Highly Mobile Two-Dimensional Electron Gases with a Strong Gating

Oct 3, 2017 - Two-dimensional electron gas (2DEG) at the perovskite oxide interface exhibits a lot of exotic properties, presenting a promising platfo...
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Highly Mobile Two-Dimensional Electron Gases with a Strong Gating Effect at the Amorphous LaAlO3/KTaO3 Interface Hui Zhang, Hongrui Zhang, Xi Yan, Xuejing Zhang, Qinghua Zhang, Jing Zhang, Furong Han, Lin Gu, Banggui Liu, Yuansha Chen, Baogen Shen, and Jirong Sun* Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, Peoples’ Republic of China School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, Peoples’ Republic of China S Supporting Information *

ABSTRACT: Two-dimensional electron gas (2DEG) at the perovskite oxide interface exhibits a lot of exotic properties, presenting a promising platform for the exploration of emergent phenomena. While most of the previous works focused on SrTiO3-based 2DEG, here we report on the fabrication of high-quality 2DEGs by growing an amorphous LaAlO3 layer on a (001)-orientated KTaO3 substrate, which is a 5d metal oxide with a polar surface, at a high temperature that is usually adopted for crystalline LaAlO3. Metallic 2DEGs with a Hall mobility as high as ∼2150 cm2/(V s) and a sheet carrier density as low as 2 × 1012 cm−2 are obtained. For the first time, the gating effect on the transport process is studied, and its influence on spin relaxation and inelastic and elastic scattering is determined. Remarkably, the spin relaxation time can be strongly tuned by a back gate. It is reduced by a factor of ∼69 while the gate voltage is swept from −25 to +100 V. The mechanism that dominates the spin relaxation is elucidated. KEYWORDS: two-dimensional electron gas, Hall mobility, gating effect, spin relaxation, weak localization, weak antilocalization

1. INTRODUCTION Highly mobile two-dimensional electron gases (2DEGs) at the LaAlO3/SrTiO3 (LAO/STO) interface have been intensively studied in the past decade,1−12 and exotic properties such as two-dimensional superconductivity,5 two-dimensional magnetism,6 giant gating effects,7−9 and efficient spin-to-charge conversion 11,12 have been observed, demonstrating the importance of the oxide 2DEG for both fundamental and applied research. In addition to LAO/STO, the anticipation for the 2DEG at the interface of other special oxides (this kind of 2DEG may inherit the distinct properties of its parent compounds) has aroused extensive investigations. Various heterostructures were synthesized and found to host a 2DEG as long as STO was involved, such as LaTiO 3 /STO, 13 GdTiO 3 /STO, 14−16 NdGaO3/STO,17 CaZrO3/STO,18 γ-Al2O3/STO,19,20 and even amorphous LAO/STO21,22 and amorphous YSZ/STO.23 Unfortunately, attempts to replace STO with other perovskite oxides are mostly unsuccessful except with CaTiO324 or KTaO3 (KTO).25 KTO shares many properties with STO, particularly the high permittivity which is desired by high-quality oxide 2DEG. Meanwhile, KTO is different from STO in the sense that it is a 5d transition-metal oxide with a strong spin−orbit coupling (SOC) which may lead to the formation of J = 1/2 Mott insulators,26,27 correlated topological insulators,28−30 and spin-triplet superconductors.31 Moreover, the strong SOC can © XXXX American Chemical Society

