Structural and optical properties of CsI thin films: Influence of film thickness and humidity

Abstract

Structural and optical studies have been performed on the thermally-evaporated "as-deposited" and "humid air aged" CsI thin films. The structural analysis for both "as-deposited" and "humid air aged" films shows a well-oriented peaks position of (110) and (220) lattice planes with a compressive stress in the films. The crystal quality has been investigated through the structural parameters. The increase in peak intensity as well as sharpness with film thickness implies the improvement of crystallinity. The optical absorbance of CsI films has been analyzed in the wavelength range of 190 nm -900 nm in order to estimate the band gap energy of the films. Slater's model has also been used to explain the degradation of band gap energy with the increase in crystallite size. been tested successfully under liquid xenon atmosphere for its use in dark matter search experiment [21]. The detectors used for these experiments generally work in two modes; reflective (opaque) and semitransparent mode. The Quantum Yield (QY) of reflective films has been observed to be higher in comparison with the semitransparent films. This is because the semitransparent films have a small escape length (L≤ 10 nm) [16]. Also, it has been revealed that the reflective films have more chemical stability against the moisture compared to the semitransparent films [17].Keeping these facts in mind, the main issue related to CsI photocathode is the aging which causes a degradation in the photoemission properties and thus limits their life. In order to develop and optimize large area photocathodes based detector, the study of the optical and structural properties of the "as-deposited" and "aged" CsI photocathodes is ultimately required. In literature, there are several studies on CsI ageing, but its mechanism does not have a clear understanding yet. A. S. Tremsin et al. [23] have used TEM technique to study the structural properties of polycrystalline CsI thin film of 100 nm thickness under the impact of humid air and UV irradiation. M. A. Nitti et al. [24] have examined the bulk structure and the surface of freshly and humid air exposed 100 nm CsI film by XRD and XPS measurements. Recently, Triloki et al. [25] have studied the structural properties of thermally-evaporated CsI thin films of different thicknesses by using XRD and TEM techniques. C. Lu and K.T. McDonald [16] have previously reported the optical properties of freshly CsI thin films up to 31 nm thickness. This work is an extension of the previous works, where we have systematically studied the structural and optical properties of semitransparent and reflective CsI thin films grown under similar vacuum-environment in case of "as-deposited" and "humid air aged". Experimental Details Deposition of CsI thin filmsThe deposition of reflective (500 nm and 300 nm) and semitransparent (50 nm and 30 nm) CsI thin films was done in a stainless steel 18" diameter spherical vacuum chamber. Prior to the deposition process, the substrates were cleaned by treating with distilled water, acetone, and alcohol to...

Luminescence of CsI and CsI:Na Films under LED and X-ray Excitation

“…The luminescence properties and preferred crystal orientations are different because of the different fabrications and thicknesses of the films [13]. In our research, the problem of different crystal orientations has also plagued us for many years [14][15][16][17].…”