Internal stress-induced changes of impurity coordination and doping mechanisms in a-Ge : H doped with column III metals

Extended X-ray absorption fine structure (EXAFS) measurements in Ga- and In -doped hydrogenated amorphous germanium (a-Ge:H) reveal that practically al l highly diluted Ga and In impurities (less than or equal to 1.5 x 10(18) c m(-3)) adopt the four-fold coordination of the host network. However, only less than 1% of them are electronically active. As the impurity concentrati on increases, their mean coordination rapidly decreases from 4 to less than 3, for doping levels which are different by one order of magnitude for Ga and In. The analysis of the overall EXAFS data suggests that this effect is triggered by the relaxation of the internal stress accumulated in the a-Ge :H network due to the increasing Ga or In incorporation. (C) 2000 Elsevier Science Ltd. All rights reserved.