As-grown LiNbO3 Research Articles

Исследование свойств монокристаллов ниобата лития одинарного и двойного легирования — функциональных материалов оптоэлектроники

A series of six LiNbO3:Tb ([Tb] = 0.1 — 2.89 wt%) crystals and seven LiNbO3:Er ([Er] = 0.08÷2.71 wt%) crystals were grown according to a single methodology. Optical uniformity and optical resistance were compared in LiNbO3:Tb and LiNbO3:Er of various chemical composition. The periods of the crystal lattice were determined by the methods of a full -profile analysis of the XRD patterns of polycrystals; models of the atomic structure of LiNbO3:Tb and LiNbO3:Er crystals were analyzed with a change in the dopant concentration. In a series of LiNbO3:Tb crystals, a concentration threshold near the concentration of terbium ~ 2.2 — 2.3 wt% was discovered for the first time. A concentration threshold near the concentration of erbium ~ 2.4 — 2.5 wt% was discovered in LiNbO3:Er. In the area of the concentration threshold, pronounced anomalies of physicochemical, optical and structural characteristics are observed. Growth irregular and regular domain microstructures were revealed in as-grown LiNbO3:Tb and LiNbO3:Er crystals by optical and atomic force microscopy. Structural characteristics and threshold effects are studied in LiNbO3:Gd and LiNbO3:Gd,Cu crystals by Raman spectroscopy, PILS, laser conoscopy and optical microscopy. In LiNbO3:Gd crystals the photorefractive effect is suppressed at as low concentration as [Gd] = 0.05 wt%. For LiNbO3:Gd,Cu crystals a distinct photorefractive response is observed, it increases with increasing Cu concentration. The effect of the association of defects (carriers) is experimentally confirmed and thermodynamically justified by the example of oxygen-octahedron structures such as perovskite and pseudoi:lmental during ion conduction in a certain temperature interval. The discovered phenomenon is extremely important for creating and evaluating the temperature range of operability of ion current sources.

Growth, microstructure and optical characteristics of doped LiNbO3:Gd and LiNbO3:Cu:Gd lithium niobate crystals

Double- and single-doped lithium niobate (LN) crystals were grown by Czochralski at air from platinum crucibles: sample 1 ([Cu] = 0.57, [Gd] = 0.07 wt%), sample 2 ([Cu] = 0.041, [Gd] = 0.076 wt%), sample 3 ([Gd] = 0.003 wt%), sample 4 ([Gd] = 0.01 wt%), sample 5 ([Gd] = 0.05 wt%) and sample 6 ([Gd] = 0.26 wt%). Macro- and microstructure of as-grown LiNbO3:Cu:Gd and LiNbO3:Gd crystals have been studied by optical microscopy. Laser conoscopy, photoinduced light scattering (PILS) and optical spectroscopy have studied optical transmission, optical uniformity and photorefraction in LN crystals. Such a composition have been determined for a four-component system Li2O–Nb2O5–CuO—Gd2O3, at which a highly compositionally and optically uniform LiNbO3:Cu:Gd crystal (sample 2, [Cu] = 0.041, [Gd] = 0.076 wt%) can be grown. Raman spectra in 180-grade scattering geometry have determined conditions for the excitation of transverse and longitudinal polar modes during Raman scattering in LiNbO3:Cu:Gd crystals. A great increase in 175 and 603 cm−1 Raman bands intensities has been detected at an increase in copper concentration; it has been explained as a manifestation of photorefractive effect.

Growth, structure, physical and chemical characteristics in a series of LiNbO3:Er crystals of different composition grown in one technological cycle

A series of LiNbO3:Er ([Er] = 0.08–2.71 wt%) crystals were grown due to one technology in a single technological cycle. A near-stoichiometric lithium niobate (NSLN) was grown as a control. Concentration dependences of LiNbO3:Er and NSLN crystals structure characteristics were studied. A full-profile analysis of polycrystals X-ray patterns was used to analyze models of LiNbO3:Er atom structure. Structure characteristics of LiNbO3:Er crystals are compares with that of NSLN crystal. Optical and atomic-force microscopy were used to study growth irregular and regular domain microstructures and periodic nano-structures in as-grown LiNbO3:Er crystals.

