半導體材料GaSe1-xSx ( 0 ≦ x ≦ 1)之光譜性質研究

Abstract

我們研究非線性光學半導體GaSe1-xSx (x = 0.00、0.01、0.03、0.14、0.18、0.26、0.37 及 1.00 ) 單晶塊材的光譜性質。首先，GaSe的拉曼散射光譜顯示四個拉曼活性振動模，其頻率位置在134 cm-1、212 cm-1、250 cm-1及307 cm-1，拉曼峰頻率位置隨著摻硫離子濃度上升而有藍移的現象；當x ≧ 0.18 時，我們觀察到多了一個160 cm-1拉曼峰，隨著摻雜硫離子濃度增加而對應到GaS的188.5 cm-1拉曼峰，這些拉曼散射光譜的變化與GaSe1-xSx層狀堆疊結構的改變有緊密的關聯性。此外，我們觀察到光激螢光光譜在x ≧ 0.18 時，其螢光峰的半高寬明顯變寬，且峰值的光子能量大於能隙，推測此時樣品與GaS同屬間接半導體，而多出來的能量即為聲子放射所造成。 我們進一步研究GaSe1-xSx的室溫全頻光譜與變溫穿透光譜，其中紅外聲子吸收峰隨著摻雜硫離子濃度上升而有藍移現象；由於摻雜硫離子後層間距離縮小，電子與離子作用距離較短因此作用力較強，所以造成能隙上升的現象；我們觀察到不同硫離子濃度樣品能隙的溫度變化率，在x ≧ 0.18 時略為上升後下降與低摻雜樣品的變化不同，此結果亦呼應堆疊結構的變化。最後，我們藉由使用第一原理理論計算GaSe在Γ點的聲子振動特性，並與拉曼散射光譜實驗及紅外光活性振動的實驗結果進行比較。

We report the optical properties of GaSe1-xSx (x = 0, 0.01, 0.03, 0.14, 0.18, 0.26, 0.37, and 1.00) single crystals. Room-temperature Raman-scattering spectrum of GaSe exhibits four phonon peaks at about 134 cm-1, 212 cm-1, 250 cm-1, and 307 cm-1. With increasing sulfur concentration, these Raman lines shift toward higher frequencies. When x ≧ 0.18, additional phonon mode appears at about 160 cm-1 and this phonon gradually hardens to 188.5 cm-1 in GaS, reflecting GaSe1-xSx crystallizes in the different kinds of stacking phases as the sulfur doping increases. Moreover, the linewidth of photoluminescence spectra broadens significantly and its peak position becomes larger than the values of energy gap at x ≧ 0.18. These changes are due to the modification of electronic structures when doped with S on Se in GaSe1-xSx. Additionally, the room-temperature infrared and optical reflectance and transmittance spectra of GaSe show one phonon resonance at about 210 cm-1 and an electronic transition at about 1.98 eV. Notably, their peak positions shift toward higher frequencies with an increase of sulfur concentration. The temperature variation of the values of energy gap reveals different trends at x ≧ 0.18. All of these observables suggest that sulfur doping causes significant changes in the stack phases of GaSe1-xSx. Finally, the first-principles theoretical calculations were used to predict the frequencies of vibrational modes at the Γ point in GaSe, and compare with the experimental results.