Electrical control of circular photogalvanic spin-valley photocurrent in

Subscriber access provided by University of South Dakota

Functional Nanostructured Materials (including low-D carbon)

Electrical control of circular photogalvanic spinvalley photocurrent in a monolayer semiconductor Lei Liu, Erik Lenferink, Guohua Wei, Teodor K. Stanev, Nathaniel Speiser, and Nathaniel P. Stern ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17476 • Publication Date (Web): 24 Dec 2018 Downloaded from http://pubs.acs.org on December 24, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Electrical control of circular photogalvanic spinvalley photocurrent in a monolayer semiconductor Lei Liu,*,†,‡ Erik J. Lenferink,† Guohua Wei, § Teodor K. Stanev,† Nathaniel Speiser,† and Nathaniel P. Stern*,†,§

† Department of Physics and Astronomy, Northwestern University, 2145 Sheridan Road, Evanston,

IL 60208, USA ‡

Department of Materials Science and Engineering, College of Engineering, Peking University,

Beijing 100871, P. R. China §

Applied Physics Program, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208,

USA

KEYWORDS: circular photogalvanic effect, monolayer transition metal dichalcogenide, spin-valley photocurrent, gate control, electrostatic screening

ACS Paragon Plus Environment

1

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 39

ABSTRACT: In a monolayer transition metal dichalcogenide (TMDC) that lacks structural inversion symmetry, spin degeneracy is lifted by strong spin-orbit coupling, and a distinctive spin-valley locking allows for the creation of valley-locked spin-polarized carriers with a circularly polarized optical excitation. When excited carriers also have net in-plane momentum, spin-polarized photocurrents can be generated at ambient temperature without magnetic fields or materials. The behavior of these spin-polarized photocurrents in monolayer TMDC remains largely unexplored. In this work, we demonstrate the tuning of spin-valley photocurrent generated from the circularly polarized photogalvanic effect in monolayer MoS2, including magnitude and polarization degree, by purely electric means at room temperature. The magnitude of spin-polarized photocurrent can be modulated up to 45 times larger and the polarization degree of the total photocurrent can be tuned significantly (here from 0.5% to 16.6%) by gate control. Combined with the atomic thickness and wafer-scale growth capabilities of monolayer TMDC, the efficient electrical tuning of spin-valley photocurrent suggests a pathway to achieve spin logic processing by local gate architectures in monolayer opto-spintronic devices.


2

Page 3 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



3


1.

Page 4 of 39

INTRODUCTION

The fundamental processes required for spintronic applications, such as creation and manipulation of a spin current, have been demonstrated at ambient temperature both by electrical and optical methods in semiconductors.1-4 Spin injection from ferromagnetic contacts typically requires magnetic fields or additional magnetic materials that pose challenges for device integration.5 Optical methods such as the circular photogalvanic effect (CPGE) can be utilized to drive spin currents using circularly polarized light, creating imbalanced spin populations of photo-excited carriers and spin photocurrent at room temperature applicable to both proof-of-principle spin manipulation and spin-based optoelectronic devices.6-10 Monolayers of the group VI transition metal dichalcogenide (TMDC) provide a unique direct bandgap semiconductor platform to explore spintronics because of the intrinsic link between polarized optical transitions and the spin texture of free and bound carriers in the band structure.1114 The inversion symmetry breaking and strong spin-orbit coupling (SOC) in monolayer TMDCs give rise to two degenerate but inequivalent K and K′ valleys well-separated in k-space, spin splitting in the valence band (~ 160 meV for monolayer MoS2) and the conduction band (about 1 order of magnitude


4

Page 5 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


smaller), and spin-valley locking such that the spin splitting has opposite sign between two valleys.15-17 These properties have intriguing valley-contrasting consequences, including magnetic moment and Berry curvature, which have the same magnitudes but opposite signs in the two valleys.15 Exploiting this contrast, circularly polarized light can selectively pump the intra-band transition in one specific valley (Figure 1a), and in turn enable optical orientation of valley-locked spin-polarized photocurrent by CPGE.18-20 Intervalley scattering requires spin flip and momentum conservation simultaneously, leading to a long spin-valley lifetime in pristine monolayers that can allow robust polarized photocurrents.11,16,21 In group VI TMDCs, the dichroic spin-valley photocurrent induced by CPGE at room temperature has been demonstrated in an electric-double-layer transistor in a bulk crystal and in a CVD-grown monolayer, and it has been confirmed by detecting spin polarization using a spin-valve structure and ferromagnetic contacts.22-25 The spin-valley photocurrent has also been observed in a TMDC-graphene-topological insulator heterostructure26. Recently, the spectral and electrical behavior of the helicity-dependent CPGE photocurrent has been explored in boron nitride-encapsulated monolayer MoSe2.27 Although gate voltage has


5


Page 6 of 39

been utilized to induce and modulate spin photocurrents in bulk WSe2 devices, an interesting next step would be to electrically manipulate spin photocurrents and polarization in monolayer TMDC devices in which the requisite symmetry for CPGE generation is intrinsic to the material. Here, we report electric tuning of the magnitude and the polarization degree of spin photocurrent in monolayer MoS2 devices at room temperature without any direct magnetic methods involved. We show that the magnitude and polarization degree of spin-polarized photocurrent can be modulated strongly by source-drain voltage and electrostatic gate tuning. Gate-controlled charging induces substantial screening for defects, which is confirmed by the quenching of defect emission in gate-dependent photoluminescence (PL) experiments. This screening effect reduces defect-associated intervalley scattering, which can enhance the generation of CPGE spinvalley photocurrent and thus spin photocurrent polarization. This capability for tuning and optimization could be useful for future spin-related devices made from large-area TMDC thin films and heterostructures.

