Tunable Surface Electron Spin Splitting with ... - ACS Publications

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Tunable Surface Electron Spin Splitting with Electric Double-Layer Transistors Based on InN Chunming Yin,† Hongtao Yuan,‡ Xinqiang Wang,*,† Shitao Liu,† Shan Zhang,† Ning Tang,† Fujun Xu,† Zhuoyu Chen,‡ Hidekazu Shimotani,‡ Yoshihiro Iwasa,‡ Yonghai Chen,§ Weikun Ge,∥ and Bo Shen*,† †

State Key Laboratory of Artificial Microstructure and Mesoscopic Physics, School of Physics, Peking University, Beijing, 100871 China ‡ Quantum-Phase Electronics Center and Department of Applied Physics, The University of Tokyo, Tokyo, 113-8656 Japan § Laboratory of Semiconductor Materials Science, Institute of Semiconductors, CAS, Beijing, 100083 China ∥ Department of Physics, Tsinghua University, Beijing, 100084 China S Supporting Information *

ABSTRACT: Electrically manipulating electron spins based on Rashba spin−orbit coupling (SOC) is a key pathway for applications of spintronics and spin-based quantum computation. Two-dimensional electron systems (2DESs) offer a particularly important SOC platform, where spin polarization can be tuned with an electric field perpendicular to the 2DES. Here, by measuring the tunable circular photogalvanic effect (CPGE), we present a room-temperature electric-fieldmodulated spin splitting of surface electrons on InN epitaxial thin films that is a good candidate to realize spin injection. The surface band bending and resulting CPGE current are successfully modulated by ionic liquid gating within an electric double-layer transistor configuration. The clear gate voltage dependence of CPGE current indicates that the spin splitting of the surface electron accumulation layer is effectively tuned, providing a way to modulate the injected spin polarization in potential spintronic devices. KEYWORDS: Surface electron accumulation, InN, ionic liquid, circular photogalvanic effect

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Actually the tuning of surface electron density and band bending is of great importance but quite difficult due to the electrostatic screening effect of high-density electrons accumulated on the surface.11 Although it is possible to deposit a gate dielectric on the InN for a field effect transistor, it may destroy the surface electron accumulation.20,21 In contrast, ionic liquid is able to greatly tune the surface band bending as well as the electron density at liquid/solid interface by forming an electric double layer (EDL)22,23 and thus serves as a more effective dielectric to modulate the interfacial band alignment. Meanwhile, the circular photogalvanic effect (CPGE)24−26 is an effective experimental tool for measuring spin splitting in lowdimensional semiconductor systems at room temperature, compared to other transport measurement methods that require cryogenic temperatures (e.g., weak antilocalization or the Shubnikov−de Haas effect). In this work, we first give evidence of surface electron spin splitting in InN thin films with CPGE measurement by comparing CPGE currents in a-plane at different orientations without involving the bulk electron spin splitting effect. Next,

he generation and manipulation of spin-polarized electrons are the most critical steps for developing semiconductor spintronic applications.1,2 Spin−orbit coupling (SOC) and the resulting spin splitting in a two-dimensional electron system (2DES) have been used for filtering and generating spin-polarized carriers3,4 in nonmagnetic materials without applying any external magnetic field.5−9 Especially, Rashba spin splitting, originating from a structural inversion asymmetry (SIA) at heterointerface, can be tuned by field effect transistors through modulating the interfacial profile of confining potential. Semiconductor materials such as InN and InAs, which show not only a high-density electron accumulation layer on the surface10,11 but also a large tunable SIA at interfaces, could be good candidates to produce spin splitting for potential spintronic applications. More importantly, such a surface accumulation layer can serve as a homogeneous layer, instead of using a heterojunction, for injecting spin-polarized electrons to the bulk semiconductor with high spin injection efficiency. To date, Rashba spin splitting has been reported in quantum wells and other heterostructures based on InN12−15 and InAs,16−19 however, there is no experimental report of the spin splitting in surface electron accumulation layer on either of the two materials due to the difficulty in manipulating the surface band bending. © 2013 American Chemical Society

