Optical Identification of Materials Transformations in Oxide Thin Films

Duncan R. Sutherland,1, ∗ Aine Boyer Connolly,1, ∗ Maximilian Amsler,1, 2 Ming-Chiang

Chang,1 Katie Rose Gann,1 Vidit Gupta,1 Sebastian Ament,3 Dan Guevarra,4 John

M. Gregoire,4 Carla P. Gomes,3 R. B. van Dover,1, † and Michael O. Thompson1, ‡

1Department of Materials Science and Engineering,Cornell University, Ithaca, NY 14853, United States

2Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland3Department of Computer Science, Cornell University, Ithaca, NY 14853, United States

4Joint Center for Artificial Photosynthesis, California Institute of Technology, Pasadena, CA 91125(Dated: July 20, 2021)

Recent advances in high-throughput experimentation for combinatorial studies have acceleratedthe discovery and analysis of materials across a wide range of compositions and synthesis conditions.However, many of the more powerful characterization methods are limited by speed, cost, availability,and/or resolution. To make efficient use of these methods, there is value in developing approachesfor identifying critical compositions and conditions to be used as a-priori knowledge for follow-up characterization with high-precision techniques, such as micron-scale synchrotron based X-raydiffraction (XRD). Here we demonstrate the use of optical microscopy and reflectance spectroscopyto identify likely phase-change boundaries in thin film libraries. These methods are used to delineatepossible metastable phase boundaries following lateral-gradient Laser Spike Annealing (lg-LSA)of oxide materials. The set of boundaries are then compared with definitive determinations ofstructural transformations obtained using high-resolution XRD. We demonstrate that the opticalmethods detect more than 95% of the structural transformations in a composition-gradient La-Mn-O library and a Ga2O3 sample, both subject to an extensive set of lg-LSA anneals. Our resultsprovide quantitative support for the value of optically-detected transformations as a priori datato guide subsequent structural characterization, ultimately accelerating and enhancing the efficientimplementation of µm-resolution XRD experiments.

I. INTRODUCTION

Combinatorial materials science research employs highthroughput experimentation to rapidly probe and ana-lyze large regions of synthesis and processing space [1, 2].The exploration of metastable phases is of particularinterest in the context of designing new materials [3]and synthesis of metastable materials requires precisecontrol of transformation kinetics. Structural charac-terization during metal oxide synthesis has revealed acomplex interplay between thermodynamics and kineticsas a function of composition, temperature, and time[4],which is consistent with the general understanding ofphase behavior/evolution in materials. The composition-temperature-time synthesis phase space is rich in struc-ture and largely unexplored due to its complexity andthe limited ability to process materials on fast time scales.Conventional methods for annealing thin film samples in-clude hot plate, furnace, and rapid thermal annealing [5],although the 1 - 100 K/s cooling rates available withthese techniques are insufficient to kinetically trap manyhigh-temperature metastable phases for room tempera-ture characterization. Methods for studying metastablematerials include characterization after quenching dur-ing thin film deposition, both with and without subse-

∗ These authors contributed equally to this work† rbv2@cornell.edu‡ mot1@cornell.edu

quent annealing[6, 7], and nanocalorimetric processingthat provides quench rates on the order of 104 K/s[8].

Recently, lateral gradient Laser Spike Annealing (lg-LSA) has been developed as a high-throughput tech-nique in which thin film samples are annealed in thesub-millisecond [9] time domain. The specific Gaussian-like laser beam profile in lg-LSA generates a reproduciblespatial temperature gradient across the scan direction,typically ∼ 1 mm in width (FWHM). With an accessi-ble domain of peak temperatures (300 − 1410 C) anddwell/quench rates (200 µs− 10 ms), a wide range of an-neal conditions can be probed rapidly in a single thin filmsample. We have demonstrated that lg-LSA can form andquench metastable phases in functional oxide materials,for example, by stabilizing high-temperature polymorphsat ambient conditions [9]. Due to the large thermal gra-dient across an lg-LSA stripe, µm-scale spatial resolu-tion is required to analyze the processed thin film prop-erties. Structural characterization using X-ray diffrac-tion (XRD) allows the identification of the cyrstallinephases present in addition to providing information onthe overall degree of crystallinity[10]. However, typi-cal lab-source diffractometers are unsuitable for resolvingstructural transformations across an lg-LSA stripe in ahigh-throughput fashion since they (a) produce spot sizeson the order of 1 mm or larger, and (b) provide insuffi-cient X-ray intensity to account for the weak scattering ofthin films, demanding long integration times. Using slitsto improve spatial resolution of the X-ray source furtherdegrades the throughput. On the other hand synchrotron

