We present a study of co-sputtered VO2-SiO2 nanocomposite dielectric thin-film media possessing continuous temperature tunability of the dielectric constant. The smooth thermal tunability is a result of the insulator-metal transition in the VO2 inclusions dispersed within an insulating matrix. We present a detailed comparison of the dielectric characteristics of this nanocomposite with those of a VO2 control layer and of VO2/SiO2 laminate multilayers of comparable overall thickness. We demonstrated a nanocomposite capacitor that has a thermal capacitance tunability of ∼60% between 25 °C and 100 °C at 1 MHz, with low leakage current. Such thermally tunable capacitors could find potential use in applications such as sensing, thermal cloaks, and phase-change energy storage devices.
I. INTRODUCTION
Tunable dielectrics are of broad interest for a range of devices, including capacitive sensors, thermal metamaterials, signal processing, and energy storage systems. Temperature tunable materials are being widely explored in the context of thermal concentrators1 and thermal cloaks.2,3 Previous studies have demonstrated that nanocomposite dielectrics can be used for thermally tunable metamaterials and switchable capacitors.4–14 The tunable dielectric properties (e.g., permittivity and loss tangent) of a nanocomposite strongly depend on the composition, particle sizes, morphology, distribution, ratio, and interaction among constituents.15,16
The phase-transition material vanadium dioxide (VO2) displays a strong and sharp temperature-driven insulator-metal transition (MIT) that features large changes in carrier density from its low-temperature monoclinic structure to the high-temperature tetragonal rutile structure.17–24 The dielectric constant of VO2 varies by orders of magnitude across the MIT temperature (TMIT).25,26 The transition behavior of thin-film VO2 can be further engineered via doping,27 defect engineering,28 or altering the deposition conditions.29–31 Consequently, VO2-containing systems are promising for the realization of thermally tunable devices. However, the highly conducting nature of the metal phase of VO2 can lead to current leakage, which is detrimental for capacitor applications. Thereby, it is important to keep the metal-phase VO2 spatially constrained to minimize leakage losses in a capacitor, especially when the VO2 is in the metallic state. The controlled dispersion of VO2 in an insulating dielectric matrix (such as SiO2) is one potential approach.32–35 Pioneering studies on VO2-based composites fabricated by ion implantation of vanadium into silica, followed by annealing in an oxygen environment, illustrated a phase-transition evolution that was strongly dependent on the size of VO2 inclusions.36 However, the use of such composites in tunable capacitors has not yet been explored.
In this study, VO2-SiO2 nanocomposites were prepared by a co-sputtering approach, and sandwiched between thin SiO2 layers. For comparison, a VO2 control layer and a SiO2/VO2 multilayer were also deposited under identical conditions. We found that dispersing VO2 inclusions in a silica matrix results in gradual tuning of the dielectric properties as opposed to an abrupt phase transition seen in pure VO2.
II. EXPERIMENTAL DETAILS
The schematics of our metal-insulator-metal (MIM) capacitor structures are shown in Fig. 1. Radio-frequency (RF) sputtering was used to fabricate each dielectric layer. First, a 10 nm SiO2 layer was deposited onto a Pt (111) (150 nm) / Ti (10 nm) /SiO2 (300 nm) / n-Si substrate at 400 °C using an SiO2 target. Then, three types of structures were fabricated using V2O5 and SiO2 targets: a VO2 control layer (100 nm), a superlattice VO2/SiO2 multilayer (5 nm for each, 20 layers in total), and a composite VO2-SiO2 layer (100 nm). The composite film is the primary focus of this study. The growth temperature and pressure were 600 °C and 5 mTorr, respectively. The RF power supplied to the SiO2 was 150 W, and that to the V2O5 was varied to achieve different VO2/SiO2 ratios in the composite layers. (V2O5 power of 100 W, 46 W, and 12 W, for 50%, 30%, and 10% VO2 molar content respectively). Finally, another SiO2 layer of 10 nm was deposited on top to form a sandwich structure. Gas flows of 49.75 SCCM of argon (Ar) and 0.25 SCCM of oxygen (O2) were used during deposition. Ti (5 nm in thickness) / Au (150 nm in thickness) were deposited on the SiO2 layer as top contacts using an electron-beam evaporator. Round metal contacts with a diameter of 200 μm were defined using a shadow mask.
