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Structural defects and electronic phase diagram of topological insulatorbismuth telluride epitaxial filmsTo cite this article: C I Fornari et al 2018 Mater. Res. Express 5 116410

 

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Mater. Res. Express 5 (2018) 116410 https://doi.org/10.1088/2053-1591/aadeb7

PAPER

Structural defects and electronic phase diagram of topologicalinsulator bismuth telluride epitaxial films

C I Fornari1,5 , PHORappl1, S LMorelhão2 , G Fornari3, J S Travelho3, S deCastro4,M J PPirralho4,F S Pena4,MLPeres4 andEAbramof1

1 Laboratório Associado de Sensores eMateriais, InstitutoNacional de Pesquisas Espaciais, 12245-970, São José dos Campos, SP, Brazil2 Instituto de Física, Universidade de São Paulo, 05315-970, São Paulo, SP, Brazil3 Laboratório Associado deComputação eMatemática Aplicada, InstitutoNacional de Pesquisas Espaciais, 12245-970, São José dos

Campos, SP, Brazil4 Instituto de Física eQuímica, Universidade Federal de Itajubá, 37500-903, Itajubá,MG, Brazil5 Author towhomany correspondence should be addressed.

E-mail: celso.fornari@inpe.br

Keywords: bismuth telluride,molecular beam epitaxy, structural properties, electrical properties

AbstractIn thiswork, bismuth telluridefilms are grownbymolecular beamepitaxy (MBE)on (111)BaF2 substrates,using stoichiometricBi2Te3 andadditionalTe solid sources.Thegrowthdynamics and structural defects areinvestigated indetail as functionof substrate temperature,Bi2Te3fluxandextraTe supply, bymeansofatomic forcemicroscopy,Raman spectroscopy and reciprocal spacemapping.Thegrowth rate increaseslinearlywith theBi2Te3fluxand themost appropriate conditions to growhigh-qualityBi2Te3 single layers isfound tobe in anarrowwindowofMBEparameters.At lowgrowth temperaturesTe clusters are formed,while theTedeficit increaseswith raising substrate temperature anddecreasingdeposition rate. It results infilmswithBi-richer phasesdue to the formationofBidouble layers inbetweenBi2Te3quintuple layers. Theelectronic transport properties are also studiedby temperaturedependent resistivity andHallmeasure-ments.Byproperly changing the substrate temperature and/or the extraTe supply, thebehaviorof thefilmscanvary from insulating tometallic aswell as themajor carriers fromp- ton-type.The electronicphasediagrampresentedhereprovides a fast route to control thebulk conductanceproperties of bismuthtelluride,which enables theproductionof intrinsic bulk insulatingfilms. In addition, the results suggest thepossibility of growing intrinsic sharpp-n junctionsofBi2Te3 byproperlymonitoring theoccurrenceofstructural defects,which is thefirst step forpractical applicationsof this topological insulatormaterial.

Introduction

Three-dimensional (3D) topological insulator (TI)materials have attracted great attention in scientific community inthepast fewyears due to its extraordinary surface properties. Since the theoretical prediction [1–3], severalexperimentalworks have reportedon topological insulator properties of bismuth chalcogenides, likeBi2Te3 andBi2Se3 [4–7].However, the surface properties of these topological insulatormaterials remain so far unexplored fordevice applications. There are twomajor challenges facing applications of this newelectronic state ofmatter: surfacecontamination andbulk conduction.Thefirst changes the chemical potential of thematerial, inhibiting to exploresuchproperties at roomconditions [8, 9]. The latter overwhelms surface spin currents, resulting in anordinarysemiconductormaterial behavior [10]. In this sense, several efforts havebeing employedby experimental scientists inorder to control these surface properties [11], but a lackof systematic studies focusingongrowthdynamics still exists.Such studies provide routes for fast control of the bulkproperties, enabling producing truly bulk-insulatingfilms.

Bismuth telluride (Bi2Te3) andbismuth selenide (Bi2Se3) are archetypes of 3DTIs. In thesematerials, thebulkband structure is crossedby topological surface states (TSS)with lineardispersion, resulting inmetallic surface stateswithbulk insulatingbehavior [12–14]. The surface particle possesses spin locked to themomentumandare protectedfromscatteringdue tonon-magnetic impurities or structural disorders by the time-reversal symmetry [15–17]. The

RECEIVED

20 July 2018

REVISED

30August 2018

ACCEPTED FOR PUBLICATION

4 September 2018

PUBLISHED

14 September 2018

© 2018 IOPPublishing Ltd

linear dispersionof theTSS results inmasslessDirac fermionswith extremelyhighFermi velocities.Consequently,high-mobility spinpolarized surface currents canbeobtainedwithout externalmagneticfields at roomtemperature,which canpermitnovel dissipation-less spindevices tobeproduced, offeringpossibilities to newapplications inspintronics or quantumcomputing [18–23].