Received: January 13, 2013 Revised: April 22, 2013 Published: April 24, 2013 2024

dx.doi.org/10.1021/nl400153p | Nano Lett. 2013, 13, 2024−2029

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Figure 1. Spin splitting and CPGE measurement. Schematic diagrams for the CPGE current caused by spin splitting of energy bands induced by right-handed circularly polarized light, which excites (a) surface electrons to higher energy band and (b) valence electrons to the conduction band. (c) The schematic configuration of the CPGE measurement along [1120̅ ] direction (jy corresponds to CPGE current) in c-plane InN films. The quarter wave plate is used to modulate the helicity of the light Pcirc = sin 2φ. (d) The photocurrent measured with configuration shown in panel c. Dashed line and dotted line represent the CPGE current and the LPGE current, respectively. (e) The amplitude of the CPGE current measured under different incident angles.

proportional to the Rashba SOC coefficient.28 Similarly, as shown in Figure 1b, the interband transitions excited by the circularly polarized light generate the asymmetric momentum distribution of electrons/holes in the bulk layer and induces a CPGE current proportional to the Dresselhaus SOC coefficient.24 Here, we take the CPGE in widely investigated c-plane InN films as an example. Generally, the CPGE current can be quantitatively described by jλ = ΣμχλμêμE20Pcirc,27 where j is the photocurrent density, χ is the CPGE second-rank pseudotensor, E0 is the complex amplitude of the electric field of the electromagnetic wave, and Pcirc is the degree of circular polarization, while ê = q/q and q are the unit vector pointing in the direction of light propagation and the light wave vector inside the medium, respectively. In c-plane InN films, the surface electron accumulation layer, in which electrons are confined in a triangle well, is a 2DES in C6v symmetry, the same point group as for the bulk InN. As a result, both the surface electrons and the bulk electrons contribute to CPGE currents in the same form. Therefore it is hard to distinguish the contributions from electrons in the surface or bulk layer by detecting CPGE current, since the spin splitting of surface and bulk electrons leads to CPGE currents with the same features.13,29 In an xyz-coordinate system (Figure 1c), nonzero items of the CPGE second-rank pseudotensor χ are χxy and χyx with χxy = −χyx.30 With the measurement configuration shown in Figure 1c, the CPGE currents along [112̅0] direction contributed by both surface electrons and bulk electrons can be expressed as25

within an electric double-layer transistor (EDLT) configuration, we demonstrate an effective tuning of surface electron spin splitting by ionic liquid gating, as indicated by the modulation of CPGE current in InN, which provides a new way of tuning spin polarization in spintronic devices. Let us start from the inversion asymmetry caused energy band spin splitting and its detection by CPGE measurement. The surface band bending in InN results in a strong surface electric field and structural inversion asymmetry (SIA), and thus leads to the spin splitting of surface electrons due to the Rashba spin−orbit coupling (SOC) (Figure 1a). Here we reduced consideration of the CPGE for surface electrons to SIA, since the spin splitting of surface electrons is dominated by the large surface electric field. The spin splitting energy ΔE of surface electrons can be described by ΔE(k) = 2αk, where α is the Rashba SOC coefficient, and k is the wave vector.25,27 In contrast, the spin splitting of electrons in bulk layer (Figure 1b) is induced by the bulk inversion asymmetry (BIA), determined by the Dresselhaus SOC coefficient and associated with the crystal symmetry.25,27 The spin splitting energy of bulk electrons can be described by ΔE(k) = 2βk, where β is the Dresselhaus SOC coefficient.25,27 The SIA and BIA induced spin splitting of the surface and bulk electrons are described by different phenomenological equations but both result in a band spin-splitting.25,27 The CPGE measurement has proven to be an effective technique to manifest the spin splitting and to evaluate the SOC coefficient.24,28 As shown in Figure 1a, the intersubband transition excited by the circularly polarized light generates the asymmetric momentum distribution of electrons/holes in the surface layer along a specific direction in the band structure and induces a related net current (named as CPGE current) along the corresponding crystal direction in real space without an external bias. The amplitude of the CPGE current is

jy = χyx ex̂ E02Pcirc = ηγI cos θ sin 2φ

(1)

where η is the CPGE optical transition index, γ is the total SOC coefficient, I is the intensity of the refractive light, θ is the refractive angle, and φ is the rotation angle of the quarter wave 2025


