High reflectivity and broad bandwidth AlN/GaN distributed Bragg reflectors grown by molecular-beam epitaxy

Abstract

A number of distributed Bragg reflectors (DBRs) based on AlN/GaN quarterwave layers have been grown on (0001) sapphire by electron cyclotron resonance plasma-assisted molecular-beam epitaxy. The number of periods for the DBRs ranges from 20.5 to 25.5 and the thickness of the quarterwave layers were chosen such that the peak reflectance occurs from the near ultraviolet to green wavelength regions. Peak reflectance values between 97% and 99% were obtained for these DBRs. The best sample has a peak reflectance up to 99% centered at 467 nm with a bandwidth of 45 nm. The experimental reflectance data for this sample were compared with simulations using the transmission matrix method and show excellent agreement with respect to peak reflectance, bandwidth of high reflectance, and the locations of the sidelobes. The thickness of the quarterwave layers and uniform periodicity of the bilayers were confirmed by cross-section transmission electron microscopy. A network of cracks was observed in some of the samples and this is attributed to tensile stress in the AlN layers. We have grown asymmetric DBRs with thicker AlN layers and thinner GaN layers to reduce the tensile strength in the AlN layers. Such an approach resulted in samples that have significantly less cracks or even crack-free.