Metal Organic Vapour Phase Epitaxy of Strained InP-Ga(x)In(1-x)As(y)P(1-y) Quantum Well Lasers

Smith, Arthur David (1998) Metal Organic Vapour Phase Epitaxy of Strained InP-Ga(x)In(1-x)As(y)P(1-y) Quantum Well Lasers. PhD thesis, University of Glasgow.

Abstract

This thesis describes experimental investigations into the application of strain to the wells and barriers of InP-Ga(x)In(1-x)As(y)P(1-y) based, multiple-quantum well (MQW) lasers. Material characterisation and device performance results are used to evaluate, both qualitatively and quantitatively, the effects of strain incorporation into structures grown by low-pressure metal organic vapour phase epitaxy. Multiple-quantum well laser structures, operating at wavelengths around 1.5?m, are grown with either strain compensated (zero-net strain) or non-compensated active regions. Strain compensated Ga(x)In(1-x)As(y)P(1-y)-Ga(x)In(1-x)As(y)P(1-y) multiple-quantum well structures are shown to suffer from thickness modulations after a given number of wells and barriers are deposited. The thickness modulations in the compressively strained wells are in anti-phase to those observed in the tensile strained barriers. Experimental evidence is presented which relates the severity of this effect to the composition of the alloys used and the conditions under which they are grown. The source of the thickness modulations is attributed to two effects: compositional clustering (spinodal decomposition) of Ga(x)In(1-x)As(y)P(1-y) alloys and strain relaxation via the expansion and contraction of lattice planes. Further experimental data is presented that describes the optimisation in growth conditions to suppress thickness modulations, enabling larger numbers of planar wells and barriers to be grown. The effects of compositional clustering can be minimised by using higher growth temperatures and selecting alloy compositions outside the miscibility gap. In contrast, strain relief via the expansion and contraction of lattice planes is eliminated by using lower growth temperatures and higher group V pre-cursor gas flows. The growth conditions which best overcome these two effects in +/-1% Ga(x)In(1-x)As and +/-1% Ga(x)In(1-x)As(y)P(1-y) strain-compensated, multiple-quantum well laser structure are described. The strain relief behaviour of non-compensated InP-Ga(x)In(1-x)As(y)P(1-y) based, multiple-quantum well structures is presented. Dislocation generation is identified as the primary strain relief mechanism in Ga(x)Inl-[x]As(y)P(1-y)/Ga(x)In(1-x)As MQW structures with tensile strain in the wells. Thickness modulations are identified as a secondary strain relief mechanism in such structures, occurring at higher values of tensile strain. The growth optimisation techniques developed for strain compensated structures are also shown to suppress thickness modulations in non-compensated structures. Strain relief in non-compensated structures is influenced by the type of barrier material used: a Ga(x)In(1-x)As(y)P(1-y) well with a Ga(x)In(1-x)As(y)P(1-y) barrier suffers from thickness modulated growth, whereas the same Ga(x)In(1-x)As(y)P(1-y) well with an InP barrier is shown to relax via the introduction of strain relieving dislocations. Broad-area 50?m stripe, ridge waveguide Fabry-Perot, ridge waveguide DFB lasers and electro-absorption modulators are used to study the effects of strain on device performance. Consistent with theoretical predictions, improvements in the threshold current and gain are observed in strain compensated Ga(x)In(1-x)As(y)P(1-y)-Ga(x)In(1-x)As(y)P(1-y) MQW lasers with compressively strained wells, when compared to similar structures containing lattice matched or compressively strained ternary wells. Further performance improvements are observed in high speed, directly modulated lasers when similar MQW active regions are used. A fourfold improvement in differential gain is observed over a similar lattice matched structure, this contributed to a 3dB frequency response of 22GHz measured on a DFB laser. MQW lasers containing tensile strained wells demonstrated the highest gain performance of all of the strained laser structures examined. This was achieved when sufficient tensile strain, greater than 1%, was introduced to the wells. An improvement in gain of up to 100% was observed over similar structures containing 1.1% compressively strained, ternary wells. In addition to improving the material performance of InP-based alloys, strain is used to engineer the valence band of an MQW, electro-absorption modulator structure. By introducing compressive strain in the wells and tensile strain in the barriers, the hole confinement in the wells is reduced. This alleviates the hole trapping effects observed in similar lattice matched structures and results in improved power handling characteristics and high speed (10 Gbits/sec) performance.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Adviser: John Marsh
• Keywords: Electrical engineering, Condensed matter physics, Materials science
• Date of Award: 1998
• Depositing User: Enlighten Team
• Unique ID: glathesis:1998-75379
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 19 Nov 2019 20:21
• Last Modified: 19 Nov 2019 20:21
• URI: https://theses.gla.ac.uk/id/eprint/75379