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facilities produce X-rays with very high intensity, ap-propriate wavelength, and µm-scale spatial extent, butare only available during limited beam times. The typi-cally short duration of synchrotron access compared to acomplete synthesis and processing cycle limits the abilityto adapt the synthesis and processing strategy based onevolving knowledge of structural transitions.

To address these limitations, we have developed twocomplementary, spatially resolved optical analysis meth-ods that permit rapid identification of the transitionsoccurring in lg-LSA annealed samples, data that thencan guide follow-up, structural characterization: (a)white-light or narrow wavelength (e.g., LED) optical mi-croscopy and (b) spectroscopic reflectometry. By com-paring these methods to spatially-resolved XRD refer-ence data for two materials systems (single-compositionGa2O3 and composition-gradient La-Mn-O) we demon-strate that the presence and location of phase transi-tions can be reliably identified using optical techniquesprior to X-ray characterization. These readily accessi-ble benchtop methods drastically improve the through-put of materials analysis, allowing precious synchrotronbeam time to be used more effectively. Because they arereadily available, our methods can speed up the itera-tive feedback cycle between characterization and synthe-sis/processing by orders of magnitude.

In Sec. II we discuss the methods. The two spatiallyresolved optical analysis methods developed in this workare described in detail in Sec. II B. Application of thesemethods to two materials systems is discussed in Sec III,which are then assessed for accuracy using two metricsin Secs III A and III B. Finally, we summarize our resultsand conclude in Sec. IV.

II. METHODS

A. Thin film deposition and lg-LSA processing

Thin film libraries were deposited onto thermally oxi-dixed (20 nm) highly doped (p-type, 0.02 Ω cm), litho-graphically patterned Si wafers with gold alignmentmarks. The Ga2O3 sample was deposited via reactivesputtering from a Ga2O3 target in an atmosphere of4.5 mTorr Ar and 0.5mTorr O2 in an AJA sputter sys-tem. The target was operated at an RF power of 210 Wwhile the substrate was rotated to create a 160 nm thickfilm with a < 5% thickness variation.

The La-Mn-O system was deposited by reactive co-sputtering from La and Mn targets offset to generate alateral composition gradient using a custom depositionsystem described previously [11]. In an atmosphere of5.4 mTorr Ar and 0.6 mTorr O2, the La and Mn targetswere energized with an RF power of 83 W and 140 W,respectively. XRF measurements, calibrated using thinfilm standards, revealed that La/(La+Mn) concentrationin the as-deposited film varied from 12 to 78% across thelibrary with a metals loading at substrate center of 1.4

micromol/cm2, corresponding to a thickness of approxi-mately 250 nm.

The beam of a CO2 laser operating at λ = 10.6µmand maximum power of 125 W was shaped to provide a320 µm-wide (full width at half maximum) lateral beamprofile and used to generate lg-LSA stripes on the twomaterials system samples. Each anneal stripe was 5 mmlong, with peak temperatures ranging from 600C to1410C and processing dwell times between 250 µs and10 ms. The dwell is defined as the FWHM of the laserin the scan direction divided by the scan velocity; thisis approximately the time the temperature is within 5 %of the peak temperature [9]. The anneal locations wererandomized across the thin film library to avoid locationbias on the wafer and to evenly distribute the time andtemperature conditions with respect to the compositiongradient in the La-Mn-O sample. A 100 mm diameterwafer offers space for a total of up to 625 stripes withdistinct anneal conditions.

B. Spatially resolved techniques

1. Optical microscopy

Optical micrographs were collected using a ThorlabsCMOS camera (RGB channels with 1024 × 1280 pix-els) aligned normal to the sample. The sample was il-luminated coaxially with white light over a spot size ap-proximately 1 mm in diameter. The R,G,B color chan-nels were summed using the weights of (0.3, 0.59, 0.11)and (0.33, 0.33, 0.33) for the Ga2O3 and La-Mn-O sam-ples, respectively, to create high-contrast grayscale im-ages. The magnification was set to provide a 0.79 µmspacing between pixels in the final image; this producedimages with an approximately 1 mm horizontal field ofview (FOV). A representative RGB micrograph is shownin Figure 1 a), which is horizontally cropped to be consis-tent with the displays associated with spectroscopic anddiffraction techniques.