provide a feasible approach for the electric tuning of the spin state, offering opportunities to obtain all oxide devices with spintronic functionality. In a word, 5d oxide-based 2DEGs are an ideal platform to uncover the intertwined effects of SOC, quantum confinement, and electronic correlation, which are important topics of condensed matter physics. Because of the difficulty to generate the high-quality 2DEGs residing in 5d oxides via interface engineering, works in this regard are scarce. In the beginning, researchers tried to generate 2DEG by irradiating the vacuum-cleaved surface of KTO with an ultraviolet light, and detecting the 2DEG thus resulted by the technique of angle-resolved photoemission spectroscopy (ARPES).32,33 Unexpectedly, the observed Rashba-like SOC is very weak. Soon after that, Harashima et al.34 found that irradiating the surface of KTO by Ar+ ions could also induce a two-dimension-like electron gas, and the mobility was as high as ∼2 × 104 cm2/(V s). In 2015, Zou et al.25 reported the first KTO-based conducting heterointerface, LTO/KTO (LTO = LaTiO3). The authors demonstrated the metallic behavior of the 2DEG, the highest Hall mobility gained is ∼300 cm2/(V s), and the effect of the layer thickness of LTO on transport properties. So far, LTO/KTO is the only reported KTO-based Received: August 25, 2017 Accepted: September 27, 2017

A

DOI: 10.1021/acsami.7b12814 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces 2DEG obtained via interface engineering. Obviously, many important effects that have been observed in the LAO/STO system remain uncovered, such as interfacial magnetism, the effect of self-interference of wave packets, and the gating effect on spin relaxation. Herein we report on the fabrication of the high-quality KTO-based 2DEG by growing an amorphous LAO film on KTO at a high temperature. Metallic 2DEGs with the electron mobility as high as ∼2150 cm2/(V s) and the carrier density as low as 2 × 1012 cm−2 are obtained. For the first time, the gating effects on spin relaxation, inelastic scattering, and elastic scattering are systematically studied. Remarkably, the spin relaxation time is strongly tuned by a back gate, varying by a factor of 69 when the gate voltage is swept from −25 to 100 V (KTO thickness 0.5 mm), in sharp contrast to the factor of 25 for the LAO/STO system.10

Figure 1. (a) Small-angle X-ray reflectivity of the LAO layer (symbols) and the results of data fitting (red curve) based on a layer thickness of 8.4 nm. (b) High-angle annular dark-field (left panel) and annular bright-field (right panel) images of the cross section of the LAO/KTO sample, showing the amorphous structure of LAO. Dashed lines mark the interfacial region with lattice defects. (c) Atomic force microscopy (AFM) image of the LAO film on the KTO substrate.

2. EXPERIMENTAL SECTION

of the amorphous LAO (a-LAO) cap layer has caused lattice defects within a layer of ∼2 nm (Figure 1b). As expected, the amorphous LAO layer is rather smooth. Figure 1c is a representative AFM image which shows a root-mean-square roughness as small as 0.2 nm over an area of 5 × 5 μm2. 3.2. Transport Behavior Tuned by the Oxygen Pressure of Sample Growth. Figure 2a shows the sheet resistance (Rs) of the 2DEG as a function of the temperature. All samples are well metallic throughout the temperature range from 2 to 300 K. With an increase of the oxygen pressure, the Rs−T dependence displays an upward shift, while the Rs(300 K)/Rs(2 K) ratio undergoes a dramatic increase, varying from ∼11 for 1 × 10−3 Pa to ∼136 for 5 × 10−3 Pa. A high Rs(300 K)/Rs(2 K) ratio implies a better metallic character of the 2DEG. Figure 2b presents the temperature dependence of the sheet carrier density (ns) deduced from Hall resistance. Two remarkable features can be identified from this figure. First, the carrier density is nearly temperature independent. This means the absence of charge localization at low temperatures. This feature was observed before only at the crystalline LAO/ STO interface,1 indicating the high interfacial quality of our 2DEG. It considerably differs from the amorphous LAO/STO interface prepared at room temperature, for which a 60% reduction in ns takes place when the sample is cooled from 100 to 2 K.23 Second, the 2DEG remains well metallic when the carrier density is as low as 2 × 1012 cm−2. As shown in Figure 2b, ns is stable in the PO2 range from 2 × 10−4 to 1 × 10−3 Pa, taking a value around ∼6 × 1013 cm−2, and undergoes a sudden drop from 6 × 1013 to 2 × 1012 cm−2 as PO2 grows from 1 × 10−3 to 5 × 10−3 Pa. For the KTO-based 2DEG, the reported carrier density is generally in the order of 1013 or 1014 cm−2, and such a low carrier density has never been achieved.25 For the LAO/STO system, a so low ns was only obtained by scanning the sample surface with a biased tip (∼1.5 × 1012 cm−2)35 or inserting a La1−xSrxMnO3 buffer layer at the a-LAO/ STO interface (∼6.9 × 1012 cm−2).36 Notably, our sample stays in a steady conducting state, without considerable degeneration for months even though ns is low. Shown in Figure 2c is the Hall mobility (μ) of our 2DEG. It exhibits the familiar characteristics of the power law temperature dependence similar to that of LAO/STO, due to the electron scattering by optical phonons. The highest Hall mobility appears in the sample with the lowest carrier density (∼2 × 1012 cm−2). It is 1712.1 cm2/(V s), appearing at 2 K. It is greater by a factor of 5 than the reported value for the LTO/

