The nature of fundamental absorption edge of WO3

The fundamental absorption edge of amorphous, polycrystalline and crystalline tungsten trioxide (WO3) thin films obtained by different techniques (thermal evaporation, rf plasma sputtering, chemical gas transport) was investigated. Special attention was paid to correct measurements of absorption values of all WO3 thin films taking into account the scattered light, interference effects and reflection losses. The indirect edge at 2.70 eV was determined for crystalline WO3, but for crystal WO3:Ti the direct edge in the same place (2.72 eV) was found. For polycrystalline WO3 thin film first edge transition located at 2.76 eV had a quadratic dependence similar to the amorphous film at a higher photon energy (3.26 eV). Localized states are involved in these transitions with quadratic dependence. The gap energy shift from crystalline to amorphous WO3 is explained by changes in the first coordination sphere of octahedral WO3 cluster. Higher direct allowed transition at 4.1 eV is estimated both for amorphous and polycrystalline thin WO3 films.