This dissertation describes the behavior of crystallization of hydrogenated amorphous silicon (a-Si:H) films prepared by Plasma Enhanced Chemical Vapor Deposition (PECVD) using $H_2$ diluted 10% $SiH_4$ gas.
In order to study the mechanism of crystallization of a-Si:H, films were deposited at various substrate temperature and operating pressure. Then those were annealed in the conventional furnace under the high vacuum condition (<$5\times10^5$ torr) at 600℃. Crystallinity of as-deposited and annealed films was identified with Raman spectroscopy, Transmission Electron Microscopy (TEM) and X-ray diffractomer (XRD). The Si-Si bond characteristics were observed with Raman spectroscopy and Si-H bond characteristics with FTIR. Grain size were measured from the TEM images. Especially, modified gas chromatography was adopted to investigate the SiH bonding energy by detecting hydrogen thermal desorption from a-Si:H films.
By varing the substrate temperature ($T_s$) from 100℃ to 300℃ at constant pressure of 0.3 torr, a-Si:H films were deposited. Grain size of crystallized films were changed with substrate temperature. The lower is the substrate temperature, the larger is the grain size. The network structure and Si-H bonding characteristics of a-Si:H films are changed with $T_s$; as $T_s$ is decrease, structural disorderness of Si network slightly increases, the clustered $SiH_2$ increases, the isolated SiH and the internal compressive stress decrease. And the Si-H bonding state is separated in two. In crystallization of a-Si:H films, those result in decreasing nucleation rate. So the final grain size of crystallized films deposited at low temperature (100℃) had the largest size of 0.7㎛.
To study the effect of deposition pressure on the grain size of crystallized films, a-Si:H films were deposited under pressure range from 0.1 torr to 1.0 torr at constant substrate temperature of 100℃ and then annealed. Grain size of those films increase with pressure increasement. So maximum grain size of 1.6㎛ were obtained from the a-Si:H films deposited under 1.0 torr. As the pressure increases, the structural characteristics of a-Si:H films are changed; the structural disorderness slightly increases, the clustered $SiH_2$ increases, the isolated SiH decreases and the Si-H bond state is separated in two. One is SiH and the other is $SiH_2$. In crystallization of a-Si:H films, those result in decreasing nucleation rate in the films deposited under higher pressure. So the final grain size of crystallized films deposited at high pressure (1.0 torr) has the largest size.
These results suggest that not only structural disorderness of amorphous Si network but also the Si-H bond characteristic affects on the behavior of crystallization of a-Si:H films. That is to say, the nucleation of crystalline silicon might occur preferentially at the region of isolated SiH in Si network. The clustered $SiH_2$ region may affect only on the relaxation of internal compressive stress but not serve as the nucleation site.
