Research Article Cite This: ACS Appl. Mater. Interfaces 2017, 9, 39011-39017
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Modulated Transport Behavior of Two-Dimensional Electron Gas at Ni-Doped LaAlO3/SrTiO3 Heterointerfaces Hong Yan, Zhaoting Zhang, Shuanhu Wang, Hongrui Zhang, Changle Chen, and Kexin Jin* Shaanxi Key Laboratory of Condensed Matter Structures and Properties, School of Science, Northwestern Polytechnical University, Xi’an 710072, China S Supporting Information *
ABSTRACT: Modulating transport behaviors of two-dimensional electron gases are of critical importance for applications of the next-generation multifunctional oxide electronics. In this study, transport behaviors of LaAlO3/SrTiO3 heterointerfaces modified through the Ni dopant and the light irradiation have been investigated. Through the Ni dopant, the resistances increase significantly and a resistance upturn phenomenon due to the Kondo effect is observed at T < 40 K. Under a 360 nm light irradiation, the interfaces exhibit a persistent photoconductivity and a suppressed Kondo effect at low temperature due to the increased mobility measured through the photoHall method. Moreover, the relative changes in resistance of interfaces induced by light are increased from 800 to 6600% at T = 12 K with increasing the substitution of Ni, which is discussed by the band bending and the lattice effect due to the Ni dopant. This work paves the way for better controlling the emerging properties of complex oxide heterointerfaces and would be helpful for photoelectric device applications based on all-oxides. KEYWORDS: LaAlO3/SrTiO3 heterointerfaces, two-dimensional electron gas, persistent photoconductivity, photo-Hall effect, Kondo effect revealed.29−32 Apart from this, substitution or dopant is another method to modify the carrier concentration and induce dramatic changes in the properties of the LAO/STO interface.33 Fix et al. have located the position of carriers within the STO surface layers by inserting Mn dopant at different distances from the interface.34 Gray et al. have demonstrated that the doping at La site with Tm and Lu acts as the source of localized moment and spin−orbit scattering centers.35 Kumar et al. have reported a metal-to-insulator transition and a gradual suppression of 2DEG at the LAO/STO interface by the substitution of Cr at Al sites, showing a distinct change of the photoresistive properties of LaAl0.6Cr0.4O3/ SrTiO3 heterostructures.36−38 Additionally, LaNiO3 is a perovskite oxide with metallic properties in its bulk form and shows electron−electron interactions that cause low electron diffusion at low temperature.39 Thereby, we choose the low doping level and ensure that the doped films are insulating themselves. Meanwhile, Ni ions with magnetic properties are expected to enhance electron correlations at the LAO/STO heterointerfaces. Thus, considering the control of the Kondo effect through the two methods mentioned above and the specific feature of Ni element, we attempt to modify the transport behavior of LAO/STO
1. INTRODUCTION As a well-known phenomenon, modulated transport has been widely studied because of its potential applications in multifunctional materials and devices.1,2 The control of transport is a challenging issue in complex oxide heterointerfaces, although it has been observed and modulated by magnetic and electrostatic fields or light.3−6 In fact, the discovery of two-dimensional electron gas (2DEG) at the LaAlO3/SrTiO3 (LAO/STO) interface of complex oxides has triggered a series of subsequent studies.7 Moreover, many exotic properties, including magnetism, novel quantum phases, such as superconductivity and strong spin−orbit coupling, and coexistence of superconductivity and ferromagnetism, have been observed at the interface between insulating materials.8−12 Particularly, one of the most fascinating aspects at the interface is the metallic transport, which can be explained in terms of the polar catastrophe, oxygen vacancies, and cation intermixing.13−18 Moreover, this metallicity can be tuned by various methods, such as the external perturbations, substitution or dopant, and electrostatic fields.19−29 As a powerful external perturbation, light can modify the transport properties of 2DEG. For example, Tebano et al. observed that the 2DEG at the LAO/STO interface exhibits giant persistent photoconductivity.19 An enhanced photoresponse by surface modification using Pd nanoparticles was observed by Chan et al.28 In addition, insulator−metal transition and the suppression of the Kondo effect at the interface induced by light have been © 2017 American Chemical Society
Received: August 6, 2017 Accepted: October 16, 2017 Published: October 16, 2017 39011
DOI: 10.1021/acsami.7b11727 ACS Appl. Mater. Interfaces 2017, 9, 39011−39017
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) AFM image of the LANO (x = 0) film on the STO substrate. The scale bar is 500 nm. The inset shows the RHEED pattern after the growth of the LANO (x = 0) film. (b) XRR image of the sample LANO (x = 0).