(a) Schematic of three different types of dielectric layers grown on the platinized silicon substrate.
(a) Schematic of three different types of dielectric layers grown on the platinized silicon substrate.
The effective permittivity of dielectric media can be calculated from the capacitance as
where C is the capacitance of device, A is the Ti/Au contact area, d is the dielectric media thickness, ξ is the effective permittivity of the dielectric material, and ξ0 is the vacuum permittivity of 8.86 × 10−12 F m−1.
The detailed parameters of each sample are shown in Table I.
Parameters of the different dielectric layers .
Sample names . | Dielectric description . | Capacitor structure . | Schematic . |
---|---|---|---|
VO2-SiO2 composite | VO2 + SiO2 composite layer (100 nm) sandwiched by two SiO2 layers (10 nm each) | Au/Ti/SiO2/composite/SiO2/substrate | ![]() |
Superlattice VO2/SiO2 multilayer | Multilayers of (VO2/SiO2) (5 nm each, 100 nm in total) sandwiched by two SiO2 layers (10 nm each) | Au/Ti/SiO2/multilayer/SiO2/substrate | ![]() |
VO2 control layer | Single VO2 layer (100 nm) sandwiched by two SiO2 layers (10 nm each) | Au/Ti/SiO2/VO2/SiO2/substrate | ![]() |
VO2 single layer | Single VO2 layer (100 nm) for material-level characterization and growth optimization | Au/Ti/VO2/SiO2/substrate | ![]() |
SiO2 reference | Single SiO2 layer of 34 nm | Au/Ti/SiO2/substrate | ![]() |
Sample names . | Dielectric description . | Capacitor structure . | Schematic . |
---|---|---|---|
VO2-SiO2 composite | VO2 + SiO2 composite layer (100 nm) sandwiched by two SiO2 layers (10 nm each) | Au/Ti/SiO2/composite/SiO2/substrate | ![]() |
Superlattice VO2/SiO2 multilayer | Multilayers of (VO2/SiO2) (5 nm each, 100 nm in total) sandwiched by two SiO2 layers (10 nm each) | Au/Ti/SiO2/multilayer/SiO2/substrate | ![]() |
VO2 control layer | Single VO2 layer (100 nm) sandwiched by two SiO2 layers (10 nm each) | Au/Ti/SiO2/VO2/SiO2/substrate | ![]() |
VO2 single layer | Single VO2 layer (100 nm) for material-level characterization and growth optimization | Au/Ti/VO2/SiO2/substrate | ![]() |
SiO2 reference | Single SiO2 layer of 34 nm | Au/Ti/SiO2/substrate | ![]() |
The current-voltage (I-V), capacitance-frequency (C-F), and capacitance-voltage (C-V) data were collected by using a Keithley 236 Source Measure Unit and an Agilent 4284A Precision LCR Meter with a temperature-controlled probe station. The conduction mechanisms in the composite medium can be analyzed by considering the temperature and voltage dependence of the leakage current through the capacitor.
Poole-Frenkel (P-F) and Schottky emission models were employed to determine the conduction mechanism of the different dielectric materials. Specifically, the current density resulting from P-F emission can be written as37
while Schottky emission can be expressed as38
J is the current density, C is the Faraday constant, A is the effective area of capacitor, e is the charge of an electron, is the carrier mobility, is the carrier concentration, E is the electric field, k is the Boltzmann constant, T is the temperature in Kelvin, is the Schottky barrier height between the metal and semiconductor dielectric. is the effective mass of carriers, and h is the Planck constant.
