Study of thermo-resistive properties of metal oxides thin films for thermal sensors

Abstract

To date, a number of thermal sensing materials have been used to develop thermal sensors, including metals and semiconductors. Among this the latter, vanadium oxide and various thicknesses of zinc oxide are commonly used due to their excellent refractory properties including a high thermal resistance coefficient (TCR). In this context, we propose a study based on TCRs measurement of vanadium oxide and zinc oxide thin films, so to determine the optimal experimental conditions for the fabrication of these layers. The structural analysis using X-Ray diffraction (XRD) and Raman spectroscopy shows that vanadium oxide thin films contain V₂O₅ as a main phase with a secondary phase of V 6 O 13 . While the electrical measurements evidences the presence of a VO₂ phase. XRD shows also that ZnO thin films are of high quality and the crystallinity is thickness dependent. The electrical measurements of films resistance by 04-point method as a function of temperature shows that TCR of vanadium oxide thin films depends on the bias current, contrary to ZnO. At room temperature (27°C), the TCR of Vanadium oxide thin films is 2.7 %/°C and 7% /°C, for respectively, 1 µA and 10 µA bias currents and relatively large resistivity of 35 and 367 Ωcm. The TCR is high up to 30%/°C at 47°C corresponding to the metal-insulator transition temperature of VO2 phase. On the other side, ZnO thin films shows a very little TCR of 0.06%/°C for 1200 nm thickness and near Zero TCR for 500 and 100 nm thicknesses with a resistivity in the range of 10-2 to 10-4 Ωcm. Further, no hysteresis loop is observed in ZnO thin films. In vanadium oxide thin films, the hysteresis loop is controlled by a threshold temperature that corroborates the initiation of the transition in VO₂ . Finally and according to our results, vanadium oxide nanocomposite active layers are promising candidates for highly sensitive thermal sensors, whereas ZnO is of a great interest to design piezoresistive thin-film sensors.