Local coordination and electronic doping of column III metals in hydrogenated amorphous germanium

The relationship between the local coordination, No, and NI,, of gallium and indium atoms in a-Ge:H, as a function of the impurity content ([Ga], [In]) and the transport and structural properties of the films, is discussed. Extended X-ray absorption fine structure (EXAFS) data indicate that for [Ga], [In] less than or equal to 1.5 x 10(18) cm(-3) (where the symbol [] indicates concentration), the impurity atoms are always fourfold coordinated. As [Ga], [In] increase, N decreases from 4 to <3, the change of N-Ga being different from that of NI,. The Ga-Ge first shell distance in a-Ge:H films is always larger than in Ga doped c-Ge, yet it decreases with increasing [Ga]. The change from fourfold to threefold Ga and In coordination is attributed to the relaxation of compressive stress added by Ga and In to the a-Ge:H network. The static disorder around the impurities (from EXAFS) decreases with increasing doping (C) 2000 Elsevier Science B.V. All rights reserved.