All-perovskite transparent high mobility field effect using epitaxial BaSnO3 andLaInO3Useong Kim, Chulkwon Park, Taewoo Ha, Young Mo Kim, Namwook Kim, Chanjong Ju, Jisung Park, JaejunYu, Jae Hoon Kim, and Kookrin Char Citation: APL Materials 3, 036101 (2015); doi: 10.1063/1.4913587 View online: http://dx.doi.org/10.1063/1.4913587 View Table of Contents: http://scitation.aip.org/content/aip/journal/aplmater/3/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Infrared-optical spectroscopy of transparent conducting perovskite (La,Ba)SnO3 thin films Appl. Phys. Lett. 104, 022102 (2014); 10.1063/1.4861776 Epitaxial growth of (111)-oriented BaTiO3/SrTiO3 perovskite superlattices on Pt(111)/Ti/Al2O3(0001)substrates Appl. Phys. Lett. 103, 112902 (2013); 10.1063/1.4820780 Large effects of dislocations on high mobility of epitaxial perovskite Ba0.96La0.04SnO3 films Appl. Phys. Lett. 102, 252105 (2013); 10.1063/1.4812642 Influence of octahedral tilting on the microwave dielectric properties of A 3 LaNb 3 O 12 hexagonalperovskites ( A = Ba , Sr) Appl. Phys. Lett. 94, 192904 (2009); 10.1063/1.3129867 Structural and dielectric properties of epitaxial Ba 1 − x Sr x Ti O 3 films grown on La Al O 3 substrates bypolymer-assisted deposition Appl. Phys. Lett. 85, 5007 (2004); 10.1063/1.1827927

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APL MATERIALS 3, 036101 (2015)

All-perovskite transparent high mobility field effect usingepitaxial BaSnO3 and LaInO3

Useong Kim,1 Chulkwon Park,1 Taewoo Ha,2 Young Mo Kim,1Namwook Kim,3 Chanjong Ju,1 Jisung Park,1 Jaejun Yu,3 Jae Hoon Kim,2and Kookrin Char1,a1Institute of Applied Physics, Department of Physics and Astronomy, Seoul NationalUniversity, Seoul 151-747, South Korea2Department of Physics, Yonsei University, Seoul 120-749, South Korea3Center for Theoretical Physics, Department of Physics and Astronomy, Seoul NationalUniversity, Seoul 151-747, South Korea

(Received 13 January 2015; accepted 16 February 2015; published online 25 February 2015)

We demonstrate an all-perovskite transparent heterojunction field effect transistormade of two lattice-matched perovskite oxides: BaSnO3 and LaInO3. We havedeveloped epitaxial LaInO3 as the gate oxide on top of BaSnO3, which were recentlyreported to possess high thermal stability and electron mobility when doped withLa. We measured the dielectric properties of the epitaxial LaInO3 films, such as theband gap, dielectric constant, and the dielectric breakdown field. Using the LaInO3as a gate dielectric and the La-doped BaSnO3 as a channel layer, we fabricated fieldeffect device structure. The field effect mobility of such device was higher than90 cm2 V−1 s−1, the on/off ratio was larger than 107, and the subthreshold swingwas 0.65 V dec−1. We discuss the possible origins for such device performance andthe future directions for further improvement. C 2015 Author(s). All article content,except where otherwise noted, is licensed under a Creative Commons Attribution 3.0Unported License. [http://dx.doi.org/10.1063/1.4913587]

The most fundamental building block of a complex circuitry in the field of electronics, amongthe diverse forms of devices, is a field effect transistor (FET) which controls the conductivity ofchannel by applying voltage to the gate electrode. In this respect, demonstration of the FET madeentirely of perovskite oxide layers, which we may call all-perovskite FET, is the first step towardsfull-fledged oxide electronics exploiting versatile novel properties emerging at the oxide inter-faces.1–3 There have been endeavors to realize all-perovskite devices ranging from a ferroelectricFET applicable to nonvolatile memory elements4 to the LaAlO3/SrTiO3 heterostructure controllingthe superconductivity.5 However, no conventional thin film FET, to the extent of our knowledge, hasbeen demonstrated using epitaxial perovskite layers exclusively.

