THE METAL-INSULATOR-TRANSITION IN SIP PROBED BY THE THERMOELECTRIC-POWER AT VERY-LOW TEMPERATURES

Measurements of the thermoelectric power S of heavily doped Si:P at ve ry low temperatures (0.04 K less-than-or-equal-to T less-than-or-equal -to 3 K) allow a clear distinction between metallic and insulating sam ples. While metallic samples with P concentration N far above the crit ical concentration N(c) exhibit nearly-free electron behavior with neg ative S approximately T, metallic samples in the vicinity of N(c) show a sign change below 1 K and a maximum of S with S --> 0 for T --> 0. This maximum is suppressed in large magnetic fields and is attributed to scattering by localized magnetic moments. Finally, S for insulating samples appears to diverge for T --> 0, and the temperature T0 of the sign change increases monotonically with decreasing N.