Gate-tunable trion switch for excitonic device applications

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Gate-tunable trion switch for excitonic device applications

Sarthak Das, Sangeeth Kallatt, Nithin Abraham, and Kausik Majumdar

Phys. Rev. B 101, 081413(R) – Published 27 February 2020

Abstract

Trions are excitonic species with a positive or negative charge, and thus, unlike neutral excitons, the flow of trions can generate a net detectable charge current. Trions under favorable doping conditions can be created in a coherent manner using resonant excitation. In this work, we exploit these properties to demonstrate a gate controlled trion switch in a few-layer graphene/monolayer ${WS}_{2}$ /monolayer graphene vertical heterojunction. By using a high-resolution spectral scan through a temperature controlled variation of the band gap of the ${WS}_{2}$ sandwich layer, we obtain a gate voltage dependent vertical photocurrent strongly relying on the spectral position of the excitation, and the photocurrent maximizes when the excitation energy is resonant with the trion peak position. Further, the resonant photocurrent thus generated can be effectively controlled by a back gate voltage applied through the incomplete screening of the bottom monolayer graphene, and the photocurrent strongly correlates with the gate dependent trion intensity, while the nonresonant photocurrent exhibits only a weak gate dependence—unambiguously proving a trion driven photocurrent generation under resonance. We estimate a sub-100 fs switching time of the device. The findings are useful towards demonstration of ultrafast excitonic devices in layered materials.

Received 2 December 2019
Revised 21 January 2020
Accepted 10 February 2020

DOI:https://doi.org/10.1103/PhysRevB.101.081413

Physics Subject Headings (PhySH)

Carrier dynamics Carrier generation & recombination Excitons Optoelectronics Trions

Transition metal dichalcogenides

Coherent control Luminescence

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Sarthak Das ¹, Sangeeth Kallatt², Nithin Abraham¹, and Kausik Majumdar ^1,*

¹Department of Electrical Communication Engineering, Indian Institute of Science, Bangalore 560012, India
²Center for Quantum Devices, Niels Bohr Institute, University of Copenhagen, Denmark

^*Corresponding author: kausikm@iisc.ac.in

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Issue

Vol. 101, Iss. 8 — 15 February 2020

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Images

Figure 1
Photocurrent in FLG/monolayer ${WS}_{2} / MLG$ vertical heterojunction. (a) Schematic of the experimental device structure where 1L- ${WS}_{2}$ is sandwiched between monolayer graphene (MLG) in the bottom and few-layer graphene (FLG) on the top. (b) The optical micrograph of the heterostructure where the MLG, 1L- ${WS}_{2}$ , and the FLG are marked with blue, yellow, and black outlines, respectively. (c) The transfer characteristics of the device under dark condition at different temperatures with $V_{d} = 20$ mV. Inset: The dark current-voltage characteristics at 293 K with $V_{g} = 0 V$ . (d) The color plot of temperature dependent PL spectra with $2.33 eV$ laser excitation. The band gap decreases monotonically with an increase in temperature. The vertical red dashed line corresponds to $1.9591 eV$ excitation. (e) Variation of exciton $(X^{0})$ and trion $(X^{-})$ peak positions as a function of temperature showing enhanced separation (and hence higher trion stability) between the two at higher temperatures. (f) The transient photocurrent at zero external bias with $1.9591 eV$ laser excitation at different temperatures starting from 303 K and increasing up to 423 K in steps of 20 K.
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Figure 2
Resonant trion mediated $I_{p h}$ generation with $1.9591 eV$ excitation. (a) Temperature dependent PL spectra of the heterojunction with $2.33 eV$ excitation. The $X^{-}$ and $X^{0}$ energy states, fitted with two Voigt curves (cyan and violet, respectively) are shown separately. The vertical red dashed line shows the spectral position of $1.9591 eV$ excitation. The $X^{-}$ state comes in resonance with $1.9591 eV$ excitation at 343 K while for the $X^{0}$ , it is around 393 K (not shown in the figure). (b)–(h) Schematic of the photocurrent generation mechanism with $1.9591 eV$ excitation at different temperatures. The off-resonance condition is shown in (b) and (h). (b) shows the situation at 303 K with the excitation in the suboptical-band-gap range of 1L ${WS}_{2}$ (at 303 K, the optical band gap is $2.014 eV$ ), while (h) shows the situation at 423 K where the excitation is above the optical band gap. The excitation is in resonance with the $X^{-}$ at 343 K (c)–(e). (c) shows the coherent formation of bright intervalley trions with linear polarization. The resonantly formed $X^{-}$ can either be transferred directly to the bottom MLG [shown in (d)] or it can recombine radiatively, releasing an electron, which in turn gets transferred to the bottom MLG [shown in (e)]. Both of the processes generate photocurrent; however, the latter process is of weaker efficiency. (f) and (g) show the $X^{0}$ resonance condition at 393 K, which does not contribute to the photocurrent generation due to the charge neutral nature of $X^{0}$ .
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Figure 3
Gate voltage modulation of generated photocurrent. (a) Modulation of transient photocurrent at 343 K (trion resonance condition) with different gate voltages ( $V_{g}$ ) ranging from $- 40$ to $+ 40 V$ , showing a monotonic increase in the photocurrent ( $I_{p h}$ ). (b) The color plot of $I_{p h}$ for the simultaneous modulation of temperature ( $T$ ) and $V_{g}$ , which reveals stronger $V_{g}$ modulation of $I_{p h}$ under $X^{-}$ resonance. (c) Horizontal line cuts [shown in dashed lines in (b)] at three different $V_{g}$ values ( $- 40$ , 0, and $+ 40 V$ ). (d) Relative separation of $X^{0}$ (in solid symbols) and $X^{-}$ (in open symbols) from $1.9591 eV$ excitation at different temperatures. The resonance conditions with $X^{0}$ and $X^{-}$ are indicated by the vertical dashed lines.
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Figure 4
Correlation of $V_{g}$ modulation with $X^{-}$ intensity under resonance. (a) The color plot of the PL intensity obtained with $1.9591 eV$ excitation as a function of $V_{g}$ (in horizontal axis) and the relative emission energy positions of $X^{-}$ with respect to the excitation energy (in vertical axis). The green open symbols denote the position of the $X^{-}$ peak obtained from $2.33 eV$ excitation. (b) Normalized PL spectra with $2.33 eV$ excitation at 343 K showing the $X^{-}$ and $X^{0}$ energy states separately at five different $V_{g}$ conditions. The red arrows indicate the position of the $1.9591 eV$ excitation, with which photocurrent is measured. (c) Normalized $I_{p h}$ (in brown symbols, left axis) and trion intensity along the horizontal dashed line in (a) (in orange symbols, right axis) for different $V_{g}$ with $1.9591 eV$ excitation at 343 K, showing a strong correlation between the two independent measurements.
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