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

Laser interference crystallization of amorphous silicon (a-Si) thin films, a technique that combines pulsed laser crystallization with holography, ena bles the fabrication of periodic arrays of polycrystalline silicon (poly-Si ) lines with lateral dimensions between 0.5 and 20 mu m. 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 mu m for a 300 nm thick film). We investigated the dynamics of the crystallization process by two-dimensional finite element computer simulat ions of the heat transport and phase transitions during laser crystallizati on. The theoretical results were compared to: (i) measurements of the cryst allization kinetics, determined by recording the transient changes of the r eflectance during laser exposure, and to (ii) the structural properties of the crystallized films, determined by scanning force and transmission elect ron microscopy. The simulations indicate that the crystallization front res ponsible for the large grains propagates laterally from the edges of the mo lten silicon lines to their centers with a velocity of similar to 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 pa rticipates in the solidification process. Using a transfer matrix approach, we converted the time-dependent phase and temperature distributions genera ted by the simulation program into values for the reflection and transmissi on 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 o f laser interference crystallization. (C) 1999 American Institute of Physic s. [S0021-8979(99)02808-X].