Anti-Reflectance Optimization of Secondary Nanostructured Black Silicon Grown on Micro-Structured Arrays

Abstract

Owing to its extremely low light absorption, black silicon has been widely investigated and reported in recent years, and simultaneously applied to various disciplines. Black silicon is, in general, fabricated on flat surfaces based on the silicon substrate. However, with three normal fabrication methods-plasma dry etching, metal-assisted wet etching, and femtosecond laser pulse etching-black silicon cannot perform easily due to its lowest absorption and thus some studies remained in the laboratory stage. This paper puts forward a novel secondary nanostructured black silicon, which uses the dry-wet hybrid fabrication method to achieve secondary nanostructures. In consideration of the influence of the structure's size, this paper fabricated different sizes of secondary nanostructured black silicon and compared their absorptions with each other. A total of 0.5% reflectance and 98% absorption efficiency of the pit sample were achieved with a diameter of 117.1 μm and a depth of 72.6 μm. In addition, the variation tendency of the absorption efficiency is not solely monotone increasing or monotone decreasing, but firstly increasing and then decreasing. By using a statistical image processing method, nanostructures with diameters between 20 and 30 nm are the majority and nanostructures with a diameter between 10 and 40 nm account for 81% of the diameters.

Keywords: absorption; black silicon; secondary nanostructures.

Figure 1

The process flow of secondary structures. (a) Piranha washing for 10 min, (b) spin coat photoresist, and prebake 30 min in nitrogen atmosphere oven, (c) pattern photoresist and post bake for 15 min in a nitrogen atmosphere oven, (d) using a buffer oxide etching fluid to etch silicon oxide, (e) piranha washing for 10 min to remove the photoresist, (f) silicon dioxide removed, (g) solution preparation, (h) solution (AgNO3/0.02-mol/L: HF/5-mol/L) configuring and soaking for 80 min, and (j) solution (HNO3/65%) configuring and soaking for 70 min.

Figure 2

The schematic diagram of the mask size parameter of the two samples. (a) Array tips sample mask. (b) Array pits sample mask.

Figure 3

The schematic diagram of parameters d₁, a₁, d₂, and a₂ in the SEM view. (a) a₁, the distance between the two center point of tips masks is 240 μm, that is, the No. 5 tip sample, (b) a₂, the distance between the two center point of pits masks is 100 μm, that is, the No. 1 pit sample, (c) d₁, the distance between the two center point of tips masks, is 33.6 μm, that is, the No. 2 tip sample, (d) d₂, the distance between the two center point of pits masks is 100 μm, that is, the No. 2 pit sample.

Figure 4

The nanostructures on the micro pits obtained using the wet etching method: (a) The top view of the low-magnification micrograph, (b) silver on the bottom surface of the pits, and (c) the high-magnification SEM image of nanostructures on the bottom surface of the pits.

Figure 5

The nanostructures SEM image and frequency, and the number distribution histogram of the nanostructures.

Figure 6

The schematic of the reaction in a microhole.

Figure 7

The hemispherical reflectance spectra of black silicon micro-nano hybrid structures obtained for different tips size structures. (a) The reflectance results of the five samples with #1 depth size, (b) the reflectance results of the five samples with #2 depth size, (c) the reflectance results of the five samples with #3 depth size, and (d) the reflectance results of the five samples with #4 depth size.

Figure 8

The hemispherical reflectance spectra of the black silicon micro-nano hybrid structures obtained for the different pits size structures. (a) The reflectance results of the five samples with #5 depth size, (b) the reflectance results of the five samples with #6 depth size, (c) the reflectance results of the five samples with #7 depth size, and (d) the reflectance results of the five samples with #8 depth size.

Figure 9

The reflectance spectrum results of polished silicon, micro tips, micro pits, nanowires nano-micro pit, and nano-micro tip structures.

Figure 10

The approximate curve of actual solar radiation energy on the Earth’s surface from 200 nm to 1200 nm.

Figure 11

The absorption efficiency of the micro-nano hybrid structures obtained for different tip and pit sizes. (a) The relational graph between the efficiency and diameter of the tips (first designed trench size) and (b) the relational graph between the efficiency and depth of the tips (first designed trench size). (c) The relational graph between the efficiency and diameter of the pits (first designed trench size) and (d) the relational graph between the efficiency and depth of the pits (first designed trench size).