椭偏光谱法研究不同硅烷浓度制备微晶硅薄膜的微结构和生长机制

Ellipsometric investigation on microstructure and growth mechanism of microcrystalline silicon thin films prepared with different silane concentrations

本文采用VHF-PECVD技术制备了两个不同硅烷浓度(SC)系列的微晶硅薄膜,通过椭圆偏振技术研究了微晶硅薄膜的微结构和表面粗糙度随沉积时间的变化。实验结果表明:随着薄膜厚度的增加,两个系列硅薄膜的晶化度增加,当薄膜增加到一定厚度时内部开始出现微空洞,这是由于随着薄膜厚度的增加,薄膜晶化度增加,晶粒增大,大晶粒边界之间更容易形成空洞。硅薄膜的表面粗糙层厚度ds与薄膜厚度d满足指数关系:ds^dβ,β为生长指数,与薄膜生长机制有关,当硅烷浓度SC为4%时,β=0.33,对应有限扩散生长模式。硅烷浓度SC为5%时,β=0.52,对应为零扩散随机生长模式。硅烷浓度降低,生长指数β减小,这是由于随着硅烷浓度的降低,氢原子浓度增加,薄膜表面氢覆盖扩大,从而有利于反应前驱物的扩散,因此薄膜表面更为光滑,生长指数β减小。

Hydrogenated microcrystalline silicon（μc-Si：H） thin films with two series of different silane concentrations（SC：4% and 5%） were prepared for different depositing time by very high frequency plasma-enhanced chemical vapor deposition（VHF-PECVD）.The evolution of microstructure and surface roughness of the films were investigated with ellipsometric technique.The results showed that the crystalline volume fraction Xc of all films and grain size both increase with the film thickness,thus resulting in the appearance of micro-voids in the films when the film thickness increases to a certain extent.The reason is probably that the increasing Xc value/grain size tends to form voids between big grains.An exponential relationship is found between the thickness of film surface rough layer ds and film thickness,ie.,the ds～dβ relation,where β is the growth exponent that is related to the film growth mechanism.When the films are deposited with SC=4% and 5%,β is about 0.33 and 0.52,which corresponds to the definite and zero diffusion growth,respectively,ie.,β decreases with decreasing SC.The reason is that the H atom concentration increases with decreasing SC,then the hydrogen atoms coverage on the film surface increases,which is beneficial to the diffusion of reactive precursors on film surface,thus making film surface smoother with decreased β value.