Growth and application of planar III-V semiconductor nanowires grown with MOCVD

The semiconductor nanowire has been widely studied over the past decade and identified as a promising nanotechnology building block with application in photonics and electronics. The flexible bottom-up approach to nanowire growth allows for straightforward fabrication of complex 1D nanostructures with interesting optical, electrical, and mechanical properties. III-V nanowires in particular are useful because of their direct bandgap, high carrier mobility, and ability to form heterojunctions and have been used to make devices such as light-emitting diodes, lasers, and field-effect transistors. However, crystal defects are widely reported for III-V nanowires when grown in the common out-of-plane <111>B direction. Furthermore, commercialization of nanowires has been limited by the difficulty of assembling nanowires with predetermined position and alignment on a wafer-scale. In this thesis, planar III-V nanowires are introduced as a low-defect and integratable nanotechnology building block grown with metalorganic chemical vapor deposition. Planar GaAs nanowires grown with gold seed particles self-align along the <110> direction on the (001) GaAs substrate. Transmission electron microscopy reveals that planar GaAs nanowires are nearly free of crystal defects and grow laterally and epitaxially on the substrate surface. The nanowire morphology is shown to be primarily controlled through growth temperature and an ideal growth window of 470 +\- 10 °C is identified for planar GaAs nanowires. Extension of the planar growth mode to other materials is demonstrated through growth of planar InAs nanowires. Using a sacrificial layer, the transfer of planar GaAs nanowires onto silicon substrates with control over the alignment and position is presented. A metal-semiconductor field-effect transistor fabricated with a planar GaAs nanowire shows bulk-like low-field electron transport characteristics with high mobility. The aligned planar geometry and excellent material quality of planar III-V nanowires may lead to highly integrated III-V nanophotonics and nanoelectronics.

The pre-reproductive period in a tropical bird: parental care, dispersal, survival, and avian life histories

The period between offspring birth and recruitment into the breeding population is considered one of the least understood components of animal life histories. Yet, examining this period is essential for studies of parental care, dispersal, demography, and life histories. Studies of the pre-reproductive period are particularly few in tropical regions, where the organization of life histories are predicted to differ compared to northern hemisphere species. For my dissertation I used radio-telemetry, mark-resighting, and field observations to study the pre-reproductive period in a Neotropical bird, the western slaty-antshrike (Thamnophilus atrinucha), in Panama. First, I found that parental care after offspring left the nest (the post-fledging period) was greater than care during the nestling period. Prolonged care resulted in a clear trade-off for parents as they did not nest again until fledglings from the first brood were independent. Parents fed offspring for a prolonged duration during the post-fledging period and higher post-fledging survival was observed compared to many northern hemisphere species. Second, I observed that offspring that remained with parents for longer periods on the natal territory had higher survival both while on the natal territory and after dispersal compared to those dispersing earlier. Parental aggression towards offspring increased with offspring age and offspring dispersed earlier when parents renested. Contrary to other family living species, only a small proportion of antshrike offspring remained on the natal territory until the following year and all dispersed to float. Floating is when juveniles wander within other breeding pairs’ territories. These results suggest that the benefits of delayed dispersal declined with offspring age and with renesting by parents. Third, I observed that survival during the dependent period and first year was greater in slaty antshrikes compared to that of northern hemisphere species. Pre-reproductive survival relative to adult survival was equal or greater than that observed in northern hemisphere species. The date offspring left the nest, mass, and age at dispersal influenced offspring survival, whereas offspring sex and year did not. Relatively high survival during the pre-reproductive period coupled with comparatively low annual productivity clarifies how many tropical species achieve replacement. High juvenile survival appears to obtain from extended post-fledging parental care, delayed dispersal, low costs of dispersal, and a less seasonal environment. Lastly, I experimentally manipulated begging at the nest to examine changes in parental behavior. Under elevated begging, parents increased provisioning rates and reduced the time between arrival to the nest and feeding of nestlings, potentially to reduce begging sounds. Furthermore, parents switched to preferentially feed the closest offspring during the begging treatment. This suggests parents either allowed sibling competition to influence feeding decisions, or feeding the closer nestling increased the efficiency of provisioning. In summary, I found that slaty antshrikes have delayed age at reproduction, higher post-fledging and first year survival, extended post-fledging parental care, equal or greater pre-reproductive survival relative to adult survival, and delayed dispersal compared to many northern hemisphere passerines. These results suggest that this tropical species has a strategy of high investment into few offspring. Furthermore, reproductive effort is equal or greater at least in slaty antshrikes compared to northern hemisphere species, suggesting that the latitudinal gradient in clutch size is not explained by a gradient in reproductive effort.