We present an approach to improving the performance of solution processed organic semiconductor transistors based on a dual solvent system. We here apply this to a blend containing the p-conjugated small molecule 6,13 bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) and polystyrene, which acts as an inert binder. Using a semiconductor-binder solution of two solvents, where the main solvent is a better solvent of the small molecule and second solvent is a better solvent of the polymer, crystal morphologies can be altered and transistor mobilities increased by almost an order of magnitude. In this way, air-ambient and solution-processed transistors with linear and saturation mobilities higher than 1 cm 2 V À1 s À1 have been fabricated. We discuss how the solubility properties of the formulation components can be used to identify solvent candidates that promote an efficient self-assembly of the small molecule.

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Reflectometry using a white light source has been applied to in situ monitoring of metal organic vapour phase epitaxy of InGaN alloy structures on GaN buffer layers. Both InGaN epilayers 60-350 nm in thickness and InGaN/GaN multi-quantum-well (MQW) structures with periods of order 10 nm were studied. The InGaN epilayers have indium mole fractions between 0.105 and 0.240, determined principally by the growth temperature. The standard method of deriving film growth rates from in situ reflectance data is a useful predictor of InGaN epilayer thicknesses, and monitoring at wavelengths of 600 or 800 nm minimizes complications caused by absorption and scattering. For a set of seven InGaN epilayers, the average agreement between reflectance-derived thicknesses and estimates based on Rutherford backscattering is within 5%. Uncertainties in these measurements arise from the significant surface roughness of the films, an imprecise knowledge of optical constants and apparent short-term fluctuations in growth rates. Growth rates obtained from in situ monitoring of InGaN epilayers and GaN grown under the same conditions as MQW barriers can be used to successfully predict layer thicknesses in actual QW structures. We illustrate this methodology by comparing predicted layer thicknesses in 10-and 18-period MQW structures with results from conventional ex situ characterization, using transmission electron microscopy and x-ray diffraction.

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GaN low temperature nucleation and high temperature buffer layers have been grown by metalorganic vapour phase epitaxy. The effect of nucleation layer annealing temperature and buffer layer growth temperature on their microstructure and morphology has been examined by atomic force microscopy and X-ray diffraction and correlated to the in-situ reflectivity obtained from the growing layers. For the nucleation layer, the anneal was observed to lead to the formation of GaN nucleites on the substrate surface, the properties of which changed markedly with temperature. Surface pits resulting from dislocations with a screw component have been observed and the variation of the density with growth temperature ascertained.