An apparatus for spatially resolved, temperature dependent reflectancemeasurements for identifying thermochromism in combinatorial thin film librariesS. C. Barron, M. P. Patel, Nam Nguyen, N. V. Nguyen, and M. L. Green Citation: Review of Scientific Instruments 86, 113903 (2015); doi: 10.1063/1.4935477 View online: http://dx.doi.org/10.1063/1.4935477 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/86/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Influence of doping with alkaline earth metals on the optical properties of thermochromic VO2 J. Appl. Phys. 117, 185301 (2015); 10.1063/1.4919433 Oxide-based chromogenic coatings and devices for energy efficient fenestration: Brief survey and update onthermochromics and electrochromics J. Vac. Sci. Technol. B 32, 060801 (2014); 10.1116/1.4896489 Thermochromic undoped and Mg-doped VO2 thin films and nanoparticles: Optical properties andperformance limits for energy efficient windows J. Appl. Phys. 115, 053513 (2014); 10.1063/1.4862930 Bandgap widening in thermochromic Mg-doped VO2 thin films: Quantitative data based on optical absorption Appl. Phys. Lett. 103, 161907 (2013); 10.1063/1.4826444 Effects of Ti – W codoping on the optical and electrical switching of vanadium dioxide thin films grown by areactive pulsed laser deposition Appl. Phys. Lett. 85, 1958 (2004); 10.1063/1.1788883

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REVIEW OF SCIENTIFIC INSTRUMENTS 86, 113903 (2015)

An apparatus for spatially resolved, temperature dependentreflectance measurements for identifying thermochromismin combinatorial thin film libraries

S. C. Barron,1,a) M. P. Patel,1,b) Nam Nguyen,1,b) N. V. Nguyen,2 and M. L. Green11Material Measurement Laboratory, National Institute of Standards and Technology,Gaithersburg, Maryland 20899, USA2Physical Measurement Laboratory, National Institute of Standards and Technology,Gaithersburg, Maryland 20899, USA

(Received 15 June 2015; accepted 28 October 2015; published online 18 November 2015)

A metrology and data analysis protocol is described for high throughput determination of ther-mochromic metal-insulator phase diagrams for lightly substituted VO2 thin films. The techniqueexploits the abrupt change in near infrared optical properties, measured in reflection, as an indicatorof the temperature- or impurity-driven metal-insulator transition. Transition metal impurities wereintroduced in a complementary combinatorial synthesis process for producing thin film libraries withthe general composition space V1−x−yMxM′yO2, with M and M′ being transition metals and x and yvarying continuously across the library. The measurement apparatus acquires reflectance spectra inthe visible or near infrared at arbitrarily many library locations, each with a unique film composition,at temperatures of 1 ◦C–85 ◦C. Data collection is rapid and automated; the measurement protocolis computer controlled to automate the collection of thousands of reflectance spectra, representinghundreds of film compositions at tens of different temperatures. A straightforward analysis algorithmis implemented to extract key information from the thousands of spectra such as near infraredthermochromic transition temperatures and regions of no thermochromic transition; similarly, reflec-tance to the visible spectrum generates key information for materials selection of smart windowmaterials. The thermochromic transition for 160 unique compositions on a thin film library with thegeneral formula V1−x−yMxM′yO2 can be measured and described in a single 20 h experiment. Theresulting impurity composition–temperature phase diagrams will contribute to the understanding ofmetal-insulator transitions in doped VO2 systems and to the development of thermochromic smartwindows. [http://dx.doi.org/10.1063/1.4935477]

I. INTRODUCTION

The development of functional property diagrams,namely, diagrams relating material structure, composition,temperature, and functional properties of interest, is a signifi-cant step in the commercialization of new materials. Property-phase diagram generation is a labor and resource intensivetask to identify with precision the temperature and composi-tion of phase boundaries and the properties within the phaseregion and therefore is a materials research problem partic-ularly well suited to the strengths of thin film combinato-rial science because the continuously varying compositionspermit probing phase boundaries with high compositionalresolution. Numerous examples of combinatorially investi-gated phase diagrams in functional thin film materials havebeen reported.1–7

The metal-insulator transition and concomitant tetrag-onal-monoclinic structural transition in VO2 have been ofsignificant interest since its discovery.8 Because of the tran-sition’s accessible temperature (68 ◦C) and the profound andabrupt shift in functional properties, VO2 has the potential for

a)Author to whom correspondence should be addressed. Electronic mail:sara.barron@nist.gov.

b)M. P. Patel and N. Nguyen contributed equally to this work.

widespread use in applications such as bolometers,9 opticalswitches,10 and thermochromic smart windows.11–13 Phasediagrams have been reported based on temperature,14–16

strain,15,17 electron density,18,19 oxygen stoichiometry,20 andchemical impurities.14,21 The transition has been observeddirectly and indirectly, inferring the structural or electronictransition from calorimetry22 or abrupt changes in measure-ments of electrical conductivity,8,14,15,23,24 near infrared (NIR)optical constants,23–28 magnetic susceptibility,8,14 diffrac-tion,14 or Raman shifts.19,25,29

While depositing thin films with continuously varyingcomposition gradients, for example, from multiple chemicallydistinct magnetron sputter sources, has long been possible,30,31

combinatorial synthesis has recently benefitted from progressin inkjet printing32 and predictive models for combinatorialphysical vapor depositions33,34 that increase the capabilityfor design of libraries with the desired composition space.Throughput-matched metrologies in recent years include bothgeneral use tools (structure, composition, etc.),35 as well asthose that are driven by a desired materials function or applica-tion, such as electronic and optical properties,36–38 thermoelec-tric properties,39–41 or catalytic activity for electrochemicalreactions.42,43 Advanced data mining tools44–47 can increasethe throughput and type of knowledge gained from theseexperiments.