2EDGs were fabricated by growing LAO films on (001)-oriented KTO single-crystal substrates (3 × 3 × 0.5 mm3) using the technique of pulsed laser deposition (PLD) with an LAO single crystal as the target. During deposition, the substrate temperature was maintained at 750 °C and the oxygen pressure was fixed to a value between PO2 = 2 × 10−4 Pa and PO2 = 5 × 10−3 Pa. After deposition, the samples were furnace-cooled to room temperature without changing the oxygen pressure. The film thickness was determined by the number of laser pulses, which has been carefully calibrated by the technique of smallangle X-ray reflectivity and scanning transmission electron microscopy (STEM). The fluence of the laser pulse was 2 J/cm2, and the repetition rate was 2 Hz (KrF excimer laser, wavelength 248 nm). The surface morphology of the film was analyzed by atomic force microscopy (AFM, SPI 3800N, Seiko) at the ambient conditions. The crystal structure of the film was determined by a Bruker X-ray diffractometer equipped with thin film accessories (D8 Discover, Cu Kα radiation). Lattice images were recorded by a high-resolution STEM with double CS correctors (JEM-ARM200F). Resistance measurements were performed by a Quantum-designed physical property measurement system (PPMS) in the temperature interval from 2 to 300 K. The van der Pauw geometry was adopted for electric measurements. Ultrasonic wire bonding (Al wire of 20 μm diameter) was used for electrode contact. The applied current for resistance measurements was 10 μA. For the investigation of the gating effect, a gate voltage was applied to a copper back gate while the 2DEG was grounded. The leakage current is lower than 10 nA. The X-ray photoelectron spectroscopy (XPS) measurements were performed in a Thermo Scientific ESCALAB 250X instrument using a monochromatic Al Kα X-ray source.

3. RESULTS AND DISCUSSION 3.1. Structure Characteristics of the LAO Overlayer. The result of small-angle X-ray reflectivity confirms the formation of an LAO film on KTO, with a layer thickness of 8.4 ± 0.4 nm (Figure 1a). However, no Bragg reflections of LAO were detected by X-ray diffraction (not shown), suggesting that the LAO layer is not in a well-crystallized state. Further analysis based on the STEM technique indicates that the LAO film presents in the form of an amorphous phase (Figure 1b). This is an unexpected result since amorphous oxides usually form at low temperatures. Probably, large lattice mismatch has prevented the crystallization of LAO. The lattice constant is 3.792 Å for LAO and 3.989 Å for KTO, with a lattice mismatch as large as 5.2%. According to Figure 1b, there is an interfacial layer between LAO and KTO, as indicated by the blurred color contrast in the high-angle annular dark-field (HAADF) image or the annular bright-field (ABF) image (marked by dashed lines). Formation B

DOI: 10.1021/acsami.7b12814 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 2. (a) Temperature dependence of the sheet resistance for the a-LAO/KTO heterostructures prepared under different oxygen pressures. (b, c) Corresponding sheet carrier density and Hall mobility, respectively, as functions of the temperature.