Nevertheless, the spontaneous presence of structural defects on these compounds leads to the occurrence offree carriers in the bulk, turning on bulk conduction and hampering the surface states from electricalmeasurements [24, 25]. Thefirst attempt to control the Fermi level inside the band gapwas to apply counterdoping. By increasing theCa and Sn concentration onBi2Se3 and Bi2Te3 bulk crystals, respectively [4, 6], it ispossible to suppress the free carriers, obtaining a bulk insulating behavior. However, the use of extrinsic dopingaffects the electrical properties of thematerials and shows instabilities in time scale of hours, even inside ultra-high vacuum (UHV) chambers [4].

To overcome these instabilities,molecular beam epitaxy (MBE) has been applied to grow bismuthchalcogenides thin films, since this technique allows growing filmswith very high crystalline quality andwellcontrolled properties on different substrates [25, 26]. Intrinsic conduction only through the topological surfacestates was achieved on these films, i.e., without using counter doping. Intrinsic bulk insulating behavior inBi2Te3 epitaxial films has been reported during in situ experiments [27–30] and, recently, during ex situinvestigation, after exposing the sample to atmosphere [31].

The kinetics, duringMBEgrowth, can lead to the formationof certain structural defects,which contributewithfree carriers to thebulk. Twinningdomains onBi2Te3 epitaxialfilms, for example, aremost likely to be formedathigher substrate temperatures, being suppressedby increasing the extraTe supply [32]or by reducing thedepositionrate [33]. This kindof structural defect can lead todanglingbondsbetween the rotateddomains, resulting in amodification in the chemical potential of the bulk [34]. Besides twinningdomains, thesematerials canpresent anti-site defects or vacancies. These structural defects havedifferent probabilities tobe formed, as shownbyfirst principlesdensity functional studies anddepend, besides the atomicposition, on the stoichiometrydeviation fromthepureBi2Te3 phase [35]. Therefore, understanding the formationmechanismof structural defects during epitaxial growth isof extreme importance. Byproperly controlling thedefects occurrence it is possible toobtainbothn- or p-typefilms,passing through abulk insulator condition. Such control enables the growthof intrinsic and sharpp-n junctionswithout using extrinsic doping. Similar ideahas been recently employedonBi2Te3 bulk crystals to obtain ap-njunctionbymeansof intrinsic defect grading [36].

Bismuth telluride can exist in several stoichiometric phases, depending on growth conditions. These phasesconstitute a homologous series formed by stacking two fundamental building blocks, the Bi2Te3 quintuple layers(QLs) and the Bi2 double layers (BLs) [37]. This series spans from the Te-richest phase Bi2Te3 up to pure bismuth(Bi2). However, it is important to emphasize that Bi2Te3 is the only phase predicted to present the TSSs. TheBi2Te3 phase crystallizes in a tetradymite-type structure and its unit cell consists of stacking threeTe1-Bi-Te2-Bi-Te1 quintuple layers (QLs) along the [0001] direction. The atoms are ionic bound inside theQLs,while theQLs are van derWaals coupled between theTe1 atoms, giving a layered character to thismaterial. Thepresence of Bi2 double layers in between Bi2Te3 quintuple layers prevents the films to completely relax the laterallatticemismatch, which can result in a difference of the lateral lattice parameter up to about 0.6%between aBi2Te3 and a Bi2Te2.6film [38]. Although the van derWaals gaps relaxes the latticemismatch conditions,avoiding the formation ofmisfits dislocations [39], latticemismatch somehow influences the overall quality ofthe epitaxialfilms [25]. The precise control on growth conditions leads to different dynamics during deposition,which favors varied kinds of structural defects, resulting in different electrical characteristics on thesefilms.

In thiswork, a systematic investigationon theproperties of the bismuth telluride epitaxialfilms is presented.Thefilmswere grownbymolecular beamepitaxy on freshly cleaved (111)BaF2 substrates byusing anominalBi2Te3 solidsource and twoextraTe cells. Byfixing thedeposition rate, a systematic studyof the electrical resistivity andHall effectmeasurements is presented.These experiments lead to an electronic phase diagram,which canprovide a fast anddirectway for experimental scientists to control electrical properties of bismuth telluride thinfilms.

