Formation of a miscibility gap in laser-crystallized poly-SiGe thin films
- PDF / 135,610 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 73 Downloads / 228 Views
A2.5.1
Formation of a miscibility gap in laser-crystallized poly-SiGe thin films M. Weizman1, N. H. Nickel1, I. Sieber1, and B. Yan 1 Hahn-Meitner-Institut Berlin, Kekuléstr. 5, 12489 Berlin, Germany United Solar Systems Corp. 1100 West Maple Road Troy, MI 48084, USA ABSTRACT Laser-crystallized polycrystalline silicon-germanium (poly-SiGe) thin films on glass substrates were characterized with energy dispersive X-ray and Raman spectroscopy. In the course of the crystallization strong lateral segregation occurs for laser-crystallized poly-Si1-xGex with 0.33 < x < 0.7, causing the local Ge content to differ by as much as 40 % from the average value. The segregation manifests itself in the appearance of well-resolved peaks in the Raman phonon modes. This mode splitting in the Raman spectra is interpreted as the formation of well defined alloy phases with a miscibility gap in between. INTRODUCTION Polycrystalline silicon-germanium alloys exhibit enhanced optical absorption, lower thermal conductivity and lower processing temperatures in comparison to poly-Si. Additionally, the production methods of SiGe are perfectly compatible with the well established Si technology. This motivates the recently rising interest in poly-SiGe as a substitute for poly-Si in thin film solar cells, thin film electronic devices, and micro electro mechanical systems (MEMS). Laser crystallization of amorphous SiGe alloys is one of the methods of choice to prepare poly-SiGe thin films on foreign substrates. Commonly, pulsed excimer lasers are used for this task because their UV irradiation is strongly absorbed in the near surface layer of the films. Crystallization occurs on a short nanosecond time scale and hence, prevents excessive heating of the substrate. Consequently, low cost substrates such as glass or plastics can be used, which are limited to process temperatures below 600 °C. According to the phase diagram of SiGe [1], in a quasi equilibrium cooling process the system travels from a temperature region where a homogeneous SiGe liquid exists to a region where a Ge poor solid and a Ge rich liquid co-exist. As cooling down proceeds, the Ge poor solid phase is enriched with Ge and finally when no liquid phase exists anymore, a homogeneous SiGe solid is formed. Hence, in thermodynamic equilibrium it is possible to create a perfect miscible SiGe alloy for every desirable composition. However, because of very slow concentration equalization in the solid a very long time is needed for the system to reach thermodynamic equilibrium and the quasi equilibrium cooling process is not very realistic. As a matter of fact, in most of the conventional growth methods of SiGe from the liquid phase, diffusion in the solid is negligible on the time scales of the growth. Therefore, the areas first to be solidified, as Ge poor areas, are not equalized by diffusion in the solid during growth and the resulting alloy exhibits some segregation. Therefore, the observation of segregation is common for SiGe alloys, produced by numerous methods based on growth from t
Data Loading...
