Influence of Annealing on Thin Film/Substrate Interface and Vacuum Ultraviolet Photoconductivity of Neodymium Fluoride Thin Films

Abstract

High photon energy vacuum ultraviolet radiation (VUV, 100−200 nm wavelength) is challenging to detect. It easily degrades conventional silicon and semiconductor photodetectors. Fluoride photodetectors can be the answer, but the correlation between fabrication parameters and photodetector performance is not known. Here, the effect of annealing is investigated on the characteristics of neodymium trifluoride thin film/quartz substrate interface and NdF3 photoconductivity within the VUV. Thin films are deposited on unheated and heated (600 °C) substrates with post-deposition annealing. Dark current of films on unheated substrates decreases by as much as 1/10 as resistance increases from 1 −12 TΩ after annealing. Dark current of films on heated substrates increases even after annealing, resulting in similar photo and dark currents of ≈303.7 nA and poor detectors. Fluorine diffuses from the film to the substrate during deposition, exacerbated by substrate heating but not by annealing. Fluorine diffusion degrades crystallinity near the interface, increasing the dark current. Fluorine diffusion is absent when MgF2 is used as the heated substrate. Unannealed NdF3/MgF2 detector on 600 °C-heated substrate and 600 °C-annealed NdF3/SiO2 detector on unheated substrate exhibit similar resistances of ≈14 TΩ. Considering the film/substrate interface and annealing is crucial when developing VUV photodetectors.