Ž . Thin Solid Films 318 1998 108–112 Structural and optical studies of CdS nanocrystals embedded in silicon dioxide films A.G. Rolo a, ) , O. Conde b,1 , M.J.M. Gomes a a Departamento de Fısica, UniÕersidade do Minho, Largo do Pac ¸o, 4709 Braga Codex, Portugal ´ b Departamento de Fısica, UniÕersidade de Lisboa, Ed. C , Campo Grande, 1700 Lisboa, Portugal ´ 1 Abstract ˚ Silicon dioxide films doped with nanocrystallites of CdS with a mean radius value in the range of 20 to 40 A have been grown using the magnetron rf-sputtering technique. We have studied the combined effects of rf-power deposition, the number of semiconductor chips on the SiO target, and post-annealing temperature on the CdS crystal size. The optical transmission spectra display a marked blue shift of 2 the absorption band edge of up to 400 meV which was attributed to quantum size confinement effects. The luminescence spectra show a narrow emission peak tentatively explained as being due to the recombination of electrons on shallow donor levels with confined valence band holes. q 1998 Elsevier Science S.A. Keywords: CdS-doped glass films; II–VI semiconductor; Rf-sputtering; Quantum dots; Optical absorption; Photoluminescence 1. Introduction Small semiconductor crystals embedded in a dielectric matrix, which have the properties of zero-dimensional quantum dots, have been extensively studied for several w x years 1,2 . Optical glasses doped with semiconductor nanocrystallites are one example of these systems. Optical and electronic properties of these semiconductor quantum dots have attracted much attention because, due to their low dimensions, these systems exhibit quantum confine- ment effects, which can give rise to interesting linear and nonlinear optical properties and have the potential of being wx developed into novel photonic devices 3 . 2. Experimental procedure CdS-doped glass films were prepared by a conventional rf-magnetron co-sputtering method using Alcatel SCM650 Ž . apparatus. A SiO 99.99% plate of 50 mm diameter, 2 partially covered by polycrystalline chips of cadmium Ž . sulphide c-CdS , was used as target. Prior to sputtering, the chamber was always evacuated to 2 = 10 y6 mbar. In situ Ar pre-sputtering of the target and substrates was ) Corresponding author. Tel.: q351-53604331; fax: q351-53678981; e-mail: arolo@fisica.uminho.pt. 1 E-mail: olinda.conde@fc.ul.pt. performed in order to clean and remove any impurities. The deposition parameters are presented in Table 1. The film deposition was carried out in highly pure Ar Ž . y3 99.996% pressure of 5 = 10 mbar on glass slides, with the substrates kept at room temperature, resulting in typical ˚ y1 deposition rates of 0.1 to 0.9 A s . The target–substrate distance was kept at 60 mm. Relatively low rf-powers have been used in order to reduce the target heating that could cause a decomposition of the CdS chips and result in the loss of sulphur. In order to investigate the growth of semiconductor nanocrystals in the deposited films, post-an- nealing was carried out in a Kantal furnace in ambient atmosphere. Annealing temperature was varied between 1008C and 5508C for annealing times from 300 s to a few Ž . hours. X-ray diffraction XRD spectra of the films were recorded with a Siemens D5000 diffractometer, employing Ž . CuK a radiation in glazing incidence geometry GIXRD . Since the fraction of CdS crystals embedded in the glass films is rather small, a counting time longer than 15 s per 0.048 step and incidence angles of 1 and 28 were required. The XRD data were obtained between 2 u s 78 and 708. Optical absorption spectra were measured at room temper- ature using a double-beam Shimadzu model UV-3101 PC in the range from 200 to 3200 nm. Photoluminescence spectra in the range from 300 to 800 nm were obtained at room temperature by exciting the samples in the range 380 to 425 nm using a continuum Xenon lamp using a 0.22-m 0040-6090r98r$19.00 q 1998 Elsevier Science S.A. All rights reserved.