Electron Microscopy Characterization of III-Nitride Nanowires grown on Graphene
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- Institutt for fysikk 
GaN nanowires grown on graphene glass using molecular beam epitaxy have been studiedusing scanning electron microscopy and different transmission electron microscopy techniques.The nanowires have an AlN nucleation particle at the bottom. The nanowires were found tobe quite evenly distributed on the substrate, and of approximately the same length (ca. 400nm). Their cross-sectional shape varied from thin (less than 40 nm) perfect hexagonal, to thicker(40 - 70 nm) distorted hexagonal, to coalesced into larger clusters. It is assumed that thedistribution of AlN nucleation particles contribute to the even distribution of nanowires. TheAlN particles are dome-shaped with a typical height of 10-15 nm. Furthermore, very littlelateral GaN growth around the base of the AlN nucleation particles was found, indicatingthat the size and shape of the AlN particle control the size and shape of the nanowire.Although the AlN nucleation particles show promising properties as nucleation particles forGaN nanowire growth on graphene, and an ability to control the shape of the nanowires, thepresence of a material with a relatively high band gap pose challenges for the use of thesegraphene/AlN/GaN nanowire heterostructures in applications.The studied nanowires can be categorized, and types occurring most often were studied infurther detail. The nanowires of type I (diameter larger than 40 nm) and II (diameter less than 40 nm) areconsidered to be closest to the targeted structure, with a single nanowire growing from asingle nucleation particle. Furthermore, they are defect-free above the nucleation particle.The type III nanowire is a single nanowire grown from multiple nucleation particles, creatinga foot at the bottom of the nanowire. Type III has approximately the same length as typeI and II, but the diameter is in generally larger, and varies more (74 +- 32 nm). The type IIInanowires are also defect-free above the foot.Compositional analysis was performed using energy-dispersive spectroscopy, which confirmedthe presence of AlN at the nucleation particle, and GaN in the rest of the nanowire. Quantification using the Cliff-Lorimer method was attempted, but the use of calculated k-factorsfrom black-box software and absorption lead to less accurate compositional quantification. Inthe future, the zeta-method should be investigated for quantification, as it could aid in solvingsome of the shortcomings that the Cliff-Lorimer method suffer from. For example, the zeta-method enable thickness independent element maps and absorption correction, which couldpossibly quantify the composition where the AlN nucleation particle and the GaN nanowireoverlap.Images and diffraction patterns were obtained at two different tilts rotated 30 degrees relative toeach other for all the studied nanowires. It is shown that this was useful for obtainingadditional structural and morphological information, which together with the EDS analysisgave a more accurate impression of the studied nanowires, than single 2D projections usuallyobtained in the TEM would do. For example, these tilt series could determine that thewurtzite GaN nanowire facets are [-1100], and not [-2110] as commonly observed in wurtziteGaAs nanowires.