A semiconductor light emitting device includes a multi-quantum-well structure, a first potential barrier layer, a first capping layer, a second capping layer, and an electron barrier layer stacked in order on a growth substrate. The multi-quantum-well structure includes a plurality of alternately-stacked second potential barrier layers and potential well layers. The first capping layer is an undoped semiconductor layer and the second capping layer is a p-doped semiconductor layer. Each of the first and second capping layers has a band gap larger than that of each of the second potential barrier layers and the electron barrier layer. A method of preparing the semiconductor light emitting device is also provided.