The Effect of Nitrogen Plasma Species on the Growth Morphology and Mechanism of Gan Nanocolumns Deposited by Plasma-Assisted Molecular-Beam Epitaxy
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This dissertation addresses GaN nanocolumns (NCs) growth on Si (111) via a SixNy intermediate layer by radio-frequency molecular-beam epitaxy. In the existing literature, the GaN NC’s morphology was shown to be influenced by growth parameters such as Ga flux, substrate type, growth temperature, buffer layer types, sticking coefficient of Ga adatoms in different planes, etc. The one aspect that so far has been understudied is the role of the plasma-generated species. Due to the multiple states and concentrations of the available excited species in a nitrogen plasma, it has been difficult to quantify and correlate their role with the morphology of the resulting NCs. To address this issue, the nitrogen plasma source has been investigated by optical emission spectroscopy, in order to quantify the relative abundances of the molecular and atomic nitrogen species, and then to examine the effect of these species on the growth morphology and mechanism of GaN NCs grown on Si (111). The length, diameter, and density of NCs were analyzed as a function of the nitrogen species Cmol/Cat concentration ratio during epitaxy. Growth rate and diameter are found to increase and density to decrease up to a certain value of Cmol/Cat nitrogen ratio but plateaued beyond that. The effect of the Cmol/Cat nitrogen ratio seems to be an additional factor that has to be taken into account in the modeling of GaN NC growth. The structural and optical characterization of GaN NCs by PL, XRD, and RHEED show that GaN NC samples are strained. This strain decreases with increasing Cmol/Cat ratio. The growth mechanism of the GaN NCs was investigated in terms of Cmol/Cat ratio using the SEM and TEM results. Under our growth conditions, the NC growth appeared to be driven by direct impingement of adatoms rather than diffusion. The NCs have a core shell structure where the difference in growth rate of shell and core diminishes as Cmol/Cat ratio increases, which results in NCs with different radial growth rates.