Figure 3. (a) Gating effect on the Rs−T dependence. The low-temperature resistance upturn indicates the appearance of a Kondo effect when charge carriers are depleted. The dashed line corresponding to VG = −50 V is obtained by inference. (b) Sheet resistance as a function of the temperature (red symbols), measured under a VG of −20 V, and the results of curve fitting (solid black line) based on the Hamman model for the Kondo effect. (c) Ta 4f core level spectrum of a-LAO/KTO, obtained by the technique of X-ray photoelectron spectroscopy (symbols). The main peaks are the contributions of the Ta5+ valence state, while the right side shoulder comes from the Ta4+ state. The black solid line through the symbols is the fitting result by the two components corresponding to Ta4+ (blue curve) and Ta5+ (red curve), respectively. On the basis of the fitting results, the concentration of Ta4+ is determined. (d) Gating effect on Hall resistance, measured at a fixed temperature of 2 K. (e−g) Sheet resistance, carrier density, and Hall mobility as functions of the gate biases, obtained at 2 K. Red symbols mark the initial state before gating. The sample does not return to its original state after electric gating due to the release of initially trapped charges.

KTO 2DEG.25 Notably, it is also much higher than that of the 2DEG at the a-LAO/STO interface (∼300 cm2/(V s)).23 These results are suggestive, indicating that the conducting channel in KTO, which locates underneath the interfacial layer marked in Figure 1b, is of high quality when an a-LAO cap layer is formed at a high temperature. Probably, the high-temperature growth has eliminated the lattice defects in KTO produced by PLD; i.e., growing the amorphous layer at high temperatures could be a promising approach to obtain high-quality oxide 2DEG for polar substrates. 3.3. Transport Behaviors Tuned by Gating Field. To reveal the hidden aspects of our 2DEG, a systematic investigation on gating effect is desirable. The low sheet carrier density of our 2DEG makes this possible. Figure 3a shows the gating effect on the sheet resistance for the sample with the lowest ns. As expected, a positive gate voltage (VG), which was applied to the back gate, causes a downward shift of the Rs−T curve, indicative of an improvement of the metallicity of the 2DEG. Take the sheet resistance at 2 K as an example. Rs decreases from ∼2425.7 to ∼431.8 Ω/□ as VG sweeps from 0

to 100 V, reduced by a factor of 5.6 (Figure 3e). Meanwhile, the subtle resistance upturn below ∼5 K is completely depressed. Corresponding to the variation of VG from 0 to −25 V, on the contrary, the sheet resistance grows from ∼2425.7 to 17191.6 Ω/□, enhanced by a factor of 7.1 (Figure 3e). Accompanying the upward shift of the Rs−T curve, a resistance upturn reminiscent of the Kondo effect emerges. On the basis of a general expression37 R s(T ) = R 0 + AT q + RK(T /TK )

(1)

the Rs−T dependence below 50 K can be well reproduced (solid curve in Figure 3b), where the first term is the residual resistance, the second term describes the transport behavior determined by electron−electron and electron−phonon interactions, and the last term is the Kondo term related to the scattering by magnetic impurities. For RK(T/TK), we use the zero-field generalized Hamann expression38 C

DOI: 10.1021/acsami.7b12814 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces ⎛ RK(T /TK ) = C ⎜⎜1 − ⎝

⎞ ⎟ ⎟ (ln(T /TK ))2 + π 2(S(S + 1)) ⎠

identified from the data in Figure 4a. The first one is the occurrence of a sharp minimum at H = 0 in all MR−H curves (inset plot in Figure 4a). This is a fingerprint of weak antilocalization owing to the Rashba-like SOC. As H deviates from zero, MR first grows and then decreases, corresponding to the depression of weak antilocalization and weak localization, respectively. The second feature is the smooth growth of the MR when the applied field is high enough, a classical orbital effect of the magnetic field. To obtain a deep understanding of the SOC of our 2DEG, we performed a quantitative analysis of the [Δσ(H)/G0]−H relation based on the expression10,39

ln(T /TK )

