Modelo de histerese para transição semicondutor-metal em filmes finos de VO2.

Abstract

Microbolometers, operating at room temperature and employing vanadium-dioxide (V02) thin film as the heat sensing material, have emerged as a promising solution for far infrared imaging, providing a wide range of military, industrial and commercial applications. The V 0 2 thin film is characterized by a solid-state phase transformation from semiconducting type at low temperatures into metallic type at higher temperatures. Recent fabrication techniques permit the deposition of vanadium dioxide films, having a temperature coefficient of resistance (TCR = ;|gp) of 3%C_ 1 in the semiconducting region that increases to values as large as 60%-C- 1 in the hystcretic transition region. Despite this pronounced increase of the TCR, V02 - t h i n film microbolometers are commonly operated outside the hysterctic region, in the semiconducting part of its resistance-temperature (R x T) curve. Experimental results, indicating a considerable increase in the sensitivity, have been recently reported for a V 0 2 - t h i n film microbolometer operating in the hystcretic region. The theoretical prediction of bolometric performance in the transition region is quite complex, due to the hysteresis in the RxT characteristics. Thus, the analysis, design, and optimization of a V 0 2 microbolometer, operating in the hystcretic region, is critically dependent on the availability of a hysteresis model that can mathematically describe both the major and minor loops. This thesis is focused on the development of a hysteresis model which can be employed as an analysis tool in the design of V 0 2 microbolometer and other related applications of this material. For this purpose, it is first proposed a new algebraic model to describe magnetic hysteresis, which I call Limiting Loop Proximity (L2P) model. With only four parameters, it has low computational cost and reduced mathematical complexity, thus permitting a fast numerical implementation and simple parameter estimation procedure. Considering the V 0 2 thin film as a composite medium, containing semiconducting and metallic microcrystals, the wellknown effective-medium approximation theory is employed to relate the volume fraction of the semiconducting microcrystals to the effective film resistance. The L2P model is first proposed for describing the hystcretic dependence of volume fraction on temperature. From this, a model for hysteresis in the resistance-temperature characteristic is then derived, and a procedure for estimating the model parameters is outlined. The model reproduces the more important hysteretic characteristics such as the major, minor, and nested loops, in good agreement with the experimental characteristics.