Figure 2. (a) Temperature dependence of the resistance at LANO/STO (x = 0, 0.01, 0.03, and 0.05) heterointerfaces in darkness. The solid lines are fit to eq 1 in the temperature range of 12−100 K. Carrier density (ns) (b) and mobility (μ) (c) as a function of temperature for LANO/STO (x = 0, 0.01, 0.03, and 0.05) heterointerfaces in darkness. 10−3 Pa, respectively. The films without the annealing treatment were in situ cooled down to room temperature at a rate of 5 °C/min under the same oxygen pressure. The MFP-3D atomic force microscopy (AFM) was used to characterize surface morphologies of the samples. X-ray reflectivity (XRR) was carried out by a PANAlytical X’Pert Pro X-ray diffractometer with Cu Kα X-ray source to estimate the thickness of films. Ultrasonic Al wire bonding was used for electrode contact. The Hall and photo-Hall measurements were measured using the van de Pauw configuration, and the applied current was 10 μA. All of the low-temperature measurements were carried out in a closedcycle He refrigerator with quartz glass windows. Films were irradiated using a light with a wavelength of 360 nm (∼3.44 eV), and the power densities used are about 0.5, 0.05, and 0.025 W/cm2. During the photo-Hall measurement, the light spot covered the entire surface of samples. The resistance as a function of time was measured after the samples were kept in the objective temperature for 2 h in darkness. For this process, we irradiated samples for 20 s and then switched off the light for 180 s. This operation is repeated two times. The samples were warmed up to 300 K and kept for 2 h before the irradiation.
heterointerfaces by a small amount of Ni dopant at Al site of LAO and further explore the properties of LaAl1−xNixO3 (LANO)/SrTiO3 (x = 0, 0.01, 0.03, and 0.05) heterointerfaces under the light irradiation. It is observed that the Kondo effect is modulated and the relative changes in resistance of interfaces induced by light are significantly enhanced by the substitution of Ni modified by a small amount of Ni dopant at Al site in LAO.
2. EXPERIMENTAL DETAILS The polycrystalline targets of nominal LANO were prepared by the solid-state reaction method. Films of LANO were deposited on the TiO2-terminated SrTiO3 (STO) substrates using the pulsed laser deposition method, with a KrF excimer laser (λ = 248 nm) operating at 1 Hz and 1.6 J/cm2. The deposition was monitored by in situ reflection high-energy electron diffraction (RHEED). STO (001) substrates were etched in the standard 1:10 buffered hydrofluoric acid with NH4F for 41 s and then annealed for 2 h at 970 °C in air to achieve a smooth TiO2 termination (Figure S1). The deposited temperature and the oxygen partial pressure were 780 °C and of 1.7 × 39012
DOI: 10.1021/acsami.7b11727 ACS Appl. Mater. Interfaces 2017, 9, 39011−39017
Research Article
ACS Applied Materials & Interfaces
Table 1. Obtained Fitting Parameters on Fitting the R vs T Data at LANO/STO (x = 0, 0.01, 0.03, and 0.05) Heterointerfaces in Darkness by Using Equation 1 in the Temperature Range of 12−100 K sample (x)
R0 (Ω)
Ra (Ω/K2)
0 0.01 0.03 0.05
2936.9398 3131.3761 3756.7498 3980.4212
0.26705 0.36334 1.68904 1.70979
Rb (Ω/K5) 7.82313 1.72347 8.20687 2.16207
× × × ×
10−8 10−7 10−7 10−6
RK,0 (Ω)
TK (K)
4212.62 19 241.99 35 122.25
37.3 38.1 39.4
Figure 3. (a) Time evolution of the resistance at LANO/STO (x = 0) heterointerfaces under irradiation of light with a power density of 0.5 W/cm2 at different temperatures. (b) Temperature dependence of PR values for the LANO/STO (x = 0, 0.01, 0.03, and 0.05) heterointerfaces. (c) Temperature dependence of the resistance at LANO/STO (x = 0, 0.01, 0.03, and 0.05) heterointerfaces under irradiation. (d) Temperature dependence of the resistance at LANO/STO (x = 0.01) heterointerfaces under irradiation with different power densities.
