| Abstract Scope |
Thin film microcrystalline Si (µc-Si) is an attractive material due to improved optical absorption compared to single crystal Si, as well as, increased mobility when compared to amorphous Si (a-Si). Producing high quality µc-Si at an affordable price is a particular challenge. Metal-induced growth (MIG) is a method of producing thin film µc-Si at temperatures of 625 <sup>o</sup>C and below. Similar to metal-induced crystallization, MIG has the advantage of depositing and crystallizing Si in a single step, while simultaneously forming an ohmic back contact. While MIG with a Ni catalyst has typically yielded good results, the process does not occur below 575 <sup>o</sup>C. Al and Cu offer significantly reduced crystallization temperatures, and consequently, lower cost. The mechanism by which crystallization occurs differs depending on the metal properties. Metals like Ni and Cu initially form a silicide compound. With Ni, this silicide precipitates through the Si and crystallizes it by the diffusion of Ni and Si atoms. Since Cu forms a metal-rich silicide, this compound does not precipitate and instead provides enough latent heat to activate the phase transition to crystallized Si. Al does not form a silicide. Instead, Si atoms diffuse into the metal and form nucleation sites where Si grains will grow that eventually come into contact with each other and form a continuous film. The MIG process began with thermal evaporation of the metal film. This was then coated with Co to modulate the Si grain growth and improve film quality. When Si was subsequently sputtered at an elevated temperature, the metal film acted as a catalyst to crystallize the Si and form a continuous µc-Si film. After deposition, films were annealed at 700 <sup>o</sup>C for 2 hr in forming gas. The appearance of distinct facets along the surface of the film indicated strong crystallization throughout the film. A strong presence of metal near the surface of films using Al compared to films using Ni and Cu signified metal contamination throughout the film. While x-ray diffraction demonstrated strong Si peaks with each metal, the film using Al observed extraneous peaks not indicative of µc-Si. Application of these films was explored by fabricating Schottky photodiodes, which were measured under dark and one-sun illumination conditions. Linearity in the log-log plot of dark forward bias indicated space charge limited conduction in films using Al likely due to considerable metal contamination. This trend also applied to the photo-response, where the fill factor greatly suffered. Films using Cu obtained performance closer to those with Ni. While Al is capable of reducing the crystallization temperature the most, its properties may not make it suitable for MIG. With optimized thickness, Cu may serve to realize a lower cost MIG process, while maintaining film quality. |