A novel crystalline form of silicon could potentially be used to make next generation electronics and energy devices
Washington, DC – A team led by Thomas Shiell and Timothy Strobel of Carnegie developed a new method to synthesize a novel crystalline form of silicon with a hexagonal structure that could potentially be used to make next generation electronic and power devices with improved properties that exceed the “normal” cubic form of silicon used today.
Your work is published in Physical review letters.
Silicon plays an outsized role in human life. It is the second most abundant element in the earth’s crust. In combination with other elements, it is indispensable for many construction and infrastructure projects. And in pure elementary form, it is crucial enough for computer science that the long-standing technology center of the USA – California’s Silicon Valley – was given the nickname in honor.
Like all elements, silicon can take various crystalline forms called allotropes, just as soft graphite and super hard diamond are both forms of carbon. The form of silicon, most commonly used in electronic devices, including computers and solar panels, has the same structure as diamond. Despite its ubiquity, this form of silicon is not fully optimized for next generation applications, including high power transistors and some photovoltaic devices.
While many different silicon allotropes with improved physical properties are theoretically possible, only a few exist in practice because no known synthetic routes are currently available.
Strobel’s laboratory had previously developed a revolutionary new form of silicon called Si24, which has an open framework made up of a series of one-dimensional channels. In this new work, Shiell and Strobel led a team that used Si24 as the starting point for a multi-step synthetic route that resulted in highly oriented crystals in a shape called 4H silicon, named for its four repeating layers in a hexagonal structure.
“Interest in hexagonal silicon dates back to the 1960s because of the possibility of tunable electronic properties that could improve performance beyond the cubic shape,” explained Strobel.
Hexagonal forms of silicon were synthesized earlier, but only through the deposition of thin films or as nanocrystals that coexist with disordered material. The newly demonstrated Si24 path produces the first high-quality bulk crystals that serve as the basis for future research activities.
Using the advanced computer tool PALLAS, previously developed by members of the team to predict structural transition pathways – such as how water turns into steam when heated or ice when frozen – the group was able to understand the transition mechanism from Si24 to 4H-Si. understand, and the structural relationship that enables the maintenance of highly oriented product crystals.
“In addition to expanding our fundamental control over the synthesis of novel structures, the discovery of massive 4H silicon crystals opens the door to exciting future research perspectives on the coordination of optical and electronic properties through strain engineering and elemental substitution,” said Shiell. “We could potentially use this method to create seed crystals to grow large quantities of the 4H structure with properties that may well exceed those of diamond silicon.”
Carnegie’s Li Zhu was also on the research team, along with Brenton Cook and Dougal McCulloch from RMIT University and Jodie Bradby from Australian National University.
This work was supported by the National Science Foundation, Division of Materials Research.
Parts of this work were carried out at HPCAT (Sector 16), Advanced Photon Source (APS), Argonne National Laboratory. The HPCAT operation is supported by the Office of Experimental Sciences of the DOE-NNSA. The Advanced Photon Source is a user facility of the US Department of Energy’s Office of Science (DOE) operated by Argonne National Laboratory for the DOE Office of Science.