2.

EXPERIMENTAL SECTION


6

Page 7 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The MoS2 crystal (SPI Supplies) was exfoliated by a thermally-assisted transfer technique and transferred onto SiO2/Si substrates with pre-fabricated alignment marks.28 Monolayers were identified by optical microscopy, AFM, and room-temperature photoluminescence. The devices were fabricated by standard electron beam lithography (Tescan MIRA with NPGS), followed by thermal metal deposition of 3 nm/100 nm of Ti/Au and lift-off processes. Before loading into the measurement chamber, the device was annealing in the forming gas at 180 C for one hour, to clean the surface and improve the contact resistance. Then the devices went through in situ 350 K annealing overnight under the high vacuum (10-5 Pa). The excitation source was provided by a tunable laser diode (Thorlabs) with wavelength of ~ 650 nm. The laser was chopped at a frequency of 1033 Hz and passed through a linear polarizer and quarter wave plate mounted on a motorized rotation stage (Thorlabs) to programmably rotate the angle  from 0 to 360, corresponding to continuous tuning of photon polarization between linear and circular configurations. The laser was focused by a lens onto the device through the side window. The channel current was detected by a Keithley source/meter, while the photocurrent was measured using a low noise current preamplifier (Stanford Research Systems, SR570)


7


Page 8 of 39

and a lock-in amplifier (Signal Recovery, 7230). Considering the relatively long recording duration (~ 15 min) for one data point, a petty linear background accounting for the lowfrequency slight drift was removed first before the fitting. All photocurrent measurements were performed at room temperature.

3.

RESULTS AND DISCUSSION

3.1 Helicity-dependent photocurrent

Figure 1. Schematic of measurement set-up and representative photon-polarizationdependent photocurrent. (a) Electronic structure of monolayer MoS2 at the two valleys, with SOC-induced spin degeneracy lifting at both valence and conduction bands. The


8

Page 9 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


valley-contrasting circular dichroism is illustrated, with the intervalley scattering indicated by dashed black line. (b) Schematic of the experimental set-up and device geometry. The angle of incidence with respect to the monolayer surface, , is fixed to be 45. (c) Photocurrent as a function of laser intensity. The data (open red squares) is well-fit by a linear response (black line). Inset is a representative optical image of a monolayer MoS2 device with metal contacts. (d) A typical photon helicity-dependent normalized photocurrent Ipc represented by black open circles. The solid red line is a fit to equation (1) for the photocurrent. The equivalent polarization of the incident light for different  is shown on the top axis, while the rotation angle of the quarter wave plate is displayed on the bottom axis.

The schematic for the light helicity-dependent photocurrent measurement is shown in Figure 1b. The photocurrent was recorded with a lock-in technique using a chopped, onexciton-resonance laser of wavelength ~ 650 nm. The wavelength dependence of CPGE in TMDCs has been explored previously23,27; here our electrical results are reported for a fixed excitation wavelength. The interband transition between the valence band maximum


9


Page 10 of 39

and conduction band minimum is pumped selectively at the K or K′ valley by circularly polarized light, resulting in imbalanced populations of valley-polarized free carriers that are accelerated by an applied electric field to form the polarized photocurrent. The inset of Figure 1c is an optical microscope image of a representative monolayer device with a channel length of ~ 5 µm. Note that compared to the device dimensions, the laser spots (~ mm2) used in the experiment are much larger, minimizing laser-induced heat gradients or thermoelectric current contribution.29 In Figure 1c we show that the photocurrent scales linearly with the laser intensity used in the experiment; all subsequent photocurrent measurements were conducted in this linear region with low excitation intensity (I < 3 mW cm-2) to eliminate any heating effects. Accounting for the possible microscopic mechanisms of photocurrent,7,9,30 the total photocurrent Ipc in the geometry of Figure 1b can be described by a phenomenological function of the angle  of the quarter wave plate (QWP) which controls the degree of circular polarization:

𝐼pc = 𝐶sin2φ + 𝐿1sin4φ + 𝐿2cos4φ + 𝐴 (1)


10

Page 11 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


where C, L1, and L2 are the coefficients of the CPGE current, linear photogalvanic effect (LPGE) current induced by linear polarized irradiation and the asymmetric scattering of electrons on material disorder,31 and linear photon drag effect (LPDE) current caused by the transfer of light momentum to electrons, and A is the polarization-independent term. The components of the photocurrent arise from different underlying mechanisms and each exhibits different angular symmetry.7,9,30 These key parameters can be extracted from a fit of the angular dependence of the photocurrent. Figure 1d shows a typical photon-polarization-dependent photocurrent Ipc (normalized by laser intensity) and fit, where C, L1, L2 and A are -20, 18, -36, and 90 pA mW-1 cm2, respectively.

