Fabrication of Metal-Semiconductor Heterostructures in Silicon Nanowires
The increasing demand for fossil fuels and the need to reduce greenhouse gases require clean energy sources and more efficient utilization of energy. Thermoelectric materials provide a means toward achieving these goals since they convert heat, including waste heat, directly into an electric potential difference. Metal-semiconductor heterostructures can work as Schottky barriers in thermoelectric materials to increase thermoelectric efficiency.
In this project, nickel silicide phases were introduced into silicon nanowires (SiNWs) to build up the Schottky barrier. SiNW arrays were fabricated using a metal-assisted chemical process, creating SiNWs about 200 nm in diameter and 30ìm in length. Different methods were adopted for nickel deposition: electroless nickel deposition, electro nickel deposition, E-beam deposition, and thermal evaporation. The samples were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that depositing nickel on SiNWs in an aqueous solution without electricity is a simple way to deposit nickel particles, and the morphology of nickel particles depends on the concentration of the deposition bath. However, an aqueous solution will cause oxidation of the SiNWs and hinder the formation of nickel silicide. To solve this problem, depositing nickel on SiNWs in organic solutions inside an oxygen-free glove box is a way to prevent oxidation, and nickel can diffuse into silicon substrates easily via annealing when there no oxidation layer on the surface of SiNWs. The dominant phase formed in these samples is NiSi2 after being annealed at 650°C for one hour in a tube furnace.