Radio-Frequency Characteristics of Ge-doped Vanadium Dioxide Thin Films with Increased Transition Temperature

This work investigates and reports on the radio-frequency (RF) behavior in the frequency range of 5 – 35 GHz of germanium-doped (Ge-doped VO2) vanadium dioxide thin films, deposited on silicon substrates via sputtering and pulsed laser deposition (PLD) with estimated Ge concentrations of 5 % and 5.5 %. Both films exhibit critical transition temperatures (Tc) of 76.2 and 72 °C, respectively, which are higher compared to that of the undoped VO2 which undergoes reversible insulator-to-metal phase transition at 68°C. Both types of Ge-doped films show low hysteresis (< 5 °C) in their conductivity versus temperature characteristics and preserve an high off-state DC-conductivities (corresponding to the insulating state of the phase change material) of 13 S/m, for the sputtered, and, 55 S/m, for the PLD deposited film, respectively. The DC on-state (corresponding to the conductive state of the phase change material) conductivity reaches 145,000 S/m in the case of the PLD film, which represents a significant increase compared to the state-of-the art values measured for undoped VO2 thin films deposited on identical substrates. In order to further understand the off-state dissimilarities and RF behavior of the deposited Ge-doped VO2 films, we propose an original methodology for the experimental extraction of the dielectric constant (ɛr) in the GHz range of the films below 60 °C. This is achieved by exploiting the frequency shift of resonant filters. For this purpose we have fabricated coplanar waveguide (CPW) structures incorporating ultra-compact Peano space filling curves, each resonating at a different frequency between 5 and 35 GHz on two types of substrates, one with the Ge-doped VO2 thin films and another one using only SiO2 to serve as reference. The reported results and analysis contribute to the advancement of the field of metal-insulator-transition material technology with high Tc for RF industrial applications.