Two-dimensional (2D) transition metal dichalcogenides (TMDCs) such as molybdenum disulfide (MoS2) and tungsten disulfide (WS2) are very promising candidates for future optoelectronics due to their enhanced light-matter interactions and stability at atomic thicknesses. However, as-fabricated 2D TMDC devices suffer from low induced current, severely limiting their application. Several post-processing treatments have been proposed to address this problem and increase 2D TMDC quality and device performance. We report here a 50x increase in the photocurrent response of photodetector MoS2 devices using a non-destructive 1,2-dichloroethane (DCE) treatment. This treatment significantly improves 2D MoS2 optoelectronic device performance without using contact-corroding acids. In addition, the treatment works well on as-grown CVD MoS2 and does not require any secondary transfers to be effective. These features are here experimentally verified for the first time, to the best of our knowledge, enabling a straightforward process for 50x enhancement in photocurrent generation of large-scale MoS2 optoelectronic devices post-fabrication.
Trilayer MoS2 was grown using a thermal vapor sulfurization (TVS) process developed in previous works. The growth yields centimeter-scale MoS2 with high crystallinity as measured by Raman spectroscopy and photoluminescence. Back-gated field effect transistor (FET) devices with 2 um channel length and 7 um channel width were fabricated using e-beam lithography, and Ti/Au source and drain contacts were deposited using e-beam evaporation. The fabricated devices were soaked in DCE at 60° C for 20 hours. Spectral photocurrent was measured with 6V source-drain bias under illumination from a supercontinuum light source and a laser line tunable filter. Photocurrent was measured before and immediately after the DCE treatment, revealing a 50x photocurrent increase at the 420nm C-peak (from 0.5 nA before treatment to 25 nA immediately after treatment) and a 30x enhancement throughout the rest of the visible spectrum. However, this enhancement degraded over time when exposed to ambient air and subsequent measurements in a regular time interval show a gradual decrease in the peak height until it settles at ~30x enhancement (15 nA) for the C-peak; encapsulation methods are currently under investigation to mitigate this degradation.
The enhanced photocurrent is caused by a combination of reduced contact resistance between the metal contacts and the semiconductor, improved n-dopant concentration in the MoS2 layer, and the passivation of dangling bonds to reduce Shockley-Reed-Hall (SRH) recombination. The interplay of these three components will inform the implementation of this treatment in optoelectronic device applications and as such, it is expected to greatly enhance the performance of 2D TMDC detectors, emitters, and photovoltaics and accelerate their development into practical technologies.