报告人：Kazuhito Tsukagoshi 研究员（National Institute for Materials Science, Japan）
报告题目：Self-assembled hetero-structure based on two-dimensional transition metal dichalcogenides
Growth of a uniform oxide ﬁlm with a tunable thickness on two-dimensional transition metal dichalcogenides is of great importance for electronic and optoelectronic applications in next generation atomically-controlled hetero semiconducting structure. Here we demonstrate homogeneous surface oxidation of atomically thin WSe2 with a self-limiting thickness from single- to trilayers. Exposure to ozone (O3) below 100 °C leads to the lateral growth of tungsten oxide selectively along selenium zigzag-edge orientations on WSe2. With further O3 exposure, the oxide regions coalesce and oxidation terminates leaving a uniform thickness oxide ﬁlm on top of unoxidized WSe2. At higher temperatures, oxidation evolves in the layer-by-layer regime up to trilayers. The oxide ﬁlms formed on WSe2 are nearly atomically ﬂat. Using photoluminescence and Raman spectroscopy, we ﬁnd that the underlying single-layer WSe2 is decoupled from the top oxide but hole-doped. The hole-doping by the under-stoichiometric tungsten oxides (WOx with x < 3) grown on WSe2 can be used as both controlled charge transfer dopants and low-barrier contacts for p-type WSe2 transistors. WOx-covered WSe2 is highly hole-doped due to surface electron transfer from the underlying WSe2 to the high electron affinity WOx. The dopant concentration can be reduced by suppressing the electron affinity of WOx by air exposure, but exposure to O3 at room temperature leads to the recovery of the electron affinity. Hence, surface transfer doping with WOx is virtually controllable. Transistors based on WSe2 covered with WOx show only p-type conductions with orders of magnitude better on-current, on-off current ratio, and carrier mobility than without WOx, suggesting that the surface WOx serves as a p-type contact with a low hole Schottky barrier. Our findings point to a simple and effective strategy for creating p-type devices based on two-dimensional transition metal dichalcogenides with controlled dopant concentrations.
1. Self-limiting surface oxidation of atomically thin WSe2, M.Yamamoto, S.Dutta, K. Wakabayashi, M. S. Fuhrer, K.Ueno, K.Tsukagoshi, Nano Letters 15, 2067–2073 (2015).
2. Surface Oxides on Single- and Few-layer WSe2 as Controlled Dopants and Low-Barrier Contacts, M.Yamamoto, S.Nakaharai, K.Ueno, K.Tsukagoshi, Nano Letters,16, 2720–2727 (2016).
Dr. Kazuhito Tsukagoshi, Principal Investigator of National Institute for Materials Science (NIMS). He studied experimental research on transport physics in semiconductor microstructure, completing his PhD in 1995. After that he worked as a visiting associate in Cavendish laboratory (University of Cambridge, U.K.) and then in Hitachi Cambridge Laboratory (Hitachi Europe Ltd, U.K.). In 1999, he joined RIKEN (Japan) where he carried out research on carbon nanotube and organic electronics. He continued this research in AIST in 2008, and moved to WPI-MANA, NIMS in 2009. His current research focuses on ultra-thin functional devices to realize the next generation electronics. He was awarded The MEXT Young Scientists' Prize (2006) and JSPS prize (2013).