Sapphire substrate controls crystal growth over large areas

Inspired by the remarkable properties of graphene, scientists have increasingly focused on trying to discover other two-dimensional (2D) materials, both those found in nature and those concocted in the lab. However, growing high-quality, crystalline 2D materials at scale has proven a significant challenge.

Now, a team of researchers from Penn State and Rensselaer Polytechnic Institute has developed a multistep process for making single crystal, atomically thin films of tungsten diselenide across large-area sapphire substrates. The team was led by Joan Redwing, director of the Two-Dimensional Crystal Consortium – Materials Innovation Platform, and professor of materials science and engineering and electrical engineering at Penn State.

"Up until now, the majority of 2D devices have been fabricated using small flakes that are exfoliated off of bulk crystals," Redwing said. "To develop a device-ready technology, you have to be able to make devices on large-area substrates and they have to have good crystal quality."

The novel process is a form of epitaxy that uses sapphire as the substrate because of its crystalline structure, which orients the growth of the tungsten diselenide film such that it forms a specific crystal pattern. Initially, small islands of tungsten diselenide form on the substrate. When the substrate is heated, these islands spread out across the substrate in a uniform pattern, forming a large-area film without gaps and with very few defects. The key advance in this process is the use of gas-source chemical vapor deposition to precisely control the island density and the rate of spreading to achieve a single layer of the 2D material.

Redwing and her team report this work in a paper in Nano Letters. In a related paper in ACS Nano, a team led by Joshua Robinson, associate professor of materials science and engineering at Penn State, provides the foundational understanding for the fabrication of device-ready synthetic 2D semiconductors based on the epitaxial large-area films in future industrial-scale electronics.

"The primary significance of this work is we were able to achieve an understanding of the extrinsic factors that go into having a high-quality 2D material," Robinson said. "What we found was that even when you grow oriented crystals on a surface, there are other factors that impact the ability to get high electron mobility or fast transistors."

In particular, they found a strong interaction between the sapphire substrate and the monolayer film, with the substrate dominating the film’s properties. To overcome these challenges, the researchers grew two or three layers, which improved electron mobility in the film by factors of between 20 and 100. "This is the first real evidence of the effect of the substrate on the transport properties of 2D layers," Robinson said.

This story is adapted from material from Penn State, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.