Effect of Uniaxial Compression on Impurity Conduction inn-Type Germanium

Abstract

Shear strains, which change the donor wave functions, greatly affect impurity conduction, which depends sensitively on the wave-function overlap of neighboring impurity states. Using uniaxial compression along [111], we investigated as a function of stress the critical impurity separation $d_c$ for the transition from non-metallic to metallic conduction and impurity conduction in the intermediate concentration range, $7\times {10}^{16}<N<\mathrm{}3\mathrm{}\times {10}^{17}$ ${\mathrm{cm}}^{-3\mathrm{}}$. The largest stress applied was 2×${10}^9$ dyne ${\mathrm{cm}}^{-2\mathrm{}}$. The main effect of stress is a change of the activation energy ${\epsilon }_2$ of impurity conduction. In arsenic- and phosphorus-doped germanium, [111] compression decreases ${\epsilon }_2$ and increases $d_c$. In antimony-doped germanium the opposite is observed; compression increases ${\epsilon }_2$ and decreases $d_c$. At 1.2°K, [111] compression can increase the resistivity of antimony-doped germanium by a factor of ${10}^7$. Using the same orientation and temperature, a decrease of the resistivity of arsenic-doped germanium by a factor of 5×${10}^{-4\mathrm{}}$ is observed. These results suggest that (i) the activation energy ${\epsilon }_2$ depends strongly on the wave function overlap, and (ii) shear strains change the donor wave functions originating from the individual valleys by an amount which is proportional to the valley-orbit splitting of the donor element.