Tungsten Diselenide Transistors: Layer Number Dependence, Channel Passivation and In‐Plane Gates

ABSTRACT High‐performance and few‐layer WSe ₂ field‐effect transistors with p ‐type channels are fabricated through sequential mono‐layer WSe ₂ film transferring. With 30 nm Al ₂ O ₃ passivation layer and polycrystalline gold electrodes, increasing field‐effect mobility values from 16.3 (1‐layer WSe ₂ ), 45.4 (2‐layer WSe ₂ ) to 81.3 (3‐layer WSe ₂ ) cm ² · V ⁻¹ · s ⁻¹ are observed with increasing WSe ₂ layers. To further improve the device performances, tri‐layer WSe ₂ transistors with in‐plane gates (IPGTs) and reduced channel width/length 400/500 nm are fabricated. High field‐effect mobility value 79.7 cm ² · V ⁻¹ · s ⁻¹ and ON/OFF ratio > 10 ⁸ are observed for the device, which demonstrates that the in‐plane gates can more effectively modulate the carrier concentration in the WSe ₂ channel. When the IPGTs are operated as photodetectors, since the Au electrode is deposited on the topmost WSe ₂ layer of the tri‐layer WSe ₂ channel, the topmost WSe ₂ layer may act as the main current flow path. The bottom WSe ₂ layers in the tri‐layer WSe ₂ channel would act as a light absorption layer to the photodetector. The hole accumulation resulted from the photo‐excited electron extraction from the bottom to top WSe ₂ layers would lead to a Fermi level shift of the top WSe ₂ layer. High responsivity 585 A/W and detectivities 10 ¹² Jones are, therefore, observed for the device.