3.2. Bias-dependent CPGE current


11


Page 12 of 39

Figure 2. Bias-dependent photocurrent. (a) The normalized photocurrent Ipc (open circles) and the corresponding fits (solid line) as a function of polarization rotator angle with two different biases of 4 V and 9 V, showing the distinct magnitude and shape of 180 degree-period photocurrent oscillation. (b) The linear responses of C, L1, and L2 on bias with the slopes of ~ 140, 230, and -160 pA mW-1 cm2 V-1, respectively. (c) The plot of A versus bias, showing monotonic near-linear dependence. (d) The drain current (left y axis) and degree of spin-polarized photocurrent P (right y axis) as a function of bias. The blue rectangle region indicates the saturation working mode for the monolayer device.

Focusing first on the bias effect on the CPGE current, Figure 2a shows the normalized photocurrent Ipc of one device and the corresponding fitting results from Equation 1 when bias voltages Vds = 4


12

Page 13 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


V and 9 V are applied, representing a dissimilar response to left- and right-handed laser excitation. We sweep the bias from 4 V to 9 V, keeping other parameters such as the incident angle and power constant. The absolute value of parameters C, L1, L2, and A all increase nearly linearly with applied bias as shown in Figure 2b,c. This increasing behavior originates from the source-drain bias facilitating the dissociation of photo-excited excitons and accelerating the free carriers to generate a significant drift current.32 The contributions from polarization-dependent photocurrents, i.e. C, L1, and L2, show comparable absolute values of their slopes (Figure 2b); in sharp contrast, a ~ ten times larger slope is found in the Vds-A curve (Figure 2c), indicating a reduced capability of tuning photogalvanic effects by lateral electric field compared to polarization-insensitive currents such as photovoltaic effect-induced photocurrent.7,29 To characterize the efficacy of spin photocurrent generation, we consider the portion of the CPGE-induced spin-polarized photocurrent in the total photocurrent. For the sake of comparison, we fix the light-polarization angle  to be 45 (circularly polarized excitation) where the magnitude of the CPGE signal approaches its maximum. Then the degree of spin-polarized photocurrent P is given by 𝑃(𝜑 = 45°) =

𝐼CPGE 𝐼PC

(2) where ICPGE is the first term (Csin2) in equation 1 and IPC is total photocurrent. Note that the spin photocurrent polarization P here has the distinct definition and consequent


13


Page 14 of 39

physics meaning when compared with photocurrent dichroism introduced in the previously reported work.29 As a function of source-drain bias, non-linear behavior of the spin photocurrent polarization degree is observed. In Figure 2d, the P is non-monotonic with Vds, showing a peak value of 9.3% at 7 V, which primarily originates from deviations from linearity in A with Vds. This bias range corresponds to a transition from the linear conduction regime to the saturation regime at a similar threshold (~ 6 V) as observed from electrical drain current output characterization. This overlap is reproducible across multiple devices. This correspondence implies a potential relevance of the electronic conduction mode to determining spin photocurrent generation efficiency, suggesting the potential for optimization of CPGE polarization in devices. Figure S1 shows the results from another device, in which the CPGE-induced spin photocurrent was tuned linearly by voltage bias (up to 4.3 times), and the P has a maximum value of 5.5 % at 2.5 V where the device goes into the saturation regime. The microscopic origin of the non-monotonic behavior of P is not yet well understood. It could be due to the formation and growth of the depletion region near the drain upon the application of a large bias,33 which can modify the dependence of the unpolarized


14

Page 15 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


photocurrent A. Further theoretical and experimental investigation of the impact of the depletion region on spin photocurrent polarization is potentially interesting for understanding monolayer spin devices.

3.3. Gate-dependence of the CPGE current

Figure 3. Gate-dependent photocurrent. (a) The normalized photocurrent Ipc (unfilled symbols) and the corresponding fits (solid lines) as a function of polarization rotator angle

 under three gate voltages of -20 V, 40 V, and 100 V (Vds = 10 V). The amplitude and shape of photocurrent curves are largely tuned by Vg. (b) The transfer curves with Vds of 2 V, 5 V, and 10 V, showing n-doped semiconducting characteristics. (c) The coefficient


15


Page 16 of 39

of CPGE current under three source-drain voltages, as a function of Vg, demonstrating continuous control capabilities. With small Vds (2 V and 5 V), C shows almost linear dependence with gate voltage, with larger fluctuation at larger bias Vds (10 V). (d) The plot of spin photocurrent polarization degree P versus gate voltage. Nearly linear response with Vg and similar slope for varied source-drain bias Vds are observed, showing the capability of tuning the spin-polarized photocurrent to both on and off regimes with gate.

