Effects of the doping level in the production of silicon nanowalls by metal assisted chemical etching

Abstract

Si nanowalls have been prepared out of Si substrates in a wide range of resistivities, by Metal Assisted Chemical Etching (MACE). Si (100) boron doped wafers with three resistivities 15-25 Ω·cm, 0.01-0.02 Ω·cm and 0.0005-0.0007 Ω·cm were used in this work. MACE was carried out in two steps: first, silver particles were deposited; and second, the etching solutions were optimized by varying the HF and H2O2 concentrations. This is the first time that nanowalls are obtained out of substrates of resistivity smaller than 0.001 Ω·cm. The relation of sizes and characteristics of the walls with the doping level of the Si substrates has been emphasized.

Doping density, not valency, influences catalytic metal-assisted plasma etching of silicon

“…27 Because of the irregularity in etch depth and rate in MACE processing depending on the Si doping, the concentration of the MACE etchants must be adjusted to accommodate for the resistivity and doping type of the Si substrates to achieve uniform etching. 26,28–30…”

“…In MACE, two competing processes determine the etch features and etch rate in highly doped Si: (1) the characteristic metal-catalysed vertical etching, and (2) porosification – the introduction of additional etching pathways due to increased availability of electronic holes at higher doping levels. 26 This porosification occurs across the Si substrate and causes unintended etching in non-metallised areas. Overall, MACE etching creates greater etch depths in highly doped n-type Si than lightly doped n- and p-type samples, which in turn exhibit greater etch depths than highly doped p-type Si.…”

“…27 Because of the irregularity in etch depth and rate in MACE processing depending on the Si doping, the concentration of the MACE etchants must be adjusted to accommodate for the resistivity and doping type of the Si substrates to achieve uniform etching. 26,[28][29][30] In MACE, several proposed explanations exist for the differences in etch porosity and etch rate at high doping levels. Higher doping concentrations increase the amount of weak defective sites in the Si lattice, which is proposed to initiate the formation of additional etching pathways.…”

Optimization of Metal-Assisted Chemical Etching for Deep Silicon Nanostructures

“…Although conceptually simple, MACE can become complicated since many parameters influence the process like etching solution composition [11], etching temperature [12], doping concentration of the Si substrate [13,14], catalyst thickness [15] or catalyst morphology [16,17]. These parameters have to be carefully adjusted to obtain the desired etching performance.…”