Black silicon: Formation in the trampoline mode of ion flow, surface properties, and performance perspectives

By irradiating the (111) surface of single-crystalline silicon wafers with Ar+ ions in the trampoline mode (high-density flows of low-energy ions) and with the assistance of metal initiator atoms, a transition of those wafers into the “black silicon” optical state was stimulated. This state is characterized by low values of light reflectance (less than 2%) from the irradiated wafer side in a spectral interval from 400 to 1000 nm. Scanning electron microscopy studies showed that this is a result of the emergence of a hierarchical submicron/nanoscale structure at the treated surface of the wafers. This transition can be performed only with the assistance of metal initiator atoms simultaneously evaporated from an auxiliary target in the operating chamber. At the same time, the application of metals initiators affects the effective lifetime of the minority charge carriers in the black state silicon, which is crucial for the efficiency of black silicon application in solar cells. Among the examined metals initiators (Al, Cr, Fe, Ni, Mo, and Cu), only the application of copper made it possible to preserve or even increase the value of this parameter. The electrostatic interaction between free electric charges and dipoles (playing the role of excitons in this case) near the interface in a two-layer system was considered theoretically. It was shown how the system parameters affect the preferable location of excitons and their orientation, which can be applied, e.g., to govern the solar cell efficiency. The formation of black silicon in the trampoline mode is environmentally friendly at all production stages.