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In this paper, we describe a high throughput non-contactmetrology for developing temperature-composition phase di-agrams of the thermochromic transition in combinatorial thinfilm libraries of lightly substituted VO2. The metrology uti-lizes the abrupt and significant change in NIR optical con-stants, measured in reflection, as an indicator of the metal-insulator and structural phase transition. The phase diagramsproduced with the apparatus described here contribute tounderstanding the factors driving the VO2-type transition indoped systems.

The optical measurements made, both in the visible andNIR, are most applicable to the use of substituted VO2 mate-rials as thermochromic window coatings. These energy-savingsmart windows dynamically adjust their transparency to NIRsolar radiation based on ambient temperature, with designcriteria such as the temperature, width, and magnitude of theNIR thermochromic transition as well as maintaining hightransparency to visible radiation.11 Though pure VO2 exhibitsthe desired transition in NIR optical properties, it does so at atemperature too high (68 ◦C) for window applications, and so,transition metal impurities such as W and Mo at cation substi-tution levels of <5% are used to depress the transition temper-ature to 20-30 ◦C.11–13 Other impurities, for example, Mg, maybe used to enhance transparency in the visible range.27,48

II. LIBRARY SAMPLE PREPARATION

Thin film combinatorial libraries were deposited bypulsed laser deposition (PLD) using a technique previouslydescribed for our system.49,50 Briefly, a KrF laser (248 nm) waspulsed at ceramic targets of V2O5, WO3, CaO, and/or Nb2O5,cyclically and in rapid succession with deposition from eachtarget centered on a different part of the 76.2 mm diametersubstrate. The thin film libraries for this study were in thegeneral composition space V1−xNbxO2 and V1−x−yWxCayO2,with x and y varying continuously with position on the library.Substrates were single-side polished p-type silicon wafers(1015–1018 cm−3), with the native silicon dioxide layer. Duringdeposition, they were radiatively heated to a substrate temper-ature of 530 ◦C, and films were deposited in a partial oxygenpressure of 0.7 Pa.

The oxide film libraries deposited in this way have a lateralcation composition gradient in the range of 0.1%/mm–3%/mmover the cation composition range of greater than 99% V to50% V,49,50 with a coincident gradient in film thickness. Themaximum film thickness of approximately 200–300 nm wasin the most V-rich region of each library. In this V-rich regionof the libraries, the film microstructure was polycrystallineVO2 with no strong crystallographic texture.50 In analyzing thethermochromic behavior, we assume that the library films aresmooth and that there are no nanostructural features that affectmeasurements of the thermochromic transition.

To map the varying film compositions on the library,a measurement appropriate to the materials system must bemade at many locations on the library; this could be x-rayfluorescence (XRF), energy dispersive spectroscopy (EDS)or wavelength dispersive spectroscopy (WDS), or x-rayphotoelectron spectroscopy (XPS). XPS has been used to find

the ratio of metal cations for selected locations on theV1−x−yWxCayO2 library, while the V1−xNbxO2 library hasbeen mapped more thoroughly, with the details and resultspresented elsewhere.50

III. INSTRUMENT ASSEMBLY AND VALIDATION

A. Apparatus

We have assembled a high-throughput measurement toolfor characterizing thermochromic transitions in VO2-basedthin films. Analysis of the thermochromic behavior data isautomated and rapid, implemented in a custom MATLABscript. The tool collects reflectance spectra at numerous arbi-trary locations across a combinatorial thin film library whileit is held at temperatures between 1 ◦C and 85 ◦C. A similarinstrument was developed by researchers for constructingphase diagrams of Mott insulators in the La1−xSrxMnO3 sys-tem;1 our system has been designed to provide informationof greater applicability to smart windows, including ther-mochromism at visible and near infrared frequencies.

Figures 1(a) and 1(b) are digital photographs of the assem-bled apparatus, comprising a thermoelectric heater/chiller, anx-y translation stage, a near infrared spectrometer, a whitelight source, an optical fiber bundle designed for reflectance,and an enclosure for atmospheric control. Each component62

is described in further detail below. The operation of eachcomponent is controlled by LabVIEW subprograms providedby its manufacturer. The coordination among the components

FIG. 1. The temperature-controlled, near infrared reflectance apparatus.(a) The library sample rests on the thermoelectric heater/chiller, which ismounted to an x-y translation stage on the optical table. A fiber bundlereflectance probe is held stationary and normal to the sample plane. Com-ponents such as the NIR spectrometer and white light source are also insidethe moisture-controlled enclosure. (b) A close-up of the reflectance probeholder and the heater plate.

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and data storage is then automated in an envelope LabVIEWprogram.

