On the Metal-Insulator Transition in n-Type Doped CuGaSe sub 2

Transport properties of n-type CuGaSe sub 2 single crystals are investigated in the temperature range from 2 to 350K. The effective donor concentration is varied between 2x10 sup (12) and 4.4x10 sup (17) cm sup (-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 sub c of 1.4x10 sup (17) cm sup (-3) is estimated, which is in good agreement with the Mott criterion (Nc~(0.25/a sub H) sup (1/3)). 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 7x10 sup (17) cm sup (-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 CuGaSe sub 2 is limited by self-compensation due to intrinsic defects (mainly Cu-vacancies). Furthermore, at low tempertures a crossover from Mott- to Efros-Shklovskii-type variable range hopping is observed on the dielectric side of the transition.