Effect of Compensation on Low-Temperature Spin Ordering in Ge:As Semiconductor Near the Insulator-Metal Phase Transition

An electron-spin resonance technique has been used to examine the influence exerted by the degree of compensation on spin effects in a set of nonmagnetic Ge:As semiconductor samples in the vicinity of the insulator-metal phase transition at temperatures 2 K <= T <= 80 K. It was found that the behavior of the paramagnetic susceptibility, specific to the Pauli paramagnetism and manifested in the spin density resonance, is drawn deep into the insulator state and is observed up to rather large degrees of compensation K approximate to 0.7 at temperature 20 K >= T >= 5-10 K. Outside this temperature range, both the quantities rapidly grow with increasing compensation and their behavior approaches the Curie law characteristic of the strong insulator state. The reason for this behavior is that the degeneracy is lifted at high temperatures and, at low temperatures, electron states are localized at the Fermi level as a result of the Coulomb blockade by compensating acceptors of the most closely lying donor states and the resulting formation of a narrow Coulomb band in the metallized impurity band. At temperature T >= 5-10 K, the gap is blurred by thermal excitations and has no effect on the manifestation of the Pauli paramagnetism. The behavior of the line width of the electron-spin resonance is determined by the scattering on quasi-stationary electric dipoles created by the Coulomb blockade at low temperatures and by the scattering on phonons at high temperatures. The behavior of the g-factor is qualitatively correlated with the specific features of the paramagnetic susceptibility and spin density: with decreasing temperature it weakly grows in the range of the Pauli paramagnetism and then rapidly increases in the range in which the Coulomb blockade is manifested and a transition occurs to the Curie law. By an analogy with ferromagnetic materials in which such a fast increase in the g-factor is observed, the possibility of a compensation caused ferromagnetic coupling of spins localized on donors. The microscopic model of such a coupling is discussed.