The measurement protocol has three primary steps: (1)ramp to and stabilize at a desired sample temperature; (2) in-plane translation of the thin film library to numerous differentlocations below a stationary reflectance probe; and (3) at eachlocation, measure the infrared reflectance. In this way, a datasetis built up of the infrared reflectance as function of temperatureand sample location. Since the infrared reflectance is charac-teristic of the crystalline phase, the resulting data representa series of composition-phase diagrams at different tempera-tures, or with some analysis, the thermochromic phase transi-tion characteristics (e.g., transition temperature, hysteresis) asa function of elemental composition.

We note that in order to achieve high-throughput datacollection on this instrument, we evaluate thermochromicbehavior from reflection measurements only. This is in contrastto many studies of VO2 thermochromism in which a VO2 thinfilm is prepared on an IR transparent substrate and measure-ments are made of its infrared transmission and, sometimes,reflection.12,25,26,51 These “one-off” experiments requirepreparing films on small substrates which are individuallyheated and measured in a commercial spectrophotometer.Often these instruments lack the necessary capabilities toaccommodate a 76.2 mm combinatorial library and to auto-mate the collection of many localized measurements. To makethese accommodations in our high-throughput instrument, welimit optical access to one side of the substrate and hencemeasure only in reflection.

The thermoelectric heater/chiller is a TECMount 284 andis controlled by a TECSource 5 A/12 V power supply, bothfrom Arroyo Instruments (San Luis Obispo, CA). The coldplate has lateral dimensions of 90 mm × 90 mm. To ensureconsistent substrate placement, an L-shaped copper brace wasmachined and mounted on two edges of the cold plate. The twoflats of the silicon substrate are placed against the L-shapedbrace, and a copper clip holds the exposed corner, as is shownin Fig. 1(b) and schematically in the inset to Fig. 2(b). Thetemperature of the cold plate is reported from the thermistorembedded within it.

The heater/chiller was mounted on the x-y translationstage, which was mounted to the optical table (breadboard).The translation stage (Griffin Motion, Holly Springs, NC)allows 100 mm of travel in two directions in a plane parallel tothe optical table surface and to the cold plate. It is controlledand powered by a stepper motor controller (BSC 102, Thor-Labs, Newton, NJ).

A fiber-optic coupled NIR spectrometer with a detectionrange of 900–2500 nm (NIRQuest 256-2.5, Ocean Optics,Dunedin, FL) was employed. The reflectance probe is heldstationary, normal to and approximately 2 mm from the sam-ple. It is mounted to the optical table and cantilevered overthe plane in which the sample translates with standard opticalcomponents. A fiber bundle probe from Ocean Optics (R200-7-VIS-NIR) contains a center fiber that returns reflected lightto the spectrometer and six surrounding fibers that deliverradiation from the white light source (HL-2000 or LS-1, OceanOptics). Each fiber is 0.2 mm diameter. The measurementarea is estimated to be 1.5 mm2. The integration time for the

FIG. 2. The temperature of the thermoelectric plate, as reported by theembedded thermistor and thermocouples (TC) pasted in the center and edgeof bare silicon substrate. (a) The temperature of the sample surface and ofthe embedded thermistor as the heater is ramped to a new setpoint. (b) Thedifference between the temperature of the surface thermocouples and theinternal temperature. Inset: the placement of the thermocouples.

detector is 1 ms, and an average of ten integrations is used foreach library location.

To prevent moisture condensation on the library wa-fers at temperatures as low as 1 ◦C, the apparatus was en-closed in a PlexiGlass box with flowing nitrogen-enrichedair (Figure 1(a)). The dimensions of the enclosure are 94 cm× 48 cm × 64 cm with a volume of 0.29 m3. Nitrogen-enrichedair from a Balston 75-72 nitrogen generator flowed into theenclosure from a green tube of 0.64 mm (0.25 in.) diameteron the left of Fig. 1(a) at a rate of about 0.37 m3/h (13standard ft3/h). The nitrogen generator filtered the compressedair (supplied at 0.7 MPa (100 psi) from the building source)to nominally 99% pure N2. After 15 min of flow, the airinside the enclosure was oxygen deficient and low humidity,as a noted by an Altair O2 sensor and a household Acuritehumidity sensor, respectively. There was no apparent watercondensation on the cold plate at temperatures as low as 1 ◦Cfor up to 30 min.

B. Temperature calibration

To evaluate the difference between the temperature re-ported by the thermistor and the surface temperature ofa thin film library, fine gauge (80 µm diameter) K typethermocouples (TCs) were pasted to a 76.2 mm diameter

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bare silicon wafer and monitored by a handheld thermocouplereader, while the wafer was heated and cooled through a seriesof temperature steps. Simultaneously, the internal thermistortemperature was recorded every 0.2 s. Several temperaturesteps are shown for two separate calibration runs in Figure 2(a).The magnitude of each step was in the range of 2 ◦C–4 ◦C, andthe temperature was allowed to stabilize for 120 s–600 s, whilethe surface temperatures were recorded.

