Surface nonradiative recombination centers

Optical minority carrier lifetime was also measured in the samples grown at 30 pm h-1. The measurements were performed by F. Yan and W.J. Choyke (University of Pittsburgh). For this, a pulsed NdYAG laser was employed as the excitation source. The values around 1 ps were obtained for both the epilayers on porous and the epilayers on standard surfaces indicating that growing on porous did not change the concentration of nonradiative recombination centers, and the intensity enhancement observed in photoluminescence is due to other reasons. 

While the surfaces of the planar o-plane GaN films described above were smooth, the TEM images shown in Figure 2.7 and X-ray diffraction data shown in Figure 2.6 indicated that their microstructural quality needed improvement. Threading dislocations in c-plane GaN have been found to act as nonradiative recombination centers, carrier scattering centers, and possibly fast diffusion pathways, limiting device efficiency, speed, and reliabihty. Less is known... 

Earlier observations by Cesario et al. [60] of a decay in fluorescence for arrays of Au nanoparticles spaced above a Ag film by a Si02 layer of increasing thickness, were interpreted as due to the finite vertical extent of the evanescent fields associated with a surface plasmon. In this model the coupling results in an enhanced interaction between individual localized plasmons at individual nanostructures [61] and thus an enhancement in the radiative efficiency increasing the spacer layer thickness moves the nanowires out of the evanescent field of the surface plasmon. A possible physical mechanism for the experimentally observed decay involves nonradiative decay of the excited states. The aluminum oxide deposited in these experiments was likely to be nonstoichio-metric, and defects in the oxide could act as recombination centers. Thicker oxides would result in higher areal densities of defects, and decay in fluorescence. A definitive assignment of the mechanism for the observed fall off of fluorescence would require determination of the complex dielectric function of our oxides (as deposited onto an Ag film), and inclusion in the field-square calculations. Finally it should be noted that at very small thicknesses quenching of the fluorescence is expected [38,62] consistent with observations of an optimum nanowire-substrate spacer thickness. 

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