Fabrication and Assessment of GaAs/AlGaAs Multiple Quantum Well Optical Modulators

McIlvaney, Karen (1991) Fabrication and Assessment of GaAs/AlGaAs Multiple Quantum Well Optical Modulators. PhD thesis, University of Glasgow.

Abstract

This thesis outlines the design, fabrication and testing of electroabsorption modulators and modulator arrays in GaAs/AIGaAs multiple quantum well material. Single electroabsorption modulators were designed to have a high contrast ratio and low insertion loss. For this purpose a 200 period structure, consisting of isolated wells 50 A wide, was grown by MOVPE. The electric field was applied, and light was incident, perpendicular to the epitaxial layers of the device. On applying a reverse bias, a red shift of the absorption edge was observed. The performance of the modulator was as follows: for an applied voltage of 25 V and working at an optimised wavelength of 837 nm, the modulation depth was 8.95 dB accompanied by an insertion loss of 4.9 dB for a device with no anti-reflection coatings. As a method of assessing the suitability MQW modulators for the routing of optical signals in a communications system, individual modulators were configured into a 4x4 array, thus forming a two dimensional spatial light modulator. One of the key requirements of such a device is to eliminate the need for many electrical connections which hinders the operation of large arrays. A row and column line addressing system was used which is capable of expansion to large arrays, but which reduces the maximum contrast ratio and alters the optimum wavelength of operation from that of single modulators. The modulation depth of the array was reduced to 6.1 dB at an optimised operating wavelength of 838.1 nm by a combination of the addressing system chosen and of nonuniformities in the epitaxial material. Theoretical calculations have predicted that coupled well MQW structures have advantages over uncoupled MQW structures in terms of achieving a high contrast ratio without the need of a high drive voltage. Two devices containing coupled well structures were designed and fabricated. The first device contained a superlattice with 200 x 50 A wells coupled through 30 A barriers. Although changes in the absorption spectrum were seen for voltages as low as 2 V, the superlattice exhibited low modulation depths (8%). The second design used pairs of coupled wells isolated by thick barriers to enhance the electroabsorption effect. In contrast to structures containing isolated wells, under a reverse bias this structure exhibited an effective blue shift in the absorption edge at low electric fields, > 9 x 10e3 Vcm -1, and an effective red shift at fields > 3 x 10e4 Vcm-1. This feature can be optimised to provide novel operation of modulator arrays and a modulation depth of ~ 2 dB was predicted for a working voltage of 8 V. For single modulators, a maximum modulation depth of 3.8 dB was inferred from photocurrent data for an applied voltage switching between 1 V and 8 V and an operating wavelength of 818 nm.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Electrical engineering, Materials science, Condensed matter physics
• Date of Award: 1991
• Depositing User: Enlighten Team
• Unique ID: glathesis:1991-78209
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 30 Jan 2020 15:37
• Last Modified: 30 Jan 2020 15:37
• URI: https://theses.gla.ac.uk/id/eprint/78209