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

Abstract

Besides the intensity and wavelength, the ability to analyze the optical polarization of 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, conventional filter-integrated polarimetric sensing systems require complex optical components and a complicated fabrication process, severely limiting their on-chip miniaturization and functionalities. Herein, the reconfigurable polarimetric photodetection with photovoltaic mode is developed based on a few-layer MoS₂/PdSe₂ heterostructure channel and a charge-trap structure composed of Al₂O₃/HfO₂/Al₂O₃ (AHA)-stacked dielectrics. Because of the remarkable charge-trapping ability of carriers in the AHA stack, the MoS₂/PdSe₂ channel exhibits a high program/erase current ratio of 10⁵ and a memory window exceeding 20 V. Moreover, the photovoltaic mode of the MoS₂/PdSe₂ Schottky diode can be operated and manipulable, resulting in high and distinct responsivities in the visible broadband. Interestingly, the linear polarization of the 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 the capability of both tunable responsivity and linear polarization, demonstrating great potential application toward reconfigurable photosensing and polarization-resolved imaging applications.

1. Introduction

The technology of polarization-sensitive photodetection plays a vital role in both civilian and military fields, such as biomedical imaging, quantum communication, and three-dimensional (3D) holographic displays [1,2,3]. In the past, conventional polarimetric photodetectors required the integration of a prepositive polarizer, lens, or polarization coding system, which increased the fabrication complexity and cost of the imaging systems [4]. Therefore, developing the architecture of polarizer-free polarimetric photodetection becomes crucial for satisfying the needs of on-chip integration, miniaturization, and multi-functionalities.

Recently, low symmetric two-dimensional (2D) semiconductors with in-plane anisotropic crystal structures have shown great potential for linear polarization photodetection. This capability arises 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.

Some previous architectures of the device have been proposed; however, the existing technologies have still been unable to realize multifunctional photodetection with tunable responsivity and polarization sensitivity. The capability for reconfigurable polarimetric photodetection could enable higher-resolution polarimetric imaging. To achieve tunable responsivity and polarization sensitivity, configurations such as a split-gate configuration [16] or ferroelectric polarization [17] have been employed. However, these approaches suffer from the complicated design of four electrical terminals and the high energy consumption required. At the same time, nonvolatile polarimetric photodetection, in which the reconfigurable responsivity and polarization sensitivity are necessary schemes for the integration of multi-functional modules, realizes an “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 devices still remains rarely studied.

This work demonstrates a novel multifunctional photodetector engineered to offer reconfiguration in both responsivity and polarization sensitivity. It is developed on a few-layer MoS₂/PdSe₂ heterostructure and an Al₂O₃/HfO₂/Al₂O₃ (AHA) charge-trap gate stack. Our elaborately designed photodetector exhibits a remarkable photovoltaic photodetection performance under visible illumination, which can be attributed to the built-in electrical driving effect of the MoS₂/PdSe₂-based Schottky diode. Under modulation of the AHA charge-trap gate stack, the electrical characteristics of the MoS₂/PdSe₂ Schottky diode can be tuned and maintained at the program/erase state, exhibiting an unprecedented memory window exceeding 20 V and the program/erase current ratio of 10⁵. Moreover, the photovoltaic mode of the MoS₂/PdSe₂ Schottky diode is operated and switchable, resulting in high and distinct responsivities in the visible spectral band. Interestingly, linear polarization can be further modulated under the 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. Importantly, the charge-trap gate stack was first 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 the ability to have remarkable electronic and optoelectronic properties, which makes it a great potential photodetector candidate. Considering the large carrier density and high work function of PdSe₂, a depletion region of the junction can be formed by stacking the MoS₂/PdSe₂ heterostructure. Multilayers of MoS₂ and PdSe₂ were subsequently exfoliated and stacked together (Figure 1a). The details of device fabrication are provided in Section S1 of Supplementary Materials. The schematic of the MoS₂/PdSe₂ photodetector is shown in Figure 1b. Figure 1c shows the distinct Raman peaks of the MoS₂/PdSe₂ heterostructure, which correspond to 382 cm⁻¹ (${\mathrm{E}}_{2\mathrm{g}}^1$) and 407 cm⁻¹ (${\mathrm{A}}_{1\mathrm{g}}$) for MoS₂ and 144 cm⁻¹ (${\mathrm{A}}_{\mathrm{g}}^1$), 203 cm⁻¹ (${\mathrm{A}}_{\mathrm{g}}^2$), 222 cm⁻¹ (${\mathrm{B}}_{1\mathrm{g}}^2$), and 258 cm⁻¹ (${\mathrm{A}}_{\mathrm{g}}^3$) for PdSe₂. Figure 1d and e show the I_ds–V_ds curves under different gate voltages V_G for MoS₂ and PdSe₂ field-effect transistor (FET), respectively. Notably, both I_ds–V_ds 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 the MoS₂/PdSe₂ photodetector. Furthermore, the output curves of I_ds–V_ds for MoS₂ FET exhibit a n-type ambipolar conducting behavior at V_G from −5 to 5 V. By contrast, the current slightly increases with decreasing negative V_G for PdSe₂ FET, which indicates the semi-metallic behavior. Given the bandgaps of multilayer MoS₂ and PdSe₂ are previously reported to be 1.2 and 0.03 eV [21,22], the band alignment of the MoS₂/PdSe₂ heterojunction is illustrated in Figure 1f. The Schottky barrier can be formed at the interface of the MoS₂/PdSe₂ heterojunction, and the I_ds–V_ds curve exhibits a rectification behavior, demonstrating a rectification ratio of I_on/I_off up to 10.

