Effects of the overlap between wave functions of impurity centers on the activation energy of hopping conduction

The hopping conductivity sigma(3) has been studied in samples of slightly c ounterdoped crystalline Si : B with a boron concentration of 2 x 10(16) cm( -3) < N < 10(17) cm(-3) and a compensation of 10(-4) less than or equal to K less than or equal to 10(-2). It is found that at K less than or equal to 10(-3) the activation energy epsilon(3) is not lower (as it must be accord ing to classical notions at finite K) but larger than the value epsilon(N) = e(2)N(1/3)/kappa, where e is the electronic charge and kappa is the diele ctric constant. With decreasing N, the energy epsilon(3) drops slower and, with decreasing K, grows faster than follows from the standard theory. At K less than or equal to 10(-4), epsilon(3) is higher than epsilon(N) by a fa ctor of 1.5-2. The result is explained by the effect of the overlap between wave functions of neighboring impurity centers on the structure of the imp urity band. (C) 2000 MAIK "Nauka/Interperiodica".