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非晶硅薄膜光学性质的研究及铝诱导晶化法制备多晶硅薄膜
【作者】 方晓玲；
【导师】 高斐；
【作者基本信息】 陕西师范大学 ， 光学， 2008， 硕士
【摘要】 太阳能电池在缓解能源危机和解决环境污染方面有重要的研究和应用价值。在各类太阳电池中薄膜太阳电池以低成本和较高转换效率的特点成为当前的研究和开发热点,特别是多晶硅薄膜太阳电池,具有原料来源丰富、成本低廉、能耗低、性能稳定高效和应用范围广泛的优点,有望成为替代晶体硅的第二代太阳电池。本论文首先采用等离子体增强化学气相沉积(PECVD)法制备非晶硅薄膜,并对非晶硅薄膜的光学性质进行了研究,利用自制的夹具改进分光光度计的装置,用来测量非晶硅薄膜的透射光谱,通过对透射光谱的拟合和计算,确定出它的光学性质。其次用金属铝诱导晶化法制备多晶硅薄膜,采用glass/a-Si:H/Al的结构,用拉曼(Raman)光谱、X射线衍射(XRD)光谱、原子力显微镜(AFM)等表征手段研究了铝诱导晶化过程中各种因素对多晶硅薄膜结构的影响,得出如下结论:1.所制备非晶硅薄膜折射率与入射光波长的关系为:n=1.6×10~5/λ~2+2.88;吸收系数与入射光波长的关系为:lgα=6.48×105/λ~2-4.8以及非晶硅薄膜的光学带隙为1.56 eV.2.在铝诱导晶化过程中,退火温度越高、退火时间越长,越有利于薄膜结晶及晶粒长大,并且发现退火温度与退火时间是相互关联的。高的退火温度需要短的时间完全晶化;低的退火温度需要较长时间才能完全晶化。3.首次提出两步退火,发现在两步退火过程中,存在晶化效果好的优化点,如先在300℃下退火2h,然后继续升温到500℃退火3h,制备出的多晶硅颗粒大,表面光滑,有一定的使用价值。4.硅膜厚度越厚越有利于薄膜晶化,但存在一个极值点,当硅膜厚度大于这个极值点时,薄膜的晶化率下降。5.Al与a-Si:H之间的氧化层形成阻挡层。氧化层越厚,Al和Si原子互扩散难,成核密度越少,形成的硅晶粒越大;反之,氧化层越薄,成核密度越大,形成硅晶粒越小。6.在铝诱导层交换过程(ALILE)中,最大驱动力是非晶硅晶化能的释放,由于硅晶化过程的局部性,这部分能量对层交换的发生并没有贡献,而发生层交换的驱动力是由铝相宏观应力和微观应变的松弛及铝颗粒的生长引起弹性能的释放。
【Abstract】 The wide use of solar cell exhibits important role to solve the resource and environmental crisis. Thin film cells have drawn great interests due to their tow cost and higher efficiency.Among them the polycrystalline silicon(p-Si) thin film has the advantages such as abundant raw materials,low production and energy cost,stable performance,extensive application and so on,which enable it promising in replacing crystalline silicon solar cells and becoming the next generation solar cells.In this thesis,firstly amorphous silicon(a-Si:H) thin films were fabricated by plasma enhanced chemical vapor deposition(PECVD) system.We investigated the optical properties of the a-Si:H film.The spectrophotometer was improved by using a clamp.Transmission spectrum of a-Si:H film were measured by the modified spectrophotometer.Its optical properties are obtained by spectrum curve fitting.And then,the a-Si:H films with a stack of glass/a-Si:H/A1 were crystallized by aluminium-inducing.It were analyzed by Raman spectrum,X-ray diffraction,and Atomic Force Microscope.It has been studied that factors affect on the structures of polycrystalline silicon in the process of aluminum induced crystallization.The study results were obtained as fellow:1.The relation between refractive index and incident wavelength of a-Si:H thin film is: n=1.6×10~5/λ~2+2.88;The relation between absorption coefficient and incident wavelength of a-Si:H thin film is:1gα=6.48×105/λ~2-4.8;The optical band gap of a-Si:H thin film is 1.56eV.2.Higher annealing temperature and longer annealing time are prone to make crystallization appear in the aluminum-induced process.Annealing temperature and time were found to be connected.Higher annealing temperature corresponds to shorter time,lower annealing temperature corresponds to longer time in the process of complete crystallization.3.Two steps annealing was put out first time.There existed an optimal point in the process of the two steps annealing,for example,annealed at 300℃for 2h and then reheated to 500℃annealing for 3h.The results showed that the p-Si thin film had big grain size and smooth surface.It is useful in practice.4.The thicker Si film,the higher crystallized fraction of p-Si films.However,there exist a critical point,crystallized fraction of p-Si films is reduced beyond the thick.5.The oxide layer between the a-Si:H and Al forms a barrier for atomic diffusion.The thicker oxide interface layer,the harder diffusion of Al and Si atoms and the smaller nucleation density of Si could produce the larger Si grain.Otherwise,the thinner oxide interface layer,the higher nucleation density resulted in the smaller Si grain.6.In the aluminum-induced layer exchange(ALILE) process the largest contribution to the driving force for the transformation is the release of the crystallization energy of the amorphous Si. Considering that the Si crystallization process takes place locally,the release of the crystallization energy of the amorphous Si has no contribution to a driving force for the layer exchange.However, The layer exchange may be driven by the release of elastic strain energy associated with the relaxation of macrostress and the microstrain and the grain growth of the A1 phase.