3. RESULTS AND DISCUSSION As illustrated in Figure 1a, the surface morphology of the LANO/STO (x = 0) heterointerface shows a flat terracelike surface. The inset is the reflection high-energy electron diffraction (RHEED) pattern after the growth of the LANO (x = 0) film. As shown in Figure 1b, the X-ray reflectivity (XRR) indicates a high-quality film of LANO (x = 0), and the thickness of the layer is estimated to be about 8.5 nm. Other samples have the same smooth morphology and similar growth rates with the thickness of about 8.5 nm (Figures S2 and S3). The Ni 2p core levels for doping samples reveal that the Ni3+ and Ni2+ ions coexist (Figure S4). Figure 2a shows the resistance−temperature curves of the LANO/STO heterointerfaces in darkness. It is obvious that a perfect metallic behavior is observed for x = 0 and the resistance upturn phenomenon at low temperature emerges for x = 0.01, 0.03, and 0.05 samples. Meanwhile, the resistances are increased and the positions corresponding to the minimum resistances slightly shift toward higher temperature with the increasing
Ni dopant. Ordinarily, the upturn of resistance for the pure LAO/STO interface is related with the oxygen pressure at the deposition process. When the pressure is above 0.01 Pa, the LAO/STO interfaces generally show a clear upturn, which is attributed to the interaction between the localized Ti3+ ions with local magnetic moments and the itinerant 2DEG.40,41 Here, the LANO/STO (x = 0) interface shows no upturn of resistance, whereas the Ni-doped interfaces favor the Kondolike feature, suggesting that the appearance of resistance upturn with Ni doping is mainly due to the contribution of the Ni dopant. The Ni element at the interface acts as the Kondo scattering center and induces an increase of resistance at low temperatures. We fit the resistance−temperature curves in the range of 12−100 K by the expression42
39013
R = R 0 + R aT 2 + R bT 5 + RK(T /TK )
(1)
⎛ ⎞s 1 ⎟ RK(T /TK ) = RK,0⎜⎜ 1/ s 2⎟ ⎝ 1 + (2 − 1)(T /TK ) ⎠
(2)
DOI: 10.1021/acsami.7b11727 ACS Appl. Mater. Interfaces 2017, 9, 39011−39017
Research Article
ACS Applied Materials & Interfaces
Table 2. Obtained Fitting Parameters on Fitting the R vs T Data at LANO/STO (x = 0.01) Heterointerfaces under Irradiation with Different Power Densities by Using Equation 1 in the Temperature Range of 12−100 K power density (W/cm2)
R0 (Ω)
Ra (Ω/K2)
0 0.025 0.05 0.5
3131.3761 2153.91681 1523.94641 584.68727
0.36334 0.36971 0.38947 0.62845
Rb (Ω/K5) 1.72347 3.92722 7.49649 1.30302
× × × ×
10−7 10−7 10−7 10−6
RK,0 (Ω)
TK (K)
4212.62
37.3
Figure 4. (a) Schematic diagram of the photo-Hall effect measurements. (b) Temperature dependence of carrier density (ns) and mobility (μ) of x = 0.01 in dark and under irradiation with a power density of 0.5 W/cm2. (c) Sketch of the movement of conduction electrons localized around the impurity spin irradiated by light at T > TK and T < TK.
