No abstract

No abstract

In this paper, we have made a clear differentiation among bandgap, diffraction, interference, and refraction effects in photonic crystal structures (PhCs). For observing bandgap, diffraction, and refraction effects, PhCs are considered on the top p-GaN surface of light emitting diodes (LEDs), whereas for interference effect, hole type PhCs are considered to be embedded within n-GaN layer of LED. From analysis, it is observed that at a particular lattice periodicity, for which bandgap lies within the wavelength of interest shows a significant light extraction due to inhibition of guided mode. Beyond a certain periodicity, diffraction effect starts dominating and light extraction improves further. The interference effect is observed in embedded photonic crystal LEDs, where depth of etching supports constructive interference of outward light waves. We have also shed light on refraction effects exhibited by the PhCs and whether negative refraction properties of PhCs may be useful in case of LED light extraction.

Aluminum nitride was grown on two-dimensional hexagonal boron nitride (hBN) and sapphire substrates using metal organic vapor phase epitaxy. hBN itself was first deposited directly on 2"  sapphire substrates using the same method, which served as templates for AlN growth. For a direct comparison and understanding of the influence of hBN, AlN was grown on both hBN and sapphire, under identical conditions. Growth parameters were varied to study their effect on AlN’s surface morphology and crystallinity. AlN growth on hBN, using a standard two-step process involving a low-temperature nucleation layer resulted in polycrystalline AlN, with a rough morphology. Hence, a modified, multistep high-temperature growth process was utilized for obtaining planar AlN films, with improved crystallinity. AlN films grown on hBN were easily delaminated from the sapphire wafers and showed a relaxation in compressive strain, which was studied using Raman spectroscopy.