Stoichiometry and microstructural effects on electrical conduction in pulsed dc sputtered vanadium oxide thin films

Abstract

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 × 10−3 to 2.98 × 104 Ω·cm. 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.

Metal-insulator transition in nanocomposite VOx films formed by anodic electrodeposition

“…This performance compares favorably with that of VO 2 films prepared by vapor deposition methods. [10][11][12] The gradual change in resistivity is in contrast with the sharp MIT phenomena observed in sputtered, laser deposited, or PVD films 12 and is consistent with a multi-phasic film with high defect density, as suggested by structural data (Fig. 2).…”

“…7 Amorphous VO 2 has also found application in bolometer devices for infrared detectors, 8 where a high temperature coefficient of resistance (TCR) 9 facilitates temperature determination from the measured resistance; values as high as À2%/ C have been obtained by physical vapor deposition (PVD) methods for amorphous VO x (x $ ¼ 1.8À2.0) films. [10][11][12] In this case, a gradual change in resistance, allowing operation over a large temperature range, is required.…”

Thermochromic undoped and Mg-doped VO2 thin films and nanoparticles: Optical properties and performance limits for energy efficient windows

“…[42][43][44][45][46] (ii) The film thickness was important, and a large value of d promoted the formation of surface nanostructures, as also seen elsewhere. 47,48 More generally, the surface of a VO 2 film may be different from its main part 49 and the same may be true for the substrate/film interface.…”

Microstructural evolution of thin film vanadium oxide prepared by pulsed-direct current magnetron sputtering

“…All films studied in this work exhibited a linear behavior in ln q as a function of temperature in this range, indicating that these materials do not undergo a metal-to-semiconductor transition at 68 C as expected for crystalline vanadium dioxide. [4][5][6][7] The near-room temperature (25 C) TCR is presented in Figure 1. The 2901 Å thick VO x film studied using RTSE was measured using a four-point probe and found to have q ¼ 0:02260:001 X cm.…”

“…4 The nanocrystalline VO x films in this study were fabricated via pulsed-DC reactive magnetron sputtering, and exhibit the face centered cubic rock-salt vanadium monoxide phase as has been previously determined by transmission electron microscopy and grazing incidence x-ray diffraction. 5 Films of differing thicknesses were produced to test if the electrical properties and microstructure evolve during growth. q of the VO x films were measured either by use of nickel coplanar contacts or via a four-point probe, and TCR is obtained from the temperature dependence of q. Spectroscopic ellipsometry (SE) has been applied to characterize the optical properties, in the form of the complex dielectric function spectra (e ¼ e 1 þ i e 2 ), over the spectral range from 0.75 to 5.15 eV.…”