磷摻雜效應於超高密度矽量子點薄膜之研究

Abstract

近年來，具奈米結晶態的矽量子點薄膜於太陽能電池(solar cell, SC)之應用受到廣泛的研究；由於其能隙可調的特性，能克服矽基材料的能隙限制，以解決的短波長光子的嚴重損耗議題。目前最普遍的結構為矽量子點埋入於二氧化矽之多層膜結構，而由於其高阻抗之特性，使得光生載子不易傳輸而復合；因此，縮短量子點間距、提高矽量子點生成密度、以及雜質(硼或磷等元素)的摻雜，皆被視為提升薄膜導電性之可行方案。 在2013年我們已開發出漸變矽過多氧化矽多層膜(Gradient Si-rich oxide multilayer, GSRO-ML)結構製作出超高密度矽量子點薄膜，成功的縮短了矽量子點間距、提升載子穿隧機率。為了近一步提升樣品之光電特性，我們研究了硼摻雜於GSRO-ML之效應，在硼摻雜效應下，薄膜元件之光電特性有明顯的提升。 在此篇論文中，我們利用POC3熱擴散的方式使磷摻雜於GSRO-ML結構中。並進一步的研究磷摻雜效應以及缺陷修補效應於超高密度矽量子點薄膜中。在磷摻雜效應下，樣品保持了磷摻雜前之結晶率及高吸收係數，此外，隨著磷的濃度從280 sccm增加至880 sccm，光伏特性有明顯的提升，且矽量子點中之磷原子在POCl3流量880 sccm時達到最高的活化程度，使元件擁有最佳之光伏特性，將流量提升至1000 sccm時，因為過多的磷原子析出形成缺陷，導致元件的整體效益明顯下降。為了探討熱擴散中產生之對薄膜有害物質，PCl5，對薄膜之影響，我們提高氧流量來使PCl5反應完全，改善缺陷的效果明顯，卻導致元件效益下降，指出在我們的元件中缺陷是有助於載子傳輸的。因此，我們驗證了磷摻雜於矽量子點GSRO-ML薄膜中對光電特性提升效應顯著。

Recently, the solar cells (SCs) integrating nano-crystalline Si quantum dot (nc-Si QD) thin films have been researched widely due to the highly-tunable ability of bandgap (Eg). Si-QD SCs can overcome the issue for energy loss from the high-energy photon due to the larger Eg than monocrystalline and amorphous Si. The general structure is Si QD embedded in SiO2 matrix utilizing [Si dioxide/Si rich oxide] multilayer ([SiO2/SRO]-ML) thin film structure. However, the poor photovoltaic (PV) properties were obtained due to naturally highly resistive properties of SiO2 matrix. Therefore, the reduction of QD separation, the increasing of QD density and the heavily impurities (B, P atoms, etc.) doping are available methods to enhance PV properties of devices. In 2013, we have proposed a new structure, gradient Si-rich oxide mutilayer (GSRO-ML) structure and demonstrated a super-high density Si QD thin film to reduce the separation between Si QDs which leads to the higher probability of carrier tunneling. To further enhance the PV properties, the boron doping effect on the super-high density Si QD thin film have been studied and the obvious improvement on PV properties can be observed. In this thesis, we propose to dope P atoms into GSRO-ML thin films by means of thermal diffusion of phosphorus oxide trichloride (POCl3). The P-doped effect and the issue of defect reduced of the P-doped super-high density Si QD thin film will be investigated and discussed. Under P-doping effect, the preserved crystallinity of Si QDs and high absorption coefficient are maintained. In addition, the electrical and PV properties are enhanced with increasing POCl3 flow rate from 280 to 880 sccm, and the best performance is obtained at 880 sccm due to the largest active P-doped atoms but decreased at 1000 sccm due to the increased inactive P-doped atoms. However, a harmful material for Si QD thin films, PCl5, was produced during the high temperature P doping process. We raise the O2 flow rate for the PCl5 reacted absolutely. The clear effect of defect reduced is observed, and the declined PV properties of devices have been observed. The phenomenon means the defect in matrix is helpful for carrier transport. Therefore, our result demonstrated the P-doping effect in the Si QD thin films by using a GSRO-ML structure.