Rare-Earth Monopnictide Alloys for Tunable, Epitaxial, DesignerPlasmonicsE. M. Krivoy,† A. P. Vasudev,‡ S. Rahimi,† R. A. Synowicki,§ K. M. McNicholas,† D. J. Ironside,†

R. Salas,† G. Kelp,∥,⊥ D. Jung,# H. P. Nair,† G. Shvets,⊥ D. Akinwande,† M. L. Lee,○

M. L. Brongersma,*,‡ and S. R. Bank*,†

†Department of Electrical and Computer Engineering, The University of Texas at Austin, Austin, Texas 78712, United States‡Geballe Laboratory for Advanced Materials, Stanford University, Palo Alto, California 94305, United States§J.A. Woollam Co., Inc., Lincoln, Nebraska 68508, United States∥Department of Physics, The University of Texas at Austin, Austin, Texas 78712, United States⊥School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14850, United States#Institute for Energy Efficiency, University of California Santa Barbara, Santa Barbara, California 93106, United States○Department of Electrical and Computer Engineering, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, UnitedStates

*S Supporting Information

ABSTRACT: We demonstrate that plasmonic response canbe tuned spectrally via the alloy composition of an epitaxialsemimetallic film. Specifically, attenuated total reflectancestudies show that the surface plasmon resonance of rare-earthmonopnictide LaxLu1−xAs alloy films can be tuned across themid-IR, while lattice-matching to technologically importantsubstrates. The electrical properties are a strong function ofthe composition, which, in turn, manifests as tunability of theoptical properties. The ability to produce tunable (semi)-metals that can be epitaxially integrated with III−V emittersand absorbers is expected to enable a new generation of novelnanophotonic device architectures and functionality.

KEYWORDS: rare-earth monopnictide, semimetal, tunable metal, epitaxial metal, mid-infrared optoelectronics, plasmonics,molecular beam epitaxy

A variety of opportunities motivate the development ofepitaxial (semi)metals, including the ability to design fullyintegrated structures where metallic films and nanostructurescan be seamlessly integrated into the heart of devices.Applications include high-performance tunnel-junctions1 forsolar cells, epitaxial transparent Ohmic contacts,2 photomixerTHz sources,3,4 and thermoelectrics,5 to name only a few.Additionally, the integration of metallic nanostructures andfilms into optoelectronic devices has shown potential forimproving device performance and functionality throughsubwavelength confinement of plasmonic modes and enhance-ment of light/matter interactions, for applications includingsensing,6 energy harvesting,7 communications,8 and so on.Tailoring the plasmonic response is essential for optimum

device performance, motivating the pursuit of spectraltunability via alloying, such as Ag−Au alloys,9,10 silicides,11germanides,12 and so on. However, single crystal materials arehighly desirable for (1) their greatly enhanced plasmonicresponse13 and (2) the potential for epitaxial integration intodevices, rather than being restricted to their periphery.14 As an

epitaxial alternative, heavily doped semiconductors, such asInAs15 and silicon,16 are being explored for their plasmonicproperties. While these materials can be grown epitaxially, theyare thus far limited to wavelengths >5 μm. Extensive work hasalso been conducted on the growth and characterization of theternary II−VI materials, ZnCdTe17 and HgCdTe,18−20 in theirsemimetallic regimes, and the dependence of structural,optical, and electrical properties on composition. However,these material systems are difficult to integrate with conven-tional III−V substrates because of a large lattice mismatch andincompatible growth parameters. The transition metalnitrides21−23 and conductive oxides21,24 are also attractivecandidates for tunable, epitaxially compatible plasmonicmaterials; however, their epitaxial integration with III−Vmaterials has yet to be explored. Additionally, despite theexcellent plasmonic properties of Ag25 and tunability of Ag−

Received: March 2, 2018Published: July 2, 2018

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Au9 alloys, monolithic integration of these materials with activeIII−V semiconductors remains an open question.The inability of current approaches to offer the flexibility of

epitaxial integration into III−V-based optoelectronic devices,response in the mid-infrared (3−5 μm), and wavelengthtunability motivates this study. We demonstrate a significantstep forward in this vein with the realization of a tunableepitaxial metallic material system, creating a path towarddesigner (semi)metallic films that can be seamlessly integratedwith traditional III−V semiconductor photonic materials.These tunable metals enable lattice-matching across a varietyof relevant substrates, as well as tunability of both the opticaltransparency windows and plasmonic response, all whilemaintaining moderate resistivity and carrier mobility.Many of the rare-earth monopnictides (RE-V) are rocksalt

semimetals26−29 and can be epitaxially integrated with III−Vsemiconductors as nanoparticles and thin films.30,31 Addition-ally, it has been shown that ErAs films can be overgrown withhigh-quality III−V materials via a nanoparticle-seeded growthtechnique,32 providing a path toward the integration ofepitaxial metals into the core of devices. The RE-V system isa viable pathway toward epitaxially integrated metals, havingwidely varying structural, electrical, and optical properties.33