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Figure 2. CPGE measurements in a-plane InN films. CPGE measurements in a-plane InN films along [0001] direction: (a) the schematic diagram (jX corresponds to CPGE current), (b) the photocurrent measured under an incident angle of θ0 = 45°, and (c) the amplitude of CPGE current under different incident angles, which can be well fitted with eq 2, indicating that the CPGE current comes from the surface electrons. CPGE measurements in a-plane InN films along [11̅00] direction: (d) the schematic diagram (jY corresponds to CPGE current), (e) the photocurrent measured with θ0 = 45°, and (f) the amplitude of CPGE current, which can be well fitted with eq 3, indicating that the CPGE current mainly comes from the bulk electrons.

the nonzero items of the CPGE second-rank pseudotensor χ are χYZ and χZY with χYZ=−χZY. In the configuration shown in Figure 2d, the bulk electrons contribute to a CPGE current along [11̅00] direction:25

plate. The photocurrent, measured between two circular electrodes, oscillates with the rotation angle of the quarter wave plate because of CPGE and linear photogalvanic effect (LPGE).25 Here, the LPGE current denotes the linearly polarized light induced currents from both the linear photogalvanic effect and the photon drag effect.25 As shown in Figure 1d, the observed oscillation of the current with a period of π corresponds to the CPGE current contributed by the surface electrons and bulk electrons, which is proportional to the total SOC coefficient. The oscillation of the current with a period of π/2 corresponds to the LPGE current25 of which the contribution is negligible in this plot. Figure 1e shows the CPGE current dependence on incident angle, which can be well fitted with eq 1. In order to distinguish the spin splitting of surface electrons from that of bulk electrons, we proposed a novel way by detecting the anisotropic CPGE current along different crystal directions in nonpolar a-plane InN films, where the band bending induced electric field is not the same as (perpendicular to) the spontaneous polarized electric field (along [0001] direction) in InN. This configuration provides a straightforward measurement to distinguish the spin splitting with different origins. Briefly, as shown in Figure 2d, the CPGE current detected along [0001] direction in a-plane InN films, is only contributed by the SIA from the surface electron accumulation layer. As shown in Figure 2a,d, an XYZ-coordinate system is employed for the analysis of the CPGE in a-plane InN films. The surface electron accumulation layer of a-plane InN in which electrons are confined in a triangle potential well is a typical 2DES with Cs symmetry, so that the nonzero items of χ are χYZ, χZY, χYX, and χXY.30 The χXY component leads to the CPGE current along [0001] direction with the configuration shown in Figure 2a25 jX = χXY eŶ E02Pcirc = ηαI sin θ sin 2φ

jY = χYZ eẐ E02Pcirc = ηβI cos θ sin 2φ

(3)

where β is the Dresselhaus SOC coefficient for bulk electrons. Note that the bulk electrons have no contribution to CPGE current along [0001] direction. In other words, the SIA due to the surface electric field along −Z direction leads to spin polarization within the XY plane. Therefore, when the light obliquely irradiates the sample, its propagation vector component êY leads to an asymmetric distribution of electrons along +X direction in the momentum space and consequently induces the CPGE current along +X direction (Figure 1a). Consequently, the CPGE current contributed by the surface electrons is distinguished from that contributed by the bulk electrons by detecting the photocurrent along different crystal directions in nonpolar a-plane InN films. Figure 2b shows the total photocurrent detected along [0001] direction in an a-plane InN film under an incident angle of θ0 = 45° (Figure 2a), and the CPGE current with a value of 0.15 nA/W is obtained from the fitting line. As the sin 4φ (LPGE) component of the photocurrent is considerably larger than its sin 2φ component, we measured the incident-angle dependence of the photocurrent to confirm that the latter is indeed due to the CPGE. As shown in Figure 2c, the CPGE current along [0001] direction first increases with the incident angle, reaches a maximum at ∼45°, and then finally decreases with the incident angle. This feature can be well fitted with eq 2, unambiguously indicating that the sin 2φ component of the photocurrent corresponds to the CPGE current contributed by the surface electrons, based on the mechanism shown in Figure 1a. In comparison, as shown in Figure 2d−f, the CPGE current along [110̅ 0] direction under different incident angles is fitted well with eq 3, which corresponds to the CPGE current contributed by bulk electrons. Accordingly, the CPGE current along [11̅00] direction in a-plane InN films should be mostly

