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Hot electron transport in a stronglycorrelated transition-metal oxideKumari Gaurav Rana1, Takeaki Yajima2,3, Subir Parui1, Alexander F. Kemper3, Thomas P. Devereaux3,Yasuyuki Hikita3, Harold Y. Hwang3,4 & Tamalika Banerjee1

1Physics of Nanodevices, Zernike Institute for Advanced Materials, University of Groningen, The Netherlands,2Department ofMaterials Engineering, The University of Tokyo, Bunkyo-ku, Tokyo, 113-8656, Japan,3Stanford Institute for Materials and EnergySciences, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA and, 4Geballe Laboratory for AdvancedMaterials, Department of Applied Physics, Stanford University, Stanford, California 94305, USA.

Oxide heterointerfaces are ideal for investigating strong correlation effects to electron transport, relevant foroxide-electronics. Using hot-electrons, we probe electron transport perpendicular to the La0.7Sr0.3MnO3(LSMO)- Nb-doped SrTiO3(Nb:STO) interface and find the characteristic hot-electron attenuation lengthin LSMO to be 1.48 6 0.10 unit cells (u.c.) at 21.9 V, increasing to 2.02 6 0.16 u.c. at 21.3 V at roomtemperature. Theoretical analysis of this energy dispersion reveals the dominance of electron-electron andpolaron scattering. Direct visualization of the local electron transport shows different transmission at theterraces and at the step-edges.

Heterointerfaces between strongly correlated transition-metal oxides have proven to be ideal platforms forinvestigating new physical phenomena in condensed-matter and for designing multifunctional devices foroxide electronics1. Increasingly, many of the exciting device prospects with such materials involve hot electron

transport, such as in photovoltaic effects in multiferroics, manganite transistors, ferroelectric tunnel-junctions, etc26.Hot electrons, characterized by an energy higher than the Fermi energy, EF, by more than a few times the thermalenergy, are an interesting probe to investigate the physics of electron transport in different material systems. At suchenergies, the fundamental scattering processes are different than at EFand include elastic/quasielastic scattering, andinelastic scattering via electron, phonon and spin wave excitations. Experimental techniques and devices that exploithot electron transport are found in a variety of electron spectroscopy techniques 7, in spintronic devices such as thespin-valve transistor8, in spin-transfer torque devices9, in Si spin injection devices10, and in graphene based optoelec-tronic devices11, and have yielded crucial insights into the transport properties in this energy regime.

In this context, very little is known about hot electron transport in oxide heterointerfaces with transition-metaloxides, particularly in the presence of strong correlations between the electrons charge, spin and orbital degreesof freedom1. Such interfaces are also attractive for designing multifunctional devices that do not necessarily scaleaccording to Moores law. Here we address this by using the technique of Ballistic Electron Emission Microscopy(BEEM)12 and probe hot electron transport across an archetypal oxide ferromagnet La0.7Sr0.3MnO3(LSMO) onn-typesemiconducting Nb-doped SrTiO3 (Nb:STO). We find thecharacteristic hot electron attenuation lengthinLSMO to be 1.486 0.10 unit cells (u.c.; 1 u.c. 5 0.39 nm) at21.9 V, increasing to 2.026 0.16 u.c. at21.3 V at

room temperature. Theoretical analysis of this dispersion reveals the dominance of electron-electron and polaronscattering at these energies.

In an oxide heterojunction, electrical transport has been commonly studied using a Schottky diode involvingunconventional semiconductors, often derived by doping Mott or band insulators. Probing electron transportusing a Schottky interface with transition-metal oxides has provided useful insights into the band bending, bandoffsets and their sensitivity to interface states, chemical doping and external magnetic and electric fields13.However, the contribution of long range correlation effects to the transport of electrons (depth-resolved) andquantification of transport parameters such as the hot electron attenuation length, carrier lifetime, etc., intransition-metal oxides has not been explored. Such studies involving pervoskite metal-semiconductor (M-S)interfaces are important as they are the building blocks of most oxide electronic devices.