The calibration runs in Figure 2(a) differed in both theheater control (proportional-integral-derivative, or PID) set-tings and the placement of the surface thermocouples. In anearly study using this instrument,50 prior to optimizing the PIDsettings, the heater overshot the setpoint temperature by up toabout 0.6 ◦C for up to 100 s. After optimizing the PID settings,the temperature overshoot was less than 0.15 ◦C. In bothcases, the surface temperatures recorded at 120 s after settinga new setpoint were essentially identical. (During reflectancemeasurements for thermochromic behavior, the temperaturewas allowed to settle for 180 s after reaching each new set-point.)

Five calibration runs were performed over different rangesof heater temperature. In three, the thermocouples were pastedto the surface with an alumina epoxy in the center of the siliconwafer and on the edge (B) of the wafer close to the minor flat,as shown in Figure 2(b) inset. In the others, the thermocoupleswere attached with silver paste to the center of the wafer andto the other edge (A).

In Figure 2(b), the average surface temperatures are re-ported over the full range of the heater operation with errorbars given as the standard deviation of 4-8 measurements madeover two or three calibration runs. As expected, we observedsurface temperatures higher than the internal plate temperatureas the plate was cooled below ambient temperature (22 ± 2 ◦C),and lower surface temperatures for heater setpoints greaterthan ambient temperature. Further, the difference between theinternal temperature and the surface temperatures and betweenthe two surface temperatures increased as the difference be-tween the heater temperature and the ambient temperatureincreased. At the highest temperatures evaluated (heater set-point of 83.5 ◦C), the surface temperature at the center of thewafer was 0.8 ± 0.1 ◦C cooler and at edge B was 1.1 ± 0.1 ◦Ccooler.

A linear fit was made between the surface temperatures(Tsurf) and the temperature recorded by the internal thermistor(Tint),

Tsurf = 0.98 × Tint + 0.63 ◦C. (1)

For thermochromic reflectance measurements, only thetemperature of the internal thermistor was recorded automat-ically; the surface (library) temperature was then calculatedusing this linear fit.

As an alternative to the linear fit, a two dimensional modelcould be made to account for the spatial variation in surfacetemperatures. However, the correction is quite limited; at thehighest setpoint temperatures (84 ◦C), the spatial variation insurface temperatures across the wafer is at most 0.3 ◦C, asseen in Fig. 2(b), while the difference between the surfacetemperature reported by the linear model here and the twodimensional model is at most 0.12 ◦C. Instead, we account

for this variation by reporting surface temperatures with anuncertainty of ±0.2 ◦C.

C. Normalization of the spectral reflectance

The reflectance spectra were normalized to the reflectancefrom a Au film deposited on a silicon wafer, according theequation

R =I − Idark

IAu − Idark, (2)

where I was the raw reflectance spectrum reported by thespectrometer, IAu was the raw reflectance from the Au, andIdark was the spectrum reported when the light source wascut off. Gold was chosen for its high reflectivity in the nearinfrared. The optical fiber bundle reflectance probe collectsmostly specularly reflected light, and no attempt was made,e.g., via an integrating sphere, to collect diffusely scatteredlight.

We note that the thin film libraries were deposited onsilicon substrates, and that silicon is partially transparent inthe near infrared, with both a transmittance and a reflectanceof about 40% for a single-side polished silicon wafer.52 Thelightly substituted VO2 based films of our study also havea partial but temperature-dependent transparency in the nearinfrared. Hence, the reflectances reported by our technique arenot characteristic solely of the VO2 based film, but rather of theensemble of the VO2 based film and the silicon substrate. Nev-ertheless, temperature-dependent changes in the NIR reflec-tance are attributed to changes in the optical properties of thefilm, as the reflectance from a bare silicon wafer did not changewith temperature up to 85 ◦C.

To validate our metrology for characterizing the ther-mochromic transition in lightly substituted VO2 films, selectedreflectance spectra from a pure VO2 film of about 280 nmthickness are presented in Figure 3(a), along with the reflec-tance spectrum from a bare silicon substrate at 22 ± 2 ◦C. Thesilicon reflectance was relatively constant at about 35% acrossthe full wavelength range of the detector.

In contrast, the reflectance from the VO2 film on sili-con increased substantially between 63.4 ± 0.2 ◦C and 79.0± 0.2 ◦C during heating. For finer details, Figure 3(b) displaysthe reflectance spectrum at each of the 41 temperature stepsthis film experienced. The spectra show an increase in reflec-tance across a broad range of near infrared wavelengths, fromroughly 1500 nm–2400 nm.

Given the large number of reflectance spectra in thecombinatorial libraries, it is desirable to decrease the dimen-sionality of the data for automation of data analysis and ease ofvisualization. In Figure 3(c), we plot the transition temperatureon heating and on cooling derived using the sigmoidal fit anal-ysis described below from reflectances at 1064 nm, 1550 nm,1700 nm, 2000 nm, 2200 nm, and 2500 nm, representing twowidely available NIR lasers and wavelengths that have beenused in the thermochromic literature.13,21,24–26,50,53 For ther-mochromic smart window applications, we additionally plotthe transition temperatures derived from an average reflectanceweighted by the solar irradiation spectrum on the surface ofthe earth.23,54 There was some hysteresis in the thermochromic

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FIG. 3. (a) The NIR reflectance spectra of a bare silicon substrate at 22±2 ◦Cand of a VO2 film with a thickness of 280 nm at selected temperatures onheating. (b) The NIR reflectance spectra for the VO2 film at all 41 temper-ature steps on heating and cooling, with reflectance given by color. (c) Thetransition temperatures on heating and cooling, derived from a sigmoidal fitto the reflectance at six different wavelengths and for an average reflectanceweighted by the solar radiation spectrum. Error bars represent the 95%confidence intervals to the fit parameter. (d) The reflectance at 1700 nm isplotted as a function of temperature, with a dumbbell to indicate the inferredtransition temperatures on heating and cooling.

transition seen here, with the transition on heating occurringat 5 ◦C higher than the transition on cooling. The error barson this plot represent the 95% confidence intervals in the fitparameter for the sigmoidal fit described in Sec. IV; they arein the range of ±0.4 ◦C to ±1 ◦C.