Electrostatic gating is a common method to modulate the carrier density and barrier height at contacts, offering an approach to tune the photoresponse of monolayer MoS2 optoelectronic devices.32 We performed gate-dependent photocurrent measurements at

Vds = 2 V, 5 V, and 10 V. At a 2 V bias the photocurrent falls within the linear regime, but at higher source-drain bias of 5 V and 10 V we observe current saturation with Vg = 0 V (Figure S2a). Figure 3a shows the photocurrent versus QWP angle  at three gate voltages with Vds = 10 V, demonstrating a diverse response, while the n-doped semiconducting features are indicated by the transfer curves (Figure 3b). More


16

Page 17 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


specifically, CPGE, LPGE, and LPDE components were extracted and are plotted as functions of gate voltage (Figure 3c, S2c, and S2d). As shown in Figure 3c, the amplitude of the CPGE current, C, can be largely tuned by the gate voltage monotonically, demonstrating ~ 6.3 times, 6 times, and 45 times modulation, respectively. In sharp contrast, the LPGE component shows weaker dependence on the perpendicular electric field (Figure S2c). The microscopic origin of LPGE has been ascribed to asymmetric electron scattering from crystal defects and charged impurities.31 The relative stability of the LPGE signal with gate tuning indicates the presence of a gate-insensitive scattering source such as the dielectric substrate. Although beyond the scope of this current experiment, encapsulation by hexagonal boron nitride could screen the material from substrate disorder and modulate the LPGE contribution to the response.27 The LPDE current undergoes a polarity reversal (Figure S2d).9 As a result, the CPGE current dominates the overall polarization-dependent photocurrent for positive gate voltage. Note that here we discuss the gate voltage tunability of C, L1, and L2, distinct from the bias voltage control of the photocurrent components shown in Figure 2b.


17


Page 18 of 39

The spin-polarized photocurrent degree P versus Vg is shown in Figure 3d. The P can be tuned from 2% to 24%, 4% to 20%, and 0.5% to 17% with Vds of 2 V, 5 V, and 10 V, respectively. Compared with similar tuning of gate voltage when applying 2 V and 5 V of bias, a higher source-drain voltage can enhance the gate effect. The negligible photocurrent spin polarization under large negative gate voltage and strong gate modulation leads to a transistor-like on/off behavior of spin photocurrent polarization (the maximum “on/off ratio” is 34 here), demonstrating purely electrical spin control at room temperature without magnetic fields or materials. Within most of the tunable gate voltage region (-40 V to 80 V), the highest polarization degree was found with moderate Vds of 5 V (close to the device working mode transition), which is consistent with the result of previous bias dependence (Figure 2d and Figure S1d).

3.4 DISCUSSION We have shown that the magnitude and polarization degree of the spin-polarized photocurrent in semiconducting monolayer MoS2 can be controlled by purely electric means, with a spin photocurrent “on/off ratio” of 34 achievable at ambient temperature.


18

Page 19 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Although previously-reported works have demonstrated the gate-tuning effect in bulk TMDC where this top-gate field is mainly used to induce inversion asymmetry in the layered materials and generation in monolayer semiconductors,22,23 the electric control of CPGE spin photocurrent in a monolayer demonstrated here is an intriguing development since the monolayer crystal with intrinsic inversion asymmetry can act as a subnanometer building block for spin photocurrent logic devices. Extending recent detailed Hall bar measurements of CPGE,27 the electrical characterization reported here shows that in addition to modulating the magnitude of photocurrents, gate and bias tuning can also significantly control the polarization magnitude of generated photocurrents over orders of magnitude of fractional polarization. This insight can be useful for adopting CPGE as a generation mechanism for polarized currents in optoelectronics.24 Despite sharing a similar origin in circularly-polarized optical excitation, the valley Hall effect should not contribute to the photocurrents described here. In monolayer TMDCs the valley-dependent Berry curvature (Ω) gives rise to an effective magnetic field with the same magnitude but opposite sign for the two valleys. Consequently, electrons gain an


19


Page 20 of 39

𝑒

anomalous velocity (𝒗𝑎 = ― ℏ𝑬 × 𝛀 ) perpendicular to the applied electric field.11,17 In contrast to a Hall bar geometry27, the photocurrent here is parallel to the applied in-plane electric field, and no contribution from this anomalous velocity is expected. By increasing the gate voltage, the Fermi level EF is tuned to be closer to the conduction band minimum reducing the tunneling barrier for electron/holes at the monolayer-metal contact interface. The reduction of the contact barrier increases the injection/collection efficiency of carriers, enhancing the photocurrent with the gate voltage (see Figure S2b for the polarization-insensitive photocurrent A as a function of Vg).32 However, the distinct modulation capability of the CPGE current, the linear photogalvanic, and photon drag currents by gate voltage indicates additional tuning mechanisms other than the modulation of the Schottky barrier only. In an electric-double-layer field-effect transistor (FET) made from bulk WSe2 which naturally has inversion symmetry, the CPGE originates from the band bending induced in the top few layers. Band mixing plays an important role in the polarization generation. The spin photocurrent shows a nonlinear dependence on the gate-induced electric field.22 In