ResultsWe use a current-perpendicular-to-plane (CPP) configuration to probe vertical transport of hot electrons in the

metallic oxide ferromagnet LSMO, across an epitaxial Schottky interface with Nb:STO, using the versatile

SUBJECT AREAS:

ELECTRONIC PROPERTIESAND MATERIALS

CHARACTERIZATION ANDANALYTICAL

TECHNIQUES

ELECTRONIC DEVICES

SEMICONDUCTORS

Received7 January 2013

Accepted29 January 2013

Published14 February 2013

Correspondence and

requests for materials

should be addressed to

T.B. (T.Banerjee@rug.

nl)

SCIENTIFICREPORTS | 3 : 1274 | DOI: 10.1038/srep01274 1

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around 1.06 6 0.02 eV, which thereafter increases with increasingsample-tip bias . This corresponds to the local Schottky barrier height(wB) extracted using the Bell-Kaiser model

12, by plotting the square

root ofIBwith sample-tip bias,VT, IB

IT! VT{wB

2

as shown in

the inset of Fig. 2b. A homogeneous distribution of the local wBin alldevices is obtained which compares well with that obtained frommacroscopic IV, CV and IPE measurements (see the Supple-mentary Information). We also observed an almost linear trend inIBwith the injected tunnel current,IT, as shown in Fig. 2b, in accordwith BEEM theory invoking planar tunneling19.

The collected BEEM current depends not only on the tunnelingcurrent injected into LSMO but also on the energy and momentumdistribution of the carriers reaching the interface and the transmis-sion probability at the LSMO/Nb:STO interface. Inelastic scattering,such as due to electron-electron interactions, can reduce the energyof the injected electron by, 50%, whereas elastic scattering fromimpurities and defects or quasielastic scattering from acoustic pho-nons render the electron distribution isotropic. The epitaxial LSMO/Nb:STO interfaces studied here have been optimized to be fullystrained, atomically abrupt, and with high crystalline perfection20.Thus they have fewer elastic scattering sites than the typical poly-crystalline metal Schottky interfaces studied by BEEM, implying the

conservation of transverse momentum of the transmitted electronsacrossthe interface andwith minimal influence on IB. IB alsodependson the acceptance angle for electron collection at the Nb:STO inter-face which is determined by the ratio of the effective masses of theNb:STO and LSMO. This is calculated to be larger here than foundfor most standard M-S interfaces (such as Au on Si). Availability ofallowed states in the conduction band minima (CBM) ink space inNb:STO is another criterion that further governs collection. Elec-tronic band structure calculations21 show that the projected conduc-tion band minima in doped STO are at the zone center (C), thuselectrons with small parallel momenta should be easily collected inthe available phase space. Despite these favourable conditions, acentral observation of this first application of BEEM to epitaxialperovskite heterostructures is the strong attenuation observed here,

as compared to the highly disordered M-S structures previouslystudied by this technique. By way of comparison, IB/ITis almost 2orders of magnitude higher for a similarly thick polycrystalline Nifilm on a n-Si/Au M-S interface22. Thus we conclude that intrinsiccorrelation effects are dominant in the measurement.

From the data in Fig. 2, we can extract the hot electron attenuationlength, l, in LSMO and study its energy dependence. For electron

transmission in LSMO, l is obtained fromIB(t,E)5 IB(0,E)exp[2t/l(E)], whereEis the sample-tip bias andtis the thickness of LSMO.FromFig. 3a, wefindl inLSMO tobe 1.486 0.10 u.c. at21.9 V andincreases to 2.026 0.16 u.c. at21.3 V. Using Matthiessens rule, thehot electron attenuation length, l(E), can be written as the sum ofelasticlelasticand inelasticlinelastic(E) scattering lengths as:

1

l E ~

1

lelasticz

1

linelastic E 1

As argued above, elastic scattering is minimal at such epitaxial het-erostructures andl(E) is thus dominated bylinelastic(E). The energydependence ofl in LSMO is shown in Fig. 3b.

