Structural studies of amorphous metal-metalloid thin films.

Amorphous materials offer not only novel technological applications, but a valuable insight into the condensed state. Interest in this relatively new field has created a demand for new theories to describe the subtle electronic, optical and physical properties associated with disorder necessary if new devices are to be exploited. Metal-metalloid alloys in particular have found use in the electronics industry because of their wide-ranging electronic properties, from metallic to semiconducting, via the so-called metal-insulator transition. The underlying mechanism of this process is still widely disputed and a comprehensive theory remains to be found. Most researchers agree that to fully understand these materials a knowledge of the atomic structure must be obtained. Furthermore, recent work has shown that this information should be considered alongside a picture of the general homogeneity of any material being investigated. We present in this work the results of a structural study of four metal-metalloid systems; a-Ge1-xTix, a-Si1-xTix, a-Si1-xNix and a-Ge1-xNix, prepared in thin-film form across a wide composition range by RF Sputtering, encompassing the metal-insulator transition. These samples have been subjected to an optical study to determine the extent of the band gap and hence the composition of the MIT. An EXAFS study has been performed to determine the atomic structure, such as interatomic distances and the number and type of near-neighbours. A relatively new atomic simulation code, RMC, has been applied to the analysis of EXAFS data with the hope that information such as partial radial distribution functions may be directly obtained. SAXS measurements have been made to probe any medium-range structure and to assess the homogeneity of the samples. Optical results show that the MIT occurs at compositions in the 0-20 at.% metal range. We find no obvious structural changes to accompany this event, but observe the high coodination of metal atoms at all compositions, in three of the four systems (a-Si1-xTix, a-Si1-xNix, a-Ge1-xNix). This is characteristic of a close-packed, conducting structure. Metalloid atoms, meanwhile, appear to exist within a tetrahedral random network at low metal content, rising to close-packed at metal rich compositions. SAXS results clarify these findings by revealing the presence of phase separation, suggesting that the homogeneity of samples should not automatically be assumed. We conclude that our samples contain regions of a conducting phase embedded in a semiconducting host network and suggest that the MIT proceeds not through the traditional Anderson mechanism but by the percolation of these regions at some threshold composition.