X-ray irradiation induced effects on the chemical and electronic properties of MoO₃ thin films

Highlights: • X-ray induced "beam damage" of MoO₃ thin films is systematically studied. • Reduction of MoO₃ is depending on x-ray photon flux and exposure time. • Appearance of Mo 4d derived above-VBM states related to degree of reduction. • "Beam damage" free XPS possible if photon flux and measurement time limited. - Abstract: The time and photon-flux dependent x-ray irradiation induced effects on the chemical and electronic properties of MoO₃ thin films have been systematically investigated by ultraviolet photoelectron spectroscopy (UPS) and x-ray photoelectron spectroscopy (XPS). After exposure to Mg K_α x-ray irradiation of different photon flux (realized by using different x-ray source powers: 30, 100, and 300 W), the Mo 3d XPS spectra indicate a partial conversion of the Mo⁶⁺ states in MoO₃ to Mo⁵⁺ states. The degree of reduction significantly depends on exposure time and x-ray source power (i.e., photon flux). To minimize x-ray irradiation induced effects on the collected data, a measurement time of 1–2 h at a low photon flux should not be exceeded. Related UPS analysis shows that no effects of UV irradiation can be detected and reveals the x-ray irradiation induced appearance of defect states above the valence band maximum (VBM). The intensity of these above-VBM states increases linearly inverse with the XPS derived Mo⁶⁺/(Mo⁵⁺ + Mo⁶⁺) ratio, suggesting an x-ray induced loss of oxygen as the common explanation for both effects.