Transmission Electron Microscopy of Semiconductor Nanowires
Abstract: Semiconductor nanowires are studied by using transmission electron microscopy (TEM) based methods in this work. In the first section, the growth mechanism of gallium arsenide nanowires grown by chemical beam epitaxy is investigated. The nanowires are epitaxially grown from a gallium arsenide substrate by using gold seed particles as catalysts. The growth of gold particle catalysed nanowires is often described by the vapour-liquid-solid mechanism with the gold seed particle as a eutectic alloy in the liquid state. Here, in-situ heating microscopy together with varied growth procedures and nanoscale analysis on the constituents shows that the gold seed particle is in a solid state during growth. Based on a mass transport model, it is also demonstrated here that the diffusion of gallium through a solid gold seed particle occurs at fast enough rates to support the observed growth rates. The gold seed particle is believed to act as a sink for growth species and the interface between the particle and the substrate becomes a preferred nucleation site, resulting in higher growth rate of the nanowires compared to the growth on the surrounding substrate surface. More complex structures can be created by using nanowires as building blocks to create branched, and possibly interconnecting, structures. Here, branched nano structures, named ?nanotrees? due to their resemblance to trees in nature, are produced by sequential seeding of nano particles. The described method offers a high level of control in each growth step in terms of diameter, length, position and composition. Positioning the height of the branches on the trunk can be precisely controlled by the developed technique which is based on a spun on polymer film masking the lower part of the trunk. This also prevents growth of branches on the substrate surface. This technique is fundamentally material independent. The crystallographic orientation of the branches in relation to the trunk and the substrate surface is also investigated. Heterostructured nanowires are a key component of newly developed electronic and optical devices based on the nanowire technology. It is possible to grow heterostructures with a large lattice mismatch without dislocations due to lateral relaxation. The strain at the interfaces affects the bandgap of the structure locally which directly changes the properties of the device. The strain in InAs/InP nanowire heterostructures has been measured and modelled using high resolution TEM techniques and finite element calculations as well as multi slice simulations. The TEM experimental measurement results correspond well with the finite element model which also generates simulated HRTEM images verifying the image processing technique. For a 20 nm thick wire a 10 nm long dislocation free region over the interface is strained with the rest of the wire having a relaxed crystal structure. The in-situ microscopy applied in this work is utilising a scanning tunneling microscopy (STM) sample holder combined with TEM imaging to characterise the electrical properties of InAs nanowires. Ohmic contact between the STM-tip and the nanowires has been achieved and electrical measurements have been performed. The current-voltage measurement results are in good agreement with the data obtained by traditional measurements based on lithographic processing techniques.
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