Giant persistent photoconductivity in monolayer MoS₂ field-effect transistors

The study investigates whether persistent photoconductivity (PPC) observed in monolayer TMDs, particularly MoS2, can originate predominantly from intrinsic material properties rather than extrinsic effects (e.g., substrate traps or adsorbates). Prior reports showed PPC with time constants of 10^2–10^4 s in ML-MoS2 under visible light and ~10^6 s in few-layer MoS2 under UV, often attributed to extrinsic charge traps. The authors hypothesize that intrinsic lattice defects in ML-MoS2 create significant spatial fluctuations of carrier potential energy, leading to spatial separation of photo-generated electrons and holes and, hence, dramatically prolonged recombination times. The purpose is to demonstrate an extremely long-lived giant PPC (GPPC) in ML-MoS2 FETs under UV (365 nm) and to elucidate its intrinsic origin through transport measurements, modeling, and atomic-scale spectroscopy/microscopy, establishing its significance for TMD optoelectronics and defect engineering.

PPC has been widely studied in amorphous and highly compensated wide-bandgap semiconductors, explained by large spatial potential fluctuations for carriers (Refs. 1–3, 12–14). In ML-MoS2, PPC with τ ~ 10^2–10^4 s under visible light at room temperature and up to ~10^6 s in few-layer MoS2 under 254 nm UV has been reported (Refs. 4–8). These earlier works generally attributed PPC to extrinsic effects: substrate charge traps, surface adsorbates, and photogating (Refs. 4–6, 8–9). There is also substantial literature on disorder-induced transport in MoS2 (variable-range hopping, mobility edge concepts; Refs. 12–20) and on intrinsic defect states (S and S2 vacancies) and strain in CVD-grown ML-MoS2 affecting electronic structure (Refs. 21–31). The present work builds on these foundations to argue for an intrinsic, defect-dominated GPPC mechanism in ML-MoS2.

• Material growth: ML-MoS2 and ML-WS2 crystals were synthesized by CVD on Si/SiO2 (300 nm oxide) using a two-zone furnace with sulfur and metal oxides (MoO3 or WO3+NaCl) precursors. Growth temperatures: 770 °C (MoS2) and 860 °C (WS2), argon flow 100 sccm with 10 sccm H2 during growth; rapid cooldown to room temperature.
• Device fabrication: Monolayers were PMMA-assisted transferred to heavily p-doped Si/SiO2 (300 nm) device substrates. E-beam lithography defined Au/Ti (30/5 nm) source-drain electrodes. Back-gate was the Si substrate; SiO2 served as gate dielectric.
• Electrical measurements: Transfer characteristics (IDS–VG) measured in vacuum (~10^-6 to 10^-1 mbar) and dark at room temperature and 6 K using Keithley SMUs and a cryogenic probe station. UV irradiation via 365 nm LED (up to ~30 mW cm^-2) for 5 min; additional LEDs at 455 and 617 nm were available. Mobility extracted from the linear regime of transfer curves.
• Modeling: A disorder potential model with large spatial fluctuations U(r)=U0 f(r/rcorr) was applied. At RT: thermal activation over a percolation (mobility edge) energy EP with σRT=σ00 exp(-EP/kBT). Electron population determined from equilibrium and photo-generated carriers via density of states of localized gap states g(E) approximated by an exponential tail. At low temperature: 2D Mott variable-range hopping σ(T)=σ0 exp[-(T0/T)^{1/3}]. Fits to pristine and irradiated transfer curves yielded U0, n0, rcorr, and photoelectron density evolution.
• STS/STM: ML-MoS2 transferred onto monolayer h-BN grown on Pt(111). dI/dV spectra recorded at 1.1 K on a 50×50 nm^2 grid to map local density of states and visualize band-edge/trap state fluctuations.
• HRTEM: 60 kV Cc/Cs-corrected TEM imaged intrinsic defects; Fourier-filtering assisted vacancy counting and contrast analysis to distinguish S2 vacancies. Clean regions were analyzed to avoid contamination and beam damage artifacts.
• PL mapping: Confocal PL at 532 nm excitation collected 650–720 nm emission before and after UV irradiation to assess optical changes associated with trap-state population.
• Controls: Raman before/after UV showed no damage; vacuum annealing at 170 °C accelerated PPC decay; reproducibility across >10 devices; comparative WS2-FETs examined.

