Hopping conduction through the 2s states of copper acceptors in uniaxially stressed germanium

We have studied hopping conduction in uniaxially stressed Cu-doped Ge. The electronic ground state of neutral Cu acceptors, normally accommodating three 1s holes, undergoes a transformation to a 1s22s1 configuration with the application of uniaxial stress greater than 4 kbar. Upon undergoing this transformation, the copper acceptors form a system of lithium-like impurities in which hopping transitions of carriers take place through 2s rather than 1s states. The significant overlap between the more extended 2s wavefunctions located on different sites results in a decrease of the resistivity in the hopping regime by eight orders of magnitude. Good agreement was obtained between the experimental results and our calculations.