Amorphous Silicon Precipitates in (100) c-SI Films Grown by ECRCVD
- PDF / 2,016,280 Bytes
- 6 Pages / 384.12 x 617.04 pts Page_size
- 27 Downloads / 148 Views
ABSTRACT Silicon films were grown on (100) n-Si with an electron-cyclotron resonance chemical vapor deposition (ECRCVD) system by decomposition of SiH 4 at 325 0 C. Structure and composition of thin films were investigated by SEM, Raman spectroscopy, elastic recoil detection analysis (ERDA) and TEM. Excellent epitaxial growth was achieved for some hundred nm thickness. For more than 1 pm thick films, however, SEM revealed the occurrence of conical structures orientated upside-down with their basal plane in the film surface. Depth-profiling of the elemental composition of thin films by means of ERDA showed the hydrogen content CH to exhibit a pronounced increase with increasing film thickness. Raman spectroscopy evidenced the coexistence of c-Si and a-Si:H by the occurrence of two bands at 520 and 480 cm , the ratio of which was found to depend sensitively upon the position of the laser spot on the sample. All experimental results could be consistently explained by assuming the conical precipitates to consist of a-Si:H which was finally proven by coherent electron beam diffraction (CEBD).
INTRODUCTION Thin-film crystalline silicon is currently investigated and optimized with respect to its suitability for solar cell applications [1, 2]. Various processes like PECVD, ECRCVD, laser crystallization etc. are used for this purpose and different device concepts are being considered which would even include other thin-film materials. In this work we report on epitaxial Si thin films as prepared on CZ-grown Si wafers by virtue of electron cyclotron resonance (ECR) CVD
at low temperatures, see also [3]. Our activities aim at the development of a thin film silicon solar cell deposited on glass and on crystalline seed layers. In the following the preparation of nominally undoped films and their structural characterization by scanning electron microscopy (SEM), elastic recoil detection analysis (ERDA), Raman spectroscopy and transmission electron microscopy (TEM) will be presented. It will be shown that the growth of epitaxial film was accompanied by the formation of a-Si:H precipitates after the film's thickness had passed a critical value.
EXPERIMENTS AND RESULTS Thin Si film depositions were performed in a high-vacuum ECRCVD chamber which is described in detail in [4]. The base pressure before the deposition was less than 10-5 Pa. Although the process was performed at 0.66 Pa, which is much lower than in normal PECVD systems, growth rates of 10-12 nm/min were obtained. The frequency of the exciting microwave was set to 2.45 GHz to obtain resonant absorption of the plasma which was enclosed by a magnetic field of 87.5 mT. The microwave power was adjusted to 1000 W and the susceptor temperature
555 Mat. Res. Soc. Symp. Proc. Vol. 557 © 1999 Materials Research Society
Figure 1 (a) scanning electron micrographof as-grown homoeptiaxial 1.3 pm thick Sifilm. (b) SEM picture of a comparablyproducedfilm after Secco etching. amounted to 325°C. Intentionally undoped films were grown with gas flow rates of 20 sccm SiH 4 and 60 scc
Data Loading...
