High Photoresponsivity and Extrinsic Quantum Efficiency in Tri-Layer Tungsten Diselenide Phototransistors

We report on the photoresponse properties of three-layer tungsten diselenide field-effect transistors (FETs) fabricated by mechanical exfoliation of bulk crystals on SiO2/Si substrates connected with Au/Ti contacts. These devices exhibit two-terminal field-effect hole mobilities of about 350 cm2/Vs at 300K increasing up to 500 cm2/Vs at T\textless 50 K. The photoresponse properties were carried out at room temperature using various excitation wavelengths (405nm, 532nm, 670nm). In addition, we mapped photocurrent of these device with a laser spot size considerably smaller than the area of the conducting channel which indicate the photo-electrical response is not from the area surrounding the electrical contacts. It was found that the three-layer WSe2 FETs display a strong photocurrent response leading up to 0.5 A/W photoresponsivities and high external quantum efficiencies of up to $\sim$ 90{\%}. Also, these transistors display fast photoresponsive transient times of several ms.