鎂鐵雙摻近等化學計量比鉭酸鋰晶體生長及其特性之研究

Abstract

近年來由於光電產業的蓬勃發展，許多新穎的材料都被陸續開發，其中因為鉭酸鋰晶體具備有數種優秀的性質，如高抗光損傷閥值、高非線性光學係數、高焦電係數、高電光係數以及容易生長等優點，成為最近熱門的研究話題，隨著光電技術迅速發展，超大容量的儲存技術將成為為電子技術後之新興產業，資料儲存的高密度化和資料傳輸的高速化將是今後科技發展的重點，而利用鉭酸鋰晶體做為媒介進行體積全像儲存已被實現，其理論儲存密度可達Tb/cm3等級，而傳輸速度也可達Gb/s。此外鉭酸鋰晶體的另外一種應用為變頻(倍頻、混頻和差頻)雷射，隨著準相位匹配技術的發展，PPLT可望取代GaN而主導下一代的藍綠光市場，有鑑於此近年來有許多廠商積極投入這塊市場。 市場上所販售的鉭酸鋰晶片，應多以共熔組成為主，然而共融組成的鉭酸鋰(CLT)晶體中通常有相當多的本質缺陷，此缺陷將導致晶體的品質下降而影響晶體的性質，如光學吸收係數增加、雙折效應減少、居禮溫度下降、矯頑電場增加、抗光折變能力下降等，大幅降低晶體的應用價值。而等化學計量比組成之鉭酸鋰(SLT)晶體在熱力學上較CLT晶體穩定且晶體中的本質缺陷少，因此SLT晶體的性質較CLT晶體優越，是一種相當值得投資及研究的光學材料。然而生長SLT晶體有其難度，因為生長時會因偏析而導致晶體組成偏移，無法使用使用傳統的柴式提拉法生長SLT單晶，因此於本論文中將嘗試使用我們實驗室自行開發的區熔提拉法進行SLT晶體生長，此外使用區熔提發法所生長之晶體，則藉由光學、物理或是化學分析方法來評價晶體之軸向、徑向組成及均勻性，此外我們也針對鉭酸鋰晶體做摻雜處理，以探討晶體摻雜後的特性。 藉著使用鋰過量之原料做為熔區，且調配適當濃度比例的鉭酸鋰做為底部進料，配合精密的熱場設計下，我們成功使用區熔提拉法生長無摻雜的近等化學計量比鉭酸鋰晶體，透過數種濃度分析技術，皆顯示晶體軸向濃度梯度僅為1.1×10-2 %/mm，而晶體的光學和物理性質在和文獻值比較後，顯示晶體品質以達到商品化的水準，因此我們可得知區熔提拉法是一個相當傑出的連續進料製程。 一般而言，鉭酸鋰晶體的居禮溫度低於熔點溫度，在長晶過程後，通常需要對鉭酸鋰晶體施以一外加電壓，以得到單鐵電域的鉭酸鋰晶體，於本論文中也討論高品質的單鐵電域晶體之製備方式，我們透過使用緩衝層來避免雜質擴散進入晶體後而導致晶體劈裂，極化過程中須施加適當的電流密度於晶體上，過強的電流密度將導致晶體碎裂，而過小的電流密度導致極化不完全。 我們也生長鎂鐵雙摻的SLT晶體，其中摻鐵的目的是能增加晶體的繞射效率以及提供大的動態範圍，而摻鎂則是增加晶體的光導電度而縮短反應時間，控制化學計量比目的則是提高敏感度。透過ZLCz法我們也成功生長不同濃度的鎂鐵雙摻SLT晶體，其晶體外觀則隨著鐵摻濃度的增加而加深，此外晶體的氧化還原狀態於穿透吸收光譜上的變化也被探討，靠著適當的鐵摻雜量以及氧化程度，Fe2+ 所引起的inter-valence transfer吸收峰被觀察在位於425nm處。高鐵摻的雙摻晶體則於OH-吸收光譜上，觀察到特殊吸收峰值(3504 cm2)，此吸收峰值被歸MgLi+-OH--FeTa3-模式所產生的吸收。最後晶體的全像儲存性質也被討論，在特定的摻雜濃度和光強下，鎂鐵雙摻晶體可提供繞射效率為83.96%，寫入時間為93.75 s，抹除時間為452 s，動態範圍為4.3，敏感度為4.07×10-2 cm/J，和其摻雜之鉭酸鋰或鈮酸鋰晶體比較下，鎂鐵雙摻鉭酸鋰晶體提供不錯的全像儲存特性。

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.