In this article, we demonstrate a fully functioning all-perovskite FET which has a conven-tional metal-insulator-semiconductor structure. We chose BaSnO3 (BSO) as a channel layer becauserecent rediscovery of BSO as a perovskite oxide semiconductor exhibits excellent transport prop-erties of BSO among perovskite oxides.6,7 When compared with SrTiO3 (STO), which is themost popular perovskite oxide semiconductor at present, BSO is 7 orders-of-magnitude more ther-mally stable in terms of the oxygen diffusion constant and almost about 30 times more mobileat room temperature in spite of its dislocation-limited transport at present.6,8,9 As a dielectriclayer, a perovskite LaInO3 (LIO) was chosen in our device because LIO, in its pseudo-cubicstructure, has similar in-plain lattice parameters (aLIO = 4.124 Å and bLIO = 4.108 Å) to BSO(aBSO = bBSO = 4.116 Å).6,10 By combining these two epitaxial perovskite layers, we fabricatedexcellent interfaces for the carrier accumulation in our devices.

aAuthor to whom correspondence should be addressed. Electronic mail: kchar@phya.snu.ac.kr

2166-532X/2015/3(3)/036101/7 3, 036101-1 ©Author(s) 2015

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036101-2 Kim et al. APL Mater. 3, 036101 (2015)

FIG. 1. Structural properties of LIO thin film. (a) θ-2θ diffraction pattern shows three diffraction peaks correspondingto (001), (002), and (003) of pseudocubic LIO. ω-rocking curve presented as an inset was measured at around the (002)diffraction peak. (b) The reciprocal space map shows the pseudocubic (103) plane of LIO.

Since LIO has not been reported in the form of an epitaxial film and has been relatively un-known material as a dielectric, we present our studies in this regard before discussing the fabricationand performance of our device. We therefore discuss first, the epitaxial growth of LIO films on BSOlayer and the lattice mismatch between two layers. Next, we introduce the optical band gap and theelectronic structure of LIO. It follows that the dielectric properties of our LIO epitaxial films suchas the dielectric constant (κ) and the dielectric strength (FBD) to discuss how eligible our LIO layeris as a dielectric. Finally, we demonstrate the excellent performance of our all-perovskite FET madeof LIO/BSO epitaxial heterojunction in terms of three conventional parameters: field effect mobility(µFE), on/off ratio (Ion/Ioff), and subthreshold swing (S).

All samples, including the layers in our device, were deposited on STO or LaAlO3 (LAO)substrates using laser ablation technique with following conditions: 750 ◦C temperature, 100 mTorrO2 pressure, and 1 ∼ 1.5 J/cm2 energy fluence. To fabricate lateral patterns on the device, we usedstencil masks made of Si or stainless steel.

We performed the X-ray diffraction measurement to examine the crystallinity of the LIO layerdeposited on the BSO layer. The sample under investigation was the 100 nm thick LIO layerdeposited on the 10 nm thick BSO buffer layer on a STO substrate. In Figure 1(a) is presentedθ-2θ diffraction pattern of the LIO layer where the pseudocubic phase of LIO is clearly seen withno secondary phases and no other orientation than (00l). Additionally, the small full width at halfmaximum of ω-rocking curve, presented in the inset, confirms high crystalline epitaxial growth ofLIO film on the BSO layer. Reciprocal space mapping was performed to obtain the lattice parame-ters of our LIO epitaxial film and to estimate the structural quality of the LIO/BSO interface. Theresult is shown in Figure 1(b). The pseudocubic (103) plane of LIO is clearly seen with the recip-rocal space vectors Qx = 1.528 Å−1 and Qz = 4.542 Å−1, corresponding to pseudocubic in-planelattice and out-of-plane lattice parameters apc

LIO = 4.111 Å and cpcLIO = 4.150 Å, respectively. apc

LIO isnearly identical to the previously reported in-plane lattice parameters of La- or Sb- doped BSO filmsgrown on STO substrates.10,11 This result implies that the interface between the LIO and BSO layersis almost free of misfit dislocations caused by the lattice mismatch.

To determine the optical band gap (Eg) of our LIO epitaxial film, we measured transmittance ofthe film deposited on the LAO substrate which is known to have larger bandgap (Eg,LAO = 5.6 eV)than LIO.12 Assuming no reflection, we extracted the absorption coefficient (α) of the film from theformula I = I0 exp(−α∆), where I, I0, and ∆ are the intensity of transmitted light, the intensity oflight source, and the film thickness, respectively. In Figure 2(a) is presented the plot of (α~ω)2 vs.

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036101-3 Kim et al. APL Mater. 3, 036101 (2015)

FIG. 2. Optical Absorption and electronic band structure of LIO. (a) The absorbance of LIO (40 nm)/LAO substrate wasmeasured. Assuming zero reflectance, (α~ω)2 vs. ~ω plot is constructed. The black line is the line of extrapolation whichshows the LIO film has a direct band gap with the magnitude of about 5.0 eV. (b) We performed the first-principle calculationsand obtained the electronic band structure of LIO. The DOS diagram on the right side is the projection of the band diagraminto the energy space. The inset is a zoomed-in diagram for the region from 3 to 5 eV.

