posted on 2021-09-02, 19:09authored byHanjie Yang, Yang Wang, Xingli Zou, Rong-Xu Bai, Sheng Han, Zecheng Wu, Qi Han, Yu Zhang, Hao Zhu, Lin Chen, Xionggang Lu, Qingqing Sun, Jack C. Lee, Edward T. Yu, Deji Akinwande, Li Ji
Transition-metal
dichalcogenides (TMDs) have attracted intense
research interest for a broad range of device applications. Atomic
layer deposition (ALD), a CMOS compatible technique, can enable the
preparation of high-quality TMD films on 8 to 12 in. wafers for large-scale
circuit integration. However, the ALD growth mechanisms are still
not fully understood. In this work, we systematically investigated
the growth mechanisms for WS2 and found them to be strongly
affected by nucleation density and film thickness. Transmission electron
microscope imaging reveals the coexistence and competition of lateral
and vertical growth mechanisms at different growth stages, and the
critical thicknesses for each mechanism are obtained. The in-plane
lateral growth mode dominates when the film thickness remains less
than 5.6 nm (8 layers), while the vertical growth mode dominates when
the thickness is greater than 20 nm. From the resulting understanding
of these growth mechanisms, the conditions for film deposition were
optimized and a maximum grain size of 108 nm was achieved. WS2-based field-effect transistors were fabricated with electron
mobility and on/off current ratio up to 3.21 cm2 V–1 s–1 and 105, respectively.
Particularly, this work proves the capability of synthesis of TMD
films in a wafer scale with excellent controllability of thickness
and morphology, enabling many potential applications other than transistors,
such as nanowire- or nanosheet-based supercapacitors, batteries, sensors,
and catalysis.