A new method of forming shallow $P^+$-N junctions was studied by using ${BF_2};^{+};&;_{11}B^+$ mixed ion implantaion at various mixing ratios. Mixed $P^+$ implantation enables us to control the defect profiles including the depth of A-C(amprphous-crystalline) interface, the depth density of residual dislocation loops as well as the amounts of fluorine-related cluster formation without varying the total dose of boron implanted. The residual defect density of 1.0E15 ions/$ extrm{cm}^2BF_2 ; ^+$ + 2.0E15ions/$ extrm{cm}^2; _{11}B^+$ implanted sample was reduced about 1/3 of conventional 3.0e15ions/$ extrm{cm}^2 BF_2 ; ^+$ ion implanted one. The depth of residual defects was 13nm shallower than that of conventional $BF_2 ; ^+$ implantation with no increase in junction depth and no deterroration of electrical properties.