Generation of Rashba Spin–Orbit Coupling in ... - ACS Publications

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Generation of Rashba Spin−Orbit Coupling in CdSe Nanowire by Ionic Liquid Gate Shan Zhang,† Ning Tang,*,† Weifeng Jin,† Junxi Duan,† Xin He,† Xin Rong,† Chenguang He,† Lisheng Zhang,† Xudong Qin,† Lun Dai,†,‡ Yonghai Chen,§ Weikun Ge,*,†,∥ and Bo Shen*,†,‡ †

State Key Laboratory of Artificial Microstructure and Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, People’s Republic of China ‡ Collaborative Innovation Center of Quantum Matter, Beijing 100871, People’s Republic of China § Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences (CAS), Beijing 100083, People’s Republic of China ∥ Department of Physics, Tsinghua University, Beijing 100084, People’s Republic of China S Supporting Information *

ABSTRACT: Spintronic devices rely on the spin degree of freedom (DOF), and spin orbit coupling (SOC) is the key to manipulate spin DOF. Quasi-one-dimensional structures, possessing marked anisotropy gives more choice for the manipulation of the spin DOF since the concrete SOC form varies along with crystallographic directions. The anisotropy of the Dresselhaus SOC in cadmium selenide (CdSe) nanobelt and nanowire was studied by circular photogalvanic effect. It was demonstrated that the Dresselhaus SOC parameter is zero along the [0001] crystallographic direction, which suppresses the spin relaxation and increases the spin diffusion length, and thus is beneficial to the spin manipulation. To achieve a device structure with Rashba SOC presence and Dresselhaus SOC absence for manipulating the spin DOF, an ionic liquid gate was produced on a nanowire grown along the [0001] crystallographic direction, and the Rashba SOC was induced by gating, as expected. KEYWORDS: CdSe nanobelt/nanowire, circular photogalvanic effect, ionic liquid, Rashba spin orbit coupling (SOC), Dresselhaus SOC

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of the electron wave vector influences the Dresselhaus SOC.14 It has also been reported that the spin relaxation is suppressed by a dimensionally constrained mechanism when transform occurs from 2D to 1D system and the spin relaxation time is closely related to crystallographic directions.15 As far as the Dresselhaus SOC is concerned, crystal point group symmetry allows different components in its second rank tensor, resulting in anisotropy.16 More importantly, it may give rise to absence of the Dresselhaus SOC in 1D structure along with certain corresponding crystallographic directions and thus induce different characteristics from those in bulk and in quantum wells. In a quantum wire whose Dresselhaus SOC is absent, the Dyakonov−Perel (D−P) spin relaxation mechanism17 is suppressed. The D−P relaxation adversely affects the spin manipulation by inducing current leakage in the off state. The absence of the Dresselhaus SOC leaves alone the Rashba SOC induced by symmetry breaking electric field to produce a fixed effective magnetic field. When spin polarized electrons are injected, they would precess about that magnetic field. As a

he spin degree of freedom (DOF) is the heart of spintronics. Spin−orbit coupling (SOC), enabling electrical manipulation of electron spin DOF in semiconductors, is crucial in the realization of spintronic devices.1 Generally, SOC is believed to come from either structural inversion asymmetry (SIA) or bulk inversion asymmetry (BIA). The former is known as the Rashba SOC and the latter the Dresselhaus SOC. The Rashba SOC can be tailored by artificial microstructure design such as heterostructures or delta-doping.2 In comparison to the Dresselhaus SOC the Rashba SOC has attracted much concerns because it can be modulated by external electric field.3,4 The Dresselhaus SOC could, however, also be important due to its dependence on the crystal symmetry.5−7 Recently Ganichev et al. reviewed the interplay of Rashba/ Dresselhaus spin splittings in heterostructures of different crystallographic directions and addressed the requirements for suppression of spin relaxation.8 While many initial studies on spintronics were on planar twodimensional systems,4,9−11 semiconductors with quasi-onedimensional (1D) structure have also attracted immense attention as significant progress has been made in their growth techniques and nanofabrication. Strong and tunable Rashba SOC has been achieved in InAs nanowire12 and Ge/Si core− shell nanowires.13 It has been found that the size quantization © XXXX American Chemical Society

