Trigonal hydrogen-related acceptor complexes in germanium

In germanium, an interstitial hydrogen atom may bind at a substitutional atom of carbon, silicon, beryllium, or zinc to form a shallow, monovalent acceptor complex. Photothermal ionization spectroscopy under uniaxial stress reveals that the complexes A(H,C), A(H,Si), A(Be,H), and A(Zn,H) have trigonal ($C_{3v}$) symmetry. Each has two (1s)-like acceptor levels which shift, but do not split, under stress. In the fourfold basis for a ${\Gamma }_8$($T_d$) level, simultaneous diagonalization of the perturbations of applied stress, and of a trigonal lowering of symmetry, yields theoretical piezo- spectroscopic behavior in quantitative agreement with all available experimental data. This procedure has been extended to predict the stress-induced shifts of (1s)-like shallow acceptor levels associated with tetragonal ($D_{2d}$) and rhombic-I ($C_{2v}$) complexes in germanium, should these ever be observed experimentally. The four trigonal complexes in germanium are to be contrasted with A(Be,H) in silicon, in which the rapid tunneling of hydrogen leads to recovery of tetrahedral symmetry and a much more complicated energy-level structure.