“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.”Thomas Shiell
New form of silicon advances semiconductor technology
New form of silicon advances semiconductor technology

An innovative approach to synthesizing a novel crystalline form of silicon with a hexagonal structure has been developed by researchers at the Carnegie Institution for Science, RMIT University and the Australian National University. The new form could lead to electronic and energy devices with enhanced properties that improve upon the standard cubic form of silicon currently being used.

With the global drive to advance semiconductor technology for both renewable energy conversion and new electronics, it is hoped enhanced forms of silicon, both allotropes and compounds, will offer more effective optoelectronic properties that compliment and/or exceed those of diamond-cubic(DC)-Si. Although silicon can take different crystalline forms, the standard form used in electronic devices such as computers and solar panels is not fully optimized for these new applications.

New synthetic methods are therefore needed, and here a team led by Thomas Shiell and Timothy Strobel applied novel pressure/temperature processing pathways to access these materials. Strobel’s lab had previously developed a new form of silicon called Si24 with an open framework comprising a series of one-dimensional channels. As reported in Physical Review Letters [Shiell et al. Phys. Rev. Lett. (2021) DOI: 10.1103/PhysRevLett.126.215701], here they used Si24 in a multi-stage synthesis pathway. This allowed highly oriented crystals in a form called 4H-silicon, as hexagonal silicon has the potential for tunable electronic properties that could improve performance beyond the cubic form.

While hexagonal forms of silicon have already been synthesized, this was only achieved by the deposition of thin films or as nanocrystals that coexist with disordered material. However, the newly demonstrated Si24 pathway offers the first high-quality, bulk crystals, while the 4H-Si structure opens up new opportunities for semiconductor devices.

Their findings provide a bulk path to the 4H-Si structure and also show the importance of metastability for discovering new phases beyond DC-Si. The application of anisotropic stress could lead to new direct-gap semiconductors for photovoltaic and transistor devices, while the improved elastic properties could help advance micro-electromechanical systems.

As Thomas Shiell said, “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”. There is also potential for using the approach to develop seed crystals to grow large volumes of the 4H structure with beneficial properties.

The team hope the work will encourage further research to scale-up and produce usable devices, and now plan to perform detailed characterization to gain a better understanding of the fundamental optoelectronic and mechanical properties of the 4H structure.