WS2 p-MOSFETs with channels deposited by plasma-enhanced atomic-layer deposition

We report the demonstration of p-channel WS2 metal oxide field effect transistors (MOSFETs) using channels deposited by plasma-enhanced atomic layer deposition (PE-ALD). Substrate-gated devices using PE-ALD WS2 films deposited at 300 °C were fabricated with source-to-drain spacing, LDS, ranging from 0.1 to 1.1 µm. Despite being undoped, both 3.4-nm and 2.1-nm-thick films displayed p-type conduction. Substrate-gated devices with LDS = 0.1 µm had on-state current, Ion, of 0.86 µA/µm (0.40 µA/µm) at a drain voltage of VD = −1 V for a channel thickness of 3.4 nm (2.1 nm). It was found that the thinner films displayed improved on/off current ratio, Ion/Ioff, and MOSFETs with 2.1-nm-thick channels had Ion/Ioff = 16.7 compared to only 6.8 for 3.4-nm-thick channels. Devices with 1.4-nm-thick WS2 channels were also fabricated by incorporating a 1.6-nm capping layer of NbxW1−xSy, where NbxW1−xSy provided a heavily doped layer underneath the metal contact and was oxidized in the region between source and drain contacts using low-temperature O2 annealing. These devices had Ion/Ioff as high as 28 at room temperature, a substantial improvement to the WS2-only devices. Measurement at 77 K showed Ion/Ioff values as high as 260, and extraction of the contact barrier height using temperature-dependent measurements showed that a small energy barrier still exists between the NbxW1−xSy and the WS2.