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UU1.09 - Impact of Nano-Patterned Oxide Templates on the Growth of Ordered Self-Catalyzed GaAs Nanowire Arrays on Silicon 
April 22, 2014   11:00am - 11:15am

A main driving force behind the interest in semiconducting nanowires is the unique capability to engineer novel heterostructures for various high-performance electronic devices. In the same way, it is also possible to achieve high-quality growth despite using lattice mismatched substrate materials. This work is focused on the growth of patterned self-catalyzed GaAs nanowire arrays on silicon substrates by gas source molecular beam epitaxy (MBE), which is expected to make an excellent candidate for high-efficiency photovoltaic applications. Patterning is used to produce the controlled nanowire morphology, uniformity and areal densities necessary for optimal ensemble nanowire devices. A template of nanoscale holes can be defined in a thin (100-300 Ã…) oxide layer, facilitating the growth of positioned, epitaxial nanowires while avoiding accompanying parasitic film deposition. In our work we have used electron beam lithography as the patterning method and show that the silicon oxide film may be both thermally grown or may be deposited by chemical vapor deposition. The yield and morphology of vertically aligned nanowires has been studied as a function of the pattern parameters such as hole diameter and inter-hole spacing. Using cross-sectional transmission electron microscopy (TEM) samples prepared using a focused ion beam technique, important features of the nanowire nucleation, growth and structure have been studied. In particular, we show that a linearly increasing length-radius distribution, analogous to that observed for unpatterned self-catalyzed growth on substrates with thin oxides, may be obtained even when using patterned oxide masks due to an unintended residual layer of oxide, as confirmed by TEM analysis. We explain how a linear length-radius dependence can result from the individual NWs beginning their growth at different times, accompanied by significant radial growth. We then show how the spread in obtained NW dimensions is significantly decreased using improved etching practices which ensure the complete removal of the oxide layer. Our experimental results also show that the axial and lateral growth rates increase strongly with increasing the interhole spacing. We account for this by proposing that a significant proportion of growth material is supplied by a secondary flux of adatoms desorbing from the oxide surface between the nanowires. Shadowing of this flux by neighboring nanowires in the array may therefore have a strong effect on the overall growth rates and the subsequent nanowire morphology, which will be characterized in an accompanying growth model.

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Keynote Address
Panel Discussion - Different Approaches to Commercializing Materials Research
Business Challenges to Starting a Materials-Based Company
Fred Kavli Distinguished Lectureship in Nanoscience
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