Spectroscopic analysis of tungsten oxide thin films Felicia S. Manciu, a) Jose L. Enriquez, William G. Durrer, and Young Yun Department of Physics, University of Texas at El Paso, El Paso, Texas 79968 Chintalapalle V. Ramana and Satya K. Gullapalli Department of Mechanical Engineering, University of Texas at El Paso, Texas 79968 (Received 2 March 2010; accepted 14 July 2010) We present a detailed study of the morphology and composition of tungsten oxide (WO 3 ) thin films, grown by radio frequency magnetron reactive sputtering at substrate temperatures varied from room temperature (RT) to 500  C, using infrared (IR) absorption, Raman spectroscopy, and x-ray photoelectron spectroscopy (XPS). This work includes valuable new far-IR results about structural changes in microcrystalline WO 3 . Both IR absorption and Raman techniques reveal an amorphous sample grown at RT and initial crystallization into monoclinic structures for samples grown at temperatures between 100 and 300  C. The Raman spectra of the samples grown at high temperatures indicate, apart from the monoclinic structure, a strain effect, with a distribution revealed by confocal Raman mapping. XPS indicates that the film surface maintains the stoichiometry WO x , with a value of x slightly greater than 3 at RT due to oxygen contamination, which decreases with increasing temperature. I. INTRODUCTION Tungsten oxide (WO 3 ) has been extensively investi- gated, especially in the form of thin films, where con- trolling material characteristics such as crystallinity, grain size, nano- and micro-porosity, stoichiometry, and surface reactivity can be used to enhance its photo- chromic, 1–3 electrochromic, 3–6 thermochromic, 7,8 and gasochromic 9,10 properties. Important reasons for the impressive amount of research dedicated to this metal oxide are its unique properties, which consist of rela- tively high melting point, diverse, but thermodynami- cally stable structures with variations in temperature and pressure growth conditions, and low-cost manufacturing procedures. In addition, the high compatibility of WO 3 with microelectronic processing and ease of integration into portable devices with the characteristics of low power consumption and online operation, have resulted in its extensive use in nanotechnology applications. It is a primary candidate for use in photocatalysts, anti-dazzle mirrors, smart windows, and gas sensing devices. 10–14 In order to achieve stable, selective, and reliable devices, accurate preparation of the functional material is crucial; many factors must be taken into account to warrant homogeneous WO 3 grain characteristics such as shape and size, distribution, porosity, and surface condi- tions. This is a consequence of the fact that, despite the simplicity of the principle and use of metal oxides for industrial purposes, the actual mechanism is quite com- plex and not yet fully understood in a way that allows total control in a confinement regime. Thus, fundamental investigations of WO 3 at the nano- and micro-scale, where structural defects and perturbation of the elec- tronic structure often arise, need particular attention. Abundant theoretical and experimental forecasts of prop- erties of this compound, in its standard and doped form, are available in the literature, revealing that thin-film structures of WO 3 exhibit various phases depending on the thermodynamic parameters (temperature and reactive pressure) used for their growth. 2,7,15 For example, tri- clinic, monoclinic, orthorhombic, tetragonal, and hexag- onal crystalline phases of WO 3 have been reported for temperatures between absolute zero and its melting point at about 1400  C. 2,7,15 Since individual nanostructures are not only small in size, but often present in low yield and as mixtures of different phases, sensitive spectroscopy tools are required to study them. In this context, we present here detailed experimental infrared absorption, confocal Raman, and x-ray photoelectron spectroscopic (XPS) results, which demonstrate the effect of growth tempera- ture on the morphological evolution of WO 3 thin films. Apart from standard structural and microscopic charac- terizations by x-ray diffraction (XRD), scanning elec- tron microscopy (SEM), and atomic force microscopy (AFM), 15–18 spectroscopic analytical techniques have also proven to be valuable, 19–21 especially for obtaining information about the local structure (e.g., by using Raman mapping) and the chemical bonding of the stoi- chiometric polycrystalline films. a) Address all correspondence to this author. e-mail: fsmanciu@utep.edu DOI: 10.1557/JMR.2010.0294 J. Mater. Res., Vol. 25, No. 12, Dec 2010 © 2010 Materials Research Society 2401