Structural and low temperature electrical transport properties of Mo-doped vanadium oxide NTC ceramic thin films

Highlights

• Mo-doped V₂O₃ single layer films with various Mo concentrations have been prepared.

• Enhanced NTC property of films at below 273.15 K was studied.

• Joule heat induced metal-insulator formation Mo-V₂O₃ system was observed.

Abstract

Mo doped V₂O₃ [V_1−xMo_xO_2−x/2 (x = 0, 0.5–1)] ceramic thin films were prepared on metal substrates by sol-gel dip coating and the influence of Mo addition on their microstructure, negative temperature coefficient (NTC) electrical transport properties and metal to insulator phase transition behavior were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscopy (HR-TEM), and resistance-temperature measurements. Resistivity-temperature curves (over a temperature range of 273.15–253.15 K) indicated that all of the prepared thin films have NTC effects, after annealing with 20 sccm N₂ at 673.15 K. It was demonstrated through microstructure analysis at Mo high concentration, (i.e., x > 0.07) it segregates at the V₂O₃ grain boundaries, causing scattering and distortion of the crystal lattice. Compared with the other V₂O₃ films, the films prepared at Mo x > 0.07 offered the high resistivity and moderate thermal constant (B) values. In particular, V₂O₃ doped with 10 mol % Mo showed excellent NTC properties and high resistivity (0.072 Ω cm). At sub-zero temperatures, the variation of electrical transport properties of the V₂O₃ films is correlated with Mo concentration, micro-structure and Joule effect.

Introduction

There is currently great interest in identifying new oxide thin film ceramic materials with enhanced negative temperature coefficient (NTC) of resistivity. These ceramic materials can be applied in low temperature sensors and thermal imaging tools in military and medical fields, where precise temperature measurement and control are necessary [1], [2]. However, it is essential to develop highly efficient NTC ceramic oxide materials with better performance characteristics than those of the well-known multi-component NTC ceramic oxides (i.e., spinel oxides, skutterudites, half-Hesular alloys, clathrates, and pentatellurides) [3], [4], [5], [6], [7], particularly for low temperature sensor applications.

Recent studies have been focused on tuning single component (i.e., preferably with one dopant) NTC ceramic oxide materials [8], [9], [10], [11], [12], [13], doping the NTC materials with transition metals, and optimizing device design for low temperature operation, because it is difficult to control the porosity and stoichiometry of multi-component NTC ceramics. Single component NTC oxide ceramic thin films offer many advantages over multi-component NTC oxide materials; high sensitivity to temperature changes at the sub-zero level, high signal to noise ratio, simple operation, and low cost. Moreover, single component NTC oxide ceramics are reliable and last long.

Several classes of single component oxide ceramics including silicon [14], silicon-germanium alloys [15], and vanadium oxide compounds [8], [16], [17] are currently under investigation. Among these, special attention has been given to vanadium compounds because of their high NTC of resistance, extreme ruggedness, and the ease with which their electrical and thermal properties can be modified. Vanadium oxide NTC ceramics have efficient electrical/thermal properties and can be used in sensing elements on any type of substrate, semiconductor, or other ceramic material with considerable ease. Hence, vanadium oxide compounds can be used in the development of low temperature sensor devices. Furthermore, the NTC electrical property of vanadium oxide ceramics can be tailored by various factors (e.g., dopant concentration, annealing process) for low temperature sensing applications.

Vanadium oxide compounds undergo metal-insulator transitions accompanied by structural changes at sub-zero temperatures [18], [19]. The transition temperature must be modified to satisfy the specific temperature requirements of the device. Dopants act as extrinsic impurity that allow for precise control over charge carrier densities. To this end, it was reported that doping and dopant concentration can greatly influence the transition temperatures of vanadium oxides [20], [21]. High impurity dopant concentrations are key to reduce the potential of ceramics for a variety of applications. In addition, the transition temperature and corresponding NTC characteristics of vanadium based ceramics depend on the crystal and micro-structures of these oxides. The crystal and micro-structures of vanadium oxide thin film ceramics can be tuned for low temperature applications by adding suitable dopants and/or varying the processing conditions (e.g., ambient gas flow rates and annealing temperatures) [22], [23]. Therefore, process conditions for the fabrication of vanadium oxide compound ceramic thin films with the desired electrical properties for low temperature applications should be carefully chosen.

In the present work, we have shown that the resistivity of vanadium oxide ceramic thin films can be increased by introducing highly concentrated Mo-dopant as an insulating material into the vanadium oxides. To this end, we fabricated single layer pure vanadium oxide (V₂O₃) and molybdenum doped vanadium oxide (V_1−xMo_xO_2−x/2) thin films on metal substrates using a sol-gel dip coating method. A series of measurements were conducted over a temperature range of 253.15–273.15 K to monitor the electrical transport properties of the oxide films as a function of Mo concentration. This study describes a new avenue to create thin ceramic NTC Mo/vanadium oxide materials for low temperature applications.