Dwe Allsopp

The impact of trench defects in InGaN/GaN light emitting diodes and implications for the “green gap” problem

The impact of trench defects in blue InGaN/GaN light emitting diodes (LEDs) has been investigated. Two mechanisms responsible for the structural degradation of the multiple quantum well (MQW) active region were identified. It was found that during the growth of the p-type GaN capping layer, loss of part of the active region enclosed within a trench defect occurred, affecting the top-most QWs in the MQW stack. Indium platelets and voids were also found to form preferentially at the bottom of the MQW stack. The presence of high densities of trench defects in the LEDs was found to relate to a significant reduction in photoluminescence and electroluminescence emission efficiency, for a range of excitation power densities and drive currents. This reduction in emission efficiency was attributed to an increase in the density of non-radiative recombination centres within the MQW stack, believed to be associated with the stacking mismatch boundaries which form part of the sub-surface structure of the trench defects. Investigation of the surface of green-emitting QW structures found a two decade increase in the density of trench defects, compared to its blue-emitting counterpart, suggesting that the efficiency of green-emitting LEDs may be strongly affected by the presence of these defects. Our results are therefore consistent with a model that the “green gap” problem might relate to localized strain relaxation occurring through defects. V C 2014 AIP Publishing LLC .[ http://dx.doi.org/10.1063/1.4896279]

Structural impact on the nanoscale optical properties of InGaN core-shell nanorods

The authors would like to thank OSRAM Opto Semiconductors for the provision of the GaN/Silicon templates and acknowledge the financial support from the European Union FP7 under Contract Nos. 228999 (SMASH) and 279361 (MACONS) and the EPSRC, UK (EP/M015181/1 “Manufacturing of nanoengineered III Nitride semiconductors”).

Electroabsorption in extremely shallow quantum wells: Comparison between theory and experiment

It is shown, via detailed comparison between measured electroabsorption spectra and electroabsorption spectra calculated by a full excitonic Green’s function method, a decoupled excitonic Green’s function method and a variational method, that accurate description of the Coulombic coupling between different subband pairs, including unbound quantum well states above the quantum well edge, is crucial for a correct prediction of electroabsorption in extremely shallow quantum well structures.

The ABC model of recombination reinterpreted: Impact on understanding carrier transport and efficiency droop in InGaN/GaN light emitting diodes

The efficiency of light emitting diodes (LEDs) remains a topic of great contemporary interest due to their potential to reduce the amount of energy consumed in lighting. The current consensus is that electrons and holes distribute themselves through the emissive region by a drift-diffusion process which results in a highly non-uniform distribution of the light emission and can reduce efficiency. In this paper, the measured variations in the external quantum efficiency of a range of InGaN/GaN LEDs with different numbers of quantum wells (QWs) are shown to compare closely with the predictions of a revised ABC model, in which it is assumed that the electrically injected electrons and holes are uniformly distributed through the multi-quantum well (MQW) region, or nearly so, and hence carrier recombination occurs equally in all the quantum wells. The implications of the reported results are that drift-diffusion plays a far lesser role in cross-well carrier transport than previously thought; that the dominant c...

The role of Coulombic coupling in electroabsorption of square quantum wells

The impact of Coulombic coupling between different subband pairs, on the quantum confined Stark effect in square quantum wells, has been studied. It is shown, via detailed comparison between electroabsorption spectra obtained from a full excitonic Green function calculation, a decoupled excitonic Green function calculation and measurement, that inclusion of the Coulombic coupling between different subband pairs is generally required for correct prediction of electroabsorption in square wells, especially for shallow or narrow square quantum wells. These results are of particular significance for the accurate calculation of electrorefraction, by Kramers-Kronig transformation, of quantum well electroabsorption spectra.

Transition from two-dimensional to three-dimensional electroabsorption in extremely shallow quantum wells

Calculations of electroabsorption in extremely shallow quantum wells are performed, accurately incorporating mixing of different subband pairs due to the Coulombic interaction. As the AlAs mole fraction is varied in the barriers of a 100 A wide AlxGa1−xAs/GaAs square quantum well, a transition from red to blueshift of the absorption edge with applied electric field occurs at x ~ 0.003. In a 20 A wide square well, which more strongly confines the excitons, the redshift of the absorption edge is still observable at low electric field strength in the simulated absorption spectra, even for AlAs mole fractions as low as ~0.001. However, on increasing the strength of the applied electric field the blueshift becomes apparent. It is demonstrated that inclusion of Coulombic coupling between different subband pairs, particularly between confined states and quasi-continuum states, is essential for a correct prediction of the absorption edge behaviour in extremely shallow quantum wells.