Column III and V elements as substitutional dopants in hydrogenated amorphous germanium

The doping properties of group III(B, Al, Ga, In) and V (N, P, As) impurities in a-Ge:H films deposited by rf sputtering were systematically studied. The Fermi level (E(F)), band-gap (E(04)), Urbach (E(0)) tail energies, density of midgap defects, N,, and hydrogen content were determined as a function of the impurity concentration by standard methods. It was found that, for a constant amount of different dopant impurities, different E(F) shifts are obtained, indicating different doping efficiencies. For Al, Ga, and In, the defect density displays a common behavior as a function of N(imp), N(D) increasing linearly with N(imp) for N(imp) > 2 x 10(19) cm(-3). In contrast, for B and all group V elements a N(D) alpha (N(imp))(1/2) relationship is found. These findings indicate that, for the case of p-type heavy metal doping of a-Ge:H, deep defects are induced by inactive impurities, whereas n-type doping appears to be consistent with a charge-induced bond breaking mechanism. A series of different doping-induced effects highlights the importance of the chemical aspects of substitutional doping in a-semiconductors. (C) 1998 Elsevier Science B.V. All rights reserved.