Investigation of dual-wavelength selective self-powered photo response of ZnO/Si heterojunction with insertion of thin TiO2 layer

Abstract

In the current work, ZnO film (~ 143 nm) is deposited on the CMOS compatible Si substrate by using RF sputtering technique to form a p-n type heterojunction device and further, thin buffer layer of TiO₂ (~ 52 nm) is inserted in between ZnO and Si to reduce the interfacial defects remarkably in ZnO/TiO₂/Si bilayer heterojunction for UV/Visible sensing applications. Smooth interface, and highly crystallite qualities of TiO₂-brookite and ZnO-hexagonal wurtzite phases are observed from atomic force microscopy (AFM) and Field-emission gun scanning electron microscopy (FEG-SEM) and X-ray diffraction (XRD), respectively. X-ray photoelectron spectroscopy (XPS) clarifies the chemical states of elements, which indicate a significant amount of the reduction in oxygen vacancies after insertion of the TiO₂ thin layer. An enhanced absorption in UV region and slight red shift in emission are observed in UV-Visible spectroscopy and Photoluminescence (PL) plots for TiO₂/ZnO bilayer heterojunction, and oxygen related complex defects are reduced due to TiO₂ insertion. Finally, Current-voltage (I-V) measurements demonstrate the dark current reduction in ZnO/Si heterojunction with TiO₂ incorporation and such heterojunction exhibits photovoltaic behavior under specific wavelengths (380 nm and 420 nm). A superior responsivity (0.17 A/W), detectivity (7.80 × 10¹³ Jones) with rapid response speed (< 1 s) at zero bias are achieved in the UV-A/Visible wavelength range for ZnO/TiO₂/Si heterojunctions. The corresponding photo-carrier generation and transport mechanism are analyzed in detail by using corresponding energy band diagram. Therefore, all the results indicate that such TiO₂ thin layer inserted ZnO/Si heterojunction may provide a cost-effective feasible design strategy for the development of interfacial defect less, self-powered wavelength selective UV/Visible detectors in recent future.