Experimental

Bismuth telluride epitaxial filmswere grownon freshly cleaved (111)BaF2 substrates using a Riber 32Pmolecular beam epitaxial system. Effusion cells chargedwith a nominal stoichiometric Bi2Te3 solid source andtwo extra Te sources were used here. The solid sources are produced at our laboratory using commerciallyavailable Bi (99.999%) andTe (99.9999%) elements. The beam equivalent pressure (BEP) of the effusion cellswasmonitored by a Bayer-Alpert ion gauge. The extra tellurium supply (ΦR) is determined as the ratio betweenthe Te andBi2Te3 BEP, asΦR=ΣBEPTe/BEPBi2Te3. In this case,ΦR=0 indicates that no extra tellurium isprovided during the growth, i.e., thefilms are grown by using only the Bi2Te3 cell. The background pressure ofthe growing chamber never exceeded 10−9 Torr during growth. Thefilm surface ismonitored in situ during

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growth by reflection high-energy electron diffraction (RHEED) equipment, using a 35 keV electron cannon. Thefilms thickness wasmeasured from scanning electronmicroscopy (TescanmodelMIRA3) images of the samplescross section and by x-ray reflectometry.

For the structural analyses, a high-resolution x-ray diffractometer (PANalytical X’PertMRD)was employedwith the x-ray tube set to point focus, a (220)Ge four-crystalsmonochromator for the incident beam, and anopen proportional Xe detector with acceptance of 1° after the diffracted beam. Long rangeQz-scans along the[0001] bismuth telluride directionweremeasured for all samples. Reciprocal spacemapswere acquired on theXRD2 beamline of the Brazilian Synchrotron Light Laboratory (LNLS). The photon energy was tuned to 8004 eV(λ=1.549 038Å) by using a double-crystal (111) Simonochromator. The beamwas vertically focusedwith abent Rh-coatedmirror, placed before themonochromator, into the sample, with a spot of 0.6mm (vertical) and2mm (axial). The sample wasmounted on aHuber 4+2-circle diffractometer and the diffracted beamwasanalyzed by a Pilatus 100k area detector, with a pixel size of 172 μm.The distance between the sample and thedetector was set to 910mm.To avoid absorption of the diffracted beam into the air, an evacuated tubewasinstalled between the sample and the detector.

Raman spectroscopymeasurementswere performed inhigh-spectral resolutionLabRamHREvolutionequipment, fromHoriba. Theoptical system is composedof aHe-Ne laser,withwavelengthof 633nm, and a 100xobjectivewith a gratingof 1800 groves/mm,which gives a spectral resolutionof 0.3 cm−1. The roomtemperature iskept constant at 20 °Cduring allmeasurements and a low laser powerwasused to avoid local heating.

Electrical resistivity, type and density ofmajor carriers andmobility weremeasured as a function oftemperature in a Keithley 80AHall Effect system. Tomake electrical contacts in the samples, goldwires with50 μmof diameter are soldered in van der Pauw geometry usingmetallic indiumpellets. The sample ismountedin aDE-202 closedHe cryostat fromARS Inc., whose temperature is controlled by a PIDLakeshore 303 between10K and 340K. Themagnet,Walker ScientificHV4, producesmagnetic fields up to 0.72 T.

Results and discussion

Molecular beamepitaxyTo investigate the growth rate as a function of substrate temperature (TSUB) at different BEPs of the Bi2Te3 solidsource, a series of bismuth telluride filmswas grown byMBEon (111)BaF2 substrates. In this series, for eachBEPBi2Te3 value fixed at 2×10−7, 5×10−7, 1×10−6 and 3×10−6 Torr, the substrate temperature wasvaried between 220 and 330 °C.The extra tellurium cells were not used in this case (ΦR=0).

The substrate temperature dependence of the experimental growth rate, determined as the ratio between thefilm thickness and the total deposition time, is shown by open-circles infigure 1(a). For each BEPBi2Te3 value, thegrowth rate remains practically constant for TSUB between 230 and 280 °C. In this temperature range, the growthrate increases linearly with the BEPof Bi2Te3 solid source, as shown infigure 1(b) for TSUB=240 °C. For TSUB

higher than 280 °C the growth rate starts to decrease until a critical temperature (TCRIT), abovewhich nodeposition occurs at all, indicated by small arrows infigure 1(a). The dependence of TCRIT on the BEPof Bi2Te3source is shown infigure 1(c).

