Importance of crystal structure of the substrate for diffusion technologies of waveguides fabrication

Effect of different crystallographic structure of various substrate cuts on diffusion processes into optical crystals is demonstrated for lithium niobate and sapphire. After the low or moderate temperature diffusion processes the X-cuts, which are parallel to the C₃ axis, always contained much more dopants comparing with the Z-cuts, which are oriented perpendicularly to the C₃. This has been proved for annealed proton exchange (APE) technology for fabrication of optical waveguides in lithium niobate and for the moderate temperature incorporation of erbium ions into both materials. Thus the possibility of localized doping by Er³⁺ diffusion at moderate (lower then 500°C) temperature is for the first time demonstrated for sapphire single crystal wafers. The strong anisotropy of the doping is explained on the bases of suitable orientation of the cleavage planes to the substrates surfaces in the X-cuts that allows for better penetration of the diffusing particles into the material.