The structure and electrical conductivity of thin films of indium

The high electrical resistance and negative temperature coefficient that thin films of indium, in common with other metals, exhibit when prepared under moderate vacuum conditions (10 -4 to 10 -5 mm.) and without exhaustive cleaning of the substrate, are shown by electron-diffraction methods to be in no way connected with the existence of any amorphous form of the metal. The lattice of freshly deposited films is found to correspond in form with that of the bulk metal but to have a spacing 0.7 per cent greater than that given by recent X-ray work, gold films here being taken as standards. With slowly deposited films the spacing increases and the axial ratio decreases with rise in temperature up to room conditions, and exposure to air carries this process further until a film showing a cubic indium lattice with spacing 3.4 per cent greater than the bulk metal is obtained. This change is accompanied by slow oxidation, the oxide lattice being 0.6 per cent larger than X-ray values indicate. It is suggested that the high resistance of these films is due to sorption of residual gas, and that the negative temperature coefficient is the result of desorption and resorption processes; changes in crystal-size with temperature are also noted and discussed. The change in lattice constants is ascribed to the action of gas within the lattice, possibly amounting to the production of intermediate stages in oxideformation.