On the metal-insulator transition in n-type doped CuGaSe₂

Transport properties of n-type CuGaSe₂ single crystals are investigated in the temperature range from 2 to 350 K. The effective donor concentration is varied between 2×10¹² and 4.4×10¹⁷ cm^-3 by co-doping with Ge and Zn. The charge transport properties are analyzed and interpreted in the framework of an Anderson metal-insulator transition. A critical donor concentration N_c of 1.4×10¹⁷ cm^-3 is estimated, which is in good agreement with the Mott criterion (N_c^1/3≈0.25/a_H). However, the second characteristic concentration, above which the Fermi level merges into the conduction band, was not observed experimentally. This is in accordance with an estimate of 7×10¹⁷ cm^-3 according to the Matsubara-Toyozawa criterion, which exceeds the highest donor concentrations achieved in this material so far. The effective dopant density in n-type CuGaSe₂ is limited by self-compensation due to intrinsic defects (mainly Cu vacancies). Furthermore, at low temperatures a crossover from Mott- to Efros-Shklovskii-type variable range hopping is observed on the dielectric side of the transition.