Growth and concentration dependences of properties of LiNbO3:Tb crystals grown in a single technological cycle

A series of crystals LiNbO3:Tb ([Tb] = 0.1 ÷ 2.89 wt% in a crystal) and nominally pure near-stoichiometric crystal (NSLN) has been grown in a single technological cycle. Crystallization processes have been studied along with concentration dependences of physical, chemical and structural characteristics of LiNbO3:Tb crystals. LiNbO3:Tb atomic structure models have been analyzed by full-profile analysis of XRD patterns of powders. Regular and non-regular domain growth microstructures together with periodic nano-sized structures have been determined in as-grown LiNbO3:Tb crystals using optical and atomic-force microscopy.

Synthesis and properties of NiSi2-LiNbO3 heterostructures fabricated by radio-frequency magnetron sputtering

Thin LiNbO3 films were synthesized by radio-frequency the magnetron sputtering method on nickel silicide (NiSi2) substrates in a pure Ar atmosphere and an Ar+O2 gas mixture. The as-grown LN-NiSi2 heterostructures fabricated in an Ar environment exhibited high-quality rectifying current-voltage (I-V) characteristics due to the presence of an oxide charge, making the band diagram nonsymmetrical. The charge transport is determined by space-charge-limited currents via electronic traps distributed in the bandgap of LiNbO3 with a concentration of Nt=2.0•1015 cm-3. By contrast, I-V characteristics of LN-NiSi2 heterostructures fabricated in an Ar+O2 gas mixture were symmetrical due to decreasing in the oxide charge amount. The trap concentration rises to Nt=3.0•1018 cm-3 when the oxygen content in a reactive chamber increases to 60%. As-grown LiNbO3 films manifested good isolating properties with the resistivity of 8•109 Ohm•cm. The pulsed photon treatment increases the resistivity of LN films by six orders of magnitude, turning their I-V characteristics into Ohmic-like which makes this post-growth technique not suitable for the studied heterostructures.

Application of Z-scan technique for the study of nonlinear absorption in chemically reduced LiNbO3 crystals

The nonlinear absorption (NLA) was studied by open-aperture Z-scan experiments in the chemically reduced nominally pure LiNbO3 crystals at cw-illumination with the red (644 nm) and green (514.5 nm) laser beams. The magnitude of the measured NLA is considerably different from the reported Z-scan results obtained in as-grown LiNbO3. The positive sign of NLA obtained with the red light has been related to the generation of the small bound polarons absorbing in red and near-IR ranges. Application of green light results in the light-induced transparency, i.e. the Z-scan traces show negative sign of NLA. Intensity dependence of Z-scan traces allows for conclusion that the photo-induced dissociation of small NbLi4+:NbNb4+ bipolarons and sequent generation of small polarons gives the dominating contribution to the nonlinear optical absorption in reduced crystals with a large bipolarons concentration.

Influence of thermal annealing on structural properties and oxide charge of LiNbO3 films

C-oriented polycrystalline lithium niobate (LiNbO3) films have been deposited on Si substrate by the radio-frequency magnetron sputtering method in an Ar atmosphere and Ar + O2 gas mixture. All as-grown LiNbO3 films manifested positive fixed oxide charge regardless of the sputtering conditions. Donor centers are formed in Si substrate because of diffusion of O2 molecules during sputtering process. Thermal annealing (TA) of the deposited films leads to increase in the surface roughness and grain size as well as formation of LiNb3O8 phase in the studied films. Also, TA resulting in the decline of positive oxide charge due to the diffusion of oxygen molecules into LiNbO3 films from air and out-diffusion from Si substrate.