3. Results

3.1. Transfer Characteristics and Static Memory Behavior

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

3.2. Dynamic Memory Behavior of the Device

The transfer characteristics of the device are further studied under different biases. As shown in Figure 3a,b, they show that an obvious memory window occurs under both a forward bias of −1 V and a reverse bias of +1 V. A maximum program/erase current ratio of 10⁵ can be achieved under a forward bias of −1 V. Endurance and retention time of the MoS₂/PdSe₂ memory are provided in Section S4 of Supplementary Materials. To explore the dynamic transition of the device, the device was initially set into an erase state by applying a negative gate pulse (−10 V, duration of 2 s). The output curves I_ds–V_ds were read by sweeping V_ds from −1 V to +1 V after applying a series of +25 V gate pulses with different duration times. The output curve I_ds–V_ds shows a clear decrease and is nearly saturated when the width of the pulse increases to 0.2 s (Figure 3c). According to the expression of the charge-trapping rate [23], the calculated charge-trapping rate varies from 10¹⁵ cm⁻²s⁻¹ to 10¹⁴ cm⁻²s⁻¹ when the pulse width changes from 0.01 s to 0.2 s. Figure 3d shows the dependency of output curves I_ds–V_ds with the amplitude of the gate pulse. It demonstrates that output current decreases with the increase of pulse amplitude. This can be explained by the modulation of the Schottky barrier through the gate pulse, which also suggests that the charge-trapping mechanism of the AHA gate stack dominates the memory behavior of the device.

3.3. Photovoltaic Behavior and Reconfigurable Linear Polarization

Given the excellent memory switching properties of the 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 was worthy of investigation. To characterize it, a positive (+25 V) and negative (−10 V) gate pulse with a width of 0.2 s were applied to switch the device into the program and erase state, respectively. In the program/erase state, I_ds–V_ds characteristics under illumination were recorded by using the polarized 520 nm light with an intensity of 120 mW/cm². Figure 4a,b show the I_ds–V_ds characteristics of the 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). The crystalline orientation of PdSe₂ was determined by angle-resolved polarized Raman spectroscopy (ARPRS), and details of the measurement are provided in Section S3 of Supplementary Materials. It can be noticed that the device exhibits noticeable photovoltaic responses, including a short-circuit current (I_sc) of ~15 nA/30 nA and an open-circuit voltage (V_oc) of ~−0.014 V/−0.012 V in the program/erase state under parallel light excitation. After switching the polarization of light to the vertical direction, the illuminated I_ds–V_ds curves shift toward the higher value, showing that I_sc and V_oc increase to ~54 nA/82 nA and ~−0.027/−0.025 V in the program/erase state. It is noted that significant photocurrent noise occurs during photocurrent measurement, which can be attributed to the degeneration of 2D flakes and environmental noise. The protection layer of Al₂O₃ on top of the MoS₂/PdSe₂ heterostructure and the optimized measurement method can significantly reduce the noise, which is summarized in Section S5 of Supplementary Materials. In addition, gate voltage was applied to modulate the performance of the device. The responsivity was extracted and plotted in Figure 4c at different memory states and polarization of light. The noise equivalent power (NEP) and detectivity of this photodetector were measured and are provided in Section S6 of Supplementary Materials. The device shows typical transfer characteristics of an n-type MoS₂ semiconductor. As the gate voltage increases from −2 V to 2 V, all of the responsivities increase at V_ds = −1 V. A figure of merit of the linear polarized photodetection is the degree of linear polarization (LP), where LP = (I_max − I_min)/(I_max + I_min), where I_max and I_min are the photocurrents of the detected light parallel and perpendicular to the primary polarization direction, respectively. Figure 4d shows the LP results as a function of V_G. Under 120 mW/cm² light illumination, both LP values of the device gradually increase from 0.4/0.36 to 0.55/0.49 in the program/erase state. This indicates that the LP of the device can be effectively modulated by the memory state, and its gap between the program and erase state becomes more obvious under the positive gate voltage.

