Electrical transport and persistent photoconductivity in monolayer MoS2 phototransistors

Abstract: We study electrical transport properties in exfoliated molybdenum disulfide (MoS2) back-gated field effect transistors at low drain bias and under different illumination intensities. It is found that photoconductive and photogating effect as well as space charge limited conduction can simultaneously occur. We point out that the photoconductivity increases logarithmically with the light intensity and can persist with a decay time longer than 10^4 s, due to photo-charge trapping at the MoS2/SiO2 interface and in MoS2 defects. The transfer characteristics present hysteresis that is enhanced by illumination. At low drain bias, the devices feature low contact resistance of 1.4 k{\Omega}/{\mu}m, ON current as high as 1.25 nA/{\mu}m, 10^5 ON-OFF ratio, mobility of 1 cm^2/Vs and photoresponsivity R=1 A/W.

Electrical Transport and Persistent Photoconductivity in Monolayer MoS₂ Phototransistors

The paper "Electrical transport and persistent photoconductivity in monolayer MoS₂ phototransistors" presents a comprehensive investigation into the electrical and optoelectronic properties of field-effect transistors (FETs) composed of exfoliated molybdenum disulfide (MoS₂). The study focuses on the conductivity mechanisms and persistent photoconductivity observed in these devices, with particular emphasis on charge trapping phenomena.

Monolayer MoS₂ has gained significant interest as a promising two-dimensional material for optoelectronic applications due to its unique properties, such as a direct bandgap of approximately 1.8 to 1.9 eV. This feature offers advantages over gapless graphene, making it suitable for logic applications with an ON/OFF ratio exceeding 10. However, the challenge has been its relatively low mobility, which ranges from 0.01 to 50 cm²/Vs on SiO₂ substrates at room temperature.

The fabricated back-gated MoS₂ transistors exhibit n-type behavior characterized by a low contact resistance of 1.4 kΩ/μm, an ON current of 1.25 nA/μm, and a mobility of approximately 1 cm²/Vs, alongside a photoresponsivity of ~1 AW⁻¹. Crucially, the study reveals the persistent photoconductivity effect wherein the conductivity enhancement due to illumination persists even after the light source is removed. The decay time exceeds 10 seconds, attributed to the trapping of photo-generated charges at the MoS₂/SiO₂ interface and within MoS₂ defects.

The study methodically dissects the optoelectronic properties of these transistors through illumination experiments under varying light intensities. Two mechanisms are identified: photogating and photoconductive effects. The photogating effect changes the threshold voltage of the transistor, resulting in a shift caused by photocharge trapping. The photoconductive effect involves direct generation of electron-hole pairs, enhancing conductivity. Notably, the photogating effect is more dominant, particularly at lower gate voltages, while the photoconductive effect becomes significant at higher voltages.

The paper further elucidates on the transient characteristics under intermittent illumination, highlighting an abrupt increase in current upon light exposure followed by a slower rise. This behavior suggests that trapped charges gradually increase channel conductivity even after illumination ceases, leading to persistent photoconductivity.

Through the analysis, the paper identifies charge trapping at the MoS₂/SiO₂ interface as the major mechanism contributing to persistent photoconductivity. This trapping enhances the photoconductive and photogating effects and impacts overall device performance. The study underscores the complexity of charge trapping in MoS₂ devices, offering insights into the underlying physical mechanisms and opening discussions on resolving the slow photoresponse characteristic.

Future explorations in this domain could leverage these findings to improve MoS₂ device performance, potentially by altering fabrication techniques or incorporating passivation strategies to mitigate charge trapping. These advancements could bolster the application of MoS₂ in next-generation optoelectronic devices, enriching the theoretical understanding and facilitating practical innovations in semiconductor technologies.