Sub-nanometer mapping of strain-induced band structure variations in planar nanowire core-shell heterostructures

Strain relaxation mechanisms during epitaxial growth of core-shell nanostructures play a key role in determining their morphologies, crystal structure and properties. To unveil those mechanisms, we perform atomic-scale aberration-corrected scanning transmission electron microscopy studies on planar core-shell ZnSe@ZnTe nanowires on α-Al2O3 substrates. The core morphology affects the shell structure involving plane bending and the formation of low-angle polar boundaries. The origin of this phenomenon and its consequences on the electronic band structure are discussed. We further use monochromated valence electron energy-loss spectroscopy to obtain spatially resolved band-gap maps of the heterostructure with sub-nanometer spatial resolution. A decrease in band-gap energy at highly strained core-shell interfacial regions is found, along with a switch from direct to indirect band-gap. These findings represent an advance in the sub-nanometer-scale understanding of the interplay between structure and electronic properties associated with highly mismatched semiconductor heterostructures, especially with those related to the planar growth of heterostructured nanowire networks.
J.A., S.M.-S., M.B., and M.C.S. were supported by the Severo Ochoa program from Spanish MCIN/AEI /10.13039/501100011033/ (Grant No. SEV-2017-0706) and are funded by the CERCA Program / Generalitat de Catalunya. Part of the present work has been performed in the framework of Universitat Autònoma de Barcelona Materials Science Ph.D. program. J.A., S.M.-S., M.B., and M.C.S. acknowledge funding from Generalitat de Catalunya 2017SGR327. S.M.S. acknowledges funding from ICN2 SO EXEM project. The HAADF-STEM microscopy was conducted in the Laboratorio de Microscopias Avanzadas at Instituto de Nanociencia de Aragon-Universidad de Zaragoza. Additional imaging and electron energy loss spectroscopy was carried out at SuperSTEM, the UK National Research Facility for Advanced Electron Microscopy, supported by the Engineering and Physical Sciences Research Council (EPSRC). Authors acknowledge the LMA-INMA for offering access to their instruments and expertise. We acknowledge support from CSIC Research Platform on Quantum Technologies PTI-001 (QTEP). M.B. acknowledges support from SUR Generalitat de Catalunya and the EU Social Fund; project ref. 2020 FI 00103. C.K. acknowledges funding from InPhINIT Scholarships LaCaixa Foundation. M.C.S. have received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No 754510 (PROBIST). J.G.d.A. has been supported by European Research Council (Advanced Grant 789104-eNANO). V.D.G. and J.G.d.A have received funding from Spanish MCINN (PID2020-112625GB-I00 and CEX2019-000910-S), Fundació Cellex, Fundació Mir-Puig, and Generalitat de Catalunya (CERCA, AGAUR). E.J. acknowledges support from the Israel Science Foundation (Grant No. 2444/19). J.A., S.M.-S., M.B. and M.C.S. were supported by MCIN with funding from European Union NextGenerationEU (PRTR-C17.I1), by Generalitat de Catalunya and by “ERDF A way of making Europe”, by the “European Union”.