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Title: Characterisation of lateral carrier out-diffusion and surface recombination in ridge waveguide devices
Author: Naidu, Deepal
Awarding Body: Cardiff University
Current Institution: Cardiff University
Date of Award: 2009
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As laser devices are scaled down in size and involve the use of photonic structures etched through the active layer - a trend driven by the desire to improve device performance and functionality for future applications in optoelectronic integrated circuits - performance limiting mechanisms such as an increasing internal optical loss, deteriorating gain-mode overlap, lateral carrier out-diffusion and surface recombination can inflict restrictions on the further miniaturisation and overall performance. In this project I have separately evaluated the relative impact of these mechanisms using ridge waveguide devices, with particular focus on the behaviour of the lateral out-diffusion and the surface-recombination mechanism in quantum-dot and quantum-well active regions. The approach of separately evaluating the relative impact of each mechanism is made possible by using the multisection characterisation technique. By this means the investigation in this study circumvents the problems associated with previous studies on lateral out-diffusion and surface recombination. Moreover, it furthers the overall analysis by measuring the effects as a function of injection-level and quantifying the change in non-radiative current density and overall internal quantum efficiency. In quantum-dot shallow etched ridge waveguide devices (S-RWG) it is found that the mechanism of an increasing internal optical mode loss and increasing lateral out-diffusion current are the principal causes for the apparent increase in threshold current density with reducing ridge width from 10 to 4 um. The internal optical loss was found to increase by a factor of 2.3 over this range and the non-radiative current density due to lateral out diffusion increased by a factor of 1.14 at an injection-level of 121 meV. The mechanism of a deteriorating gain-mode overlap was negligible in this range. Measurements of the lateral ambipolar diffusion length found that in self-assembled quantum-dot/wetting-layer systems the lateral ambipolar diffusion process can be inhibited in one of two ways: one good and one bad. This original result showed that the good way, in terms of benefiting the overall device performance, involves the inhibition due to three-dimensional carrier confinement in the quantum-dots. The other involves populating the wetting-layer to a point where a higher order non-radiative recombination process reduces the average carrier lifetime and hence ambipolar diffusion length. Both regimes can reduce the loss of carriers to lateral carrier out-diffusion and surface recombination however the latter is at the expense of increasing other non-radiative recombination processes. The ambipolar diffusion length was also found to be temperature dependent with a smaller diffusion length at lower temperatures. At 350 K the lateral ambipolar diffusion length varied from 0.75 um to a maximum value of 1.5 um over the injection-level range 65 meV to 84 meV. At 300 K the ambipolar diffusion length was smaller than 0.75 um for injection-levels below 121 meV. In quantum-well deep etched ridge waveguide devices (D-RWG) it was found that the D-RWG structure allowed much smaller ridge width (<2.8 (am) devices than S-RWG structures before as significant an increase in internal optical loss occurred. However, once a significant interaction of the wave amplitude and rough sidewalls does occur, the scattering loss in D-RWG structures was much more strongly affected. The D-RWG structure also provided no deterioration in the gain-mode overlap in the range 29 to 1.9 um. A power law analysis of the measured non-radiative current density revealed that the principal threshold increasing mechanism in D-RWG devices was surface recombination. The fractional increase in threshold non-radiative current density for 1.5 mm lasers was significant. From a width of 29.1 to 9.6 um the non-radiative current density increased by factor of 2 to a value of 462 A/cm2, and from 29.1 to 2.8 um by a factor of 11 to 2612 A/cm2. The overall internal quantum efficiency at threshold in the 1.5 mm lasers was measured to significantly decrease as the ridge width was reduced. This is a direct consequence of an increasing fraction of applied current recombining via surface recombination. The measured decrease from a ridge width of 29.1 um to 2.8 um was 16.6 % to 1.8 %. By characterizing the performance differences in the two RWG structures and the threshold increasing mechanisms of lateral out-diffusion and surface recombination in quantum-dot and quantum-well active regions respectively, knowledge of the criteria required for designing better devices for further miniaturisation and improved threshold performance was gained.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available