On the role of laser heating and adatom diffusion in femtosecond laser induced tungsten nanogratings

摘要

We have previously reported the observation of self-organized tungsten nanogratings during chemical vapor deposition of tungsten induced by a 400-nm 80-MHz laser oscillator on a wide range of substrates. We show that the growth of nanostructures begins with a thin tungsten film, followed by a rapid formation of periodic texture, when the laser power exceeds a threshold value. The threshold power is found strongly substrate dependent. The ubiquitous presence of thin films prior to nanograting growth suggests adatom diffusion induced by laser heating is vital, as the strong electron-phonon coupling in tungsten is expected to turn absorbed photon energy rapidly into heat. Using a simplified 1D heat diffusion model, we estimate the critical surface temperature on various substrates at the onset of nanograting formation, based on substrate-specific threshold power and material properties. We found interesting correlation of critical temperatures: all the covalent substrates (A1N, A1_2O_3, quartz, silica, and glass) exhibit a common critical temperature while the ionic substrates (MgO, MgF_2, and CaF_2) share another yet different critical temperature. The critical temperature of covalent substrates is found higher than that of ionic substrates, indicating the former possesses larger activation energy for adatom diffusion. Based on this model, we can also extract a substrate-independent enthalpy for nanograting formation. Although the present ID model overestimates the surface temperature, the correlation of critical temperatures among substrates and the presence of a unique enthalpy independent of substrates strongly support the role of laser heating and adatom diffusion in the formation of tungsten nanogratings.