Nanometer- and Micrometer-scale LED arrays are useful not only for display applications, but also for specialized applications like lens-less microscopy, mask-less lithography or optogenetics. In these contexts, the spatial resolution of the optical field and precise control over the illumination pattern at the object plane is of special importance. We have studied numerically different GaN LED array designs, calculating light extraction, optical near field and crosstalk between pixels. We find that 3D-patterning can help in shaping the light emission, while optical crosstalk becomes a critical issue for small LEDs and pitches below 300 nm.
During the last decade a number of both theoretical and experimental studies have shown the importance and the possible effects of random alloy fluctuations in InGaN. Interesting results have been obtained in particular with atomistic simulation models. Based on experimental evidence, most theoretical studies so far concentrated on a uniform random alloy, i.e. where the probability of finding an indium instead of a gallium atom is spatially constant.
In this work, we calculated the density of states, the spontaneous emission spectrum and the radiative coefficient for InGaN/GaN single quantum wells and for bulk InGaN in presence of alloy non-uniformity, using an empirical tight binding approach. We considered an indium concentration of 20%, and 10 nm large supercells. The non-uniform indium distribution has been obtained by distributing a certain percentage of all indium atoms with uniform probability, and the rest with a probability that depends on the number of indium atoms already present locally. This allows to produce structures ranging from random alloy up to strong clustering.
We find that non-uniformity reduces the band gap and the peak energy of the optical emission spectrum. Moreover, increasing degree of clustering decreases the average value of the ground state transition matrix element, which can be explained by the carriers’ spatial localization, combined with quantum confined Stark effect in quantum wells. The radiative coefficient on the other hand is not substantially influenced by light non-uniformity, while it increases for stronger degree of clustering, compatible with a transition to a quantum dot system.
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