Bryan D. Gauntt

Electrical and optical properties of sputtered amorphous vanadium oxide thin films

Amorphous vanadium oxide (VOx) is a component found in composite nanocrystalline VOx thin films. These types of composite films are used as thermistors in pulsed biased uncooled infrared imaging devices when containing face centered cubic vanadium monoxide phase crystallites, and substantial fractions of amorphous material in the composite are necessary to optimize device electrical properties. Similarly, optoelectronic devices exploiting the metal-to-semiconductor transition contain the room-temperature monoclinic or high-temperature (>68 °C) rutile vanadium dioxide phase. Thin films of VOx exhibiting the metal-to-semiconductor transition are typically polycrystalline or nanocrystalline, implying that significant amounts of disordered, amorphous material is present at grain boundaries or surrounding the crystallites and can impact the overall optical or electronic properties of the film. The performance of thin film material for either application depends on both the nature of the crystalline and amorpho...

Comparison of ion beam and magnetron sputtered vanadium oxide thin films for uncooled IR imaging

Uncooled Infrared (IR) focal plane arrays are an enabling technology for both military and commercial high sensitivity night vision cameras. IR imaging is accomplished using MEMS microbolometers fabricated on read-out integrated circuits and depends critically on the material used to absorb the incoming IR radiation. Suitable detector materials must exhibit a large temperature coefficient of resistance (TCR) and low noise characteristics to efficiently detect IR photons while also maintaining compatibility with standard integrated circuit (IC) processing. The most commonly used material in uncooled infrared imaging detectors is vanadium oxide deposited by reactive ion beam sputtering. Here we present a comparison of vanadium oxide thin films grown via commercial reactive ion beam sputtering to films grown using reactive pulsed DC magnetron sputtering. Films deposited using both methods were optically and structurally characterized using Raman spectroscopy, transmission electron microscopy, atomic force microscopy and grazing incidence X-ray diffraction. The measured electrical properties of the films were found to be very sensitive to the deposition conditions used. The ion beam sputtered films contained twinned FCC VOx nanocrystals with sub-nanometer twin spacing, in the form of large 10-20 nm wide columnar/conical grains. In contrast, the un-biased magnetron sputtered films consisted of equiax grains of FCC VOx (5-10 nm) encapsulated in an amorphous matrix. However, applying an RF bias to the sample substrate during the magnetron sputtering process, resulted in films that are similar in structure to ion beam deposited VOx. These differences in microstructure and composition were then correlated to the measured resistivities and TCRs of the films.

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

Vanadium oxide (VOx) thin films have been deposited by pulsed-DC magnetron sputtering using a metallic vanadium target in a reactive argon and oxygen environment. While the process parameters (power, total pressure, oxygen-to-argon ratio) remained constant, the deposition time was varied to produce films between 75 ± 6 and 2901 ± 30 A thick, which were then optically and electrically characterized. The complex dielectric function spectra (e = e1 + ie2) of the films from 0.75 to 5.15 eV were extracted by ex situ, multiple-angle spectroscopic ellipsometry (SE) measurements for the series of varied thickness VOx samples. Significant changes in e and resistivity occur as a function of thickness, indicating the correlations exist between the electrical and the optical properties over this spectral range. In addition, in situ measurements via real time SE (RTSE) were made on the film grown to the largest thickness to track optical property and structural variations during growth. RTSE was also used to character...

Microstructure of vanadium oxide used in microbolometers

Reactive pulsed DC sputtering was used to grow a systematic series of films with resistivity ranging from 1 × 10-3 to 6.8 × 104 Ohm cm and TCR varying from 0 to -4% K-1. Throughout the parameter space studied a transition from amorphous to nano-crystalline growth was observed. Films in the resistivity range of interest for microbolometers contained a FCC VOx (0.8 < x < 1.3) phase. Altering the sputtering energetics via substrate biasing resulted in highlycolumnar, nano-twinned grains of FCC VOx, providing a microstructure reminiscent of ion beam sputtered bolometer material. Electron diffraction in the TEM confirmed the presence of a secondary, oxygen-rich amorphous phase. Micro- Raman spectroscopy, which was also found to be sensitive to the secondary amorphous phase, was used to probe the chemical composition and morphology of VOx thin films. Raman spectra from high resistivity amorphous films show a broad feature around ~890 cm-1, while spectra from lower resistivity nano-crystalline films exhibit this same amorphous feature and a second broad feature at ~320 cm-1. The resulting microstructure can be described as a nano-composite material composed of a low-resistivity crystalline phase embedded in a high-resistivity amorphous matrix. Our results suggest that both phases are required to achieve a high TCR, low resistivity material.

In-situ TEM Study of Thin-film Vanadium Oxide Stability

Vanadium oxide thin films were co-deposited via reactive pulsed-dc sputtering onto both formvarcoated Cu TEM grids and SiO2-coated Si wafers. A trend was observed throughout the series whereby increasing oxygen partial pressure in the growth chamber caused an increase in both the oxygen content and the disorder in the films [1]. Films deposited below a PO2 of 5% exhibited the presence of nanocrystals and films deposited above a PO2 of 5% were amorphous. Two films from the series, one nano-crystalline with an amorphous component, and the other completely amorphous, were found to have the same stoichiometry via rutherford backscattering spectroscopy. Though the bulk stoichiometry of both films was found to be VO2, the nanocrystalline phase was determined to be the FCC phase, which at equilibrium is characteristic of VOx, where 0.8<x<1.3.