LaAlO3

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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 20950−20958

Largely Enhanced Mobility in Trilayered LaAlO3/SrTiO3/LaAlO3 Heterostructures Hai-Long Hu,† Anh Pham,† Richard Tilley,‡ Rong Zeng,† Thiam Teck Tan,† Chun-Hua (Charlie) Kong,‡ Richard Webster,‡ Danyang Wang,*,† and Sean Li*,† †

School of Materials Science and Engineering and ‡Mark Wainwright Analytical Centre, University of New South Wales, Sydney, New South Wales 2052, Australia

ACS Appl. Mater. Interfaces 2018.10:20950-20958. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/05/19. For personal use only.

S Supporting Information *

ABSTRACT: LaAlO3 (LAO)/SrTiO3 (STO)/LaAlO3 (LAO) heterostructures were epitaxially deposited on TiO2-terminated (100) SrTiO3 single-crystal substrates by laser molecular beam epitaxy. The electron Hall mobility of 1.2 × 104 cm2/V s at 2 K was obtained in our trilayered heterostructures grown under 1 × 10−5 Torr, which was significantly higher than that in single-layer 5 unit cells LAO (∼4 × 103 cm2/V s) epitaxially grown on (100) STO substrates under the same conditions. It is believed that the enhancement of dielectric permittivity in the polar insulating trilayer can screen the electric field, thus reducing the carrier effective mass of the two-dimensional electron gas formed at the TiO2 interfacial layer in the substrate, resulting in a largely enhanced mobility, as suggested by the firstprinciple calculation. Our results will pave the way for designing highmobility oxide nanoelectronic devices based on LAO/STO heterostructures. KEYWORDS: laser molecular beam epitaxy, LaAlO3/SrTiO3/LaAlO3 heterostructures, dielectric permittivity, carrier effective mass, two-dimensional electron gas, electron mobility

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INTRODUCTION The discovery of two-dimensional electron gas (2DEG) at the interface between the two insulating oxides SrTiO3 (STO) and LaAlO3 (LAO) has triggered intensive research in this area due to the remarkable functional properties, such as superconductivity, ferromagnetism, and tunable conductivity.1−5 The electron reconstruction due to a polar discontinuity at the interface was considered to be the possible origin of 2DEG at LAO/STO interface.6−8 Other mechanisms, such as cation intermixing across the interface9 and oxygen deficiency,10,11 have also been reported to explain the formation of 2DEG at LAO/STO interface. The advances of the microelectronic industry over the last decade have required new devices to exponentially increase their performance by lowering the resistance of the semiconducting materials with simultaneously increased electron mobility.12−17 Oxides are one of the most promising material candidates for next generation nanoelectronic materials as they can function at elevated temperatures. Despite the controversy with respect to the origin of 2DEG at the interface of LAO/ STO, establishing electron confinement with increased electron mobility in LAO/STO system has become a major focus. Enhancement of mobility in complex oxide 2DEGs remains a very challenging task. The reported electron mobility at the interface between LAO and STO was often just a few thousand cm2/V s, with the sheet carrier density around 1013 cm−2.18 To © 2018 American Chemical Society

increase the electron mobility in LAO/STO system, different methods to control defect scattering,19 surface control,20 modulation-doping,21 and growth strategies22,23 have been attempted. Up to date, the highest mobility in LAO/STO 2DEG reported in the literature was ∼104 cm2/V s at 2 K.21 In addition to control of the growth method, an alternative strategy to improve carrier mobility of the 2D conducting channel in the LAO/STO interface is to increase carrier confinement on the STO surface by enhancing the dielectric permittivity at the interface of the STO substrate and the LAO layer. Different buffer layers like LaTiO3 and La-doped SrTiO3 have been employed to module the carrier screening at the interface of the LAO/STO to enhance the carrier mobility in the 2DEG system.24,25 Recently, much attention has been paid to the theoretical and experimental studies of LAO/STO/LAO quantum well structure containing two n-type interfaces with the LaO and TiO2 stacking.26−28 It was predicted that the excess charge of 0.5e at the polar interface might bring a charge-ordered phase with dxy orbital polarization, or the excess of 0.5e was delocalized throughout this trilayered heterostructure.28 The multilayered LAO/STO/LAO system has also been predicted Received: February 14, 2018 Accepted: May 31, 2018 Published: May 31, 2018 20950

