Effects of microstructure and oxidation state of multi-valent vanadium oxide thin films for use in infrared microbolometers

Uncooled, resistive bolometry has been a widely used method to detect infrared radiation for several years. The thermistor in the material heats up upon absorbing infrared radiation. The resistivity (rho) of this material is lowered by the heat and is detected by a readout circuitry. Improvement of device performance may arise from improving the temperature coefficient of resistivity (TCR). For several years, vanadium oxide (VO x) thin films have been used as the thermally active material in these bolometer devices due to the somewhat controllable rho and high TCR nature.;Prior research has shown that often VOx thin films with more desirable electrical properties exhibit a nanocomposite structure consisting of highly defective nanocrystalline domains of face-centered cubic VO x (0.8 ≤ x ≤ 1.3) phase embedded within an amorphous matrix of VOx with x > 2. Attempts were made to fabricate reference materials of each of these constituents of the nanocomposite by means of reactive pulsed-DC magnetron sputtering in order to obtain a reference database of the electrical properties and the optical properties obtained via spectroscopic ellipsometry (SE). In doing so, it has been discovered that many deposition parameters such as oxygen flow rates, substrate temperature, substrate bias, and the substrate surface, itself, affect the resulting VOx thin film growth and nucleation. The strong substrate dependence of these films dictates the crystallinity, overall phase, and structural evolution of the VOx thin films. VOx thin films grown on single crystal Al2O 3 have exhibited higher degrees of crystallinity and predominately V 2O3 like in structure, which lead to undesirably low TCR magnitudes (0.09 -- 0.59 --%/K). VOx thin films grown on single crystal MgO have been shown to have different results than that of the VOx on Al2O3 as the TCR is much higher, and thus more desirable (2.23- 3.59 --%/K). These films on MgO were highly disordered and had the highest values of TCR for given resistivity known to the author to date. VOx thin films grown on LPCVD SiNx coated c-Si wafers exhibited intermediate effects of mixed V2O3 and FCC-VOx nanocrystallites with TCR magnitudes ranging from 0.59 to 1.56 --%/K. The difference between temperature and substrate bias effects on these phenomena was also evaluated.;VOx growth and evolution was monitored in situ via real time spectroscopic ellipsometry (RTSE) for unbiased films grown on unheated dielectric coated c-Si wafers. By modeling the data obtained from RTSE, it has been shown that morphology of the as-deposited VOx thin film and post deposition conditions, such as post-sputtering and atmospheric exposure, lead to an overall phase reconstruction of nanocomposite VO x. Amorphous VOx thin films exhibit more stability with respect to post-deposition exposure to process gases and atmosphere than the nanocomposite VOx thin films. This behavior implies that the nanocrystallites or grain boundary components in unbiased VOx thin films are more susceptible to these types of changes.;Post-deposition annealing in previous studies has been shown to be a potential method to improve the electrical properties of VOx thin films deposited without a substrate bias. The post-deposition annealing in this work was conducted with a different experimental setup on VOx thin films deposited with a substrate bias. The annealing process was monitored via RTSE in order to non-destructively observe the optical properties in the form of the dielectric function spectra, epsilon = epsilon1 + iepsilon 2, vary as a function of temperature. By annealing the as-deposited VOx thin films in different ambients, more favorable TCR values for a given resistivity were observed.;By obtaining epsilon for various phases of VOx, effective medium) theory has been applied in order to try to obtain initial models of nanocrystalline volume fractions within nanocomposite VOx thin films. These models are preliminary in that a specific set of epsilon for nanocrystalline and amorphous VOx was implemented to represent the "end points" components. Each of these constituents varies a great deal with oxygen incorporation and is reflected in epsilon. Despite this, initial representative models of nanocrystalline content have been obtained.