Effects of In composition on ultraviolet emission efficiency in quaternary InAlGaN light-emitting diodes on freestanding GaN substrates and sapphire substrates

350 - nm -band quaternary InAlGaN quantum-well (QW) structures have been fabricated on low-dislocation-density GaN substrates and sapphire substrates, and the impact of the In composition on ultraviolet emission efficiency has been investigated. Photoluminescence, cathodoluminescence, and atomic force microscopy measurements have revealed that with increasing In concentration, band-gap inhomogeneity in InAlGaN layers is promoted and quantum efficiency becomes less sensitive to dislocation densities. However, this does not necessarily result in the enhancement of QW emission efficiency, because both a decrease in growth temperature and an increase in Al concentration degrade the InAlGaN crystalline quality. It is found that improving the InAlGaN crystalline quality by increasing the growth temperature while maintaining an In fluctuation effect with minimal In composition is essential for highly efficient InAlGaN QWs. The utilization of the optimized InAlGaN growth condition significantly improves the output power of 350 - nm -band light-emitting diodes on GaN and sapphire substrates to 7.4 mW at 400 mA and 2.9 mW at 200 mA , respectively.