Multiple Quantum Well Structures As Optical Waveguides

MacBean, Myles D. A (1986) Multiple Quantum Well Structures As Optical Waveguides. PhD thesis, University of Glasgow.

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This thesis is concerned with the design, fabrication and characterisation of semiconductor optical waveguides in which the high index guiding layer is a multiple quantum well structure (MQWS), consisting of alternate layers of high and low band gap semiconductors with the electrons and holes in the MQWS being confined to the low band gap material. This confinement in two dimensions alters greatly the electronic and optical properties of the MQWS in comparison to the bulk properties of the constituent layers. The basic concepts involved in MQW waveguides are introduced using an elementary quantum mechanical analysis of quantum wells together with a brief description of the properties of dielectric waveguides, A more detailed treatment of the electronic and optical properties of MQWS and a review of published experimental work is used to show that the fundamental absorption edge is much more abrupt than that in the corresponding bulk material with strong excitonic characteristics being evident even at room temperature. In addition, the absorption edge is seen to be anisotropic with the fundamental energy gap being larger for light polarised perpendicular to the MQW layers. This anisotropic absorption edge, together with the layered dielectric nature of MQWS, makes them birefringent with a smaller refractive index for light polarised perpendicular to the MQW layers. The quantum confinement of carriers in MQWS also enhances their electroabsorption and electro-optic properties through the quantum confined Stark effect. Standard techniques used in the design, fabrication and analysis of bulk semiconductor waveguides are developed for application to MQW waveguides. These include analytical and numerical techniques for the design of dielectric waveguides; dry etching and metallisation processes for the fabrication of devices; and a laser/optics system to analyse the waveguide devices. To verify these techniques they are first applied to the well-understood case of n/n+ GaAs waveguides and are used to successfully fabricate and analyse single-mode, passive, rib waveguides at l=1.15mum. The electro-optic coefficient is also measured in an active, planar n/n+ waveguide and found to be close to that reported by other workers. The design techniques are then applied to MOWS waveguides resulting in the design of a MQW double heterostructure (MQW-DH), p-i-n diode which was predicted to produce the required Quantum properties (strong, room temperature, excitonic behaviour), waveguide properties (single-mode propagation up to the fundamental absorption edge) and electronic properties (a high reverse bias breakdown voltage and uniform applied electric field). Most of the theoretical work and all the experimental work included is devoted to MQWS in the (Al,Ga)As, III-V semiconductor alloy system. Accordingly, the methods available for growing MQWS in this system are reviewed with Molecular Beam Epitaxy (MBE) being found the most likely method to satisfactorily reproduce the desired structure. MQW-DH were grown at two establishments and are initially studied by photoluminescence and scanning electron microscopy before their planar optical waveguide characteristics are checked using the laser system. Only one sample is found to satisfy all the design requirements, and then only partially. Detailed analysis of the properties of MQW waveguides is therefore limited to this structure. Passive MQW-DH waveguides are demonstrated to exhibit an anisotropic absorption edge as predicted, and it is shown that the design and fabrication techniques developed can be successfully used to obtain single, double and multi-mode strip loaded waveguides. Single-mode waveguides are also used to fabricate passive directional couplers with coupling lengths in good agreement with theoretically predicted values. A semi-empirical model is put forward to describe the band edge electro-absorption of MQWS. Although simple, the model is in qualitative and approximate quantitative agreement with published results. (Abstract shortened by ProQuest.).

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Electrical engineering
Date of Award: 1986
Depositing User: Enlighten Team
Unique ID: glathesis:1986-77375
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 14 Jan 2020 09:10
Last Modified: 14 Jan 2020 09:10

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