Fabrication and characterization of cerium oxide thin films for ultraviolet sensing applications

Abstract

Cerium oxides are of interest in many applications including optical coatings, solid-state fuel cells, and catalyst supports. Due to excellent absorption in ultraviolet (UV), these materials are also widely used as UV blocking layers in medical glassware and aerospace windows. In this paper, we present the fabrication and characterization of cerium oxide thin films and their potential application in UV sensing. Cerium oxides were deposited by reactive oxygen ion beam assisted e-beam evaporation. Comparing with the films obtained without ion assistance, oxygen plasma assisted deposition enhances refractive index from ~ 1.8 to ~ 2.2 and improves electrical resistivity from ~ 10⁶ Ωcm to ~ 10¹⁰ Ωcm. More importantly, the film deposited with ion assistance shows stronger blue to UV absorption observed from the transmittance spectra, which presumably is better for UV sensing. X-ray photoelectron spectroscopy and X-ray diffraction measurements further suggest insights in stoichiometry and crystal structures. A concept-proving photodiode was fabricated by employing a p⁺-Si/CeO₂/n-In₂O₃ heterostructure. The current-voltage characteristics exhibit obvious diode-like behavior with a current rectification ratio of ~ 10⁵. A low leakage current in a range of 10^-10 ~ 10^-8 A was achieved at reverse biases of 0 to 10 V, respectively. The diode demonstrates high photocurrent gain of ~ 100 times and fast photoresponse under a 405 nm UV exposure.