Observation and Elucidation of Enhanced Spectral Blue-Shift in InGaN/GaN Quantum Well Microstructures upon Thermal Annealing

Abstract InGaN/GaN multiquantum wells grown on c-plane sapphire substrates exhibit strong quantum-confined Stark effects owing to the lattice mismatch, resulting in a built-in piezoelectric field in the strained heterostructure. One of the commonly observed consequences of the QCSE is the spectral blue-shifting of the emission spectra with dimensional down-scaling of the structures. The extent of the spectral blue-shifts increases with decreasing dimensions, with blue-shifts of over 100 nm observed in nanometer-scale structures, while the shifts are significantly milder at the micrometer scale. In the present study, large cathodoluminescence spectral blue-shifts are observed from etched microstructures following an annealing treatment at 800°C in a N2 environment for 1 h. Specifically, the peak wavelength of the cathodoluminescence spectra shifts from ∼625 nm to ∼469 nm as the diameter of a microdisk is scaled from 20 to 1.5 μm. With the aid of molecular dynamics simulations and scanning transmission electron microscopy imaging and elemental mapping, the broadened indium profile at the multiquantum well region is interpreted as evidence of well-barrier intermixing and indium depletion. The reduction in the local indium fraction increases the InGaN bandgap and transition energy, while associated changes in strain, polarization, and the QCSE may further contribute to the blue-shift. Meanwhile, indium out-diffusion is facilitated by the exposed sidewalls near the edges of the microstructures. Based on these observations and analysis, polychromatic emission is demonstrated from an array of annealed microdisks with dimensions ranging from 1 to 20 μm under photoluminescence. This work provides insight into the role of annealing in etched InGaN/GaN MQWs microstructures and serves as a proof-of-concept demonstration of monolithic polychromatic emission.