Pulsed Laser Crystallization of a-Si:H on Glass: A Comparative Study of 1064 and 532 nm Excitation

Abstract

Amorphous silicon films of a few hundred nm thickness on glass substrates have been crystallized by pulsed Nd‐YAG laser excitation of 6 to 8 ns duration at wavelengths of λ = 532 nm and λ = 1064 nm. The structural properties of the crystallized films obtained from Raman spectra, X‐ray diffraction data and transmission electron microscopy micrographs reveal crystallites of good structural quality and several hundred nanometers in size. The best quality and size of the crystallites is obtained by complete melting of the film. For λ = 532 nm excitation no sharp threshold for crystallization is obtained and incomplete melting of the films at low energy density leads to small crystallites and large strain in the films. For λ = 1064 nm pulses the threshold for crystallization is very sharp due to nonlinear absorption effects and the energy density coupled into the film is very sensitive to the film thickness because of interference effects. When films, crystallized at the two wavelengths under optimized conditions are compared, the gross structural properties and crystallite sizes differ only slightly but films crystallized with λ = 532 nm show much smoother surfaces and a larger tensile strain.

Monitoring explosive crystallization phenomenon of amorphous silicon thin films during short pulse duration XeF excimer laser annealing using real-time optical diagnostic measurements

“…8(b).The SLG-crystallized area have RMS roughness of approximately 8.28 nm. This result indicates that a ridge is observed at this region because 10% density change between the solid (2.30 g/cm 3 ) and the liquid (2.53 g/cm 3 ) phases of Si films provides a driving force for the formation of capillary wave [31][32][33]. The SLG regime requires tight control of the melting process because of the narrow processing window.…”

Crystallization of amorphous‐Si using nanosecond laser interference method

“…Considering that the current wavelength of 1064 nm corresponds to an extremely weak absorption rate in an a‐Si or crystalline Si, the direct absorption of the laser beam is possibly limited . Instead, the laser energy is absorbed by the nonlinear effect because of the high laser energy density of 2.2 J/cm 2 …”

“…As can be seen in Figure A, the intensity of the crystalline Si peak gradually increases as it moves away from the middle position and finally forms two maximum peaks near the positions of the highest laser intensity. This result is different from the previous results where two grains from both directions meet to form a single ripple shaped by lateral solidification . Owing to the sinusoidal laser interference, the thermal gradient to the lateral direction becomes significant.…”

“…However, it is known that it is difficult to use a 1064‐nm wavelength laser for processing a‐Si thin films because of extremely low absorption coefficients of a‐Si and crystalline Si . Carius et al conducted a laser crystallization experiment using a laser with a wavelength of 1064 nm on an a‐Si thin film with a thickness of approximately 500 nm and suggested that crystallization could be achieved by the nonlinear absorption of the laser . Actually, it was reported that the absorption coefficient of Si could increase at high laser intensity, assisted with two‐photon absorption process …”

Laser micro-machining and applications of glasses in optoelectronics

“…The glass was sliced in 2mm thick and polished finely to be irradiated with Nd:YAG laser [3], [4]. The glass specimen were irradiated with 355nm and 1064nm wavelength.…”