Laser Beam Crystallization

Laser beam annealing of a-MLs under cw or pulse exposure has also been applied as an alternative of thermal crystallization, in order to produce nanocrystalline films [100, 103, 106, 109, 110, 121, 126-130]. In a series of studies on crystallization with an excimer laser of a-Si:H/a-SiNx :H MLs [126-130], ability for fabrication of electroluminescence cells has been shown having visible light emission. This approach provides several advantages such as selected area processing, rapid crystallization, and avoidance of the surface contamination. It is also possible to crystallize a thin film while keeping the substrate temperature low enough to prevent its thermal damage.

The primary interaction between laser light and semiconductors is based on excitation of electrons and phonons [131]. The dominating interaction mechanisms depend on the laser parameters and the particular type of semiconductor, its microstructure (amorphous/crystalline), doping, etc. The interaction mechanisms may be thermal or non-thermal depending on the relative size of the thermal relaxation time and the time for structural rearrangements of material atoms or molecules. In most cases of semiconductor processing, the excitation energy is rapidly dissipated into heat (thermal laser processing previously mentioned). In spite of the thermal character, laser-driven processes may be quite different from those initiated by a conventional heat source. The laser-induced temperature rise can be localized in space and time. Temperature changes of more than 104 K can be induced in a small volume at a heat rating of 1015 K/s. For example, heat flow calculations have been performed for a-Si:H thin film for the case of single-shot laser annealing with an excimer laser [132]. It has been shown that the laser irradiation is able to melt and crystallize the film but even the melting process could not be described by pure heat flow calculation. It is interesting to note that the results of crystallization of a-Si:H thin films with a cw Ar+ laser and a pilsed Nd:YAG laser have indicated [133] that the mechanisms of crystallization are different and, in particular, the pulsed crystallization is a nonthermal process. Another example of the complex interaction of the light with matter is laser-induced crystallization of a-Se. Polarization-dependent pho-tocrystallization has been observed and though it seems to be optical in origin [134], an alternative, essentially thermal, mechanism has also been discussed [135].

Both thermal and pure photo-induced crystallization has been observed in a-MLs. Compositional and thickness dependent changes of the crystallization process in amorphous Se/Se100-xTex multilayers (x = 7.5, 15, and 30) have also been studied under light treatments [109, 110]. The treatments have been performed using the 647.1 and 530.1 nm Kr+ laser lines. The thermal effect of the laser exposure has been investigated at room temperature in Se/ Se70Te30 a-ML, using the red laser line (which is strongly absorbed by the Se70Te30 sublayers but weakly so in the Se ones), as well as on Se/Se100-xTex a-MLs using the green laser line [110]. In these experiments, it has been established that the threshold power for crystallization (and, hence, the crystallization temperature) of Se70Te30 sublayers increases with decreasing their sublayer thickness. The result is in accordance with those reported previously for a-Se [100, 106].

"Pure" photo-induced structural changes in Se/Se100-xTex a-MLs have been explored at a low temperature (38 K) at which thermally induced photo crystallization is not anticipated. It has been found [110] that exposure to the green laser line lead to an increase of the strength of photo-induced crystallization in Se/Se100-xTex a-MLs with increasing Te content, which is in agreement with previous reports on three-dimensional Se100-xTex alloys [136-138]. This result implied that there is no considerable change in Se-Se and Se-Te bond energies when a dimensionality reduction (from 3 to 2) takes place in the alloys. Furthermore, the thickness dependence of the strength of photo-induced crystallization was studied in Se/Se70Te30 a-MLs for which the strongest effect has been observed. A series of Raman scattering spectra measured with the laser beam focused on the same spot of each sample are shown in Figure 4. One can see that the intensity of the 237 cm-1 band, which corresponds to scattering from trygonal Se, and hence, the relative part of the crystalline phase in both Se70Te30 and Se layers increases when the time of preliminary exposure increases. A reduction of overall Raman intensity has also been observed. The latter has been ascribed to a deterioration of the ML structure and/or the surface quality of the sample due to crystallization. A conclusion has been derived [109, 110] that, as in the case of thermally induced crystallization, the rate of photo crystallization in Se70Te30decreases with decreasing sublayer thickness but the cause of this dependence in the case of photo crystallization is different from that responsible for thermal crystallization. The last suggestion is based on the mechanism of photo-induced crystallization developed for aSe [134]. According to this model, the photo crystallization strength in a-Se is controlled by the production of electron hole pairs and not by the density of absorbed energy. This implies that structural disorder may play a significant role in the process of photo crystallization. Thus, the observed lower rate of crystallization in Se/Se70Te30 a-ML having thinner sublayers has been attributed [110] to a decrease in the lifetime and, hence, the concentration of photo-excited free carriers due to a disorder level increase. Moreover, in thinner sublayers, photo crystallization starts at a higher level of structural disorder and, hence, at the same light exposure, a longer time is necessary in order to achieve the same crystallization effect.

Laser annealing along with both previously described thermal post-treatment methods (furnace annealing and rapid thermal annealing) were applied for crystallization of a series of a-Si:H/a-SiNx:H MLs [139]. It has been shown that when the thickness of a-Si:H layers is larger than 4.0 nm, the constrained a-Si:H layers were well crystallized by each of three methods. However, when the thickness of a-Si:H layers is smaller than 4.0 nm, the laser annealing method is the most advantageous for the formation of the nuclei and

150 200 250 300 Raman shift (cm1)

Figure 4. Raman scattering spectra of Se(4 nm)/Se70Te30 MLs having Se70Te30 layer thickness of (a) 3.5 nm and (b) 7.0 nm. The spectra were measured on a constant spot for each sample after a preliminary laser beam exposure (647.1 nm, 55 W/cm2) for various times indicated in the figure.

150 200 250 300 Raman shift (cm1)

Figure 4. Raman scattering spectra of Se(4 nm)/Se70Te30 MLs having Se70Te30 layer thickness of (a) 3.5 nm and (b) 7.0 nm. The spectra were measured on a constant spot for each sample after a preliminary laser beam exposure (647.1 nm, 55 W/cm2) for various times indicated in the figure.

crystallization of the samples compared to the other methods. Using laser annealing, ultra-thin a-Si:H layers can be well crystallized even when their thickness decreases down to 2.0 nm. In contrast, the nucleation become more difficult upon furnace annealing, so that few Si NCs can be formed by this method.

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