DOI: 10.1021/acsami.7b11218 ACS Appl. Mater. Interfaces 2018, 10, 20950−20958

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Schematic diagram of the trilayered, five-layered, and seven-layered heterostructures grown on TiO2-terminated STO substrate; (b) AFM image of trilayered LAO/STO/LAO grown under 1 × 10−5 Torr, showing step flow surface morphology; (c) XRD θ−2θ diffraction pattern for trilayered, five-layered, and seven-layered LAO/STO heterostructures grown under oxygen partial pressure of 1 × 10−5 Torr (S is for substrate STO, F is for film LAO, both −1, −2 and 1, 2 are for the first-order superlative satellites); XRD RSM around (103) reflection for (d) trilayered, (e) fivelayered, and (f) seven-layered LAO/STO heterostructures grown under 1 × 10−5 Torr.

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to exhibit topological superconductivity.29 In addition, the trilayer system includes two LAO layers separated by one STO layer, which can significantly modify the electric field on the STO substrate surface due to the large dielectric constant of STO at low temperature.30 Consequently, such a system might have enhanced electron mobility due to the screening effect originated from large dielectric permittivity in comparison with the conventional LAO/STO heterostructure.31 In this work, we obtained a high electron Hall mobility and carrier density of 1.2 × 104 cm2/V s and 3.0 × 1013 cm−2 at 2 K, respectively, in a trilayered LAO/STO/LAO 2DEG. We also theoretically demonstrated the high mobility can be attributed to the enhanced polar field and the reduced carrier effective mass in this particular materials system. The high mobility in our LAO/STO/LAO heterostructures may provide a new paradigm for the development of all-oxide nanoelectronic devices, such as tunnel junctions and high mobility field-effect transistors.32

EXPERIMENTAL DETAILS

The trilayered LAO/STO/LAO heterostructures were grown on the TiO2-terminated (100) STO single-crystal substrates by laser molecular beam epitaxy with a KrF excimer laser (λ = 248 nm). The thickness of each layer is 5 unit cells (uc). Single-crystal LAO and STO targets were used for the thin-film deposition. The heterostructures were deposited at 850 °C under oxygen partial pressure of 1 × 10−5 Torr. The laser energy density was 1.275 J/cm2, and the repetition rate was 0.5 Hz. The film growth was real-time monitored by in situ reflection high-energy electron diffraction (RHEED). After deposition, the samples were cooled to room temperature under the deposition oxygen partial pressures at a rate of 20 °C/min without postannealing treatment. We also deposited 5 uc LAO thin film, fivelayered (5 uc LAO/5 uc STO/5 uc LAO/5 uc STO/5 uc LAO) and seven-layered (5 uc LAO/5 uc STO/5 uc LAO/5 uc STO/5 uc LAO/ 5 uc STO/5 uc LAO) heterostructures on TiO2-terminated STO(001) substrates under the same conditions for comparison purpose. X-ray diffraction (XRD) with CuKα radiation and four-bounce Ge(220) monochromator was used to perform crystallographic 20951