2. Reflectance spectroscopy

For reflectance spectroscopy, the sample was illumi-nated with a white light source (400 < λ < 90 nm).A seven linear fiber bundle was imaged onto the sam-ple to collect reflected light from an area approximately10µm×70µm, aligned along the laser scan direction. Thefiber bundle (a) improves the signal to noise ratio and(b) reduces erroneous signal from defects and particles.A reference dark spectrum and a spectrum from an Ag-coated mirror were used to calibrate the sample spectra.The fiber image was scanned across laser stripes at 10 µmincrement for 201 samples, each with an optimized inte-gration time of ≈ 250 ms. Reflectance traces from threerepresentative scans are shown in Figure 1 c).

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3. X-ray diffraction

The data from the two optical techniques were com-pared to reference data collected using the ID3B beam-line at the Cornell High Energy Synchrotron Source(CHESS). A 9.7 keV beam was focused to yield a spot sizewith a 20µm× 40µm footprint on the sample at a 2 an-gle of incidence. The diffracted signal was captured witha Pilatus 300K detector. The X-ray beam was scannedalong the same path as in the reflectance spectroscopytechnique, collecting data every 10 µm across the an-neal stripe with a 50 ms integration time. The 2D de-tector data was integrated along the χ direction usingpyFAI [12].

C. Analysis

Systematic data processing protocols were developedin order to compare the different techniques. Since theyvary in their spatial extent and resolution, a key step in-volved carefully aligning the resulting data maps acrossthe techniques to allow a quantitative comparison. Theoptical micrographs were cropped to conform to the di-mensions of the region probed by the reflectance spec-troscopy. Each map was smoothed with the same spatialdegree of filtering to decrease noise. An unsharp mask fil-ter was applied to the x-ray diffraction maps to enhancesharpness and emphasize featuring.

III. RESULTS AND DISCUSSION

To demonstrate the value of the novel optical meth-ods presented in Secs II B 1 and II B 2, we apply themto study phase transitions of (meta)stable phases in twomaterials systems: (a) single-composition Ga2O3 and (b)composition-gradient La-Mn-O.

Ga2O3 has four known accessible crystalline poly-morphs: α, β, γ, and ε [13–15] with distinct tempera-ture regimes of thermodynamic stability, allowing clearinterpretation of the phase space regions. β-Ga2O3

is of particular interest due to its commercial applica-tion in electronic and power devices as a wide bandgapsemiconductor [16]. α- and ε-Ga2O3 polymorphs have,for example, applications in functional heterostructures,as transparent conductive oxides and exhibits tunablebandgaps [14, 15].

The La-Mn-O system has been the subject of experi-mental interest for many years due to its application insolid oxide fuel cells [17] and the colossal magnetoresis-tance found in lanthanum manganite perovskites [18, 19].The equilibrium pseudo-binary phase diagram in air wasreviewed by Grundy et al. [17] who identify four poly-morphs of MnOx , two polymorphs of La2O3, and the per-ovskite LaMnO3. However, non-equilibrium metastablephases of the system have not been studied extensively.The richness of the chemical system along with interest

in potential metastable phases makes it an excellent can-didate for a combinatorial high-throughput study.

A. Root mean square signal gradients

We commence our analysis of the data arising fromthe three different experimental sources by deriving keyquantities that can be compared across the samplingtechniques. The images and heat maps, as described inSec. II B, share a common horizontal axis correspondingto the spatial location along the temperature-gradient(lateral) axis of a given lg-LSA stripe. Figure 1 shows avisual comparison of the resulting images/maps for a rep-resentative sample from the La-Mn-O library, where wealign the x-axis with respect to the center of the lg-LSAstripe.

For a quantitative comparison we reduce the dimen-sionality of these images/maps by integrating along they-dimension of the grayscale intensity in the microscopyimages, the reflected photon energy in the spectroscopymaps, and the signal intensity in reciprocal space in theXRD maps. Note that for the microscopy images there isessentially no variation along the y axis in the region im-aged since the annealing conditions have reached steadystate (i.e., along the longitudinal axis). Integration there-fore simply averages over redundant information alongthat direction.

