We have investigated the characteristics of the surface of the GaP window layer of 630 nm AlGaInP LED, which was improved by post-Zn diffusion process. The measured resistance and the amount of hole concentration of the post-Zn-diffused GaP window layer have remarkably decreased and increased, respectively. Moreover, the ECV system showed that the amount of doping concentrations on the surface of the GaP window layer was significantly increased because of the diffusion of Zn atoms. The amount of surface defects observed on the post-Zn-diffused GaP window layer was also reduced. Furthermore, it was found out that the efficiency of 630 nm AlGaInP LED chip was increased due to the surface improvement of the GaP window layer. At an injection current of 40 mA, the LED chip with a Zn diffusion layer obtained a higher output power of 11 mW compared to the 7.5 mW output of the conventional LED chip. (C) 2013 Elsevier B.V. All rights reserved.