The growth rate (RG) can be described as the difference between the deposition rate (D) and the desorptionrate (RD), described by a Polanyi-Wigner law [40]:

R T D R . 1G SUB D= -( ) ( )

In our case, the deposition rateD can be considered as themeasured growth rate at low temperatures, whichdepends exclusively on the BEP of the Bi2Te3 effusion cell (figure 1(b)). The desorption rate (RD) is determinedas the product of the surface atomic density (ND) available for desorption, in atoms/cm2, with theArrheniusterm:

R T N exp , 2D SUB D

Ek TB SUBu= -( ) · · ( )·

where E is the activation energy for desorption, kB the Boltzman constant andTSUB the substratetemperature. Considering the hexagonal lattice of bismuth telluride, the (0001) surface atomic density isND=6.013×1014 atoms/cm2. To convert RD in units of Å/s, ND must bemultiplied by the substrate area(1.5×1.5 cm2) and by the height of onemonolayer that can be considered as themean inter-plane atomicdistance along the [0001] growth direction, which equals to 2.035Å for Bi2Te3 [32]. The typical vibrationalatomic frequency u is defined as

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Mater. Res. Express 5 (2018) 116410 C I Fornari et al

k T

h

2, 3Bu = ( )

where h is the Planck constant. The calculated vibrational frequency is 1013Hz for typical substrate temperaturesaround 250 °C. It is related to the frequency inwhich the bond between adjacent atoms is stretched and can beinterpreted as an attempt to break the bond and desorb.

Within these assumptions, the film growth rate was calculated as a function of substrate temperature foreach BEPof the Bi2Te3 effusion cell. The bestfits to the experimental data are plotted as dotted lines infigure 1(a). The goodness of the fit is determined by themean square deviationχ2 between the calculated andmeasured data. The activation energy E is the singlefitting parameter andwas refined up to the second decimaldigit. It was found to beE=1.55 eV for all BEPBi2Te3 values without extra Te supply (ΦR=0).

In order tomake a detailed investigation of the growth rate as a function of substrate temperature and extraTe supplyΦR, a series of bismuth telluride samples was producedwith the beam equivalent pressure of theBi2Te3 cellfixed at 5×10−7 Torr (deposition rate equal to 0.2 Å s−1). In this series, TSUBwas varied from180 to310 °C forΦR=0 and 1. The growth rate is exhibited as open circles infigure 1(d) and can be divided in threeregions, i.e., three different temperature regimes [32]. Thefirst regime occurs for low substrate temperatures(TSUB�215 °C) and the transition point to the next region is indicated by a vertical dotted line. In this region,due to low kinetic energy, all extra tellurium atoms stick to the growing surface, increasing the growth rate and

Figure 1. (a)Growth rate (open-circles) of bismuth telluridefilms on (111)BaF2 as a function of substrate temperature (TSUB) for fourdifferent values of the Bi2Te3 solid source beam equivalent pressure (BEP) and no extra Te supply (ΦR=0). The short-dashed linesare the bestfit to the experimental data. (b)Growth rate as a function of the BEPBi2Te3 for TSUB=240 °C,which shows a lineardependency. (c)Critical temperature (TCRIT) abovewhich no deposition occurs as a function of the BEPBi2Te3, presenting a negativeexponential dependency. (d)Growth rate (open-circles) as a function of substrate temperature for two different Te supplies,ΦR=0andΦR=1. The dashed lines represent the calculated curves that bestfitted to the experimental data.

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resulting in bismuth telluride filmswith Te-clusters and low crystalline quality [32]. Since the increase on thegrowth rate of the thin films is due to the excess of Te atoms, the activation energy value for desorption, obtainedbyfitting this curve, can be related to Te. The purple dashed line infigure 1(d) represents the calculated curvethat best fitted to the experimental data in this region. The activation energy value foundwas E 1.27Te = eV.

At substrate temperatures from220 °C to 270 °C, the growth rate is practically constant independent of theΦR value. This region is found to be themost suitable to growhigh quality Bi2Te3 epitaxial films. Above 270 °C,the growth rate starts to decrease until it vanishes above the critical temperature, showing a transition from thesecond to the third growth regime. In this region thefilms present thickness inhomogeneities along thewholesubstrate area. The critical temperature depends on theΦR value, as can be observed in figure 1(d). Since filmsgrownwithΦR=1 present a higher critical temperature, the activation energy for this set of datawas alsocalculated. The best fit is exhibited as a blue dashed line infigure 1(d) and the energy valuewas found to beE 1.59= eV forΦR=1.

All these activation energy values are very close to each other, which explains the narrowparameters windowobserved here during theMBE growth of bismuth telluride films. This set of values can also be useful incomputational growthmodels to study nucleation, surface features and formation of structural defects [41].

