Piezotronic spin and valley transistors based on monolayer MoS2
Graphical abstract
Piezotronic effect on monolayer MoS2 has been investigated based on a normal-ferromagnetic-normal structure. The piezoelectric field at the interface of two-dimensional materials can modulate quantum transport by applied strain, which can effectively control spin and valley polarization. The quantum piezotronic transistor can act as a block unit of spin and valley filters for quantum information and storage.
Introduction
Piezotronics and piezo-phototronics are two emerging fields which can be used for self-powered systems, flexible electronic devices, and human-computer interface [[1], [2], [3]]. Owing to coupling characteristics of piezoelectricity, semiconductor and photoexcitation, piezotronic and piezo-phototronic effect can be used to control electronic transport and optical excitation process by strain-induced polarized charges [[4], [5], [6], [7]]. A variety of high performance piezotronic devices such as nanogenerators [8], flexible piezotronic strain sensors [9], piezotronic logic nanodevices [10], photoluminescence devices [11] have been developed based on piezoelectric semiconductor materials, such as ZnO, GaN and monolayer MoS2 [12]. Recently, piezotronic effect on quantum materials has been studied theoretically, and piezo topological insulator devices have ultrahigh ON/OFF ratio and low power consumption [[13], [14], [15]].
Transition metal dichalcogenides (TMDs) have been paid much attention in both fundamental physics and practical devices applications [16,17]. The quantum spin Hall effect is found in WSe2 [17]. Quantum wells based on two-dimensional (2D) layer-structured TMDs have unique optical features of intersubband transition [18]. The symmetry and strong spin-orbit coupling of TMDs can obtain spin and valley polarization [19,20]. Valley is a local extremum point of the Bloch. For example, valley is a local maximum in the valence band, or local minimum in the conduction, and can be used to design new quantum devices as an internal quantum degree of freedom due to breaking center inversion symmetry [20], such as reversible valley NAND gate, valley filter and valley valve [21,22]. Spin and valley quantum devices are good candidates for quantum information process and storage [23], for example, a valley sensitively responses to different polarization of light due to the strong valley-light coupling [24].
Single quantum state of spin or valley need ultra-high electric field in 2D materials [[25], [26], [27], [28]], which is a great challenge [26]. Gate voltage provides an electric field intensity of 100 kV/cm [29,30]. It is very difficult to further increase the electric field. Using MoS2 transistor with gate length of 1 nm, electric field can reach up to 1 MV/cm [31]. There are ultra-high electric field in piezoelectric semiconductor quantum wells, which can reach over 10 MV/cm [29,32]. Ultra-high electric field will offer unique modulation for quantum devices, which is important for quantum piezotronics.
In this paper, a piezotronic normal-ferromagnetic-normal monolayer MoS2 is studied theoretically. The ferromagnetic material in the middle structure can magnetize MoS2 and bring in an exchange field under the magnetic field [33]. Transport characteristics of spin and valley are controlled by strain, which is superior to the gate voltage control mode [34]. This work paved a new way to the control of the spin and valley by piezotronic effect.
Section snippets
Theoretical model of piezotronic effect on quantum transport
Take monolayer MoS2 piezotronic transistor as an example, we use normal-ferromagnetic-normal MoS2 junction. The ferromagnetic region with the exchange field connects to the two normal MoS2 regions in the monolayer MoS2 transistor, as shown in Fig. 1(a). The ratio of piezoelectric coefficient to relative permittivity of MoS2 is ~18 pC/m, higher than MoSe2, WS2 and WSe2 (16, 15 and 12 pC/m). MoS2 has been successfully synthesized to integrate into ferromagnetic materials [35]. The model can be
Results and discussion
To compute the transport properties, we use the quantum transport package KWANT which is based on the wave function method. This approach is mathematically equivalent to the commonly used nonequilibrium Green's function [43]. Owing to size effect, electronic transport properties depend on the strip length and width [44,45]. According to typical normal-ferromagnetic-normal structure [46], we assume that the strip width is 40 nm and the length is 140 nm while the width of the ferromagnetic region
Summary
In this study, piezotronic effect on electronic spin and valley transport properties have been studied by using a normal-ferromagnetic-normal monolayer MoS2 transistor. The transport characteristics are significantly affected by the strain-induced strong piezoelectric field. Spin and valley polarization can be induced by strain. Spin and valley filters have novel selectivity of piezotronic spin and valley transistors, which have potential application in quantum information. This study provides
CRediT authorship contribution statement
Ruhao Liu: Conceptualization, Methodology, Formal analysis, Writing - original draft. Gongwei Hu: Conceptualization, Formal analysis, Validation, Writing - original draft. Minjiang Dan: Formal analysis, Validation. Yaming Zhang: Formal analysis, Validation. Lijie Li: Writing - review & editing. Yan Zhang: Supervision, Conceptualization, Methodology, Formal analysis, Writing - review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors are thankful for the support from University of Electronic Science and Technology of China (ZYGX2015KYQD063), Swansea University, SPARC II project.
Ruhao Liu received his B.S. in applied physics in the School of Physics at University of Electronic Science and Technology of China in 2018. He is currently a M.S. student in group of Prof. Yan Zhang at UESTC. His research focuses on the field of piezotronics and piezo-phototronics, especially quantum piezotronics.
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Ruhao Liu received his B.S. in applied physics in the School of Physics at University of Electronic Science and Technology of China in 2018. He is currently a M.S. student in group of Prof. Yan Zhang at UESTC. His research focuses on the field of piezotronics and piezo-phototronics, especially quantum piezotronics.
Gongwei Hu received his B.S. degree (2014) from China Three Gorges University and M.S. degree in Theoretical Physics (2017) from Lanzhou University. He is currently pursuing the Ph.D degree under the guidance of Professor Yan Zhang in School of Physics in UESTC. His research focuses on the field of functional nanostructures and their physics.
Minjiang Dan is currently pursuing his B.S. degree in the University of Electronic Science and Technology of China (UESTC). He is now simulating piezotronic devices under the guidance of Professor Yan Zhang in School of Physics. His interests focus on the quantum piezotronics, semiconductor device and solar cell.
Yaming Zhang received his B.S. in the School of Physics at University of Electronic Science and Technology of China in 2018. He is currently a M.S. student in group of Prof. Yan Zhang at UESTC. He is interested in new semiconductor nanodevices, and focuses on theory of piezotronics and piezophototronics.
Lijie Li is a professor at Swansea University, UK. His research interests are design, modeling, fabrication, and characterization of MEMS, NEMS, sensors and actuators. He is Fellow of IET, and senior member of IEEE.
Yan Zhang is a professor at University of Electronic Science and Technology of China. He received his B. S. degree (1995) and Ph.D degree in Theoretical Physics (2004) from Lanzhou University. His research interests include self-powered nano/micro system, piezotronic and modeling of nonlinear dynamics of NEMS. He is senior member of IEEE.
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This author contributes equally to this work.