High mobility thin film transistors by Nd:YVO4-laser crystallization

Abstract

Laser crystallization of amorphous silicon is one of the most interesting ways to obtain high-quality polycrystalline silicon films on glass. We crystallized the channel region of n- and p-type thin film transistors (TFTs) with a frequency-doubled Nd:YVO4 laser utilizing a sequential lateral solidification process. The high repetition rate of the laser of up to 100 kHz allows for high scanning speeds of up to 5 cm s−1. The laser irradiation was performed in air at room temperature. The resulting polycrystalline films showed longitudinally elongated grains with a length of up to 100 μm in the scanning direction of the laser beam and a width of up to 2 μm perpendicular to the scanning direction. Due to the anisotropic grain dimensions, the TFT performance depends on the orientation of the channel with respect to the scanning direction. Furthermore, a scale down of the TFT dimensions results in a better device performance because the number of grain boundaries within the channel of a TFT is reduced. For example, a decrease in the width W and length L of the channel from W=63 and L=22 μm to W=30 and L=15 μm increases the field-effect electron mobility μN of the TFTs from μN=410 to 510 cm2 V−1 s−1. The high mobility μ and low sub-threshold slope S=0.45 V decade−1 obtained with a gate oxide thickness of 100 nm show the high quality of laser-crystallized polycrystalline silicon.