When fabricating TeOx thin films by vacuum deposition, it is common to use a tungsten (W) boat. However, with this method, it is difficult to perform an accurate evaluation because the raw materials—TeO₂ and W—react chemically. Therefore, we used an alumina crucible, which is less prone to reaction, and fabricated thin films using a mixture of TeO₂ and W powder as the raw material to investigate the effect of W. We found that samples containing 15 wt% or more of W exhibited TFT characteristics, whereas those containing 10 wt% or less behaved as insulators and did not function as TFTs (fig. 1). XPS measurements revealed that peaks originating from Te-Te bonds were observed in the thin films exhibiting TFT characteristics, whereas they were scarcely detected in the insulating films (fig. 2). Furthermore, TEM observations and X-ray diffraction patterns revealed the presence of metallic Te nanoparticles within the amorphous TeOx. Results from SIMS and other techniques also indicated that W was not present in the thin films. Therefore, it is believed that W reduces the TeO₂ in the raw material, thereby diluting the oxygen concentration in the thin film; the dispersion of metallic Te nanoparticles then enables electrical conduction via hopping.