Reduction of interface state density in coaxial TiO2/Ge nanowire assembly-based heterostructure and superior photodetection

Abstract

This paper compares the advantages of a catalyst-free vertically aligned coaxial titanium dioxide nanowire/germanium nanowire (TiO2-NW/Ge-NW) heterostructure compared to basic germanium nanowire (Ge-NW) on silicon (Si) substrate. Both were fabricated using the glancing angle deposition (GLAD) technique integrated into an electron beam evaporation system. The study is focussed on the successful suppression of native germanium oxide (GeO2) as well as the reduction of interface state density (Dit) by incorporating TiO2-NW. X-ray diffraction (XRD) patterns confirm the successful growth of rutile phase TiO2-NW in the coaxial TiO2-NW/Ge-NW assembly at room temperature. The crystal size analysis was carried out using the Scherrer equation and it confirmed the decrease in crystal size of GeO2 in the coaxial TiO2-NW/Ge-NW assembly. FEGSEM reveals the growth of vertically oriented NWs. Capacitance and conduction voltage data were used to extract the interface state density (Dit). The presence of TiO2 reduced the interface defect in the TiO2-NW/Ge-NW heterostructure compared to the bare Ge-NW sample. The optical absorption study showed that TiO2-NW/Ge-NW samples exhibited enhancement in the UV region primarily due to the presence of TiO2. The electrical characterization of Ag/TiO2-NW/Ge-NW/Si and Ag/Ge-NW/Si revealed a low dark current of −1.056 × 10−7 A/cm2 for Ag/TiO2-NW/Ge-NW/Si compared to −8.39 × 10−7 A/cm2 for Ag/Ge-NW/Si. Ag/TiO2-NW/Ge-NW/Si exhibited a superior rectification ratio, increased detectivity, and enhanced noise equivalent power compared to the Ag/Ge-NW/Si device. The TiO2-NW/Ge-NW heterostructure exhibited a fast temporal response with rise and fall times of ∼0.047 s and ∼0.037 s, respectively.