Quantitative piezospectroscopy of neutral copper in germanium

Experimental results of the optical-absorption spectrum of neutral copper impurity in germanium with and without an applied uniaxial compressive force F are described. The zero-stress spectrum agrees with previous work and indicates that the absorption lines observed correspond to the excitation of one of the three bound holes to single-acceptor-like, higher-lying states. This is used as the basis of a model (described elsewhere) to explain the piezospectroscopic behavior of the spectrum observed with F∥〈100〉, F∥〈111〉, and F∥〈110〉; this model is similar to that already exploited to describe the behavior of neutral group-II acceptors in germanium. The results obtained are much simpler than might be expected for such a complex system, although it has been shown that the antisymmetric product of three single-hole ${\Gamma }_8$(T${¯}_d$) ground states is itself a ${\Gamma }_8$(T${¯}_d$) state and not a multiplet as is the case for neutral group-II impurities. A detailed comparison between the results predicted by the symmetry analysis of the model permits the intensity parameters of some of the transitions to be determined and also deformation-potential constants of some of the energy states. The latter are found to be consistent with those for single-hole states in agreement with the model. However, several discrepancies remain between the model and the observations. A characteristic difference between the piezospectra of this impurity and the shallower group-III acceptors is that the intensities of the lower-energy G components increase with increasing stress whereas for a group-III impurity the higher-energy G components survive at large stress.