Photo-induced cubic-to-hexagonal polytype transition in silicon nanowires

Transformation of the crystalline lattice in silicon nanowires from cubic diamond (cub-Si) to hexagonal diamond (hex-Si) was observed under laser irradiation at intensities above 10 kW cm−2 (wavelength of 473 nm) by appearance of an additional peak in their Raman spectra in the range from 490 to 505 cm−1. Formation of the hex-Si phase in SiNWs is favoured by strong mechanical stresses caused by inhomogeneous photo-induced heating, which results in a singlet–doublet splitting of the Raman peaks for LO and TO phonons at about 517 and 510 cm−1, respectively. The estimated values of the photo-induced mechanical stresses and temperatures required for the polytype transformation in SiNWs correspond to those for bulk Si. The formation of the hex-Si phase in SiNWs is further illustrated by huge photoluminescence (PL) enhancement at laser intensities above 10 kW cm−2, which correlates with the appearance of the Raman peak at about 500 cm−1. The spectral position of the PL band at about 1.5 eV is close to the direct band gap transition in the stressed hex-Si.