Superlattices and Microstructures, Vol. 4, No. 6. 1988 717 INELASTIC LIGHT SCATTERING FROM STRAINED-LAYER SI/GE SUPERLATTICES W. Bacsa, II. v. Kane& K. A. Milder, M.Ospelt and P. Wachter Laboratorium fiir FestkBrperphysik, ETH Honggerberg, CH 8093 Zurich (Received 1 June 1988) We have studied strained-layer Si/Ge superlattices (SL) composed of ultrathin (3-6 ML) Ge layers by inelastic light (Raman) scattering. The SL’s with periods ranging from 27 to 72A have been grown on Si (100) by MBE. Folded longitudinal acoustic phonons up to the fourth order have been observed indicating abrupt interfaces and exellent uniformity in the growth direction. The energy of the optical modes of the Ge layers is shifted due to strain. The observed shift is consistent with the calculated value obtained using uniaxial stress parameters. 1. Introduction With the advent of new growth techniques, such as MBE and MOCVD the fabrication of artificial superlattices (SL) composed of different materials has become a highly successful endeavour. The superlattice periodicity leads to a reduction of the Brillouin zone dimension in the growth direction. As a result, new Bragg planes are formed which affect the propagation of waves in the crystal. In particular new phonon modes arise which to a first approximation may be described by the folding back of the dispersion curves of the constituent materials. Inelastic light scattering by folded longitudinal acous- tic (LA) phonons has been reported both for lattice matched SL’s (e.g. GaAs - Al,Gal_,As lv2) and for strained-layer SL’s (e.g. GaSb - AZSb 3,4). Si/Ge superlattices are charac- terised by a rather large mismatch of 4% between Si and Ge at room temperature. This large mismatch can be reduced by using Si,Ger_, alloy layers. D.J. Lookwood et aL5 and G. Abstreiter et al6 have investigated such Si - S&Gel_, superlattices by inelastic light scattering. Fasolino and Moli- nari ’ have shown theoretically how the phonon properties of Si/Ge superlattices consisting of pure Si and Ge layers can be derived from those of the constituent materials. In the energy range of overlapping dispersion branches of Si and Ge, the corresponding folded phonon modes of the Si/Ge su- perlattice can propagate throughout the entire superlattice. This happens mainly in the acoustic region of the bulk com- pounds. The optical phonon branches of Si and Ge do not overlap. But those of Ge overlap with the acoustical ones of Si. The corresponding modes of the superlattice have reduced amplitudes in the Si layers. The optical phonons 07494036/88/060717+05$02.00/0 of Si are beyond the phonon edge of Ge and are therefore strictly confined to the Si layers, their amplitudes being ex- ponentially damped in the Ge layers. The bonds at the interface lead to different force constants coinciding with neither those of Ge nor those of Si. They give rise to local- ized phonon modes. The various modes are thus a measure for the superlattice periodicity (extended modes), the indi- vidual layers (confined modes) and the interfaces (interface modes). Recently, the experimental realisation of short period Si/Ge superlattices consisting of pure Si and Ge layers has become possible s. In the following we report on Si/Ge superiattices with periods in the range between 30 to 70A, investigated by inelastic light (Raman) scattering. 2. Experimental The [loo] oriented, n-type (0.05-5Rcm) Si-wavers have been cleaned in situ by thermal annealing to 830°C and by subjecting them to a small Si-flux. In addition a buffer layer of 500 - 1OOOA thickness was grown at 700°C. Substrates prepared in this manner exhibit a smooth (2 x 1 + 1 x 2) reconstructed surface as verified by RHEED. Ge and Si have been evaporated from a Knudsen cell and an electron beam gun, respectively. The base preassure is lo-“mbar, not exceeding 10-‘Ombar during growth. Our Si,Ge, superlattices are grown at temperatures of 460 - 480°C with evaporation rates of about 0.5A/s for both Si and Ge. There exist two critical thicknesses for such systems 9. First, the thickness of an individual Ge-layer has to be kept below the critical thickness h, for pseudomorphic growth of Ge on Si. Experimentally h, has been found to be 0 1988 Academic Press Limited