The effects of Be on the II-VI/GaAs interface and on CdSe quantum dot (QD) formation were investigated. A (1 X 2) surface reconstruction was observed after a Be-Zn coirradiation of the (0 0 1) GaAs (2 X 4) surface. ZnBeSe epilayers grown after the Be-Zn coirradiation show very high crystalline quality with x-ray rocking curve linewidths down to 23 arcsec and a low etch pit density of 4 X 10(4) cm(-2), and good optical quality with a band-edge photoluminescence (PL) emission peak linewidth of 2.5 meV at 13 K. However, ZnBeSe epilayers grown after Zn irradiation alone have poor crystalline quality and poor optical properties. Atomic force microscopy measurements show that CdSe QDs grown on ZnBeSe have higher density and smaller size than those grown on ZnSe. A narrower PL emission peak with higher emission energy was observed for the CdSe QDS sandwiched by ZnBeSe. These results indicate that the formation of CdSe QDs as well as the II-VI/GaAs interface are modified by the presence of Be.