The Te deficit (δ), defined as Bi2Te3-δ, of this sample series was determined from the peaks positions of the( L0 0 0 ), L 15= and 30, x-ray diffraction curves [32, 42]. The highest telluriumdeficit, resulting in astoichiometry close to Bi4Te5 (δ=0.5), was achieved for thefilms grownwith growth rate close to 0.1 Å s−1 andsubstrate temperatures near to TCRIT. For afixed substrate temperature, δmonotonically decreases as thedeposition rate increases, i.e. the BEPBi2Te3 value rises. This result is shown in the graph of figure 2. It isimportant to emphasize that the phase BiTe (δ=1)was never obtained in this work. This result is different fromprevious ones published for the epitaxial growth of bismuth telluride films on (111)BaF2 substrates using aBi2Te3 solid source and extra Te cell. A possible explanation for this fact is a variation in the Bi2Te3 solid sourcesstoichiometry used in the differentMBEmachines.

The following physical picture can explain the Te deficit behavior. By increasing the deposition rate, theatoms on the growing surface remains less time available for surface diffusion or desorption processes, i.e., thesurface is rapidly covered by a new layer of atoms, which difficult desorption processes. By diminishing thedeposition rate, the surface-available-time increases, favoring Te desorption due to its low sticking coefficient[43] and resulting in Bi-richer phases. By increasing the surface-available-timewith enough kinetic energy andproviding sufficient Te atoms to compensate the low sticking coefficient, i.e., with high substrate temperatureandΦR, the occurrence of structural defects is suppressed, since the atoms possess sufficient time toaccommodate in the lowest energy positions in the lattice. This picture is in good agreement to experimentalreports that observed suppression of structural defects [33] and bulk insulating behavior in Bi2Te3 epitaxial films[27–30].

Figure 2.Telluriumdeficit δ of the Bi2Te3-δfilms as a function of the beam equivalent pressure (BEP) from the Bi2Te3 solid source andof the substrate temperature (TSUB)with no extra Te supply (ΦR=0). The dotted lines roughly indicate the transition region betweenbismuth telluride phaseswith decreasing δ values of 0.1. The dashed line indicates the critical regionwhere no deposition occurs at all.

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Toaccomplish the second stepof this investigation,which consists of a systematic studyon the structural andelectronicproperties of theBi2Te3 epitaxialfilms grownon (111)BaF2, afixed value of deposition rate of 0.2 Å s−1waschosen. Films grownwith this deposition rate exhibited the smoothest surface, lowdensity of twiningdomains,reducedmosaicity, evidencing anoptimized condition to grow thebismuth telluride thinfilms.

Structural propertiesByfixing the deposition rate in 0.2 Å s−1 (BEPBi2Te3=5×10−7 Torr), the substrate temperature was rangedbetween 180 and 310 °C,with a step of 10 °Cand the extra tellurium supply was varied between 0 and 2, with astep of 0.5. In this study,more than 30 samples of bismuth telluride epitaxial films grown on (111)BaF2substrates were produced. All samples were grown for 2 h. The structural properties of this set of samples wereinvestigated by reciprocal spacemapping (RSM), atomic forcemicroscopy (AFM) andRaman spectroscopy.

TheAFMsurface images, shown infigure 3(a), indicate that bismuth telluridefilms grownatTSUBbelow220 °CpresentTe agglomerates,which increases the growth rate and leads to rough surfaces. As canbe clearly seen in theseAFMimages, the surface of thefilmgrownatTSUB=200 °C is completely coveredwithTe agglomerates,whosedensity decreases sensibly asTSUB rises to 210 °C.Forfilms grownatTSUB above 220 °C,Te agglomerates are nolonger observed.AtTSUB>220 °C, the extra telluriumdesorbs from the surface, resulting inBi2Te3filmswithflatsurfaces andhomogeneous thickness. This growthdynamicswas confirmedby theRaman spectra, shown infigure 3(b), acquired for thefilms grownat this temperature range. The violet shaded areas in the graphs offigure 3(b)correspond toRamanpeaks relative to theA1, E1 andA2normal vibrationmodesof pure tellurium [44], while theblue shaded areas representRamanpeaks relative to theA1

g andA2g out-of-plane vibrationmodes and to theEg in-

plane vibrationalmodeofBi2Te3 [45, 46]. TheRaman spectrumof the bismuth telluridefilmgrownatTSUB=200 °Cexhibits onlypeaks corresponding to theTemodes, demonstrating that its surface is completelycoveredwith tellurium. For thefilmgrownatTSUB=210 °C, theRaman spectrumshows the threepeaks relative totheBi2Te3modes and, less pronouncedly, theTeRamanpeaks, indicating the formationof theBi2Te3 phase togetherwithTe clusters.Now, for the epitaxialfilmgrownatTSUB=220 °C,onlyBi2Te3 relatedRamanpeaks are visible,proving that in this condition the exceedingTe atomspossess sufficient energy todesorb fromthe surface andnoclusters are formedat all. Asfilms grownbelowTSUB=220 °CpresentTe agglomerates andpoor crystalline quality,theywere discarded for theupcoming analyses.

