Semiconductor quantum dots give atom-like energy levels in a semiconductor material. This project aims to investigate the electronic structure of QDs, its consequences for gain and loss mechanisms in devices, and approaches to the design of devices which will optimize the device gain, while also suppressing the intrinsic loss mechanisms. It is part of a combined experimental and theoretical investigation of QD materials development. We seek to expand the wavelength range accessible to QDs into the mid-infrared, to investigate the control of carrier dynamics and loss processess by careful dot design, and to study the unexplored physical properties of dot within dot systems. The students work will focus on theoretical analysis of the electronic band structure of these materials and its consequences for gain and loss mechanisms. The work will also involve close collaboration with experimental colleagues investigating the gain and loss mechanisms, and relaxation on an ultra-fast timescale in near and mid-infrared QD devices.
Much progress has been accomplished with compound-semiconductor quantum dot (QD) materials based on self-assembled growth techniques. Several types of optoelectronic devices have been realized using such QD materials, including light emitting diodes, lasers and optical amplifiers. However, progress in QD lasers has saturated for a variety of reasons, including the continued presence of significant intrinsic loss mechanisms such as Auger recombination and intervalence band absorption in lasers designed for telecomm applications.
This project aims to investigate the electronic structure of QDs, its consequences for gain and loss mechanisms in devices, and approaches to design devices which will optimize the device gain, while also suppressing the intrinsic loss mechanisms.
The group at Tyndall has developed efficient techniques to calculate the electronic structure of realistic QD structures, including the effects of dot shape, size, composition and built-in strain. These programs will first be used to determine the electronic structure of archetypal semiconductor QDs. The project will then use the calculated electronic structure as a starting point to address the following research issues:
1) Existing QD lasers have achieved excellent threshold characteristics, but the threshold current in 1.3 m lasers still appears to be dominated by Auger recombination, which leads to a strong temperature dependence of the laser characteristics. Using the band structure, we will calculate the magnitude and wavelength dependence of Auger recombination in typical QDs. How does the recombination rate vary between the ground and excited states, and how does it depend on dot size? Overall, can we engineer the dot electronic structure to miinimise the intrinsic losses at 1.3 m?
2) At longer wavelengths, Auger is an even greater issue, and is the limiting factor which has prevented the development of conventional lasers in the 3 V 4 m range for gas-sensing applications. It has been suggested that the Auger rate can be reduced in antimonide-containing QDs. This may occur, because the dot can be designed so that the energy gap and recombination energy is smaller both than the band offset and also than the valence band spin-orbit splitting. Auger recombination processes can then only be associated with transitions within the dot. Having established in step 1) the factors determining Auger in 1.3 m lasers, we shall then extend our calculations to identify the extent to which antimonide-based QDs can bring benefits at longer wavelengths, opening the possibility of room temperature semiconductor lasers at these longer wavelengths.
3) The calculations above will all be undertaken assuming conventional quantum dot structures. Once we have established the dependence of gain and loss mechanisms on dot composition and band structure for conventional QDs, the final part of the project will investigate whether the dot structure can be further engineered to achieve optimum performance, including the impact of deliberate n- or p-doping, and the creation for instance of dot-within-a-dot structures.
The Tyndall group has a well established collaboration both with theorists and experimentalists at the University of Surrey investigating gain and loss processes in semiconductor lasers. It is anticipated that this collaboration will continue, and we therefore request funds for several short visits to Surrey, both for training and exchange of information.
Overall, the outcome of this project should lead to a considerably improved understanding of gain and loss mechanisms in semiconductor QD lasers, and identify paths towards their optimization both in the key telecomm wavelength range and also at longer wavelengths for sensor applications.
Informal enquiries concerning this studentship can be made to: Mary O'Regan (tel. +353 (0)21 490 4372)
To apply for this position, please submit a CV to the e-mail address below, quoting the reference number. |