4. Discussion

To understand the photoresponse mechanism of the MoS₂/PdSe₂ device, the energy band structure diagram is illustrated in Figure 5. Since the fermi level of PdSe₂ is lower than MoS₂, the electrons will flow from MoS₂ to PdSe₂, and the holes diffuse in opposite directions to MoS₂. The opposite diffusion of electrons and holes introduces a Schottky barrier with the build-in field E_in 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 by the conduction bands in both materials. Then, with the help of build-in field E_in, the electrons occupied in PdSe₂ can be driven to the conduction band of MoS₂, while E_in will force the holes within the MoS₂ valence band to flow into the valence band of PdSe₂, resulting in the photovoltaic behavior. When the negative gate voltage pulse is applied to switch the device into an erase state (Figure 5b), the energy band of MoS₂ is lowered, and E_in will increase, which results in the enhancement of separation of photo-generated electron-hole pairs as well as short-circuit current. Meanwhile, the photocurrent generated from MoS₂ increases in the erase state, which results in the decrease of LP because of the intrinsic polarization-insensitivity of MoS₂. On the other hand, the E_in will be reduced in the program state (Figure 5c), which induces 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 was demonstrated, which was composed of in-plane anisotropic PdSe₂ and MoS₂ with an 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.G. and X.H. conceived the research, and Q.B. and T.L. fabricated the samples. Q.B. conducted the measurements. X.H., Q.B., Y.G., Q.L., T.L., W.L., A.J., B.Q., H.Y., B.L. and C.G. discussed the data and contributed to the manuscript. All authors have read and agreed to the published version of the manuscript.

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.

Footnotes

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Figures

Figure 1. Schematics and characterization of the MoS2/PdSe2 heterostructure device: (a,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,e) Ids–Vds relationship of MoS2 and PdSe2, respectively; (f) the transfer characteristics of the MoS2/PdSe2 heterostructure under the bias of −3 V and schematic of the MoS2/PdSe2 heterostructure-based Schottky barrier.

Figure 2. The statical behavior of the nonvolatile gate charge-trap memory based on MoS2/PdSe2 heterostructure: (a) Ids–VG characteristics of the 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 3. The dynamic behavior of the nonvolatile gate charge-trap memory based on MoS2/PdSe2 heterostructure: (a,b) Ids–VG characteristics of the device under different VG under the forward bias of −1 V and reverse bias of +1 V, respectively; (c,d) Ids–Vds characteristics of the device under different pulse durations and amplitudes.

Figure 4. The polarization-modulated photovoltaic behavior of MoS2/PdSe2 photodetector: (a,b) Short-circuit current Isc and open-circuit voltage Voc of MoS2/PdSe2 photodetector under the program and erase state, respectively. (c,d) Dependency of responsibility and linear polarization with gate voltage VG under different program/erase states and polarization directions of light.

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,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 [less than] 0) and program states (VG [greater than] 0), respectively.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/nano14231936/s1: Figure S1. Thickness of (a) MoS₂ and (b) PdSe₂ measured by atomic force microscopy. Figure S2. Angle-resolved polarized Raman spectroscopy of a PdSe2 flake. Figure S3. Endurance of the MoS₂/PdSe₂ memory. (a) Endurance of the memory device for 100 cycles with the program/erase voltage being +20 V, 1 s and −10 V, 1 s. (b) The stability of the program/erase state of the device after programming at +20 V, 1 s duration, and erasing at −10 V, 1 s duration (Vds = -1 V). Figure S4. The polarization-modulated photovoltaic behavior of the MoS2/PdSe2 photodetector.: (a) and (b) Short-circuit current Isc and open-circuit voltage Voc of the MoS2/PdSe2 photodetector under the program and erase state, respectively. Figure S5. The NEP and detection limit of the MoS2/PdSe2 photodetector. (a) NEP and (b) detectivity tuned by the gate voltage, indicating the detection limit of our photodetector.

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