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Title: Efficiency limiting processes and optimisation of silicon compatible lasers for optoelectronic integration and optical interconnects
Author: Read, Graham W.
ISNI:       0000 0004 5918 3884
Awarding Body: University of Surrey
Current Institution: University of Surrey
Date of Award: 2015
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Optoelectronic integration on silicon is an area of increasing interest for both physicists and the microelectronics industry. Due to the limitations of silicon as an optical gain medium, the integration of III-Vs with silicon microelectronics has become a prominent area of research. However, the fundamental physical differences between these materials has caused such lasers to be strongly limited by non-radiative recombination. Studies of these mechanisms are therefore essential for solutions to be developed that will allow commercially viable III-V/Si lasers to be fabricated. This thesis presents such studies for three of the four leading approaches to producing III-V/Si lasers (quantum dots on silicon are not studied), with conclusions on the relative performance of each presented in the final chapter. AlGaInAs/InP laser active regions wafer bonded onto pre-processed silicon-on-insulator waveguides have exhibited strong performance, with electrical injection lasing demonstrated at room temperature. However, large and temperature sensitive threshold current densities of ~ 3.4-6.16 kAcm⁻² indicate that the devices are not yet optimised. Defect current fractions of 22-39% suggest that significant densities of threading dislocations propagate to the active region during bonding. In addition low T0 and T1 values, a lack of carrier density pinning and Z parameter values in excess of three suggest the presence of both carrier leakage and inter valence band absorption. Development of the GaNAsP active material, has allowed lattice matched, direct epitaxial growth on silicon. The polar/non-polar III-V/Si interface and thermal expansion coefficient mismatch however, cause large densities of defects to form. As such, up to 68% of carriers are found to recombine non-radiatively via defect states. Improvements to performance are achieved by the use of MQW structures and by optimising the silicon surface orientation to minimise the formation of anti-phase domains, leading to an 18% increase in radiative current fraction. However, defect densities remain large, with additional thermally activated defects potentially caused by the diffusion of nitrogen from the QW, forming defect states at the QW/barrier interface. These states may also form the carrier leakage path, responsible for up to 27% of recombination. Optimisation of the GaNAsP lasers by optical simulation predicts a potential increase in optical confinement factor from 0.35% to 0.6% and 1.33% to 1.73%, corresponding to reductions in threshold current of 41% and 27% for single and multiple quantum well structures, respectively. Poor electrical performance was investigated by SEM of the contacts. This also identified limitations to the lithography, etching and metalisation, which caused among other effects, the burning of contacts under electrical injection. A processing optimisation study eliminated the contact burning, improved the IV characteristics and increased the facet output power by almost an order of magnitude. The addition of a post metalisation annealing step was also found to reduce the p and n-type contact resistances by 64% and 20% respectively. A final method studied in this thesis is the use of GaInSb composite quantum wells grown on GaSb. III-Sbs have been demonstrated to accommodate significant strain as well as tending to prevent dislocations from propagating in the growth direction. Lasing at room temperature with a threshold current density of 426 Acm⁻² is observed, with a radiative current fraction of 41%. Carrier leakage is found to be the dominant source of loss, accounting for 39% of recombination. However, based on pressure dependence studies an increase in carrier confinement of only 20 meV may be enough to halve this. The remaining 20% of carriers are thought to recombine via Auger processes, particularly CHSH due to the small difference between the bandgap and the spin-orbit splitting.
Supervisor: Sweeney, Stephen J. Sponsor: Engineering and Physical Sciences Research Council (EPSRC)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available