Process Development of Zone-Leveling Pulling Growth of Stoichiometric Lithium Niobate Single Crystals
Date Issued
2005
Date
2005
Author(s)
Tsai, Chung-Bo
DOI
zh-TW
Abstract
Lithium niobate (LN) single crystal is a potential opto-electro material with various properties. The usage of the doping or the increase of the Li/Nb ratio in the LN crystal could be highly enhanced the properties for different applications, such as the photorefractive enhancement for holographic storage devices, or the larger photoresistance for the laser application. However, the crystal having non-uniformity along the grown boules was found to be a serious problem due to the segregation. The conventional technique for growing stoichiometric lithium niobate (SLN) crystals was the double crucible Czochralski (DCCz) method (Oxide Inc., Japan). However, because the usage of the powder feeding requires a complicate design, this is not convenient for a conventional Czochralski puller. In this study, the zone-leveling Czochalski method (ZLCz) was developed, and could be convenient for the conventional Czochralski puller. The ZLCz method was then the first time using on the growth of SLN crystals with uniform MgO or ZnO doping. The 1”-diameter MgO or ZnO-doped SLN crystals was grown to investigate the characteristics of the ZLCz method and the segregation behavior of the doping. The ZLCz process was then further improved to grow near 2”-diameter high quality SLN crystals.
There are some problems in the ZLCz technique, such as the bubble incorporation, volume change of the solution zone, and the cracking problem of the grown crystal. The bubble formation from the solid feed was found inevitable for the ZLCz method with continuous fed from below of the solution zone. The usage of the inner crucible could be used to avoid the bubble incorporation. The volume change of the solution zone during growth was found to be another serious problem which can cause the axial non-uniformity of the grown crystal. A proper hot zone design could crucially reduce the change of the heat lose condition, and thus the zone change during crystal growth. In addition, with a suitable thermal configuration, the temperature gradient could be controlled under 20℃/cm, and this was found necessary to avoid crystal cracking. 1 mol% MgO-doped SLN crystal pulled from a Li-rich (59-60 mol% Li2O) solution with SLN feeding was found to be 0.975 in Li/Nb ratio within 1% in deviation. Moreover, the uniformity of the as-grown crystal could be enhanced if we pulled the crystal from a K2O-added (16 mol%) solution zone, or fed by using Li-excess (51 mol% Li2O) solid feed. The deviation of the Li/Nb ratio was reduced to less than 0.5%. However, with a Li-excess solid feed, the grown crystal was found with a higher Li/Nb ratio being about 0.98. With the increase of the doping concentration, the Li/Nb ratio of the SLN crystal decreased. And the theoretical limit of the Li/Nb ratio in the 1 mol% MgO-doped SLN crystal is also about 0.98. The stoichiometry of the as-grown MgO-doped crystals was close to the theoretical limit value. In addition, the MgO concentration along the grown crystal was varied less than 5%. And the compositional change in radial direction was found within only 0.1% deviations from the absorption edge measurement.
On the other hand, the zinc-doped SLN crystal with a larger doping variation was found due to the segregation of ZnO. The segregation coefficient was about 0.58. With a higher pre-doped concentration in the solution zone, the doping variation in grown crystal could be significantly reduced to less than 5%. In addition, for ZnO-doped SLN, the properties were varied with the doping concentration, and showed a transition with the zinc concentration at about 1 mol%. Based on the defect model, at a low doping level, the doping was mainly replaced the anti-site defect. At a higher doping level, the Zn ions replaced the normal Nb and Li sites. Thus, the anti-site defects of the grown crystals were about at 1 mol%.
The ZLCz method was successfully developed for MgO or ZnO doped SLN growth. The crystal could be used for the preparation of the periodically poled SLN (PPSLN) chip. The properties and the homogeneity of the grown crystals show good results in this study.
Subjects
區熔提拉法
鈮酸鋰
單晶生長
偏析
等化學計量比
Zone-leveling Czochralski method
Lithium niobate
Single crystal growth
Segregation
Stoichiometry
Type
thesis
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