(2)

where α is the Rashba SOC coefficient for surface electrons. Since the point group of bulk InN is C6v as mentioned above, 2026


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Figure 3. CPGE measurements with ionic liquid gating. (a) The schematic diagram of the ionic liquid gating EDL transistor and resulting modification of interfacial band bending. When a voltage is applied between the gate electrode and the InN channel, the mobile cation and anion of the ionic liquid will move toward oppositely charged electrodes to form the EDLs. (b) The cross-section and oblique drawing diagrams for the modulation of the CPGE with the ionic liquid gating EDL transistor. The laser light irradiates the InN film under an incident angle of 45°, and the CPGE current jy is detected along [112̅0] direction. The gate voltage is applied between the Pt gate electrode and the InN film with a dry battery. (c) The CPGE current contributed by surface electrons and the surface electric field, as a function of applied gate voltage. The surface electric field could also reflect the Rashba spin splitting of surface electrons, since the Rashba SOC coefficient is proportional to the surface electric field.

surface electrons is proportional to the surface electric field,25 the intensity of the CPGE current should also linearly increase with increasing gate voltage, which clearly interpreted our observation in Figure 3c. Therefore, the linear increase of CPGE current with increasing gate voltage provides with the firm evidence that ionic liquid gating can modulate the surface electron spin splitting. The maximum CPGE current from the surface layer is about 0.15 nA/W under an incident angle of 45° (Figure 2c), which is 1 order of magnitude lower than that from the bulk InN. However, this does not mean that the spin splitting of surface electrons is weaker than that in bulk InN, if we consider that most of incident light is absorbed by the bulk layer. Moreover, the optical transition probability, which determines the amplitude of photocurrent, is much higher in the bulk (caused by interband transition) than that in surface layer (cause by intersubband transition). It is also worth noting that, there should be CPGE current with similar magnitude from surface electrons in c-plane InN films as well for its similar surface electron density to a-plane InN films,32 even though it cannot be distinguished from the bulk-induced CPGE current. With the ionic liquid gating CPGE measurement, the relative polarity of CPGE currents in c-plane InN films can be determined. As discussed above, the CPGE current detected in c-plane InN films includes contribution from both bulk and surface layer. Since the ionic liquid gating only modulates surface electrons,22 the modulation of CPGE current by the gate voltage should originate from the surface electron accumulation layer alone. Regarding that the CPGE current contributed by surface electrons increases with increasing the gate voltage, the reduction of the total CPGE current with increasing gate voltage (see Supporting Information) indicates that the polarity of CPGE current induced by the surface layer is opposite to that induced by the bulk layer. In conclusion, the CPGE current along [0001] direction contributed only by the surface electrons was detected in the aplane InN films, which shows direct evidence for the spin splitting of the surface electron accumulation layer in InN. An effective method to distinguish the CPGE currents from the