20

Page 21 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the conventional monolayer MoS2 FET studied here, the lack of intrinsic inversion symmetry gives rise to valley-dependent optical selection rules, and both the spin photocurrent and spin polarization show a linear response to gate voltage, indicating distinct mechanisms. This is not surprising given that the gating field induces the asymmetry that gives rise to CPGE generation in bulk devices, whereas this effect is naturally present in the monolayer device.23,24,27 In a monolayer FET, the gating tunes the charge density and screening, which influence not only the spin photocurrent response but also the monolayer photoluminescence (PL) emission. As a prototypical Group VI TMDC, the inversion asymmetry of monolayer MoS2 manifests experimentally in the circular polarization of the PL. The temperature dependence of polarized PL measurements indicates that the optical phonon-assisted intervalley scattering acts as the major spin/valley relaxation channel at room temperature,20 which strongly depresses the degree of valley polarization of PL emission. Unlike the emission processes, the dynamics of CPGE photocurrents more likely originate from impurities rather than phonons. The natural crystal sources of MoS2 used for exfoliation have abundant defects such as Mo substitution for S,34,35 single chalcogen


21


Page 22 of 39

vacancies, and vacancy chains,35,36 which introduce mid-gap defect states with variable energies.36,37 Moreover, trapped charges, like the impurities of immobile metal ions at the SiO2 surface, are well known to exist at the interface for monolayer MoS2.38 Intervalley scattering assisted by these local defects can provide additional spin-valley relaxation mechanisms.

Figure 4. Schematic diagram of electrostatic screening effect and gate voltage-tuned bound defect state luminescence. (a) The representative S vacancy (dashed circle, both top and bottom S atoms are removed intentionally for clarity) and surface traps (red dot), like immobile Na ions on the SiO2 surface,38 can be screened by free electrons (green


22

Page 23 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


dot) tuned by gate voltage. The charged defects have large scattering cross sections due to the long-range Coulomb interaction between defects and electrons; fewer scattering events are expected due to the electrostatic screening effect.4142 (b) Density of states changes, demonstrating that with the Fermi level EF sweeping controlled by gate voltage, the defects-induced mid-gap state can be unfilled and filled by electrons, corresponding to

the

pristine

and

screening

scenarios,

respectively.

(c)

Gate

dependent

photoluminescence spectra of monolayer MoS2 with a Vg step of 20 V at 10 K (to eliminate the effect of optical phonon-assisted intervalley scattering). Two peaks, an exciton emission peak X0 located at ~ 1.94 eV and a broad defect-related luminescence centered at ~ 1.85 eV, are observed at Vg = -100 V. With increasing gate voltage, the neutral peak X0 red-shifts and the negatively charged exciton X- peak dominates because of larger electron density; the gradually vanishing luminescence from defect states can be ascribed to the screening effect from the increasing free carrier density. The screened defects cannot serve as recombination centers, leading to the decrease of defect state emission. The full PL gate voltage dependence appears in the Supporting Information.


23


Page 24 of 39

To overcome the point defect-induced effects, several strategies have been demonstrated, such as hexagonal boron nitride as an almost charge trap-free substrate to achieve up to 100% of polarization at low temperature,18 super-acid treatment to reduce defect-associated nonradiative recombination,39 and electrostatic screening by the contact with an ionic liquid.40 Increased gate voltage boosts the free electron density and can cause an analogous screening of impurities which could be the origin of the enhanced CPGE polarization with gating. With the device geometry used here, the carrier density change ∆𝑛 = 𝐶gate∆𝑉g/𝑒, where Cgate is 12.2 nF/cm2 for 285 nm SiO2, ∆𝑉g is 140 – 160 V for gate voltage range, and e is elementary charge, is expected to be 1.05 – 1.26 × 1013 cm-2. These nonlocalized electrons are also able to effectively screen the positively charged defects, e. g. the S vacancy and immobile metal ions as shown in Figure 4a, to eliminate the spin relaxation mechanism associated with defect-mediated scattering during the electric-field-driven drift of spin-polarized carriers. This mechanism suggested here can be understood in momentum space as well. By sweeping the gate voltage, the corresponding shift of the Fermi level EF due to the field effect is estimated to be 90 – 108 meV. Due to the wide energy distribution for various types of defects in MoS2 confirmed


24

Page 25 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


by Scanning Tunneling Microscopy/Spectroscopy experiments,3637 the defect levels can exist within this energy window and therefore influence the scattering mechanisms for CPGE. Figure 4b illustrates a schematic of the filling of defect-induced mid-gap state by tuning the gate voltage. When the Fermi energy is tuned below a certain defect state energy level by negative Vg, the defects are intrinsically charged and can serve as scattering centers with large scattering cross sections due to the long-range Coulomb potential.41,42 With more positive Vg the Fermi level is tuned above the defect state energy level and, in turn, the defects are fully screened by the surrounding electrons, resulting in the reduced scattering cross section and higher spin polarization. We confirm this gating effect on defects by a PL measurement at low temperature (Room-temperature PL result is displayed in Figure S3). Figure 4c shows the typical gatedependent PL spectra of mechanically exfoliated monolayer MoS2. Apart from the exciton emission peak (X0) and trion peak (X-), the broad peak around 1.84 eV is associated with bound defect states. At deep negative Vg, the defects state emission is quite prominent (even higher than exciton peak), when the defects could serve as recombination centers and thus facilitate the emission. With increasing gate voltage (from -100 V to -20 V), the


