Efficiency droop in InGaN/GaN-based LEDs with a gradually varying In composition in each InGaN well layer

Temperature-dependent and driving current-dependent electroluminescence spectra of two different InGaN/GaN multiple quantum well structures SA and SB are investigated,with the In composition in each well layer(WL)along the growth direction progressively increasing for SA and progressively decreasing for SB.The results show that SB exhibits an improved efficiency droop compared with SA.This phenomenon can be explained as follows:owing to the difference in growth pattern of the WL between these two samples,the terminal region of the WL in SB contains fewer In atoms than in SA,and therefore the former undergoes less In volatilization than the latter during the waiting period required for warming-up due to the difference in the growth temperature between well and barrier layers.This results in SB having a deeper triangular-shaped potential well in its WL than SA,which strongly confines the carriers to the initial region of the WL to prevent them from leaking to the p-GaN side,thus improving the efficiency droop.Moreover,the improvement in the efficiency droop for SB is also partly attributed to its stronger Coulomb screening effect and carrier localization effect.