Received: November 4, 2014 Revised: December 31, 2014

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Figure 1. Morphology of the CdSe nanobelt and nanowire. (a) TEM image, (b) HRTEM image, and (c) electron diffraction pattern of nanobelts. (d) TEM image, (e) HRTEM image, and (f) electron diffraction pattern of nanowires.

TEM images demonstrate clearly a single-crystalline structure. Electron diffraction patterns reveal that the nanobelts grow along the [112̅0] direction, while the nanowires grow along the [0001] direction. Given the crystalline growth direction, we conducted the CPGE measurement on the CdSe nanobelt and nanowire, respectively. Figure 2a, d shows the morphology of the nanobelt and nanowire with ohmic contacts, respectively. As shown in Figure 2b, e, the CPGE current can be detected in the CdSe nanobelt but not in the CdSe nanowire, demonstrating that the Dresselhaus SOC is present in the former and absent in the latter. We define the [11̅00] direction as the x-axis, [112̅0] direction as the y-axis, and [0001] direction as the z-axis. Generally, the CPGE current can be quantitatively described by jλ = ∑μχλμêμE02Pcirc, where j is the photocurrent density, χ is the CPGE second-rank pseudo tensor, E0 is the complex amplitude of the electric field of the electromagnetic wave of the light, Pcirc is the degree of the circular polarization of the light, ê = q/q is the unit vector pointing in the direction of light propagation, and q is the light wave vector inside the medium. χ has the same form with Dresselhaus SOC. The wurtzite semiconductor CdSe belongs to the C6ν point group. With this symmetry the nonzero items of the CPGE second-rank pseudo tensor χ are χxy and χyx, with χxy = −χyx.16 The nonzero components cause spin spitting in the momentum space, as shown in Figure 3c. The interband transitions excited by the circularly polarized light generate asymmetric momentum distribution of electrons/holes in the bulk layer. Consequently it leads to a spinpolarized net current in the y direction, namely, the CPGE current. The CPGE current contributed by the bulk electrons can then be expressed as

result, the D−P spin relaxation is inhibited, thus enabling a longer spin relaxation length, as desired in spintronics applications.18−20 Cadmium selenide (CdSe) is a wurtzite semiconductor of group II−VI with a direct bandgap of 1.74 eV at room temperature, in good match with the visible spectrum. CdSe has made a wide range of applications such as photo detectors, solar cells, and field-effect transistors.21−24 However, the investigation of SOC in CdSe is almost blank. In this study the Dresselhaus SOC in the wurtzite 1D CdSe semiconductor materials was studied by the circular photogalvanic effect (CPGE) measurement. CPGE is a sensitive method to evaluate SOC coefficient at room temperature.25,26 Our experiments show that the CPGE current can be detected in 1D CdSe (either nanobelt or nanowire) grown along [112̅0] direction but not in 1D CdSe grown along [0001] direction, demonstrating the anisotropy of the Dresselhaus SOC. To realize the spin manipulation, an ionic liquid (IL) gate was applied on the CdSe along [0001] direction. The IL, serving as an effective dielectric, accumulates electric charges at the sample surface much more effectively comparing with conventional solid gate dieletric.27 The large contact area between the IL and 1D sample further enhances the gate control capability.12 In addition, the IL is transparent to the laser beam, well aligned with the CPGE measurement.10,23 Accumulation of the surface electrons generates electric field at the interface, which then induces the Rashba SOC and results in spin splitting of the surface electrons. Well detectable CPGE signals prove the existence of the Rashba SOC. To investigate the anisotropy of the Dresselhaus SOC, the crystalline growth direction was determined by transmission electron microscopy (TEM). Figure 1 shows the morphology of the CdSe nanobelt and nanowire. The high-resolution (HR) B

