Fabrication And Antireflection Properties Of Silicon Nanowires By Confined Metal-assisted Chemical Etching

Solar energy is a kind of clean, no-pollution and renewable energy. Photovoltaictechnology is one of the effective ways to solve energy crisis and environmentalpollution. Due to their unique structure, optical and electrical properties, siliconnanowires (SiNWs) are attracting immense interest as a promising material toimprove solar cells’ efficiency and reduce the cost. On the one hand, with the averagereflection lower than5%, SiNWs have excellent antireflection and light trappingproperties due to their unique structure. On the other hand, co-axial solar cells basedon SiNWs have the advantage of orthogonal directions for carrier separation andcarrier collection. The carrier separation takes place in the radial versus the longeraxial direction. Hence, photogenerated carriers can reach the junction with highefficiency without substantial bulk recombination. Therefore, the research on theapplication of SiNWs in photovoltaic devices is meaningful. In this paper, we studiedthe fabrication and antireflection properties of flat surface SiNWs by confinedmetal-assisted chemical etching (CMACE), as well as the adjustment of thereflectance by coating an oxide decoration layer. The results we obtained are asfollows:1. In the fabrication process of SiNWs based on CMACE method, polystyrenespheres are used as the models whose diameter can be controlled by oxygen plasmaetching. The diameter of SiNWs can be controlled by the diameter of polystyrenespheres and the oxygen plasma etching parameters.With the increase of etching poweror etching time, the diameter of polystyrene spheres decreases. But the influence ofoxygen pressure is not obvious. The concentration of HF and H2O2etching solutionshas significant effect on the growth of SiNWs. With the concentration increase of HFor H2O2solution, the etching rate increases and the influence of H2O2concentration ismore obvious. Therefore, proper concentrations should be chosen carefully in order toobtain an appropriate etching rate. In addition, the length of SiNWs is increased byextending the etching time. And the etching rate of SiNWs with larger diameter is faster than that of thinner SiNWs. Hence, to obtain different SiNWs with the samelength, the thinner SiNWs need more etching time.2. Due to the samll contact area (point contact) between polystyrene spheres andsilicon subtract, Au particles fall on the silicon subtract during sputtering, resulting inthe uneven top surface of SiNWs. To solve this problem, we proposed hot treatmenton polystyrene spheres at135℃for10min after oxygen plasma etching. Thistemperature is close to the glass transition point of the polystyrene spheres, so thespheres will distort without melting, leading to larger contact area (surface contact)between polystyrene spheres and silicon subtract.3. The reflectance of SiNWs is reduced with increasing length. Besides, with thediameter decreasing, the reflectance decreases at first, and then increases. In addition,we deposited a uniform layer of aluminum-doped zinc oxide (AZO) film on SiNWsby atomic layer deposition (ALD) to further reduce the reflectance. AZO film servesas a buffering layer, alleviating the impedance of refractive indices between siliconand air, resulting in an average reflectance as low as2.7%after a coating of50nmAZO film.