Compositionally graded InGaN layers grown on vicinal N-face GaN substrates by plasma-assisted molecular beam epitaxy

This work reports on compositionally graded (0 0 0 1 bar) N-polar In_xGa_1-xN layers. The InGaN grades with different final In compositions x_f up to 0.25 were grown by plasma-assisted molecular beam epitaxy on vicinal GaN base layers with a miscut angle of 4° towards the m-direction. When increasing x_f the surface morphology evolved from an interlacing finger structure, attributed to the Ehrlich-Schwöbel effect, towards fully strain-relaxed columnar features. Regardless of the crystal morphology and the strain state each graded sample exhibited a bright photoluminescence signal at room temperature spanning the whole visible range. Cross-sectional nanoscale cathodoluminescence evidenced a red-shift of the luminesced signal from 420 to 580 nm along the grade and also showed strong lateral emission inhomogeneities.