With the exception of the derived transition temperatureat 1064 nm, a wavelength at which there is very little differ-ence in NIR reflectance across the transition, the transitiontemperatures are relatively constant, but do decrease by 2 ◦C

for heating and by 1 ◦C for cooling with increasing wavelength.Further analysis on a greater number of samples will benecessary to determine whether this effect is more general andstatistically significant. When the reflectance spectra are aver-aged by weighting to the solar irradiation spectrum, the derivedtransition temperatures are sensitive to the smaller reflec-tance changes occurring at lower frequencies (∼1000 nm)because of the greater solar radiation at these and visiblefrequencies.

In this paper, as a compromise between the greater solarradiation at lower wavelengths and the greater heating asso-ciated with higher wavelengths, we represent the reflectancespectra by the reflectances at wavelengths of 1700 nm. Atthis wavelength, the transition occurred at 74.1 ± 0.4 ◦C onheating and at 69.2 ± 0.6 ◦C on cooling, which is in reasonableagreement with the transition temperature of 68 ◦C reported forsingle crystal and polycrystalline films of VO2.8,11 In Fig. 3(d),for the same VO2 film, the reflectance at 1700 nm is plotted ateach of the 41 temperatures measured in the heating-coolingcycle.

The abrupt change in near infrared reflectance is in-terpreted as evidence of the metal-insulator transition andconcomitant structural phase transformation and can be usedas a low cost, non-contact surrogate measurement of thetransition temperature and thermal hysteresis. We note thatour tool’s large spot size (mm2) and long temperature ramp(seconds) render it insensitive to phase co-existence or tran-sient features of the first order VO2 phase transformationthat have been observed using techniques with better spatial,temporal, and/or thermal resolution.16,55,56 Instead our interestis in thermochromic window applications and the impurityconcentrations that effect the desired thermochromic responseat application-specified temperatures. Transient features of thetransition event have limited impact on the present work.

IV. APPLICATION TO COMBINATORIAL THINFILM LIBRARIES

A. Measurements and analysis of near infraredthermochromic behavior

For combinatorial libraries, reflectance spectra arecollected at different library locations, each with a unique filmcomposition, and at many temperatures. Figure 4 displays the7520 reflectance spectra collected from a thin film library ofthe composition space V1−x−yWxCayO2, with the magnitude ofthe reflectance given by color. As represented in Figure 4(a),for this library, 160 library locations were measured in a snake-like pattern at each temperature, and then the temperature wasstepped to the next setpoint temperature over 47 temperaturesteps. The total time to collect the 7520 spectra was 19.8 h.

In Figure 4(b), the reflectance spectra are displayed inthe order in which they were collected, starting at the lowesttemperature (16.3 ± 0.2 ◦C) (bottom), heating to 82.7 ± 0.2 ◦C,and then cooling again (top). At each of the 47 measure-ment temperatures, reflectance spectra were collected at 160different library locations, each representing a unique filmcomposition. As the library was heated, some regionsof the library sample, i.e., some regions of the represented

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FIG. 4. (a) Schematics of the measurement protocol: the snake-like path ofthe translation stage to measure 160 locations on the combinatorial libraryat each measurement temperature, and the temperature ramp in which 160spectra are collected before the temperature was stepped to the next setpoint.(b) The 7520 NIR reflectance spectra from a V1−x−yWxCayO2 library film,in the order in which the spectra were collected, with the magnitude of thereflectance given as color. (c) A close up of the first 480 spectra collected.(d) The 47 reflectance spectra collected on heating and cooling for the librarylocation x= 53 mm, y= 35 mm.

composition space, transitioned from a low to a high reflec-tance state. At increasing temperatures, more and more ofthe library film transitioned to the high reflectance state. Thetransition was reversible, with the low reflectance returning asthe library was cooled to 16.3 ± 0.2 ◦C.

To clarify the measurement protocol, Figure 4(c) zooms inon the reflectance spectra taken at the first three measurementtemperatures. Here, we can see that even at a uniform temper-ature, 16.3 ± 0.2 ◦C, e.g., the 160 different library locationsexhibit different levels of reflectivity. This is because eachlibrary location has a different film composition and film thick-ness. These same 160 library locations were probed at 18.3± 0.2 ◦C, and then at 20.2 ± 0.2 ◦C, with no discernible change

in reflectance with temperature at these low temperatures.However, the variations within the library give rise to the repeatpattern with a periodicity of 160 spectra per temperature whichis seen in Fig. 4(b). (Because of the NIR transparency of the sil-icon substrate, we do not measure absolute reflectance; rather,we analyze the temperature-dependent change in reflectanceat each location to determine the transition temperatures.)

