A substrate for semiconductor light emitting devices is provided. The substrate is characterized in that the substrate is a single crystal material and has a nanocrystal structure capable of diffracting an electromagnetic wave. The nanocrystal structure is disposed on a surface portion of the substrate and includes an etched region and an unetched region, wherein the etched region has a depth of 10-200 nm. Due to the periodicity of the nanocrystal structure, the semiconductor material grown on the substrate has fewer defects, and the material stress is reduced.