PhD defense: Martin Espiñeira Cachaza
Title: Selective Area Growth Modes of Semiconductor Networks and Defect Characterization
This thesis presents results on the different selective area growth (SAG) modes of in-plane III-V semiconductor nanowire (NW) networks, their modelling and their structural characterization. SAG networks are candidates to become a scalable hosting platform of a universal quantum computer based on the topologically protected qubit. Challenging requirements are demanded for this approach, such as the reproducible growth of homogeneous NW networks and very low density of crystal defects.
The first part of the thesis explores the different growth modes arising during SAG and their impact in NW material incorporation. The transition that defines the growth mode during SAG is the sign of the flux of adatoms between the mask and crystal ΔΓa_ma_c. SAG growth modes are experimentally observed and defined as source if ΔΓa_ma_c < 0, sink if ΔΓa_ma_c > 0 and balance if ΔΓa_ma_c =0. These growth modes affect the incorporation of NWs in arrays and their faceting. Two proposals for the growth of reproducible arrays of NWs are presented: a fine compensation of growth modes on buffer layers and to increase the number of structures around the transport NWs creating an adatom density saturation region. In addition, two models are presented based on mass conservation and coupled diffusion equations, predicting the observed experiments.
The second part studies the defects appearing in SAG NWs and a new characterization technique to observe them. DFT calculations are performed on InAs, suggesting a mild effect of stacking faults in the semiconductor properties and a more detrimental effect of dislocation cores, which introduce electron states in the band gap. It is also developed a new characterization technique based in electron channeling contrast SEM, using the design properties of the immersion lens and allowing for a high resolution characterization of NW stacking faults.
In summary, the findings presented expand the understanding of the SAG mechanism, the effect on the semiconductor of the defects generated and a new way of observing them. Although this helps to accomplish some of the material requirements for a topological qubit, more optimization needs to be performed in the defect reduction of the structures.