Direct evidence of 2H hexagonal Si in Si nanowires

Hexagonal Si (2H polytype) has attracted great interest because of its unique physical properties and wide range of potential applications. For example, it might be used in heterojunctions based on hexagonal and cubic Si. Although hexagonal Si has been reported in Si nanowires, its existence is doubted because structural defects of diamond cubic Si can produce structural signals similar to those attributed to hexagonal Si. Here, through the use of atomic resolution high-angle annular dark-field scanning transmission electron microscopy imaging, we unambiguously report the coherent intergrowth of diamond cubic (3C polytype) and 2H hexagonal Si in Si nanowires grown by chemical vapor deposition. A model describing the intergrowth of 3C and 2H Si is proposed and the reasons for the generation of 2H Si are discussed in detail.