中频孪生靶非平衡磁控溅射纳米硅薄膜的研究

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	中频孪生靶非平衡磁控溅射纳米硅薄膜的研究
	高俊华
学位类型	博士
导师	闻立时
	2012
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料加工工程
关键词	中频孪生靶非平衡磁控溅射纳米硅薄膜磁镜场 Middle Frequency Dual-target Unbalanced Magnetron Sputtering Nc-si Film Magnetic Mirror Field
摘要	" 纳米硅(nc-Si)薄膜是硅纳米晶与非晶硅(a-Si)网络复合的一种半导体纳米材料，掺入氢后在室温下具有优良的综合性能，如高的电导率、对太阳光谱吸收系数大且带隙可调等等，可用于硅基发光二极管、薄膜晶体管和薄膜太阳能电池等光电器件。然而，利用现有的气相沉积技术难以实现高质量nc-Si薄膜的大面积快速制备，使得以nc-Si薄膜作为本征吸收层的大尺度薄膜太阳能电池的开发面临较大困难。中频磁控溅射技术具有设备简单、沉积速率快、氢的掺入量易于调节以及大面积镀膜均匀性较理想等优点，因此，开展中频磁控溅射技术制备nc-Si薄膜的研究具有重要的应用价值。通常中频磁控溅射沉积nc-Si薄膜工艺的主要过程是：首先利用中频对靶磁控溅射系统制备非晶硅薄膜，然后经中高温退火获得晶化率低(<50%)的nc-Si薄膜。本工作提出了一种利用中频孪生靶非平衡磁控溅射技术直接在基底上沉积nc-Si薄膜的新方法，利用该方法在玻璃基底上沉积nc-Si薄膜时，在较低的靶功率密度(~1W/cm2)条件下可获得超过35nm/min的沉积速率。在此基础上开展了中频孪生靶非平衡磁控溅射nc-Si薄膜沉积工艺的研究，并考察了工艺参数对nc-Si薄膜生长机制、组织结构及光学性能的影响。研究结果表明：在不施加外磁场和基底偏压时，在低溅射气压(0.2Pa)、较高的基底温度(≥530ºC)下可沉积出nc-Si薄膜。薄膜微结构分析证实其生长初期存在一定厚度的非晶硅孕育层，且沿薄膜生长方向呈梯度分布，即随着沉积时间的增加，薄膜晶化率大幅度提高，晶粒尺寸也逐渐变大，薄膜表面粗糙度也会呈现单调增加趋势。生长动力学行为的研究结果显示，上述条件下nc-Si薄膜的生长呈现非稳态，且生长界面的演化与硅粒子晶化长大和阴影效应有关。在基底附近引入外磁场可在较大范围内调控辉光放电区等离子体参数，穿过基底区域的磁力线强度及磁场位形的变化影响基底的饱和离子电流密度，基底饱和离子电流密度的增加会增强薄膜生长表面吸附原子的局域扩散迁移能力，一方面可降低nc-Si薄膜的沉积温度且提高其晶化率和沉积速率，更重要的是能有效地抑制溅射沉积过程中束流随机噪声、阴影效应等生长机制的影响，降低非晶孕育层厚度及平滑薄膜生长界面。在本实验条件内，通过调节外加磁场强度和改变磁场位形，增加薄膜生长表面能量输入，利于薄膜的晶化，但过度的离子轰击则抑制了薄膜中晶体硅的生长，并导致应力的产生；当溅射气压较低时，薄膜中硅纳米晶平均尺寸较大(~10nm)，且在非晶硅网络中呈弥散分布；相应高的溅射气压时，硅纳米晶平均尺寸变小，且趋向于团聚在一起；另外，在较高的气压和外加磁场下，可实现nc-Si薄膜沉积的稳态生长。等离子体中电介质层表面充放电效应分析的结果表明，采用一定频率的脉冲电源可以消除溅射沉积过程中硅薄膜表面电荷的累积，在基底表面形成加速电场，实现其表面离子轰击状态的可控。实验证实，在玻璃和不锈钢基底上施加双极脉冲偏压可调控沉积的nc-Si薄膜结构。本工作获得的nc-Si薄膜光学带隙在1.5-1.85eV之间。
其他摘要	" As a kind of semiconductive nanophase materials, nanocrystalline silicon thin films (nc-Si), where Si nanocrystals are embedded in the amorphous silicon (a-Si) network, possess many excellent room-temperature photoelectric properties after hydrogenated, such as high electric conductivity, large optical-absorption coefficient for the sunlight spectrum and tunable optical band gap. Hydrogenated nc-Si films has been used for preparation and development of photoelectric devices such as silicon-based light emitting diodes, thin film transistors and thin film solar cells. However, it is difficult to rapidly deposit high quality nc-Si films with large-area scale by existing vapor deposition techniques, which makes it has some difficulties in developing large-scale thin film solar cells with nc-Si films as intrinsic absorb layers. Compared with the other methods, the middle frequency magnetron sputtering has some advantages such as simple equipment, high deposition rates, and better flexibility in incorporation of hydrogen and ideal uniformity for large-area films. Therefore, investigations on preparation of nc-Si films would have significant application potentials. The main processes for preparation of nc-Si thin films by a middle frequency magnetron sputtering system are as fellows: firstly, deposit a-Si films by middle frequency face target magnetron sputtering and subsequently anneal the a-Si films to acquire nc-Si thin films. In this thesis, a new method was developed to directly deposit nc-Si thin film on substrates by middle frequency dual unbalanced magnetron sputtering with external fields’ assistances. The results revealed that under a low target power density (~1W/cm2), we obtained deposition rates of beyond 35nm/min for preparing nc-Si thin films on glass substrates by above-mentioned sputtering method. Based upon the above case, the deposition of nc-Si thin films by middle frequency dual unbalanced magnetron sputtering and the effects of various process parameters on growth mechanisms, structure and optical properties of nc-Si thin films were investigated, respectively. Without external fields’ assistances, nc-Si thin films had been deposited at low sputtering gas (0.2Pa) and relatively high substrate temperature (≥530°C). The analyses for their microstructures confirmed that there is a structural gradient in the nc-Si film deposited under above deposition conditions with an amorphous incubation layer at the initial step of its growth. Namely, with increasing deposition time the crystalline volume fraction, grain sizes and surface roughness for the films significantly increased. The studies about growth dynamic scale behaviors of nc-Si films illuminated that the growth process for the nc-Si films is nonstationary and crystallization of silicon particles and shadowing effect would dominate the evolution of growth interface. The introduction of external magnetic field had a strong effect on the near-substrate plasma parameters, such as substrate saturated ion current density, resulting from the variations of magnetic field density and configuration in the substrate region. The enhancement of substrate saturated ion current density could improve the local mobility of adatoms, which can reduce the deposition temperature, and enhance the crystalline volume fraction and the deposition rate, especially could effectively compress the effects of the growth mechanisms such as random noise and shadowing effect. It also could lower the thickness of amorphous incubation layer and smooth the surface interface. In our experiments, with varying magnetic field density and configuration in the substrate region, the energy transferred into the film growth surface could be enhanced and help the crystallization of Si films; however, the excessive ion bombardment damaged the order degree of Si films and caused the stress in the films. The average sizes of nanocrystals and their distributions could be modified by the sputtering gas pressure. In addition, the growth process could become stationary under higher pressures with the strong external magnetic field. The analyses of the charging and discharging effect existed in surfaces of dielectric layers, which were immersed in a plasma, showed that the pulse power with a certain frequency could eliminate the accumulated charges on the Si films surface and steer the ion bombardment conditions to substrate. It was verified that the variations in the structure of nc-Si films deposited on glass and steel substrates could caused by the bipolar pulse bias power. In this work the optical band gap for the nc-Si films ranged from 1.5 to 1.85eV."
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64431
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	高俊华. 中频孪生靶非平衡磁控溅射纳米硅薄膜的研究[D]. 北京. 中国科学院金属研究所,2012.