(2)

where TK is the effective Kondo temperature and S is the effective spin of the magnetic scattering centers. The fitting parameters are R0 = 2723.8 Ω/□, A = 2.16 Ω/(□ K2.04), q = 2.04, C = 2509 Ω/□, TK = 15.6 K, and S = 0.12. This result indicates the appearance of Kondo scattering and thus the presence of local magnetic moments. This is consistent with the result of the XPS analysis, which shows the existence of the Ta4+ ions with a concentration of ∼7.7% (Figure 3c). Figure 3d shows the gating effect on the Hall resistance, measured at 2 K. All Rxy−H curves are well linear, indicating that there is only one species of charge carriers in the 2DEG. The charge carriers should be 5dxy electrons, the energy band of which is lowered by the symmetry breaking of the interface. The decline of the Rxy−H curve with the increase of VG specifies the increase of the carrier density. Figure 3f exemplifies the deduced sheet carrier density as a function of thegate voltage. As VG sweeps from −25 to +100 V, ns increases monotonically from 1.6 × 1012 to 6.7 × 1012 cm−2. On the basis of the data in Figure 3f,g, Hall mobility can be deduced. In the same time as charge carriers are accumulated, as shown in Figure 3g, the gate voltage tunes the mobility by a factor of 9.3, from ∼232 to ∼2150 cm2/(V s). Notably, ∼2150 cm2/(V s) is the record value for the heterointerfaces based on KTaO3, higher by a factor of 6 than previously reported values for LTO/KTO.25 A further issue to be addressed is the gating effect on quantum interference, which is a unique feature of 2DEG. After gating 2DEG to the preset state, we measured the sheet resistance in perpendicular magnetic fields. Figure 4a shows the magnetoresistance at a fixed temperature of 2 K, defined by MR = [Rs(H) − Rs(H = 0)]/Rs(H = 0). Two features can be

H H + Hso Δσ(B) 1 3 1 1 1 = −Ψ + tr + Ψ + i − Ψ G0 2 H 2 2 H 2 2 ⎡ ⎛ H + H ⎞ 1 ⎛ H + H ⎞⎤ H so so + i − ⎢ln⎜ i ⎟ + ln⎜ i ⎟⎥ ⎢⎣ ⎝ Htr ⎠ H 2 ⎝ Hi ⎠⎥⎦ − AK

σ(0) H2 G0 1 + CH2

(3)

where Δσ(H) = σ(H) − σ(H = 0), σ(H) is the conductance in an applied field of H, and G0 = e2/πh is the quantum conductance. In eq 3, quantum corrections to the conductance in the 2D limit are described by the first four terms, where Ψ(x) is the digamma function defined as Ψ(x) = ln(x) + Ψ(1/2 + 1/x) and Htr, Hi, and Hso are the effective fields related to elastic scattering, inelastic scattering, and spin−orbit scattering, respectively. The last term of eq 3 involving the parameters AK and C is the Kohler term, which gives an account of orbital MR. As shown by the solid curves in Figure 4b, eq 3 well reproduces the experimental results, adopting the parameters (Hi, Hso, and AK) shown in Figure 4c,d. At first glance, the effective field of the SOC is very weak for the a-LAO/KTO interface. According to Figure 4c, Hso takes a value of ∼0.03 T when VG = −25 V and ∼0.08 T when VG = 100 V. For the typical LAO/STO interface, in contrast, it is ∼0.5 and ∼8 T under the corresponding gate biases.10 For our 2DGE, the charge carriers should be mainly dxy electrons in nature. In this case, the vertical electron hopping between the dxy states, which determines Hso,40 is relatively difficult since it proceeds via the π-type bonding. Moreover, the interfacial quantum well could be shallow because of the low carrier density. As a result, the effective SOC field is weak. Although dxz/dyz orbitals form the σ-type bonding, electron hopping between dxz/dyz states is invalid for our 2DEG since the corresponding states are empty. As reported, the energy level of the dxz/dyz band is well above the Fermi level,25 and it cannot be reached by accumulating charge carriers using the present gate biases. Therefore, gate bias only affects the band filling of the dxy states; thus, the gating effect on Hso is weak. Although both Hso and its gate bias dependence are weak, fascinatingly, we found that the gating effect on the spin relaxation process in our 2DEG is comparable to or even stronger than that in the LAO/STO system. Estimated by the relation of Hi,so = ℏ/4eDi,so, the spin relaxation time closes to 13.7 ps when VG = −25 V and 0.2 ps when VG = 100 V (Figure 5a); i.e., gate bias has tuned τso by a factor of ∼69. Here D is the diffusion coefficient and e is the electron charge. (The diffusion coefficient D is determined on the basis of the Drude expression D = 1/2υF2τe, where υF = ℏkF/m*kF = (2πns)1/2 is the Fermi wave vector, m* ≈ 0.36m0,25 with m0 being the free

