利用脈衝雷射濺鍍硫化鎘與硒化鋅微晶玻璃薄膜之製成與特性研究

Abstract

我們利用脈衝雷射濺鍍成功地成長出硫化鎘及硒化鋅微晶玻璃薄膜，並且以各種不同的量測方法分析這些樣品光學與結構上的特性。由X光繞射測量得到硫化鎘微晶玻璃薄膜中硫化鎘微晶為六角型 wurzite 結構，其大小約為40 nm。吸收及螢光光譜顯示量子侷限效應所造成的光譜藍位移，而由拉曼光譜中第一階縱向聲子（1LO）的軟化 (softening) 可以進一步驗證量子侷限效應的存在。以電子微探儀（EPMA）及能量分散型光譜分析（EDX），發現樣品中硫的含量大於鎘，與螢光光譜分析結果相符。 在硒化鋅微晶薄膜研究方面，利用空間相干（spatial correlation）模型分析樣品拉曼光譜的第一階縱向聲子，得到用以代表單晶大小的相干長度（correlation length）小於掃瞄式電子顯微鏡所直接觀察的單晶大小。造成這樣差別的原因可能是電子顯微鏡觀察到的一個奈米硒化鋅微粒子中含有硒化鋅複晶，這些複晶間則有明顯的分界。相干長度代表的則是微粒中單晶的平均晶域大小，因此小於掃瞄式顯微鏡之觀察結果。樣品中光激載子可以穿過這些晶界而存在於其他方向的複晶晶域，所以由螢光光譜得到的粒子大小仍近似於掃瞄式電子顯微鏡的分析。低溫（低於200C）的退火過程，並無法消除晶界及樣品的應力。 在不同溫度下樣品的拉曼及螢光光譜譜線位移及半高頻寬變化的分析中，藉由模擬螢光光譜強度隨溫度變化的趨勢，我們得到能帶激發（edge-emission）及深層雜質激發（deep-level）譜線的活化能分別為 237 cm-1 及 266 cm-1，分別近似於硒化鋅的第一階橫向模及縱向模聲子能量。而能帶激發譜線隨溫度變化的偏移，明顯比硒化鋅塊狀樣品要大，這額外的偏移在量測溫度較高時呈現相當的線性，相信是相同於塊材因為溫度降低時微晶間晶格距離隨溫度變化造成的。硒化鋅在高溫時熱脹冷縮且膨脹係數大於玻璃，低溫時( 100K 以下 )開始隨溫度降低而膨脹，可能由於膨脹與玻璃界面擠壓造成譜線反而偏離線性往高能量偏移。雖然這個現象無法以硒化鋅塊材與玻璃之膨脹係數解釋，這樣的假設仍可以由拉曼光譜譜線隨溫度變化的情況得到進一步的證實，顯示硒化鋅微晶的膨脹係數可能異於塊材。

We have successfully fabricated CdS and ZnSe microcrystal-doped silica-glass thin films by pulsed laser deposition from CdS and ZnSe powder embedded in sol-gel glass targets, respectively. The as-deposited and post-annealed semiconductor doped glass (SDG) thin films were examined by various measurements to investigate the structural and optical properties of these samples. The CdS SDG thin films show a wurzite structure with a mean diameter of CdS nanocrystalline of about 40nm by X-ray diffraction. We observed the quantum size effect of these nanocrystallites from the blue shifts of the absorption and photoluminescence spectra which can also be confirmed by the softening of the 1LO Raman mode. The EPMA and EDX indicate that the composition of the thin films is richer in S than Cd. We also showed that the spatial correlation model can be used to analyze the LO Raman mode of ZnSe SDG thin films. The reason why the correlation lengths obtained here differ from the real size of the particulates observed by SEM may be due to the particulates is consisted of polycrystalline with different orientations separated by grain walls. The correlation length of LO phonon provides a measure of separated averaged grain dimension. However, the diameters of quantum dots evaluated from the PL spectra indicate that the carriers may tunnel through the grain walls in the polycrystalline particulates. Low temperature annealing only cause aggregate of particulates without merging the grain boundaries and releasing the stress. The temperature dependence of the shifts and broadening of emission spectra in both Raman and PL measurements for these thin films were also analyzed. By fitting the temperature dependent PL intensity curves, we obtained the activation energies of the edge-emission and deep-level bands as 237 cm-1 (close to 1TO of ZnSe) and 266 cm-1 (close to 1LO of ZnSe). The excess shift of the edge-emission band to higher energy is quite linearly when the temperature is high, which may result from the lattice constant of bulk ZnSe changing with decreasing temperature. As the temperature decreasing furthermore and being below 100 K, the peak shifts to high energy apparently may due to increasing stress in the ZnSe microcrystal and glass interface. The stress was thought to be due to the different thermal expansion of the ZnSe microcrystal and the glass matrix, which can be further clarified by the Raman measurement.