Controlled Pore Formation on Mesoporous Single Crystalline Silicon Nanowires: Threshold and Mechanisms

Abstract

Silicon nanowires are prepared by the method of the two-step metal-assisted wet chemical etching. We have analyzed the structure of solid, rough and porous nanowire surfaces of borondoped silicon substrates with resistivities of ρ > 1000 Ωcm, ρ = 14-23 Ωcm, ρ < 0.01 Ωcm by scanning electron microscopy and nitrogen gas adsorption. Silicon nanowires prepared from highly-doped silicon reveal mesopores on their surface. However, we found a limit for pore formation. Pores were only formed by etching below a critical H 2 O 2 concentration (c H2O2 < 0.3 M). Furthermore, we have determined the pore size distribution in dependence on the etching parameters and characterized the morphology of the pores on the nanowire surface. The pores are in the regime of small mesopores with a mean diameter of 9-13 nm. Crystal and surface structure of individual mesoporous nanowires have been investigated by transmission electron microscopy. The vibrational properties of nanowire ensembles have been investigated by Raman spectroscopy. Heavily boron-doped silicon nanowires are highly porous and the remaining single crystalline silicon nanoscale mesh leads to a redshift and a strong asymmetric line broadening for Raman scattering by optical phonons at 520 cm -1 . This redshift, λ Si bulk = 520 cm -1  λ Si nanowire = 512 cm -1 , hints to a phonon confinement in mesoporous single crystalline silicon nanowire.

Unraveling the Morphological Evolution and Etching Kinetics of Porous Silicon Nanowires During Metal-Assisted Chemical Etching

“…Figure 5a shows that the length of the formed SiNWs increases with the MACE reaction time, consistent with observations in previous studies [14, 16–19, 21, 23]. However, it can be seen that the increase in SiNW length diminishes over time, i.e., the etch rate is decreasing (see Additional file 1: Figure S5a), which could be due to increasing etch rates at the SiNW tips or decreasing etch rates at the base or both.…”

Surface effects on thermoelectric properties of metallic and semiconducting nanowires

“…Although the NWs differ in their surface morphology, the measured SAED patterns in Fig. d–f prove them to be single crystalline and grown along the (Ag), (), and (Si) direction, which is in agreement with previous studies .…”

“…The single crystalline Ag, , and Si NWs are synthesized as described in Refs. and brought into aqueous solution. The NWs are assembled on the TNCP by a dielectrophoretic‐assisted approach () and by direct mechanical transfer.…”

“…However, these devices miss the possibility to combine a comprehensive thermoelectric characterization with a structural and chemical analysis. In this work, we review our recent results based on a novel measurement platform () allowing analytical transmission electron microscopy (TEM) subsequently to the thermoelectric characterization. Therefore, we can unambiguously demonstrate how the NW's transport properties correlate with the structural data and in particular with the surface morphology.…”

“…The crystal structure along the NWs is analyzed by selected area electron diffraction (SAED) . Although the NWs differ in their surface morphology, the measured SAED patterns in Fig.…”