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Figure 2. (a) Typical RHEED intensity oscillations for deposition of trilayered LAO/STO/LAO on TiO2-terminated STO. RHEED patterns for (b) before deposition and (c) after deposition of trilayered LAO/STO/LAO grown under oxygen partial pressure of 1 × 10−5 Torr; (d) high-angle annular dark field STEM image and EDX analysis of trilayered LAO/STO/LAO grown under oxygen partial pressure of 1 × 10−5 Torr. characterization of the LAO/STO/LAO heterostructures. Surface morphology of the samples was imaged by an atomic force microscopy (AFM, Bruker Dimension ICON SPM) operating in tapping mode. The interfacial structures of LAO/STO heterostructures were analyzed by using the high-resolution transmission electron microscopy. Energy-dispersive X-ray spectroscopy was employed to study the elemental distribution across the interfaces. The sheet resistance, carrier density, and hall mobility were measured using the physical property measurement system (Quantum Design, San Diego, CA) with van der Pauw geometry. Direct contact with the interface of heterostructures/STO substrate was made by aluminum wire using an ultrasonic wire-bonding technique. A magnetic field perpendicular to the sample surface of up to 10 T was applied to conduct the magnetoresistance (MR) measurements. The nominal sheet carrier density n2D was determined by the n2D = −B/eRxy, where B is the applied magnetic field, Rxy is the Hall resistance, and e is the charge of an electron. The mobility μ was determined from the sheet resistance Rs and n2D by μ = 1/en2DRs. To understand the effect of dielectric permittivity in the two different heterostructures, the dielectric permittivity was measured.

Although the peak of STO layer cannot be resolved from the strong STO substrate peak, the characteristic spots of LAO are clearly observed. The satellite spots (marked with “−1, −2, 1, and 2”) correspond to the superlattice structure. The thickness of the as-deposited films was determined by RHEED intensity oscillations, as shown in Figure 2a. The streaky RHEED patterns shown in Figure 2b,c confirmed the layer-by-layer growth mode. Figure 2d shows the scanning transmission electron microscopy (STEM) image of trilayered LAO/STO/LAO heterostructures. The highly coherent lattice across the interface was revealed. Atomic-scale energydispersive X-ray spectroscopy (EDX) spectral mapping was also performed on the sample, indicating a slight cation intermixing at the bottom LAO layer. Scanning transmission electron microscopy image and atomic-scale energy-dispersive X-ray spectroscopy results of seven-layered heterostructures are illustrated in Figure S1. Figure 3a shows the Hall resistance as a function of the applied magnetic field at different temperatures for the trilayered heterostructure. A linear relationship between the Hall resistance and the applied field in the temperature range from 100 to 300 K suggests a normal Hall effect. The Hall resistance shows a strong nonlinearity against the applied magnetic field at temperatures below 50 K. Such a nonlinear behavior was associated with the double electron channels where spatially separated multichannels of carriers with different mobilities existed in this trilayered LAO/STO/LAO heterostructure. A two-band model was used to extract the multichannel conduction arising from the different electronic

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RESULTS AND DISCUSSION The schematic diagram of the trilayered, five-layered, and seven-layered heterostructures is shown in Figure 1a. Figure 1b shows the AFM image of the trilayered sample. Typical terrace morphology was observed with a very low surface roughness Ra of ∼0.164 nm. Figure 1c shows the XRD θ−2θ patterns of the trilayered, five-layered and seven-layered samples. The satellite peaks of the superlattice structure were also clearly seen in the spectra. XRD reciprocal space mappings (RSM) around (103) plane of the heterostructures are shown in Figure 1d−f. 20952


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Figure 3. (a) Hall resistance Rxy of trilayered LAO/STO/LAO grown under 1 × 10−5 Torr; (b) carrier density and (c) mobility as a function of temperature below 50 K. The bold fitting lines based on two-band model are also shown in (a).

electron mobility in this LAO/STO system was typically ∼1000 cm2/V s at 2 K.18 Figure 4a shows sheet resistance Rs of the trilayered heterostructure against magnetic field along various directions (φ = 0, 30, and 60°) measured at 2 K. The inset of Figure 4a shows the angular relationship between the direction of the applied magnetic field and sample surface. φ = 0 and 90° are the out-of-plane and in-plane directions, respectively. It is apparent that the perpendicular component of the applied magnetic field contributed to the 2DEG quantum oscillation. The Shubnikov−de Haas (SdH) oscillation was resulted from the high mobility in the light effective mass sub-band with low electron carrier density, representing a direct measurement of the area of the Fermi surface.35 After subtracting the magnetoresistance background, pronounced oscillations as a function of 1/B are shown in Figure 4b. In Figure 4c, the angular dependence of sheet resistance clearly illustrates the anisotropic nature of trilayered LAO/STO/LAO. The resistance as a function of sample position φ with respect to the applied magnetic field exhibits a dip at 90 and 270°, which is a feature of 2D electron transport.36 The anisotropy and MR were suppressed at temperatures above 100 K. In addition, a closer look at angular dependence of magnetoresistance at 2 K and 10 T reveals a negative MR with in-plane geometry (90 and 270°) in Figure 4d. Temperature dependence of sheet resistance, sheet carrier density, and mobility of this trilayered LAO/STO/LAO grown under different oxygen partial pressures were also investigated to verify its conducting behavior, as shown in Figure S2. In addition, the magnetoresistance behavior of this trilayered LAO/STO/LAO grown