where R0 is the residual resistance due to the disorder, the T2 term comes from the electron−electron scattering, and the third term originates from electron−photon interactions. The last term is the Kondo contribution. RK,0 is the Kondo resistance at zero temperature, and TK is the Kondo temperature. The parameter s is fixed at 0.225 according to the theoretical result obtained from the numerical renormalization group.43,44 As the solid lines show in Figure 2a, the fitting curves are well consistent with the experimental data and the fitting parameters are listed in Table 1. The fitted TK values are 37.3, 38.1, and 39.4 K for x = 0.01, 0.03, and 0.05, respectively, agreeing well with the experimental results. By introducing the Ni dopant, the cloud of opposite spin-polarization around the impurity spin formed by the conduction electrons would become larger and the constraint would be strengthened. Thus, the Kondo temperatures are raised with increasing the Ni
dopant. The scattering from the localized conduction electrons within the scope of the local area is enhanced, resulting in an increase of Ra with increasing the Ni dopant. In Figure 2b,c, the temperature dependencies of carrier density (ns) and mobility (μ) for LANO/STO heterointerfaces in darkness are shown. The carrier density decreases with increasing Ni dopant, whereas the mobility increases over all of the temperature. To investigate the effect of doping on photoresponsive properties, the experiments on temporal evolution of the resistance of LANO/STO heterointerfaces under the irradiation of a 360 nm light with a power density of 0.5 W/cm2 are performed at different temperatures. As shown in Figure 3a, the resistances of heterointerfaces under the irradiation decrease at different temperatures and cannot restore their originating values. The temporal evolution of resistance under irradiation of x = 0.01, 0.03, and 0.05 heterointerfaces are shown in Figures 39014
DOI: 10.1021/acsami.7b11727 ACS Appl. Mater. Interfaces 2017, 9, 39011−39017
Research Article
ACS Applied Materials & Interfaces
the scattering is weakened, thus increasing the mobility and reducing the resistance. Another feature is the persistent photoconductivity, which is observed in other systems.19,32 Various models have been proposed to describe the photoconductivity effect, such as defect-cluster, random fluctuation, and lattice relaxation.51−56 Here, one of the most important factors is the band bending via electronic reconstruction, which produces the inbuilt electric field and therefore induces the macroscopic potential barriers.57,58 The persistent photoconductivity is related with the macroscopic potential barriers, which spatially separate the photoexcited electrons and holes, and one type of carrier is recaptured by the barriers, resulting in the persistent photoconductivity.59 As for the Ni-doping interface, the PR values are enhanced with increasing Ni dopant. It is reported that a persistent photoconductivity in ZnSe/(Zn, Cd, and Mn)Se 2DEG systems becomes more pronounced as the Mn2+ concentration is increased.60 Therefore, besides the band bending effect, other effects mentioned above should be considered. In particular, the Ni dopant would cause the lattice effect. The ionic radius of Ni3+ (60.0 pm) is larger than that of Al3+ (53.5 pm), which would cause the ionic mismatch and form the defects. Hence, this would create a barrier, which prevents the recapture of mobile carriers, resulting in a persistent photoconductivity. As increasing the Ni dopant, these defects become more and the larger PR is obtained. In addition, it has been reported that Ti3+ ions appear at the Ti/STO interface with metallic transport properties. Thus, Ni dopant would also have an effect on the valence evolution of the interface.61 Further studies of intrinsic mechanisms are underway.
