Photoinduced effect in Ga–Ge–S based thin films S.H. Messaddeq a, * , M. Siu Li a , S. Inoue b , S.J.L. Ribeiro c , Y. Messaddeq c a Instituto de Fı ´sica de Sa ˜o Carlos, Universidade de Sa ˜o Paulo, C.P. 369, CEP 13560-970, Sa ˜o Carlos, SP, Brazil b National Institute of Research in Inorganic Materials, Namiki, Tsukuba, Ibaraki, Japan c Instituto de Quı ´mica, UNESP, Araraquara, C.P. 355, CEP 14801-970, Araraquara, SP, Brazil Received 30 August 2005; received in revised form 18 November 2005; accepted 8 December 2005 Available online 18 January 2006 Abstract Glassy films of Ga 10 Ge 25 S 65 with 4 mm thickness were deposited on quartz substrates by electron beam evaporation. Photoexpansion (PE) (photoinduced increase in volume) and photobleaching (PB) (blue shift of the bandgap) effects have been examined. The exposed areas have been analyzed using perfilometer and an expansion of 1.7 mm(DV/V  30%) is observed for composition Ga 10 Ge 25 S 65 exposed during 180 min and 3 mW/cm 2 power density. The optical absorption edge measured for the film Ge 25 Ga 10 S 65 above and below the bandgap show that the blue shift of the gap by below bandgap photon illumination is considerable higher (DE g = 440 meV) than DE g induced by above bandgap illumination (DE g = 190 meV). The distribution of the refraction index profile showed a negative change of the refraction index in the irradiated samples (Dn = 0.6). The morphology was examined using a scanning electron microscopy (SEM). The chemical compositions measured using an energy dispersive analyzer (EDX) indicate an increase of the oxygen atoms into the irradiated area. Using a Lloyd’s mirror setup for continuous wave holography it was possible to record holographic gratings using the photoinduced effects that occur in them. Diffraction efficiency up to 25% was achieved for the recorded gratings and atomic force microscopy images are presented. # 2005 Elsevier B.V. All rights reserved. Keywords: Chalcogenide; Thin films; Photoexpansion; Photobleacing; Refraction index; Diffraction gratings 1. Introduction Because the density of information storage in optical media can be up to two orders of magnitude better than in magnetic discs, there has been extensive research into materials capable of optical recording. Chalcogenide films have potential in this area and have been investigated as optical recording media for mass-memory applications [1,2]. In these applications the basic mechanism used to record information is generally photo- induced phenomena. It is well known that photoinduced changes in chalcogenide thin films are induced by exposing a sample to near bandgap light [3–5]. Such changes can be structural (e.g. changes in the density) [6], mechanical (e.g. rheological properties) [7] and optical (photodarkening) [8]. Also, it has been reported that these materials has a volume change upon light illumination [9,10]. Ganjoo et al. [11] have reported that when a chalcogenide film is photodarkened by bandgap illumination, the material has a macroscopic expansion, which is recovered with annealing at 400 8C for 5 h. The fractional expansion, DV/ V , is about 0.5% in As 2 S 3 illuminated with 2 eV at room temperature [12]. Hisakuni and Tanaka have detected that, upon illumination with light whose energy is less than bandgap, chalcogenide films have larger expansion, which they labeled ‘‘giant photoexpansion’’ [12]. At the present, the phenomenon is detected in As 2 S 3 and GeS 2 and hence it may occur in other chalcogenide films. For instance, As 2 S 3 exposed at room temperature to light from a He–Ne laser has a 5% volume expansion, which has been used for the development of micro lens devices [13]. In the present paper, we reported the photoexpansion (PE) and photobleaching (PB) effect observed on GaGeS amorphous films when the sample was irradiated using the 351 nm (3.54 eV) Ar + ion laser line, varying power density (2–6 mW/ cm 2 ) and exposure time (0–180 min). The photoexpansion effect was used to produce relief gratings on the film surface using a Lloyd’s mirror setup. Analyses of the morphological and chemical compositions were performed to determine the origin of the photoinduced effect. www.elsevier.com/locate/apsusc Applied Surface Science 252 (2006) 8738–8744 * Corresponding author. Tel.: +55 16 33016632; fax: +55 16 33016636. E-mail address: sandra@posgrad.iq.unesp.br (S.H. Messaddeq). URL: http://www.iq.unesp.br/pesquisa/LaMF/ 0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.12.074