(A) transmission electron microscopy study on the crystallization of low pressure chemical vapor deposition amorphous silicon thin films

Abstract

The solid phase crystallization of amorphous Si thin films and the formation mechanism of crystallites observed in as-deposited mixed-phase silicon thin films deposited by low pressure chemical vapor deposition were studied using a transmission electron microscope. Furthermore to reduce the crystallization time of amorphous Si films, Al-induced crystallization of an a-Si thin film in the Al/native-$SiO_2/a-Si$ structure and crystallization of $Si/Si_{0.7}Ge_{0.3}/Si$ triple layer were investigated. TFTs that used the $Si/Si_{0.7}Ge_{0.3}/Si$ triple layer as an active layer were fabricated and device characteristics of them were compared to those of Si TFTs and of SiGe TFTs. The solid phase crystallization of the amorphous silicon thin films, deposited on $SiO_2$ at 520℃ by low pressure chemical vapor deposition and annealed at 550℃ ambient, was carried out, so that the grain growth mechanism, various types of defects, and origins of defects formation could be defined on an atomic level. A crystallite formed at the initial stage of the crystallization had a circular shape and the grains grew preferentially in one direction. In all the grains, there were twins of which {111} coherent boundaries were parallel to the long axis of the grain. In addition to twins, the following defects were observed in the grain: intrinsic stacking faults, extrinsic stacking faults, perfect dislocations, extended screw dislocations, and Shockley partial dislocations. These defects were formed by the following reasons: the errors in the stacking sequence at the amorphous/crystalline interface, jumps of a twin plane, intersecting of two crystal growth fronts slightly misoriented, and intersecting of two twin planes at the amorphous/crystalline interface. Not only twins but also stacking faults accelerated the growth rate of the crystalline silicon preferentially in the <112> direction along the {111} plane. Then the shape of a silicon grain elongated in the direction parallel to the t...