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

A number of distributed Bragg reflectors (DBRs) based on AlN/GaN quarterwav e 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 g reen wavelength regions. Peak reflectance values between 97% and 99% were o btained for these DBRs. The best sample has a peak reflectance up to 99% ce ntered at 467 nm with a bandwidth of 45 nm. The experimental reflectance da ta for this sample were compared with simulations using the transmission ma trix method and show excellent agreement with respect to peak reflectance, bandwidth of high reflectance, and the locations of the sidelobes. The thic kness of the quarterwave layers and uniform periodicity of the bilayers wer e confirmed by cross-section transmission electron microscopy. A network of cracks was observed in some of the samples and this is attributed to tensi le 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 la yers. Such an approach resulted in samples that have significantly less cra cks or even crack-free. (C) 2000 American Institute of Physics. [S0003-6951 (00)00820-2].