Molybdenum Oxide Thin Films Obtained by the Hot-Filament Metal Oxide Deposition Technique

Abstract

Molybdenum oxide thin films find diverse applications as catalysts, gas sensors, and electrochromic devices. Such films are produced mainly by reactive sputtering and thermal evaporation but other techniques such as chemical vapor deposition and electrochemical deposition have been used. In the present work, the feasibility of an alternative method for the production of molybdenum oxide films using a molybdenum filament heated in a rarefied oxygen atmosphere is demonstrated. The filament heating current, IF, and the oxygen flow rate, FO2, are the key deposition parameters and their effect on the deposition rate, R, was investigated. For IF = 12.5 A, an increase in the R-value from 7.5 to 31 nm/min was observed as FO2 was increased from 6.0 to 21 sccm. To characterize the chemical bonds, infrared spectroscopy, using both unpolarized and p-polarized infrared beams, and X-ray photoelectron spectroscopy (XPS) were employed. Line shape analysis of the Mo(3d) XPS peak revealed that the Mo atoms were in mixed valence states, Mo6+ and Mo5+, with a high predominance of the former over the latter, thus indicating an oxygen-deficient MoO3 film. From Rutherford backscattering spectroscopic analysis of the films, an average O/Mo atomic ratio of 2.9 was calculated, consistent with the XPS results. A combination of the XPS and RBS results and the data of other investigators on the oxidation of molybdenum suggests that the film is formed from MoO2 and MoO3 species desorbed from the Mo filament. The optical gap, Eg, was determined from transmission UV−visible spectra of the films. An average Eg value of 3.03 eV was found. The electrochromic properties of the films were investigated for Li+ intercalation using an electrochemical cell. A coloration efficiency of 19.5 cm2/C at a wavelength of 700 nm was observed.