Characterization and Development of Submillimeter Wavelength Devices Incorporating Vanadium Dioxide Thin Films

Vanadium dioxide exhibits an insulator-to-metal transition (IMT) accompanied by a decrease in resistivity of several magnitudes and a large increase in reflectivity. The IMT can be triggered in multiple ways. The most common method is to heat a sample above 67°C causing it to go from a semiconducting to metallic phase. However, the transition can also be triggered using current injection, a change in electric field, or an optical pump. A diverse array of potential applications exploiting VO2 including next generation CMOS, switches, thermal relays, intelligent window coatings, bolometers, filters, memory and logic devices, and reconfigurable circuit elements and antennas from dc to x-ray wavelengths have been proposed in the literature.
A better understanding of vanadium dioxide thin films and their properties is needed to develop new devices and technologies based on the material. One such property is the contact resistance. As devices become smaller, the contact interface will account for an increasing portion of the device function and have noticeable effect on its performance. The standard method for determining the contact resistance of planar metal-semiconductor interfaces involves measuring adjacent pairs of contacts with various separation lengths at a constant bias. This method relies on the resistivity of the semiconductor material remaining constant during measurement. However, the strong temperature dependence of the resistivity of VO2 requires a modified approach that maintains a constant power density dissipated within the film to account for Joule heating. A new method for measuring contact resistance in semiconductors with a high thermal coefficient of resistivity is presented and results using this new method are compared with results using the standard technique.
As new device concepts are proposed and developed, VO2 films are exposed to many standard chemicals used for semiconductor device processing, including etchants, organic solvents, and other reagents such as acetone, O2 plasma, de-ionized water, and photolithography process. During fabrication of devices for this dissertation, physical changes to the appearance of VO2 films were observed. It was hypothesized that some of the chemicals and processes used during device fabrication were etching or otherwise altering the VO2 films. Changes in thickness, stoichiometry, resistivity, and appearance of a VO2 thin film as a result of several common reagents and photolithography were systematically investigated and the results are presented.
The recent development of on-wafer terahertz measurement systems permits direct in-situ characterization of planar devices at frequencies approaching 1 THz. Such measurements are critical for assessing new and emerging device technologies for high-frequency applications, including those based on vanadium oxide, and allow accurate circuit models to be derived for use in circuit design. Vanadium oxide switches are of interest for their potential use in realizing reconfigurable circuits at these frequencies.
A single element (one port) device consisting of a thin film VO2 load at the terminus of a coplanar waveguide (CPW) transmission line and a two port switch consisting of two CPW transmission lines connected in series with a VO2 load were designed and characterized in the WR-1.5 waveguide band (500-750 GHz). These results demonstrate that the voltage-induced IMT modulates reflection or transmission of a signal by about 10 dB. The metallic state impedance of the VO2 film was calculated from the data obtained and shown to be primarily resistive making VO2 attractive for broadband applications.