Each measurement location (unique film composition)was evaluated independently for a thermochromic transition.Figure 4(d) displays only the 47 spectra that were collectedat the location x = 53 mm, y = −35 mm, at which a ther-mochromic transition was observed at 52.5 ± 1.1 ◦C duringheating and 49.3 ± 1.2 ◦C during cooling. The shape of thereflectance spectra is similar to those for pure VO2 in Fig-ure 3(b), but with a transition at a lower temperature.

Figure 5 includes a photograph of the V1−x−yWxCayO2library film and five maps of the reflectance at 1700 nm atdifferent temperatures. The library film was deposited fromthree unary metal oxide targets, with the center of each target’sdeposition plume marked on the photograph; interferencefringes from the spatial variation in film thickness give aqualitative indicator of the relative deposition rates, with thehighest deposition from the vanadium oxide target, leading toa VO2-rich film. The region with the highest CaO depositionhad greater visible transparency. The ratio of metal cationswas measured by XPS at three locations, labeled F, G, andH, on the first reflectance map, with compositions rangingfrom the most V-rich (location G: V0.929Ca0.066W0.004O2−δ) tothe more W-rich (location F: V0.850Ca0.123W0.028O2−δ) and themore Ca-rich (location H: V0.829Ca0.166W0.005O2−δ), in whichthe oxygen stoichiometry is assumed to be approximately2 because of the presence of the VO2-type thermochromictransition. In each reflectance map, the magnitude of thereflectance at 1700 nm is represented both by the color and thesize of the marker, and the sample temperature is in the lowerleft corner. An animation of reflectance maps for this library,at all 47 measurement temperatures, is available online as thesupplementary material.57

We found that the lowest transition temperatures werefound in the region with moderate W doping and display anoticeable increase in reflectance by 44.0 ± 0.2 ◦C (see, forexample, the measurement location x = 43 mm, y = −40 mm).In contrast, the region with moderate Ca doping (for example,x = 83 mm, y = −43 mm) did not transition until highertemperatures. When the library was heated to 59.4 ± 0.2 ◦C, aregion between the VO2 and WO3 depositions had transitionedto the high reflectance state. By 82.7 ± 0.2 ◦C, approximately46% (21 cm2) of the library, composed of the most VO2-richregions, had transitioned to a more highly reflective state. Thetransition was largely reversible; after the 19.8 h measure-ment when the library was cooled to the initial temperatureof 16.7 ± 0.2 ◦C, the reflectances returned to within 5.4%of their initial measured reflectance. (The reflectances wereuniformly higher in the initial measurement with a medianreflectance decrease of 2.8%, even in regions of the film thatdid not exhibit a thermochromic transition (data availablein the supplementary material57). The apparent decrease inreflectance may be from a decrease in the intensity of the lightsource over the 19.8 h measurement. In future studies, this

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FIG. 5. (Top left) Photograph of a V1−x−yWxCayO2 combinatorial thin film library, deposited on a 76.2 mm diameter silicon wafer. (Others) The reflectanceof light at a wavelength of 1700 nm from the library is represented by the color and size of the marker. The sample temperature during measurement isgiven in the lower left hand corner. The compositions at locations labeled F, G, and H on the first 16 ◦C reflectance map were V0.850Ca0.123W0.028O2−δ (F),V0.929Ca0.066W0.004O2−δ (G), and V0.829Ca0.166W0.005O2−δ (H).57 (Multimedia view) [URL: http://dx.doi.org/10.1063/1.4935477.1]

decrease could be compensated for by a change in protocol thatcalls for remeasuring the reference Au film at each temperaturesetpoint.)

To determine the transition temperature associated witheach measurement location in a high throughput and repeat-able fashion, we implemented a simple analysis protocol inMATLAB. First, we plotted the reflectance at 1700 nm versusthe temperature as the library went through a thermal cycle ofheating and then cooling, as seen for a few different librarylocations (film compositions) in Figure 6.

Second, to identify the characteristic temperatures,sigmoidal fits of the following form were made to both theheating and cooling halves of the thermal cycle and superim-posed on the curves in Fig. 6:

R = R1 +(R2 − R1)

(1 + 10m(T−To)) , (3)

where R is the NIR reflectance at 1700 nm; T is the measure-ment temperature; and R1, R2, To, and m are fitting parametersthat can be understood, respectively, as the reflectance at lowtemperature, the reflectance at high temperature, the tempera-ture of the inflection point, and parameter related to the slope atthe inflection point. The fit parameters were found with a leastsquares estimation implemented in MATLAB. The sharpnessof the transition, defined as the full width at half maximum(FWHM) of the first derivative of the heating curve,58 wasalso calculated from the first derivative of Equation (3) andthe fit parameters. The resulting To coefficients were definedas the transition temperatures on heating or cooling, with theaverage defined as the transition temperature. To account for

the uncertainty in the To fit parameters (up to ±1.2 ◦C) andthe variation in surface temperatures (up to ±0.2 ◦C), com-posite transition temperatures are reported with a combineduncertainty of ±1.7 ◦C. For each of the four film locations inFig. 6, the transition temperatures on heating and cooling wererepresented as dumbbells above the reflectance vs. T curve.

