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Pressure-induced amorphization and an amorphous–amorphous transition in densified porous silicon

Abstract

Crystalline and amorphous forms of silicon are the principal materials used for solid-state electronics and photovoltaics technologies. Silicon is therefore a well-studied material, although new structures and properties are still being discovered1,2,3,4. Compression of bulk silicon, which is tetrahedrally coordinated at atmospheric pressure, results in a transition to octahedrally coordinated metallic phases5. In compressed nanocrystalline Si particles, the initial diamond structure persists to higher pressure than for bulk material, before transforming to high-density crystals6. Here we report compression experiments on films of porous Si, which contains nanometre-sized domains of diamond-structured material7,8,9. At pressures larger than 10 GPa we observed pressure-induced amorphization10,11. Furthermore, we find from Raman spectroscopy measurements that the high-density amorphous form obtained by this process transforms to low-density amorphous silicon upon decompression. This amorphous–amorphous transition is remarkably similar to that reported previously for water12,13, which suggests an underlying transition between a high-density and a low-density liquid phase in supercooled Si (refs 10, 14, 15). The Si melting temperature decreases with increasing pressure, and the crystalline semiconductor melts to a metallic liquid with average coordination 5 (ref. 16).

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Figure 1: Energy-dispersive X-ray diffraction from porous silicon at high pressure.
Figure 2: Raman spectra of porous silicon during one compression/decompression cycle.
Figure 3: Calculated temperature–pressure (TP) diagrams of Si in stable and metastable crystalline and amorphous states.
Figure 4: Calculated viscosities of hypothetical HDL and LDL silicon liquids.

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Acknowledgements

P.F.M. is a Wolfson–Royal Society Research Merit Award holder.

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Correspondence to Paul F. McMillan.

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Deb, S., Wilding, M., Somayazulu, M. et al. Pressure-induced amorphization and an amorphous–amorphous transition in densified porous silicon. Nature 414, 528–530 (2001). https://doi.org/10.1038/35107036

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