In situ real time optical study of films grown on low temperature substrates by vacuum evaporation of iodine and silver iodide: spectral transitions during deposition and annealing processes

摘要

In order to clarify the growth process and hard to understand reported spectra of iodine film and silver iodide (AgI) film, various spectral transitions have been measured in situ: (a) optical density spectra of films grown on low temperature substrates by vacuum evaporation of an iodine lump, during deposition and annealing; (b) optical extinction spectra of vapour zones produced by gas evaporation of AgI powder, during evaporation; and (c) optical density spectra of films grown on low temperature and room temperature substrates by vacuum evaporation of AgI powder, during deposition and annealing. After these in situ real time measurements, the film specimens were allowed to recover and were examined by means of comparative measurements of the optical density and photoluminescence spectra at about 12 K. The spectra obtained were analysed and compared with those reported previously by other workers, and the following questions have been optically clarified: (1) How does annealing improve the crystal quality of iodine film? (The annealing at about 200 K improves the crystal quality considerably.) (2) What is the film that is quench deposited on low temperature sapphire surfaces by thermal evaporation of AgI powder? (It is AgmIn (m, n > 1) dispersed iodine film.) (3) How does AgI film grow in the deposited film? (Above about 220 K, iodine evaporates abruptly to grow uniform AgI film on the substrate.) (4) How does excess iodine affect the optical spectrum of AgI? (The excess iodine creates both several exciton absorptions due to polytype structures at wavelengths between 370 and 410 nm, and a tail at wavelengths longer than 430 nm.) (5) What is the best evaporation condition for the fabrication of AgI and iodine films? (The best quality of iodine and betaAgI films can be obtained at a substrate temperature of about 200 K in the quench deposition method.) Besides this, the present optical experiments have answered the following question: is there truly a strongly enhanced optical absorption in quench deposited AgI films, as reported by Kondo et al (1998 Phys. Rev. B 57 13235)? The present studies have elucidated that the enhanced absorption is not due to amorphization of AgI but due merely to intense optical absorption of iodine in the films.