The effect of stress on the acceptor ground state in germanium

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Abstract
Approximate wavefunctions and energies of the ground state of the shallow acceptor in Ge were obtained by solving the effective mass equation possessing terms proportional to the deformation tensor components. The variational approach was used involving trial wavefunctions similar to those of D Schechter, but possessing transformation properties required by the symmetry of the uniaxially deformed crystal. Results were obtained for the stresses X in the (100) directions, varying from -600 MPa to +600 MPa. At X to 0, where the energies of the impurity and the band states are expressible through the deformation potential constants (respectively b' and b), it is found that the value of b'/b is 0.55 and for sufficiently large X, b'/b to 1, in agreement with experiments. The calculated energies were further used for finding the solutions of the effective mass equation for large distances from the impurity nucleus. The solutions have two components with different spatial extent and anisotropies, and they are traceable to the light and heavy hole bands of Ge. Their behaviour with stress explains some of the observed features of the hopping piezoresistance in p-type Ge.