The Variation of Optical and Electrical Properties Due to Annealing of Thin, Vapour-Quenched Aluminium Films

Abstract

This thesis contains the results of simultaneous observations of the changes of optical and electrical properties of thin aluminium films deposited in ultra-high vacuum on to various substrates at 77°K, and subsequently raised to room temperature and then annealed to about 550°K. The optical constants n and k were measured using an ellipsometer of simple construction and the defect concentration was estimated from electrical resistivity. Oxidation of an aluminium surface was found to be significant even at a total pressure of 1.5 x 10-9 torr but observations on several films enabled a correction to be applied. Films below 15 nm thickness, deposited at 77°K on glass substrates showed an irreversible fall of resistance of up to 40% when warmed to room temperature. The techniques used in radiation damage recovery were employed and the observations were found to be consistent with the migration of 'Dumbell' ⟨100⟩ interstitials to vacancies. During the fall of resistance, a simultaneous rise of the product nk, which is proportional to optical absorption, was observed for all films at a wavelength of 0.549µ. This was not consistent with the Drude-Zener formulae and was attributed to an increase of the inter-band absorption peak centred at 0.8µ for aluminium. Optical measurements over an extended wavelength range (0.49µ to 1.94µ) for films on both glass and single crystal potassium bromide substrates also showed that the anneal from 77°K to room temperature always produced development of the inter-band absorption peak and a shift of the maximum towards higher energy. This occurred over and above the known effect of temperature. The movement of the peak is interpreted as an ordering effect producing development of the (200) component of the pseudo-potential and hence an increase of the inter-band absorption in the region of the symmetry point W on the (200) Bragg reflection plane of the Brillouin zone. During annealing to 550°K of totally oxidised films, it was found that on glass substrates the free electron value of nk increased, while on the crystal substrates the value decreased. This was interpreted as a difference in thermal expansion between the film and substrate. On the glass substrates the films were compressed beyond the elastic limit and the defect concentration was increased, whereas on the crystal substrates the films were extended and true annealing occurred. On both substrates the inter-band contribution was increased by the anneal.