脈衝雷射沉積法成長非極性摻雜鋁之氧化鋅磊晶薄膜於R面(1-102)藍寶石基板

Abstract

本論文使用脈衝雷射沉積法(pulsed laser deposition，PLD)於r面 藍寶石(sapphire)基板上成長摻雜鋁之氧化鋅(AZO)非極性(11-20)磊晶薄膜，膜厚約為200-500nm。PLD使用準分子KrF雷射(波長248nm)剝離AZO靶材組成為(ZnO: 1wt% Al2O3)。為了得到較佳品質的AZO薄膜，PLD製程除了改變雷射功率及頻率(XRD)、原子力顯微鏡(AFM)、掃描式電子顯微鏡(SEM)、穿透式電子顯微鏡(TEM)分析薄膜結構特徵與表面形貌外，並利用UV-可見光吸收光譜、光激發光譜(PL)與霍爾量測，測量薄膜之光電性質。XRD θ-2θ的分析可以發現在基板溫度高於600□C時，AZO薄膜為a面(11-20)的磊晶，而較低之基板溫度，則有c面的訊號出現。X光束沿著c軸方向所測得XRD rocking curve之半高寬為1.3o，從SEM與AFM觀察所成長的AZO磊晶薄膜，發現到具有週期性的長條狀排列，寬度約為25nm。藉由TEM的觀察，發現平行長條狀的方向為AZO的m軸 ，並且在橫截面的TEM影像中觀察到週期性的傾斜排列，可與SEM、AFM影像中表面的長條狀排列相對應。傾斜狀的排列是由界面處垂直的排列轉向而成，其傾斜方向為 。表面形狀呈現鋸齒狀，AZO/sapphire界面無中間相形成。在電性方面，電阻率介於10-3到10-4cm，載子濃度則介於1019與1020cm-3之間，最好的電子遷移率為46cm/V-s，電子遷移率隨著結晶品質的提升而增加。PL光譜的近帶隙(band edge emission)發光峰值約在3.264eV。

Non-polar a-axis orientated ZnO:Al films in 200-500nm thickness were deposited on r-sapphire at different substrate temperature and oxygen ambient by pulsed laser deposition (PLD). An excimer KrF laser (wavelength = 248nm) was used for PLD. The target composition was 1wt% Al2O3 in ZnO. In order to deposit good-quality AZO films, PLD were performed at various conditions such as varying laser power with different frequency, substrate temperature, oxygen partial pressure, and working distance. X-ray diffraction, atomic force microscopy, scanning electron microscopy, and transmission electron microscopy were utilized for microstructural characterization, while UV-visible absorption spectroscopy, photoluminescence and Hall measurements were used for measurements of optical and electrical properties. XRD shows that epitaxial a-plane AZO can be obtained at high substrate temperature, above 600oC, while deposition at lower temperature may result in formation of a small amount of c-plane AZO phase. Full-width half maximum values of XRD rocking curves show the quality of crystallinity of AZO films in the range of 1.3-1.5o for (0002) peak. Plan-view SEM and AFM clearly show stripe-like surface morphology of AZO films in a periodic width of 25nm. Cross-sectional TEM with selected area diffraction and XRD phi-scan results indentify that the elongated orientation of AZO stripes actually along m-axis . TEM in cross section revealed that AZO after 20nm growth has been tilted toward away from the surface normal, and the surface exhibits in saw-tooth morphology. The resistivity of the AZO films were measured in 10-3 to 10-4Ω.cm range, and the carrier concentration ranges from 1019 to 1020cm-3, the highest mobility is about 45cm/V-s. The band edge emission in PL is found at the peak of 3.264eV.