Ghisoni, Marco Umberto Paolo; (1993) Post-growth band gap engineering for optoelectronic integration. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis presents results on a post-growth technique for selectively altering the shape of GaAs/A1GaAs quantum well (QW) material, to produce a resultant blue shift in the optical spectra. The technique used is known as impurity free vacancy diffusion (IFVD), and involves encapsulation with SiO 2 followed by rapid thermal annealing at elevated temperature. This is shown to give rise to surface Ga vacancies, that rapidly diffuse through the structure enhancing the intermixing of the group III atoms at the well/barrier interface, and resulting in a blue shift of the lowest confined state. Work presented shows that IFVD allows large selective blue shifts while still retaining QW characteristics i.e. excitonic features and the optoelectronic response, namely the quantum confined Stark effect (QCSE). The diffused well can be successfully modelled, to a first approximation by an error function solution. The depth dependence expected is not observed and this could be due to a gallium vacancy diffusion coefficient higher than previously reported. IFVD is used to produce two different types of reflection modulators from the same wafer, demonstrating that the technique has applications to surface-normal devices and also that it can be used in conjunction with multiple well structures. The lateral resolution is found to be reasonable, and the change in well profile induced is shown to improve the carrier escape from the well and the saturation intensity. Importantly the use of dielectric caps of increasing thickness is shown to produce increased levels of intermixing and hence blue shift. This last result is of particular importance since the ability to produce selective area bandgap engineering has applications in the fabrication of optoelectronic integrated circuits (OEICs). The thesis ends with a discussion of future directions which IFVD may take including applications in the field of wavelength division multiplexing (WDM).

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