Laser Nucleation and Solid Phase Crystallization of a-Si:H

摘要

The ability to grow large-area, large-grained polycrystalline silicon on inexpensive substrates is becoming increasingly important for photovoltaic (PV) devices. With large-grained (grain size >10 μm) 10 μm thick films it is possible with light trapping to achieve PV efficiencies exceeding 15%. If crystallites could be nucleated and grown for longer times before native nucleation occurs, then potentially these much larger grain, thin film silicon material could be produced. The interaction of sub-crystallization threshold laser fluence with hydrogenated amorphous silicon (a-Si:H) has been demonstrated on a macroscopic scale to shorten the incubation time in subsequently thermally annealed films. Further examination of crystallite laser nucleation, found that nucleation was suppressed around PECVD a-Si:H thin film(50-100nm) sample edges, and scratches, in addition to laser-ablated areas, extending as much as 100-200 μm laterally from these features. Optical microscopy and stepwise high temperature thermal annealing were used to investigate this behavior for the a-Si:H films deposited on glass substrates. The nucleation rates were measured in the treated and untreated regions. The data suggests that these features (edges, scratches, and laser ablated areas) provide stress relief by interrupting the surface connectivity. We confirm the existence of stress and stress relief by μ-Raman measurements of the crystallite transverse optical peak position relative to that of c-Si. PECVD films were annealed at temperatures between 540-600C, to enable a determination of r_n at each anneal temperature. The temperature dependent measurements enabled the determination of the nucleation rate activation energies (E_A), and how they are affected by film stress.