| May 29, 2025 |
Scientists uncovered rare hexagonal silicon forms (2H, 4H, 6H) created by stress and heat, challenging assumptions about this key electronic material.
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(Nanowerk Spotlight) Scientists at SRM Institute of Science and Technology, India, IIT Mandi and Sorbonne Université, France have made a striking discovery that could reshape how we think about silicon, the foundational material of modern electronics. In a new study published in Advanced Functional Materials (“Hexagonal Silicon Formation and Its Phase Transformability”)
, the research team led by Dr. Kiran Mangalampalli has found clear evidence of rare hexagonal forms of silicon—known as 2H, 4H, and 6H polytypes—forming under controlled mechanical stress and thermal treatment.
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This is not the first time hexagonal silicon has been observed. But what sets this study apart is the clarity, reproducibility, and scale at which these hexagonal structures have been created and identified. Using a nanoindentation method followed by vacuum annealing, the team converted common diamond cubic silicon (dc-Si) into crystalline domains of hexagonal diamond (hd-Si), revealing stacking patterns that mirror those found in silicon carbide—a material already well-known in high-power electronics.
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| The high-resolution TEM analysis provides evidence of mix-phased hd-Si with 2H- 4H- and 6H-like stacking sequences. This direct experimental observation of multiple hexagonal polytypes (2H, 4H, and 6H) at 500 C temperature strongly supports a structural transformation pathway distinct from those predicted by classical models of silicon phase evolution. (Image: Dr. Abhay A. Sagade)
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From Pressure to Precision
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The researchers applied a diamond-tipped nanoindenter to generate high pressure in tiny regions of silicon wafers. At these pressures, silicon transitions into a metallic phase known as β-tin (Si-II), and upon unloading, it usually stabilizes into metastable forms like bc8 and r8 phases. However, when these deformed regions were annealed at specific temperatures (250°C to 750°C), the team observed a transformation into hd-Si with clear evidence of 2H, 4H, and 6H polytypes.
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“We combined multiple in-situ techniques—Raman spectroscopy, transmission electron microscopy, and nanoelectrical contact resistance measurements—to confirm the presence and behavior of these phases,” said Megha S. Nisha, the study’s first author and a PhD student in Dr. Kiran’s lab.
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These observations validate long-standing theoretical predictions about the structural evolution of silicon under stress and heat. Prior work had hinted at such transformations in ultra-thin films or exotic environments. The Indo-French team’s work stands out because it achieves these transitions in bulk silicon wafers using relatively standard lab equipment.
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The Beauty of Reversibility
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One of the most intriguing aspects of the study is the reversible nature of the transformation. Upon re-applying pressure via the nanoindenter, the hexagonal silicon reverts to the β-Sn phase and subsequently returns to the R8/BC8 mixture when the pressure is released. This looped phase path shows the potential for mechanical control over silicon phases.
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“What we have here is a real-world demonstration of phase engineering in silicon,” noted Dr. Kiran. “We can now think about locally tuning the crystal structure of silicon on a chip to serve different functions.”
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Why It Matters
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Hexagonal silicon isn’t just structurally novel—it has significantly different electronic properties. Unlike diamond cubic silicon, which has an indirect bandgap that limits its light-emitting potential, some hexagonal polytypes could have a direct or quasi-direct bandgap. This opens the door to optoelectronic applications like LEDs, laser diodes, and advanced photonic circuits—all built on silicon, but without its traditional limitations.
Moreover, the mechanical and thermal characteristics of hexagonal silicon are distinct. The study finds that R8/BC8 phases are harder than hd-Si or standard silicon, which could be useful in designing resilient materials for space, defense, and high-stress computing environments.
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A Pathway to Scalable Innovation
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The simplicity of the Indo-French team’s approach is part of its brilliance. Instead of relying on expensive chemical vapor deposition or exotic synthesis routes, they used nanoindentation—a method already widespread in materials labs. “This could democratize access to hexagonal silicon research,” said co-author Dr. Abhay Sagade.
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While hexagonal silicon has been previously produced in nanowires or thin films, this is one of the first demonstrations in bulk-like settings with robust identification of multiple polytypes. These polytypes exhibit large, oriented domains with low defect densities—a prerequisite for real-world application.
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Collaborative Spirit and Future Directions
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The research was supported by the Anusandhan National Research Foundation (ANRF), Govt. of India, with technical collaboration from French researchers. Dr. Kiran emphasized the role of mentorship and collaboration in this success: “I am grateful to my mentors Prof. Jodie Bradby, and Prof. Jim Williams, for guiding me into this fascinating field.”
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Commenting on the international significance of the work, Dr. Alexandre Courac of Sorbonne Université, France, noted: “This study showcases how targeted mechanical, and thermal stimuli can unlock complex phase transformations in elemental materials like silicon. The clarity of polytype identification and reproducibility makes it a valuable contribution to the global semiconductor research landscape.”
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Prof. Viswanath Balakrishnan from IIT Mandi, a collaborator on the study, remarked, “This work is a fine example of how deep structural analysis and collaborative science can lead to tangible advances in silicon-based materials research. It reinforces the value of cross-institutional efforts in addressing longstanding questions.”
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The team plans to explore doping and strain effects in these hexagonal domains, aiming to fine-tune their optical and electronic behavior. If scalable production is achieved, hexagonal silicon could become a key material in next-generation electronics.
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Conclusion: A Crystal-Clear Future
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In a field often dominated by incremental progress, this study brings refreshing clarity and possibility. It takes us one step closer to silicon-based optoelectronics, resilient computing materials, and tunable crystal architectures. And all this from a lab in India with a collaboration from French researchers, demonstrating that world-class innovation can come from anywhere pressure is applied—literally.
As the paper aptly concludes, the discovery “paves the way for future studies on phase engineering and stabilization for advanced semiconductor applications.” Silicon may never look the same again.
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Source: Provided by SRM Research Institute of Science and Technology
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