Visible light photoresistance and response time of samarium doped zinc oxide thin films grown by pulsed laser deposition

Samarium doped zinc oxide (Sm:ZnO) thin films were fabricated on c-oriented sapphire substrates by pulsed-laser deposition with Sm dopant from 0 to 10 at.%. X-ray diffraction patterns, X-ray fluorescence mapping, and X-ray absorption near-edge structure spectra indicated uniform incorporation of Sm into ZnO lattice without secondary phase. With increasing Sm concentration, zinc vacancy or oxygen interstitial is formed to maintain electrical neutrality, which are identified by photoluminescence spectroscopy. The increasing defect concentration and decreasing exciton binding energy enhanced the photoresistance response in visible regime, where the absolute value of normalized resistance response was between 2 % and 8.79 % by 1 mW blue laser (wavelength of 450 nm) illumination for Sm:ZnO thin films. Time-dependent photoresistance measurements revealed two response times. The shorter response time, around 1.2–26.7 s, is contributed by the free electron-hole pair generation/recombination and the response of carriers trapped in deep level defects. The longer response time, around 76–858 s, is contributed by the captured oxygen on the thin film surface. Overall, the 5 at.% Sm:ZnO thin film is the best for photodetection applications due to its high photoresistance response and short response time.