Study on growth kinetics of Si clusters in silane plasmas

尽管a-Si：H基太阳能电池有望作为太阳能电池作为电源，但高速沉积的a-Si：H薄膜的光致退化一直是二十多年来最重要的问题。最近，在硅烷射频(RF)放电中形成的小于几纳米的团簇被指出可能是利用该放电沉积的a-Si：H薄膜的光致退化的原因。本研究旨在利用两种新的原位测量团簇尺寸和密度的方法来研究硅烷射频放电中硅团簇的生长动力学，并揭示团簇生长抑制与沉积薄膜质量之间的关系。主要研究结果如下：1)即使在器件质量的沉积条件下，放电中也存在大量的团簇。2)利用热电泳力、气体粘滞力、放电的脉冲调制和氢稀释，可以显著地抑制团簇的数量。3)研制了一台团簇抑制等离子体CVD反应器。利用该反应器，以0.2 nm/S的速率沉积了微结构参数小于0.02，肖特基电池光降解后填充因子大于0.53的a-Si：H薄膜，表明团簇的抑制使沉积超高质量的a-Si：H薄膜成为可能。作为未来的工作，通过发展上述成果，建立更高速率沉积太阳能电池用超高质量a-Si：H薄膜的技术是非常重要的。

Although a-Si : H-based solar cells are promissing as solar cells for electrical power source, the light-induced degradation of a-Si : H films deposited at a high rate has been the most important issue over twenty years. Recently, clusters below a few nanometers in size formed in silane radio frequency (rf) discharges have been pointed out to be a possible cause of light-induced degradation of a-Si : H films deposited using the discharges. This research has been carried out in order to clarify growth kinetics of Si clusters in silane rf discharges using two novel in-situ methods for measurements of size and density of clusters, and also to reveal correlation between growth suppression of clusters and quality of deposited films. Main results obtain in this research are as follows :1) Large amount of clusters exist in the discharges even under the device-quality deposition conditions.2) Cluster amount can be significantly suppressed using the thermophoretic force, gas viscous force, pulse modulation of discharges, and hydrogen dilution.3) A cluster-suppressed plasma CVD reactor is developed. By using this reactor, a-Si : H films of microstructure parameter below 0.02 and fill factors of Schottky cell after light degradation above 0.53 can be deposited at a rate of 0.2 nm/s. These results show that suppression of clusters makes it possible to deposite ultra-high quality a-Si : H films. As the future work, it is important to establish techniques for depositing ultra-high quality a-Si : H films for solar cells at higher rate by developing results mentioned above.