Pressure-Raman studies of substrate-generated internal strain in heterostructures

The effect of pseudomorphic constraints on phonon pressure response in 2-constituent semiconductor heterostructures is investigated. Several hundred pressure-Raman measurements on different superlattices and bulk samples belonging to the GaAs/AlAs system are reported. Calculations of the internal superlattice strain in each constituent as a function of applied pressure are used to predict the induced shift of the LO( Gamma ) phonons. In agreement with theory, no statistically significant substrate or layer-thickness related differences could be measured for the LO( Gamma ) pressure coefficients in GaAs/AlAs heterostructures. Surprisingly, however, differences are observed for the 2TA(X) peak. The calculations indicate that internal-strain effects can be an order of magnitude larger in other common epitaxial systems.