Growth and Properties of Mg, Fe Codoped near Stoichiometric Lithium Tantalate Single Crystals

• Main Authors: Wei-Tse Hsu, 徐為哲
• Other Authors: 藍崇文
• Format: Others
• Language: zh-TW
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/59624127984795964481

博士 === 臺灣大學 === 化學工程學研究所 === 98 === Recently, the electro-optics industry has developed vigorously, thus several new materials with excellent properties have been discovered. Lithium tantalate (LT) crystal is one of the most important materials for industrial application due to its excellent properties such as high damage threshold, large nonlinear optical constant, large electro-optic constant, and ease of growth; it has been, therefore, the focus of research of many companies and laboratories. With the rapid expansion of digital information, a new storage technique with extra-large storage density and high transfer rate is needed. A promising technique named holographic data storage technique could satisfy the requirements. The theoretical storage density by using holographic technique on the LT crystal could be estimated at a value of Tb/cm3, and transfer rate of Gb/s. The LT crystal is also often used in laser application. By employing the quasi phase matched (QPM) technique, the periodic poled lithium tantalate (PPLT) device could replace GaN to lead the laser market. Therefore, many companies have zealously investigated the new material. The congruent lithium tantalate (CLT) crystal is the main product in the market, because the CLT crystal could be grown easily though the traditional Czochralski (CZ) method. However, the CLT crystal often contains a lot of intrinsic defects (anti-site defects), which will damage the potencies of the LT crystal, that is, increase the cutoff wavelength and coercive field, and decrease the birefringence and optical damage threshold. It is well known that the stoichiometric lithium tantalate (SLT) crystal provides better properties. Due to the segregation issue, growing SLT crystal by traditional Cz puller is very difficult. The conventional technique for growing SLT crystals was the double-crucible Czochralski (DCCz) method. However, because the use of the powder feeding requires a complicate design, this is not convenient for a conventional Czochralski puller. In this study, the zone-leveling Czochralski (ZLCz) method was used to grow the SLT crystals successfully. Additionally, the axial and radial composition distributions in these grown crystals are characterized by using several optical, physical, and chemical methods, which show that these crystals show high uniformity in radial and axial distribution. The axial composition segregation is very slight (1.1×10-2 %/mm) in the crystals, and no radial segregation is observed in the SLT crystals. The SLT crystals grown by ZLCz technique have comparable quality with commercial products. Furthermore, we also studied the doping effects for the SLT crystals. For the laser or hologram application, the as grown SLT crystals need to be further treated by an electrical field to be of single ferroelectric domain. In this study, we also discuss the method for producing high-quality single-domain wafers. We found that the LT buffer wafers needed to be used to prevent cracks caused by the ions diffusion issue; the high-quality crystals could then be obtained by applying a moderate current density (2.5 mA/cm2). However, the smaller or larger current density could cause the domain inverse to become incomplete or the crystal to crash, respectively. The Mg and Fe co-doped SLT crystals were also grown by the ZLCz method. The purpose of doping Fe ions into the crystal is to enhance the diffraction efficiency and dynamic range. Doping Mg and the increase in Li/Ta ratio would enhance the photoconductivity and improve the sensitivity and response time of the SLT crystal. The crystal color becomes darker with the iron concentration. The oxidation and reduction condition of Mg as well as the Fe co-doped SLT crystal are also studied. By controlling the Fe concentration in the crystal and the oxidation time, a special absorption peak attributed to inter-valence transfer of Fe2+ to Ta5+ could be shown at the wavelength of 425 nm. For the high iron doping crystal, an extra vibrational peak located at 3504 cm-1 is observed in the OH-absorption spectrum, which is attributed to the MgLi+-OH--FeTa3- vibrational mode. Finally, we investigated the photorefractive properties of a series of Mg/Fe co-doped near-stoichiometric lithium tantalate (SLT) crystals under varying light intensities of the two-beam coupling method. In the photorefractive experiment, the recording time constant, erasing time constant, dynamic range, and sensitivity decreased with light intensity; however, the diffraction efficiency showed an opposite trend. Furthermore, the photorefractive properties of crystals were enhanced with the increasing Fe/Ta ratio. The effects of the Li/Ta, Mg/Ta, and Fe/Ta ratios on the holographic parameters were also studied. The recording time constant and erasing time constant were shortened by doping Mg and controlling the stoichiometry. The experimental results showed that the Mg/Fe co-doped SLT crystal could be an excellent candidate for lifetime data storage media.