Terahertz spectroscopy studies on epitaxial vanadiumdioxide thin films across the metal-insulator transition

Pankaj Mandal,1 Andrew Speck,1 Changhyun Ko,2 and Shriram Ramanathan2,*1Rowland Institute at Harvard, Cambridge Massachusetts 02142, USA

2School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA*Corresponding author: shriram@seas.harvard.edu

Received December 13, 2010; revised April 13, 2011; accepted April 15, 2011;posted April 18, 2011 (Doc. ID 139292); published May 13, 2011

We present results on terahertz (THz) spectroscopy on epitaxial vanadium dioxide (VO2) films grown on sapphireacross the metal-insulator transition. X-ray diffraction indicates the VO2 film is highly oriented with the crystallo-graphic relationship: ð002Þfilm==ð0006Þsub and ½010�film==½2�1�10�sub. THz studies measuring the change in transmissionas a function of temperature demonstrate an 85% reduction in transmission as the thin film completes its phasetransition to the conducting phase, which is much greater than the previous observation on polycrystalline films.This indicates the crucial role of microstructure and phase homogeneity in influencing THz properties. © 2011Optical Society of AmericaOCIS codes: 300.6340, 000.2190, 300.6450, 160.3918.

Vanadium dioxide (VO2) experiences a sharp metal-insulator transition (MIT) near room temperatureaccompanied by impressive changes in the electricalconductivity [1], optical reflectance [2], and dielectricproperties [3]. In recent years, there has been tremendousinterest in exploiting such effects in novel solid statedevices, such as, but not limited to, electronic switches[4], temperature sensors [5], and metamaterials [6].Studies on functional materials in the THz frequency

range are growing rapidly. Anticipated applicationsinclude security and novel optoelectronic device technol-ogies [7]. Also, for exploring MIT phenomena in VO2, THztechniques are relevant, such as time-resolved opticalstudies and THz apertureless near-field optical micro-scopy experiments [8,9].The dielectric properties of VO2 thin films at high fre-

quencies have been investigated and the results wereanalyzed on the basis of effective medium theories(EMTs) considering the presence of a mixture of con-ducting and dielectric phases in the VO2 films [2,10]. Choiet al. conducted temperature-variable midinfrared spec-tra characterizations on VO2 thin films grown by pulsedlaser deposition on sapphire substrates [2]. The dielectricproperties were described by an EMT based on theBruggeman approximation that includes percolative be-havior [2,11]. Jepsen et al. investigated transmission inpolycrystalline VO2 films prepared from annealing amor-phous films by THz time-domain spectroscopy (THz-TDS), which enables monitoring high-frequency conduc-tivity without electrical contacts to the sample, and foundthat the THz response could be modeled with a Maxwell–Garnett (MG) EMT where the percolation aspect isexcluded [10,12].While there is great interest in understanding THz re-

sponse of VO2, to the best of our knowledge there are noprior THz-TDS studies on epitaxial VO2 films. It is knownthat the electrical properties of single crystalline VO2 canbe dramatically different from that of polycrystallinefilms or films with nonstoichiometric phases [13], how-ever, no such literature exists for THz spectroscopy.Here, we present THz-TDS studies on highly orientedVO2 films spanning the MIT phase boundary.

VO2 thin films of nominal thickness ∼200 nm weregrown by RF-sputtering from a VO2 target onto ∼500 μm-thick (0001) sapphire substrates. The sputtering gunpower was set as 270W and the chamber pressure andsubstrate temperature were controlled to be ∼10mTorrand 550 °C, respectively, during synthesis. The films werecharacterized by x-ray diffraction (XRD) 2θ − ω scans, φ-scans, rocking curve measurements, and electrical resis-tance measurements as a function of temperature in anenvironmental probe station.

