Dielectric losses in libr films at low temperatures

Abstract

Thin films of lithium bromide have been prepared by vacuum deposition at both 78°k and room temperatures, and the dielectric properties have been measured over a frequency range of 0·01 Hz to 1000 Hz and temperatures from 78°K to room temperature. Three separate dielectric loss mechanisms have been identified. Two of these mechanisms have been observed previously but the third occurs only in films which have cracked or crazed. It has an activation energy of only 0·20±0·02 ev and is due to surface migration over the crack surfaces. When it occurs, other mechanisms are likely to be obscured. A simple space-charge polarization due to blocking of charge-carriers at the electrodes is observed both in films deposited at room temperature and in films freshly deposited at 78°K. Blocking of charge-carriers at inter-crystalline boundaries does not occur in films deposited a,t 78°K or as the temperature is raised gradually to room temperatures. It appears only after an appreciable period of ageing or annealing and it is explained in terms of a Frcnkcl-Lehovec type of potential at the crystallite boundaries. The formation of this potential involves defect migration and can occur only for temperatures at which defects are mobile. This mechanism of blocking at intercrystalline boundaries has an activation energy of 0·40±0·02 ev which is in good agreement with the value of 0·39 ev for jumping of cation vacancies in LiBr.