• Giant persistent photoconductivity (GPPC) in ML-MoS2 FETs after 365 nm UV exposure (≈30 mW cm^-2, 5 min): conductivity enhancement up to ~10^7 at RT; immediate ΔIDS/IDS ≈ 10^2 at VG ≈ 40 V near threshold.
• Persistence and kinetics: Bi-exponential decay of persistent photocurrent at VG=0 V with τ1 ≈ 1 day and τ2 = 34 days; overall GPPC persists for ~30 days at RT; full recovery to pristine state over months; vacuum annealing (170 °C) accelerates decay.
• Temperature dependence: Enhanced IDS with increasing UV dose at both RT and 6 K; lower absolute IDS at 6 K indicative of variable-range hopping transport.
• Transport model parameters (from fits): Disorder amplitude U0 ≈ 0.18 eV; maximum CB electron density n0 ≈ 2.7 × 10^18 m^-2; correlation radius rcorr ≈ 5 nm (from 2D Mott VRH analysis). STS maps show trap-state patches of 5–10 nm with energy variation ~0.25 eV near the CB edge, consistent with U0 and rcorr.
• Defect quantification (HRTEM): Total vacancy concentration in ML-MoS2 ≈ 0.79(6) vacancies/nm^2; S2 (double sulfur) vacancies ≈ 0.067(2) vacancies/nm^2 (~8.5% of total). For ML-WS2: total ≈ 0.49(9) vacancies/nm^2; S2 ≈ 0.022(5) vacancies/nm^2 (≈3× lower relative to MoS2).
• Strain: Raman indicates biaxial strain in CVD ML-MoS2 of 0.34 ± 0.08% vs. exfoliated reference; likely contributes to faster decay component (τ1) along with photogating.
• Comparative WS2-FETs: PPC significantly weaker with τ ~ 6 h; largely influenced by extrinsic factors (adsorbates, substrate interaction, photogating) rather than intrinsic defects.
• Optical response: PL intensity of ML-MoS2 significantly quenched after UV irradiation, consistent with carrier trapping in deep states below the CB.
• Device baseline: Pristine ML-MoS2 field-effect mobility ≈ 1.5 cm^2 V^-1 s^-1 (typical for CVD ML-MoS2).

The results support an intrinsic origin of GPPC in ML-MoS2 arising from large spatial fluctuations in the carrier potential energy landscape due to a high density of intrinsic defects (primarily S and S2 vacancies) and, to a lesser extent, strain. These fluctuations create localized trap states forming nanoscale patches, which spatially separate photo-generated electrons and holes (electrons accumulating in potential minima near the CB, holes in maxima near the VB), suppressing recombination and yielding extremely long time constants. Transport manifests as thermally activated conduction over a mobility edge at RT and 2D variable-range hopping at low temperature; quantitative agreement between model and measurements yields U0 and rcorr values that match STS-observed energy and length scales. The bi-exponential decay arises from a distribution of electron–hole separations and possibly a minor photogating contribution, with the longer τ2 reflecting deeper traps associated with vacancies. Control experiments and the weaker, short-lived PPC in WS2 with lower vacancy densities corroborate the defect-driven mechanism in MoS2. Annealing-enhanced decay further aligns with thermally assisted recombination. These insights clarify that intrinsic defect engineering is pivotal in tuning optoelectronic responses, challenging the notion that PPC in ML-MoS2 is primarily extrinsic.

This work demonstrates an extremely long-lived giant persistent photoconductivity in CVD-grown ML-MoS2 FETs induced by 365 nm UV irradiation, with a slow decay component of ~34 days at room temperature and conductivity enhancements up to ~10^7. A disorder-based transport model, supported by STS and HRTEM, attributes GPPC mainly to intrinsic defects that generate significant spatial fluctuations in the electronic potential, causing spatial separation of photo-carriers and extended recombination times. PL quenching after UV exposure further indicates deep trap-state occupation. The comparative WS2 study, exhibiting fewer defects and weaker, short-lived PPC, reinforces the intrinsic-defect origin in MoS2. These findings highlight the central role of atomic defects in ML-TMD optoelectronics and point to defect engineering as a route to tailor electronic and optical device performance. Future work could focus on controlled defect introduction/passivation, correlating specific defect types with PPC dynamics, and leveraging GPPC for non-volatile optoelectronic memory and sensing applications.

• While extrinsic effects (adsorbates, substrate/interface charges, photogating) are argued to play a secondary role in ML-MoS2, they may still contribute to the faster decay component and cannot be completely excluded.
• The disorder model uses simplified assumptions (e.g., exponential density-of-states tail, effective parameters U0 and rcorr); extracted values represent effective averages over inhomogeneous samples.
• STS measurements were performed on MoS2 transferred to hBN/Pt(111), which differs from the SiO2 device substrate; substrate interactions may alter local electronic structure.
• PPC kinetics and magnitudes were measured under specific illumination (365 nm, defined doses) and environmental conditions (vacuum, dark); behavior may differ under ambient conditions or different wavelengths/powers.
• Defect identification via HRTEM quantifies vacancy densities but does not assign all defect species or their charge states; beam effects were mitigated but cannot be entirely ruled out across all regions.