Conduction electrons in an impurity band: NMR in Ge:As

The 73 Ge spin-lattice relaxation time T 1 has been studied at 4.2K and below in single-crystal specimens of arsenic-doped germanium (Ge:As), with doping densities spanning the metal-non-metal transition. An increase of T 1 with the applied magnetic field is observed for both semiconducting and the low-density metallic 'impurity-banded' materials, although T 1 is independent of field for heavily doped metallic samples. The behaviour in the metallic region is interpreted here using two models of the band in which the conduction electrons move, based on the tight-binding and rigid-band approaches, and the results are compared with those of other workers in the related Si:P system. Very little support can be adduced from this work for the dominance of correlation as the driving mechanism for the metal-non-metal transition in Ge:As. The results indicate that the metal-non-metal transition in Ge:As occurs when the impurity band heavily overlaps the germanium conduction band.