Persistent photoconductivity (PPC), the prolonged enhancement of conductivity following light exposure, has been extensively studied in various semiconductors. In bulk semiconductors, PPC is often linked to spatial potential energy fluctuations of charge carriers. Previous studies on transition metal dichalcogenides (TMDs), specifically monolayer MoS₂ (ML-MoS₂), reported PPC with time constants ranging from 10² to 10⁴ seconds after visible light irradiation. Even longer time constants (~10⁶ s) were observed in few-layer MoS₂ with UV irradiation. These effects were generally attributed to extrinsic factors such as substrate inhomogeneities or surface adsorbates. This research explores the possibility that long-lived photogenerated charge carriers could originate from intrinsic material properties, such as lattice defects, resulting in extended carrier recombination times. The central hypothesis is that the observed GPPC in ML-MoS₂ FETs is primarily an intrinsic material effect caused by lattice defects which trap photogenerated charge carriers and prevent their rapid recombination. This will be investigated through a combined experimental and theoretical approach, involving transport measurements, spectroscopy, and microscopy.

The existing literature on persistent photoconductivity (PPC) in semiconductors highlights its dependence on material properties and external factors. Studies on amorphous and compensated wide-bandgap bulk semiconductors attribute PPC to large spatial fluctuations in the potential energy of charge carriers, hindering recombination. In monolayer MoS₂, PPC has been reported, with time constants ranging from 10² to 10⁴ seconds after visible light illumination and even longer times after UV irradiation in multi-layer MoS₂. However, previous explanations often focus on extrinsic factors like substrate imperfections or surface adsorbates. The current work aims to differentiate the contributions of intrinsic and extrinsic factors to the observed GPPC effect in ML-MoS₂ FETs. The lack of conclusive evidence regarding the role of intrinsic defects in driving GPPC in monolayer MoS₂ necessitates this detailed investigation.

Monolayer MoS₂ and WS₂ crystals were grown using chemical vapor deposition (CVD). The MoS₂ FET devices were fabricated using e-beam lithography to define source and drain electrodes (Au/Ti). The heavily p-doped silicon substrate acted as the back gate. Electrical transport measurements were performed using two Keithley 2634B source measure units in high vacuum to determine the source-drain current (I_DS) as a function of gate voltage (V_G). UV irradiation (λ = 365 nm) was applied to induce GPPC. The decay of I_DS over time was measured at V_G = 0 V. To investigate the temperature dependence, measurements were also conducted at 6 K. A theoretical model incorporating spatial fluctuations in the potential energy of charge carriers was developed to explain the experimental observations. Scanning tunneling spectroscopy (STS) at 1.1 K was employed to visualize spatial inhomogeneities in the band structure of ML-MoS₂ transferred onto a hBN/Pt(111) substrate. Aberration-corrected high-resolution transmission electron microscopy (HRTEM) at 60 kV was used to determine the density and type of point defects, specifically sulfur vacancies, in ML-MoS₂ and ML-WS₂. Photoluminescence (PL) mapping was performed before and after UV irradiation to study changes in optical properties. The CVD growth involved a two-zone tube furnace with sulfur powder and metal oxide (MoO₃ or WO₃) precursors. For STS, a high-quality h-BN layer on Pt(111) was prepared using borazine as a precursor, and its quality was verified using X-ray photoelectron spectroscopy (XPS). Raman spectroscopy and atomic force microscopy (AFM) were used for basic characterization of the grown monolayers.

The study reveals a giant persistent photoconductivity (GPPC) effect in monolayer MoS₂ FETs after UV irradiation (λ = 365 nm). The conductivity increases by a factor of up to 10⁷, persisting for approximately 30 days at room temperature. The decay of the photocurrent follows a bi-exponential function, with time constants of about 1 day and 34 days, respectively. Low-temperature (6 K) measurements reveal a variable-range hopping transport regime, indicating strong carrier localization. A theoretical model incorporating spatial fluctuations in the potential energy landscape successfully describes the experimental data, extracting parameters such as the amplitude and correlation radius of the potential fluctuations. STS measurements confirm the presence of spatially inhomogeneous localized states within the bandgap, consistent with the theoretical model's predictions. HRTEM analysis shows a high density of sulfur vacancies in ML-MoS₂ (0.79 vacancies/nm²), with a smaller density in ML-WS₂ (0.49 vacancies/nm²). The observed GPPC is significantly weaker in WS₂ FETs, with a time constant of only ~6 hours. UV irradiation leads to a substantial quenching of the photoluminescence (PL) emission in ML-MoS₂, further supporting the trapping of photogenerated carriers in localized states.

The findings strongly support the hypothesis that the GPPC effect in ML-MoS₂ originates predominantly from intrinsic lattice defects, primarily sulfur vacancies, which create a large number of localized states in the bandgap. These localized states act as traps for photogenerated carriers, significantly extending their recombination time and leading to the observed long-lasting enhancement of conductivity. The difference in GPPC behavior between MoS₂ and WS₂ further emphasizes the role of intrinsic defects, as the lower vacancy density in WS₂ correlates with a much shorter PPC time constant. The theoretical model's success in reproducing the experimental transport data confirms the importance of spatial potential energy fluctuations arising from these defects. The observed quenching of PL after UV irradiation provides additional evidence for the trapping of charge carriers in the localized states introduced by the defects. This research demonstrates that the control of intrinsic defects is crucial for manipulating the optoelectronic properties of TMDs, providing a pathway to design high-performance devices.

This work demonstrates an exceptionally long-lived GPPC effect in monolayer MoS₂ FETs, lasting for about 30 days at room temperature. The observed GPPC is shown to be primarily an intrinsic effect stemming from the presence of lattice defects, particularly sulfur vacancies, which introduce a large number of localized states in the bandgap, hindering charge carrier recombination. The findings highlight the crucial role of intrinsic defects in determining the optoelectronic properties of TMDs and offer opportunities for defect engineering to optimize device performance. Future studies could investigate the impact of other types of defects and the potential for controlled defect introduction to further enhance the GPPC effect for applications in memory devices, photodetectors, and other optoelectronic technologies.

The study primarily focuses on CVD-grown ML-MoS₂, and the generalizability of the findings to other growth methods or TMD materials requires further investigation. The theoretical model, while successfully describing the experimental data, employs approximations, such as the simplified density of states. The precise identification and quantification of different types of defects remains challenging. The influence of other external factors, such as adsorbed molecules, on the GPPC effect might not be completely excluded although the study minimizes these factors with high vacuum conditions. Future studies could use more sophisticated theoretical models and techniques to address these limitations and refine the understanding of the GPPC effect.