bands.33,34 In this model, the magnetic field (B) dependence of hall resistance Rxy(B) can be described as R xy(B) = B[(μ12 n1 + μ22 n2) + (μ1μ2 B)2 (n1 + n2)] /e[(μ1|n1| + μ2 |n2|)2 + (μ1μ2 B)2 (n1 + n2)2 ] (1)

The longitudinal resistance Rxx(0) at zero magnetic field can be calculated by R xx(0) =

1 (n1μ1 + n2μ2 ) e

(2)

where e is the electron charge, B is the applied magnetic field, n1 and n2 and μ1 and μ2 are carrier density and mobility, respectively. The carrier density n1 and mobility μ1 are associated with the main conducting channels, whereas n2 and μ2 are from the minor conducting channels. The Hall resistance below 50 K can be fitted with the two-band model very well, and the resultant data for the carrier density and mobility are shown in Figure 3b,c, respectively. These results show that the main channel has a carrier density of ∼1014 cm−2, whereas the carrier density of the other channel is 1012−1013 cm−2. The mobilities of the two channels also render 1 order of magnitude difference. Meanwhile, the mobility μ1 of ∼1.2 × 104 cm2/V s at 2 K is very similar to that deduced from the Hall measurement. A lower mobility μ2 of ∼1.5 × 103 cm2/V s at 2 K was observed. The carrier mobility of our trilayered heterostructures is among the highest values of the LAO/ STO 2DEG system reported in the literature, given that 20953


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Figure 4. Two-dimensional quantum oscillation of the conduction at the interface (LAO/STO) of trilayered LAO/STO/LAO grown under 1 × 10−5 Torr. (a) Sheet resistance Rs as a function of magnetic fields applied along different directions; (φ is depicted with the directions of both the current and applied field being considered, where 0° is out-ofplane and 90° is in-plane); (b) amplitude of the SdH oscillations, ΔRSdH as a function of reciprocal magnetic field 1/B. The measurements were conducted at 2 K. Electrical measurements for the 1 × 10−5 Torr sample: (c) sheet resistance as a function of angular position at different temperatures at 10 T magnetic field. (d) Magnetoresistance with respect to angular position at 2 K under a magnetic field of 10 T.

Figure 5. Comparison of single 5 uc LAO, trilayered, five-layered, and seven-layered LAO/STO heterostructures grown under 1 × 10−5 Torr on TiO2-terminated STO. (a) Temperature dependence of sheet resistance (Rs); (b) The Hall resistance Rxy of single 5 uc LAO; (c) temperature dependence of sheet carrier density (ns) and (d) mobility (μ). 20954


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Figure 6. Spin-polarized charge density and the partial density of states with the contribution of Ti’s d and O’s p of the trilayered 5 uc LaAlO3/5 uc SrTiO3/5 uc LaAlO3/7 uc SrTiO3.

Figure 7. Partial density of states with the contribution of Ti’s d and O’s p of the 5 uc LaAlO3/7 uc SrTiO3.