The key quantity derived from these 1-D reduceddatasets is the root mean square of the signal gradi-ents (RMSG) with respect to the lateral position acrossa stripe. Regions with a high RMSG value correspondto rapid variations in the underlying measured propertiesassociated with materials transformations. For the twooptical methods, the physical source of these variations inthe transparent films stems from varying interference dueto changes in the film thickness d and/or the refractiveindex n of the film, and possible diffuse scattering of thelight due to formation of a non-specular surface. In par-ticular, small changes in either n or d markedly alter theoptical reflectance of the material. Changes in the RMSGin the XRD patterns are directly associated to structuralchanges, including local ordering of the amorphous filmand changes in crystalline phase of the material.

Figure 2 shows how the images/maps compare witheach other for the same representative lg-LSA stripe asin Figure 1, and presents the corresponding RMSG in theright-hand panels. The height and widths of the peaks inthe RMSG vary from technique to technique due to thesensitivity and spatial resolution of each method. Thelocation of the peaks in the RMSG are indicated by ver-tical teal lines in the signal maps; these align well acrossthe three experimental methods. With the XRD pat-terns of known LaMnO3 phases [20, 21], we can identifythe structural transformations in the film and associatethem with the processing conditions on the stripe. As themaximum temperature experienced by the film rangesfrom room temperature at the position x = −400 µm to-

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FIG. 1. Acquisition and construction of the spatially resolved technique maps for a representative lg-LSA stripe (vertical scan)on the La-Mn-O sample, annealed at a peak temperature of 1175 C for 3000µs with a nominal La atomic fraction of 0.7. Dueto the lateral profile of the incident laser, the center of the stripe corresponds to the peak temperature, decreasing smoothly toambient on either side, panel b). Panel a) shows an optical micrograph with reference probe sizes for reflectance spectroscopy(yellow bar, 10µm×70µm) and XRD measurements (blue bar, 20µm×40µm), and measurement scan direction (gray arrow);Panel c) shows the spectroscopic reflectance at three specific locations on the lg-LSA stripe. Panel e) shows the reflectancespectroscopy heatmap (reflection coefficient mapped to color) with locations corresponding to those of panel c) indicated withvertical lines. Panel d) shows the XRD spectra (diffraction intensity vs. scattering vector, Q) at the same three locations onthe lg-LSA stripe, and panel f) shows the corresponding XRD heatmap (diffraction intensity mapped to color).

wards the peak temperature of the entire lg-LSA stripeat the position x = 0 µm, the film undergoes two defin-able transformations. First, the amorphous as-depositedLa0.7Mn0.3O3 film begin to change to an optically densercondition x = −250 µm. Second, the fully dense amor-phous film transforms to a crystalline cubic LaMnO3

structure at x = −100 µm. The symmetric nature of thelateral thermal gradient leads to the same set of trans-formations in reverse order on the right-hand half of thestripe.

Ideally, any materials transformation should be cap-tured with all three methods, leading to a one-to-onemapping of the associated RMSG. In general the samesymmetries and transformations are observed in the op-tical micrograph and the reflectance spectroscopy map,with RMSG peaks locations consistent with those ob-served in the XRD map. To quantify the difference ofthe RMSG across the techniques for all the experimen-tal conditions, we use the Pearson correlation coefficient(PCC) between each pair of techniques as a measureof the similarity in the overall material transformationcharacterization. A PCC value of 0 equates to no lin-ear correlation, whereas a value 1 or -1 equates to totalpositive or negative linear correlation, respectively. Forthe PCC calculation the RMSGs are interpolated with acubic spline function spanning the same spatial extent.

Note that the comparison is sensitive to the difference inspatial resolution of each technique (the effective probesizes in Figure 1), which can cause a sharp transformationboundary to appear indistinct, or could create or obscurethe appearance of a multi-phase region. Further, inaccu-rate calibration of the lateral anneal space can compressor expand the RMSG and significantly alter the calcu-lated correlation coefficients. Hence, we carefully cali-brated our spatial scales to minimize the effect of suchartifacts.

