Investigation on the valence state of Te ions in the Bi₆Ti₅TeO₂₂ thin film using X-ray photoelectron spectroscopy

Bi₆Ti₅TeO₂₂ (BTT) thin films were grown on a Pt/Ti/SiO₂/Si(1 0 0) substrate under various conditions and the valence state of the Te ion was investigated. For the BTT films grown at 300 °C, most of the Te ions existed as Te⁴⁺ ions. However, for the 10 mol% Mn-added BTT films grown at 300 °C, Te⁶⁺ ions were found even in the film grown under low oxygen partial pressure (OPP) and their number increased with increasing OPP. This increase was attributed to the presence of Mn²⁺ ions, which assisted the transition of Te⁴⁺ ions to Te⁶⁺ ions in order to maintain the charge balance of the Ti⁴⁺ sites. Furthermore, in the films grown at 300 °C under a high OPP of 80.0 Pa and subsequently annealed at 600 °C under a high oxygen pressure of 101 kPa, most of the Te ions existed as Te⁶⁺ ions. However, for the film grown at 300 °C under low OPP, even though the film was annealed under a high oxygen pressure of 101 kPa, only a few of Te⁶⁺ ions were formed, whereas most of Te ions remained as the Te⁴⁺ ions.