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Figure 2. CPGE measurement in the CdSe nanobelt and nanowire. (a) The morphology of the nanobelt with ohmic contact. (b) CPGE measured on the CdSe nanobelt. The red line is the fitting line. A CPGE current of 25.6 pA is extracted from the measurement. The insets show the schematic configuration of the CPGE measurement. (c) Schematic diagram for the CPGE current caused by spin splitting of energy bands induced by righthanded circularly polarized light exciting the valence electrons to the conduction band. (d) The morphology of the nanowire with ohmic contact. (e) The CPGE measured on the CdSe nanowire. The red line is the fitting line. No CPGE current is extracted from the CdSe nanowire. (f) Schematic diagrams for the right-handed circularly polarized light exciting the valence electrons to the conduction band. Since the electron spin is degenerate in kz direction, the interband transition generates a symmetric momentum distribution of electrons/holes in the bulk layer; therefore, no CPGE signal was detected.

jy = χyx ex̂ E0 2Pcirc = −ηγI sin θ sin 2φ

the electron spin is degenerate in the kz direction, as shown in Figure 2f. The interband transitions excited by the circularly polarized light thus generate symmetric momentum distribution of electrons/holes in the bulk layer, and therefore, no CPGE can be detected. Figure 2e shows the photocurrent of such a CdSe nanowire as a function of the rotation angle of the quarter wave plate. In this case, JLPGE and J0 are dominated, while the CPGE is insignificant and JCPGE can be ignored. The undetectable JCPGE gives a clear evidence of the absence of the Dresselhaus SOC along the [0001] direction. Symmetry analysis reveals that the Dresselhaus SOC exists in a 1D structure along with [112̅0] direction but is absent in that along with [0001] direction, which corresponds well to the CPGE experimental results, suggesting that the CPGE measurement is a simple but effective way to evaluate the Dresselhaus SOC and to distinguish the growth orientations for wurtzite semiconductors. Having identified the 1D CdSe nanowire whose Dresselhaus SOC parameter equals zero, an IL gate was used to induce the Rashba SOC. The gate voltage is applied between the Au gate pad and the CdSe nanowire, as shown in Figure 3a. The inset of Figure 3e shows the mobile cation and anion of the IL move toward oppositely charged electrodes. The IL together with the sample and the gate electrode form an electric double layer geometry, where the interface could be regarded as a nanothin capacitor with huge capacitance, resulting in high electrondensity in the surface layer of the nanowire. The electric charge

(1)

where η is the CPGE optical transition index, γ is the SOC coefficient with γ = χxy = −χyx, I is the intensity of the refractive light, θ is the refractive angle, and φ is the rotation angle of the quarter wave plate, representing the helicity of the light. The measured photocurrent between the two electrodes of the CdSe nanobelt as a function of the rotation angle φ of the quarter wave plate, which is used for varying the light polarization, is shown in Figure 2b, where the angle of incidence θ0 = 50°. The measured photocurrent is fitted by the following formula jtotal = JCPGE sin 2φ + JLPGE sin 2φ cos 2φ + J0

(2)

where JCPGE is the CPGE current of which we are concerned, JLPGE is the amplitude of the linear photogalvanic effect (LPGE) current, which results from asymmetric scattering of electrons, and J0 counts for the background photocurrent originating from the Dember effect and/or other photovoltaic effects.25 The LPGE current oscillates with angle φ in a period of π/2, while the CPGE current oscillates with angle φ in a period of π. As can be seen from the fitting line, JCPGE and J0 are predominant, while the LPGE is insignificant. The observable CPGE coincides with the symmetry analysis where χyx is nonzero. However, no CPGE occurs in a CdSe nanowire grown along the [0001] direction (z-axis) because both χzx and χzy equal zero in this case, leading to jz = (χzx + χzy)êxE02Pcirc = 0). Thus, C

