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Reconfigurable Polarimetric Photodetector Based on MoS2/PdSe2 Heterostructure with Charge-Trap Gate Stack

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31 October 2024

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31 October 2024

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Abstract
Besides the intensity and wavelength, the ability to analyze the optical polarization of the detected light can provide a new degree of freedom for numerous applications, such as object recognition, biomedical applications, environmental monitoring and remote sensing imaging. However, the conventional filter-integrated polarimetric sensing systems require complex optical components and complicated fabrication process, severely limiting its on-chip miniaturization and functionalities. Herein, the reconfigurable polarimetric photodetection with photovoltaic mode are developed based on a few-layer MoS2/PdSe2 heterostructure channel and a charge-trap structure composed of Al2O3/HfO2/Al2O3 (AHA) stacked dielectric. Because of the remarkable charge-trapping ability of carriers in AHA stack, the MoS2/PdSe2 channel exhibits a high program/erase current ratio of 105 and memory window exceeding 20 V. Moreover, photovoltaic mode of MoS2/PdSe2 Schottky diode can be operated and manipulable, resulting into high and distinct responsivities in the visible broadband. Interestingly, the linear polarization of device can be modulated under program/erase states, enabling the reconfigurable capability of linearly polarized photodetection. This study demonstrates a new prototype heterostructure-based photodetector with capability of both tunable responsivity and linear polarization, demonstrating great potential application toward reconfigurable photosensing and polarization-resolved imaging applications.
Keywords: 
Subject: Physical Sciences  -   Optics and Photonics

1. Introduction

Technology of polarization-sensitive photodetection plays a vital role in both civilian fields and military such as biomedical imaging, quantum communication and three-dimensional (3D) holographic displays [1,2,3]. In the past, the conventional polarimetric photodetectors require the integration of prepositive polarizer, lens, or polarization coding systems, which increases the fabrication complexity and cost of the imaging systems [4]. Therefore, developing the architecture of polarizer-free polarimetric photodetection become crucial for satisfying the needs of on-chip integration, miniaturization and multi-functionalities.
Recently, the low symmetric two-dimensional (2D) semiconductors with in-plane anisotropic crystal structure have shown great promise for linear polarization photodetection, which benefit from their intrinsic linear dichroism as well as the absence of surface dangling bonds which allows for integration into complex heterostructures regardless of lattice mismatch [5,6,7,8]. Among 2D materials and their heterojunctions, including black phosphorous [9], ReS2 [10], GeAs [11], 1T′-MoTe2 [12], 1T′-WTe2 [13] as well as WSe2/ReSe2 [14], WS2/GeAs [15] and graphene/PdSe2/Ge [2], they have been widely used to build polarization-sensitive photodetectors with high detectivity, fast speed and broad-band sensitivity.
Considering some previous architectures of device has been proposed, however, the existing technologies have still been unable to realize the multifunctional photodetection with tunable responsivity and polarization sensitivity. The ability of reconfigurable polarimetric photodetection could realize the higher-resolved polarimetric imaging. To achieve the responsivity and polarization sensitivity change, a split-gate configuration [16] or ferroelectric polarization [17] have been adopted, which however suffer from the complicated design of four electrical terminals and high energy consumption required. At the same time, nonvolatile polarimetric photodetection, in which the reconfigurable responsivity and polarization sensitivity, are necessary schemes for integration of multi-functional modules, realizing “All-in-one” system, such as in-memory sensing technology [18], vision acquisition [19] and high-level cognitive computing [20]. However, the tunable nonvolatile and reconfigurable polarimetric photodetection in 2D device still remain rarely studied.
This work demonstrates a novel multifunctional photodetector engineered to offer reconfiguration in both responsivity and polarization sensitivity, which is developed on a few-layer MoS2/PdSe2 heterostructure and a Al2O3/HfO2/ Al2O3 (AHA) charge-trap gate stack. Our elaborately designed photodetector exhibits a remarkable photovoltaic photodetection performance under visible illumination, which results from the build-in electrical driving effect of MoS2/PdSe2 based Schottky diode. Under modulation of the AHA charge-trap gate stack, the electrical characteristics of MoS2/PdSe2 Schottky diode can be tuned and maintained at program/erase state, exhibiting an unprecedented memory window exceeding 20 V and the program/erase current ratio of 105. Moreover, the photovoltaic mode of MoS2/PdSe2 Schottky diode are operated and switchable, contributing to high and distinct responsivities in the wide visible spectral band (420 – 650 nm). Interestingly, the linear polarization can be further modulated under program/erase state, enabling the reconfigurable capability of linearly polarized photodetection. Our work provides promising solutions for increasing the versatility of applications for reconfigurable photodetection. More importantly, the charge-trap gate stack is firstly applied on the 2D heterostructure to engineer the band alignment type, enhance the photodetection performance, and enrich functionalities.

