Synthesis of new Si₉ material with a direct bandgap and its unique physical properties

Even though economic advantages of silicon still keep it as the dominant material for the solar cell industry in the near future, crystal silicon in the diamond structure (d-Si) is an indirect bandgap semiconductor which prevents to consider it as a next-generation platform for optical material technologies. Here, we report the formation of a new allotrope of silicon on surface, Si 9 , using a novel two-step synthesis methodology. First, a film of amorphous silicon was produced by using pulsed laser deposition method, and second, new Si 9 was synthesized under irradiation of coherent electron beam on the amorphous Si film. It is important that the structure of Si 9 , forming six-membered sp3 silicon rings and involving 9 silicon atoms in one unit, possesses a direct bandgap near 1.59 eV, around which we have measured the emission peak in photoluminescence spectra on the pure Si 9 . It is discovered that Si 9 can be easily doped as both p- and n-type on surface, where boron and nitrogen are demonstrated as the most promising elements for the p-type and n-type doping in Si 9 , respectively, due to their low formation energies and reductions in the band gap. These properties suggest great potential in constructing a novel Si 9 -based p-n junction which is highly desired for future industrial application of optoelectronic technologies and photovoltaic devices.