Microstructure and residual stress in hydrogenated amorphous silicon (a-Si:H) layers
Master Thesis
2006
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University of Cape Town
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Hydrogenated amorphous silicon (a-Si:H) is known to be highly disordered. The disorder introduces a high amount of defects in the network, such as bond length and angle deviation, non-coordinated bonds or voids. In this work the microstructural characterization and a new approach for strain determination in hydrogenated amorphous silicon in the layers is determined by synchrotron diffraction. The a-Si:H layers were deposited by hotwire chemical vapour deposition (HW -CVD) on a glass substrate at a growth temperature of 300°C and 500°C, respectively. The microstructural state from the diffraction phase has been experimentally determined using the pair correlation function, calculated using Fourier transformation of the scattered intensity. Indication of the residual stress in the layers was obtained using the conventional sin²ψ method, normally used and especially developed for polycrystalline materials, but here applied to an amorphous structure. It is found that residual stress and microstructure of the layers are correlated The pair distribution function (PDF) of the short range order reveals a significant shift in the nearest neighbour distance of Si-Si pairs leading to bond strain in the layers. The PDF reveals that the short-range bonding of a Si:H is tetrahedral but it does not give much information about the intermediate region which relates to the structural topology. We observed a deviation in the nearest-neighbour and the second nearest-neighbour separation, independent of the growth temperature. The observed strain curves deviate from the linear prediction of the conventional sin²ψ method. The layers were found to be generally under compressive stress, with strong gradients dominant in the interface region of the sample, and the resulting stress is highly dependent on the details of the deposition process.
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Ramukosi, F. 2006. Microstructure and residual stress in hydrogenated amorphous silicon (a-Si:H) layers. University of Cape Town.