Inverted V-shaped size-dependent emission wavelength shift in InGaN micro-LEDs with size down to 1 μm

The size-dependent emission wavelength shift of micro-scale light emitting diodes (micro-LEDs) has been frequently reported in recent publications, but its underlying physical mechanism has not yet been thoroughly elucidated. Here, we fabricate and characterize the red, green, and blue InGaN micro-LED mesas with different diameters down to 1 μm. As the size decreases, all the samples of different colors show an inverted V-shaped photoluminescence wavelength variation trend, first a red shift and then a blue shift, and the shifting range is larger for samples with longer wavelengths. Micro-Raman spectrum confirms that the stress was significantly released after scaling down the size from epitaxial wafer to 1 μm. The theoretical simulations show that the red and blue shifts are, respectively, attributed to the bandgap narrowing and the weakening of the quantum-confined Stark effect caused by strain relaxation, which dominate successively.