Elucidating the Effect of Etching Time Key-Parameter toward Optically and Electrically-Active Silicon Nanowires

Abstract

In this work, vertically aligned silicon nanowires (SiNWs) with relatively high crystallinity have been fabricated through a facile, reliable, and cost-effective metal assisted chemical etching method. After introducing an itemized elucidation of the fabrication process, the effect of varying etching time on morphological, structural, optical, and electrical properties of SiNWs was analysed. The NWs length increased with increasing etching time, whereas the wires filling ratio decreased. The broadband photoluminescence (PL) emission was originated from self-generated silicon nanocrystallites (SiNCs) and their size were derived through an analytical model. FTIR spectroscopy confirms that the PL deterioration for extended time is owing to the restriction of excitation volume and therefore reduction of effective light-emitting crystallites. These SiNWs are very effective in reducing the reflectance to 9-15% in comparison with Si wafer. I-V characteristics revealed that the rectifying behaviour and the diode parameters calculated from conventional thermionic emission and Cheung's model depend on the geometry of SiNWs. We deduce that judicious control of etching time or otherwise SiNWs' length is the key to ensure better optical and electrical properties of SiNWs. Our findings demonstrate that shorter SiNWs are much more optically and electrically active which is auspicious for the use in optoelectronic devices and solar cells applications.

Keywords: electrical properties; etching time; metal assisted chemical etching; optical properties; silicon nanowires.

Figure 1

Schematic of the potential relationship between bands in a silicon (Si) substrate, Ag⁺/Ag, and H₂O₂/H₂O redox pairs.

Figure 2

Schematic illustration of the formation mechanism of silicon nanowires (SiNWs) via two-step MACE. (a) Reduction of Ag⁺ ions and formation of Ag nuclei at the Si surface. (b) Further silver nuclei growth, oxidative dissolution of Silicon atoms, and production of porous layer. (c) Vertical propagation of silver nanoparticles and faster etching leading to SiNWs formation. (d) Silver removal and vertical aligned SiNW arrays production.

Figure 3

SEM images of SiNWs etched at different durations showing top view surface (a–f), tilt view 35° (g–l), and cross-sectional (m–r).

Figure 4

Variation of the (a) filling ratio and (b) length of SiNWs as a function of etching time.

Figure 5

XRD patterns of silicon nanowires and untreated Si wafer.

Figure 6

PL spectra of SiNWs synthesized at different etching times.

Figure 7

Variation of integrated PL intensities versus etching duration.

Figure 8

FTIR spectra of SiNWs obtained at different etching time.

Figure 9

Evolution of peaks intensity of dominant FTIR bands versus etching time.

Figure 10

Reflectance spectra of SiNWs with various lengths etched during different times and corresponding Si wafer. The inset displays images of Si wafer and SiNWs.

Figure 11

Curves of the as-prepared heterojunction based on SiNWs synthesised at various etching times. The inset displays a schematic illustration of Ag/SiNWs/Si/Al device.

Figure 12

Logarithm of forward current versus voltage (Ln (I) vs. V) curves of SiNWs synthesized at different etching times.

Figure 13

Plots of dV/d(LnI) vs. I of SiNWs samples (S1–S6).