Fig. 2: Material characterization.

a High-resolution X-ray diffraction reciprocal space maps (11-24 reflection) illustrate the strain relaxation in the AlGaN and AlN layers. Two distinct peaks correspond to the AlN and AlGaN layers, respectively. Analysis of these peaks reveals an Al-mole fraction x = 0.69 and a mere 3% relaxation of in-plane compressive strain in the AlGaN layer, indicating a predominantly pseudomorphic growth on the AlN template and avoiding the generation of strain-relief defects. b Illustration of the basic heterostructure on a sapphire substrate. c Supported TE₀ and TE₁ modes for a 250 nm thick AlGaN layer on AlN at 632 nm. d The optical loss spectrum of these modes in the AlGaN/AlN heterostructure and the TE₀ mode of a 0.4 μm thick AlN single layer on sapphire film (AlNoS), measured by prism coupling, where shaded areas indicate the uncertainty bounds.