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

Abstract

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.

Highly Sensitive Plasmonic Biosensors with Precise Phase Singularity Coupling on the Metastructures

“…These techniques are shown to alter the optical properties of the phase-change material; for instance, the change in refractive index is affected by the percentage of the doping ions. However, at low concentrations of dopants and at visible frequencies, no significant change is recorded [ 25 , 26 , 27 , 28 , 29 , 32 , 33 ]. Doping 1% of tungsten and 0.4% of niobium can lower the transition temperature by 25 °C and 13 °C respectively [ 25 ].…”

Temperature Effect on a Lumped Element Balanced Dual-Band Band-Stop Filter

“…Moreover, it is a dual-band filter response whereas all other designs incorporate with only one frequency band (third column). From the fourth column, the filter response is tested for a much higher temperature variation range (80 • C) than [16][17][18] (35 • C-40 • C). For this wide temperature range, the proposed filter shows only 0.2-0.3% shift in center frequency whereas other designs (except [20]) demonstrate at least 0.6% variation for a temperature sweep of only 35 • C (fifth column).…”

“…In [17], a master-slave filter circuit has been discussed to alleviate the impact of temperature on resonant frequency. Moreover, the stopband rejection in [16] is not satisfactory (maximum 10 dB). Likewise, the temperature change in [19] and [20] affects the filter designs significantly in terms of stopband resonant frequency and stopband rejection, respectively.…”

“…Similarly, in [16][17][18][19][20], the impact of temperature is examined on various band-stop filter architectures. Among them, the designs in [16][17][18] have been tested for a temperature variation range of less than 40 • C, and in this range, they exhibited at least 0.9% shift in their corresponding resonant frequencies. In [17], a master-slave filter circuit has been discussed to alleviate the impact of temperature on resonant frequency.…”

High-Strain-Induced Local Modification of the Electronic Properties of VO2 Thin Films

“…The reversible semiconductor-to-metal transition, often referred to as metal–insulator transition (MIT), can be triggered by various stimuli, such as heat, light, mechanical pressure, and electric and magnetic fields, which opens the possibility for realizing switching devices between well-defined off (insulating) and on (metallic) states. In order to better understand the microscopic origin of the MIT, VO 2 has been investigated with state-of-the-art (pump–probe) spectroscopy techniques employing radiation all across the electromagnetic spectrum from hard and soft X-rays, to XUV, infrared, THz, microwave, and radio frequency, as well as electron microscopy. , The interested reader is referred to the review by Shao et al Our current understanding is that the electronic phase transition of VO 2 is inherently coupled to an accompanying structural phase transition from a monoclinic crystal structure at low temperature to a tetragonal structure at high temperature. The monoclinic phase is defined by a characteristic dimerization and tilt motif of the vanadium ions, whereas the tetragonal phase is isostructural to rutile TiO 2 .…”