Characterization of reactively sputtered molybdenum oxide films for solar cell application

Molybdenum oxide (MoO3) thin films were prepared via Radio Frequency (RF) sputtering at different ambient composition and post-deposition annealing. The effects on the structural, optical, and surface properties of the deposited films were investigated. The ambient oxygen concentration O-2/(O-2+Ar) was varied from 10% to 100% at 10 mTorr. Post deposition anneals were performed in Ar at 300-500 degrees C. The films were analyzed using glancing incidence x-ray diffraction (GIXRD), UV/Vis/NIR spectrophotometry, and x-ray photoelectron spectroscopy (XPS). As-deposited films have amorphous structures, independent of the oxygen partial pressure. Annealing at 300 degrees C in air resulted in crystallization of the molybdenum oxide films to the monoclinic beta-MoO3 phase. Samples annealed at 400 and 500 degrees C were identified as pure orthorhombic alpha-MoO3 phase with (020) preferred orientation. High resolution XPS studies showed the presence of Mo6+ (MoO3) and Mo5+ (Mo4O11) oxidation states at the surface of as deposited and low temperature (300 degrees C) annealed films, and the Mo6+ to Mo5+ did not change much with deposition oxygen partial pressure. Annealing at 400-500 degrees C suppressed the oxygen deficiency at the surface, resulting in films with composition close to stoichiometric phases. UV/Vis/NIR spectrophotometry revealed that all films have a high optical transmittance (>80%) in the visible range, followed by a steep drop at lambda approximate to 400 nm indicating a strong absorption due to band-to-band transition. Increasing the oxygen partial pressure had no significant effect on optical transmittance of the films, and the bandgaps in the range of 2.6 eV to 2.9 eV were obtained. Annealing at 300 degrees C had a negligible effect on the optical properties of the MoO3 films, but samples annealed at 400 degrees C and 500 degrees C exhibited wider bandgaps within the range of 3.1-3.4 eV. (C) 2013 AIP Publishing LLC.