Growth Mechanisms And In Situ Annealing Observation Of Silicon Nanowires For Photovoltaic Solar Cells

Solar cells based on radial junctions built on nanowires have many advantageous. Effective structural and morphological control of silicon nanowires(SiNWs) is critical to promote the use of these one dimensional building blocks in a new generation of photovoltaic devices. The present subject is centered on the study of the SiNWs fabricated by PECVD at low temperature via vapor-liquid-solid method. Samples of SiNWs with different growth duration were prepared. And by TEM and SEM characterization, unique structure and morphology were observed and its growth mechanism was revealed.There are three stages in the process of SiNW growth, and each stage has its unique structure features. In stage 1(up to 30 min), SiNW is composed of Sn outside layer and Si core inside. The growth is mainly along the axial direction. In addition, the hexagonal structure of Si crystal was also observed, which has never been found in SiNWs. Kink and bending is very common in this stage, counting up to 1/3 respect. In stage 2(from a growth duration of 30-90 min), SiNWs are composed of crystalline Si core and polycrystalline Si shell and the growth of SiNW was both along axial and radial direction.The SiNWs become tapered. Kink also happens in this stage, as the percentage of kink NWs remains the same as in stage 1. In stage 3, the Sn liquid wetting layer is exhausted(growth time above 90 min), the axial growth of SiNWs begins to saturate and the subsequent plasma exposure leads to a-Si:H deposition on the SiNWs, transforming them into thicker pillars. SiNWs exhibit the core-1st shell-2nd shell structure, composed of crystalline Si core, polycrystalline Si shell and amorphous Si shell at stage 3. Bending is quite dominating, and the curvature of the bending SiNW was measured, confirmed that the the local curvature increases as close to the top. The curvature was made by the stress between the crystalline Si and the deposited a-Si.In situ annealing has been done for the SiNWs in stage 1 and stage 2. Melting behavior was observed below the melting point. For the 2 min growth SiNWs, the hexagonal Si core was melt by 700�C annealing. The test of its stability at high temperature was failed. For the 30 min SiNWs, atoms evaporation was observed with amorphous contrast, and dislocation and sub-grain boundaries happens in the edge with disorientation of atomic planes.In the process, growth mechanism was confirmed and revealed, which could help further investigation and optimization of the fabrication procedure.