Optical and electronic properties of amorphous WO 3 thin film irradiated by laser in air

Abstract

To study the potential in optical recording, the laser induced chromism of amorphous WO 3 thin films has been investigated. The original film can be colored from grey to purple by one pulse of the KrF excimer laser at 248 nm and bleached to dark grey by one pulse of Nd-YAG laser at 1.06 μm in air. Spectroscopy measurements were applied to study the films at the three alternate states: original, colored and bleached. The changes of refractive index (n) and extinction coefficient (k) from colored states to bleached states, could be measured by ellipsometry spectroscopy that showed increasing and decreasing tendency, respectively in luminous range at colored state. X-ray photoelectron spectroscopy (XPS) was used to study the chemical states. With some W 5+ ions existed in the original films by pulsed laser deposition, more lower states ions, such as the W 3+ ions were produced along with decreasing W 6+ states in films colored by KrF excimer. Laser bleaching was accompanied with decreasing of W 5+ , W 3+ states and increasing of W 6+ states in films. The purple color was thought due to the polaron transition between W 3+ states and W 4+ states or, W 5+ states and W 4+ states. Scanning tunneling spectroscopy (STS) showed that the colored films had more characteristics of n type semiconductor after coloring. We attribute the coloring and bleaching process to photochemical activation and photothermal oxidation, respectively. Raman spectroscopy showed slight crystallization after coloration in films. From temperature field calculation, the crystallization is suggested most likely caused by ion intercalation instead of thermal crystallization. The color states in films are very stable after longtime exposure in air, as well as in oxygen.