Hexagonal forms of silicon have been synthesized previously, but only through the deposition of thin films or as nanocrystals that coexist with disordered material. A team led by Carnegie Institution of Science’s Thomas Shiell and Timothy Strobel, and including colleagues from RMIT University (Australia) and The Australian National University, developed a new method for synthesizing a novel crystalline form of silicon, called 4H-silicon, or 4H-Si. This novel 4H-Si form has a hexagonal structure and could potentially be used to create next-generation electronic and energy devices with enhanced properties that exceed those of the “normal” cubic form of silicon used today. This newly reported synthesis pathway produces the first high-quality, bulk crystals that serve as the basis for future research activities. Their work, based in large part on research carried out at the U.S. Department of Energy’s Advanced Photon Source (APS), was 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. When mixed with other elements, it is essential for many construction and infrastructure projects. And in pure elemental form, it is crucial enough to computing that the longstanding technological hub of the U.S.—California’s Silicon Valley—was nicknamed in honor of it.
Like all elements, silicon can take different crystalline forms, called allotropes, in the same way that 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 actually fully optimized for next-generation applications, including high-performance transistors and some photovoltaic devices. While many different silicon allotropes with enhanced physical properties are theoretically possible, only a handful exist in practice given the lack of known synthetic pathways that are currently accessible. Strobel’s lab had previously developed a revolutionary new form of silicon, called Si24, which has an open framework composed of a series of one-dimensional channels. In this new work, Shiell and Strobel’s team used Si24 as the starting point in a multi-stage synthesis pathway that resulted in highly oriented crystals in a form 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, which could enhance performance beyond the cubic form” Strobel explained.
The 4H-Si samples were measured (Fig. 1) using synchrotron x-ray diffraction (XRD) at the HPCAT-XSD 16-ID-B x-ray beamline of the APS, an Office of Science user facility at Argonne National Laboratory. In both cases (XRD and transmission electron microscopy), the high-quality 4H-Si crystals were synthesized using high-pressure laser heating in a diamond anvil cell at HPCAT-XSD, also on the 16-ID-B beamline.
Using the advanced computing tool called PALLAS, which was previously developed by members of the team to predict structural transition pathways—like how water becomes steam when heated or ice when frozen—the group was able to understand the transition mechanism from Si24 to 4H-Si, and the structural relationship that allows the preservation of highly oriented product crystals.
“In addition to expanding our fundamental control over the synthesis of novel structures, the discovery of bulk 4H-silicon crystals opens the door to exciting future research prospects for tuning the optical and electronic properties through strain engineering and elemental substitution,” Shiell said. “We could potentially use this method to create seed crystals to grow large volumes of the 4H structure with properties that potentially exceed those of diamond silicon.”
See: Thomas B. Shiell1*, Li Zhu1, Brenton A. Cook2, Jodie E. Bradby3, Dougal G. McCulloch2, and Timothy A. Strobel1**, “Bulk Crystalline 4H-Silicon through a Metastable Allotropic Transition,” Phys. Rev. Lett. 126, 215701 (2021). DOI: 10.1103/PhysRevLett.126.215701
Author affiliations: 1Carnegie Institution for Science, 2RMIT University, 3The Australian National University
The authors thank Yue Meng and Piotr Guńka for their assistance with experimental measurements. The authors gratefully acknowledge the RMIT Microscopy and Microanalysis Facility. This work was supported by the National Science Foundation, Division of Materials Research (NSF-DMR) under Grant No. 1809756. HPCAT-XSD operations are supported by the U.S. Department of Energy National Nuclear Security Administration Office of Experimental Sciences. The Advanced Photon Source is a U.S. Department of Energy Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.
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