contributed by bulk electrons. As clearly shown in Figure 2, the CPGE contributed by surface electrons is distinguished from that from the bulk layer in a-plane InN films, which shows the direct evidence that the surface electrons are spin split. After knowing that surface electrons are spin split, ionic liquid gated EDLTs22 are used to modulate the magnitude of the surface electron spin splitting. Figure 3a shows the schematic diagram of the InN EDL transistors, where the cation and anion of the ionic liquid are driven toward oppositely charged electrodes and thus to form the EDL simply by applying a small gate voltage.23 The EDLs at the liquid/semiconductor interface can be regarded as a nanogap capacitor with a huge capacitance,31 which is able to accumulate charges at the surface of InN to a very high density that is unreachable by conventional solid gate dielectrics. Hall effect measurement shows that the surface electron density of the cplane InN film is successfully tuned by the ionic liquid gating (see Supporting Information). It is worth noting that the cplane InN thin film is chosen here since it shows atomically flat surface that is necessary for EDL transistors.23,31 Since the ionic liquid gate can effectively tune the density of surface electrons, indicating the tuning of band bending, its electric field should also be able to tune the spin splitting of the surface electrons. To confirm that, we measured the ionic liquid gating modulated CPGE current in c-plane InN films, based on the experimental setup shown in Figure 3b. The bias is applied between Pt gate electrode and the InN film, and the incident laser beam goes through the ionic liquid and irradiates on the InN film. Since the ionic liquid is transparent to the laser beam, it would not influence the CPGE measurement. Figure 3c shows the CPGE current contributed by the surface electrons, as well as the surface electric field due to the band bending. The CPGE current increases from 0 to 0.35 nA/W approximately linearly with increasing gate voltage from −0.9 to 1.0 V, while further increase of gate voltage leads to saturation of the CPGE current. The surface electric field is obtained according to the Poisson Equation as ES = qNS/εInN, where q is the elementary charge, NS is the surface electron density, and εInN is the dielectric constant of InN. Since the Rashba SOC coefficient of 2027


Nano Letters surface and bulk electrons was also demonstrated by detecting the photocurrent along different crystal directions. Moreover, the surface-electron-induced CPGE current was successfully modulated by ionic liquid gating with an InN EDL transistor and further proves that the spin splitting of the surface electron accumulation layer can be effectively tuned, providing a way to modulate the spin polarization and thus beneficial for spintronic manipulation. Methods. The a-plane InN films were grown on r-plane sapphire substrates by molecular beam epitaxy (MBE). Sapphire nitridation was performed at 400 °C for 1 h to get a very thin AlN layer. Then a low temperature GaN buffer layer and a 250-nm thick high-temperature GaN epilayer were grown in sequence. The GaN layer was confirmed to be a-plane and acted as a template for the following InN epitaxy. The a-plane InN layer was directly grown on the GaN template at 400 °C with a thickness of 300 nm. For comparison, c-plane InN films were grown on c-plane sapphire substrates using the same growth process as that of the a-plane samples. All the samples were cut into rectangles, and two ohmic contacts were made with In metal at a distance of 4 mm. Hall effect was measured on InN thin films patterned into Hall bar configuration with Ti/Au electrodes. All measurements were performed at room temperature. The CPGE measurement configurations were shown in Figures 1c, 2a,d, and 3b. A solid state laser with a wavelength of 1064 nm was used to excite electrons in lower energy band to higher energy band. The laser beam passed through a rotatable quarter wave plate and then irradiated the samples with a 1-mm diameter spot at the center between the two electrodes. The helicity of laser light was modulated by the quarter wave plate as Pcirc = sin 2φ. The photocurrent was collected between the two electrodes and related results were fitted well based on the CPGE and LPGE mechanisms. As shown in Figure 3b, for the ionic liquid gating modulated CPGE measurement, the ionic liquid (DEME-TFSI) was sandwiched between the surface of InN film and Pt plate electrode, which were finally covered by a quartz coverslip.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the National Basic Research Program of China (Nos. 2012CB619306 and 2012CB619303), the National Natural Science Foundation of China (Nos. 61225019, 11023003, 10990102, 60990313, and 11174008), the National High Technology Research and Development Project of China (Nos. 2011AA050514 and 2011AA03A103), and the Research Fund for the Doctoral Program of Higher Education.

