Optimization of NC-LSPR coupling for high-performance wide-spectrum MoS2 phototransistors

1. Introduction

Due to its unique lattice structure, adjustable energy band and excellent optoelectronic properties, as well as van der Waals interface compatible with CMOS process, two-dimensional MoS ₂ phototransistors have become a research hotspot in recent years. However, due to the weak light absorption and limited spectral response range ( < 680 nm ) of atomically thin MoS ₂ , its low detectivity has hindered the development of MoS ₂ phototransistors. In previous research work (ACS Photonics 2024, 11, 2308 − 2315) , we designed and proposed a MoS ₂ phototransistor based on Hf _0.5 Zr _0.5 O ₂ (HZO) /Au nanoparticles (AuNPs) /Al ₂ O ₃  /MoS 2 _gate  stack structure , realizing the coupling of negative capacitance ( Negative capacitance NC) effect and local surface plasmon resonance ( LSPR ) effect, achieving the goal of simultaneously achieving high detectivity and wide spectral response range. However, it is necessary to analyze in detail the influence of the key passivation layer _{Al2O3 thickness} on the _LSPR effect , that is, the tensile strain of Au nanoparticles on MoS2 and the regulation of Au nanoparticles on HZO ferroelectric polarization, the NC electric field amplification capability and the ferroelectric / dielectric capacitance matching-related factors, and how these factors ultimately regulate the optoelectronic performance of the phototransistor.

Therefore, in this paper, we prepared MoS ₂ phototransistors with different Al ₂ O ₃ thicknesses and studied their effects on the LSPR effect, the strain on MoS ₂ , the device capacitance matching, and the photoelectric properties of the transistor. The results show that when the Al ₂ O ₃ thickness is 3nm , the device achieves the best performance and has good uniformity. This work provides an effective solution for large-area, high-performance, and wide-spectrum-response two-dimensional phototransistor arrays.

2. Research background

In order to optimize the detection performance of MoS ₂ phototransistors, the current mainstream optimization methods include charge transfer-assisted strategies that form composite structures with macromolecules, quantum dots, nanowires, etc. and MoS ₂ ; and using the plasma elementary resonance of metal nanoparticles (gold nanoparticles / silver nanoparticles, etc.) to enhance light absorption. The above two optimization strategies can often effectively improve the detection range of MoS ₂ phototransistors, but the introduction of composite structures or nanoparticles often easily leads to a significant increase in the leakage current (dark current) of the device, limiting its detection rate to a low level. In 2020 , negative capacitance field-effect transistors were reported to be used to reduce the dark current of MoS ₂ photodetectors and to increase their detection rate to more than 10 ¹⁴ Jones . Unfortunately, the negative capacitance optimization strategy cannot go any further in the detection range. Therefore, the problem that needs to be solved urgently is how to achieve a MoS ₂ photodetector with both high detectivity and wide spectrum response .

3. Innovative research

To realize a MoS ₂ photodetector with high detectivity and wide spectrum response , we designed and proposed a new NC-LSPR coupled gate stack structure HZO/AuNPs/Al ₂ O ₃ /MoS _2. The in-situ preparation of Au nanoparticles with controllable size and uniform distribution on the surface of HZO ferroelectric film can enhance its ferroelectric polarization performance; the insertion of Al ₂ O ₃ film to form AuNPs/Al ₂ O ₃ /MoS ₂ structure effectively avoids the complex low-quality interface of AuNPs/MoS ₂ while maintaining the light absorption amplification advantage of LSPR effect; Al ₂ O ₃ is used as a passivation layer to achieve capacitance matching with HZO , stabilize the NC effect and reduce transistor hysteresis, thereby improving device performance. Furthermore, we explored the effects of the thickness of Al ₂ O ₃ as a key passivation layer on the LSPR effect, the tensile strain applied to MoS _2, and the capacitance matching of the HZO ferroelectric layer, and how these factors ultimately regulate the photoelectric performance of the phototransistor.

Using COMSOL software to simulate the electric field intensity distribution in the MoS ₂ /Al ₂ O ₃ /AuNPs structure at different Al ₂ O ₃ thicknesses, it can be seen that as the Al ₂ O ₃ thickness increases, the electric field intensity representing the LSPR effect gradually decreases. When the thickness does not exceed 3nm , a strong LSPR effect can still be maintained. When it exceeds 3nm , the electric field intensity decreases significantly, and the light absorption spectrum also shows the same trend of change (Figure 2 ). PL and Raman are used to characterize the strain effect on MoS _2. The strain effect will stretch the MoS ₂ lattice, causing the band gap to narrow, thereby improving the long-wave absorption capacity. When the Al ₂ O ₃ thickness increases, the strain on MoS ₂ also decreases.

We prepared NC-LSPR MoS ₂ phototransistors with different thicknesses of Al ₂ O ₃ and tested the electrical and optical properties of the devices. After the AuNPs were inserted alone , the transistor performance decreased due to the formation of leakage channels at the AuNPs/MoS ₂ interface; with the addition of the passivation layer Al ₂ O ₃ , the subthreshold slope and current switching ratio of the transistor were improved, and a good capacitance match was obtained when the thickness was 3~5nm . After that, as the thickness of Al ₂ O ₃ continued to increase, the electrical performance degraded due to capacitance mismatch. At the same time, the optoelectronic properties showed that when the thickness of Al ₂ O ₃ was 3nm , the device achieved the best responsivity ( 140.1A/W ) and detectivity ( 3.83 × 10 ¹⁴ Jones ), and the performance indicators were at the leading level.

4. Application and Prospect

This study effectively combines the high detectivity brought by the NC effect and the larger detection range brought by the LSPR effect. The thickness of Al ₂ O ₃ passivation layer is optimized by comprehensively considering the influence of the thickness of the Al _{2 O 3} passivation layer on the LSPR effect, strain effect, NC capacitance matching effect and photoelectric property modulation . Finally, under 740 nm illumination, the NC-LSPR coupled phototransistor of HZO/AuNPs/ 3nm Al 2 O ₃ / _MoS 2 _achieved a high detectivity of more than 10 ¹⁴ Jones and excellent uniformity. This provides a theoretical basis and experimental support for the further application of the NC-LSPR coupling structure, and paves the way for the preparation of a two-dimensional high-performance photodetector array with a broad spectrum response.

The research results were published online in Nanophotonics under the title “Optimization of NC-LSPR Coupled MoS ₂ Phototransistors for High-Performance Broad-Spectrum Detection” .

The authors of this article are Weichao Jiang, Yuheng Deng, Rui Su, Jingping Xu, and Lu Liu , of which Weichao Jiang is the first author and Lu Liu is the corresponding author. Lu Liu is the head of the Institute of Micro-Nano Electronic Devices, School of Integrated Circuits, Huazhong University of Science and Technology.