Electrical generation and control of the valley carriers in a monolayer transition metal dichalcogenide

Journal name:
Nature Nanotechnology
Volume:
11,
Pages:
598–602
Year published:
DOI:
doi:10.1038/nnano.2016.49
Received
Accepted
Published online

Electrically controlling the flow of charge carriers is the foundation of modern electronics. By accessing the extra spin degree of freedom (DOF) in electronics, spintronics allows for information processes such as magnetoresistive random-access memory1. Recently, atomic membranes of transition metal dichalcogenides (TMDCs) were found to support unequal and distinguishable carrier distribution in different crystal momentum valleys. This valley polarization of carriers enables a new DOF for information processing2, 3, 4. A variety of valleytronic devices such as valley filters and valves have been proposed5, and optical valley excitation has been observed2, 3, 4. However, to realize its potential in electronics it is necessary to electrically control the valley DOF, which has so far remained a significant challenge. Here, we experimentally demonstrate the electrical generation and control of valley polarization. This is achieved through spin injection via a diluted ferromagnetic semiconductor and measured through the helicity of the electroluminescence due to the spin–valley locking in TMDC monolayers6. We also report a new scheme of electronic devices that combine both the spin and valley DOFs. Such direct electrical generation and control of valley carriers opens up new dimensions in utilizing both the spin and valley DOFs for next-generation electronics and computing.

At a glance

Figures

  1. Electrically driven valley polarization via spin injection and the principle of operation.
    Figure 1: Electrically driven valley polarization via spin injection and the principle of operation.

    a, Electronic structure at the K and K’ valleys of monolayer TMDC. K and K’ represent the two distinct momentum valleys in the reciprocal space of a TMDC monolayer. The spin degeneracy at the valence band edges is lifted by spin–orbit interactions. Electrical excitation and confinement of the carriers in one set of the two non-equivalent valleys are achieved through the manipulation of the injected carrier spin polarizations, due to the spin–valley locking in monolayer TMDCs. Optical selection rules give rise to opposite circularly polarized light emissions at different excited valleys. b, Schematic of the monolayer TMDC/(Ga,Mn)As heterojunction for electrical valley polarization devices. (Ga,Mn)As was used as a spin aligner under an external magnetic field. The valley polarization can be directly determined from the helicity of the emitted electroluminescence as a result of the recombination between the electrically injected spin-polarized holes and the selected degenerate electrons in TMDC monolayers.

  2. Electroluminescence of the monolayer WS2/(Ga,Mn)As heterojunctions.
    Figure 2: Electroluminescence of the monolayer WS2/(Ga,Mn)As heterojunctions.

    a, Electrical characteristics of the WS2 diode, showing clear rectifying behaviour for bias voltage between −1.5 to +1.5 V. Inset: Surface plot of the electroluminescent emission overlaid with the scanning electron micrograph image at a forward bias of 5 V. The electroluminescence is localized at the edge of heterojunction. By applying an in-plane bias voltage, the largest voltage drop naturally occurs across the heterojunction edge due to the semiconducting characteristics of monolayer WS2. The white dashed line indicates the interface between the SiO2 and (Ga,Mn)As film. b, Electroluminescence intensity of the WS2/(Ga,Mn)As heterojunction as a function of injection current and emission photon wavelength. The strong electroluminescence resonance peak at 1.97 eV (A exciton) is clearly observed and increases with the carrier injection rate. No B exciton and defect-related emission features are observed.

  3. Electrical control of valley polarization in monolayer WS2.
    Figure 3: Electrical control of valley polarization in monolayer WS2.

    Spectra of a, σ− and σ+ resolved electroluminescence polarized under an outward magnetic field of 400 Oe perpendicular to the surface with a current of 15 µA. The electroluminescence helicity, ρ = (Iσ − Iσ+)/(Iσ + Iσ+), is found to be as large as 16.2% at the peak, indicating strong valley polarization in the monolayer WS2/(Ga,Mn)As heterojunction as a result of spin-polarized hole injections. Inset: Schematic representation of electrical excitation and emission processes. The K valley is populated by spin-up hole injection from spin-polarized (Ga,Mn)As due to spin–valley locking, resulting in σ− light emission as a result of the optical selection rules. b, Spectra of σ− and σ+ resolved electroluminescence polarized under an inward magnetic field of 400 Oe perpendicular to the surface. An opposite electroluminescence helicity of ρ = −14.8% is observed. Inset: Schematic representation of electrical excitation and emission processes. The reversed magnetic field allows injection of the opposite (spin-down) holes and populating the inversed valley, K’, resulting in σ+ light emission. c, Out-of-plane magnetic field dependence of electroluminescence helicity of a new device operated at 10 K with a current of 30 µA. The helicity of the electroluminescence has a hysteresis loop that agrees with the SQUID magnetometer and anomalous Hall effect measurement. The clear squared hysteresis of electroluminescence helicity confirms the electrical generation of valley polarization. Spin switching causes a zero intensity difference in the σ− and σ+ light emission. As the lock-in amplifier cannot detect the signal, a large outlier occurs at −100 Oe.

  4. Valley dynamics measurement in monolayer WS2 on (Ga,Mn)As.
    Figure 4: Valley dynamics measurement in monolayer WS2 on (Ga,Mn)As.

    a, Time-resolved total photoluminescence using a σ+ polarization femtosecond excitation laser pulse with an energy of 2.21 eV. Convolution fitting with the laser pulse (green dashed line), yields two exciton lifetimes of 2.9 ps and 20.0 ps. b, Time-resolved σ+ and σ− photoluminescence components excited by a σ+ polarized laser. Blue and green lines indicate the different decay rates for these two components. Inset: Convolution fitting (red curve) for the time-resolved valley exciton population (σ+ − σ−, black circles). The intervalley scattering time is estimated to be 2.5 ps.

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Affiliations

  1. NSF Nanoscale Science and Engineering Center, University of California, 3112 Etcheverry Hall, Berkeley, California 94720, USA

    • Yu Ye,
    • Jun Xiao,
    • Ziliang Ye,
    • Hanyu Zhu,
    • Mervin Zhao,
    • Yuan Wang &
    • Xiang Zhang
  2. State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, PO Box 912, Beijing 10083, China

    • Hailong Wang &
    • Jianhua Zhao
  3. Department of Mechanical Engineering and Materials Science and Engineering Program, University of Colorado Boulder, Boulder, Colorado 80309, USA

    • Xiaobo Yin
  4. Materials Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, USA

    • Xiang Zhang
  5. Department of Physics, King Abdulaziz University, Jeddah 21589, Saudi Arabia

    • Xiang Zhang

Contributions

Y.Y., X.Y., Z.Y. and X.Z. conceived the project. H.W. and J.Z. grew and characterized (Ga,Mn)As films. Y.Y., H.Z. and M.Z. developed the sample design and fabricated the samples. Y.Y., J.X. and Z.Y. performed the measurements. Y.Y. and J.X. carried out the data analysis. Y.Y., X.Y. and J.X. wrote the manuscript. X.Z., X.Y. and Y.W. guided the research. All authors discussed the results and commented on the manuscript.

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The authors declare no competing financial interests.

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