Figure 4 presents the structural characterization results of bismuth telluride films grown at substratetemperatures of 220, 240, 260 and 280 °Cwithfixed extra Te supplyΦR=1. TheRSMswere acquired in the

Figure 3. (a)Atomic forcemicroscopy images and (b)Raman spectra of bismuth telluride films grownon (111)BaF2 at a depositionrate of 0.2 Å s−1 (BEPBi2Te3=5×10−7 Torr)withΦR=1 andTSUB=200, 210 and 220 °C.The blue and violet shaded areas in (b)indicate the position of Bi2Te3- andTe-related Raman peaks, respectively.

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vicinity of the symmetric 0 0 0 18( )Bi2Te3 diffraction peak and are shown infigure 4(a) in reciprocal space unitsQz parallel to the [0 0 0 1] growth direction andQx along the in-plane direction. In all RSMs, scattering along thecrystal truncation rod, parallel toQz, and along the Ewald sphere are visible. The latter is attenuated for samplesgrownwith higher substrate temperatures, indicating awidening of the diffraction peaks. TheRSMof thesample grown at TSUB=220 °Cpresents the lowest scattering alongQz, with a full-width at half-maximum(FWHM) around 230 arcseconds. However, alongQx an intense scattering is observed, which indicates thepresence of structural defects, likemosaicity. It is possible to observe through the RSMs that the FWHMvaluesalongQz rises as TSUB increases. For films grownwith TSUB=260 and 280 °C, the observedwidening onQz

scattering occurs due to a split in the diffraction peak [42, 47], indicated by arrows in the graphs. It results fromthe formation of Bi double layers in betweenBi2Te3 quintuple layers due to a Te deficit induced by the elevatedsubstrate temperatures, leading tofilmswith Bi-richer phases.

The corresponding AFM images were acquired over areas of 3×3 μm2 and are displayed infigure 4(b). Thetypical pyramidal triangular Bi2Te3 surfacemorphology [42, 47, 48], with step-heights around 1 nm, are clearlyobserved in all images. Thewhite triangles drawn on the images correspond to the average edge-size of thetriangular domain basis oriented along the symmetry directions of the hexagonal lattice. The presence of 60°-rotated domains indicate the occurrence of twinning [26]. The density of these structural defects is found toincrease by raising the substrate temperature with afixed extra Te supply [32]. This behavior can be explained bythe fact that formation of twinned domains is energetically less favorable and the energy necessary to form suchdomains can be achieved by increasing the substrate temperature. The edge-size of the basis of the triangulardomains is alsomodified by changing the substrate temperature. As shown infigure 4(b), the size of the domainsincreases substantially as a function of substrate temperature. Themean edge-size of the triangular basis valueswere extracted from theAFM images and are exhibited infigure 5(a) as a function of TSUB. An exponentialbehaviorwas found and the dotted line represents the exponential curve that bestfitted to themeasured data.According to abovementioned comments, this increase in the triangular domains size as the growthtemperature rises can occur due to higher surface diffusion.

To complement the investigation, the Raman spectra of thesefilmsweremeasured and are displayed infigure 4(c). The Raman spectra of all samples grown in this temperature range exhibits clearly the Bi2Te3 Ramanpeaks relative to the A1

g andA2g out-of-plane vibrationmodes, located near 61 and 134 cm−1, respectively, and

Figure 4. (a)Reciprocal spacemaps in the vicinity of the 0 0 0 18( ) Bi2Te3 X-ray diffraction peak, (b) atomic forcemicroscopy surfaceimages, and (c)Raman spectra acquired for bismuth telluride films grownon (111)BaF2 substrates at a deposition rate of 0.2 Å s−1

(BEPBi2Te3=5×10−7 Torr)withΦR=1 andTSUB=220, 240, 260 and 280 °C.Thewhite arrows in (a) indicate a peak splitting dueto the formation of afilmwith a bismuth richer phase. Thewhite triangles in (b) correspond to themean edge value of the triangulardomain basis oriented along the three symmetry directions of the hexagonal lattice. The inset in theAFM image of the sample grown atTSUB=220 °C is a viewwithmagnification 2x higher. In the graphs of (c), the Raman peaks relative to the out-of-plane vibrationmodes (A1

g andA2g) and the in-plane vibrationalmode (Eg) of Bi2Te3 are clearly visible. The threemain Bi2Te3 Raman peaks are

adjustedwith Lorentz andGaussian curves (dashed lines) and the full-width at half-maximumvalues of the Eg peak are given.

