This
work investigates and reports on the radio-frequency (rf)
behavior in the frequency range of 5–35 GHz of germanium-doped
vanadium dioxide (Ge-doped VO2) 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 (T
c)
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 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 (εr) 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 structures incorporating ultracompact
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 the reference. The reported results and analysis contribute
to the advancement of the field of metal–insulator–transition-material
technology with high T
c for rf industrial
applications.