The transition temperatures for the four film locationsin Fig. 6 were 65.2 ± 1.7 ◦C, 59.5 ± 1.7 ◦C, 50.9 ± 1.7 ◦C,and 37.2 ± 1.7 ◦C, representing a transition temperature

FIG. 6. For the four locations (compositions) on the V1−x−yWxCayO2 libraryfilm, the reflectance at 1700 nm as a function of temperature. The sigmoidalfits described in the text are superimposed, with a fine solid line (red) forthe reflectance on heating and a fine dotted line (red) for the reflectanceon cooling. The dumbbells above the reflectance loops denote the inferredtransition temperatures, on heating and cooling, for each of the four locations.

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depression of more than 30 ◦C from the undoped polycrys-talline VO2. The library location with the highest transitiontemperatures was in the region with the highest deposition rateof VO2 and hence had the lowest dopant concentration, whilelocations further from the V oxide deposition center had higherdopant concentrations and transition at lower temperatures.We also found that the compositions with the highest transitiontemperatures also had the sharpest transitions, occurring overthe smallest temperature range, while the transition at 37.2± 1.7 ◦C occurred gradually over a 29 ◦C range (i.e., a 29 ◦Csharpness).

Some regions of the film do not exhibit a thermochromictransition, for example, the lower left corner of Fig. 5 in whichthe reflectance does not change appreciably with temperature.The hysteresis curves for this region are not well-fit by Equa-tion (3) and are excluded from further analysis. Within theMATLAB script, such sample locations were identified usingcriteria such as high temperature reflectances that are less than10% greater than the low temperature reflectances; derivedtransition temperatures outside the bounds of the measurementtemperatures; or differences of greater than 50 ◦C between thederived transition temperatures on heating and on cooling.

Figure 7(a) presents the transition temperatures derivedfrom the sigmoidal fit analysis for the library whose reflectanceis given in Fig. 5. Regions of the film that did not exhibit athermochromic transition as defined above are represented byopen circles and occurred in regions of the highest CaO andWO3 concentrations. In contrast, thermochromic transitionswere observed in the region of the library with the highest VO2concentrations. This is expected since within this compositionspace, thermochromic behavior is only observed in VO2-typematerials. The highest transition temperatures were 65 ◦Cand were observed in the region of the film furthest from Wdeposition. Transition temperatures decreased continuously asthe measurement location moved closer to the center of WO3deposition plume and to a less extent to the center of CaOdeposition plume.

Tungsten impurities are known to depress the VO2 tran-sition temperature at the lowest concentrations among thosetransition metals that have been explored, at efficiencies of upto 23 ◦C/at. % W.21 Calcium, in contrast, depresses the transi-tion temperature at rates of only 0.5 ◦C/at. % Ca but increasestransparency in the visible range,48 similar to the effect of Mgimpurities.27,28 Qualitatively, these reports are in agreementwith the thermochromic library measurements here that theW doping was more effective at depressing the VO2 transitiontemperature than Ca, with observed transition temperaturesof 20–30 ◦C for regions of the library with intermediate Wdoping.

The difference between the inferred transition temper-atures on heating and cooling is shown in Fig. 7(b). Thisdifference can be interpreted as the width of the hysteresis ofthe thermochromic behavior. For this library, this differencewas usually less than 5 ◦C, but increased to about 10 ◦C at thehighest W dopant concentrations (lowest transition tempera-tures). The sharpness of the transition during heating is plottedin Fig. 7(c) and indicates the temperature range over which thetransition occurs. The sharpness in most of the thermochromicregion was in the range of 8 ◦C–11 ◦C, but increased to 35 ◦Cin those regions with the highest W dopant concentrationsand lowest transition temperatures. The shift to more gradualtransitions with higher dopant concentrations is in agreementwith the curves in Fig. 6 and those observed by other re-searchers,12–14,51 but may also be influenced by the crystal-linity (grain size and orientation).58

B. Measurements of thermochromic behaviorin the visible spectrum

Finally, for thermochromic window applications, it isvaluable to characterize thermochromic behavior in the lumi-nous (visible) spectrum, since temperature-driven or im-purity-driven changes to visible light transmission willaffect the aesthetic appeal of the window coating.11 Though the

FIG. 7. For 160 locations on the V1−x−yWxCayO2 combinatorial library in Figure 5, the (a) transition temperature, (b) hysteresis width, and (c) the sharpnessof the transition during heating derived from the sigmoidal fit of Equation (3). Open circles indicate regions of the combinatorial library that did not exhibit athermochromic transition.

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metrology tool we developed does not allow transmissionmeasurements, reflectance spectra in the visible spectrum canclarify whether a thermochromic transition that affects opticalproperties in the near infrared has a similar effect in the visible.

For these measurements, the spectrometer is switchedout to a UV-VIS-NIR detector (USB 2000, Ocean Optics).Because of the greater sensitivity of the detector, a less intenselight source (LS-1, Ocean Optics) was used with a neutraldensity filter.

