Since the quantum confinement in nanosilicon occurs at diameters below 5 nm, you still can have plenty of Si nanowires with indirect band gap. And as far as I know the growth of really thin Si nanowires is not that easy, so to get Si nanowires with direct band gap would require some post fabrication processes such as etching.

Direct band gaps in semiconductors are the result of the rather complex electronic bandstructure, which generally does NOT become easier to compute for a spatially confined system such as a nanowire. The reason is that the Fourier transform of a space charge distribtution of systems confined in real space is extending further out in reciprocal space.

See this paper by world class researcher and educator Professor M. P. Anantram. It has detailed modeling calculations taking account of indirect transition in silicon nanowire. My understanding is Si itself and Si:Ge alloyed nanowire both perform well with indirect band gap transitions just like In As and InAlAs heterojunction nanowires which should be mostly direct band-gap engineered transitions. Si and Si:Ge nanowires are still providing reasonably useful device performance even when quantum confinement is considered.

Sincerely,

Dr. Nabil Shovon Ashraf.

Dear Dr. Nabil

Thank you very much for sharing the valuable knowledge and link.