DiscussionsOptimally doped LSMO is a ferromagnet and a transport half-metalwith an insulating gap for minority spins at the Fermi level,EF, andconducting for majority spins23. However, at energies higher thanEFandrelevant forour studies, thedensity of states forboth themajorityand minority spin electrons increases24,25. Conduction and ferromag-netism in LSMO is governed by the interaction of localized electronsfrom the incomplete 3dshell in Mn, by a process commonly referredto as the Zener double-exchange mechanism26. Furthermore, as sug-gested by neutron scattering experiments27, LSMO at room temper-

ature consists of dynamic nanoscale polarons, which do not freezeout below the Curie temperature (as opposed to the situation inLa0.7Ca0.3MnO3). LSMO separates into regions where the electronsare trapped in a local Jahn-Teller distortion, and a conductive net-work without distortions28. The transport then occurs as the hotelectrons move through the conductive network, while being scat-tered by the local Jahn-Teller distortions, in addition to the electron-electron scattering of the injected charges themselves. Here, thepolaron scattering can be regarded as static because the time scaleof lattice motion (picoseconds) is much larger than that of hot elec-tron transport through the LSMO film (femtoseconds)29.

We show that both mechanisms are present in our experiments inFig. 3b. First, we have attempted a Fermi-liquid theory fit based onthedensity of states (DOS) previouslyreported. However, the experi-

mentally determined scattering rate decreases slower than that pre-dicted by this model. Incorporating an energy-independent fittingparameter that accounts for the polaronic scattering processes, wecan fit our experimental data by the solid red curve (le2e1polaron) inFig. 3b. The obtained fitting constants are well within the rangesexpected based on estimates of the band masses and DOS fromdensity functional theory25. The mean free path due to just the

Figure 3| Energy dependence of the hot electron attenuation length in LSMO. (a), BEEM transmission normalized per nA of injected tunnel currentversus the thickness of LSMO obtained from the data in Fig. 2a. This is plotted for sample-tip bias of21.9 V (black circles),21.7 V (red circles),21.5 V

(bluecircles) and21.3 V (browncircles). Dottedlinesat each energyrepresent exponential decay ofIBwithattenuation length of 1.486 0.10 u.c., 1.516

0.12 u.c., 1.77 6 0.12 u.c. and 2.02 6 0.16 u.c. at these energies. Error bars represent the deviation from the best exponential fit. These lines can be

extrapolated to zero LSMO thickness, which corresponds to attenuation due to the interface. (b), Attenuation length, l, extracted from Fig. 2a for

different energy values. The solid red curve which fits the experimental data is a theoretical estimate taking into account electron-electron scattering

within Fermi liquid theory based on the DOS for the materials and an energy-independent (at the energies shown) polaron scattering term. The dotted

curves depict the two contributions independently, where blue (green) shows the electron-electron (polaron) contribution to the full estimate.

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polaronic scattering is expected to be on the order of the polaronseparation (or the polaron size). A previous study with neutronmeasurements estimated this value to be,2 u.c. (0.77 nm)27 whichis roughly consistentwith the constant value obtained here. Thus, thehot electron energy dependence enables us to quantitatively isolatetwo different scattering mechanisms: electron-electron (blue dottedcurve) and polaronic scattering (green dotted line).

Further, using the imaging capabilities in BEEM we visualise localhot electron transport across a LSMO/Nb:STO heterointerface. STMtopography of the LSMO (9 u.c.)/Nb:STO interface along withsimultaneously recorded spatial map of the transmitted current, atVT522.5 V andIT5 8 nA are shown in Fig. 4. An atomically flatsingly terminated TiO2 surface with a step-height of 0.4 nm isobserved from the STM topography whereas, a cross-section profilein the same location reveals different transmission at the terrace andat the step-edge withITbeing constant. A histogram of the transmit-ted current at the terrace (area under the blue box in b) is shown inthe inset in Fig. 4d. The mean value ofIB matches well with that of theBEEM spectra for this film (Fig. 2). The reduction inIBat the step-edge as compared to that at the terrace arises due to the sensitivity ofthe propagating hot electrons to momentum scattering at the step-

edges. This broadens thehot electron distribution andconcomitantlyreducesIBat such locations. This observation highlights the uniquecapability of the BEEM to study and directly visualize local electrontransport in oxide heterointerfaces at the nanoscale.

