Morphology changes of CuCl thin films induced by photo‐irradiation

Abstract

We have irradiated CuCl films, which are prepared by a vacuum vapor deposition method, with picosecond light pulses of 82 MHz repetition rate at the photon energy of 3.290 eV. After a long time of photo-irradiation, the photo-irradiated region is recognized by eye for thin films. We observe the surface morphology by atomic force microscopy at room temperature. For a 50 nm thick film, surface structure changes such as surface smoothing, nanometer-scale and micrometer-scale structures formation are observed, which depend on the position in the photo-irradiated region, while such changes are not detected for a 200 nm thick film. The effects of ablation and local heating by the photo-irradiation are excluded for the mechanism of the observed morphology change. The abundance change of Cu and Cl in the film is also detected by the Auger microprobe measurement. . Characteristics of the films are determined by the growth conditions, such as a substrate surface, a substrate temperature and a growth speed, and have been thought to be free from effects of photoirradiation for the photo-luminescence observation.Recently, we found that a decrease in the exciton luminescence intensity and appearance of new bound exciton luminescence of CuCl bulk and vapor-evaporated films on TiO 2 , Al 2 O 3 and synthesized quartz were induced by photo-irradiation with ultra-violet light pulses at 4.6 K [7,8]. In the series of the experiments we recognized the photo-irradiated spot in thin films by eye after the photo-irradiation. Usually, such changes in the exciton luminescence and the sample may be regarded as radiation damage or photo-degradation. However, we observed the well-characterized Z 3 exciton luminescence of the intrinsic nature even after a long time of photo-irradiation without substantial changes of the spectral profile, which is known to reflect the exciton dynamics in the film [6,9,10]. Therefore, observation of the film structure change may be of interest. In this work we present the morphology changes of the CuCl film observed by atomic force microscopy together with the spatially resolved luminescence spectra. The Auger microprobe measurement of the compositional change in the film is also presented.