Design and characterisation of quantum dot lasers

Alsayyadi, Maryam 2024. Design and characterisation of quantum dot lasers. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

InAs quantum dot (QD) lasers grown on GaAs substrates have demonstrated significant advantages over quantum well devices, including low threshold currents and long-term reliability, particularly for applications on silicon platforms. For longer wavelengths around 1550 nm, InAs QD lasers on InP substrates have been explored; however, limited studies exist for achieving high-performance QD lasers in this range. This work primarily focuses on lasers fabricated from InAs quantum dots (QDs) on InP substrates grown by metal-organic chemical vapor deposition (MOCVD). High performance AlGaInAs/InP multi-quantum-well (MQW) laser diodes are first examined for comparison, with the QW structures exhibiting a gain parameter (NG0) as high as 94.4 cm⁻¹ for 9-QW layer samples. Simulations of AlInGaAs/InP MQW laser diodes highlight the poor performance at high temperature and demonstrate the benefits of incorporating an electron stopper layer. This layer may be useful to reduce thermionic emission and electron leakage while maintaining efficient carrier injection. For samples intended to operate as quantum dots the laser performance is explored and the question as to whether dots, dashes or modulated quantum wells are formed under varying growth conditions is examined. Here performance limitations related to a small optical gain are identified. Epitaxial structures are grown on n-type InP substrates using MOCVD, and broad-area lasers with oxide-isolated stripe widths of 50 μm are fabricated using standard photolithography. The lasers feature uncoated facets, cavity lengths ranging from 300 μm to 2000 μm, and emission wavelengths around 1550 nm. A 5-layer InAs/InP QD laser structure with 0.7 nm capping layers achieved optical gain of approximately 60 cm⁻¹ and ground-state lasing up to 390 K for cavity lengths as short as 330 μm. These structures demonstrate minimal gain saturation, with a G0 per layer similar to the quantum well lasers, showcasing their potential as optical sources for silicon-photonics. Additionally, 3-layer and 5-layer QD structures with 4 nm capping layers are investigated, with their performance under various growth conditions evaluated, including the use of (Al)InGaAs capping layers. Three distinct laser diode structures, each with two fabrication orientations, are analyzed to differentiate between dot and dash morphologies. A multi-section technique is employed to measure modal gain and absorption spectra. At room temperature, internal optical losses ranged from to 15±0.2 cm⁻¹, for quantum dashes (QDH) to 18±2 cm⁻¹, for quantum wells (QW). Peak modal gains of 40±2 cm⁻¹ and 24±2 cm⁻¹ were observed for QW and QDH lasers, respectively, under injected current densities. These findings highlight the potential of InAs/InP QDH lasers for operation on silicon at 1.55μm.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Schools > Physics and Astronomy
Subjects:	Q Science > QC Physics
Uncontrolled Keywords:	Quantum dot lasers, quantum-dash laser, semiconductor lasers, laser diode, C-band, L-band, Optical gain, MOCVD
Funders:	SACB
Date of First Compliant Deposit:	17 June 2025
Last Modified:	17 Jun 2025 14:28
URI:	https://orca.cardiff.ac.uk/id/eprint/179118

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