Enhanced Efficiency InGaN/GaN Multiple Quantum Well Structures via Strain Engineering and Ultrathin Subwells Formed by V‑Pit Sidewalls

We study the impact of strain engineering by exploring the influence of the number of superlattice (SL) layers underneath InGaN/GaN multiple quantum wells (MQWs) on the optical properties of In_xGa_1–xN/GaN MQWs grown on patterned sapphire by metal–organic chemical vapor deposition while retaining the same composition and MQW periods. X-ray diffraction and reciprocal space mapping show that the strain initially increases with the number of SLs in the structure followed by a slight relaxation. Scanning electron microscopy analysis indicates that the desired strain is obtained by increasing the number of SL pairs up to 12 due to which the V-pit density and size (>270 nm in diameter) increase. Scanning transmission electron microscopy reveals that such large-sized V-pits [with large sidewalls comprising ultrathin MQWs and SLs (<1 nm)] emerge in the n-GaN layer below the SLs, leading to high n-GaN quality as confirmed by temperature-dependent photoluminescence (PL) and PL excitation measurements as defect-related emission in n-GaN decreases as the V-pit density increases. Low-temperature PL spectra show a higher-energy emission centered at 402 nm besides the MQW emission at ∼454–458 nm, while room-temperature cathodoluminescence mapping reveals that this higher-energy emission is due to the ultrathin MQW + SL structures surrounding V-pits, forming ultrathin subquantum well (sub-QW). We show, for the first time, that the carrier repopulation process between MQWs and sub-QW caused by a high density of V-pits through the strain engineering process can be a significant factor in enhancing the optical quality and efficiency. These findings provide valuable insight into the impact of strain engineering that can govern high-efficiency light-emitting diode (LED) performance.