- The paper identifies drift-diffusion currents as the primary driver of photoresponse in MoS₂ FETs, emphasizing field-assisted carrier separation.
- It employs spectrally and temporally resolved scanning photocurrent microscopy to analyze carrier dynamics across single, triple, and robust 4L MoS₂ devices.
- Findings reveal significant band bending and a ~400 meV Schottky barrier at the Au/MoS₂ interface, offering insights for optimizing 2D optoelectronic device designs.
An Evaluation of Ultrafast MoS Field-Effect Transistor Photoresponse Mechanisms
The paper explores the photoresponse dynamics of ultrathin molybdenum disulfide (MoS₂)-based field-effect transistors (FETs). Utilizing scanning photocurrent microscopy (SPCM), the authors systematically explore the different mechanisms that influence the behavior of photoexcited carriers under various external electric fields and carrier densities. In particular, the research underscores the dominance of drift-diffusion currents in determining the photocurrent characteristics of these MoS₂ devices, especially under applied bias.
The investigation is rooted in the context of transition metal dichalcogenides (TMDCs), which have garnered significant attention owing to their unique optoelectrical properties and high potential for applications in electronic and optoelectronic systems. Previous studies highlight both unipolar and ambipolar charge transport within TMDC-based FETs, as well as impressive on/off ratios and electron mobilities. However, while earlier research often attributed MoS₂ photoresponse to photoconductivity, photovoltaic, or photothermoelectric (PTE) effects, this paper specifically zeroes in on the field-assisted separation of carriers.
Key findings are derived from spectrally and temporally resolved SPCM, focusing on devices with varying MoS₂ thickness. Although the paper includes data on single, triple, and quadruple-layer devices, the analysis primarily emphasizes 4L devices due to their robustness under bias. The responses in these devices, primarily driven by drift and diffusion mechanisms, are noted across high electric field regions, both within the channel and at the MoS₂ and Au contact interfaces. Notably, the photocurrent spectra align with MoS₂'s optical absorption, suggesting that photocurrents primarily result from interband photoexcitation, as opposed to effects like metal contact absorption.
Detailed evaluation of the 4L MoS₂ FETs reveals significant band bending-induced photocurrent near contacts; a phenomenon mirrored in previously documented silicon nanowires, carbon nanotubes, and graphene devices. The reported Schottky barrier (~400 meV) at the Au contact contributes to the upward band bending in the n-type semiconductor, influencing the movement of electrons and holes upon illumination near the contacts.
A fundamental insight from the paper is the dynamic interplay between applied biases and the resulting photocurrent profiles. The researchers identified shifts in photocurrent localization and intensity, particularly in regimes transitioning from linear to saturated states. At elevated drain-source biases, the resulting space-charge region facilitates more effective separation and collection of carriers, thus amplifying the photocurrent. The bias-dependence findings, supported by meticulous carrier lifetime and diffusion length measurements, underscore the preeminence of electric field effects in these photoresponses, even as PTE effects linger as secondary contributors near contact areas.
This research offers a refined understanding of the field-induced photoresponse mechanisms within MoS₂ FETs, producing a nuanced portrayal of carrier dynamics in 2D semiconductors. The mechanistic revelations have significant implications for the design and optimization of MoS₂-based optoelectronic devices. Moreover, the paper elucidates how chemical functionalization and hybrid structures could potentially modulate these phototransistor characteristics, fostering advancements in interface engineering for tailored photoresponses. As the methods and findings presented become foundational in further explorations, future research is poised to extend these principles into novel configurations and emergent materials within the semiconductor domain.
