequisite for understanding lattice dynamical properties and electron-phonon interactions in QD. The first-order resonant Raman scattering via the Fr6lich interaction has been investigated for spherical QD [9]. The electronic intermediate state was considered as uncorrelated electron-hole pairs. Electron- hole correlation effects via Coulomb interaction on Raman amplitude for spherical QD has also been reported [10,11]. It was shown that the resonant scattering efficiencies depend strongly on the Coloumb correlation and only excitonic states and vibrational modes with zero angular momentum were allowed in the Raman process. The effect of carrier confinement on optical phonons in Si QD has been widely studied by Raman scattering [6-8]. The size effect is reflected in a finite wave vector of QD and 235 Mat. Res. Soc. Symp. Proc. Vol. 571 © 2000 Materials Research Society