EFFECT OF HYDROSTATIC-PRESSURE ON THE RAMAN-SPECTRUM OF GENSIM MULTIPLE-QUANTUM WELLS WITH N-LESS-THAN-OR-EQUAL-TO-4 AND M-LESS-THAN-OR-EQUAL-TO-7

We report a Raman study of the effects pressure has on the vibrational structure of GenSim multiple quantum wells (MQW's) with n less than o r equal to 4 and m less than or equal to 7. Three primary phonon bands are studied: Ge-Ge within the germanium layers, Si-Si within the sili con layers, and the Ge-Si interface mode. Pressure shifts each of thes e bands consistent with a mode-Gruneisen constant of unity for all sam ples and laser excitations used. We observe resonance effects with the confined Ge-like E-1 transition for the Ge4Si5 sample. The transition is near 2.4 eV at ambient pressure and blueshifts at approximate to 4 +/-1 meV/kbar. This pressure coefficient is smaller than the correspon ding quantity in bulk germanium. This is attributed to the fact that s ilicon dictates the in-plane contraction of the Ge layer that is at a smaller rate than the corresponding quantity in bulk germanium. We see no evidence of resonance enhancement in samples with thinner Ge layer s in each MQW period. This implies that at least four Ge atoms are nec essary to form the states producing the E-1 transition, consistent wit h previous studies. An additional feature seen in the spectra near 310 cm(-1) is identified by the pressure study to be 2TA Raman scattering from silicon.