~ω, where ~ω is the incident photon energy. The plotted experiment data near the absorption edgewere well fitted with a line, which implies the direct optical transition at 5.0 eV.

We performed the first-principle calculations and obtained the electronic band structure aspresented in Figure 2(b). According to the electronic band structure and the associated density ofstates (DOS) diagram, the valence band maximum (VBM) is almost composed of O 2p state; theconduction minimum (CBM) has significant contributions from both In 5s and O 2p states. Thedirect optical transition can occur at Γ point, which supports the result of our optical absorptionexperiment. The calculated band gap is 3.1 eV and about 40% smaller than the optical gap wemeasured. The underestimation of the band gap, however, is not uncommon in the calculation withthe local density approximation.13 We also calculated the hole and electron effective masses of LIO(m∗h,LIO and m∗e,LIO), respectively, for later use in this article. The results are presented in Table I.

To measure dielectric properties of LIO epitaxial films, we made a junction structure in whicha 244 nm thick LIO dielectric layer was sandwiched between two highly conducting 4% La-dopedBSO (BLSO) layers with an overlapped area of 17 680 µm2. In order to measure the capacitance weapplied AC voltage, the root-mean-square magnitude of which was 30 mV with frequency variationfrom 103 to 105 Hz. Admittance (Y ) and phase shift (π − δ/2) were measured and analyzed with aparallel circuit model in which the parallel capacitance (Cp) and the dissipation factor (tan δ) arederived from the following relation: |Y | = ωCp

√1 + tan2δ. The results are presented in Figure 3(a).

TABLE I. LIO Effective masses calculated by the first-principlescalculations.

Direction from Γ m∗h,LIO/m0 m∗e,LIO/m0

X 0.53 0.41Z 1.5 0.52Y 4.6 0.50R 0.95 0.48T 0.80 0.47S 0.78 0.45

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036101-4 Kim et al. APL Mater. 3, 036101 (2015)

FIG. 3. The dielectric properties of LIO. (a) J -F characteristic of the LIO dielectric layer interposed by 4% BLSO contactswas obtained. The inset graph of ln(J/ηF2) vs. F−1 is plotted to analyze the FN tunneling process in the LIO layer. Theblack line is the linear fit from which we calculated the barrier height in LIO/BLSO interface. (b) The capacitance of thesame LIO dielectric layer was measured with respect to the applied frequencies of AC voltage. κ can be calculated from themeasured capacitance and the given dimensions. (c) The schematic of the band alignment between LIO and BSO systems isconstructed according to the experimental results discussed in Figs. 2(a) and 3(b).

Cp remains almost constant in the given frequency range, while tan δ is kept lower than 0.1 exceptthe verge of high frequency region. Consequently, κ is calculated to be 38.7 with the given geometryof the junction structure. This somewhat large value for κ is in the range of κ values reported forLIO ceramics with the orthorhombic phase.14

Next, we tested the dielectric strength (FBD) of LIO epitaxial films, which has not been reportedin previous literatures, for the subsequent field effect study. We measured the leakage current den-sity (J) flowing through the same junction structure with respect to the applied electric field (F)between the two BLSO electrodes. The result is presented in Figure 3(b). The dielectric breakdownoccurred at FBD = 3.13 MV cm−1. From the FBD and κ of the LIO epitaxial film, we deducedthe maximum carrier density modulation nmax = 7.4 × 1013 cm−2 in accordance with the definitionnmax = κε0FBD/e, where ε0 and e are the permittivity of the vacuum and the elementary charge,respectively.

As shown in the inset of Figure 3(b), J can be roughly divided into two regions. In the low-fieldregion, from 0 to about 2 MV cm−1, J seems to have no special feature. In the high-field regionwhere F is higher than 2 MV cm−1, however, J can be described by the Fowler-Nordheim (FN)tunneling process:

J =ηe3m∗BLSO

16π2~m∗e,LIOΦF2 exp

*..,−

4

2m∗e,LIOΦ3

3e~F+//-, (1)

where η, Φ, and m∗BLSO are the coefficients defined as the ratio of the effective FN tunneling area tothe total junction area, the height of tunneling barrier, and the effective mass of conduction electronsin the 4% BLSO contact layers, respectively. To extract Φ, we construct a plot of ln(J/ηF2) againstF−1 shown in the inset of Figure 3(b). From the slope of the fitting line, we obtain Φ = 1.15 eVusing m∗e,LIO = 0.46 m0, which is the harmonic mean of the effective masses along the directions ofT and S (Table I).

