EXAFS determination of the local environment of column III and V elements in amorphous germanium thin films

G. DALBA; P. FORNASINI; R. GRISENTI; I. CHAMBOULEYRON; D. COMEDI

ESRF: Highlights 1999

Año: 2000 p. 33 - 34

Single doped hydrogenated amorphous germanium thin films were prepared with high dilutions extents ranging from 1.5*1018 up to 4.5*1020 cm3. The average thickness of the films was 3 µm with a number of doping atoms exposed to the X-ray beam (4 mm x 1.5 mm) of the order of 3*1013 to 8*1015. The K-edge of the doping elements (Ga, In, Sb) has been measured in fluorescence mode at BM8 (GILDA CRG beamline). Interesting results were obtained for the coordination number (N) and Debye-Waller (DW) factor. A general behaviour of N as a function of impurity concentration was found for all the dopant species. In general, the rising of concentration results in a lower coordination of the dopant atom. At the lowest concentrations, for Ga and In a four-fold coordination is apparent, while for Sb a slightly smaller value has been obtained. Values close to 2 were found for all the impurity atoms at the highest concentrations. The abrupt decrease in coordination doesn´t follow the modified "8-N Mott´s rule" model proposed by Street, which was achieved for P and B impurities in a-Si:H. In addition, only 1% of four-fold coordinated sites was found electrically active. Moreover, the comparison between conductivity and N shows only a partial agreement, which suggests that the conduction mechanism cannot be completely related to the substitutional doping. On the basis of these results, a new model has been proposed to explain the doping properties of group III metals in a-Ge:H. The proposed mechanism considers the effect on the coordination and electrical conductivity of the compressive stress induced by the atomic and ionic size differences between the germanium atom and the four-fold coordinated impurity.