Stoichiometry and microstructural effects on electrical conduction in pulsed dc sputtered vanadium oxide thin films
- PDF / 1,361,373 Bytes
- 10 Pages / 584.957 x 782.986 pts Page_size
- 106 Downloads / 213 Views
Mark W. Horn Department of Engineering Sciences and Mechanics, Pennsylvania State University, University Park, Pennsylvania 16802 (Received 17 October 2008; accepted 13 January 2009)
Vanadium oxide thin films were deposited using pulsed direct current (dc) magnetron sputtering in an atmosphere containing argon and oxygen. The total pressure was varied from 2.5 to 15 mTorr, and the oxygen-to-argon ratio was varied from 2.5 to 30%. The resulting films were characterized using Rutherford backscattering spectroscopy (RBS), transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), and glancing incidence x-ray diffraction (GIXRD). Electrical resistivity was calculated from I–V curves acquired from two-point-probe measurements and thicknesses measured from bright-field TEM images of cross-sectioned samples. TEM and GIXRD were used to characterize the crystallinity of each film. A transition from nanocrystalline to amorphous growth was observed with increasing partial pressure of oxygen. In all samples, the only crystalline phase observed was cubic vanadium oxide with the sodium chloride structure. Though the cubic VOx equilibrium phase field is limited to a maximum of x = 1.3, the cubic phase was observed with a value of x up to 2 in the present work. It was apparent from electron diffraction data that increased oxygen content correlated with an increase in the film disorder. The increase in oxygen content also corresponded with an increase in the film resistivity, which varied over 7 orders of magnitude from 1.18 103 to 2.98 104 Ocm. The temperature coefficient of resistance was found to increase with increasing oxygen content from 0.1 to 3.5%/ C. A direct correlation between film disorder and temperature coefficient of resistivity (TCR) was observed and could be exploited to engineer materials with the desired TCR.
I. INTRODUCTION
Resistive bolometers are used to detect infrared radiation through the absorption of incident energy that excites phonons.1 The resulting increase in the temperature of the material is sufficient to cause a change in the resistivity, the magnitude of which is characterized by the temperature coefficient of resistivity (TCR). The resistance change is detected as a change in the current of a biased device. Typically, these materials are applied to free-standing, thermally isolated pixels arranged in arrays,1 which do not require the continuous cooling necessary in photonic detectors.2 Vanadium oxide (VOx) thin films are of particular interest for use in these devices due to their large negative TCRs.3 TCR values of large magnitude increase the minimum detectable temperature difference in the scene being imaged. a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0183
The bulk phase diagram in the vanadium–oxygen system indicates multiple crystalline phases depending on stoichiometry.4 Many of the crystalline vanadium oxide phases have sharp metal-to-insulator transitions (MITs) in which an abrupt change in conductivity
Data Loading...