THz-TDS studies were conducted on the samples as afunction of temperature using a custom-designed setup.THz-TDS relies on the generation and detection of ultra-short electromagnetic transients, which in our setup havepulse widths of less than 1ps, spectral content from 0.1 to1:0THz, and average power of 375 μW [14]. Femtosecondoptical pulses from an 80MHz KMLabs Chinook titanium:sapphire oscillator are split with 450mW being used togenerate the THz radiation by exciting a semi-insulatingGaAs photoconductive switch biased at 2kV=cm usingaluminum strip lines with a gap of 1mm. The remaining5mW is used to detect the THz radiation by measuringthe polarization rotation of the probe pulse from inducedbirefringence within a ZnTe crystal resulting from theelectric field of the incoming THz pulse. Off-axis para-bolic mirrors are used to focus the generated THzradiation onto the VO2 sample and then refocus the trans-mitted radiation back onto the ZnTe detector.

The VO2 sample was mounted in a holder whose tem-perature was controlled by a Peltier element heater andmonitored by a platinum resistance temperature detec-tor. To ensure a minimum temperature gradient acrossthe sample, the sample block was covered by expandedfoam insulation, which creates minimal absorption of theincoming THz radiation. To account for any temperatureeffects of the sapphire substrate, the sample holder holdsboth a sapphire substrate coated with a VO2 thin film aswell as a bare substrate. All data points are normalized tothe transmission through the bare substrate. The THz-TDS system measures the amplitude, A, and phase, δ,of the transmission function EVO2

=Eref ¼ A expðiδÞwhere Eref and EVO2

indicate the transmitted electric

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fields through the bare substrate and the VO2 thin filmsample, respectively. This function is linked to the dielec-tric functions of the VO2 thin film and the substrate, andcan be calculated analytically [15]. Reflections within thesubstrate are removed experimentally by stopping thetime delay scan before the reflected pulse arrivesfrom the sapphire substrate 10 ps after the nonreflectedpulse. The transmission function in this configuration isgiven by

EVO2=Eref ¼ð1þnsubstrateÞ=ð1þnsubstrateþZ0~σðωf ÞdÞ; ð1Þ

where ωf is the angular frequency, Z0 ¼ 377Ω is the vac-uum impedance, d is the thickness of the VO2 film, and~σ ¼ σ1 þ iσ2 ¼ −iε0ωf ðε − 1Þ is the complex conductivityof the VO2 thin film [16].Representative XRD 2θ − ω scan of the film shows re-

flections only from (002) and (004) planes of VO2 phaseas displayed in Fig. 1(a), indicating that the single phaseand epitaxial VO2 thin film with the c-axis perpendicularto the substrate surface was successfully grown on c-sapphire. In addition, as shown in the Fig. 1(a) inset,the full width at half-maximum (FWHM) of the rockingcurve was estimated as ∼0:03° at the (002) reflectionpeak verifying a high crystalline quality of the VO2 thinfilm. The XRD φ-scan displayed in Fig. 1(b) shows sixoff-axis reflections at Ψ ¼ 45° and 2θ ¼ 27°, correspond-ing to (011) orientation of VO2. This sixfold symmetry isdue to the presence of three orientation variants of VO2that are rotated in-plane by 60° about [020] direction ofthe VO2. These energetically equivalent orientations arisebecause of different symmetries of monoclinic VO2 andrhombohedral (0001) sapphire across the interface withthe orientation relationship: ð002Þfilm==ð0006Þsub and½010�film==½2�1�10�sub. Electrical measurements on the iden-tical film showed nearly 4 orders of magnitude resistancechange across the transition, indicating very high-qualityfilms. The onset temperatures for the transition were es-timated to be ∼68 °C and ∼64 °C during heating and cool-ing, respectively, consistent with typical observationson VO2.

THz-TDS data were collected as the temperature, T , isslowly increased and then decreased, as shown in Fig. 2.Part (a) shows that, as T increases, the transmitted elec-tric field directly measured by the ZnTe detector is re-duced through both reflection and absorption due tothe increased conductivity of the VO2 thin film. This datais then Fourier transformed and normalized to the baresubstrate at the same T . As expected, there are nochanges seen in the transmission through the bare sap-phire substrate as a function of T . Parts (b) and (c) showthe amplitude and phase of the transmission function,respectively. The transmitted amplitude is flat as a func-tion of frequency, ν, and decreases with increasing T asexpected; the phase is relatively insensitive to both T andν except for the peak at 0:1THz, which corresponds toreflections within the substrate and suggests that thetemporal cutoff of this reflection is not complete. Reflec-tions within the substrate also explain the increasedphase shift with T at 0:1THz as this reflection willbecome larger as the VO2 film becomes more conductive,increasing the amplitude of the pulses from multiplereflections.

