Ultraviolet Photoresponse Properties of Li-Doped ZnO Thin Films Prepared by Sol–Gel Spin-Coating Method

In this study, we investigated the influence of Li doping on the optical, structural, and ultraviolet photoresponse properties of ZnO thin films prepared using the sol–gel spin-coating method on Si substrates. Photoluminescence analysis showed that a visible emission peak related to O vacancy increased with increasing Li doping concentration, resulting in an increase in the formation of substitutional Li at the Zn site. In X-ray diffraction, the intensity of the ZnO (002) peak increased with increasing Li doping concentration, which indicates an improved c-axis orientation. The full width at half maximum (FWHM) of the ZnO (002) diffraction peaks was the lowest at 2.5 at.% Li content, which indicates the highest crystal quality. For measuring UV photoresponse, metal-semiconductor-metal UV sensors were fabricated through the deposition of indium onto Li-doped ZnO (LZO) thin films using a thermal evaporator. The UV sensors based on LZO thin films exhibited enhanced UV sensitivity as well as faster rise and decay times compared with those based on ZnO thin films. The responsivity of the UV sensor based on LZO thin film with 2.5 at.% Li content exponentially increased as the bias voltage increased from 1 to 5 V.