Effects on the NMR Properties of the Metal-Nonmetal Transistion in Impure Semiconductors

The nuclear relaxation of ${}_{}{}^{29}{}_{}{}^{}\mathrm{Si}$ in phosphorus-doped silicon is studied from the metal-nonmetal transition point of view. Going from the nonmetal to the metal there is a rapid decrease of relaxation time at the transition, donor impurity ≈ ${10}^{18}$ P atoms/${\mathrm{cm}}^3$. For the insulating side of the transition the dominant process of relaxation is due to the modulation of the dipolar interaction between the impurity electron spin hopping from one impurity center to another, and the nuclear spins. This type of relaxation can only take place if the material is compensated. For the metallic side of the transition the dominant process of relaxation is due to the modulation of the hyperfine coupling between the nuclei and the highly degenerate electron gas in a narrow band. The electron density in the vicinity of the donor centers is measured in a 2.5× ${10}^{18}$ P atoms/${\mathrm{cm}}^3$ sample. The measurement shows that it remains strongly localized around the impurities. A comparison is made of our results with the $D^{-}$ band model of conductivity.