The slopes of ln(J) vs V1/2 and ln(J/V) vs V1/2 plots should be constant for Schottky emission and the Poole-Frenkel effect, respectively.
X-ray diffraction (XRD) patterns were collected using a PANalytical MRD X'Pert Pro High Resolution XRD instrument. X-ray photoelectron spectroscopy (XPS) was carried out on the composite thin films using a Kratos spectrometer with an Al Kα of 1.4866 keV as the x-ray source. The carbonaceous C1s line at 284.6 eV was used as a reference to calibrate the binding energies. The reflectance of the patterned samples in the mid-infrared wavelength range was investigated via near-normal incidence measurements using a mid-infrared microscope (Hyperion 2000) with a Cassegrain objective (NA = 0.4), attached to a Fourier transform infrared (FTIR) spectrometer (Bruker Vertex 70). The mid-infrared source in our FTIR setup is a Globar (a heated silicon carbide rod).
High-angle annular dark field-scanning transmission electron microscope (HAADF-STEM) images were collected using FEI TALOS F200X TEM operated at 200 kV. The high-resolution electron energy loss spectroscopy (EELS) maps and energy dispersive spectroscopy (EDS) mapping results were collected using an FEI Titan3 G2* TEM operating at 80 kV, equipped with a built-in Super X-EDS system, and Gatan Quantum 966 instrumentation. The cross-sectional transmission electron microscope (TEM) samples were prepared using a Helios NanoLab 660 field emission scanning electron microscope (FESEM) with a focused ion beam (FIB). The preparation procedure included ion-beam gas-assisted chemical vapor deposition of a 2- m-thick carbon layer to protect the sample during milling.
III. RESULTS AND DISCUSSION
Figure 2(a) displays the cross-sectional STEM image taken in HAADF mode from a composite capacitor device, which clearly shows the interface between dielectric layers (SiO2/composite/SiO2) and underlayers (Pt/Ti/SiO2/Si). The corresponding EDS mapping shows the elemental composition of each layer [Fig. 2(b)]. High resolution TEM-EELS and EDS elemental mapping of the composite layer is collectively shown in Fig. 2(c). As expected from co-sputtering, the Si and V elements are uniformly mixed and dispersed throughout the film. The particle size of VO2 clusters is smaller than 5 nm. The temperature-dependent normalized in-plane resistance evolution (during heating) of a VO2 single layer is shown in Fig. 3(a). The film resistance decreases by more than two orders of magnitude from 25 °C to 100 °C, with the MIT temperature at around 72 °C. The VO2 single layer shows the characteristic ( 11) diffraction peak at 36.22°, which is ascribed to the vanadium dioxide (VO2) phase [Fig. 3(b)].39 By comparison, neither a sharp conductance transition nor significant VO2 XRD peaks could be found on the composite sample. However, high-resolution XPS analysis of the pure VO2 film and composite films with different V/Si ratios [Fig. 3(c)] clearly shows the V2p3/2 peak at ∼516 eV which we attribute to V4+ in VO2.30,40–42 The combination of XRD and XPS characterization indicates the presence of small VO2 particles dispersed throughout the composite layer, which is consistent with the STEM results. The ultra-small particle size is likely responsible for the graded switching behavior.
(a) STEM image and (b) corresponding EDS mapping of a cross-section of the film/Pt/Ti/SiO2/Si interface. (c) High resolution EFTEM and EDS elemental mapping images of the composite layer. The VO2 is distributed uniformly in the composite.
(a) STEM image and (b) corresponding EDS mapping of a cross-section of the film/Pt/Ti/SiO2/Si interface. (c) High resolution EFTEM and EDS elemental mapping images of the composite layer. The VO2 is distributed uniformly in the composite.