The subplots a)–c) and d)–f) in Figure 3 summarizethe PCC values between each pair of techniques (op-tical to spectroscopy, optical to XRD, spectroscopy toXRD) for the 612 and 603 LSA conditions analyzed in theGa2O3 and La-Mn-O samples, respectively. The right-hand panels in each subplot show the scatter of PCC val-ues between the specified techniques over all LSA condi-tions. For the Ga2O3 system, the variation in dwell con-ditions are encoded with different opacity (light and darkfor short and long dwells, respectively), while for the La-Mn-O system the color encodes the compositional varia-tion of the La/(La+Mn) atom fraction. The left panelsin each subplot show the histogram of the PCC from thecorresponding panel on the right. The red horizontal linedenotes the median value of the PCC.

The histograms for the Ga2O3 system clearly show a

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bimodal distribution, which indicates the presence of dis-tinct, systematic features in the PCCs over the investi-gated LSA conditions. The first mode exhibits a peakwith a median of ≈ 0.1, while the peak of the secondmode is located at a value of ≈ 0.7−0.8, with some vari-ation depending on the compared techniques. A com-parison with the corresponding scatter plot shows thatthe two modes stem from samples that are separated dis-tinctly at a critical temperature Tcrit ≈ 600C that is in-dependent of the anneal time as observed in Figure 3 a)–c). We ascribe this sharp increase in the PCC to theonset of crystallization from the amorphous Ga2O3 film,consistent with the findings reported previously [22]. Be-low the threshold value Tcrit, there is essentially no vari-ation in the RMSG since the signal stems from purelyamorphous samples, leading to correlation values closeto zero. Above Tcrit, the crystallization events lead toclear changes in the RMSG that correlate strongly acrossthe techniques. Note that the correlations between thespectroscopy and XRD characterization has the tightestdistribution for the second mode (above Tcrit), which weattribute to the very similar spatial resolutions of the twotechniques.

A similarly sharp onset is neither expected nor ob-served for the La-Mn-O system due to the increased com-plexity associated with compositional variation across thewafer. The PCC gradually increases as a function of an-

neal temperature between 500C and 800C, leading toa unimodal, skewed distribution in the histograms. Themedian value is ≈ 0.6, which is lower than the secondmode of the Ga2O3 system but still indicates a strongcorrelation. Thin films of La2O3 and MnO2 crystallizefrom amorphous as-deposited films at 600C and 450C[23, 24], respectively. While various lanthanum man-ganate materials have been investigated, they typicallyincorporate various substitutions [25] that complicatecomparative measurements, or are grown on a heatedsubstrate at 900C to synthesize as-deposited crystallinefilms [26].

Overall, we observe a high degree of correlation (>0.5) in the PCC analysis, which clearly indicates thatthe optical and spectroscopic measurements are sensitiveenough to identify the material transformations observedby XRD. For both materials systems, the strongest corre-lation is observed when comparing spectroscopy to XRD.This result is not surprising, since these two methods con-tain a broader spectrum of information (photon energyand diffraction angles) compared to microscopy (singlevalue from the three color channels). Nevertheless, mi-croscopy imaging is a simple and powerful method torapidly assess the RMSG features since the timescale tocapture a single RGB image is 2-3 orders of magnitudeshorter than that needed for reflectance spectroscopy orXRD (when available).

B. Completeness of the material transformations

The PCC serves as a statistical method to assess andcompare the RMSG, but does not reflect whether or notthe optical methods are sensitive to a particular typeof crystallographic transformation occurring in the LSAprocessed thin films, i.e., it does not clarify what kind oftransformations are identified or missed. To address thisissue, we employ a metric complementary to the PCCby analyzing the completeness of the material transfor-mations identified by XRD to those identified by eitherof the optical techniques. Of particular concern are thecases where crystalline structure changes occur (based onthe complete XRD data) without detectable changes inthe optical techniques, i.e., “false-negatives”. To assessthe presence of such cases, we manually compiled a list oftransformations detected in any or all sets of techniquesin a given stripe through visual inspection and identifiedthe false negatives. “False-positives” correspond to tran-sitions observed in either of the optical methods corre-sponding to putatuve phase transformations that do notappear in the XRD measurements. The completeness ofobserved structural transformations are best visualizedwith a Venn diagram (Figure 4). The subsets representthe portion of the transformations identified by one, two,or all three characterization techniques. The total num-ber of identified transformations, independent of trans-formation type, within each set are listed in Table I.

