Scanning Tunneling Microscopy and Atomic Force Microscopy Studies of Laser Irradiation of Amorphous WO3 Thin Films

Abstract

The effect of irradiating amorphous WO3-x thin films by an excimer laser and a Nd-YAG (yttrium-aluminum-garnet) laser on the evolution of electronic states was investigated by scanning tunneling spectroscopy (STS) and X-ray photoelectron spectroscopy (XPS). The original films prepared by pulsed-laser deposition (PLD) were first irradiated by a KrF excimer laser at 248 nm in air. The surface was reduced to a quasi-metallic one with oxygen deficiency and the number of W 5d occupied states was increased. A number of surface states appeared on the films. When the films were subsequently irradiated by the Nd-YAG laser at 1.06 µm in air, the number of W 5d occupied states was reduced due to oxygen restoration. The number of surface states was also greatly reduced. A water layer on the surface, which played a key role in photolysis, was monitored by STS by trapping charges in the Schottky barrier. Atomic force microscopy (AFM) and scanning tunneling microscopy (STM) images of the films at three states: original, surface reduced, and surface restored, were compared in terms of average profile roughness and height. Thermal effects (thermochromism) dominated in the Nd-YAG laser irradiation, resulting in a maximum average roughness in the AFM images, whereas photon effects (photochromism) dominated in excimer laser irradiation, which led to the highest conductance. Slight nanocrystallization occurred in the laser-irradiated films.