25


Page 26 of 39

defect state peak gradually vanishes. In this regime the excitons are also negatively charged by excess electrons, thus forming the trion peak in PL spectrum.43 The gatinginduced free carrier density is responsible for this behavior through screening, and this electrostatic screening effect continue working with the improved gate voltages based on the results from photocurrent analysis as shown in Figure 3c and 3d because the new defects-related states with Fermi level sweeping start to impact on intervalley scattering, though these new defects levels don’t dominate the PL process and even cannot be observed from luminescence results. Moreover, the defect emission in MoS2 shows no circularly-polarized PL,18,44,45 indicating that defect-induced scattering in transport is not expected to preserve optically-pumped valley index. The defect screening evident in the PL spectrum would also impact the transport of optically-generated free carriers, suggesting a possible mechanism for the strong gate dependence of CPGE and polarization. Defects are important to the mechanisms of both LPGE and CPGE. Because the CPGE in monolayer TMDCs arises from the interband transitions at valley minima, CPGE photocurrents are limited by defect-dominated spin/valley relaxation times rather than


26

Page 27 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


band mixing as in ionic gated bulk WSe2.22 Achieving ultralow defect densities should result in larger CPGE photocurrents and higher polarizations. In practice, point defects are always abundant in mechanically exfoliated and chemical vapor deposited MoS2, making defect modulation of CPGE a challenging possibility.

4.

CONCLUSION

In summary, our experiment shows that the spin photocurrent induced by CPGE and the degree of polarization can be tuned by an electrostatic gate voltage in a monolayer TMDC, allowing spin photocurrent switching with maximum “on/off ratio” of 34. The screening of defects at the interface of monolayer and the substrate can account for this tuning capability. Optimized top gate design and other more efficient tuning methods such as using high- dielectrics could enhance this tunability. Our findings could enhance exploration of spin-polarized optoelectronic properties in monolayer TMDCs.

ASSOCIATED CONTENT


27


Page 28 of 39

Supporting Information.

The following files are available free of charge. Bias-dependent photocurrent from a second device, characterization of the device shown in the main manuscript, the representative room-temperature photoluminescence spectrum of as-exfoliated monolayer MoS2 (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected] (L. L.)

*E-mail: [email protected] (N. S.)

ACKNOWLEDGMENT

This work was supported by the Office of Naval Research under Grant No. N00014-161-3055 (L.L.), the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under award No. DE-SC0012130 (E.J.L., G.W.), and the National Science Foundation MRSEC program under grant No. DMR-1720139


28

Page 29 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(T.K.S.) at the Materials Research Center of Northwestern University. This work utilized Northwestern University Micro/Nano Fabrication Facility (NUFAB) and the EPIC facility of Northwestern University’s NUANCE Center, which have received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205); the MRSEC program (NSF DMR-1720139) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois.

REFERENCES 1. Zutic, I.; Fabian, J.; Das Sarma, S. Spintronics: Fundamentals and applications.

Rev. Mod. Phys. 2004, 76, 323-410.

2. Awschalom, D. D.; Flatte, M. E. Challenges for Semiconductor Spintronics. Nat.

Phys. 2007, 3, 153-159.

3. Appelbaum, I.; Huang, B. Q.; Monsma, D. J. Electronic Measurement and Control of Spin Transport in Silicon. Nature, 2007, 447, 295-298.


29


Page 30 of 39

4. Koo, H. C.; Kwon, J. H.; Eom, J.; Chang, J.; Han, S. H.; Johnson, M. Control of Spin Precession in a Spin-Injected Field Effect Transistor. Science, 2009, 325, 15151518.

5. Zhu, H. J.; Ramsteiner, M.; Kostial, H.; Wassermeier, M.; Schönherr, H. P.; Ploog, K. H. Room-temperature Spin Injection from Fe into GaAs. Phys. Rev. Lett. 2001,

87, 016601.

6. Weber, W.; Ganichev, S. D.; Danilov, S. N.; Weiss, D.; Prettl, W. Demonstration of Rashba Spin Splitting in GaN-based Heterostructures. Appl. Phys. Lett. 2005, 87, 262106.

7. Ganichev, S. D.; Prettl, W. Spin Photocurrents in Quantum Wells. J. Phys-Condens.

Mat. 2003, 15, R935-R983.

8. Ganichev, S. D.; Ivchenko, E. L.; Bel’Kov, V. V.; Tarasenko, S. A.; Sollinger, M.; Weiss, D.; Wegscheidere, W.; Prettl, W. Spin-Galvanic Effect. Nature 2002, 417, 153-156.


30

Page 31 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


9. McIver, J. W.; Hsieh, D.; Steinberg, H.; Jarillo-Herrero, P.; Gedik, N. Control over Topological Insulator Photocurrents with Light Polarization. Nat. Nanotechnol. 2002,

7, 96-100.

10. Dhara, S.; Mele, E. J.; Agarwal, R. Voltage-Tunable Circular Photogalvanic Effect in Silicon Nanowires. Science 2015, 349, 726-729.

11. Xu, X. D.; Yao, W.; Xiao, D.; Heinz, T. F. Spin and Pseudospins in Layered Transition Metal Dichalcogenides. Nat. Phys. 2014, 10, 343-350.

12. Schaibley, J. R.; Yu, H.; Clark, G.; Rivera, P.; Ross, J. S.; Seyler, K. L.; Yao, W.; Xu, X. Valleytronics in 2D Materials. Nat. Rev. Mater. 2016, 1, 16055.