In contrast to what one would expect from the PCC

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FIG. 3. The PCCs comparing the RMSG between each technique across all peak temperature and dwell time conditions. Panelsa), b), and c) show the histograms and temperature-sorted scatter plots of the PCCs for all LSA experiments conducted forthe Ga2O3 sample, while panels d), e), and f) show the corresponding results for the La-Mn-O sample. The color gradient inthe scatter plot corresponds to the La cation fraction.

analysis, the anneal stripes in the Ga2O3 system displayvery gradual materials transformations across the stripethat lead to a relatively large false negative rate of ap-proximately 4.3% for each of the optical techniques. Thisvalue is substantially higher than for the La-Mn-O sys-tem, but is low on an absolute scale. A detailed investiga-tion of this subset of false negatives reveals that they oc-cur primarily when the γ phase transforms to the β phasewith increasing temperature. This result confirms thatthe two optical methodologies may be insensitive to cer-

Optical Spectroscopy

XRDXRD

Optical

La-Mn-OGa2O3a) b)

FIG. 4. Venn diagrams illustrating the intersections betweenall three techniques for the La-Mn-O and Ga2O3 samples inthe panels a) and b), respectively. The optical microscopy,reflectance spectroscopy and XRD areas are colored in red,yellow and blue, respectively. False negatives are representedby as small blue slivers at the bottom of each Venn diagram.

tain types of materials transformation, particularly whenit occurs gradually between 2 phases with similar opti-cal properties. Both optical microscopy and reflectancespectroscopy identified almost identical sets of materialtransformations, i.e., the number of transformations onlyobserved in microscopy (O \ (S∪X))) and only observedin spectroscopy (S \ (O ∪ X)) are very small. The totalfalse positive rate for the Ga2O3 sample, i.e., (S∪O)\X,is 9.8%.

For the La-Mn-O system, 0.7% of the structural trans-formations are demonstrably false negatives (X\ (S∪O);see tiny blue sliver at bottom of Figure 4 b)). The sen-sitivities of each optical method applied to the La-Mn-Osystem are similar to those for Ga2O3 with regard to cap-turing identical material transformations. The La-Mn-Osystem shows a larger false positive rate, 25.1%, higherthan that in Ga2O3. In general, such false positives canbe attributed to dust, delamination, or other defects inthe thin film that do not pertain to the underlying crys-tallographic transformations.

As method for pre-screening phase transitions prior toXRD experiments, both optical methods are effective de-spite the large number of false positives. Such false pos-itives are undesirable because they motivate redundantsampling but they do not lead to any loss of informationabout crystallographic material transformations. For ex-ample, exhaustive XRD sampling would require 125,000individual measurements in the La-Mn-O compositionspread, while focussing only spectroscopy-detected tran-

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TABLE I. Venn diagram values for material transformations identified by the optical microscopy (O), reflectance spectroscopy(S), and/or XRD (X) techniques. The critical false negatives are X \ (O∪ S), while false positives leading to unnecessary XRDmeasurements are (S ∪O) \X.

Library S ∪O ∪X (S ∪O) \X (O ∪X) \ S (S ∪X) \O O \ (S ∪X) S \ (O ∪X) X \ (O ∪ S)Ga2O3 848 78 5 12 2 18 39

La-Mn-O 943 279 8 2 39 3 7

sitions would reduce the the number of required mea-surements to as few as 960. Thus the false positive rateof 23% nevertheless enables substantial resource savingsin terms of the total number of required XRD measure-ments.

On the other hand, false negatives may result in lossof information in the synthesis phase diagram and shouldbe minimized. The low rate false-negative rate of approx-imately 0.7% in the La-Mn-O sample demonstrates thecapability to effectively pre-screen transition boundariesprior to XRD experiments. The higher false-negative rateof 4.3% observed in the Ga2O3 sample stems from a singletype of phase transformation that is especially difficult todetect optically, i.e., in which the optical absorption andoptical thickness (the product of n and d) both remainapproximately constant. Such instances are coinciden-tal and expected to be rare in higher order compositionspaces.

C. Accelerating experimental throughput

To assess the value of this work in accelerated ex-ploration of materials synthesis spaces, we consider thedemonstrated workflow of i) composition library deposi-tion, ii) LSA-based processing with approximately 600LSA stripes (peak temperature and dwell time condi-tions) per composition library, and iii) characterizationwithin each LSA stripe to identify transitions in temper-ature, dwell time, and composition.

