Patterned epitaxial vapor-liquid-solid growth of silicon nanowires on Si(111) using silane

Abstract

We have carried out a detailed study on the vapour-liquid-solid growth of silicon nanowires (SiNWs) on (111)-oriented Si substrates using Au as catalytic seed material. Arrays of individual seeds were patterned by electron-beam lithography, followed by Au evaporation and lift-off. SiNWs were grown using diluted silane as precursor gas in a low-pressure chemical vapor deposition system. The silane partial pressure, substrate temperature, and seed diameter were systematically varied to obtain the growth rate of the NWs and the rate of sidewall deposition. Activation energies of 19kcal∕mol for the axial SiNW growth and 29kcal∕mol for the radial deposition on the SiNW surface are derived from the data. SiNW growth at elevated temperatures is accompanied by significant Au surface diffusion, leading to a loss of Au from the tips of the SiNWs that depends on the layout and density of the Au seeds patterned. In contrast to NWs grown from a thin-film-nucleated substrate, the deterministic patterning of identical Au seeds of varying diameters allows accurate measurements of the nucleation yield of the SiNW, which is close to 100%, and analysis of the epitaxial relationship with the substrate. In addition to the vertical and the three 70.5°-inclined ⟨111⟩ epitaxial growth directions, we observe three additional 70.5°-inclined directions, which are rotated by 60°. The 60° rotation is explained by the occurrence of stacking faults in the SiNWs. The overall yield of vertically grown ⟨111⟩ NWs depends sensitively on the partial pressure of the silane and, to a lesser extent, on the growth temperature. At 80mTorr partial pressure and 470°C, up to 60% of the SiNWs grow in the vertical ⟨111⟩ direction. In situ doping of SiNWs using arsine resulted in a significant reduction of nucleation and wire growth, whereas doping with trimethylboron and phosphine exhibited no difference in growth and epitaxy compared with undoped samples.

Growth, Thermodynamics, and Electrical Properties of Silicon Nanowires

“…Lew et al 12 investigated nanowire growth at a similar temperature range and reported an activation energy of 22 kcal/mol. However, their data seem to indicate a somewhat smaller value, closer to the 19 kcal/mol of Schmid et al 29 From the data of Bootsma and Gassen, 30 who studied Si nanowires growth at temperatures of 600-800°C, an activation energy of 11.1 ( 0.4 kcal/mol (0.48 ( 0.02 eV) can be deduced.…”

“…Si nanowires obtained by using Au and silane are usually only slightly tapered, which indicates that radial growth is slow compared to axial growth. According to the data by Schmid et al, 29 radial growth rates are about 2 orders of magnitude smaller than axial growth rates. The activation energy for radial growth is found to be larger than that for axial growth.…”

“…Nanowire growth velocities related to the use of silane as precursor show an exponential dependence on inverse temperature (∝exp(-E a /kT)). From observations at growth temperatures of 450-600°C, Schmid et al 29 deduced an activation energy, E a , of 19 ( 1.5 kcal/mol (0.82 ( 0.07 eV). Lew et al 12 investigated nanowire growth at a similar temperature range and reported an activation energy of 22 kcal/mol.…”

“…Typical silane partial pressures range from 0.1 mbar to 1 mbar, 12,27,29 which results in growth velocities on the order of nanometers per second. Concerning the pressure dependence, the data published by Lew et al 12 indicate that the growth velocity increases approximately linearly with silane partial pressure.…”

“…Combining the pressure and temperature dependencies, 12,29 one can obtain the following estimate for the temperature and pressure dependence of the growth velocity of Aucatalyzed nanowires:…”