Quantum Dot bilayer laser diodes

Optical communication was developed to allow high-speed and long-distance data transmission and is currently a £6bn market. This has also led to the adoption of optical technologies in other areas including the CD, DVD and medical imaging systems. Standardisation of components means that these systems require light sources that operate near the 1310 and 1550 nm telecommunications windows but existing lasers here are expensive due to their high temperature sensitivity. The exploitation of quantum con¯nement has led to the development of \quan- tum dot" (QD) laser material because of predictions of huge gains in performance. Emission wavelengths of InAs/GaAs QD lasers have been extended to the telecom- munications window near 1300 nm by various growth technologies and the first commercial devices have recently been brought to the market. However, progress to longer wavelengths has been stalled for several years as well as the speed and tem- perature sensitivity of these devices falling short of the predictions; partly because QDs are grown by self-assembly resulting in a random distribution of sizes, compo- sitions and strain-states, leading to inhomogeneous broadening which is a departure from the ideal \atom-like" system. This work details the growth, design and development of QD bilayer laser devices, which o®er a unique approach to fixing these shortcomings. When two QD layers are grown close together; the first layer provides a template that allows larger, more uniform QDs to be grown in the second layer, giving greater uniformity and deeper confinement. This has the potential to increase the efficiency and to achieve emission wavelengths out towards the more-commonly used telecommunications window at 1550 nm directly on GaAs substrates. Multiple bilayer laser diodes with inhomge- neous broadening of less than 30meV, lasing at up to 1430 nm and room-temperature photoluminescence at 1515 nm are shown. Despite the vastly reduced inhomogeneous broadening of QD bilayers, it is still found to be a relevant factor due to the change from de-localised geometries of quantum wells to an ensemble of separate QDs. It will be shown that understanding this is essential for describing the observed optical and electrical behaviour of the laser diodes

Simulation of facet heating in high-power red lasers

A two-dimensional self-consistent laser model has been used for the simulation of the facet heating of red emitting AlGaInP lasers. It solves in the steady-state the complete semiconductor optoelectronic and thermal equations in the epitaxial and longitudinal directions and takes into account the population of different conduction band valleys. The model considers the possibility of two independent mechanisms contributing to the facet heating: recombination at surface traps and optical absorption at the facet. The simulation parameters have been calibrated by comparison with measurements of the temperature dependence of the threshold current and slope efficiency of broad-area lasers. Facet temperature has been measured by micro-Raman spectrometry in devices with standard and non absorbing mirrors evidencing an effective decrease of the facet heating due to the non absorbing mirrors. A good agreement between experimental values and calculations is obtained for both devices when a certain amount of surface traps and optical absorption is assumed. A simulation analysis of the effect of non absorbing mirrors in the reduction of facet heating in terms of temperature, carrier density, material gain and Shockly-Read-Hall recombination rate profiles is provided

Impact of the capture time on the series resistance of quantum-well diode lasers

Electrons and holes injected into a semiconductor heterostructure containing quantum wellsare captured with a finite time. We show theoretically that this very fact can cause a considerableexcess contribution to the series resistivity and this is one of the main limiting factors to higherefficiency for GaAs based high-power lasers. The theory combines a standard microscopic-basedmodel for the capture-escape processes in the quantum well with a drift-diffusion description ofcurrent flow outside the quantum well. Simulations of five GaAs-based devices differing in theirAl-content reveal the root-cause of the unexpected and until now unexplained increase of theseries resistance with decreasing heat sink temperature measured recently. The finite capturetime results in resistances in excess of the bulk layer resistances (decreasing with increasingtemperature) from 1 mΩ up to 30 mΩ in good agreement with experiment

Monolithic 1.3 μm InAs/GaAs Quantum Dot Lasers on Silicon: Simulation and Experiment

1.3 μm quantum dot (QD) lasers epitaxially grown on silicon have attracted great interest as light source for silicon photonics and other optical communication applications. This work focuses on improving the understanding of the physical mechanisms limiting the performance of these devices, and on studying the laser dynamics with respect to data transmission potential through simulation and experiment. Dislocation-induced carrier loss is a major concern for the performance of QD lasers on silicon. Part of this work aims, therefore, at identifying the processes degrading the laser characteristics as well as on understanding the performance disparity between silicon-based QD and quantum well (QW) devices. By using two specially extended types of rate equation travelling-wave models it is found that enhanced carrier loss at higher dislocation densities leads to a much larger laser threshold increase in QW than in QD lasers. The QD laser’s increased tolerance to dislocations can be explained based on efficient and ultrafast carrier capture into the QDs, where high energy barriers prevent them from migrating into defects. The carrier density reduction in the higher energy continuum layers is eventually reflected in a lower current injection efficiency and thus reduced light-current slope, confirming experimentally observed trends of 1.3 μm QD lasers on silicon. In particular, a large minority carrier diffusion length is identified as a key parameter inhibiting laser operation in QW-based devices. The other part of this work focuses on investigating the QD lasers’ dynamics by means of gain switching, small-signal and large-signal modulation. 150 ps short gain-switched pulses, modulation bandwidths of 1.6 GHz to 2.3 GHz, and optical eyes at 1.5 Gb/s are obtained from 2.5 mm long ridge-waveguide lasers grown and fabricated at University College London. Numerical simulations reveal that the observed high-speed limitations are a result of limited gain and a long photon lifetime, whereas suitability for 10 Gb/s operation is predicted in an optimised laser design. It was, furthermore, found that neither dislocation-induced carrier loss nor optical loss limit the modulation characteristics fundamentally. The reduced carrier lifetime is, however, reflected in stronger damping of the small-signal modulation curves. Apart from that, the overall device dynamics remain governed by the QD physics.Qualcomm Inc. School of Technology PhD scholarshi

Semiconductor Laser Based on Thermoelectrophotonics

This dissertation presents to our knowledge the first demonstration of a quantum well (QW) laser monolithically integrated with internal optical pump based on a light emitting diode (LED). The LED with high efficiency is operated in a thermoelectrophotonic (TEP) regime for which it can absorb both its own emitted light and heat. The LED optical pump can reduce internal optical loss in the QW laser, and enables monolithically integrated TEP heat pumps to the semiconductor laser. The design, growth and fabrication processes of the laser chip are discussed, and its experimental data is presented. In order to further increase the TEP laser efficiency the development of QDs as the active region for TEP edge emitting laser (EEL) is studied. The usage of QD as TEP laser\u27s active region is significant in terms of its low threshold current density, low internal optical loss and high reliability, which are mainly due to low transparency in QD laser. The crystal growth of self-organized QDs in molecular beam epitaxial (MBE) system and characterization of QDs are mentioned. The design, growth, processing and fabrication of a QD laser structure are detailed. The characteristics of laser devices with different cavity length are reported. QD active regions with different amount of material are grown to improve the active region performance. Theoretical calculations based on material parameters and semiconductor physics indicate that with proper design, the combination of high efficiency LED in TEP regime with a QD laser can result in the integrated laser chip power conversion efficiency exceeding unity

Optical and structural properties of InGaAs self assembled quantum dots

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