Hybrid single-layer/bulk tungsten diselenide transistors by lithographic encoding of material thickness in chemical vapor deposition

Schottky-like barriers are an important limitation of the performance of single-layer transition metal dichalcogenide (TMD) transistors; because of the small number of charge carriers in a 2D semiconductor, the screening of metal contacts is inefficient leading to large depletion zones and enhanced reduction of performance compared to conventional bulk Schottky Barriers. Here we demonstrate that lithographic pre-patterning of a growth substrate prior to chemical vapor deposition of a TMD film can shape the TMD material into nanoscale hybrid 2D/3D structures whose bulk-like (3D) portion can be used for metal contacts and efficient charge injection into the single-layer (2D) areas which serve as transistor channels with excellent mobilities and on-off ratios. We observe mobilities of nearly 100 cm2 V−1 s−1 with an on/off ratio >105 for bottom-gated devices (through 300 nm of oxide) at realistic operation temperatures near 100 °C using comparatively long channels (>5 microns) and absent other contact optimization. Our process involves lithographic patterning of a hafnium (IV) dioxide film onto the SiO2/Si substrate prior to TMD growth. Bulk-like 3D WSe2 is observed to grow at the location of the hafnia, while 2D single-layer material is grown in regions of bare SiO2. Systematic evaluation of transport data allows us to extract Schottky barrier heights and other fundamental properties of our hybrid devices.