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the Eg in-plane vibrationalmode, around 101 cm−1. At TSUB between 260 and 290 °C, the Eg andA2g FWHM

values are typically around 2.2 and 6 cm−1, respectively, which are 40% lower than the ones reported inliterature [48], reinforcing the excellent quality of the epitaxial films grown in this work.NoRaman peaksrelative to pure Temodes are present in this investigated range of substrate temperature. The veryweak bandslocated between the A1

g and Eg vibrationalmodes can be attributed to surface oxidation [48].By adjusting Lorentz andGaussian curves to the experimental data, dashed lines in the graphs offigure 4(c),

the full-width at half-maximumvalues were extracted from the threemain Raman peaks. The FWHMvalues ofEg peak, relative to the Bi2Te3 in-plane vibrationalmode, and the A2

g peak, related to the Bi2Te3 out-of-planevibrationalmode, are plotted infigure 5(b) as a function of the substrate temperature. Two parabolas wereadjusted to the experimental set of data and are displayed in the graph as dashed lines. The extrema of theFWHMvalue of the in-plane Eg and the out-of-plane A

2g Raman peaks are located aroundTSUB=270 °C. The

enlargement of the in-plane related peak indicates the presence of vacancies in the crystalline lattice, while theout-of-plane peakwidth can be related tomosaicity-tilt or even the presence of Bi double-layers in the structure.

Electronic transportThe charge transport in the bismuth telluridefilmswere investigated throughmeasurements of conductivity, in therangebetween12 and300K, andHall effect performedat afixed temperature of 12K. Figure 6presents the electricalconductivity as a functionof inverse temperature for bismuth telluridefilms grownat severalTSUB andfixed extraTesupplyΦR=1. Forfilms grownatTSUBbetween220and240 °C, an insulatingbehavior is observed, inwhich theconductivity values increase by raising the sample temperature.However, at TSUB=250 °C, the resistivity behaves asametal above 120Kandas an insulator below this temperature, indicating that amixed electrical behavior occurs atthis temperature. For higher substrate temperatures,TSUB� 260 °C, a completemetallic behavior is achieved.

The carrier type and concentration at 12K are displayed infigure 6(b) as a function of substrate temperature.Forfilms grown belowTSUB=260 °C, a p-type bulk behavior is observed, inwhich the carrier concentrationincreases almost linearly from2×1019 cm−3 up to 2×1020 cm−3 by raising the growth temperature. Filmsgrown at substrate temperatures higher than 270 °Chave an n-type character with electron density increasing

Figure 5. (a)Mean edge-size of the triangular domain basis extracted from the AFM images as a function of substrate temperaturewithfixed extra Te supplyΦR=1. The dashed line indicates the exponential curve that bestfitted to the data. (b) Full-width at half-maximum (FWHM) of the in-planemode Eg and out-of-planemodeA2

g Bi2Te3 Raman peaks as a function of TSUB andΦR fixed at 1.The dashed lines correspond to the parabolas that bestfitted to the data.

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from4×1019 cm−3 to 9×1020 cm−3 as TSUB rises up to 310 °C. In this sense, this transition fromp-type ton-typewas observed to occur at TSUB=265 °C for afixed extra Te supplyΦR=1.

Bismuth valence band states contributewithfive electrons, which indicates that a Bi vacancywill result in atriple acceptor, i.e., a bismuth vacancy contributewith three holes. Telluriumpossesses six valence electrons andthen, a telluriumvacancy acts like a double donator, i.e., a tellurium vacancy contribute with two free electronsto the lattice [35]. At high substrate temperatures, tellurium ismore favorable to desorb from the growingsurface, since it possess a lower sticking coefficient compared to Bi atoms [43]. This fact can leads to telluriumvacancies on thefilm structure, resulting in free electrons in the bulk (n-type). This picture is in good agreementto the n-branch offigure 6(b), inwhich the free electrons density increases by increasing the substratetemperature, i.e., favoring Te desorption.However, in the p-branch such picture is not sufficient to explain theincreasing density of charge carriers as TSUB rises, indicating that other structural defects, besides vacancies, arealso contributingwith positive charges.

In order to illustrate the electronic properties of the bismuth telluride epitaxial films as a function of theMBE growth parameters, a diagram summarizing the electronic transport characteristics was constructed.Figure 7 presents the electronic behavior, type and concentration of themajor carriers of theMBE grownbismuth telluride films as a function of substrate temperature and extra Te supply. The dashed lines indicate thetransition from insulating tometallic behavior, while in between these lines amixed electronic behavior wasfound. The solid lines indicate the transitions of the obtained Bi2Te3-δ phases in steps of the Te-deficit δ=0.1.

