Structural and optical properties of high quality zinc-blende/wurtzite GaAs hetero-nanowires

The structural and optical properties of 3 different kinds of GaAs nanowires with 100% zinc-blende structure and with an average of 30% and 70% wurtzite are presented. A variety of shorter and longer segments of zinc-blende or wurtzite crystal phases are observed by transmission electron microscopy in the nanowires. Sharp photoluminescence lines are observed with emission energies tuned from 1.515 eV down to 1.43 eV when the percentage of wurtzite is increased. The downward shift of the emission peaks can be understood by carrier confinement at the interfaces, in quantum wells and in random short period superlattices existent in these nanowires, assuming a staggered band-offset between wurtzite and zinc-blende GaAs. The latter is confirmed also by time resolved measurements. The extremely local nature of these optical transitions is evidenced also by cathodoluminescence measurements. Raman spectroscopy on single wires shows different strain conditions, depending on the wurtzite content which affects also the band alignments. Finally, the occurrence of the two crystallographic phases is discussed in thermodynamic terms.Comment: 24 page

Cluster scale composition determination in a boron-rich compound

Composition metrology is an important issue for compound nanostructure and devices. By imaging the internal structure of boron clusters in a boron-rich material using the high angle annular dark field technique in an aberration-corrected scanning transmission electron microscope, the authors have been able to determine quantitatively the local composition to be B6O1−x (x=0.30±0.04) through a relative image contrast analysis with the aid of a dynamical electron scattering simulation. Fluctuation of the oxygen occupancy is spatially resolved. This high precision and efficient measurement allows the study of the nonstoichiometry effect on an atomic scale for boron-rich materials