Figure 8 displays the reflectance at four temperatures ofa V0.96Nb0.04O2 film deposited on silicon. In Fig. 8(a), thewavelength range of the detector was 400 nm–950 nm, whilereflectances from the same film at similar temperatures fromthe NIR spectrometer are displayed in Fig. 8(b). Because theAu film used to normalize the NIR spectra in Fig. 8(b) istransparent at wavelengths less than approximately 520 nm,it cannot be used to normalize the lower wavelength spectrain Fig. 8(a). Instead, the reflectances at visible wavelengths(Fig. 8(a)) were normalized to the reflectance from a baresilicon substrate. However, the silicon was much less reflectivethan gold at nearly all wavelengths, and hence, the reflectancein Figs. 8(a) and 8(b) is not directly quantitatively comparable.Rather, the NIR spectra in Fig. 8(b) are presented only todemonstrate that the NIR thermochromic behavior in this Nb-doped VO2 is similar to that in the pure VO2 and the VO2 co-doped with W and Ca in Figures 3 and 4, respectively.

For this film with a thickness of 140 ± 20 nm, spectro-scopic reflectometry in the visible displays interference fringesassociated with the film thickness and complex indices of

FIG. 8. Spectrographic reflectance from a V0.96Nb0.04O2 film while heatingat several temperatures in (a) visible frequencies and (b) near infrared fre-quencies. Note that the spectra in (a) are normalized to a bare silicon wafer,while the reflectance in (b) is normalized to a Au film on silicon.

refraction. From analysis of the NIR reflectances, this filmunderwent a thermochromic transition at 47.1 ± 1.7 ◦C, andit is evident that in addition to the NIR thermochromism, thetwo VO2 phases also differ in their complex optical constantsin the visible.

Though the type of visible light reflectometry displayedin Fig. 8(a) does not permit quantitative assessment of un-known optical constants, n + ik, we can make some qualitativecomparisons between the low temperature and high tempera-ture phases. To start, the high temperature reflectance spectraappear to exhibit a greater number of interference fringes(Fresnel reflections) within the visible frequencies (Fig. 8(a))than the low temperature spectra, indicating that in this spectralrange, the high temperature refractive index n may be lowerthan that for the low temperature phase. Other researchers havemade similar findings for undoped VO2,28,59 but the resultsfor doped VO2 have greater variation.28 These reports alsofound that the extinction coefficient k was higher for the hightemperature metallic phase,28,59 but our reflectometry resultsare not conclusive on this point.

The optical constants of the film at 22 ± 2 ◦C were derivedfrom spectroscopic ellipsometry measurements performed ona vacuum ultraviolet variable angle spectroscopic ellipsometer(VUV-VASE) (the supplementary material57). Because thesemeasurements were performed at 22 ± 2 ◦C, below the ther-mochromic transition temperature for this film (47.1 ± 1.7 ◦C),only the low temperature semiconducting phase can be eval-uated by ellipsometry. The results were analyzed with a four-layer model: (1) an effective medium approximation (EMA)layer being a physical mixture of Lorentz and voids on (2)a two- Lorentz oscillator layer on (3) an SiO2 layer (37 nmthick) on (4) a silicon wafer (500 µm thick), similar to themodel described by Kakiuchida et al. for VO2 films in theirlow temperature, semiconducting phase.59 The two oscillatorsin the Lorentz model are understood as interband transitions insemiconducting VO2, associated with the band gap of 2.5 eV(500 nm) between O 2p electrons and the V 3d band andwith the low temperature splitting of the V 3d at 0.6–0.7 eV(1700–2050 nm).60,61

Fig. 9 displays the inferred optical constants, n + ik,for this V0.96Nb0.04O2 film. Consistent with the results of

FIG. 9. The optical constants, n+ ik , of the V0.96Nb0.04O2 film, derivedfrom a fit to the spectroscopic ellipsometry results (data available in thesupplementary material57).

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others,28,59 both n and k are high, characteristic of the lowtransparency of VO2 materials.

V. CONCLUSIONS

We report a metrology for the high throughput generationof composition-dependent phase transition maps in lightlysubstituted VO2, by utilizing the near infrared thermo-chromism of the metal-insulator transition as a non-contactsurrogate measurement. The apparatus automates spectro-scopic reflectance measurements at sample temperatures of1 ◦C–85 ◦C, at visible (400 nm–900 nm) or NIR (950 nm–2500 nm) wavelengths. The tool can map reflectance spectra atarbitrarily numerous temperatures and locations on a thin filmcombinatorial library of 76.2 mm diameter or smaller with anareal resolution of 1.5 mm2.

From the NIR reflectance, a simple analysis has beenimplemented to identify thermochromic transition tempera-tures and regions of no transition in thin film combinatorial li-braries with composition space V1−x−yMxM′yO2. We also lookfor thermochromic behavior at visible wavelengths, generallyconsidered undesirable for smart window applications, andthe reflectance spectra in the visible can be used to quantifyfilm thicknesses, using appropriate models, and to indicatefilm transparency. When coupled with high throughput filmcomposition mapping, the metrology described enables highthroughput phase diagram generation of thermochromic tran-sitions in lightly substituted VO2; these functional phase di-agrams are a valuable contribution to the development andcommercialization of energy-saving smart windows.

ACKNOWLEDGMENTS

The authors are grateful to J. Ouyang and S. Takeuchi forcritical reading of the manuscript and to C. Amigo for designof machined components.

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