In a general view, the electronic properties can be summarized as follows. Bismuth telluride films grown atsubstrate temperatures (TSUB) lower than 250 °C, present insulating behavior, with p-typemajor carriers,independent of the extra Te supply. Increasing the substrate temperature, for values around 260 °C, it is possible

Figure 6. (a)Electrical conductivity of bismuth telluride epitaxial films, grownwithΦR=1 and varied TSUB, as a function of inversetemperature. The open (closed) circles correspond to n-type (p-type) samples. (b)Carrier concentrationmeasured at 12K as afunction of substrate temperature for bismuth telluride films grownwithΦR=1. A transition fromp-type to n-type is observed forTSUB=265 °C.

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to obtain both p and n-type carriers andmetallic or insulating behavior by simply changing the extra telluriumsupply.Now, for substrate temperatures above 270 °C, a puremetallic behavior is achieved and the type of thecharge carriers still depends on the additional Te supply.

It is important tonote that sharpp-n vertical junctions canbe fabricated forTSUBhigher thanor equal to 260 °Cby just controlling theTebeamflux, i.e., by simplyopening and closing the shutter from theTe cells. Suchbismuthtelluridep-n junctionsmaybe strain-free, sinceno extrinsic counter doping is necessary. In addition, an abrupt andsharpp-n junction canbe formed, avoiding the formationof interface defects due to thepresence ofmaterialswithdifferent lattice constants. As aplus, the electronic behavior of thep-typeportion canbe tuned from insulator tometallic by carefully choosing the substrate temperature.Controlling the electrical properties of topological insulatorcompounds is thefirst step towards practical applications of this newelectronic state ofmatter.

Conclusions

A systematic study on the growth dynamics, structural defects and electrical properties of bismuth tellurideepitaxial filmswas presented here. For this purpose, a set of bismuth telluride filmswas grown bymolecularbeam epitaxy on (111)BaF2 substrates using a stoichiometric Bi2Te3 solid source and extra Te effusion cells. Theproperties of these sampleswere investigated in detail as a function of Bi2Te3 beam equivalent pressure, substratetemperature and extra Te supply.

The growth rate was found to dependmainly on the BEPof Bi2Te3 source and to increase linearly with it. TheTe deficit δ of the grownBi2Te3-δfilmswas found to increase with raising substrate temperature and decreasingdeposition rate, and its values ranged from0 to 0.3 in thewhole investigated range. Bismuth telluride layers withBi-richer phases (higher δ values) are obtained via the formation of Bi double-layers in betweenBi2Te3quintuple-layers inside the van derWaals gap. Atomic forcemicroscopy, Raman spectroscopy and reciprocalspacemapping elucidated the structure defects present in the films.

It is possible to control the electronic transportof thebismuth telluride epitaxialfilmsbyproperly changing thesubstrate temperature and/or the extraTe supply.The electrical conductivity canvarybetweenan insulating to ametallic behavior aswell as the typeofmajor free carriers fromp- ton-type. In summary, at lowsubstrate temperaturesthe epilayers present an insulatingbehavior andp-type asmajor carriers. By raising the substrate temperature, the thinfilmspresent ametallic behaviorwithp-orn-type asmajor carriers dependingon the extraTe supply.

The electronic phase diagram constructedwith the results obtained here provides a fast route to understandthe bulk electrical properties of bismuth telluride epitaxialfilms. It indicates the growth regions to obtain bulk-insulatingfilms, which can enable the investigation of the conductance only through the topological surfacestates. The results also suggest that, by carefully adjusting the growth conditions, it is possible to grow intrinsicand sharp p-n junctions, without introducing strain and other defects due to extrinsic doping.

Figure 7.Electronic phase diagramof bismuth telluride epitaxial films in awide range ofMBE growth temperatures (TSUB) and extraTe supplyΦR for afixed deposition rate of 0.2 Å s−1 (BEPBi2Te3=5×10−7 Torr). The dashed lines indicate the transition frominsulating tometallic behavior. The gradedmap color represents the type and concentration of free carriersmeasured at 12K. Thesolid lines indicate the transition between the obtained Bi2Te3-δphases.

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Acknowledgments

The authors acknowledgeCNPq (GrantNos. 307933/2013-0, 302134/2014-0, 140694/2016-1), CAPES (GrantNo. 88881.119076/2016-01), FAPESP (GrantNo. 2016/22366-5), and LNLS (ProposalNo. XRD2-20170254)formeasurement assistance.

ORCID iDs

C I Fornari https://orcid.org/0000-0003-1765-2999S LMorelhão https://orcid.org/0000-0003-1643-0948MLPeres https://orcid.org/0000-0002-6635-0244

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