Silicon is at the heart of all modern electronics, the indispensable ingredient in microchips, solar cells, optical devices, digital circuits, and every other device that manipulates electrons with semiconducting principles. Among its many useful and fascinating characteristics are the variety of polymorphic phases it exhibits under different temperatures and pressures, many of which can have useful applications beyond the typical Si-I phase seen at ambient conditions. But because these potentially useful phases generally appear only under very high pressures and temperatures, they remain essentially impractical for applied use.
Researchers led by a team from Iowa State University have discovered an alternative path to changing silicon’s crystal structure under much lower pressures than previously thought necessary through the application of plastic shear strain. Their work appeared in Nature Communications.
Many previous studies have investigated phase transitions (PTs) in silicon under high pressure, and some have even demonstrated the synthesis of different phases, but little if any work has studied plastic strain and shear-induced PTs, especially in situ. For the first time, the present experiments demonstrate silicon PTs induced with various plastic strains at far lower pressures using a rotational diamond anvil cell (RDAC) with in situ synchrotron X-ray diffraction (XRD) studies at the 16-ID-B HPCAT beamline of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science user facility at DOE’s Argonne National Laboratory. The results revealed surprising new phenomena and transformation paths.
The experimenters used silicon with three different particle sizes, ~1 µm, 100 nanometers (nm), and 30 nm. Using a dislocation pileup-based mechanism (DPBM) model, they predicted a correlation between the direct and inverse Hall-Petch effect of particle size on the yield strength and particle size dependence of the phase transformation pressure.
The work confirmed that the plastic strain-induced PTs observed are the result of huge stresses generated at DPBM defects under plastic flow, unlike PTs under hydrostatic pressure which tend to occur at pre-existing defects such as grain boundaries or dislocations. This allows strain-induced PTs to occur at much lower pressures, which opens the possibility of manipulating phase transformation paths for the synthesis of desired polymorphs.
Upon pressure release after shear strain, simpler PT pathways to single-phase Si-III are seen in a short time of several minutes. The researchers were able to design a low-pressure compression-torsion loading path for 100 nm silicon in which a small amount of Si-II remained at ambient pressure, and in another experiment with 30 nm Si, a transition to Si-II PT was seen with Si-II recovered after unloading. This opens the possibility of synthesizing enough Si-II for detailed studies and for its potential applications.
Aside from providing experimental evidence of the DPBM model, the observation that pressure in small Si-II and Si-III regions of micron and 100 nm Si under plastic strain is ~5-7 GPa higher than in Si-I offers some intriguing possible applications. The experiments show that it is possible to effect microstructural changes in silicon and other semiconductor materials at low pressures through carefully designed pathways utilizing the application of various plastic compression and shear loading paths. Such prospects have generally been neglected in the past in silicon research where (impractical) high pressures were required to induce the PTs, but this work demonstrates that all hope is not lost. Further research could promise some possibly extremely valuable and economical industrial solutions. – Mark Wolverton
See: S. Yesudhas1, V.I. Levitas1,2, F. Lin1, K.K. Pandey3, J. Smith4, “Unusual plastic strain-induced phase transformation phenomena in silicon,” Nat Commun 15 7054 (2024)
Author affiliations: 1Iowa State University; 2Ames National Laboratory; 3Bhabha Atomic Research Center; 4Argonne National Laboratory.
Support from NSF (CMMI-1943710 and DMR-2246991), ARO (W911NF2420145), and Iowa State University (Vance Coffman Faculty Chair Professorship and Murray Harpole Chair in Engineering) is greatly appreciated. This work is performed at HPCAT (Sector 16), Advanced Photon Source (APS), and Argonne National Laboratory. HPCAT operations are supported by DOE-NNSA’s Office of Experimental Science. The Advanced Photon Source is a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. We also acknowledge CDAC-UIC for helping with the laser drilling of gaskets.
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