Dynamic impurities in ultrapure semiconductors

Abstract

Theoretical and experimental results are presented which shed light on a series of unusual shallow level impurities in ultrapure germanium. These impurities, both donors and acceptors, (1) show hydrogenic spectra with signatures identical to all other shallow impurities; (2) exhibit unusual dependence in their spectra to applied uniaxial stresses; (3) display a clear isotope effect (hydrogen to deuterium) in the electronic spectrum when the crystal growing conditions are changed -- a clear violation of the Born-Oppenheimer approximation. They are dynamic impurities consisting of a two-atom center in a fully tetrahedral environment, in which coherent nuclear motion is coupled to the electronic degrees of freedom. Experimental data, together with group-theoretical analysis, permit a complete description of these systems.