2. Materials and Methods

MoS2 is one of the most studied 2D materials and it demonstrates ability of remarkable electronic and optoelectronic properties, which shows great potential candidate as the channel material of the photodetector. Considering the large carrier density and high work function of PdSe2, a depletion region of junction can be constructed by stacking MoS2/PdSe2 heterostructure. Multilayers of MoS2 and PdSe2 are subsequently exfoliated and stacked together (Figure 1a). The schematic of MoS2/PdSe2 photodetector is shown in Figure 1b. Figure 1c shows the distinct Raman peaks of MoS2/PdSe2 heterostructure, which correspond to 382 cm−1 ( E 2 g 1 ) and 407 cm−1 ( A 1 g ) for MoS2 and 144 cm−1 ( A g 1 ), 203 cm−1 ( A g 2 ), 222 cm−1 ( B 1 g 2 ) and 258 cm−1 ( A g 3 ) for PdSe2. Figure 1d and e show the Ids–Vds curves under different gate voltages VG for MoS2 and PdSe2 field-effect transistor (FET), respectively. Notably, both of Ids–Vds curves demonstrate obvious linearity and it can be proved to be Ohmic source-drain contact for both FETs, which is necessary for optoelectronic characteristics of MoS2/PdSe2 photodetector. Furthermore, the output curves of Ids–Vds for MoS2 FET exhibit a n-type ambipolar conducting behavior at VG scanning from −5 to 5 V. By contrast, the current slightly increases with decreasing negative VG for PdSe2 FET, which indicates the semi-metallic behavior. Given the bandgaps of multilayer MoS2 and PdSe2 are previous reported to be 1.2 and 0.03 eV [21,22], the band alignment of MoS2/PdSe2 heterojunction is illustrated in Figure 1f. The Schottky barrier can be formed at the interface of MoS2/PdSe2 heterojunction and the Ids–Vds curve exhibits a rectification behavior, demonstrating a rectification ratio Ion/ Ioff up to 10.

3. Results

3.1. Transfer Characteristics and Static Memory Behavir

Charge-trap stack of Al2O3/HfO2/ Al2O3 (6 nm/8 nm/32 nm) is deposited via atomic layer deposition (ALD). Figure 2a shows the transfer curves of MoS2/PdSe2 Schottky diode acquired by sweeping the gate voltage VG in a closed loop (from negative to positive values) under a fixed Vds of -1 V, exhibiting a clear hysteresis window and the hysteresis enables to widening as VG sweep range increases from 5 to 25 V. The Ids–VG curves exhibit a clockwise memory window and the extraction of memory window ∆V increases almost linearly with the maximum VG and reaches a maximum of 20 V when the VG sweeps to 25 V (Figure 2b). The transfer curve of Ids–VG decreases with increasing negative VG, suggesting that n-type MoS2 dominates the transfer characteristics of device. Figure 2c illuminates the device operation process. When a high positive/negative VG is applied to the gate, the band alignment starts favoring the tunnelling in/out of electrons from MoS2/PdSe2 channel to the HfO2 charge-trap layer, changing the carrier concentration in MoS2 and resulting into the program and erase state, respectively.

3.2. Dynamic Memory Behavir of Device

The transfer characteristics of device are further studied under different bias. As shown in Figure 3a and 3b, they show that an obvious memory window under both forward bias of -1 V and reverse bias of +1 V, especially demonstrating a maximum program/erase current ratio of 10 5. To explore the dynamic transition of device, the device is initially set into erase state by applying a negative gate pulse (-10 V, duration of 2 s) and its output curves Ids–Vds are read by sweeping Vds from -1 V to +1 V after applying a series of +25 V gate pulses with different duration times. The output curve Ids–Vds shows clear decrease and nearly saturated when the width of pulse increases to 0.2 s (Figure 3c). According to the expression of charge-trapping rate [23], the calculated charge-trapping rate varies from 1015 to 1014 cm-2t-1 when the pulse width changes from 0.01 s to 0.2 s. Figure 3d shows the dependency of output curve Ids–Vds with amplitude of gate pulse. It demonstrates that output current decreases with the increase of pulse amplitude. It can be explained by modulation of Schottky barrier through the gate pulse, and it also suggests a charge-trapping mechanism of AHA gate stack dominates the memory behavior of device.