Sheet resistance as a function of temperature for the 5 uc LAO sample was higher than that of the trilayered heterostructure over the entire measuring temperature range, as shown in Figure 5a. Hall resistance Rxy of 5 uc LAO sample against an applied magnetic field (B) in the sample showed a linear relationship in Figure 5b. This linear relationship between Rxy and the perpendicular applied field suggested a single-band mode of transport. In addition, the carrier density between ∼4 × 1013 and 1.5 × 1014 cm−2 (in Figure 5c) indicated the twodimensional confined conducting channel.3 It should be noted that carrier mobility of the trilayered sample was largely enhanced compared with that of the single 5 uc LAO sample (Figure 5d). Specifically, the mobility of trilayered LAO/STO/ LAO at 2 K was ∼1.2 × 104 cm2/V s, whereas it was only ∼4 × 103 cm2/V s for single 5 uc LAO. Mobilities of the five-layered and seven-layered heterostructures were both ∼1.3 × 104 cm2/ V s at 2 K, i.e., no substantial change was observed compared with the trilayered sample. To further understand the mechanisms of the largely enhanced mobility in our trilayered heterostructures, density functional theory was employed to study the electronic properties of the heterostructures. The first-principle calculations were done using VASP software with the projected augmented wave method37 and the Perdew−Burke−Ernzerhof (PBE) generalized gradient approximation38 with an energy cutoff of 450 eV 9 × 9 × 1 k-points. To simulate the electronic properties of the heterostructures, slab configurations were

Figure 8. Electrostatic potential and its average for (a) STO/LAO structure and (b) trilayered STO/LAO/STO/LAO heterostructures. The in-plane average calculated for a lattice was 3.94 Å.

under different oxygen partial pressures was systematically studied in Figure S3. To understand the nature of the observed high electron mobility, transport properties of a single 5 uc LAO thin-film and five-layered and seven-layered samples were measured. 20955


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density in Figure 6. This is significantly different from the configuration when only 5 uc LAO is deposited on the STO substrate, as shown in Figure 7, where the band alignment occurs between the nonmagnetic Ti’s d interface and the surface AlO2−1 of the 5 uc LAO film. In addition, it should also be emphasized that second interface between the STO layer the top LAO film is insulating, which suggests that only one 2DEG in our trilayered system occurs due to the band alignment between second LAO film and the STO substrate. Since the second LAO layer is separated from the STO substrate by 5 uc LAO and 5 uc STO film, this can significantly influence the electric field in the two LAO layers in the trilayered system. In the normal LAO/STO structure, the electric field on the LAO film is estimated to be 0.15 eV/Å on the basis of the slope of macroscopic average electrostatic potential in Figure 8a. This is within the range of 0.15−0.24 eV/Å in the previous studies of the LAO/STO system.40−42 On the other hand, the two LAO layers in the trilayered system exhibit two significant smaller electric fields of 0.09 and 0.085 eV/Å (in Figure 8b) for the layer closest to the STO substrate (7 uc) and the top LAO layer, respectively. These significantly smaller values suggest that the inclusion of the STO film in the trilayered system has screened out the polar field due to the large dielectric permittivity of the STO material in comparison with the LAO layer. To confirm the larger dielectric permittivity in the trilayered heterostructure, the dielectric permittivity of the LAO (5 uc)/STO (5 uc)/LAO (5 uc) was measured. as shown in Figure 9. Our measurement confirms the initial hypothesis that the trilayered system has a much larger dielectric permittivity than the stand-alone LAO film. Finally, to further understand the influence of the different polar fields on the carrier mobility of the 2DEG in the two oxide heterostructures, the band structure and effective mass of the lowest occupied conduction band Ti dxy in the SrTiO3 substrate (7 uc) in the LAO/STO and LAO/STO/LAO/STO systems were calculated, as presented in Figure 10. For the single LAO layer on STO substrate, the carrier effective mass is calculated to be 0.389 me (Figure 10a), which is consistent with the previous theoretical study of the light electron effective mass in SrTiO3.43 However, this effective mass reduces significantly for the complex heterostructures LAO/STO/LAO. The light electron effective mass corresponding to the lowest occupied Ti dxy is 0.285 me for the spin-up bands and 0.316 me for the spin-down bands in (Figure 10b). The significantly lower carrier effective mass in

