ZnSe single crystals of 30mm diameter and about 20mm length were grown by the semi-open free-growth physical vapor transport method. The area of the dislocation density less than 1x10 4 cm - 2 is more than 90% of the (100) substrate, about 28mmx20mm in size, cut from the grown ZnSe crystal. Al was diffused by annealing into (100) substrates. Al concentration and carrier concentration over the region between the surface and 400μm in depth of the substrate were higher than 4x10 1 8 cm - 3 and 5x10 1 7 cm - 3 , respectively. The dislocation density did not increase after Al diffusion and was kept below 1x10 4 cm - 2 . The full-width at half maximum of the X-ray rocking curve was maintained below 10arcsec. The Al concentration dependence of the Al diffusion coefficient in ZnSe was estimated.