Investigation of optical property of tungsten-doped zinc oxide films deposited by sputtering*

Tungsten-doped zinc oxide (WZO) was deposited on quartz glass substrates by sputtering with tunable doping of tungsten for application as a phosphor material. The crystallinity and photoluminescence (PL) property of the films were characterized by X-ray diffraction (XRD) and PL measurement. The XRD patterns show that ZnWO₄ and ZnO exist at low doping concentration, and the component of the WZO phase is changed to mixture of ZnWO₄ and WO₃ at high doping concentration. The WZO films show photoluminescence by ultraviolet light, resulting in a broad emission which is useful for white light emission. The highest internal quantum efficiency (QE) in our condition is 76% when 46 at.% of tungsten is doped and the QE are different between excitation side, which suggests the segregation of materials in the film thickness direction. The decreases of QE at low and high concentrations with a boundary of 46 at.% indicate absorption of excitation light by ZnO and WO₃, that inhibits excitation of ZnWO₄. These analyses suggest that QE is determined by segregation of biproduct in the films. Therefore, the suppression of formation of biproduct is considered essential to obtain strong luminescence from WZO. Our investigation provides importance of the doping control in WZO for white-LED applications.