title: Direct measurement of coherency limits for strain relaxation in heteroepitaxial core/shell nanowires.
abstract: The growth of heteroepitaxially strained semiconductors at the nanoscale enables tailoring of material properties for enhanced device performance. For core/shell nanowires (NWs), theoretical predictions of the coherency limits and the implications they carry remain uncertain without proper identification of the mechanisms by which strains relax. We present here for the Ge/Si core/shell NW system the first experimental measurement of critical shell thickness for strain relaxation in a semiconductor NW heterostructure and the identification of the relaxation mechanisms. Axial and tangential strain relief is initiated by the formation of periodic a/2 <110> perfect dislocations via nucleation and glide on {111} slip-planes. Glide of dislocation segments is directly confirmed by real-time in situ transmission electron microscope observations and by dislocation dynamics simulations. Further shell growth leads to roughening and grain formation which provides additional strain relief. As a consequence of core/shell strain sharing in NWs, a 16 nm radius Ge NW with a 3 nm Si shell is shown to accommodate 3% coherent strain at equilibrium, a factor of 3 increase over the 1 nm equilibrium critical thickness for planar Si/Ge heteroepitaxial growth.
authors.affiliation: Center for Integrated Nanotechnologies, Los Alamos National Laboratory, P.O. Box 1663, K771 Los Alamos, New Mexico 87545, USA. shadi@lanl.gov
authors.forename: Shadi A
authors.initials: SA
authors.lastname: Dayeh
authors.forename: Wei
authors.initials: W
authors.lastname: Tang
authors.forename: Francesca
authors.initials: F
authors.lastname: Boioli
authors.forename: Karen L
authors.initials: KL
authors.lastname: Kavanagh
authors.forename: He
authors.initials: H
authors.lastname: Zheng
authors.forename: Jian
authors.initials: J
authors.lastname: Wang
authors.forename: Nathan H
authors.initials: NH
authors.lastname: Mack
authors.forename: Greg
authors.initials: G
authors.lastname: Swadener
authors.forename: Jian Yu
authors.initials: JY
authors.lastname: Huang
authors.forename: Leo
authors.initials: L
authors.lastname: Miglio
authors.forename: King-Ning
authors.initials: KN
authors.lastname: Tu
authors.forename: S Tom
authors.initials: ST
authors.lastname: Picraux