Furthermore, RE-V alloys can be used for the development oftunable, epitaxial metallic films and nanostructures, with thepotential to integrate customized metals into III−V hetero-structures devices.Most investigations thus far have studied binary RE-V

materials, with some reported use of ternary alloys to lattice-match ScErAs/GaAs,34 ScYbAs/GaAs,35 and ScErSb/InAs.36

However, the full range of optical, electrical, and structuralproperties accessible through RE-V ternary alloy growth hasnot been studied. Specifically, the properties of LaAs37 andLuAs30,38 have been shown to vary greatly in their opticalspectra, lattice parameters, and bulk room-temperatureresistivity. These divergent properties make these two materialsparticularly interesting for alloy growth and characterizationbecause they may offer a large range of tunable parameters.Here we demonstrate, with the growth of high-qualityLa1−xLuxAs films, the ability to produce tunable epitaxialmetals, with (1) a range of peak transmission spectra fromnear- to mid-IR wavelengths, (2) moderate resistivity, and (3)potential lattice-matching to many technologically relevantIII−V substrates. This investigation demonstrates the fullrange of the potential of the RE-V ternary alloys and of theirelectrical, structural, and optical properties.Samples were grown by solid-source molecular beam epitaxy

(MBE) in an EPI Mod. Gen. II system on semi-insulating(100) GaAs. Films were grown at 460 °C, measured with apyrometer with a wavelength response centered at 900 nm,with an As:RE flux ratio of 21.3:1, corresponding to a beamequivalent pressure ratio of 46:1. The reflection high-energyelectron diffraction (RHEED) patterns observed in situ duringgrowth suggest high-quality material, with rougheningobserved only from films that exceeded the critical thicknessfor lattice relaxation. We have previously reported thecomplications for growth of LaAs directly on III−V substrates,and developed a method for the growth of high qualitylanthanum-containing RE-V films, utilizing thin LuAs barrierlayers at the RE-V/III−V heterointerfaces.37 The growth oflanthanum-containing LaxLu1−xAs films therefore employedthin, 5 monolayer (ML) thick, LuAs barrier layers to ensurethe growth of high quality single-crystalline rocksalt epitaxial

LaxLu1−xAs films. We observed that for films of low lanthanumcontent,

4 K are underway, and may give more insight into the carrierscattering behavior of the LaxLu1−xAs alloys.

41

Transmittance and reflectance measurements of theLaxLu1−xAs alloys showed a similar linear dependence oftheir optical properties on alloy content. The peak trans-mission of the 0.5 μm La0.48Lu0.52As film was intermediatethose of the constituent materials, as seen in Figure 3. The

transmission peak occurred at ∼2.3 μm with a transmittance of>30%. Similarly, the reflectance spectra (Figure 3, inset)showed a sharp Drude edge at ∼4.5 μm, midway between thepreviously reported Drude edges for LuAs38 and LaAs,37 whichappeared at ∼3.1 and ∼8 μm, respectively. The reflectancespectra of several 100 nm LaxLu1−xAs samples with varyinglanthanum content (Figure 4) more clearly illustrate thetunability of the Drude edge with increasing lanthanumcontent. This shifting of the apparent Drude edge with Lacontent demonstrates a shifting of the plasma frequency withalloy composition. We would expect, then, that the surfaceplasmon frequency, which is proportional to the plasmafrequency, to undergo a similar shift. Indeed, Figure 5 plots theattenuated total reflectance (ATR) of a 0.5 μm film of

La0.48Lu0.52As grown on GaAs, compared to the transmittanceand reflectance spectra of the same film. ATR spectroscopyallows coupling to surface plasmons43 and has previously beenused to study surface plasmon polaritons (SPP) in polardielectrics.44−46 The ATR spectra were collected using aThermo Nicolet 6700 FTIR coupled to a Continuum IRmicroscope. The IR beam, generated by the FTIR, was focusedonto a ZnSe (n = 2.4) prism at a 45° angle. We controlled thesample-prism distance with submicron resolution using ananopositioning stage. The ATR spectra were then averagedover 128 collections with 4 cm−1 spectral resolution. Figure 5indicates that many of the higher energy attenuations in theATR spectrum may be attributed to peaks in the transmissionspectra. We observe an ATR dip corresponding to surfaceplasmon coupling (denoted with arrows in Figure 6) thatconsistently appears at frequencies slightly below the plasmafrequency across the range of tunable alloy films of LaxLu1−xAs.For each composition, this dip spectrally shifted as the gapbetween the film and the ZnSe crystal was varied with a piezoactuator (which in turn, altered the local dielectric environ-ment), while the other features remained spectrally unchanged,leading to its unambiguous identification as the surfaceplasmon resonance. Additionally, by comparing the ATRspectra of LuAs, LaAs, and La0.48Lu0.52As in Figure 6, weobserved that there appears to be a suppression of the short-wavelength peak observed in LuAs at ∼2.5 μm, attributed to

Figure 2. Room temperature resistivity of LaxLu1−xAs films with x =5−48% increasing in resistivity with increasing La content. Inset:Temperature dependent resistivity of 0.6 μm films of LuAs, and 0.5μm films of La0.48Lu0.52As and LaAs.