We are now in a position to estimate the band alignment between the LIO and BSO systems. Φcan be written as

Φ = ECB, LIO − EF, BLSO, (2)

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036101-5 Kim et al. APL Mater. 3, 036101 (2015)

FIG. 4. Structure and I-V characteristics of the LIO/BLSO heterostructure field effect device. (a) Schematic of the structureof our device is presented. (b) The top view of our device was pictured by an optical microscope. (c) Transfer characteristicof our FET in the linear region is presented. VDS was maintained at 1 V during the measurement. The maximum value ofµFE is higher than 90 cm2/Vs. Ion/Ioff and S are 107 and 0.65 V/dec, respectively. (d) Transfer characteristic of our FET inthe saturated region is presented. The red open circles represent I 1/2

DS which can be linearly fitted to obtain the Vth. The gmis also presented by the blue open circles. (e) The output characteristics of our device are presented by the red solid lines.7 traces are drawn in the range of VGS= 0 V to VGS= 30 V with 5 V interval. The black solid line and blue dashed linesare the semi-empirical boundary dividing the IDS into non-saturated and saturated regions and the semi-empirical outputcharacteristic of our FET, respectively. Both are estimated by the standard square-law theory.

where ECB,LIO and EF,BLSO are the energy levels of the CBM of LIO and the Fermi level of 4%BLSO, respectively. Assuming a parabolic band, we can calculate the following:

φ = EF, BLSO − ECB,BSO =~2�3π2n

�2/3

2m∗BLSO, (3)

where ECB,BSO and n are the energy levels of the CBM of BSO and the carrier density of 4% BLSO,respectively. If we use m∗BLSO = 0.42 m0 and n = 3.7 × 1020 cm−3 for the 4% BLSO,11 we obtainφ = 0.45 eV. The difference between the CBMs of LIO and BSO (∆ECB) is calculated from the sumof Φ and φ: ∆ECB = Φ + φ = 1.60 eV. Once ∆ECB is estimated, the difference between the VBMs(∆EVB) of LIO and BSO can be also estimated as 0.3 eV, using Eg’s of LIO (Eg,LIO = 5.0 eV) andBSO (Eg,BSO = 3.1 eV).6 The consequent alignment of the bands between LIO and BSO is estab-lished in Figure 3(c). Since LIO has a large enough band offset and the lattice-matched interfacewith BSO as well as high κ, it is a good candidate for epitaxial gate dielectrics of a BSO-basedFET.15

Figure 4(a) shows the structure of our FET. We deposited 100 nm-thick undoped BSO bufferlayer on the STO substrate as the first step to reduce the influence of threading dislocations on thechannel layer. As the second step, we deposited two line-patterned 0.07% BLSO channel layersusing a Si stencil mask. The thickness of the channel was controlled to be 10 nm. In the third step,the patterned 4% BLSO contact layers were deposited using a stainless steel stencil mask. As thefourth and last step, 244 nm-thick LIO dielectric and 4% BLSO gate layers were deposited in thatorder using Si stencils mask again. A top view of our FET is presented in Figure 4(b) to showthe lateral sizes of our FET. The channel width (W ) is 56 µm. The source and drain contacts wereseparated by 118 µm which became the channel length (L). The line-patterned gate had the width of143 µm.

The transfer characteristic of our FET in non-saturated region is presented in Figure 4(c). Inthe measurement of the source-drain current (IDS) and the gate leakage current (IGS), the drainvoltage (VDS) was set at 1 V while the gate voltage (VGS) was swept from −10 V to 33 V. First,µFE was calculated from the relation, µFE = (∂IDS/∂VGS) (Lt/W κε0VDS), where t is the thickness ofLIO dielectric, and plotted on the right axis in Figure 4(d); the maximum value of µFE is higherthan 90 cm2 V−1 s−1. Next, Ion/Ioff was calculated simply by dividing the maximum of IDS by theminimum of IDS; the calculated value is about 107. S was calculated as S =

�∂log10 (IDS) /∂VGS

�−1;

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036101-6 Kim et al. APL Mater. 3, 036101 (2015)

the minimum value of S is 0.65 V dec−1. These values are better than the recently reported FETusing BLSO as the channel layer and the atomic-layer-deposited AlOx as a gate oxide.16

To estimate the threshold voltage (Vth) of our FET, we measured the transfer characteristic ofour FET in the saturated region presented in Figure 4(d). VDS was set to be 20 V; VGS was swept from−10 to 20 V. When we plotted IDS

1/2 vs. VGS, it agreed well with a line fitting, which implied that ourFET conforms to the standard square-law theory of transistors: IDS,sat ∝ (VGS − Vth)2, where IDS,sat isthe saturated IDS.17 According to the theory, Vth can be estimated to be 1.9 V by reading the x-axisintercept of the fitting line. The transconductance (gm), defined as gm = dIDS/dV GS, is also plottedto show the potential of our FET as an amplifier.

