Trapped Exciton-Enhanced Response of n-TiO₂(110)/p-Si(111) Nanostructures as Photodetectors

A p-type Si(111)/n-TiO2(110) heterojunction photodetector device has been fabricated for broad spectral range detection. An environmentally friendly NaCl assisted hydrothermal route was employed for synthesizing an n-type TiO2(110) hierarchical nanostructure powder. A 1 μm thick film of TiO2 was then deposited by spin coating on the top of p-type Si(111). The role of NaCl in tuning both anionic (oxygen vacancy) and cationic (Ti mono- and divacancy, Ti interstitials) defects has been investigated. The vacancies posed a significant effect on photoresponse and junction characteristics of the device, viz., dark current, barrier height, photosensitivity, detectivity, and photoresponse gain. The TiO2 n-type layer fabricated at higher NaCl concentrations has been found to exhibit maximum responsivity of ∼550 A/W, gain of ∼2.00, specific detectivity of ∼1.27 × 1011 Jones, and fast response and recovery times of 38 and 43 ms, respectively. The device has been highly repeatable (for 12 cycles) with a stability of 60 days. These have been ascribed to the exciton, formed at the bridged oxygen adjacent to the Ti vacancy at the (110) surface. We have also resolved that the charge carrier–phonon interaction facilitates exciton stabilization, while the charge transfer across the interface follows the adiabatic process. To explain the variation of device performance with defects, a mathematical model has been proposed to correlate the device responsivity to the different vacancies.