One step reactive ion etching of black germanium conical nanostructures: Ultra-wide solar spectral absorption, finite element simulation, super hydrophilicity, photothermal conversion

Abstract

We report a reactive ion etching process based on SF 6 –CHF 3 , which can prepare black germanium at low RF power and normal temperature. The absorption of black germanium in the spectral range of 300–1750 nm is greater than 99%. Spectral selectivity optimization allows nano tapered germanium to absorb nearly 98.1% of solar radiation. This nanostructure is uniform in a large area and also supports patterning design for the application in related fields such as photothermal imaging. Here, we discuss the effect of SF 6 and CHF 3 plasma concentrations on the progress of chemical reactions on the germanium surface and their essential role in forming tapered nanostructures. We observe its surface plasmon resonance by finite-difference time-domain way. Finally evaluate its feasibility in devices by photothermal conversion and water droplet evaporation experiments. The proposal of this work provides an available method for semiconductor micromachining, and also provides experimental and theoretical support for the efficient thermal utilization of solar energy. In conclusion, we analyzed the mechanism of one-step etching of highly absorbing tapered nanostructures by SF 6 –CHF 3 -based RIE process by SEM and XPS. The germanium surface tends to form particles at the initial stage of etching, and due to the very high concentration of negative ions in the SF 6 plasma, these particles gradually form GeS x clusters. The germanium fluoride is mainly present at the surface of the germanium sample, while the germanium sulfide extends to a greater depth, so these clusters are not effective in preventing etching. The CHF 3 plasma greatly reduces the negative ion concentration and reduces the tendency of particle formation in favor of smoother GeS x and C x H z F y layers, which are very favorable for the formation of nanostructures. Aggregation of the germanium sidewalls decreases with depth, so tapered nanostructures are gradually formed with etch time. In addition, over-etching leads to additional carbon deposition, which is mainly composed of fluorocarbon polymers (C x H y F z ) formed from the decomposition products of CHF 3 . The wide spatial distribution of the sizes of the tapered nanostructures makes the absorptivity of black germanium more than 99% in the 300–1750 nm spectral range. As an indirect bandgap semiconductor, the absorption of photons makes electrons excited from the valence band to the conduction band with a great probability of releasing energy to the lattice, converting it into phonons, and turning it into heat energy. The cone-shaped nanostructured germanium can be heated to 69.4 °C in 20 min, and the heating rate can reach 7.98 °C/min in 300 s. The conical nanostructure modification makes germanium exhibit superhydrophilicity, and the water droplets make wet contact with germanium and fill the grooves on the surface of the nanostructures, making it capable of completely evaporating 0.05 ml droplets within 101 s. It is very competitive in heating and evaporation of water droplets.a) Black germanium photos, and SEM images of nano cones; b) Comparison of absorptivity between black germanium and polished germanium; c) Infrared image of graphic germanium wafer; (the letter part is nano cone structure, and the rest is polished germanium) d) With the increase of etching time, the temperature change curve of the sample under simulated sunlight irradiation (1 solar intensity). • Disordered nanocrystalline germanium was prepared by metal induced one-step self masked RIE. • The absorbance in the 300-1750 nm regions was higher than 98%. • The structure has excellent optical properties due to light trapping effect and plasma resonance. • The excellent optical properties and super hydrophilic properties make the structure have excellent photothermal properties.