(a) Normalized resistance change (heating) versus temperature for a VO2 film and a composite (SiO2-VO2) film grown on SiO2/Si substrates. The inset shows the derivative plot versus temperature, indicating that the transition temperature of the VO2 single layer is around 72 °C. In comparison, the VO2-SiO2 composite layer shows smoothly varying resistance with temperature. (b) X-ray diffraction (XRD) pattern of three different layers. Unlike the VO2 control layer and the VO2/SiO2 multilayer structure, the composite sample shows no diffraction peak from VO2 due to the very small particle size. (c) X-ray photoelectron spectroscopy (XPS) of different films.
(a) Normalized resistance change (heating) versus temperature for a VO2 film and a composite (SiO2-VO2) film grown on SiO2/Si substrates. The inset shows the derivative plot versus temperature, indicating that the transition temperature of the VO2 single layer is around 72 °C. In comparison, the VO2-SiO2 composite layer shows smoothly varying resistance with temperature. (b) X-ray diffraction (XRD) pattern of three different layers. Unlike the VO2 control layer and the VO2/SiO2 multilayer structure, the composite sample shows no diffraction peak from VO2 due to the very small particle size. (c) X-ray photoelectron spectroscopy (XPS) of different films.
It has been previously reported that the precipitation of VO2 in the SiO2 matrix also influences the optical properties of VO2.43,44 In our case, temperature-dependent near-infrared reflectance integrated over 1.5–2 m was measured to reveal the optical evolution of different thin films (Fig. 4). The VO2 control layer shows an abrupt reflectance increase (33% to 64%) across the TMIT. In comparison, the composite layer only shows a slight change of 0.6%. This small change is ascribed to the small particle size of VO2, and is consistent with the characterization data shown in Figs. 2 and 3.
Temperature-dependent reflectance of the dielectric films, integrated over the 1.5 to 2 μm wavelength range. Unlike the VO2 control layer, which shows a factor of two increase in reflectance across the TMIT, the composite sample shows a small reflectance change of only 0.6%.
Temperature-dependent reflectance of the dielectric films, integrated over the 1.5 to 2 μm wavelength range. Unlike the VO2 control layer, which shows a factor of two increase in reflectance across the TMIT, the composite sample shows a small reflectance change of only 0.6%.
Analysis of the leakage current of composite samples with different VO2 contents [Fig. 5(a)] at 1 V bias shows that the leakage current significantly decreases with the reduction of the VO2 content. Figure 5(b) shows the I-V characteristics of the composite layer (50 mol. % VO2 content) with temperature from 25 °C to 100 °C. The current density of the composite capacitor is lower than 10−6 A cm−2 at 1 V. By comparison, the current leakage of the VO2 control layer reaches 10−1 A cm−2 at 0.1 V, and the superlattice VO2/SiO2 multilayer shows leakage higher than 10−3 A cm−2 at 1 V (Fig. 6).
(a) Temperature dependence of the current leakage of composite dielectric stacks with different VO2 contents, at a bias of 1 V. A lower VO2 fraction in the composite leads to a lower current leakage level within the measured temperature range. (b) I-V characteristics of the composite dielectric in the temperature range of 25–100 °C. The leakage current increases with temperature [(c) and (d)]. Poole-Frenkel [ln (I/V) vs V1/2] plot and Schottky plot [ln (I) vs V1/2] corresponding to the I-V characteristics of the composite dielectric with 50% VO2. The electron transport in the composite layer follows the Poole-Frenkel and Schottky mechanisms at higher (e.g., 65–100 °C) and lower (e.g., 25–65 °C) temperature, respectively.
(a) Temperature dependence of the current leakage of composite dielectric stacks with different VO2 contents, at a bias of 1 V. A lower VO2 fraction in the composite leads to a lower current leakage level within the measured temperature range. (b) I-V characteristics of the composite dielectric in the temperature range of 25–100 °C. The leakage current increases with temperature [(c) and (d)]. Poole-Frenkel [ln (I/V) vs V1/2] plot and Schottky plot [ln (I) vs V1/2] corresponding to the I-V characteristics of the composite dielectric with 50% VO2. The electron transport in the composite layer follows the Poole-Frenkel and Schottky mechanisms at higher (e.g., 65–100 °C) and lower (e.g., 25–65 °C) temperature, respectively.