The throughput of these processes in terms of numberof composition libraries is 1 and 10 per day for depositionand lg-LSA, respectively. For transition characterization,the alternate approaches described in the present work– optical microscopy, reflectance spectroscopy, and µm-resolution XRD – have throughputs of 1000, 2, and 10per day, respectively, although the XRD throughput canbe substantially lower for thin films that require largeracquisition times due to weak scattering.

With these nominal throughputs, the deposition stepappears to be approximately rate-limiting until one con-siders resource availability, which is of order of 9 days peryear for the specialized synchrotron XRD measurementand 300 days per year for the other purpose-built opticalinstruments. Limited XRD availability makes this tech-nique rate-limiting for annual workflow throughput, andthis limitation is exacerbated when considering the valueof feedback–the ability to choose the deposition condi-tions based on consideration of all prior data.

Typical synchroton XRD throughput is up to 90 li-braries per year corresponding to 90 traversals of theworkflow, although deposition selection informed bycharacterization of all prior libraries only occurs in up to9 of those traversals because during the 9 days of XRDexperiments; decision throughput is limited by depositionthroughput. The ability to detect transitions with opticaltechniques increases the decision throughput from 9 to300 per year, enabling optimized allocation of the XRDresource to structural characterization of the optically-identified transitions.

This scheme is particularly effective if the XRD-based(structural) transitions are consistently identified withthe optical techniques; Figure 4 provides the most exten-sive demonstration to date that optically-detected transi-tions are most often due to structural transitions. Whilethe throughput of the optical microscopy technique ismuch higher than that of reflectance spectroscopy, inthe context of the annual workflow analysis the increasedthroughput is not the essential benefit. The value of themicroscopy lies in its spatial resolution. The spectralresolution of reflectance spectroscopy facilitates transi-tion detection but its spatial resolution is limited in prac-tice to ≈ 5µm. If multiple transitions occur within thatlength scale (which corresponds to a range in tempera-ture of up to ≈ 25 K for our lg-LSA system), optical mi-croscopy becomes a critical complementary transition de-tector. Narrowly clustered transitions were not observedin the data presented herein, but the concurrence of thetransitions identified by microscopy and spectroscopy in-dicate that the combination of these techniques providesa robust approach to transition detection in synthesis pa-rameter space.

IV. CONCLUSIONS

The methods described here enable high-throughputexperimental identification of materials phase transitionsacross an lg-LSA stripe. Using rapid, low-cost, opti-cal techniques, such as optical microscopy and spatiallyresolved reflectance spectroscopy, locations of materialstransitions can be identified and can serve as prior knowl-edge for experimentally expensive techniques such assynchrotron XRD studies. For both single-compositionand composition-gradient samples, structural transitionswere identified via optical methods. An anomalous tran-sition behavior was noted in Ga2O3, where a structuralchange – the introduction and growth of a secondary

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phase – was not always identified via optical means,demonstrating the possibility of false negatives under cer-tain circumstances. The appearance of unidentified tran-sitions and the common occurrence of optical changesnot corresponding to a structural change (false positives)suggest that further development could improve the ef-fectiveness of the methods and extends the propertiesthat can be identified optically. Collectively, the resultsdemonstrate the utility of using in-lab optical character-ization methods to accelerate iterative feedback betweenprocessing and thin film deposition in the discovery anddevelopment of new materials. The results also motivatefurther development of algorithms that leverage the op-tical characterization techniques to design synchrotronXRD experiments that enhance the efficient utilization

of that limited resource.

V. ACKNOWLEDGEMENTS

The authors acknowledge the Air Force Office of Sci-entific Research for support under award FA9550-18-1-0136. This work is based upon research conducted at theMaterials Solutions Network at CHESS (MSN-C) whichis supported by the Air Force Research Laboratory un-der award FA8650-19-2-5220. This work was also per-formed in part at the Cornell NanoScale Facility, a mem-ber of the National Nanotechnology Coordinated Infras-tructure (NNCI), which is supported by the National Sci-ence Foundation (Grant NNCI-1542081). MA acknowl-edges support from the Swiss National Science Founda-tion (project P4P4P2-180669).

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