鍺摻雜之 AgGaS2 單晶成長及其熱處理研究

Abstract

本研究主要在以垂直布氏長晶法生長鍺摻雜非線性光學材料 AgGaS2 單 晶.實驗中將以前置晶種的方式生長晶體,可長出直徑 10mm,長度 68mm 的 高品質單晶.並以 X 光粉末繞射法確認晶體是黃銅礦結構和計算出鍺摻雜 後的晶格常數為 a=5.776A,c=10.365A.複晶原料是採直接合成的方式,其 配製是以高純度的銀,鎵,硫三元素依化學計量Ag(1-x)Ga(1+x)S(1+x), x=0.01 的比例稱重(誤差小於 0.0005g).由於此晶體是沿 C 軸熱縮冷脹 的材料,為防止降溫時和長晶石英管過度擠壓而破裂氧化,所以採用熱裂解 法,在長晶石英管壁上塗佈一層石墨當作緩衝層(buffer layer).適當的長 晶條件如下:長晶溫度梯度 35C/cm,長晶速率 30mm/day,降溫速率 24 小 時降至室溫.由於配製原料比例的關係,生長完成的晶體避免不了析出物的 產生,這些針狀的析出物通常在平行(100)及(010)的方向上成核並且成長, 而成為散射中心,嚴重影響電性和光學性質,因此,必須以熱處理的方式將 析出物去除,以獲得更高品質的晶體.熱處理所使用的原料(source)是純 Ag,使用量約為 3wt%,溫度為 950C,在熱處理 12~14 天左右後能消除析出 物.在光學顯微鏡(OM),掃描式電子顯微鏡(SEM)的觀察下,除可看見析出 物,孔洞外,也可發現擠壓所造成的雙晶和微裂縫.電子微探儀(EPMA)分析 出晶體成份有愈長愈均勻的趨勢,且熱處理後的確達到均質化,利用其 mapping 的功能,亦可得到析出物有比母材較多的鎵和硫而較少銀的結果. 以感應耦合電漿-原子發射光譜分析儀(ICP-AES)量測晶體中鍺摻雜濃度發 現,鍺元素隨晶體成長時,晶體愈接近尾端鍺元素濃度愈高,分佈係數 k 值 估計約為 0.079(配料中鍺摻雜濃度為 10^20atm/cc).霍氏轉換紅外光譜 儀(FTIR)量測晶體的結果顯示,紅外穿透率因熱處理消除析出物而提升.冷 激光光譜儀(PL)量得晶體螢光光譜最高峰在 466nm 處附近,無法量得鍺摻 雜所造成的能階,推測可能的原因是鍺摻雜所造成的能階電子躍遷量少,而 室溫熱噪音(noise)嚴重,造成訊號無法判斷. Ge doped AgGaS2 single crystals of diameters up to 10mm and length up to 68mm for applications in nonlinear optics and blue light emiting diodes have been successfully grown by vertical Bridgman method.Direct high temperature synthesisof element Ag, Ga,S with nonstoichoiemetry Ag0.99Ga1.001S2.001 is employed to prepare polycrystalline source materials for the subsequent single crystal grrowth.Growth quartz ampoules were coated with pyrolytic graphite buffer layersto prevent thermal expansion of the crystal during cooling from high temperature.The recommended growth conditions are:temperature gradient at the melt/solid:35 C/cm;pulling rate:30cm/day;cooling procedure:24 hr. to room temperature after growth.Precipitates,twins and microcracks are present in the as-grown crystals.Most of these defects can eliminated by the post annealing besides twins.As-grown AgGaS2 crystals were heat treated in a sealed quartz ampoulefor 12 to 14 days at 950C,using approximately 3wt% excess pure Ag.EPMA resultsindicate that the composition of crystals are uniform even more after annealing procedure,and the precipitates are Ga- rich as revealed by EPMA mapping.ICP results indicate that the concentration of Ge-doping increases at tail with the growth of crystal,and the partition coefficient is about 0.079.FTIR results indicate that IR transmittance can be enhanced by heat treated which removes the precipitats.PL results indicate that the emission energyfor the band-edge emission at room temperature is about 2.662eV.