Layer dependent photoresponse behavior of chemical vapor deposition synthesized MoS2 films for broadband optical sensing

Abstract

The present study reports the fabrication of MoS2 based optical sensors for tunable, broadband and wavelength selective light detection with single layer and multilayer MoS2 samples. The I–V measurements are performed in a two-terminal configuration with bias voltage from −1 to +1 V and I–t at +1 V for light wavelengths ranging from UV (300–450 nm), visible (500–670 nm), to near infrared (700–1100 nm). A sigmoidal I–V behavior is observed in dark and for optically generated current in single layer and multilayer MoS2 samples. The photoconductivity is studied as a function of different number of layers of MoS2, namely, single layer (1L) and multilayer (seven layers: 7L) on SiO2/Si as the growth substrates. The high value of photoresponse (403 µA mW−1) and responsivity (0.92 A W−1) at 1000 nm exhibited by the 7L MoS2 sample makes it a suitable candidate for highly selective photodetector applications. The quantum efficiency also shows a high value of 120% for 7L sample at 900 nm. UV–visible absorption spectra collected for few layer MoS2 samples grown on sapphire and quartz substrates gives an insight into the presence of van Hove singularities, thereby, leading to high photocurrent in the UV range. An interesting phenomenon of broadband selection of 1L and specific wavelength response of 7L MoS2 samples is observed. Various photocurrent generation mechanisms are seen to be prevalent, namely, due to the inherent van Hove singularities in the band structure, band edge excitation of MoS2, trap assisted and IR generated photocurrent. Hence, based on the selection of number of layers of the active material (2D MoS2), a tunable response ranging from broadband in 1L to near infrared (upto 1000 nm) wavelength selective in 7L is possible, paving the way towards a multifunctional optical sensor.