Z-scan study of nonlinear absorption in reduced LiNbO3 crystals

The nonlinear absorption (NLA) was studied by open-aperture Z-scan experiments in the chemically reduced nominally pure LiNbO3 crystals at cw-illumination with the red (644 nm) and green (514.5 nm) laser beams. The magnitude of the measured NLA is considerably different from the reported Z-scan results obtained in as-grown LiNbO3. The positive sign of NLA obtained with the red light has been related to the generation of the small bound polarons absorbing in red and near-IR ranges. Application of green light results in the light-induced transparency, i.e., the Z-scan traces show negative sign of NLA. Intensity dependence of Z-scan traces allows for conclusion that the photo-induced dissociation of small NbLi4+:NbNb4+ bipolarons and sequent generation of small polarons gives the dominating contribution to the nonlinear optical absorption in reduced crystals with a large bipolarons concentration.

Fast photorefractive response and high sensitivity of Zr and Fe codoped LiNbO3 crystals

A series of Zr and Fe codoped LiNbO3 crystals was grown and their photorefractive properties have been investigated. Compared with LiNbO3:Fe,Hf, the concentration of Fe2+ ions is strongly increased in LiNbO3:Fe,Zr. As a result, the photorefractive response speed of these as-grown LiNbO3:Fe,Zr crystals is only 2s and the sensitivity is larger than 12cm∕J while the saturation diffraction efficiency still remains at a high level. These findings prove that LiNbO3:Fe,Zr is an excellent choice for volume holographic storage.

Broadband absorption changes and sensitization of near-infrared photorefractivity induced by ultraviolet light in LiNbO3:Mg

We observe ultraviolet-light-induced absorption changes in spectral regions from the near-ultraviolet to the near-infrared in as-grown LiNbO3:Mg. Such a broad light-induced absorption band is unstable at room temperature and decays nonmonoexponentially in the dark with a time constant on the order of tens of seconds. We attribute the appearance of the broadband light-induced absorption to the creation of the intermediate state O− in which holes excited from deep defect centers are trapped at O2− sites near cation vacancies. It is also found that near-infrared photorefractive sensitivities increase by several orders of magnitude by pre-exposing the crystal to incoherent ultraviolet light. These properties may be useful for an application of LiNbO3:Mg to nonvolatile two-color holography.

Change of the stoichiometry and electrocoloration due to the injection of Li and O ions into lithium niobate crystals

Congruently grown LiNbO3 single crystals show both high oxygen and lithium ion conductivity at temperatures above 500 °C. The high oxygen ion conductivity can be understood in terms of a certain amount of oxygen vacancies already present in congruently grown LiNbO3 single crystals. Thermal treatment of LiNbO3 produces additional oxygen vacancies. The absorption bands introduced by this procedure are investigated. It is found that the electrons which are generated during the reduction process are homogeneously distributed among all oxygen vacancies in the LiNbO3 single crystals. The electrocoloration phenomenon in LiNbO3 single crystals is due to the process of injection of lithium ions and electrons into LiNbO3 by a double charge mechanism. Investigations of the optical and electrical properties of electrocolored LiNbO3 crystals are reported. It is shown that the absorption spectra of thermally and electrochemically reduced samples are identical and that the origin of the absorption processes has to be therefore the same in both cases. That means, additional electrons produced by the double charge injection of lithium ions and electrons are also homogeneously distributed among the oxygen vacancies. This supports our hypothesis that a certain amount of oxygen vacancies has to be present already in as-grown LiNbO3 single crystals.

Thin film LiNbO3 for integrated optic devices

Abstract Highly textured optical waveguides of thin film lithium niobate have been grown on sapphire and Si, including SiO2/Si, wafers using rf magnetron sputtering, metallo-organic decomposition (MOD), and photo-induced metallo-organic decomposition (PIMOD) techniques. The properties of films made by these techniques have been characterized by a variety of methods. Properties of optical waveguides and electrooptic modulators have been measured. Depending on the mode coupled in and the quality and thickness of the film, waveguide propagation loss as low as 0.9 dB/cm has been measured at 633 nm for the LiNbO3/Al2O3 structure. Moreover, the lowest propagation loss in the LiNbO3 waveguides on SiO2/Si has been determined to be 1.9 dB/cm. The electro-optic coefficient r33 of the as-grown LiNbO3 films has been measured to be 10.7 × 10−12 m/V.