Growth and characterization of zinc oxide thin films on Ge substrate by radio frequency sputtering
|摘要:||本論文主要是研究使用射頻濺鍍法(RF sputtering)製作氧化鋅薄膜。本實驗主要區分為三部分，首先利用Si基板探討製程參數對ZnO薄膜結晶性質的影響，第二是ZnO薄膜沉積在單晶Ge(111)基板上，第三沉積摻雜Ga的ZnO (ZnO:Ga) 薄膜於單晶Ge(111)基板上。ZnO薄膜成長後，以原子力顯微鏡(AFM)觀察表面型態，以掃描式電子顯微鏡(SEM)測量薄膜成長速度，以X光繞射儀(XRD)、穿透式電子顯微鏡(TEM)分析晶體結構與晶體品質，二次離子質譜儀(SIMS)分析摻雜Ga的分布狀況，四點探針(4-point probe)分析電阻係數。
Si基板方面，發現濺鍍參數如溫度、Ar/O2比例及工作壓力對於結晶品質影響很大。當氧濃度高於約23% (氬氧比20/6)，會造成更好的結晶品質，觀察到ZnO(0002)半高寬有明顯變窄。當氧濃度高於33%(氬氧比20/10)，ZnO (0002)半高寬有明顯變寬，這說明了薄膜的結晶品質在下降。Ge基板方面，發現氧化層對於結晶品質影響很大；可以用化學濕處理(H2O2 、HCl、NH4OH/H2O2/H2O)及高溫度600 □C兩小時的熱脫附處哩，減少Ge氧化物的影響。XRD分析中有發現ZnGeO氧化物出現，顯示ZnO/Ge兩者有反應發生。GZO系統方面，使用1% 與2% Ga之靶材鍍膜；其中2% Ga靶材濺鍍結果，表面較為平整，電阻係數隨著功率的提高而降低至1.25 x 10-3 □.cm。從SIMS縱深分析，Ga在ZnO薄膜中均勻分布。Ge上濺鍍ZnO與ZnO:Ga薄膜，XRD顯示皆為C軸指向之薄膜，晶粒尺寸為18~38 nm。橫截面TEM觀察，ZnO薄膜為柱狀晶結構，電子繞射只有兩組晶向關係：(1) (0002)ZnO // (111)Ge [1 00] ZnO // Ge ， (2) (0002)ZnO // (111)Ge [ ]ZnO // Ge，薄膜與基材之界面平整；俯視圖(plan-view)之繞射圖亦證明所有晶粒皆為C軸指向。從實驗結果可知，利用RF濺鍍法在Ge基材上可以成長品質良好且具有高優選方向的ZnO薄膜。|
In this study, highly oriented crystalline ZnO thin films were deposited by radio-frequency (rf) sputtering. The thesis is divided three parts. In the first part, ZnO films were deposited on Si substrate by rf sputtering at various deposition conditions including different temperature, power, working pressure, and oxygen concentration. Secondly, ZnO films were deposited on Ge(111) substrate. Thirdly, Ga-doped ZnO (ZnO:Ga) films were deposited on Ge(111) substrate. The surface morphology was characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM); the microstructres of ZnO thin films were characterized by x-ray diffraction (XRD) and transmission electron microscopy (TEM); the Ga distribution was determined by secondary ion mass spectrometry (SIMS); and the electrical properties were measured by the 4-point probe. For the ZnO deposited on Si substrate by rf sputtering, it is found that the sputtering parameters of temperature, Ar/O2 ratio and working pressure have tremendous influence on crystallinity. With oxygen concentration higher than 23%, the ZnO films have better quality as seen in XRD pattern that the full –width at half maximum of ZnO(0002) peak becomes narrower. However, when oxygen concentration is higher than 33%, the crystallinity is degraded. For the ZnO deposited on Ge substrate, it is found that the Ge oxide surface layer plays an influential role on the ZnO film quality. To reduce the effect of the Ge oxide layer, we used wet chemical cleaning (H2O2, HCl/H2O, NH4OH/H2O2/H2O) followed by desorption by heating at 600□C for 2hr. XRD patterns show only ZnO c-plane reflections in addition to reflections of ZnGeO related oxides. For deposition of ZnO:Ga films on Ge substrate , targets of 1% and 2% Ga in ZnO were used. Sputtering of the 2% Ga target on Ge substrate can result in a smooth surface with resistivity of 1.25 x 10-3 .cm. The depth profile of SIMS analysis shows that Ga is distributed evenly in the ZnO film. For deposition of ZnO and ZnO:Ga on Ge, all the films have the c-axis preferred orientation with a grain size in 18~38 nm as shown by XRD. Cross-sectional TEM reveals that ZnO film structure is columnar with the c-axis orientation, and the interface between films and substrate is smooth. As shown by TEM diffraction pattern, there are two orientation relationships between ZnO and Ge: (1) (0002)ZnO // (111)Ge [1 00] ZnO // Ge, and (2) (0002)ZnO // (111)Ge [ ]ZnO // Ge. Plan-view diffraction pattern proves that almost all the grains are grown in the c-axis orientation. From the experimental results, it can be concluded that rf sputtering of ZnO on Ge substrate can deposit a good crystalline quality film with strongly c-axis oriented grains.