Pastrana-González, Juan J.

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    Fabrication of VO2 (B) thin films by pulsed DC reactive magnetron sputtering and characterization at low and high temperatures
    (2018) Pastrana-González, Juan J.; Fernández, Félix E.; College of Arts and Sciences – Sciences; Lu, Jun-Qiang; Marrero, Pablo J.; Department of Physics; Valentín, Ricky
    For this work, VO2 (B) thin films were fabricated using pulsed DC magnetron reactive sputtering. The technique belongs to the ionic sputtering methods which allows high deposition rates, high-purity films, excellent uniformity and high adhesion of films. Reactive sputtering suffers from one drawback called target ´´poisoning´´, which causes a hysteretic control problem and results in instabilities in the fabrication process. For the purpose of having a better control in the film´s fabrication, measurements of cathode voltage in the sputtering gun as a function of oxygen flow were performed at different gun power settings. Based on the results, the parameters which presented the best conditions for film growth were as follows: Argon working pressure of 5 - 6.2 mTorr, argon flow of 100 standard cubic centimeters per minute (sccm), 11.5 - 12.5 sccm oxygen flow and gun power of 200 Watts. Thin films were deposited over fused silica glass (SiO2) at 350 °C and 375 °C. X-ray characterization showed VO2 (B) films grown in this manner present strong preferential crystallization orientation with (00l) planes parallel to the substrate surface. In addition, peaks are shifted to higher angles, evidencing that the films are in tensile stress. Electrical measurements were done through heating-cooling cycles from 400 to 120 K. Resistivity measurements showed a drop of 6 orders of magnitude while cooling from 300 to 120 K. Graphing conductivity as a function of temperature showed no hysteresis, in agreement with recent reports. The linear behaviour was described by an Arrhenius plot in which activation energy is 137 meV. A Temperature Coefficient of Resistance (TCR) of -3.6% and -1.7% K-1 for cooling and heating cycles was calculated, respectively. Also, a detailed analysis for the heating cycle (300 - 400 K) of the samples was done. High temperature resistivity measurements for VO2 (B) films prepared at 350 °C showed a change of slope at ~68 °C indicating the admixture of VO2 (M1). On the other hand, samples prepared at 375 °C showed a change of slope at ~79 °C, representing a small concentration of VO2 (M1) with higher oxidation composition. Finally, two samples were analysed by atomic force microscopy (AFM). Surface roughness (RMS) values obtained were 10 and 13 nm respectively. The average lateral grain size was calculated using Heyn’s technique, obtaining values of 121 nm for one sample and 158 nm for the other. The grain size results in numerous imperfections, having an impact on the electrical properties of VO2 (B): it is possible that larger grain sizes will result in higher conductivity samples. This study may provide the possibility to fabricate VO2 (B) thin films that are relevant for both, infrared sensing applications and lithium-ion batteries cathode material.