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Figure 3. CPGE measurement in the CdSe nanowire with IL gate. (a) Setup for the gate modulation of the CPGE current measurement. (b) Schematic diagram for the CPGE current caused by spin splitting of the energy bands, as a result of the right-handed circularly polarized light excitation of the surface electrons to higher energy bands. (c) The measured photocurrent as a function of the phase angle φ for oblique incidence of the irradiation with IL gate but with no bias voltage at room temperature. The red line is the fitting line. Both LPGE and CPGE current can be extracted. (d) The gate voltage modulation of the CPGE current. (e) The CPGE current as a function of the applied gate voltage. The inset shows the schematic diagram of the IL dripped on the sample forming an electric field at the heterointerface. Strong surface electric field and structural inversion asymmetry (SIA) leads to spin splitting of the surface electrons due to the Rashba SOC. (f) A nanowire with rectangular cross-section. Electron transport along the axis of the wire. External electric field is applied perpendicular to the axis of wire to induce the Rashba SOC. The effective Rashba magnetic field is fixed but the effective Dresselhaus magnetic field is not. If intersubband scattering causes electron transferring from subband (1,1) to (2,1), the effective Dresselhaus magnetic field changes from B⃗ D(1,1)(ν⃗) to B⃗ D(2,1)(ν⃗). As a result, the total effective magnetic field B⃗ (ν⃗) changes, rendering the D−P spin relaxation. If, however, there is no Dresselhaus SOC, the B⃗ (ν⃗) is fixed, and thus, the D−P spin relaxation is suppressed.

the CPGE signal indicates the strength of Rashba SOC increases with gate voltage. Both the Rashba and Dresselhaus SOC contribute to effective magnetic field B⃗ R(ν⃗) and B⃗ D(ν⃗), which cause electron’s spin to precess about them. As shown in Figure 3f, in a 1D system, the electron’s velocity ν⃗ is always along the axis of the wire. B⃗ R(ν⃗) is mutually perpendicular to the wire axis and the symmetry breaking electric field E⃗ . B⃗ D(ν⃗), however, points along the wire axis.18 The magnitude of B⃗ R(ν⃗) is fixed as long as ν⃗ and E⃗ are given. However, the magnitude of B⃗ D(ν⃗) is proportional to [(mπ/Wx)2 − (nπ/Wy)2]ν⃗ with Wx and Wy as the transvers dimensions of the wire (m and n are subband indexes). If intersubband scattering occurs, as in most normal conditions of operation, the magnitude of B⃗ D(ν⃗) varies. Thus, the total magnitude and direction of the effective magnetic field B⃗ (ν⃗) = B⃗ R(ν⃗) + B⃗ D(ν⃗) would change randomly in time, rendering the D−P spin relaxation. However, in a quantum wire whose Dresselhaus SOC is absent, B⃗ (ν⃗) = B⃗ R(ν⃗), the total B⃗ (ν⃗) is fixed. As a result, the D−P relaxation is suppressed, allowing for a longer spin relaxation length. In conclusion, anisotropy of Dresselhaus SOC was studied in CdSe 1D structures with different crystalline growth directions by CPGE measurement, suggesting that the CPGE measurement is a simple but effective way to distinguish the growth

