The possibility of localized doping by Er3+ diffusion at moderate (less than 500 °C) temperature was for the first time demonstrated for sapphire single crystal wafers. The doping was achieved by immersing the substrate wafers into reaction melt containing small amounts of erbium salt. The crucial point of the presented technology was a crystallographic orientation of the used wafers. The most suitable orientation of the cuts was the “X-cut” with orientation (11–20). The strong anisotropy of the moderate temperature Er3+ doping into lithium niobate and sapphire was explained on the basis of the crystal structure of particular cuts.