Figure 4. (a) Magnetoresistance as a function of the perpendicular magnetic field, measured under different gate biases at a fixed temperature of 2 K. The inset plot is a close view of the MR curves around H = 0. (b) Corresponding reduced magnetoconductance (symbols) and the results of data fitting (solid curves) based on eq 3. (c) Effective inelastic field (Hi) and spin−orbit field (Hso), deduced from the data fitting of (b). (d) Kohler parameter describing the orbital effect of a perpendicular magnetic field. Solid lines in (c) and (d) are guides for the eye. D

DOI: 10.1021/acsami.7b12814 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b12814. Binding energies and amounts of Ta5+ and Ta4+ and gate bias dependence of the diffusion coefficient of the 2DEG at the a-LAO/KTO interface (PDF)

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AUTHOR INFORMATION

Corresponding Author

Figure 5. (a) Spin relaxation time, inelastic scattering time, and elastic scattering time as functions of the gate voltage. τso is reduced by a factor of 69 as VG sweeps from −25 to +100 V. The increase of τe with VG indicates that the charge carriers are driven away from interface. (b) Spin relaxation time as a function of the inverse inelastic scattering time. The linear relation specifies the Dyakonov−Perel mechanism for spin relaxation. The solid line is a guide for the eye.

*E-mail: [email protected]. ORCID

Lin Gu: 0000-0002-7504-031X Banggui Liu: 0000-0002-6030-6680 Jirong Sun: 0000-0003-1238-8770 Notes

The authors declare no competing financial interest.

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electron mass, and τe = m*μ/e is the elastic scattering time.) In the same VG range, in contrast, τso is varied only by a factor of ∼25 for the LAO/STO interface.10 We did not extend the measurement below a gate voltage of −25 V since the exhaustion of charge carriers makes the data unreliable. A further analysis shows that the diffusion coefficient varies by a factor of ∼27 for our 2DEG as VG sweeps from −25 to +100 V (Supporting Information, Figure S1) while it is only doubled for the 2DEG of LAO/STO across the same gate range.10 Therefore, the strong gate dependence of our τso mainly comes from D. This result indicates that a gating effect on spin relaxation can be optimized by tuning Hso or D or both of them. In Figure 5a, we also present the inelastic (τi) and elastic (τe) scattering times. On the basis of the data in Figure 5a, we can establish a relation between τso and τe (Figure 5b). The most remarkable observation is the inverse proportion of τso to τe. This means that the spin relaxation in our 2DEG is determined by the so-called Dyakonov−Perel mechanism,41 rather than the Elliott−Yafet mechanism, which is assumed for KTO because of the significant correction to the band structure of the ionic spin−orbit coupling. Probably, the coexistence of TaO2 and KO terminated layers at the a-LAO/KTO interface has deteriorated the band structure and at the same time depressed the Rashba effect.

ACKNOWLEDGMENTS This work has been supported by the National Basic Research P r o g r a m o f C h i n a ( G r a n t s 2 0 13 C B 9 2 1 70 0 a n d 2016YFA0300701), the National Natural Science Foundation of China (Grants 11520101002, 51590880, 11374348, 11134007, 11574376, and 11574366), and the Key Program of the Chinese Academy of Sciences.

