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

Abstract

Crystalline phase transformation in silicon nanowires from cubic diamond to hexagonal diamond under strong laser excitation, caused by inhomogeneous heating-induced mechanical stresses.

A Review on Metastable Silicon Allotropes

“…In contrast, Rodichkina and others [ 24 ] observed photo-induced cubic-to-hexagonal polytype transition in silicon nanowires under laser irradiation at intensities above 10 kW·cm −2 and the red-shift of PL maximum peak with the increase in photoexcitation level. The stress distribution in two-dimensional nanowires is different from that in the bulk silicon, as described in Section 3.1 ; thus, the resulting phase is different, necessitating further investigation via finite element analysis.…”

“…In Shukla’s experiment, Raman active optical modes of Si( hP 4) were around 515 cm −1 , 507 cm −1 , and 495 cm −1 , respectively [ 23 ]. Rodichkina and Lysenko [ 24 ] reported a reversible photo-induced formation of Si( hP 4) in initially metal-assisted chemical etching-fabricated diamond cubic Si nanowires, but they observed the Raman peaks centered near 490 cm −1 , 510 cm −1 , and 517 cm −1 , corresponding to the Si( hP 4) phase in their experiment. Theoretical and experimental studies suggest that the Si( hP 4) indirect band gap is in the near-infrared range, near the emission of erbium in bulk silicon [ 6 , 23 ], while a direct transition at point Γ is at around 1.4–1.6 eV [ 37 , 38 , 39 , 40 ] (see in Table 2 ) and the emission efficiency is 2–3 orders of magnitude greater than that of Si( cF 8) in the visible-light region.…”

“…O’Keeffe found the type II structure to have the lowest energy among all clathrates using empirical potentials [ 59 ]. The discovery of the Na x Si 136 clathrate in the 1960s provided the first demonstration that silicon can exist in expanded, low-density forms [ 24 ], and the new allotrope of silicon, Si( cF 136), was yielded in their work for five years afterward [ 60 ]. The cage-like structure Si( cF 136) possesses 136 Si atoms per unit cell in sp 3 -like arrangement, which consists of five- and six-membered rings [ 61 ].…”

Photothermal Effects and Heat Conduction in Nanogranular Silicon Films

“…Laser induced heating has a significant effect on phase transformation in Si nanostructures [22,62]. At room temperature, the cubic phase (c-Si) exhibits diamond-like cubic structure where each Si atom is distant from its four nearest neighbors at 2.73 Å.…”

“…Low power CW laser-induced heating of Si NPs causes softening of the first-order Raman Si-Si transverse optical phonon mode accompanied by a decrease in the corresponding phonon lifetimes [20], while the same Raman shift was observed for bulk crystalline Si heated at substantially higher incident laser powers [21] because bulk Si is much more heat conductive compared to Si NPs. Strong thermal stress causes a singletdoublet splitting of the Raman peaks into LO and TO phonons, which leads to the phase transition provoking asymmetric broadening and spectral downshift of the Raman peak [22]. Heating of the NPs can actually be quite significant even at mild laser irradiating powers leading to large phonon softening and spectral broadening accompanied by the decay of optical phonon lifetime and pronounced interatomic potential's anharmonicity in the form of 3 and 4-phonon processes [23].…”