Piezotransmission Measurements of Phonon-Assisted Transitions in Semiconductors. I. Germanium

The strain-optical constants of Ge have been determined in the vicinity of the first indirect band gap 0.60-0.74 eV. The experiments were performed at room temperature and consisted of straining single crystals of Ge in an oscillatory manner and observing the amplitude modulation which was thereby impressed upon the transmitted beam. Single-crystal samples were used, allowing the full specification to be made. The results show both sharp structure and a relatively slowly varying background signal. The structure is discussed in terms of the strain-induced changes of the various threshold energies for indirect transitions expected on the basis of the deformation-potential model. The shape of the observed structure is in agreement with this simple model and the phonon energies are in excellent agreement with earlier determinations. The general features of the experimental results are explained in terms of indirect transitions taking place via the ${\Gamma }_{2^{\prime }}$ intermediate state. A detailed calculation of the strain-optical constants made on the basis of the deformation-potential model indicates that direct transitions to ${\Gamma }_{2^{\prime }}$ may also be important in this energy range.