Tunable p- and n-type Nb:TiO 2 and performance optimizing of self-powered Nb:TiO 2 /CdS photodetectors

For the first time, Niobium-doped titanium dioxide (Nb:TiO2, NTO) and cadmium sulfide (CdS) are selected for building self-powered photodetectors for optimizing the performance. The p- or n-type NTO and the band structures of the heterojunctions can be controlled by doping Nb and tuning the sputtering power. A low sputtering power tends to produce pure anatase crystals and a high power induces additional rutile phase aligned in the (200) orientation. NTO films deposited at a sputtering power of 120 W show p-type behavior attributed to the compensation of oxygen vacancies. NTO films deposited at 180 W contain mixed anatase and rutile phases with lattice imperfections and show n-type semiconductor property after annealing. By measuring the energy band structure of n-n-type anatase-TiO2/CdS, p-n-type anatase-NTO/CdS, and n-n-type mix-NTO/CdS heterojunctions, we identify the interface carrier motion characteristics and tune the energy band structure to optimize the performance of the photodetectors. Without any external power supply, the responsivities reaches at least 0.125A/W in the mix-NTO/CdS (light source: 550 nm) and the response speed is lower than 10ms, which can be used for building a self-powered photodetector.