Dependence of Photoresponsivity and On/Off Ratio on Quantum Dot Density in Quantum Dot Sensitized MoS2 Photodetector.

Yung-Yu Lai,An-Jye Tzou,Yi-Yuan Chen,Yen-Wei Yeh,Yuh-Jen Cheng,Yewchung Sermon Wu,Hao-Chung Kuo

doi:10.3390/nano10091828

Abstract

Non-radiative energy transfer (NRET) from quantum dots (QDs) to monolayer MoS2 has been shown to greatly enhance the photoresponsivity of the MoS2 photodetector, lifting the limitations imposed by monolayer absorption thickness. Studies were often performed on a photodetector with a channel length of only a few μm and an active area of a few μm2. Here, we demonstrate a QD sensitized monolayer MoS2 photodetector with a large channel length of 40 μm and an active area of 0.13 mm2. The QD sensitizing coating greatly enhances photoresponsivity by 14-fold at 1.3 μW illumination power, as compared with a plain monolayer MoS2 photodetector without QD coating. The photoresponsivity enhancement increases as QD coating density increases. However, QD coating also causes dark current to increase due to charge doping from QD on MoS2. At low QD density, the increase of photocurrent is much larger than the increase of dark current, resulting in a significant enhancement of the signal on/off ratio. As QD density increases, the increase of photocurrent becomes slower than the increase of dark current. As a result, photoresponsivity increases, but the on/off ratio decreases. This inverse dependence on QD density is an important factor to consider in the QD sensitized photodetector design.

Highlights

Applications of two-dimensional (2D) semiconductor materials in optoelectronics have attracted great research interest due to their unique atomically thin profile, mechanical flexibility, and potential high electron mobility and gain [1,2,3,4,5]
The lack of bandgap in graphene leads to high dark currents, and a low on/off ratio, which limits its applications for active semiconducting channels in optoelectronic devices
Monolayer MoS2, in contrast, has a direct bandgap with a large absorption coefficient, but the monolayer thickness limits the absorption of incident light

Summary

Introduction

Applications of two-dimensional (2D) semiconductor materials in optoelectronics have attracted great research interest due to their unique atomically thin profile, mechanical flexibility, and potential high electron mobility and gain [1,2,3,4,5]. The lack of bandgap in graphene leads to high dark currents, and a low on/off ratio, which limits its applications for active semiconducting channels in optoelectronic devices. Multilayer MoS2 has a greater thickness to absorb incident light, but the indirect bandgap property leads to a lower absorption coefficient, which compromises overall photo response. Monolayer MoS2, in contrast, has a direct bandgap with a large absorption coefficient, but the monolayer thickness limits the absorption of incident light

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Results

Conclusion