Investigation of the relaxation of excess carriers in SiGe-heterostructures by photothermal measurement

Charge carrier waves generated by a double modulated laser beam probe the mobility, lifetime and the radiationless relaxation channels in layered structures of semiconducting materials. The information on amplitude and phase of the response is gained by modulated reflectivity measurement of the same laser beam. This double modulation method of thermal and carrier wave analysis is applied to the investigation of the relaxation of photogenerated excess carriers in SiGe-heterostructures grown with low temperature MBE on silicon substrates. One part of these excess carriers is collected by the buried superlattice region creating a laterally spreading carrier wave. Its damping is governed by the lifetime and the mobility in the superlattice. Results are presented for the determination of the dependence of the carrier and thermal wave amplitude on the Ge-content in the superlattice region. Correlations between alternatively measured Hall mobility and the carrier wave signal gained from structures with constant Ge content are presented.