Masters Defense: Anna Wulff Christensen

Statistical Analysis of Selective Area Grown Nanowire Crystal Morphologies

Errors in quantum computing arising from the decoherence of states, present in current qubit systems, provide one major challenge in the further development of a fullscale quantum computer. Majorana zero modes show potential in topological quantum computing, which inherently overcomes the obstacle of decoherence. To ensure the scalability of systems that may host these topological qubits, recent research focuses on the growth of defect free selective area grown structures. In addition to the requirement of ballistic transport in individual wires, it must also be possible to grow largearraysofidenticalstructures. Butthegrowthofnanostructuresinselectiveareas is a young field, and while growth kinetics of vapour-liquid-solid systems are relatively well studied, the catalyst-free growth in selective areas is less so. The success ofthetopologicalqubitthereforecurrentlyreliesonthefurtherstudy—bothelectrical and morphological—of such structures. This thesis work is of the latter type. Using atomic force microscopy as the main microscopy method, nanowire arrays are characterised by a number of morphological parameters, mainly height. To conduct this study and to ensure consistency with future studies of a similar kind, an automated software has been developed. This software has been used in the characterisation of more than 150 nanowires grown in four-wire arrays. Employing this same software, theselectiveareagrowthofaGaAsbufferonGaAs(001)hasbeenstudiedindepth. In this study, the height and incorporation rate are analysed as a function of wire pitch and width in similar four-wire arrays. The outer-most two wires grow consistently less than the inner-most two; an unexpected result in the view of previous vapourliquid-solid studies. It is shown that the longer the distance between the wires in the array, the less they grow in height. This effect saturates at a pitch value of around 4 µm, where outer and inner nanowires also grow consistently to the same height. A similartrendisseenintheheightasafunctionofnanowirewidth,butwithananowire orientationdependence,believedtostemfromthewell-knownanisotropicbehaviour of Ga adatoms on GaAs. In general, the behaviour is attributed to the source-like nature of the selective area grown nanowires in this growth window, where all derived incorporation rates are smaller than the expected layer by layer growth rate.