Growth issues of CdZnTe crystals doped with transition elements for photorefractive applications

摘要

A way to interconnect optical fibers in a telecommunication network working between 1.3 and 1.55 micrometer is the use of two-wave mixing mirrors. These devices use the photorefractive effect which can be described as a modification of the refractive index under an electric field created by electric charges photo-induced and then trapped in the material. The index modulation reproduces then the image projected on the material. Due to its high electro-optic factor of merit, about three times higher than the one of III-V semiconductors, and its photosensitivity in the telecommunication wavelength range, CdTe doped with transition elements is very attractive. CdTe crystals dedicated to such devices have to obey severe criteria that are shown to guide their crystal growth. Several effects are discussed, such as solubility, segregation, precipitation, purity, not only related to the transition elements incorporated in CdTe, but also to zinc which is shown to present a specific behavior in photorefractive CdTe. The stoichiometry of the crystals is shown to be a significant parameter as well: from electrical measurements on CdZnTe crystals presenting various V doping levels, an effective segregation coefficient depending on stoichiometry and purity is introduced. The deviation from stoichiometry of CdTe is estimated from lattice parameter measurements. Finally the appropriateness, for the specific application of photorefractivity, of the different techniques of crystal growth classically used for CdTe is discussed.