Figure 3. Transmittance spectra of 500 nm La0.48Lu0.52As, 600 nmLuAs, and 500 nm LaAs. Inset: Reflectance spectra of the films,showing the Drude edge of the La0.48Lu0.52As film between that ofLuAs and LaAs.

Figure 4. Reflectance spectra for 100 nm films of LaxLu1−xAs, with x= 5−48%. The apparent position of the Drude edge red-shifts (lowerenergy) with increasing La content, consistent with the electrical data.

Figure 5. Attenuated total reflectance measurement of 0.5 μmLa0.48Lu0.52As film plotted with transmittance and reflectance spectra.

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interband absorption, through composition adjustment of thealloy. Note that the ATR-excited SPP mode using thismeasurement geometry is not a pure air-metal SPP. Rather itis a perturbed mode due to the spatial overlap of the prismwith the SPP. This mode perturbation, however, does notinfluence the electron-concentration dependence of the ATRdip. These measurements further confirm the featuresobservable through specular reflectance and transmittance,indicating clear tunability of the LaxLu1−xAs alloy optical andplasmonic properties.Ellipsometry measurements of LuAs, LaAs, and

La0.48Lu0.52As were performed to extract the real and imaginarycomponents of the permittivity. Plotting ε′ and ε′′, the realand imaginary components of the permittivity, respectively, inFigure 7 demonstrates the potential to tune the low-loss

wavelength ranges of the RE-V alloys. Identifying the fullextent of tunable features for the RE-Vs will allow for betterunderstanding and development of novel device structures.Based on the linear dependence with composition of the

lattice-constant, resistivity, and optical spectra, there is strongevidence that the LaxLu1−xAs films behave as homogeneousalloys of the constituent binaries. The similar enthalpies offormation of LaAs and LuAs47 suggest that the alloy may grow

without phase-segregation of the composing materials. A studyof the films using scanning transmission electron microscopy(STEM) and energy dispersive X-ray spectroscopy (EDS) wasconducted to study sample homogeneity. Samples wereprepared for cross-sectional TEM using mechanical lappingand polishing, followed by argon ion-milling to achievetransparency. The EDS spectra showed a homogeneousdistribution of the RE elements to the order of ∼1−2 nmresolution, as seen in Figure 8b. In addition, the high-resolution TEM and selected-area diffraction pattern in Figure8c,d show the epitaxial registry between the GaAs, LuAs, andLaxLu1−xAs layers, as expected.

In conclusion, tunable epitaxial metals with a broad range ofoptical, electrical, and structural properties are accessible withternary alloy RE-V monopnictides. We have demonstrated thegrowth of high-quality La1−xLuxAs films with plasmonicproperties tunable across the mid-IR, as well as the potentialto lattice-match to a number of technologically important III−V substrates. This demonstrates the full range of the potentialof the RE-As ternary alloys, and of their electrical, structural,and optical properties. Future work will focus on utilizing thenanoparticle-seeded growth technique32 for exploring theincorporation of RE-V ternary alloys into III−V hetero-structures. A more in-depth theoretical study of the plasmonresonance is currently underway.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsphoto-nics.8b00288.

Composition dependent plasma frequency, Drudedamping, and SPP figure of merit (PDF).

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: sbank@ece.utexas.edu.

Figure 6. Attenuated total reflectance spectra for thick (0.5−0.6 μm)films of LuAs, La0.48Lu0.52As, and LaAs demonstrating tunableplasmon resonance with alloy content. Arrows denote the surfaceplasmon resonance for each composition.

Figure 7. Real (ε′) and imaginary (ε′′) components of permittivityfor LaxLu1−xAs films of varying composition.

Figure 8. (a) Layer structure of TEM sample, (b) STEM electrondispersive X-ray spectroscopy (EDS) image of 500 nm La0.48Lu0.52Asfilm, showing the 5 ML LuAs barrier layer as reference with thearsenic signal filtered out, (c) HRTEM lattice image of LaLuAs/LuAs/GaAs interface, and (d) selected area diffraction (SAD) patternof LaLuAs/LuAs/GaAs interface.

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*E-mail: markb29@stanford.edu.ORCIDK. M. McNicholas: 0000-0002-2426-2027D. J. Ironside: 0000-0002-4555-0845S. R. Bank: 0000-0002-5682-0126Author ContributionsThe manuscript was written through contributions of allauthors. All authors have given approval to the final version ofthe manuscript. All authors contributed equally.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by a Young Investigators ResearchProgram from the Air Force Scientific Office of ScientificResearch (AFOSR YIP FA9550−10−1−0182) and a Multi-disciplinary University Research Initiative from the Air ForceOffice of Scientific Research (AFOSR MURI Award No.FA9550−12- 1−0488).

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