pubs.acs.org/acsaelm Article Radio-Frequency Characteristics of Ge-Doped Vanadium Dioxide Thin Films with Increased Transition Temperature Andrei Muller,* Riyaz A. Khadar, Tobias Abel, Nour Negm, Teodor Rosca, Anna Krammer, Matteo Cavalieri, Andreas Schueler, Fatemeh Qaderi, Jens Bolten, Max Lemme, Igor Stolichnov, and Adrian M. Ionescu* Cite This: https://dx.doi.org/10.1021/acsaelm.0c00078 Read Online ACCESS Metrics & More Article Recommendations 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 fi VO2) thin lms 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 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 fi to the state-of-the art values measured for undoped VO2 thin lms deposited on identical ff fi substrates. In order to further understand the o -state dissimilarities and rf behavior of the deposited Ge-doped VO2 lms, we ε fi propose an original methodology for the experimental extraction of the dielectric constant ( r) in the GHz range of the lms 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 fi two types of substrates, one with the Ge-doped VO2 thin lms 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 Tc for rf industrial applications. KEYWORDS: phase change materials, radio frequency passive circuits, vanadium dioxide, dielectric constant, coplanar waveguide, CMOS, lossy dielectrics Downloaded via ANDREI MULLER on April 29, 2020 at 07:18:39 (UTC). 1. INTRODUCTION integration, reversible insulator to metal transition (IMT), fast See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. Recently, the applications of Mott insulators showing metal-to- switching time, and device-size shrinking capabilities, the 1−3 fi insulator transitions have been actively explored, as the employment of VO2 as a recon gurable radio-frequency (rf) phase transitions could be triggered facts, practically in sub- material has been recently investigated for a variety of rf- nanosecond timescales and also because the device operation reconfigurable devices on various substrates such as sap- 4−8 9−16 can be done purely electrically, with technologies compatible phire, or cheaper SiO2 among others, in the frequency with silicon complementary metal oxide semiconductor (f) range of 1−35 GHz. The relatively low transition (CMOS). The key feature of Mott insulators is related to ° temperature of VO2 of Tc =68 C when deposited on silicon the fact that the energy of many other interactions is on the − (Si) CMOS compatible substrates hinders its usability for same order of magnitude with electron electron correlation developing rf switches because for most rf applications, it is and kinetic energy; as a consequence, such interactions could play a relevant role on the electronic properties of the material. Received: January 31, 2020 The phase-change material, vanadium dioxide (VO2,) exhibits a monoclinic crystal structure and behaves like an insulator Accepted: April 16, 2020 Published: April 16, 2020 below its insulating-to-metal transition temperature (TITM). Subject of many controversies, it is currently agreed that the ff phase transition in VO2 is probably a combined e ect of lattice distortion and Coulomb correlations. Because of its ease of © XXXX American Chemical Society https://dx.doi.org/10.1021/acsaelm.0c00078 A ACS Appl. Electron. Mater. XXXX, XXX, XXX−XXX ACS Applied Electronic Materials pubs.acs.org/acsaelm Article ff ff Table 1. Overview of the Dielectric Loss Modeling of VO2 Thin Films in the O -State at Di erent Frequency Bands frequency dielectric constant conductivity references spectrum ε′ ()f σ (f) model, simulation, and assumptions r,VO2 ac_OFF σ σ • 1, 35, 8,rf OFF(f)= dc_OFF simulated as a resistive material • area occupied by VO2 is very small compared to the device area σ σ • 9 rf OFF(f)= dc_OFF same as above 10,13 σ σ • − rf 30 (f 0)= dc_OFF simulated as a non-causal dielectric violating Kramers Kronig relations − on SiO2 Si substrates • area occupied by VO2 is very small compared to the device area yes from 4 rf εr,VO′ ()f •on sapphire 2 ε″ ()f r,VO2 •extracted via conformal mapping, assuming infinite ground planes, simplified lossesmodel37 18,24 •obeying Kramers−Kronig relations via different models optical εr,VO′ ()f yes from ε″ ()f 2 r,VO2 (Lorentz, Tauc−Lorentz, etc.) ° 17 