| Pacific Rim Symposium on Surfaces, Coatings and Interfaces (PacSurf 2016) | |
| Thin Films | Tuesday Sessions |
| Session TF-TuE |
| Session: | Growth & Characterization of 2D Materials |
| Presenter: | Shaul Aloni, Lawrence Berkeley National Laboratory, , USA |
| Authors: | S. Aloni, Lawrence Berkeley National Laboratory, , USA C.T. Chen, Lawrence Berkeley National Laboratory, USA T.R. Kuykendall, Lawrence Berkeley National Laboratory, USA C. Kastl, Lawrence Berkeley National Laboratory, USA T.P. Darlington, Lawrence Berkeley National Laboratory, USA P.J. Schuck, Lawrence Berkeley National Laboratory, USA N.J. Borys, Lawrence Berkeley National Laboratory, USA A.M. Schwartzberg, Lawrence Berkeley Lab, USA |
| Correspondent: | Click to Email |
The promising properties of transition metal chalcogenides (TMD’s) continue to inspire great deal of research on optical and electronic devices. However, the progress in this field is limited by challenges in materials synthesis and device fabrication. In this work we present a new approach for the synthesis of TMD’s with digital control of layer thickness. This method utilises chemical transformation of solid thin films of oxides deposited with submonolayer precision by ALD. Following their deposition the films are exposed to a chalcogen containing gas resulting in smooth and continuous TMD films whose thickness is defined by the thickness of the ALD deposited oxide film.
Typical experiments involve deposition of metal oxide, WO3 or MoO3, followed by a short conversion procedure involving annealing of the oxide film in presence of a chalcogenation agent, e.g. hydrogen disulfide gas or organochalcogen vapor. Typical composition of the gas phase is equivalent to 1% of H2S in argon. However, precise control of water content in the gas phase composition provides means for controlling the reaction mechanism. At low water vapor content (2-10 ppm) metal oxide films are chalcogenized in place with the thickness of the continuous TMD film defined by the thickness of the oxide layer. The resulting WS2 thin films are nanocrystalline, and moderately luminescent . At higher water concentrations (> 200 ppm) the process is dominated by vapor transport. Under these conditions, the volatility of the oxide species is significantly enhanced, resulting in minimal residual metal disulfide after growth. Under optimized conditions, highly luminescent, triangular monolayer WS2 and MoS2 islands with good island-to-island uniformity can be grown directly on the previously metal-oxide-coated substrate or onto a bare substrate placed downstream of a source. Carefully controlled humidity (~ 100 ppm) consistently produces high quality highly luminescent triangular WS2 and MoS2 islands.
The chemical transformation of solid films by a gas phase precursors offers additional benefits. In addition to precise control of thickness and compatibility with many transition metals, it is also compatible with any substrate that is not adversely affected by the chalcogenation agent. We present deposition of WS2 on variety of substrates including amorphous SiO2 and Si3N4 as well as SiC, TiO2 and GaN. Moreover, we suggest that the use of controlled amounts of water vapor is a new knob by which to tune growth of these materials, and these results demonstrate a route to improved material quality and unprecedented reproducibility of chemical vapor transport of many transition metal dichalcogenides.