Thin Solid Films 408 (2002) 188–193 0040-6090/02/$ - see front matter  2002 Elsevier Science B.V. All rights reserved. PII: S0040-6090 Ž 02 . 00138-4 Effects of preparation conditions and thermocoloration on the optical properties of thin films of molybdenum oxide M.F. Al-Kuhaili *, S.M.A. Durrani , E.E. Khawaja a, b b Physics Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia a Center for Applied Physical Sciences, Research Institute, King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia b Received 29 November 2000; received in revised form 12 February 2002; accepted 12 February 2002 Abstract Thin films of molybdenum oxide were prepared by thermal evaporation. The effects of the preparation conditions (substrate temperature and oxygen partial pressure) on the optical constants (n and k) were studied. It was found that the optical constants increased with increasing substrate temperature and they decreased with the introduction of oxygen into the deposition chamber. Thermocoloration of the films was carried out by annealing them in vacuum. The annealing of the films produced significant changes in the transmittance of the films, and thus, in their optical constants. These changes resulted from the observed changes in film thickness and atomic oxygen-to-molybdenum ratio in the films.  2002 Elsevier Science B.V. All rights reserved. Keywords: Molybdenum oxide; Optical properties 1. Introduction Molybdenum oxide is one of a group of materials that exhibits chromogenic properties. These are materials which can be switched between two different optical states w1x. These optical changes can be prompted by the injection of charges (electrochromism), by irradia- tion with ultraviolet light (photochromism), or by heat treatment (thermochromism). For applications based on chromogenic phenomena, the materials are grown as thin films. Several studies have been carried out to investigate the electrical and structural properties of molybdenum oxide thin films. However, the chromogen- ic properties are directly related to the films’ optical properties. To study the chromogenic properties of molybdenum oxide thin films, several workers have measured the changes in the transmittance of these films as a result of the chromogenic processes w2–5x. How- ever, very little is known about the effects of chromo- genic processes on the optical constants of the films. Molybdenum oxide thin films can be produced by various growth techniques w2x. The properties of thin *Corresponding author. Tel.: q966-860-2255; fax: q966-860- 2293. E-mail address: kuhaili@kfupm.edu.sa (M.F. Al-Kuhaili). films are dependent on the growth method and the parameters involved in each method. The optimum operation of these films requires a detailed study of how these parameters affect their properties. One of the methods of growing molybdenum oxide thin films is thermal evaporation w6x. In this technique, the most important parameters are the background (base) pressure of the vacuum chamber, the source-to-substrate distance, the substrate temperature, the deposition rate and the oxygen partial pressure. In this study, molybdenum oxide thin films were produced using thermal evaporation. The effects of substrate temperature and oxygen partial pressure on the optical constants of the films were investigated. The thermocoloration of these films was produced by anneal- ing them in a vacuum and, subsequently, determining their optical constants. 2. Experiment Thin films were prepared simultaneously on fused silica and tantalum substrates in a Lybold model L560 box coater pumped by a turbomolecular pump. The system was pumped to a base pressure of 2=10 mbar. y6 The source material, MoO , of purity 99.5%, was evap- 3