Coherently Strained Si–Si_<i>x</i>Ge_1–<i>x</i> Core–Shell\nNanowire Heterostructures

Abstract

Coherently strained Si–Si_<i>x</i>Ge_1–<i>x</i> core–shell\nnanowire heterostructures are expected to possess a positive shell-to-core\nconduction band offset, allowing for quantum confinement of electrons\nin the Si core. We report the growth of epitaxial, coherently strained\nSi–Si_<i>x</i>Ge_1–<i>x</i> core–shell heterostructures through the vapor–liquid–solid\nmechanism for the Si core, followed in situ by the epitaxial Si_<i>x</i>Ge_1–<i>x</i> shell\ngrowth using ultrahigh vacuum chemical vapor deposition. The Raman\nspectra of individual nanowires reveal peaks associated with the Si–Si\noptical phonon mode in the Si core and the Si–Si, Si–Ge,\nand Ge–Ge vibrational modes of the Si_<i>x</i>Ge_1–<i>x</i> shell. The core Si–Si\nmode displays a clear red-shift compared to unstrained, bare Si nanowires\nthanks to the lattice mismatch-induced tensile strain, in agreement\nwith calculated values using a finite-element continuum elasticity\nmodel combined with lattice dynamic theory. <i>N</i>-type\nfield-effect transistors using Si–Si_<i>x</i>Ge_1–<i>x</i> core–shell\nnanowires as channel are demonstrated.