*Article* **Strain Relaxation Effect on the Peak Wavelength of Blue InGaN/GaN Multi-Quantum Well Micro-LEDs**

**Chaoqiang Zhang 1, Ke Gao 1, Fei Wang 1, Zhiming Chen 1, Philip Shields 2, Sean Lee 3, Yanqin Wang 3, Dongyan Zhang 3, Hongwei Liu 1,\* and Pingjuan Niu 1,\***


**Abstract:** In this paper, the edge strain relaxation of InGaN/GaN MQW micro-pillars is studied. Micro-pillar arrays with a diameter of 3–20 μm were prepared on a blue GaN LED wafer by inductively coupled plasma (ICP) etching. The peak wavelength shift caused by edge strain relaxation was tested using micro-LED pillar array room temperature photoluminescence (PL) spectrum measurements. The results show that there is a nearly 3 nm peak wavelength shift between the micro-pillar arrays, caused by a high range of the strain relaxation region in the small size LED pillar. Furthermore, a 19 μm micro-LED pillar's Raman spectrum was employed to observe the pillar strain relaxation. It was found that the Raman E2 <sup>H</sup> mode at the edge of the micro-LED pillar moved to high frequency, which verified an edge strain relaxation of = 0.1%. Then, the exact strain and peak wavelength distribution of the InGaN quantum wells were simulated by the finite element method, which provides effective verification of our PL and Raman strain relaxation analysis. The results and methods in this paper provide good references for the design and analysis of small-size micro-LED devices.

**Keywords:** InGaN/GaN multiple quantum well (MQW); strain relaxation; micro-LED arrays; photoluminescence (PL); Raman shift