I-V characteristics of (a) the VO2 control layer and (b) the VO2/SiO2 multilayer in the temperature range of 25–100 °C. The VO2 control layer shows a current-leakage jump across the transition temperature. Current leakage levels of both the VO2 control layer and the VO2/SiO2 multilayer are orders of magnitude higher than those of the composite layer, indicating a significant leakage suppression effect due to VO2 dispersal in the silica matrix.
I-V characteristics of (a) the VO2 control layer and (b) the VO2/SiO2 multilayer in the temperature range of 25–100 °C. The VO2 control layer shows a current-leakage jump across the transition temperature. Current leakage levels of both the VO2 control layer and the VO2/SiO2 multilayer are orders of magnitude higher than those of the composite layer, indicating a significant leakage suppression effect due to VO2 dispersal in the silica matrix.
To evaluate the temperature-dependent current transport mechanism in the composite film, Poole-Frenkel (P-F) and Schottky emission models were employed. These models describe the typical bulk-limited and electrode-limited conduction mechanism in dielectric films, respectively. The P-F and Schottky fitting plots are shown in Figs. 5(c) and 5(d), respectively. Typically, current transport following the P-F mechanism relies more on the thermal emission of electrons from the conduction band of the dielectric.45 In comparison, Schottky emission is used for explaining the electrons that are thermally activated to overcome the energy barrier at the metal-dielectric interface. Conduction in the VO2 layer is not purely from the P-F mechanism especially when the temperature crosses TMIT. This is because of the metallic state of VO2 beyond the phase-transition temperature, which leads to ohmic conduction. A better linear fitting with a slope of 2.9 using the P-F model [ln (I/V) vs V1/2] is observed in the composite sample when the temperature approaches or is above the transition temperature of VO2 (∼65 °C–100 °C), suggesting the conduction is gradually dominated by the P-F mechanism. In comparison, better linear fitting of composite data using the Schottky model [ln(I) vs V1/2] results in a slope of 5.1 when the temperature is below the transition temperature (∼25 °C–65 °C). Unlike the homogenous film of VO2, conducting VO2 particles embedded in SiO2 at high temperature can be seen as a localized electron that can be excited under bias. Consequently, P-F emission is dominant. In comparison, at low temperature, the VO2 is more insulating. Therefore, the conduction mainly relied on the transport of electrons from the electrode across the electrode-dielectric interface (following the Schottky model). When the V(1/2) is in the extremely small range (< 0.1 V) in Fig. 5(c), the conduction of the thin film may follow the ohmic mechanism.45 In this case, the ohmic conduction current intensity (I) is linearly dependent on the electric bias (V).
The temperature-dependent capacitance density for various capacitors from 25 °C to 100 °C is shown in Fig. 7. As a reference, a capacitor with only SiO2 as the dielectric was prepared (Table I) and measured. Unlike the SiO2 device, which has negligible capacitance variation (<0.13%), the capacitances of all VO2-containing devices increase with increasing temperature. The distribution of the SiO2 and VO2 has a significant effect on the capacitance evolution. The capacitance density of the composite sample smoothly increases from 84.3 nF cm−2 at 25 °C to 134.1 nF cm−2 at 100 °C, an increase of ∼60% [Fig. 7(a)]. Such tunability is close to previous studies on ceramic/metal-polymer composite dielectrics.46–48 Our VO2-SiO2 composite shows a smoothly varying thermal tunability of ∼60% spanning the TMIT region. On the other hand, the VO2 control layer displays a sharp capacitance density jump by a factor of 2 proximal to the transition region (563.1 nF cm−2 at 70 °C vs. 1038.4 nF cm−2 at 80 °C) [Fig. 7(b)].