13. Dankert, A,; Dash, S. P. Electrical Gate Control of Spin Current in van der Waals Heterostructures at Room Temperature. Nat. Commun. 2017, 8, 16093.

14. Liang, S.; Yang, H.; Renucci, P.; Tao, B.; Laczkowski, P., Mc-Murtry, S.; Wang, G.; Marie, X.; George, J.-M.; Petit-Watelot, S.; Djeffal, A.; Mangin, S.; Jaffrès, H.; Lu, Y.;


31


Page 32 of 39

Electrical Spin Injection and Detection in Molybdenum Disulfide Multilayer Channel.

Nat. Commun. 2017, 8, 14947.

15. Xiao, D.; Yao, W.; Niu, Q. Valley-contrasting Physics in Graphene: Magnetic Moment and Topological Transport. Phys. Rev. Lett. 2007, 99, 236809.

16. Yao, W.; Xiao, D.; Niu, Q. Valley-dependent Optoelectronics from Inversion Symmetry Breaking. Phys. Rev. B 2008, 77, 235406.

17. Xiao, D.; Liu, G. B.; Feng, W. X.; Xu, X. D.; Yao, W. Coupled Spin and Valley Physics in Monolayers of MoS2 and Other Group-VI Dichalcogenides. Phys. Rev. Lett. 2012,

108, 196802.

18. Mak, K, F,; He, K. L.; Shan, J.; Heinz, T. F. Control of Valley Polarization in Monolayer MoS2 by Optical Helicity. Nat. Nanotechnol. 2012, 7, 494-498.

19. Cao, T.; Wang, G.; Han, W.; Ye, H.; Zhu, C.; Shi, J.; Niu, Q.; Tan, P.; Wang, E.; Liu, B.; Feng, J. Valley-selective Circular Dichroism of Monolayer Molybdenum Disulphide. Nat. Commun. 2012, 3, 887.


32

Page 33 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20. Zeng, H. L.; Dai, J. F.; Yao, W.; Xiao, D.; Cui, X. D. Valley Polarization in MoS2 Monolayers by Optical Pumping. Nat. Nanotechnol. 2012, 7, 490-493.

21. Lagarde, D.; Bouet, L.; Marie, X.; Zhu, C. R.; Liu, B. L.; Amand, T.; Tan, P. H.; Urbaszek, B. Carrier and Polarization Dynamics in Monolayer MoS2. Phys. Rev.

Lett. 2014, 112, 047401.

22. Yuan, H.; Wang, X.; Lian, B.; Zhang, H.; Fang, X.; Shen, B.; Xu, G.; Xu, Y.; Zhang, S.-C.; Hwang, H. Y.; Cui, Y. Generation and Electric Control of Spin-valley-coupled Circular Photogalvanic Current in WSe2. Nat. Nanotechnol. 2014, 9, 851-857.

23. Eginligil, M.; Cao, B.; Wang, Z.; Shen, X.; Cong, C.; Shang, J.; Soci, C.; Yu, T. Dichroic Spin-valley Photocurrent in Monolayer Molybdenum Disulphide. Nat.

Commun. 2015, 6, 7636.

24. Xie, L.; Cui, X. D. Manipulating Spin-polarized Photocurrents in 2D Transition Metal Dichalcogenides. P. Natl. Acad. Sci. USA 2016, 113, 3746-3750.


33


Page 34 of 39

25. Guan, H.; Tang, N.; Xu, X.; Shang, L.; Huang, W.; Fu, L.; Fang, X.; Yu, J.; Zhang, C.; Zhang, X.; Dai, L.; Chen, Y.; Ge, W.; Shen, B. Photon Wavelength Dependent Valley Photocurrent in Multilayer MoS2. Phys. Rev. B 2017, 96, 241304.

26. Cha, S.; Noh, M.; Kim, J.; Son, J.; Bae, H.; Lee, D.; Kim, H.; Lee, J.; Shin, H.-S.; Sim, S.; Yang, S.; Lee, S.; Shim, W.; Lee, C.-H.; Jo, M.-H.; Kim, J. S.; Kim, D.; Choi, H. Generation, Transport and Detection of Valley-locked Spin Photocurrent in WSe2Graphene-Bi2Se3 Heterostructure. Nat. Nanotech. 2018, 13, 910-914.

27. Quereda, J; Ghiasi, T. S.; You, J-S.; Van Den Brink, J.; Van Wees, B. J.; Van Der Wal, C. H. Symmetry regimes for circular photocurrents in monolayer MoSe2. Nat.

Commun. 2018, 9, 3345.

28. Huang, Y.; Sutter, E.; Shi, N. N.; Zheng, J.; Yang, T.; Englund, D.; Gao, H.-J.; Sutter, P. Reliable Exfoliation of Large-area High-quality Flakes of Graphene and Other Two-dimensional Materials. ACS Nano, 2015, 9, 10612-10620.


34

Page 35 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


29. Zhang, Y. W.; Li, H.; Wang, L.; Wang, H.; Xie, X.; Zhang, S.; Liu, R.; Qiu, Z.-J. Photothermoelectric and Photovoltaic Effects both Present in MoS2. Sci. Rep. 2015,

5, 7938.

