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

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 discovered(1-4). Compression of bulk silicon, which is tetrahedrally coordinated at atmospheric pressure, results in a transition to octahedrally coordinated metallic phases(5). In compressed nanocrystalline Si particles, the initial diamond structure persists to higher pressure than for bulk material, before transforming to high-density crystals(6). Here we report compression experiments on films of porous Si, which contains nanometre-sized domains of diamond-structured material(7-9). At pressures larger than 10 GPa we observed pressure-induced amorphization(10,11). Furthermore, we rnd 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 water(12,13), 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 similar to5 (ref. 16).