A study of NaTi-type NaIn films prepared by the sandwich-type evaporation method

Abstract

The sandwich-type evaporation method has been developed as a technique for the preparation of films of NaTl-type compounds including one containing an active material, namely lithium. In the investigation reported here the method was applied to the preparation of NaIn films with the NaTl-type lattice structure, in which each component has a diamond-type sublattice. The method consists in the evaporation of firstly indium (99.99% pure), then sodium (99.999% pure) and indium together and finally indium. The evaporation process is continuous to prevent oxidation of the sodium. Thus, the film of codeposited sodium and indium is formed between indium films. The films were deposited onto a glass substrate by evaporating the sodium and indium from a tantalum boat and were annealed in a conventional vacuum system that employed an oil diffusion pump which maintained a vacuum of about 6×10 −6 Torr. The degree of crystallization of the films deposited at various substrate temperatures was investigated by electron diffraction and measurements of the electrical resistance during annealing. The interplanar spacings in the NaIn layers were calculated from the radii of the Debye rings on the original plates of the electron diffraction patterns of the films. From the results obtained for substrate temperatures of 120 and 150°C, there was no evidence of free indium in any of the films and the (111), (220), (311), (400), (331), (422), (440), (531) and (620) planes of NaIn were detected, in agreement with ASTM data within experimental error. For a typical NaIn films resistivity is about 8.13×10 −5 Ohms cm at room temperature and the temperature coefficient of the resistivity is about 0.48×10 −6 Ohms cm K −1, which is very similar to that for an NaTl film.