Dynamics of lateral grain growth during the laser interference crystallization of a-Si

Abstract

Laser interference crystallization of amorphous silicon (a-Si) thin films, a technique that combines pulsed laser crystallization with holography, enables the fabrication of periodic arrays of polycrystalline silicon (poly-Si) lines with lateral dimensions between 0.5 and 20 μm. The lines consist of grains with well-defined grain boundary locations and lateral dimensions that are appreciably larger than the thickness of the initial a-Si:H film (up to 2 μm for a 300 nm thick film). We investigated the dynamics of the crystallization process by two-dimensional finite element computer simulations of the heat transport and phase transitions during laser crystallization. The theoretical results were compared to: (i) measurements of the crystallization kinetics, determined by recording the transient changes of the reflectance during laser exposure, and to (ii) the structural properties of the crystallized films, determined by scanning force and transmission electron microscopy. The simulations indicate that the crystallization front responsible for the large grains propagates laterally from the edges of the molten silicon lines to their centers with a velocity of ∼14 m/s. A substantial lateral growth only occurs for laser intensities large enough to melt the a-Si film around the center of the lines down to the substrate. Vertical crystallization, which is substantially slower (0.5 m/s), also participates in the solidification process. Using a transfer matrix approach, we converted the time-dependent phase and temperature distributions generated by the simulation program into values for the reflection and transmission of the film as a function of time during and after the laser exposure. A good agreement between the simulated and measured transient reflection was obtained both in the case of homogeneous crystallization as well as that of laser interference crystallization.