In this paper we discuss theoretical modelling methods for the design of photonic crystal and photonic quasi-crystal
(PQC) LEDs - and apply them to the analysis of the extraction enhancement performance and shaping of the emitted
beam profile of PQC-LED structures. In particular we investigate the effect of the pitch of the PQC patterning, and
consider the physical mechanisms giving rise to performance improvements. In addition, we examine the relative
contributions to performance improvements from effective index reduction effects that alter the conditions for total
internal reflection at the device air interface, and from photonic crystal scattering effects that give rise to radically
improved extraction performance. Comparisons are made with the performance of recently fabricated devices.
Processing of GaN-AlInN-GaN epitaxial trilayers into 3-dimensional microstructures, using a combination of vertical
dry etching and lateral wet etching, is discussed. The AlInN layers were grown so as to have an InN mole fraction close
to the value of 17% required for lattice matching with GaN. Inductively coupled plasma etching with chlorine-argon gas
mixtures was used to define mesa features with near-vertical sidewalls. Refluxing aqueous solutions of nitric acid of 2
molar concentration allowed highly selective lateral etching of the AlInN interlayers exposed on the mesa sidewalls,
providing a novel sacrificial layer technology for the III-nitride materials. Lateral etch rates of 0.14-0.21 μm/hr were
observed for 100-nm AlInN interlayers. Two distinct applications are discussed. In one example, lateral etching of an
AlInN layer was used to expose the underside of epitaxial GaN disks for fabrication of planar microcavities. Here,
retention of an optically smooth GaN (0001) surface on the underside of the disks is critical. Microbridges with potential
for development as sensors were also demonstrated, and the deformation of these structures provides a sensitive probe
of the local strain state of the undercut GaN layer.
An array of GaN micro-pyramids containing a near-surface InxGa1-xN/GaN single quantum well has been fabricated
using selective area epitaxial overgrowth above a patterned silica mask. The pyramid array has been studied by means of
angle-resolved reflection measurements in the near- and mid- infrared optical ranges. We have found that the periodic
array of flat-topped pyramids shows marked resonances in the near-infrared optical range due to resonant Bloch modes
within the extraction cone and that the angular dispersion of these modes exhibits strong photonic crystal characteristics.
The experimental results are in good agreement with the photonic band structure calculated using a scattering matrix
formalism. The mid-infrared optical anisotropy properties of the micro-pyramids were investigated to probe the infrared
active phonons of the pyramid array. The A1(LO) phonon of the InxGa1-xN/GaN single quantum well was identified and
the InN mole fraction was estimated from the mode behaviour.
Photonic devices that exploit photonic crystal (PhC) principles in a planar environment continue to provide a fertile field of research. 2D PhC based channel waveguides can provide both strong confinement and controlled dispersion behaviour. In conjunction with, for instance, various electro-optic, thermo-optic and other effects, a range of device functionality is accessible in very compact PhC channel-guide devices that offer the potential for high-density integration. Low enough propagation losses are now being obtained with photonic crystal channel-guide structures that their use in real applications has become plausible. Photonic wires (PhWs) can also provide strong confinement and low propagation losses. Bragg-gratings imposed on photonic wires can provide dispersion and frequency selection in device structures that are intrinsically simpler than 2D PhC channel guides--and can compete with them under realistic conditions.