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REFERENCES

(1) Ohtomo, A.; Hwang, H. Y. A High-Mobility Electron Gas at the LaAlO3/SrTiO3 Heterointerface. Nature 2004, 427, 423−426. (2) Nakagawa, N.; Hwang, H. Y.; Muller, D. A. Why Some Interfaces Cannot be Sharp. Nat. Mater. 2006, 5, 204−209. (3) Herranz, G.; Basletic, M.; Bibes, M.; Carretero, C.; Tafra, E.; Jacquet, E.; Bouzehouane, K.; Deranlot, C.; Hamzic, A.; Broto, J. M.; Barthelemy, A.; Fert, A. High Mobility in LaAlO3/SrTiO3 Heterostructures: Origin, Dimensionality, and Perspectives. Phys. Rev. Lett. 2007, 98, 216803. (4) Kalabukhov, A.; Gunnarsson, R.; Borjesson, J.; Olsson, E.; Claeson, T.; Winkler, D. Effect of Oxygen Vacancies in the SrTiO3 Substrate on the Electrical Properties of the LaAlO3/SrTiO3 Interface. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 75, 121404. (5) Reyren, N.; Thiel, S.; Caviglia, A. D.; Kourkoutis, L. F.; Hammerl, G.; Richter, C.; Schneider, C. W.; Kopp, T.; Ruetschi, A. S.; Jaccard, D.; Gabay, M.; Muller, D. A.; Triscone, J. M.; Mannhart, J. Superconducting Interfaces between Insulating Oxides. Science 2007, 317, 1196−1199. (6) Brinkman, A.; Huijben, M.; Van Zalk, M.; Huijben, J.; Zeitler, U.; Maan, J. C.; Van der Wiel, W. G.; Rijnders, G.; Blank, D. H. A.; Hilgenkamp, H. Magnetic Effects at the Interface between NonMagnetic Oxides. Nat. Mater. 2007, 6, 493−496. (7) Thiel, S.; Hammerl, G.; Schmehl, A.; Schneider, C. W.; Mannhart, J. Tunable Quasi-Two-Dimensional Electron Gases in Oxide Heterostructures. Science 2006, 313, 1942−1945. (8) Bell, C.; Harashima, S.; Kozuka, Y.; Kim, M.; Kim, B. G.; Hikita, Y.; Hwang, H. Y. Dominant Mobility Modulation by the Electric Field Effect at the LaAlO3/SrTiO3 Interface. Phys. Rev. Lett. 2009, 103, 226802. (9) Caviglia, A. D.; Gariglio, S.; Cancellieri, C.; Sacepe, B.; Fete, A.; Reyren, N.; Gabay, M.; Morpurgo, A. F.; Triscone, J. M. TwoDimensional Quantum Oscillations of the Conductance at LaAlO3/ SrTiO3 Interfaces. Phys. Rev. Lett. 2010, 105, 236802. (10) Caviglia, A. D.; Gabay, M.; Gariglio, S.; Reyren, N.; Cancellieri, C.; Triscone, J. M. Tunable Rashba Spin-Orbit Interaction at Oxide Interfaces. Phys. Rev. Lett. 2010, 104, 126803.

4. SUMMARY In summary, high-quality 2DEG has been obtained by fabricating an amorphous LAO layer on a (001)-orientated KTO substrate at a high temperature that is usually adopted to grow crystalline LAO. Through optimizing the oxygen pressure for sample preparation, we obtained the 2DEGs with an electron mobility as high as ∼2150 cm2/(V s) and a carrier density as low as 2 × 1012 cm−2. A strong gating effect is observed, which drives the 2DEG from a well metallic state into a Kondo state. For the first time, the gating effect on spin relaxation and inelastic and elastic scattering is determined for the present 2DEG. More importantly, the spin relaxation time can be strongly tuned by gate bias, which demonstrates the great potential of the present 2DEG in developing all oxide devices with spintronic functionality. E

DOI: 10.1021/acsami.7b12814 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

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DOI: 10.1021/acsami.7b12814 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX