Crystallization of submicron amorphous hydrogenated silicon films with different hydrogen concentration by nanosecond ruby laser irradiation

摘要

Pulse laser annealing remains an actual problem aimed to crystallization of amorphous films on nonrefractive substrates. But, the most previous works studied laser crystallization of relatively thin (300 nm) a-Si:H films and mainly used excimer lasers. But, excimer lasers are not suitable for crystallization of submicron a-Si:H films due to low penetration depth in a-Si:H at such wavelengths. The problem can be resolved by using lasers with longer wavelengths. The desirable result of crystallization also depends on the choice of proper laser fluence, which is different for films with different hydrogen concentrations. In this work, the processes of a pulsed ruby laser induced crystallization of submicron (0.7 mu m) amorphous hydrogenated silicon films with different hydrogen concentrations (2, 12, and 39 at. %) by different laser fluences were investigated. The films were prepared on glass substrates by plasma enhanced chemical vapor deposition technique followed by isothermal annealing in nitrogen atmosphere. The laser annealing (lambda = 694 nm) was carried out at a pulse duration of 80 ns (full width at half-maximum) in the fluence range from 0.6 to 2.1 J/cm(2). The laser fluence thresholds for surface area crystallization were found for different hydrogen concentrations in the films. The increase of hydrogen concentration leads to an increase of the threshold energy density (laser fluence) for surface area crystallization due to a decrease of light absorption in the films with a higher hydrogen concentration. Also, it was shown that ruby laser radiation can penetrate and partially crystallize the full depth of the submicron a-Si:H film, but the problem of homogeneity remains.