Tombling, Craig; (1990) Design and study of gallium arsenide optical modulators exploiting quantum well excitonic quenching. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The thesis describes the development of a gallium arsenide (GaAs) optical waveguide modulator suitable for the external or remote modulation of a laser source. Design techniques, fabrication, measurement tools and assessment are detailed and the device application is discussed. In short, the device is a waveguide quantum well absorption modulator using the quenching of the excitonic resonance by an electron gas as the mechanism. A p-n or Schottky junction in the structure allows a small reverse bias to influence the electron density in a single quantum well, hence enabling controlled recovery of the excitonic absorption. Many devices have been developed which take advantage of quantum well excitonic absorption which is enhanced above that of the bulk material. This device contrasts most quantum well modulators by using the excitonic quenching mechanism rather than the conventional Stark shift of the excitonic peak. The epitaxial GaAs structures are fabricated into two device types, one allowing photocurrent to be measured and the second enabling optical waveguiding. These measurements, performed on several samples, form the bulk of the experimental assessment and demonstrate the desired excitonic quenching. Low temperature measurements show an enhanced modulation depth but more importantly highlight the features of the absorption spectrum. A model of the electrical characteristics of the device is developed and successfully acts as a design and analysis tool in conjunction with measured capacitance-voltage doping profiles. The limitations to the achievable modulation depth are discussed in view of the experimental data and projections of the performance are made. The ultimate limit to the absorption change at the band edge is investigated by modelling the subband absorption taking into account the anticipated bandedge shifts associated with the large carrier densities in the quantum well. A critical assessment of the state of the art monolithic optoelectronic integrated circuits is made and the duality of the modulation mechanism with that of a heterostructure FET is discussed in this context.

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