3.3. Photovoltaic Behavior and Reconfigurable Linear Polarization

Given the excellent memory switching properties of device (including unprecedented memory window, large program/erase current ratio and nonvolatile switchable Schottky barriers) and strong optical anisotropy of PdSe2, the polarization-modulated photovoltaic behavior in the MoS2/PdSe2-based photodetector is worthy of investigation. To characterize it, a positive (+25 V) and negative (-10 V) gate pulse with width of 0.2 s are applied to switch the device into program and erase state respectively. In program/erase state, Ids–Vds characteristics under illumination are recorded by using the polarized 520 nm light with intensity of 120 mW/cm2. Figure 4a and 4b show the Ids–Vds characteristics of device under parallel ( 0 ° ) and vertical ( 90 ° ) polarized light in the program and erase state, respectively ( 0 ° and 90 ° directions correspond to the b-axis and a-axis crystalline direction of PdSe2). It can be noticed that the device exhibits noticeable photovoltaic responses including a short-circuit current (Isc) of ~15 nA/30 nA and an open-circuit voltage (Voc) of ~-0.014 V/-0.012 V in program/erase state under parallel light excitation. After switching the polarization of light to vertical direction, the illuminated Ids–Vds curve shift toward the higher value, showing that Isc and Voc increase to ~54 nA/82 nA and ~-0.027/-0.025 V in program/erase state. In addition, gate voltage is applied to modulate the performance of device. The responsivity is extracted and plotted in Figure 4c at different memory states and polarizations of light. The device shows a typical transfer characteristics of n-type MoS2 semiconductor. As the gate voltage increases from -2 V to 2 V, all of responsivities increase at Vds = -1 V. A figure of merit of the linear polarized photodetection is the degree of linear polarization (LP), where LP = (Imax - Imin)/(Imax + Imin), where Imax and Imin are the photocurrents of the detected light parallel and perpendicular to the primary polarization direction, respectively. Figure 4d shows that the LP results as a function of VG. Under 120 mW/cm2 light illumination, both of LP of device gradually increases from 0.4/0.36 to 0.55/0.49 in the program/erase state. This indicates that the LP of device can be effectively modulated by the memory state and its gap between program and erase state become more obvious under the positive gate voltage.

4. Discussion

To understand the photoresponse mechanism of the MoS2/PdSe2 device, the energy band structure diagram was illustrated in Figure 5. Since the fermi level of PdSe2 is lower than MoS2, the electrons will flow from MoS2 to the PdSe2, while the holes diffuse in opposite directions to MoS2, introducing a Schottky barrier with the build-in field Ein pointing from MoS2 to PdSe2 which is described in Figure 1f. When the laser shines on the surface of the device, the electrons confined in the valence band will be excited to the conduction bands in both the two materials. Then with the help of build-in field Ein, the electrons occupied in PdSe2 can be driven to the conduction band of MoS2, while Ein will force the holes within the MoS2 valence band to flow into the valence band of PdSe2, resulting into the photovoltaic behavior. When the negative gate voltage pulse is applied to switch the device into erase state (Figure 5b), the energy band of MoS2 is lowered and Ein will increase, therefore enhancing the separation of photo-generated electron-hole pairs and short-circuit current. Meanwhile, the photocurrent generated from MoS2 increase in the erase state, which results into the decrease of LP because of intrinsic polarization-insensitivity of MoS2. On the other hand, the Ein will be reduced in the program state (Figure 5c), which induce to the decrease of short-circuit current and increase of LP. In this way, we can adjust the energy band structure of MoS2/PdSe2 by switching the program/erase state, thereby adjusting the photodetection performance of the device.

5. Conclusions

In summary, a multifunctional photovoltaic photodetector is demonstrated which is composed of in-plane anisotropic PdSe2 and MoS2 with AHA charge-trap gate stack. The device exhibits a nonvolatile phenomenon in both electrical and photovoltaic characteristics, resulting from the modulation of band alignment by the gate voltage pulse. Utilizing the AHA charge-trap gate stack, the memory window and program/erase current ratio of MoS2/PdSe2 can be effectively modulated. Acting as a reconfigurable polarimetric photodetector, the device exhibits a reversible performance of both responsivity and polarization-sensitive photocurrent by switching the program and erase state, rendering it a promising candidate for polarization signal recognition and imaging.

