Isotropic TFT Characteristics in the {100}-Oriented Grain-Boundary-Free Laser-Crystallized Si Thin Films

Abstract

Isotropic TFT characteristics are realized in the {100}-oriented grain-boundary-free 60 nm thick Si film obtained by the continuous-wave laser lateral crystallization, where the grain- and sub-boundaries are defined as the crystallographic boundaries having misfit angles of θ > 15° and θ < 15°, respectively. Sub-boundaries are observed in the film parallel to the scan directions; the misfit angles were 5–10° and the sub-boundary density was 0.02956 μm−1. Sub-grains, joined by the sub-boundaries, have widths of 8 ~ 69 μm. The cumulative distributions of mobility, threshold voltage, and subthreshold swing agree well between the parallel and perpendicular TFTs in the film, where parallel or perpendicular means the source-to-drain directions to the laser scan direction. The maximum mobilities of the parallel and perpendicular TFTs are 695 and 663 cm2/Vs, respectively. The trap-state density NT in the sub-boundaries estimated from the product of the bond efficiency η and the dangling bond density decreases by two decades from those of the grain boundaries. A new carrier transport model of the current flow across the sub-boundary is proposed instead of the thermionic emission model for the grain boundaries.