Laser processing of thin-film silicon is a promising approach for the realization of polycrystalline silicon for large area electronics and solar cell applications. Here we investigate the modification of a-Si:H with different hydrogen content (30, 13 and <1 at. %) by femtosecond (fs) laser materials processing. The different hydrogen content of the intrinsic a-Si:H layers with thicknesses of either 50 nm or 300 nm was achieved by varying the temperature during PECVD growth (25°C, 200°C and 520°C). Single 30 fs light pulses of an amplified Titanium Sapphire laser (1 kHz repetition rate, 790 nm centre wavelength) are attenuated and focussed to an e-2-spot width of 420 µm leading to peak fluences between 30 mJ cm-2 and 120 mJ cm-2. The modified spots were characterized by optical microscopy, imaging ellipsometry at 658 nm, Raman micro-spectroscopy and scanning electron microscopy (SEM). The intensity profile of the laser beam in combination with microscopy allows analyzing the fluence dependence of the material modification across individual spots. Qualitative depth information of the material modification is obtained from Raman micro-spectroscopy using different excitation wavelengths (473 nm and 633 nm).
The general findings are the following:
Despite the low absorption coefficient of a-Si:H at 790 nm a high local energy deposition close to the surface of the a-Si layer is achieved via nonlinear absorption. Consequently, a distinct material modification (hydrogen effusion, recrystallization, ablation, etc.) in a thin layer near the surface can be achieved enabling the realization of electronic circuits for large area electronics and modification of contact and intermediate layers for solar cell application. In addition, pulse properties (duration, energy, shape, etc.) allow to control the laser modification depth profile.
The material ablation threshold depends strongly on the hydrogen content. For low hydrogen (< 1%) content a 100% higher fluence has to be applied before material ablation sets in. Surprisingly, below ablation threshold irradiation of this sample even increases the Si-H related Raman signal (2000-2100 cm 1 Stokes shift) by more than a factor of two in comparison to an untreated area.
A detailed analysis of thin-film silicon modified by fs-laser pulses will be presented.