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

Abstract

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 x Ga 1−x N/ 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.

Investigation of Emission Heterogeneity in InGaN/GaN Micro-Light-Emitting Diodes by Photon-Correlation Cathodoluminescence Spectroscopy

“…This value matches the amount expected from Figure (2 ± 1·10 9 Vpits·cm –2 ), but is larger than previous reports for the expected V-pit density of InGaN LEDs grown on sapphire, of 1.5·10 8 /cm 2 and 3–5·10 8 /cm 2 . However, as observed by ref , the amount of V-pits grows with the number of layers, so that discrepancy could be explained by the presence of an underlayer. As observed on the STEM images, the V-pits cannot be counted by high-resolution SEM or atomic force microscopy due to the conformal growth of the p-GaN layer.…”