A Novel Method to Grow Vertically Aligned Silicon Nanowires on Si (111) and Their Optical Absorption

In this study we grew silicon nanowires (SiNWs) on Si (111) substrate by gold-catalyzed vapor liquid solid (VLS) process using tetrachlorosilane (SiCl4) in a hot-wall chemical vapor deposition reactor. SiNWs with 150-200 nm diameters were found to grow along the orientations of all < 111 > family, including the vertical and the inclined, on Si (111). The effects of various process conditions, including SiCl4 concentration, SiCl4 feeding temperature, H-2 annealing, and ramp cooling, on the crystal quality and growth orientation of SiNWs, were studied to optimize the growth conditions. Furthermore, a novel method was developed to reliably grow vertically aligned SiNWs on Si (111) utilizing the principle of liquid phase epitaxy (LPE). A ramp-cooling process was employed to slowly precipitate the epitaxial Si seeds on Si (111) after H-2 annealing at 650 degrees C. Then, after heating in SiCl4/H-2 up to 850 degrees C to grow SiNWs, almost 100% vertically aligned SiNWs could be achieved reproducibly. The high degree of vertical alignment of SiNWs is effective in reducing surface reflection of solar light with the reflectance decreasing with increasing the SiNWs length. The vertically aligned SiNWs have good potentials for solar cells and nano devices.