The oxygen deficiency effect of VO 2 thin films prepared by laser ablation
- PDF / 241,087 Bytes
- 7 Pages / 612 x 792 pts (letter) Page_size
- 98 Downloads / 193 Views
MATERIALS RESEARCH
Welcome
Comments
Help
The oxygen deficiency effect of VO2 thin films prepared by laser ablation M. Nagashima and H. Wada Second Research Center, TRDI, Japan Defense Agency, 1-2-24 Ikejiri, Setagaya, Tokyo, Japan (Received 30 May 1996; accepted 15 July 1996)
Vanadium dioxide thin films (VO2 ) have been deposited by laser ablation. The temperature dependence of resistivity and temperature coefficient of resistance (TCR) for each deposition condition were investigated. It was clarified that the TCR at room temperature (RT) can be optimized by controlling the oxygen pressure introduced during deposition as the deposition parameter. In the result, larger TCR’s at RT were observed for the oxygen deficient condition of VO2 than for oxygen-richer samples. Obtained TCR values were 0.072yK and 0.045yK at 25 ±C for VO2 thin films deposited onto R-cut sapphire and SiO2ySi, respectively.
I. INTRODUCTION
Vanadium oxide has a complicated crystal system and consists of various compositions of vanadium and oxygen.1 Most of these oxides undergo semiconductorto-metal transition with an abrupt change of resistivity, optical transmittance, and reflectance in the infrared region. Vanadium dioxide (VO2 ) is a most attractive material since its transition temperature is at nearly room temperature (RT), 68 ±C, and shows a large change of resistivity and optical transmission. For this reason it is a candidate for electrical and optical switching applications. Many thin film deposition methods such as reactive sputtering,2–4,7–10 electron beam evaporation,2,5,11 chemical vapor deposition,12 sol-gel process,6,12 and laser ablation (LA)14,15 have been employed to obtain VO2 thin films with wide magnitude of transition and narrow transition width. It has been found in these studies that one of the crucial conditions to grow VO2 thin films having such qualities is to improve the crystallinity of films and to optimize their stoichiometry. Another application of vanadium dioxide is in infrared bolometric sensors. This type of sensor utilizes the resistivity change of the material sensitive to its temperature change by infrared radiation. In this application, the value of the temperature coefficient of resistance (TCR), defined as a slope of log resistivity, for the sensor material is an important factor to achieve high sensitivity. VO2 has a large activation energy up to 0.5 eV in the semiconductor state under transition temperature Tc .17 This large activation energy is due to the decrease effect in the band gap by temperature increase, observed by its optical transmission measurements.13 Since this value is proportional to TCR, VO2 has a large TCR near RT, an appropriate material for a bolometer operating at RT. However, VO2 thin films generally prepared so far exhibit less activation energy and TCR than that 416
J. Mater. Res., Vol. 12, No. 2, Feb 1997
of a single crystal.2,7,18 For obtaining VO2 films with higher activation energy, it is important to improve their stoichiometry, because the nonstoichiometric VO2 fil
Data Loading...