13−16 σ required to have a switching temperature above 80 C. On dielectric, the dc_OFF being considered as the con- fi the other hand, the dc conductivity levels of VO2 thin lms in ductivity value at the central frequency (f 0) of the analyzed ff σ their insulating o -state ( dc_OFF) and conductive on-state frequency band of the simulations (σ (f )=σ ), whereas σ ac 0 dc_OFF ( dc_ON) vary over a wide range depending on the substrates a fixed empirical value of 30 is used for the dielectric constant, used for deposition and hence the built-in lattice mismatches.18 εrVO′ of VO2 at room temperature. Doping VO proved to be a solution for changing the critical 2 2 The 3D electromagnetic field simulation softwareAnsys transition temperature, T (which is defined as the average − c HFSS (v. 17.2), used for modeling in rf applications13 16,28,29 between the T and the metal-to-insulator transition ITM has three predefined different models, two of which are based temperature TMIT). This shift was downward while doping 19 ° on the Debye model and one other based on the Djordjevic− with Co on Al2O3 substrates or upward to Tc = 93.6 C using 30,31 Ge on Si-substrates as shown in our previous work;20 however, Sarkar frequency dispersion model. Their use avoids σ ° improper definition of the material,32 the latter one being the conductivity level, dc_ON obtained above 93.6 C was 33 below 360 S/m (for Ge concentrations of 5.9%), which is far used by default when losses exceed certain limits (unless σ deactivated by the user). When using one of these existing lower than dc_ON obtained for undoped VO2 (which are higher than 10,000 S/m) when the latter was employed for rf frequency dispersive models, the Kramers−Kronig relations − research applications.13 16 followed by all real and imaginary parts of the dielectric 34 Unlike the range of optical frequencies, where ellipsometry functions of any causal material are obeyed, but only (a far-field approach) is a powerful tool for the determination according to these particular models. Conversely, using the fi 13−16 of the relative dielectric permittivity of the VO2, given by simpli ed models in (where authors use VO2 only on a limited area of the devices) and assuming a constant ε (,)fT= εε′ (,)j fT+ ″ (,) fT r,VO22 r,VO r,VO 2 (1) ε′ (,fT=° 25C) and a nonconstant rVO2 − σ =° when thin films are deposited on different substrates,21 27 ε″ (,fT=°= 25C) ac(,fT 25C)7,35 (where ε denotes r,VO2 πε 0 fi 7 2 f 0 near- eld approach or conformal mapping are used in rf − engineering of electronic functions. the vacuum permittivity) violates the Kramers Kronig In the rf range, to the best of our knowledge, no study is relations because a frequency-independent dielectric constant available in the literature about the dielectric constant, implies a constant imaginary part, and this is obeyed only by vacuum.34 (ε′ (,fT )), of Ge-doped VO2 deposited on CMOS r,VO2 :Ge In this work, we first do a dc characterization of two compatible substrates or its relative dielectric permittivity fi germanium-doped VO2 lms with doping concentrations of 5 ε (,)fT= εε′ (,)j fT+ ″ (,) fT and 5.5% prepared via sputtering and pulsed laser deposition r,VO22 :Ge r,VO :Ge r,VO 2 :Ge (2) (PLD), respectively, on CMOS-compatible SiO2/high-resis- Even for undoped VO , models used within 1−30 GHz for tivity silicon substrates. We compare their temperature- 2 σ the relative dielectric function are in their incipient stages of dependent dc conductivity levels, dc(T) and temperature fi fi − development and exist only for the case of a VO2 thin lm on a hysteresis (de ned as TITM TMIT) with other undoped VO2 sapphire substrate obtained by using conformal mapping depositions done in similar conditions by us.16 The PLD (243 7 fi ’ approximations. nm) and sputtered (183 nm) Ge-doped VO2 lms exhibit Tc s In most cases, for the modeling of the off-state conductivity of 72 and 76.2 °C, respectively, and present σ values fi dc_ON in rf VO2 thin lms used for switches, the authors usually which are orders of magnitude higher than that of the 5.9% Ge- fi 20 consider the material as a constant valued sheet resistance doped VO2 thin lm from our previous work with Tc = 93.6 while seldom simulating it as a lossy dielectric, as depicted in °C.