III-V Quantum Wells and Dots
In the Photonics Theory Group we model the optical performance of state-of-the-art novel materials for the development of the next generation of key communications devices such as lasers and optical amplifiers. The materials of interest and capabilities vary from strained III-V quantum wells to InAs/GaAs quantum dots grown on (100) substrates as well as the very new technologies of site-controlled Pyramidal Quantum Dots.
We aim to simulate and evaluate the electronic and optical properties of existing III-V materials to improve and optimise the performance of currently available devices as well as to understand new and exciting materials developed here at Tyndall. In our analysis we contribute to the improvement of energy efficiency, increased power generation, reduction in optical losses for enhanced communications technologies as well as increases in signal speed and bandwidth. Our work is performed in conjunction with experimentalists both here in Tyndall and across Europe and is funded both by SFI and EU projects.
Site-controlled quantum dots (QDs) grown on patterned (111)B GaAs substrates here at Tyndall are suitable for the generation of single and entangled photons. These novel InGaAs/GaAs site-controlled QD structures demonstrate record-breaking spectral purity. We are developing the first comprehensive theoretical analysis of the emission energies and optical properties of this system to understand growth processes as a function of QD height and In concentrations and the ultimate limits of what can be achieved through this growth approach.
Fig. 1 The sample layer structure; 1. Al0.55Ga0.45As, 2. GaAs, 3. InxGa1-xAs.
|Fig. 2. TEM Cross sectional view of quantum
dash layers (courtesy of nanoplus).
Quantum dots offer lucrative potential benefits such as a lower and temperature insensitive threshold current compared with quantum well lasers. Difficulties arise in extending existing GaAs-based materials to the second telecommunications window, around 1.5µm, so InAs quantum dots grown on InP substrates represent a promising alternative. These InAs/InP quantum dash lasers have demonstrated many impressive characteristics such as high modal gain per dash layer and high characteristic temperature, T0. As part of the ZODIAC project we have theoretically and experimentally investigated the properties of these devices, identifying consequences of the novel band structure and providing important information to the growth partner for the next generation of device.
Gain and loss mechanisms in InAs/GaAs 1.3 µm Quantum-Dot Lasers and Optical Amplifiers
We have applied our understanding of the fundamental electronic structure of InAs/GaAs dots to the real life limitations of QD devices. We have with University of Surrey analysed both theoretically and experimentally the temperature dependence of the threshold current density of 1.3µm InAs/GaAs QD lasers. We have identified that the weak temperature variation measured for the threshold radiative current density arises because the recombination rate for excited state transitions is significantly slower than for the ground state transition. In contrast, nonradiative Auger recombination can have a similar probability for transitions involving excited states as for those involving ground state carriers. This leads to a sharp increase in the threshold current density at high temperatures, similar to that already observed in quantum well lasers.
InGaAs QW active regions for VCSELS.
High-speed short wavelength (850 nm) VCSELs are required for future high capacity, short reach data communication links. The modulation bandwidth of such devices is intrinsically limited by the differential gain of the quantum wells (QWs) used in the active region. We have performed gain calculations which show that the incorporation of 10% indium in an InGaAs/AlGaAs QW structure can approximately double the differential gain compared to a GaAs/AlGaAs QW structure. Excellent agreement is obtained between the theoretically and experimentally determined values of the differential gain, confirming the benefits of strained InGaAs QW structures for high-speed VCSEL applications.
Fig. 3 Valence band structure of GaAs based VCSEL and highspeed InGaAs based device