Study On The Electronic Structures And Optical Properties Of Doped LiNbO₃ Crystals

LiNbO₃ crystal is a photorefractive material that has very important technology applications in many areas, especially exhibits very superior performance in volume holographic storage aspect, and is considered one of the preferred material in optical storage medium. For a long time, the researchers study on the structure, performance and various doping characteristics of LiNbO₃ crystals by experiment, and show that the LiNbO₃ crystal is similar to the ilmenite structure, and has a considerable amount of intrinsic defects, which makes it possible to dope in the crystal to expand practical applications in all aspects. In this paper, the electronic structures and optical properties of variety doped LiNbO₃ crystals are studied by means of theoretical calculation, using the first principles based on the density functional theory. The result show that:1. The impurity energy levels of Fe doped LiNbO₃ appear within in the band gaps（with a width of 2.845eV）,which is contributed by Fe3d orbital and O2p orbital, The impurity levels make light absorptions in the visible area appear in the crystals. For Fe:Mg:LN crystals, the band gap are 2.901 eV and 2.805 eV respectively for the Mg ion concentration less than and equal to the threshold. The two absorption peaks at 539 nm and 477 nm are attributed to the Fe ions in crystal, Moreover, the intensities of these peaks vary with the concentration of Mg ion. The concentration of Mg ion influences the concentrations and the sites of Fe²⁺ and Fe³⁺ ions in crystal. Analysis suggests that the formations of photoelectrons not only depend on the orbit electronic state of Fe and thec_Fe²⁺/c_Fe³⁺ concentration ratio, but also the orbit electronic state of O2p bonded with Fe.2. The impurity energy levels within the band gap of Cu and Fe doped LiNbO₃ crystals, are contributed by Cu3d orbital, Fe3d orbital and O2p orbital; Two energy levels appear in forbidden gap of co-doped systems, in which the deep one iscontributed by Cu3d orbital and O2p orbital, the shallow one is contributed by Fe3d orbital and O2p orbital. The band gap width of Cu, Fe mono and co-doped systems decreases successively. Two wide absorption spectra appear at 447 nm and 630 nm in co-doped system, which show stronger light absorption property than that of mono-doped system. Analysis suggests that the co-doped samples with Fe at Nb site are more competitive than those with Fe at Li site in optical storage applications; and that reducing properly of c_Fe²⁺/c_Fe³⁺ value may help to this advantage.3. Within the forbidden band gap of Ce、Cu doped LiNbO₃ crystals, there are two impurity energy levels, which are contributed to Ce4 f orbital, Cu3d orbital and O2p orbital. The Cu ions provide deep levels（0.45 eV to valence band） and Ce ions provide shallow levels（2.41 eV to valence band）. This two-levels structure provide a possible for holographic storage application of Ce\Cu doped LiNbO₃ crystal.