ZnSe single crystals of 30 mm diameter and about 20 mm length were grown by the semi-open free-growth physical vapor transport method. The area of the dislocation density less than 1 x 10(4) cm(-2) is more than 90% of the (1 0 0) substrate, about 28 mm x 20 mm in size, cut from the grown ZnSe crystal. Al was diffused by annealing into (1 0 0) substrates. Al concentration and carrier concentration over the region between the surface and 400 mum in depth of the substrate were higher than 4 x 10(18) cm(-3) and 5 x 10(17) 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 10 arcsec. The Al concentration dependence of the Al diffusion coefficient in ZnSe was estimated.