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

Highlights: • Interface state density (D_it) of TiO₂-NW/Ge-NW/Si and Ge-NW/Si are investigated. • Suppression of native unstable GeO_x by incorporating TiO₂-NW is achieved. • Enhanced optical absorption is observed in TiO₂-NW/Ge-NW/Si compared to Ge-NW/Si. • Ag/TiO₂-NW/Ge-NW/Si device has lower leakage current compared to Ag/Ge-NW/Si device. • Ag/TiO₂-NW/Ge-NW/Si device exhibited superior detectivity, noise equivalent power and fast switching response. -- 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 (D_it) 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 (D_it). 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.