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Title: Optical properties of coupled quantum wells and their use as electro-absorptive modulators
Author: Atkinson, David
Awarding Body: University of London
Current Institution: University College London (University of London)
Date of Award: 1991
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This thesis describes some of the ways in which the coupling between closely spaced quantum wells may be used to enhance the performance of quantum well optical modulators. These are identified as having a useful role as electro-optic modulators for which operating voltage and contrast ratios are two crucial parameters. The background to Airy function solutions of Schrödinger's equation using a matrix matching technique is outlined and absorption spectra calculated at various applied electric fields for a range of quantum well structures. Through a consideration of coupling effects, the width of the barrier separating a pair of wells is optimised for optical modulation at room temperature and this is confirmed from photocurrent measurements. The electric field changes required are shown to be smaller than comparable multiple quantum well devices. Based upon this and an experimental determination of the absorption coefficient an asymmetric Fabry Perot reflection modulator employing coupled quantum wells is designed and fabricated. This exhibits the expected zero field reflectivity and similar devices should operate at low voltages giving good contrast ratios. A p-i-n diode with the wells in the intrinsic region is used to apply bias to these well structures. The diode has a built in voltage and for any low electric field device this can degrade performance. Allowance was made for this in the design of the device above by the use of an extra layer between the p and n regions. As a possible alternative, the use of a particular asymmetric well structure is investigated. Finally, the use of barriers narrower than normal in multiple quantum wells is modelled taking coupling into account. Modelling and photocurrent results show that a reduction in operating voltage through a reduction in barrier width is possible without coupling worsening the optical modulation.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available