accumulation at the surface tunes the surface band bending, and hence, the electric filed at the heterointerface is able to tune the spin splitting of the surface electrons. From the group symmetry point of view, the induced electric field lowers the sample’s symmetry from C6ν to Cs, leading to nonzero items of χ expansion to include χyz, χzy, χxy, and χyx. The nonzero χzy component is the origin of the surface electron spin splitting in kz direction. Figure 3b shows the schematic diagram for the CPGE current caused by the spin splitting induced by righthanded circularly polarized light exciting surface electrons to higher energy bands. The measured photocurrent between the two electrodes of the nanowire enwrapped with IL but with no voltage bias is shown in Figure 3c, indicating that the IL application itself has already induced a surface charge accumulation. The application of IL changed the surface character of the sample and induced SIA. The amplitude of JCPGE is fitted to be 0.6 pA, and JLPGE is fitted to be 1.7 pA. Figure 3d shows the dependence of the CPGE signal on the applied gate voltage. The CPGE increases from 0.3 to 1.2 pA with increasing gate voltage from −0.6 to 0.6 V. The CPGE signals taken with different gating voltage are shown in Figure 3d, e. The CPGE signal provides a strong evidence of the existence of the Rashba SOC. The gate voltage dependence of D

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Nano Letters orientations in wurtzite 1D materials. CPGE measurement can efficiently pick out the CdSe nanowires grown along the [0001] directions with the Dresselhaus SOC absence. The Rashba SOC was induced to the nanowire by IL gate to manipulate spin, whereupon a device structure with Rashba SOC presence and Dresselhaus SOC absence was realized. We believe that the approach proposed in this study should also work in other wurtzite semiconductor materials with 1D structure. Methods. Sample Preparation. The CdSe nanobelt and nanowire were synthesized via chemical vapor deposition (CVD). The as-synthesized CdSe exhibited n-type conductivity due to selenium vacancies or cadmium interstitial atoms, which serve as shallow donors in CdSe. The CdSe was dispersed in ethanol with an ultrasonic process. Then its suspension solution was dropped on to an oxidized Si substrate with a 600 nm SiO2 capping layer. After that, the In/Au (150/50 nm) ohmic contact electrodes were defined on both ends of the CdSe nanobelt/nanowire with electron beam lithography (EBL), followed by electron beam evaporation and lift-off process. Measurement. The morphology of the CdSe nanowire and nanobelt are characterized by TEM images. Electron diffraction patterns reveal the crystalline direction of the samples. The CPGE measurements were performed at room temperature. The setup for CPGE measurements is illustrated in the Supporting Information Figure S1. A diode-pumped solid state laser with wavelength of 532 nm served as the radiation source. A rotatable quarter-wavelength plate was employed to modulate the helicity of the incident light. The helicity of the incident light Pcirc equals sin 2φ, where the phase angle φ is the angle between the polarization direction of the incident light and the optical axis of the quarter-wave plate. After passing through a chopper, the light beam with a diameter of about 1.5 mm irradiated obliquely on the sample. Since the diameter of the light beam is much larger than the sample, the light spot covers the sample completely. The photocurrent signal was collected by a lock-in amplifier after being preamplified. For the IL gating CPGE measurement, the IL (DEME-TFSI) was carefully dripped on the CdSe nanowire. The gate voltage is applied between the Au gate pad and the CdSe nanowire.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the National Basic Research Program of China (Nos. 2013CB921901, 2012CB921304, and 2012CB932703), the National Natural Science Foundation of China (Nos. 61376095, 11174008, 61361166007, and 61125402), the National High-Tech Research and Development Program of China (No. 2014AA032606), the Beijing Higher Education Young Elite Teacher Project (No. YETP0006), and the Beijing Municipal Science and Technology Project (No. Z131100005913001).

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ASSOCIATED CONTENT

S Supporting Information *

Setup for CPGE measurement. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

S.Z. conducted the CPGE measurement. W.J. and L.D. grew the CdSe nanowires and nanobelts. S.Z., J.D., X.H., and X.Q. prepared the samples gated by IL. S.Z., X.R., and L.Z. determined the crystalline growth direction of the samples by TEM. N.T., W.G., and B.S. supervised the study. S.Z., N.T., and W.G. wrote the manuscript. S.Z., N.T, J.D., Y.C., and W.G. contributed to the analysis for the results. All the authors discussed the results and commented on the manuscript. Notes

The authors declare no competing financial interest. E

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