Imaging Atomic Scale Dynamics on III-V Nanowire Surfaces During Electrical Operation.

As semiconductor electronics keep shrinking, functionality depends on individual atomic scale surface and interface features that may change as voltages are applied. In this work we demonstrate a novel device platform that allows scanning tunneling microscopy (STM) imaging with atomic scale resolution across a device simultaneously with full electrical operation. The platform presents a significant step forward as it allows STM to be performed everywhere on the device surface and high temperature processing in reactive gases of the complete device. We demonstrate the new method through proof of principle measurements on both InAs and GaAs nanowire devices with variable biases up to 4 V. On InAs nanowires we observe a surprising removal of atomic defects and smoothing of the surface morphology under applied bias, in contrast to the expected increase in defects and electromigration-related failure. As we use only standard fabrication and scanning instrumentation our concept is widely applicable and opens up the possibility of fundamental investigations of device surface reliability as well as new electronic functionality based on restructuring during operation.

Atomic scale surface structure and morphology of InAs nanowire crystal superlattices: the effect of epitaxial overgrowth.

While shell growth engineering to the atomic scale is important for tailoring semiconductor nanowires with superior properties, a precise knowledge of the surface structure and morphology at different stages of this type of overgrowth has been lacking. We present a systematic scanning tunneling microscopy (STM) study of homoepitaxial shell growth of twinned superlattices in zinc blende InAs nanowires that transforms {111}A/B-type facets to the nonpolar {110}-type. STM imaging along the nanowires provides information on different stages of the shell growth revealing distinct differences in growth dynamics of the crystal facets and surface structures not found in the bulk. While growth of a new surface layer is initiated simultaneously (at the twin plane interface) on the {111}A and {111}B nanofacets, the step flow growth proceeds much faster on {111}A compared to {111}B leading to significant differences in roughness. Further, we observe that the atomic scale structures on the {111}B facet is different from its bulk counterpart and that shell growth on this facet occurs via steps perpendicular to the ⟨112⟩B-type directions.

Direct imaging of atomic scale structure and electronic properties of GaAs wurtzite and zinc blende nanowire surfaces.

Using scanning tunneling microscopy and spectroscopy we study the atomic scale geometry and electronic structure of GaAs nanowires exhibiting controlled axial stacking of wurtzite (Wz) and zinc blende (Zb) crystal segments. We find that the nonpolar low-index surfaces {110}, {101[overline]0}, and {112[overline]0} are unreconstructed, unpinned, and without states in the band gap region. Direct comparison between Wz and Zb GaAs reveal a type-II band alignment and a Wz GaAs band gap of 1.52 eV.

Five-year results for use of single-flanged tympanostomy tubes in children.

AIMS: To assess the five-year outcome of the use of single-flanged tympanostomy tubes in children, including the time to extrusion, rate of retained tubes and rate of persistent perforation. MATERIALS AND METHODS: The medical records relating to 640 single-flanged tympanostomy tubes intended for short-term use in paediatric patients were retrospectively reviewed. RESULTS AND ANALYSIS: We found that 36.4 per cent of the tubes had extruded within 12 months and 71.0 per cent within 24 months. Results showed that 14.1 per cent of the tubes had been removed because of prolonged retention, with a mean time to removal of 38.9 months. The time to extrusion was longer and the rate of retained tubes was higher than those reported for several other short-term tubes. We found that 4.5 per cent of tube insertions had resulted in a persistent perforation, a higher percentage than previously reported for other tubes intended for short-term use. Within five years of tube insertion, 70.5 per cent of the tympanic membranes had normalised.