*2.1. Epitaxial Growth and InGaN/GaN Micro-LED Arrays Fabrication*

The InGaN/GaN LED wafer was grown on a 4-inch c-plane sapphire substrate by metal–organic chemical vapor deposition. The epitaxy layer is illustrated in Figure 1: a 3 μm undoped GaN buffer layer; a 1.5 μm n-type GaN layer; twelve periods 3 nm MQWs separated by 12 nm GaN barrier layers; and a 0.2 μm p-type GaN top layer.

**Figure 1.** Schematic of GaN blue LED epitaxial wafer layers.

In order to study strain relaxation of InGaN/GaN MQWs, the top p-GaN layer thickness was reduced to 100 nm by inductively coupled plasma (ICP) etching. Then, we divided a 4-inch wafer into 18 areas of the same size with area sizes of 12.24 mm × 19.38 mm. We etched an equal number of micro-pillars of different sizes in each area and the etching period was 30 μm (row: 12,240 μm /30 μm = 408, column: 19,380 μm/30 μm = 646). The diameters of the etched micro-pillars ranged from 3 μm to 20 μm. (as shown in Figure 2)

**Figure 2.** Micro-pillar layout on the 4-inch wafer with diameters from 3 to 20 m.