Figure 9. Dielectric permittivity measured at room temperature for single-layer 5 uc LAO and trilayered 5 uc LaAlO3/5 uc SrTiO3/5 uc LaAlO3. Repeated measurements (labeled first and second) were performed to confirm the results.

used with 5 uc LAO/5 uc STO/5 uc LAO/7 uc STO and 5 uc LAO/7 uc STO with a minimum vacuum layer of 20 Å. The two slab configurations were done with the same volume of the supercell to avoid any size effect. The DFT + U method within the Dudarev scheme39 was used similar to the previous study,28 with U = 5.0 eV and J = 0.7 eV applied on Ti’s d and U = 9.0 eV and J = 1 eV on La’s f orbitals to correct for the localization problem in the PBE functional. The in-plane lattices of the slab structures were set at the bulk SrTiO3 value obtained from the PBE functional (3.94 Å). The carrier effective mass was calculated numerically on the basis of the band structure using 1 m*

=

1 ℏ2

∂ 2E ∂k 2

( ). The band structure was calculated without the

spin−orbit interaction, which results in symmetric effective mass of light electron band along the ΓX and ΓM directions. As shown in Figure 6, the heterostructures consisting of 5 uc LaAlO3/5 uc SrTiO3/5 uc LaAlO3 on the SrTiO3 substrate (7 uc SrTiO3) show a band alignment effect only between the top AlO2−1 layer and the top TiO2 layer of the substrate. The 2DEG TiO2 interface layer is shown to be spontaneously spinpolarized with a magnetic moment of 0.061 μB/Ti. Small magnetic moments are also observed for the O’s p surface (0.032 μB/O’s p), as illustrated in the spin-polarized charge

Figure 10. Band structure and carrier effective mass of the lowest occupied conduction band for (a) LAO/STO configuration and (b) trilayered LAO/STO/LAO/STO configuration. 20956



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the complex heterostructures leads to an improvement in carrier mobility in the trilayered heterostructure, as shown in Figure 5d. Thus, it is believed that the carrier transport can be improved by engineering the polar field through enhancing the dielectric permittivity of the polar layer, thus resulting in a lower carrier effective mass of the occupied Ti dxy orbital at the LAO/STO heterostructures, leading to the largely enhanced mobility in trilayered heterostructures.

CONCLUSIONS In conclusion, epitaxial heterostructures of LAO/STO/LAO were deposited on TiO2-terminated (100) SrTiO3 single substrates by laser molecular beam epitaxy. In contrast to the mobility of ∼103 cm2/V s in a typical LAO/STO 2DEG at 2 K, a greatly enhanced electron mobility of 1.2 × 104 cm2/V s was obtained in our trilayered heterostructures. Because of the increased dielectric permittivity of the polar layer, the carrier effective mass in the lowest conduction band in these heterostructures was significantly lower than that of a singlelayer 5 uc LAO. Our results suggest that the mobility of the oxide 2DEG system can be largely enhanced by engineering the polar field in the 2DEG through constructing trilayered LAO/ STO/LAO heterostructures. The high mobility of our 2EDG may open a new path for designing all-oxide nanoelectronic devices with the LAO/STO heterostructures. ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b11218. STEM image and EDX analysis of the seven-layered heterostructures, Figure S1; investigation of temperature dependence of sheet resistance, sheet carrier density, and mobility of the trilayered LAO/STO/LAO grown under different oxygen partial pressures, Figure S2; study of magnetoresistance behavior of this trilayered LAO/ STO/LAO grown under different oxygen partial pressures, Figure S3 (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.W.). *E-mail: [email protected] (S.L.). ORCID

Richard Tilley: 0000-0003-2097-063X Danyang Wang: 0000-0002-7883-8001 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research was supported by the Australian Research Council through projects DP150103006 and DP140104373. This work was performed in part at the NSW Node of the Australian National Fabrication Facility, a company established under the National Collaborative Research Infrastructure Strategy to provide nano- and microfabrication facilities for Australia’s researchers. 20957


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