Lattice Relaxation Effects in Si and GaAs Nanocrystals
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Physics Department, Rensselaer Polytechnic Institute, Troy, NY 12180
ABSTRACT Raman scattering results on porous silicon, and silicon and gallium arsenide nanocrystals show that almost all vibrational modes become Raman active and remarkably soft in these nanocrystal systems. The experimental results further demonstrate that the carrier-induced strain effects play an important role on the optical properties of such nanocrystal systems.
INTRODUCTION It has become more apparent that porous silicon exhibits the optical properties similar to that observed for silicon quantum dots[l,2]. The fact that surface states and the other energy relaxed states are often involved confusions the identification of the precise luminescence mechanism. We believe that there must exist a unique luminescence mechanism which not only controls the optical properties of silicon nanocrystal systems but also other semiconductor nanocrystals with indirect band gap. To this end, we report here the Raman scattering studies on porous silicon, and silicon and gallium arsenide nanocrystals. The results further prove that the carrier-induced strain is a common feature, which dominates the optical properties of semiconductor nanocrystals.
EXPERIMENTAL Silicon and GaAs nanocrystal samples were prepared by pulverizing well-characterized bulk m crystals. A particle-water or particle-alcohol colloid was then formed and precipitation and centrifugation were employed to select particles of different size. Typical tolerance for the particle size ( measured by TEM ) is about 20 % for each sample used in this study. Porous silicon was prepared by standard electrochemical anodization of p+ Si[3]. Raman and photoluminescence were carried out on particles which dried from the colloid onto a stainless steel sheet at room temperature. The excitation power of 100 mW and 150 mW from 488 nm line of an Ar+ laser was used for PS sample and semiconductor nanocrystals respectively.
RESULTS AND DISCUSSION In Fig. I we show the Raman spectra of porous and nanocrystal Si as well as bulk crystalline Si. The particle size in the nanocrystal sample is 2.0-3.0 nm. The PS sample shown here exhibited a well-defined PL peak at 1.7 eV and the Si nanocrystal sample exhibited a peak at 1.6 eV. Bulk silicon has a triply degenerate r Raman peak at 521 cm-' with FWHM of 3 cm-I. The r peak for PS and Si nanocrystals is observed at 500 cm-1 and 505 cm-1 respectively with 199
Mat. Res. Soc. Symp. Proc. Vol. 358 01995 Materials Research Society
FWHM of 22 cm-' for both. The TA(X) phonon for bulk Si is 152 cm-' obtained from the 2TA(X) overtone peak. The same acoustic phonon for PS and Si particles is at 142 cm-' and 144 cm-' respectively. We also observe that the overtone Raman peaks, the TO(L) and TO(W) phonons in PS shift down by 22 cm-1 and 20 cm-' respectively. Both modes have a downshift of 18 cm-' in the Si nanocrystal sample. All of the other phonons which we have identified ( including TA(L), LO(A) and LO(L)) show a corresponding downshift[4]. The redshift of a vibrational mo
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