(1) Ž utić, I.; Fabian, J.; Das Sarma, S. Spintronics: Fundamentals and applications. Rev. Mod. Phys. 2004, 76, 323. (2) Awschalom, D. D.; Flatté, M. E. Challenges for semiconductor spintronics. Nat. Phys. 2007, 3, 153−159. (3) Kotissek, P.; Bailleul, M.; Sperl, M.; Spitzer, A.; Schuh, D.; Wegscheider, W.; Back, C. H.; Bayreuther, G. Cross-sectional imaging of spin injection into a semiconductor. Nat. Phys. 2007, 3, 872−877. (4) Fiederling, R.; Kleim, M.; Reuscher, G.; Ossau, W.; Schmidt, G.; Waag, A.; Molenkamp, L. W. Injection and detection of a spinpolarized current in a light-emitting diode. Nature 1999, 402, 787− 790. (5) Hall, K. C.; Lau, W. H.; Gündoğdu, K.; Flatté, M. E.; Boggess, T. F. Nonmagnetic semiconductor spin transistor. Appl. Phys. Lett. 2003, 83, 2937. (6) Kato, Y. K.; Myers, R. C.; Gossard, A. C.; Awschalom, D. D. Observation of the spin Hall effect in semiconductors. Science 2004, 306, 1910−1913. (7) Valenzuela, S. O.; Tinkham, M. Direct electronic measurement of the spin Hall effect. Nature 2006, 442, 176−179. (8) Ando, K.; Saitoh, E. Observation of the inverse spin Hall effect in silicon. Nat. Commun. 2012, 3, 629. (9) Wunderlich, J.; Park, B. G.; Irvine, A. C.; Zârbo, L. P.; Rozkotová, E.; Nemec, P.; Novák, V.; Sinova, J.; Jungwirth, T. Spin Hall effect transistor. Science 2010, 330, 1801−1804. (10) Mahboob, I.; Veal, T. D.; McConville, C. F.; Lu, H.; Schaff, W. J. Intrinsic electron accumulation at clean InN surfaces. Phys. Rev. Lett. 2004, 92, 036804. (11) Noguchi, M.; Hirakawa, K.; Ikoma, T. Intrinsic Electron accumulation layers on reconstructed clean InAs (100) surfaces. Phys. Rev. Lett. 1991, 66, 2243. (12) Jia, Z. W.; Shen, W. Z.; Ogawa, H.; Guo, Q. X. Electron dephasing in wurtzite indium nitride thin films. Appl. Phys. Lett. 2006, 89, 232107. (13) Zhang, Z.; Zhang, R.; Liu, B.; Xie, Z. L.; Xiu, X. Q.; Han, P.; Lu, H.; Zheng, Y. D.; Chen, Y. H.; Tang, C. G.; Wang, Z. G. Circular photogalvanic effect at inter-band excitation in InN. Solid State Commun. 2008, 145, 159−162. (14) Petersen, G.; Estévez Hernández, S.; Calarco, R.; Demarina, N.; SchäPers, Th. Spin-orbit coupling and phase-coherent transport in InN nanowires. Phys. Rev. B 2009, 80, 125321. (15) Mei, F. H.; Tang, N.; Wang, X. Q.; Duan, J. X.; Zhang, S.; Chen, Y. H.; Ge, W. K.; Shen, B. Detection of spin-orbit coupling of surface electron layer via reciprocal spin Hall effect in InN films. Appl. Phys. Lett. 2012, 101, 132404. (16) Nitta, J.; Akazaki, T.; Takayanagi, H.; Enoki, T. Gate control of spin-orbit interaction in an inverted In0.53Ga0.47As/In0.52Al0.48As Heterostructure. Phys. Rev. Lett. 1997, 78, 1335. (17) Engels, G.; Lange, J.; SchäPers, Th.; Lüth, H. Experimental and theoretical approach to spin splitting in modulation-doped InxGa1‑xAs/ InP quantum wells for B→0. Phys. Rev. B 1997, 55, R1958. (18) Kunihashi, Y.; Kohda, M.; Sanada, H.; Gotoh, H.; Sogawa, T.; Nitta, J. Proposal of spin complementary field effect transistor. Appl. Phys. Lett. 2012, 100, 113502. (19) Kohda, M.; Lechner, V.; Kunihashi, Y.; Dollinger, T.; Olbrich, P.; Schönhuber, C.; Caspers, I.; Bel’kov, V. V.; Golub, L. E.; Weiss, D.; Richter, K.; Nitta, J.; Ganichev, S. D. Gate-controlled persistent spin

