The electrical and optical properties of amorphous thin films of Mg-Bi

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Abstract
The results of d.c. and a.c. conductivity measurements, as a function of temperature, and optical transmission through vacuum-evaporated amorphous films of Mg-Bi are presented. Films have been prepared in high-and ultra high-vacuum conditions using both liquid nitrogen-and liquid helium-cooled substrates. The properties have been measured as a function of composition, for a range of compositions extending a few atomic per cent either side of that appropriate to the intermetallic compound Mg3Bi2. The results are interpreted in terms of a rigid band model in which the density of states is invariant for small changes in composition about Mg3Bi2. An optical gap of ∼0.7 eV is apparent for these compositions and the electronic properties are discussed in terms of an amorphous semiconductor. The position of the Fermi Level, EF , is strongly dependent on composition and is not pinned near the centre of the gap as in other amorphous semiconducting systems such as Ge-Te. Conduction mechanisms due to carriers hopping between localized states at EF , carriers excited to localized states at the band edge and carriers excited to extended states at the band edge are observable for compositions very close to Mg3Bi2.

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