30. Glazov, M. M.; Ganichev, S. D. High Frequency Electric Field Induced Nonlinear Effects in Graphene. Phys. Rep. 2014, 535, 101-138.

31. Sturman, B. I.; Fridkin V. M. The Photovoltaic and Photo-refractive Effects in Noncentrosymmetric Materials. Gordon and Breach Science Publishers, 1992.

32. Lopez-Sanchez, O.; Lembke, D.; Kayci, M.; Radenovic, A.; Kis, A. Ultrasensitive Photodetectors Based on Monolayer MoS2. Nat. Nanotechnol. 2013, 8, 497-501.

33. Kim, S.; Konar, A.; Hwang, W.-S.; Lee, J. H.; Lee, J.; Yang, J.; Jung, C.; Kim, H.; Yoo, J.-B., Choi, J.-Y.; Jin, Y. W.; Lee, S. Y.; Jena, D.; Choi, W.; Kim, K. Highmobility and Low-power Thin-film Transistors Based on Multilayer MoS2 Crystals.

Nat Commun. 2012, 3, 1011.


35


Page 36 of 39

34. Hong, J.; Hu, Z.; Probert, M.; Li, K.; Lv, D.; Yang, X.; Gu, L.; Mao, N.; Feng, Q.; Xie, L.; Zhang, J.; Wu, D.; Zhang, Z.; Jin, C.; Ji. W.; Zhang, X.; Yuan, J.; Zhang, Z. Exploring Atomic Defects in Molybdenum Disulphide Monolayers. Nat. Commun. 2015, 6, 6293.

35. Zhou, W.; Zou, X.; Najmaei, S.; Liu, Z.; Shi, Y.; Kong, J.; Lou, J.; Ajayan, P. M.; Yakobson, B. I.; Idrobo, J.-C. Intrinsic Structural Defects in Monolayer Molybdenum Disulfide. Nano. Lett. 2013, 13, 2615-2622.

36. Lu, C. P.; Li, G. H.; Mao, J. H.; Wang, L. M.; Andrei, E. Y. Bandgap, Mid-Gap States, and Gating Effects in MoS2. Nano. Lett. 2014, 14, 4628-4633.

37. Liu, M.; Shi, J.; Li, Y.; Zhou, X.; Ma, D.; Qi, Y.; Zhang, Y.; Liu, Z. Temperaturetriggered Sulfur Vacancy Evolution in Monolayer MoS2/Graphene Heterostructures.

Small 2017, 13, 1602967.

38. Dolui, K.; Rungger, I.; Sanvito, S.; Origin of the n-type and p-type Conductivity of MoS2 Monolayers on a SiO2 Substrate. Phys. Rev. B 2013, 87, 165402.


36

Page 37 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


39. Amani, M.; Lien, D.-H.; Kiriya, D.; Xiao, J.; Azcatl, A.; Noh, J.; Madhvapathy, S. R.; Addou, R.; KC, S.; Dubey, M.; Cho, K.; Wallace, R. M.; Lee, S.-C.; He, J.-H.; Ager, J. W.; Zhang, X.; Yablonovitch, E.; Javey, A. Near-unity Photoluminescence Quantum Yield in MoS2. Science 2015, 350, 1065-1068.

40. Atallah, T. L.; Wang, J.; Bosch, M.; Seo, D.; Burke, R. A.; Moneer, O.; Zhu, J.; Theibault, M.; Brus, L. E.; Hone, J.; Zhu, X.-Y. Electrostatic Screening of Charged Defects in Monolayer MoS2. J. Phys. Chem. Lett. 2017, 8, 2148-2152.

41. Yan, B.; Han, Q.; Jia, Z.; Niu, J.; Cai, T.; Ya, D.; Wu, X.; Electrical Control of Intervalley Scattering in Graphene via the Charge State of Defects. Phys. Rev. B 2016, 93, 041407.

42. Brar, V. W.; Decker, R.; Solowan, H.-M.; Wang, Y.; Maserati, L.; Chan, K. T.; Lee, H.; Girit, C. O.; Zettl, A.; Louie, S. G.; Cohen, M. L.; Crommie, M. F. Gate-controlled Ionization and Screening of Cobalt Adatoms on a Graphene Surface. Nat. Phys. 2011, 7, 43-47.


37


Page 38 of 39

43. Mak, K. F.; He, K.; Lee, C.; Lee, G. H.; Hone, J.; Heinz, T. F.; Shan, J. Tightly Bound Trions in Monolayer MoS2. Nat. Mater. 2013, 12, 207-211.

44. Chen, Y.; Cain, J. D.; Stanev, T. K.; Dravid, V. P.; Stern, N. P. Valley-polarized Exciton-Polaritons in a Monolayer Semiconductor. Nat. Photonics 2017, 11, 431435.

45. Wei, G.; Czaplewski, D. A.; Lenferink, E. J.; Stanev, T. K.; Jung Ⅱ, W.; Stern, N. P. Size-tunable Lateral Confinement in Monolayer Semiconductors. Sci. Rep. 2017, 7, 3324.


38

Page 39 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table Of Contents (TOC) graphic


39

Electrical control of circular photogalvanic spin-valley photocurrent in

Recommend Documents