Summary of temperature-dependent capacitance density evolution of (a) the VO2-SiO2 composite and the control SiO2 dielectric, and (b) the VO2 control layer and the VO2/SiO2 multilayer between 25 °C and 100 °C, at 1 MHz. The VO2 control layer shows an abrupt jump across the transition temperature. By comparison, the composite dielectric shows smoothly tunable capacitance from 25 °C to 100 °C.
Summary of temperature-dependent capacitance density evolution of (a) the VO2-SiO2 composite and the control SiO2 dielectric, and (b) the VO2 control layer and the VO2/SiO2 multilayer between 25 °C and 100 °C, at 1 MHz. The VO2 control layer shows an abrupt jump across the transition temperature. By comparison, the composite dielectric shows smoothly tunable capacitance from 25 °C to 100 °C.
Next, we consider the frequency dependence of the capacitance. Figures 8 and 9 show the logarithmic plots of capacitance density vs. frequency for different devices and temperatures. The SiO2 control shows no significant change in capacitance as frequency is increased [Fig. 8(a)], which is consistent with previous reports.49 By comparison, the capacitance density of all VO2-containing samples shows a clear reduction with increasing frequency. At 25 °C, the composite layer capacitance densities decrease from 160 nF cm−2 at 104 Hz to 82 nF cm−2 at 106 Hz [Fig. 8(b)]. This frequency dependence becomes more significant at higher temperature. Similarly, the capacitance density of the VO2 control layer and the superlattice sample also decreased with increasing frequency (600 nF cm−2 to 300 nF cm−2 and 350 nF cm−2 to 225 nF cm−2 at 25 °C, respectively) (Fig. 9). This AC frequency-dependent permittivity evolution has also been noted in ceramic-polymer composites.50–53
Temperature-dependent (25 °C to 100 °C) capacitance density vs. AC frequency (C-F) plots of (a) the SiO2 control layer and (b) the nanocomposite. The inset figure shows the temperature-dependent evolution of ΔC C0.01 MHz C1MHz. The nanocomposite dielectric has substantial frequency dependence, which becomes more significant at a higher temperature range.
Temperature-dependent (25 °C to 100 °C) capacitance density vs. AC frequency (C-F) plots of (a) the SiO2 control layer and (b) the nanocomposite. The inset figure shows the temperature-dependent evolution of ΔC C0.01 MHz C1MHz. The nanocomposite dielectric has substantial frequency dependence, which becomes more significant at a higher temperature range.
Temperature-dependent (25 °C to 100 °C) capacitance density vs. frequency (C-F) plots of (a) the VO2 control layer and (b) the VO2/SiO2 multilayer.
Temperature-dependent (25 °C to 100 °C) capacitance density vs. frequency (C-F) plots of (a) the VO2 control layer and (b) the VO2/SiO2 multilayer.
The electronic properties of the materials comprising the composite are summarized in Table II.54 The frequency-dependent effective permittivity of the composite layer is shown in Fig. 10, which is much higher than that of the SiO2 control, especially at low frequency and high temperature (effective permittivity of 8.5 and 16 at 25 and 100 °C [at 104 Hz)].
Dielectric properties of VO2 and SiO2 at 1 MHz.
Component . | Dielectric constant . | Conductivity (Ω cm)−1 . | References . |
---|---|---|---|
SiO2 | 3.9 | 10−14 | 55 |
VO2 (after transition) | 6 × 104–1 × 105 | 2000 | 25 and 56 |
VO2 (before transition) | 36–40 | 5 | 56–58 |
Frequency-dependent dielectric constant of the nanocomposite and the SiO2 control at 25 °C and 100 °C, respectively. The nanocomposite shows an increased dielectric constant compared to the control due to the presence of VO2 inclusions. In our study, the value of the effective permittivity is limited by the thin silica layers sandwiching the composite film and may be further engineered in the future via atomic layer deposition (ALD) or similar ultra-thin conformal film deposition methods.
