Hydrogenated amorphous semiconductors are a class of materials with increasing applications in the photo-electronic industry. Among them, the hydrogenated amorphous germanium (a-Ge:H) is especially interesting, due to its narrow gap, for the production of infrared sensitive devices. Most of the potential advantages, however, rely on the ability of controlling the electronic properties of the active semi-conducting layer by chemical doping. Such processes in an amorphous semiconductor are much less well understood than for the crystalline material due to the lack of atomic periodicity. To shed more light on this subject, careful preparation and characterisation of intrinsic and doped hydrogenated amorphous semiconductors is being undertaken by using various techniques, among which EXAFS plays a dominant role for its local order sensitivity.

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): typical EXAFS signals are reported in Figure 37. 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, as shown in Figure 38 (solid symbols). 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 [1], 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.

For an estimation of the disorder induced by the doping process, the ratio method, based on the cumulant expansion, was used. By this procedure, it is possible to estimate the variation, with respect to a reference model, of the second cumulant, which is related to the overall disorder of the sample under investigation. The outcome of previous investigations showed that hydrogen can reduce the effect of the doping-induced network stress by completing dangling bonds [2]. Surprisingly, the adding of chemical impurities reduces the disorder even more, as can be seen in Figure 38 (open symbols).

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 is not only related to the fraction of four-fold coordinated doping atoms, but it 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. To verify the validity of these assumptions, further EXAFS measurements on heavier (Tl and Bi) and lighter (Al and P) acceptors and donors are planned in the near future.

References
[1] R.A. Street, Phys. Rev. Letters, 49, 1187 (1982).
[2] G. Dalba, P. Fornasini, R. Grisenti, F. Rocca, I. Chambouleyron, C.F.O. Graeff, J. Phys.: Condens. Matter, 9, 5875 (1997).

Principal Publications and Authors
G. Dalba (a), P. Fornasini (a), R. Grisenti (a), F. Rocca (b), D. Comedi (c), I. Chambouleyron (c), Appl. Phys. Lett., 74, 281 (1999).
I. Chambouleyron (c), D. Comedi (c), G. Dalba (a), P. Fornasini (a), R. Grisenti (a), F. Rocca (b), submitted to Phys. Rev. Lett., (1999).

(a) Dipartimento di Fisica and INFM, Università di Trento (Italy)
(b) CeFSA - Centro CNR-ITC di Fisica degli Stati Aggregati, Trento (Italy)
(c) Istituto de Fisica Gleb Wataghin, Universidade Estadual de Campinas-UNICAMP, Campinas (Brasil)