Experiment-simulation comparison of luminescence properties of GaN/InGaN/GaN double graded structures

Graded InGaN structures have been designed by increasing the indium composition from 3% to a maximum value then back to 3%. This results in a Zig-Zag quantum well (QW) structure with a composition grading and subsequent polarization doping, which forms a p-n junction. Separate structures are designed by varying the maximum In composition but maintaining a constant QW thickness. This is both in order to test the limits of the molecular beam epitaxy growth control in terms of the deposition source ramping rates and to determine the limits of the maximum In composition within a narrow QW. The results indicate that by increasing the maximum indium composition, the overall emission, while very broad, shifts to lower energies. The broadband emission covers wavelengths between 360 and 700 nm. Simulations identify the lowest energy, band-to-band transition for fully strained structures with a noticeable shift resulting from partial relaxation in samples with higher indium content. This type of novel polarization doped structure has useful applications in optoelectronic devices; therefore the presented understanding of its optical transitions will be beneficial to their design and implementation.