ASSOCIATED CONTENT

S Supporting Information *

Microstructural characterization of InN films and the Hall effect measurement and CPGE measurement data with ionic liquid gating. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: (X.W.) [email protected]. (B.S.) [email protected]. cn. Author Contributions

C.M.Y. and H.T.Y. contributed equally to this work. C.M.Y., H.T.Y., and X.Q.W. designed the experiments. X.Q.W. and S.T.L. performed sample fabrication. C.M.Y., X.Q.W., and H.T.Y. conducted the experiments and wrote the paper. S.Z., Z.Y.C., and H.S. assisted with measurements and analysis. Y.I., Y.H.C., W.K.G., and B.S. helped in data analysis. All authors contributed through scientific discussions. Notes

The authors declare no competing financial interest. 2028


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helix state in (In,Ga)As quantum wells. Phys. Rev. B 2012, 86, 081306(R). (20) Denisenko, A.; Pietzka, C.; Chuvilin, A.; Kaiser, U.; Lu, H.; Schaff, W. J.; Kohn, E. J. Depletion of surface accumulation charge in InN by anodic oxidation. J. Appl. Phys. 2009, 105, 033702. (21) Liu, W.; Tan, R. J. N.; Soh, C. B.; Chua, S. J. The effects of cap layers on electrical properties of indium nitride films. Appl. Phys. Lett. 2010, 97, 042110. (22) Yuan, H. T.; Shimotani, H.; Tsukazaki, A.; Ohtomo, A.; Kawasaki, M.; Iwasa, Y. High-density carrier accumulation in ZnO field-effect transistors gated by electric double layers of Ionic Liquids. Adv. Funct. Mater. 2009, 19, 1046−1053. (23) Ye, J. T.; Inoue, S.; Kobayashi, K.; Kasahara, Y.; Yuan, H. T.; Shimotani, H.; Iwasa, Y. Liquid-gated interface superconductivity on an atomically flat film. Nat. Mater. 2010, 9, 125−128. (24) Ganichev, S. D.; Ivchenko, E. L.; Prettl, W. Photogalvanic effects in quantum wells. Physica E 2002, 14, 166−171. (25) Ganichev, S. D.; Prettl, W. Spin photocurrents in quantum wells. J. Phys.: Condens. Matter 2003, 15, R935. (26) Ivchenko, E. L.; Ganichev, S. Spin−Photogalvanics. In Spin Physics in Semiconductors; Dyakonov, M. I., Ed.; Springer: New York, 2008; pp 245−277. (27) Winkler, R. Spin-Orbit Coupling Effects in Two-Dimensional Electron and Hole Systems; Springer: Berlin, 2003. (28) Ganichev, S. D.; Bel’kov, V. V.; Schneider, P.; Ivchenko, E. L.; Tarasenko, S. A.; Wegscheider, W.; Weiss, D.; Schuh, D.; Beregulin, E. V.; Prettl, W. Resonant inversion of the circular photogalvanic effect in n-doped quantum wells. Phys. Rev. B 2003, 68, 035319. (29) Zhang, Q.; Wang, X. Q.; He, X. W.; Yin, C. M.; Xu, F. J.; Shen, B.; Chen, Y. H.; Wang, Z. G.; Ishitani, Y.; Yoshikawa, A. Lattice polarity detection of InN by circular photogalvanic effect. Appl. Phys. Lett. 2009, 95, 031902. (30) Sturman, B. I.; Fridkin, V. M. The Photovoltaic and Photorefractive Effects. In Noncentrosymmetric Materials; translated by Bradley, J. E. S.; Gordon and Breach Science Publishers: New York, 1992. (31) Yuan, H. T.; Liu, H. W.; Shimotani, H.; Guo, H.; Chen, M. W.; Xue, Q. K.; Iwasa, Y. Liquid-gated ambipolar transport in ultrathin films of a topological insulator Bi2Te3. Nano Lett. 2011, 11, 2601. (32) Nagata, T.; Koblmuller, G.; Bierwagen, O.; Gallinat, C. S.; Speck, J. S. Surface structure and chemical states of a-plane and c-plane InN films. Appl. Phys. Lett. 2009, 95, 132104.

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