Japanese Team Makes 2D MoSe2 Phototransistors
A promising material for ultrathin photodetectors
2D layered materials are attracting a lot of interest due to their unique optoelectronic properties at atomic thicknesses. Among them, graphene has been mostly investigated, but its zero-gap limits practical applications. However, 2D layered materials with intrinsic band gaps such as MoS2, MoSe2, and MoTe2 are proving to be promising candidates for ultrathin and high-performance optoelectronic devices.
Recently Pil Ju Ko and colleagues at Toyohashi University of Technology, Japan have fabricated back-gated field-effect phototransistors made of MoSe2 crystals having a thickness of only 20nm. The research was published in the journal Nanotechnology.
The devices (the schematic of which is shown above) were made by mechanical cleavage of MoSe2 crystals into few-layered flakes, followed by transfer onto a silicon wafer with pre-deposited titanium electrodes.
Despite their ultra-thin physical size, the devices showed excellent field-effect phototransistor characteristics, according to the researchers. The measured photoresponsivity of 97.1 AW-1 at zero back gate voltage was higher than previous reports of photodetectors fabricated using GaS, GaSe, MoS2, and InSe.
The photoresponse of the MoSe2 was much faster (less than 15 msec) than ultrasensitive photodetectors based on monolayer MoS2. Furthermore, the theoretical external quantum efficiency was 280-fold higher than of commercial Si and InGaAs photodiodes.
The main graph above shows the laser power dependence of the drain current versus the drain-source voltage at zero gate voltage. Inset is the photoresponsivity extracted from the Id-Vds characteristic.
Having found that MoSe2 is a promising material for photodetector applications, the group is now optimising the device performance by studying the thickness-dependence of the photosensitivity.
'High photosensitivity few-layered MoSe2 back-gated field-effect phototransistors' by Abdelkader Abderrahmane et al, was published in Nanotechnology 25 365202 (1-5) (2014).