S5−S7. Namely, the LANO/STO heterointerfaces exhibit a persistent photoconductivity.45,46 To compare the change in resistance of samples induced by light, we define the PR = [(R0 − Rp)/Rp] × 100%, where R0 is the original resistance before light irradiation and Rp is the resistance of heterointerfaces under light irradiation. The temperature-dependent PR values of heterointerfaces are shown in Figure 3b. As for the LANO/ STO heterointerfaces, the PR values increase with the increasing substitution and the maximum values of PR are 800, 880, 2900, and 6600% at T = 12 K for x = 0, 0.01, 0.03, and 0.05 samples, respectively. It is observed that the PR values decrease monotonically with increasing temperatures and gradually saturate as the temperature is above 50 K. This tendency is caused by the thermal fluctuations at higher temperatures or the absence of lattice vibrations at low temperatures.47,48 The photon energy of light in the experiments is 3.44 eV (360 nm), which is larger than the band gap of STO and less than that of LAO. Thus, electrons within the band gap are promoted from the valence band to the conduction band of STO close to the interface under light irradiation. More electrons take part in the conduction, resulting in a decrease in the resistance in the metallic state of all of the heterointerfaces. We extract the steady resistance under the irradiation at different temperatures, as given in Figure 3c. Obviously, the Kondo effect is suppressed at LANO/ STO heterointerfaces. The same phenomena are observed when the x = 0.01 heterointerface is irradiated by the light with different power densities, as shown in Figure 3d. More interestingly, only a very small intensity of light can induce the resistance upturn disappear and the resistance is decreased with increasing the power density. Furthermore, we fit the resistance−temperature curves by eq 1 and the best fitting parameters are listed in Table 2. To obtain the deep mechanism, the Hall measurement of the x = 0.01 heterointerface under light irradiation with the power density of 0.5 W/cm2 has been performed at different temperatures. The schematic diagram of the photo-Hall effect measurement is shown in Figure 4a. The Hall resistance of LANO/STO (x = 0.01) sample in dark and under light irradiation is shown in Figure S8. The carrier density is slightly increased under light irradiation, whereas the mobility is abruptly increased 3-fold at T =12 K, as shown in Figure 4b. The mobility under light irradiation is increased from 52 cm2/V s at 40 K to 166 cm2/V s at 12 K. Above 40 K, the light-induced change in mobility can be negligible. The Kondo energy scale (kBTK, kB is Boltzmann constant) of about 3.39 meV (39.4 K) is far smaller than the photon energy used in the experiment (about 3.44 eV). Thus, the light has a more profound effect on the Kondo state. The quasibound state from the cloud of opposite spin-polarization in darkness and under the light irradiation is shown in Figure 4c. It is clear that the Kondo singlet with local spins is destroyed under the irradiation of light.31,32 Accordingly, the Kondo effect is dominant for the Nidoping interfaces at T < 40 K and the decrease in resistance is mainly attributed to the destruction of scattering centers of the doped Ni element, which is verified by the increased mobility. In addition, the increase in carrier density also contributes to the resistance decrease. Under light irradiation, the strong exchange interaction of a singlet to the spins of an adjacent Fermi sea is shut down due to the Anderson orthogonality catastrophe and leads to the destruction of Kondo resonance.49,50 Consequently, the irradiation-induced decoherence effect of localized spin states and carriers is dominant and
4. CONCLUSIONS In conclusion, we have modified the Kondo behavior of LANO/STO (x = 0, 0.01, 0.03, and 0.05) heterointerfaces through dopant and light irradiation. In darkness, the resistances are increased and the Kondo effect is produced significantly with the increasing substitution of Ni. Under irradiation, the Kondo effect is suppressed at low temperatures and the substitution of Ni causes a giant increase in PR values. Our work in this study is helpful for deep understanding of the controlling Kondo effect and has great significance for photoelectric devices based on all-oxides toward practical applications.
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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.7b11727. AFM image of the TiO2−STO substrate; AFM and XRR images of LANO heterointerfaces; Ni 2p core-level photoemission spectra for LANO; time evolution of the resistance of LANO/STO with light; Hall resistance of LANO/STO (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Kexin Jin: 0000-0001-5838-0315 Notes
The authors declare no competing financial interest. 39015
DOI: 10.1021/acsami.7b11727 ACS Appl. Mater. Interfaces 2017, 9, 39011−39017
Research Article
ACS Applied Materials & Interfaces
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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Nos. 51572222, 11604265, and 61471301).
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DOI: 10.1021/acsami.7b11727 ACS Appl. Mater. Interfaces 2017, 9, 39011−39017
Research Article
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DOI: 10.1021/acsami.7b11727 ACS Appl. Mater. Interfaces 2017, 9, 39011−39017
