Band Gap Engineering of Metal Oxide (WO<sub>3</sub>, MoO<sub>3</sub>) Thin Films through Alloying with Cadmium Telluride

Abstract

This study was concerned with the development of a synthesis technique based on thermal evaporation for engineering the band gap of some metal oxide semiconductors, namely tungsten oxide (WO3) and molybdenum oxide (MoO3) by alloying them with a narrow-band-gap II-VI semiconductor, specifically cadmium telluride (CdTe). In this study, thermal evaporation, which is considered to be one of the simplest methods for the deposition of thin films, was employed to fabricate widely-tuned band gaps of WO3 and MoO3 films by mixing them with specific concentrations of CdTe. The novelty of this technique was that the band gap engineering of these oxides required merely evaporation of a powder mixture of each oxide with controlled concentrations of CdTe. Such a study was attempted to provide the opportunity for tuning other wide band gap metal oxide semiconductors through mixing with different narrow band gap compounds. First, the influence of CdTe concentration of 5%, 10%, 15%, 20% and 25% on the band gap of WO3 thin films was studied. Second, the influence of CdTe concentration of 2%, 4%, 6%, 8% and 10% on the band gap of MoO3 thin films was investigated. X-ray diffraction showed that the obtained pure WO3 and MoO3 films had an amorphous structure, while CdTe thin films were polycrystalline. The addition of CdTe to WO3 and MoO3 did not improve their structures. However, at high CdTe concentrations, WO3 thin films still had an amorphous phase but the polycrystalinity of CdTe was decreased. The films showed an increase in the surface roughness with the increase in CdTe concentration as revealed by atomic force microscopy. Optical analysis based on transmittance measurements revealed that the band gap of WO3 thin film was significantly red-shifted due to alloying with CdTe. The band gap values of the fabricated films were found to decrease from 3.30 eV for pure WO3 to 2.47 eV at CdTe concentrations of 25%. Similarly, a remarkable red-shift in the band gap of MoO3 thin films through mixing with CdTe was observed. The band gap of the fabricated films was decreased from 2.90 to 2.60 eV as the CdTe concentration increased from 0% to 10%, respectively. The experimental variation of the band gap values with CdTe was described by the standard bowing quadratic equation for the whole range of CdTe concentration. The obtained results indicated that a wider portion of the visible spectrum can be harvested rather than only single wavelength, corresponding to the band gaps of pure WO3 or MoO3 thin films. Chemical analysis of pure WO3 and MoO3 films performed by X-ray photoelectron spectroscopy showed that a sub-stoichiometric WO2.96 and an almost stoichiometric MoO2.98 films were obtained. Depth profiling analysis of the films alloyed with CdTe confirmed a nonhomogeneous distribution of the constituents throughout the depth of the WO3 alloyed films. However, the constituents were homogeneously distributed throughout the thickness of the MoO3 alloyed films. Photocurrent measurements showed a significant increase in the photocurrent response of the CdTe-alloyed WO3 and the CdTe-alloyed MoO3 films with the increase of the CdTe concentration due to the enhancement of the light absorption in the long wavelength region. The obtained results support the potential of these alloyed films for improving the photo-to-current conversion efficiency in the photovoltaic and photoelectrochemical solar cell applications.