Figure 3 shows the T -dependence of the averageconductivity calculated from 0.3 to 0:95THz. In compar-ison, previous results with polycrystalline films on a150 μm-thick glass by Jepsen et al. are shown as dashedcurves [10]. The highly oriented VO2 samples show muchgreater real conductivity but no susceptance change.

The much greater conductivity is likely due to the epi-taxial nature of the film. The blue and magenta curves inFig. 3 show the predictions of the MG and BruggemanEMTs for the VO2 dielectric constant as the film is driventhrough the phase transition [11,12]. At the T extremes,the film is assumed to be either entirely in the insulatingphase with ~ε ¼ 9 or in the metallic phase with dielectricfunction where ~ε ¼ ~ε

∞− ω2

p=½ωf ðωf þ iΓÞ� given by theDrude model. Here we take the high-frequency value,~ε∞, to be given by the insulating phase value, ω2

p ¼ne2=ε0m� is the plasma frequency, and the scattering rateis Γ ¼ e=m�μ. We have used previously measured valuesof these parameters as estimates [2]. In particular, the ef-fective mass ism� ¼ 2me, withme the free electron mass,the best fit carrier density is n ¼ 0:87 × 1022 cm−3, and thecarrier mobility is μ ¼ 2 cm2=V · s. The excellent agree-ment with the measured values at both the low and highT extremes indicates that these parameters describe thefilm components well. To calculate the behavior of thedielectric constant through the MIT, we have assumedthe microscopic domains can be replaced by an effectivemedium where the fractional metallic part is given byf ¼ 1 − f1þ exp½ðT − T0Þ=ΔT �g−1. For the BruggemanEMT, the best fit is given for T0 ¼ 70 °C and ΔT ¼6 °C, while for the MG EMT, T0 ¼ 41 °C and ΔT ¼ 6 °C.The significantly better agreement with the BruggemanEMT is not surprising as the MG EMT only models dilutemixtures properly. The agreement also suggests that thephase transition is continuous in order for it to be mod-eled properly by an EMT. In contrast the nanogranularfilm measured by Jepsen et al. may produce a non-Drude-like conductivity from the existence of metallic is-lands and, thus, show the observed significant change insusceptibility [10].

Fig. 1. (a) X-ray diffraction (XRD) 2θ − ω scan profile obtainedfrom the monoclinic VO2 film grown on the rhombohedral sap-phire (0001) substrate. The inset includes the rocking curvemeasured at the (002) reflection peak with FWHM value of∼0:03°. (b) XRD φ-scan at ψ ¼ 45° and 2θ ¼ 27° correspondingto (011) plane reflection in monoclinic VO2 phase.

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In summary, we have conducted THz-TDS on VO2 epi-taxial thin films as a function of temperature spanningthe MIT. From the 85% reduction in transmission acrossthe phase transition, our results indicate that the transi-tion to the metallic phase is much closer to completion inepitaxial films. The results could be of potential rele-vance in designing epitaxial phase transition oxides fornovel THz applications and nanophotonics.

We acknowledge the United States Air Force Office ofScientific Research (USAFOSR) grant FA9550-08-1-0203for financial support. We thank Gulgun Aydogdu-Kuru forassistance with the x-ray diffraction experiment.

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Fig. 3. (Color online) Conductivity of the VO2 thin film aver-aged from 0.3 to 0:95THz as a function of temperature duringheating (black symbols) and cooling (red symbols). The dashedbeige curves show the behavior of a polycrystalline thin filmmeasured previously by Jepsen et al. [10]. The blue and magen-ta curves show the predictions of the Bruggeman and MGeffective medium theories, respectively.

Fig. 2. (Color online) (a) THz signals transmitted through both the VO2 thin film and the sapphire substrate as a function oftemperature. The insets show the temporal and frequency profile of the THz pulse transmitted solely through the substrate.The T -dependence of the transmitted THz electric field amplitude (b) and phase shift (c) is shown relative to the transmissionof the bare substrate as a function of frequency. All data is normalized to the transmission through the thin film at 50 °C.

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