利用脈衝雷射濺鍍之硒化鎵薄膜之製程與特性研究

Abstract

我們成功地利用脈衝雷射濺鍍在氧化鋁(Al2O3)晶片(0001)以及石英玻璃(quartz glass)上成長出(00l)單一指向性硒化鎵薄膜；以成長條件：基板溫度400度C，經過450度C退火1小時，以雷射能量密度9J/cm2(聚焦面積~0.5mm2)，重覆頻率5Hz。由X光繞射測量得到硒化鎵為六角型結構，且最佳品質的薄膜之晶粒大小約為57.2nm。從X光繞射發現並由EPMA的分析中確定，隨著溫度的降低(550~450度C)有三硒化二鎵到硒化鎵的相變化。 在室溫下做拉曼光譜分析，我們觀察硒化鎵的A1’mode(振動方向平行c軸)及E’(LO)mode (振動方向垂直c軸)的拉曼位移變化。A1’mode可以作為薄膜結晶程度好壞的指標，當A1’ mode的拉曼位移(分別為134cm-1及308.1cm-1)及半高寬越接近硒化鎵塊材時則薄膜的品質越佳。而由E’(LO) mode的拉曼位移可計算出硒化鎵薄膜與氧化鋁基板之間的應力。並且我們發現以上述條件成長高指向硒化鎵薄膜時，高品質的薄膜其厚度約為80nm~120nm左右，且同時其彼此之間應力也已消失。 從AFM的分析裡，我們得知了硒化鎵薄膜是以島狀加上層狀方式成長；若以高雷射濺鍍能量且聚焦面積小時，可以促使硒化鎵薄膜以層狀方式(00l)在氧化鋁晶片(0001)上成長。從AFM的觀察中也發現經過退火處理後明顯的薄膜品質變好；在低溫77K下所做的PL光譜分析也得到同樣的結果。 在上述成長條件之下，硒化鎵薄膜的厚度只需成長至80nm就已足夠做為氮化鎵或氧化鋅與氧化鋁晶片之間的緩衝層。

We have successfully fabricated highly oriented (00l) GaSe film on c-cut sapphire (0001) by pulsed laser deposition which posses a hexagonal and layered structure with a grain size about 572 Å determined by X-ray diffraction (XRD). We also observed phase transition from Ga2Se3 to GaSe by XRD when the growth temperature is reduced from 550 C to 400 C that is also confirmed by the EPMA analysis. The crystallinity of GaSe thin films can be also evaluated by Raman A1’ mode which vibrates along c-axis and the E’(LO) Raman mode which vibrates perpendicular to the c-axis of GaSe. The more consistent frequency and linewidth of A1’ mode with those of GaSe bulk, the better crystalline GaSe film is. And the E’(LO) mode can be used as a stress indicator which is very sensitive to the interface stress of GaSe layer and sapphire substrate. We found the best quality GaSe films have 800 Å to 1200 Å thick grown at 400 C with laser energy flux 9 J/cm2 and repetition rate 5Hz and post-annealed at 450 C in which less Raman shift as well as spectral width were found as compared with the bulk. We show in AFM images that the growth process of the PLD GaSe films is in the manner of layer plus island growth. It reveals that high laser energy and small spot size would help the GaSe film grow epitaxially on c-cut sapphire, which is also confirmed by XRD. From AFM images and the FWHM of PL at 77 K, we found the quality and morphology of GaSe films get better after annealing. Growth of GaSe on sapphire to about 80 nm thick by PLD is sufficient for a buffer layer of epitaxial growth of hexagonal structure materials like GaN and ZnO on sapphire.