Impurity Conduction and the Metal-Nonmetal Transition in Manganese-Doped Gallium Arsenide

Electrical-transport measurements between 20 and 550 K demonstrate impurity conduction below 100 K in Mn-doped GaAs samples with Nₐ>10¹⁹ cm^-3. Of a group of samples with 10¹⁹<Nₐ<3×10¹⁹ cm^-3 and moderate compensation ($K<0.1$), some display metallic (activationless) impurity conduction, whereas others have a finite activation energy of about 0.015 eV in the impurity-conduction range. It is concluded that this energy should be identified as E₂ for impurity conduction in a semiconductor with "intermediate-range" doping, and that the transition range of Nₐ in which E₂ decreases before vanishing must be a narrow one. The values for E₂ are compatible with the models of Mycielski and of Mikoshiba for this conduction. Since the bound-state wave function for a manganese acceptor has a characteristic radius aL=10.1 {}, then the critical acceptor density Nc for a metal-nonmetal transition corresponds with the condition aLNc1/3=0.28. This denotes closer spacing than the Mott-Hubbard criterion because of the compact nature of the manganese wave function. Hopping conduction was not detected with samples containing less than 10¹⁹ cm^-3 of Mn acceptors, and this also is to be expected from the charge distribution of the manganese bound state.