Author Contributions

C.Z. Gu and X. Huang conceived the research, Q.H. Bai and T. Liu fabricated the samples. Q.H. Bai conducted the measurements. All authors discussed the date and contributed to the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China under Grant Nos. 62204259, 61888102, 92265110, 62174179, 11974386, 61905274, 12074420 and U21A20140, the National Key Research and Development Program of China under Grant No. 2021YFA1400700, the Key Research Program of Frontier Sciences of CAS under Grant Nos. QYZDJ-SSWSLH042 and XDPB22, the Project for Young Scientists in Basic Research of CAS under Grant No.YSBR021, the Guangdong Basic and Applied Basic Research Foundation under Grant No. 2023A1515010693. This work is also supported by the Synergic Extreme Condition User Facility, China.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Schematic and characterizations of the MoS2/PdSe2 heterostructure device: (a) and (b) Schematic and picture of MoS2/PdSe2 heterostructure photodetector. The scale bar is 20 μm; (c) Raman spectra of the multilayer MoS2 flakes, PdSe2 flakes and heterostructure, respectively; (d) and (e) Ids–Vds relationship of MoS2 and PdSe2, respectively; (f) The transfer characteristics of MoS2/PdSe2 heterostructure under the bias of -3 V and schematic of MoS2/PdSe2 heterostructure based Schottky barrier.
Figure 1. Schematic and characterizations of the MoS2/PdSe2 heterostructure device: (a) and (b) Schematic and picture of MoS2/PdSe2 heterostructure photodetector. The scale bar is 20 μm; (c) Raman spectra of the multilayer MoS2 flakes, PdSe2 flakes and heterostructure, respectively; (d) and (e) Ids–Vds relationship of MoS2 and PdSe2, respectively; (f) The transfer characteristics of MoS2/PdSe2 heterostructure under the bias of -3 V and schematic of MoS2/PdSe2 heterostructure based Schottky barrier.
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Figure 2. The statical behavior of the nonvolatile gate charge-trap memory based on MoS2/PdSe2 heterostructure: (a) Ids-VG characteristics of device under different VG at Vds = -1 V; (b) Extraction of memory window ∆V vs VG. The memory window increases from 1 to ∼20 V in our experimental settings; (c) Band diagram of the program/erase state of the device under positive and negative VG. Positive VG programs the device. Electrons tunneling from the few-layer MoS2 channel are accumulated in the HfO2 charge-trap layer. Negative VG erases the device. Holes tunnel from the few-layer MoS2 channel to the HfO2 charge-trap layer.
Figure 2. The statical behavior of the nonvolatile gate charge-trap memory based on MoS2/PdSe2 heterostructure: (a) Ids-VG characteristics of device under different VG at Vds = -1 V; (b) Extraction of memory window ∆V vs VG. The memory window increases from 1 to ∼20 V in our experimental settings; (c) Band diagram of the program/erase state of the device under positive and negative VG. Positive VG programs the device. Electrons tunneling from the few-layer MoS2 channel are accumulated in the HfO2 charge-trap layer. Negative VG erases the device. Holes tunnel from the few-layer MoS2 channel to the HfO2 charge-trap layer.
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Figure 3. The dynamic behavior of the nonvolatile gate charge-trap memory based on MoS2/PdSe2 heterostructure: (a) and (b) Ids-VG characteristics of device under different VG under forward bias of -1 V and reverse bias of +1 V, respectively; (c) and (d) Ids–Vds characteristics of device under different pulse durations and amplitudes.
Figure 3. The dynamic behavior of the nonvolatile gate charge-trap memory based on MoS2/PdSe2 heterostructure: (a) and (b) Ids-VG characteristics of device under different VG under forward bias of -1 V and reverse bias of +1 V, respectively; (c) and (d) Ids–Vds characteristics of device under different pulse durations and amplitudes.
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Figure 4. The polarization-modulated photovoltaic behavior of MoS2/PdSe2 photodetector: (a) and (b) Short-circuit current Isc and open-circuit voltage Voc of MoS2/PdSe2 photodetector under program and erase state, respectively. (c) and (d) Dependency of responsibility and linear polarization with gate voltage VG under different program/erase states and polarization directions of light.
Figure 4. The polarization-modulated photovoltaic behavior of MoS2/PdSe2 photodetector: (a) and (b) Short-circuit current Isc and open-circuit voltage Voc of MoS2/PdSe2 photodetector under program and erase state, respectively. (c) and (d) Dependency of responsibility and linear polarization with gate voltage VG under different program/erase states and polarization directions of light.
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Figure 5. Photoresponse mechanism of reconfigurable MoS2/PdSe2 photodetector: (a) The energy band structure of AHA charge-trap stack and MoS2/PdSe2 heterostructure before contact; (b) and (c) The energy band structure of the device and the flow of photo-generated electron-holes under illumination when the device is set to erase (VG < 0) and program state (VG > 0), respectively.
Figure 5. Photoresponse mechanism of reconfigurable MoS2/PdSe2 photodetector: (a) The energy band structure of AHA charge-trap stack and MoS2/PdSe2 heterostructure before contact; (b) and (c) The energy band structure of the device and the flow of photo-generated electron-holes under illumination when the device is set to erase (VG < 0) and program state (VG > 0), respectively.
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