High-Pressure High-Temperature Synthesis: a Path Towards New Hexagonal Polytypes of Silicon

Technology is a ubiquitous component of modern life. Silicon is one of the main protagonists of this field, but its indirect bandgap imposes strict limitations on future performance improving. The most intriguing perspective for the future is bandgap engineering, accomplished by both nanostructuring or by modifying the crystal structure. Indeed, many metastable phases with properties suitable for applications have been predicted by ab-initio calculations; high-pressure could be an efficient technique and lead to the synthesis of new functional allotropes.
Different hexagonal polytypes of Si have been predicted, and their bandgap values are expected between 1 eV and 1.32 eV, a very interesting range for future applications. Despite previous reports of Si-IV (2H polytype) synthesis, a clear evidence of a purely hexagonal sample is still missing. Thanks to our recent HP-HT route for Si-III synthesis (precursor for Si-IV), we are now able to provide such evidence for the first time.
Our procedure has lead to the synthesis of a new polytype, Si 4H, that differs from the one expected and has never been reported in literature. A deep analysis of its structure and a wide physical characterization (combining experiments and theoretical calculations) will be here presented.