Vanadium oxide thin films with high midwave & longwave infrared thermo-optic coefficients and high temperature coefficients of resistance

Abstract

The development of thermo-optic modulator elements for midwave infrared (MWIR) and longwave infrared (LWIR) electro-optic systems, and the development of resistive thermometer elements for infrared imaging microbolometers require materials with high thermo-optic coefficients (TOC) and high temperature coefficients of resistance (TCR), respectively. In this paper, we synthesize novel vanadium oxide (VxOy) thin film structures and measure their MWIR/LWIR thermo-optic coefficients (TOCs) and their temperature coefficients of resistance (TCRs). The VxOy thin film are synthesized by sputter depositing interchanging, 5 nm-thick, layers of vanadium sesquioxide (V2O3) and vanadium (V) reaching a final thin film thickness of 95 nm. The sputter deposited multilayer structures are then ex-situ annealed in N2 and O2 atmospheres at 300 °C for 30 min. Infrared spectroscopic ellipsometry was used to measure the optical constants of the thin films as a function of temperature across the MWIR and LWIR bands (3000–14000 nm). The synthesized VxOy thin films exhibited high TOCs and high TCRs when operated in their semiconducting phase. A high TOC was measured reaching a maximum of 0.0278 °C−1 and 0.119 °C−1 at λ = 4000 nm for N2 and O2 annealed VxOy thin films respectively; and reaching a maximum of 0.0634 °C−1 and 0.139 °C−1 at λ = 10,000 nm for N2 and O2 annealed VxOy thin films respectively. Moreover, sheet resistance versus temperature measurements were conducted revealing room temperature sheet resistances of 1.495 k Ω/sq. and 1.516 k Ω/sq. and TCRs of −3.54%/°C and −3.46%/°C for N2 and O2 annealed VxOy thin films respectively.