The output characteristics of our FET are presented in Figure 4(e). VDS was applied up to 30V in our measurement due to the long channel length. VGS was varied from 0 V to 30 V with theinterval of 5 V. The corresponding 7 red solid lines in Figure 4(e) show the linear region in thelow-VDS limit and the saturated region in the high-VDS limit. To visualize the conformity of ourFET to the standard square-law theory, we first obtain the following semi-empirical boundary whichdivides IDS into the non-saturated and saturated regions:

IDS,sat = γ(VGS − Vth)2, (4)

where γ =�∂√

IDS/∂VGS�2 with VDS = VGS = 20 V. Once the boundary is established, IDS in the

non-saturated region (VDS < VGS − Vth) can be determined semi-empirically to be

IDS = 2γ(VGS − Vth)VDS −

V 2DS

2

, (5)

traces of which are shown as the blue dashed lines in Figure 4(e).The three device parameters, µFE, Ion/Ioff, and S, of our FET can be compared with those of a

LAO/STO-based device with the same top-gated structure.18 µFE is almost 20 times higher in ourFET and Ion/Ioff’s are comparable while S of our FET is higher than that of the LAO/STO-baseddevice which almost reaches the theoretical limit: (ln 10)(kT/e), where k and T are the Boltzmannconstant and the temperature, respectively.19 Since the higher S in our FET is probably the effectof the highly dense threading dislocations which pass through the interface and, thereby, createinterface charge traps, the S value can be lowered further if the density of threading dislocations isreduced by employing a lattice-matched substrate or more effective buffer layer than the undopedBSO layer. If our FET is compared with other perovskite-based FETs where hetero-interfacesbetween two single-crystalline perovskite oxides were employed: DyScO3/STO and CaHfO3/STOinterfaces, µFE, and Ion/Ioff are 40 times and an order higher in our FET, respectively.20,21 Theseperformances of our FET are also comparable to the best performances of In2O3:ZnO-based FET.22

The reasons for such high device performance can be attributed to the material properties ofBSO, the use of epitaxial gate dielectric, the quality of the LIO/BLSO interface, and the possibilityof the formation of two dimensional electron gas (2DEG) at the LIO/BLSO interface resulting fromits polar discontinuity. The high Hall mobility in the BSO system is definitely a basis of the highµFE in our FET. The high stability of the BSO system could play an important role in resisting theformation of oxygen vacancies, which can act as additional scattering sources and charge trappingcenters, in the interface between LIO and BLSO layers during the several high temperature thermalcycles in the device fabrication process. The epitaxy of LIO layer can also significantly reduce thedensity of interfacial or bulk charge traps as compared with amorphous gate dielectrics. The latticematching between LIO and BLSO layers contributes to reducing the density of interfacial defectssuch as the misfit dislocations and, thereby, reduces interface scattering rates of electrons. Judgingfrom the fact that the maximum value of µFE in our FET is higher than the highest Hall mobility(µH) of BLSO thin films reported to date, we may postulate that the polar discontinuity betweenthe LIO and BLSO leads to an electronic reconstruction across the LIO layer and the formation of2DEG-like highly conducting channel at the interface. More detailed investigation of such interfacewill be required.

In summary, we have successfully grown LIO epitaxial films, characterized the electronic anddielectric properties of LIO, and constructed its band alignment with BSO. Subsequently, we have

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036101-7 Kim et al. APL Mater. 3, 036101 (2015)

fabricated all-perovskite transparent FET utilizing the LIO/BLSO interface and obtained the highdevice performance despite the high density threading dislocations and the considerable lengthscale of the device which limit the ultimate mobility of the channel. Fortunately, these are solvableproblems by reasonable research efforts. In addition, we have found an indication that LIO/BLSOinterface might play an important role in the device performance. We believe further research onreduction of dislocations and the LIO/BLSO interface in detail will lead to much more opportunitiesfor science and technology.

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