Our experimental method, based on hot electron transport in avertical device structure of LSMO on Nb:STO, provides a first experi-mental measure of the hot electron attenuation length in a stronglycorrelated transition-metal oxide as LSMO. This approach to probeelectron transport, on the nanoscale, will open up exciting possibil-ities to both understand and tailor the electronic properties at oxideheterointerfaces and advance this emerging field of oxide electronics.

MethodsThe devices were fabricated by pulsed laser deposition using TiO2-terminated

Nb:SrTiO3(001) substrates (Nb5 0.01 wt.%). An SrMnO3single unit cell was first

grown to enhance the SBH and suppress reverse bias leakage 15, and subsequentlyLSMO films were grown at the O2partial pressure of 10

21 Torr, the substrate tem-perature of 850uC, and the laser fluence of 0.8 J/cm2. The deposited layer thicknesseswere controlled by using reflection high-energy electron diffraction intensity oscil-lations. For Ohmic contacts, gold was evaporated onto the LSMO, and indium wasultrasonically soldered onto the Nb:STO. A modified commercial STM system fromRHK Technology was used for the BEEM studies. All t he BEEM measurements wereperformed at 300 K using PtIr metaltips,in theconstant current mode. A Pt wire was

used to ground the top metal contact. Indium solder defined the Ohmic contact.Typically 10 devices were fabricated for each thickness. Each BEEM spectra repre-sents an average of at least 50 individualIBspectra, measured by positioning the STMtip at several different regions of the film. Approximately four devices of eachthickness were measured. The BEEM current is detected with a two-stage amplifier(1011 V/A).

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Figure 4| Spatially resolved electron transmission through LSMO (9 u.c.)/Nb:STO (001) epitaxial heterostructure. (a), (c), Topographic STM imagetaken at VT5 22.5 V and IT5 8 nA over a 0.55 3 0.55 mm

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the inset of d. The mean value of the distribution matches well with that of the BEEM transmission of this device as shown in Fig. 2 (left).



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AcknowledgementsWe acknowledge financial support from the NWO-VIDI program and the NanoNed

program coordinated by the Dutch Ministry of Economic Affairs (K.G.R., S.P. & T.B.) andthankR. Ruiter for helpwith an illustration. We acknowledge support fromthe Departmentof Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering,under contract DE-AC02-76SF00515 (T.Y., A.F.K., T.P.D., Y.H. & H.Y.H.). T.Y. alsoacknowledges support from the Japan Society for the Promotion of Science (JSPS).

Authors contributionsT.Y. performed the device fabrication, the current-voltage, capacitance-voltagemeasurements, and the internal photoemission spectroscopy. K.G.R. performed all theBEEM measurementsalongwithS.P.andcontributedto theanalysisof thedata; theoreticalfits tothe extracteddatawereperformed byA.F.K.and T.P.D.Y.H. andH.Y.H.assistedwiththe planning and the measurements. T.B. conceived and supervised the research andcontributed to the analysis of the data. All co authors extensively discussed the results and

provided important insights. T.B. wrote the manuscript with input from all co authors.

Additional informationSupplementary informationaccompanies this paper athttp://www.nature.com/scientificreports

Competing financial interests:The authors declare no competing financial interests.

License:This work is licensed under a Creative CommonsAttribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this

license, visithttp://creativecommons.org/licenses/by-nc-nd/3.0/

How to cite this article:Rana, K.G.et al. Hot electron transport in a strongly correlatedtransition-metal oxide.Sci. Rep.3, 1274; DOI:10.1038/srep01274 (2013).


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