Frequency-dependent dielectric constant of the nanocomposite and the SiO2 control at 25 °C and 100 °C, respectively. The nanocomposite shows an increased dielectric constant compared to the control due to the presence of VO2 inclusions. In our study, the value of the effective permittivity is limited by the thin silica layers sandwiching the composite film and may be further engineered in the future via atomic layer deposition (ALD) or similar ultra-thin conformal film deposition methods.
Compared to SiO2, the enhanced dielectric constant of the composite has several potential explanations. First, the introduction of higher dielectric constant VO2 phase (compared to SiO2) may increase the effective dielectric constant of the medium, especially in the metallic state of vanadium dioxide.25 The second reason could be the enhanced interfacial polarization due to difference in electrical conductivity between SiO2 and VO2. Generally, in electrically heterogeneous composite materials, the large amount of charge accumulated at interfaces between the two different phases can result in an enhanced interfacial polarization density.59 This can be explained by the Maxwell–Wagner–Sillars (MWS) effect.60–63 The MWS mechanism is suited to explain the enhanced permittivity in composite dielectrics due to the different conduction properties in the two systems [like VO2 and SiO2 (Table II)]. The composite layer contains many SiO2/VO2 interfaces. By comparison, the VO2 control layer possesses only two interfaces with the top and bottom SiO2 layers and the superlattices (SiO2/VO2) have nine parallel planar interfaces. The largest number of VO2/SiO2 interfaces in the composite layer can qualitatively explain its most sensitive response to frequency change among the three types of structures (Table III). As temperature increases, the VO2 phase becomes metallic, leading to greater difference in conductivity between the two media, which further enhances the MWS effect.
Frequency dependent capacitance evolution of different dielectrics.
. | ΔC (C0.01 MHz-C1 MHz)/C1 MHz at 25 °C (%) . | ΔC (C0.01 MHz-C1 MHz)/C1 MHz at 100 °C (%) . |
---|---|---|
VO2-SiO2 composite | 100 | 120 |
SiO2 control | 0 | 0 |
VO2 control layer | 48.2 | 55 |
VO2/SiO2 multilayer | 54.5 | 73.5 |
. | ΔC (C0.01 MHz-C1 MHz)/C1 MHz at 25 °C (%) . | ΔC (C0.01 MHz-C1 MHz)/C1 MHz at 100 °C (%) . |
---|---|---|
VO2-SiO2 composite | 100 | 120 |
SiO2 control | 0 | 0 |
VO2 control layer | 48.2 | 55 |
VO2/SiO2 multilayer | 54.5 | 73.5 |
IV. CONCLUSIONS
We used co-sputtering to fabricate nanocomposite VO2-SiO2 thin films with a thermally tunable and frequency-dependent dielectric constant. The nanocomposite dielectric film is compared to a VO2 control layer and a VO2/SiO2 superlattice to systematically investigate the influence of structure and geometry on the resulting capacitance. The nanocomposite dielectric film shows smoothly tunable dielectric properties, resulting from the insulator-metal transition in VO2. While leakage through the VO2 in its conducting state will set the upper bound for acceptable loss in capacitors in eventual applications, further optimization of such tunable capacitors could be possible by careful spatial control of the inclusions, such as by lithographic patterning in an insulating matrix such as silica or alumina. The smoothly varying phase change behavior may also find applications in areas requiring continuous tunability of electrical, optical, or thermal properties across a range of temperatures.
ACKNOWLEDGMENTS
Y.S., K.N., and S.R. acknowledge the Office of Naval Research (No. N00014-16-1-2398) and the U.S. National Science Foundation (No. DMR-1609898) for the financial support. M.A.K. acknowledges support from the Office of Naval Research (No. N00014-16-1-2556). X.S. and H.W. acknowledge the support from the U.S. National Science Foundation (No. DMR-1565822) for the TEM work. S.M. and K.C. acknowledge Ke Wang and Shih-Ying Yu for their assistance and advice.