Chapter 8: General Conclusions and Future Prospects
8.1 General conclusions
8.1.1 CI effect of YSZ layer on quality of crystallized Si film
Quality of the solid-phase crystallized Si films on the glass substrates with and without a YSZ-CI layer by PLA was investigated. The obtained results are as follows:
For the Si/YSZ/glass, at a low irradiation energy density E, nucleation occurred faster at the YSZ interface than in the bulk of the a-Si film, which is considered thanks to the CI effect of the YSZ layer. This suggests that the nucleation sites can be controlled on the YSZ interface to make crystallization growth proceed to the film surface smoothly without random nucleation.
It was revealed that the crystallization rate to E was lower for the Si/YSZ/glass than for the Si/glass. This is because the optical absorption in the Si film for the Si/YSZ/glass is lower than that for the Si/glass.117
At the same crystallization degree, crystalline quality of the Si film on the YSZ layer was better than that on the glass substrate. It can be considered that, in the Si/YSZ/glass, Si atoms are arranged more orderly during the phase transition from amorphous to crystalline and the defect density is lower than in the Si/glass. This may be due to the CI effect of the YSZ layer.
The film tensile stress behavior was explained by the mass density change from the amorphous phase to the crystalline phase.
The crystallization degrees of both the structures Si/glass and Si/YSZ/glass increased with the pulse number (or annealing time) and tended to saturate even at a short annealing time. This saturation behavior was found to be a self-limiting process, by which the crystallization of Si films can be performed in a relatively stable manner even if E fluctuates within a certain range during irradiation.From the above results, it is expected that annealing with low E produces Si films with better crystallinity. However, since the crystallization rate is so low and the crystalline fraction is saturated at a short annealing time with self-limiting behavior, it should take a very long time or might be impossible to complete the crystallization. Annealing with high E can reduce the annealing time, but the crystallized Si film quality becomes poorer or more defective due to faster random nucleation and crystallization growth.
8.1.2 By using the two-step method, improving the Si film crystalline quality On the basis of the aforementioned results, we proposed the two-step method to further improve the crystallinity of the Si films, and crystallized a-Si films on YSZ-CI layers by the two-step method with PLA without intentional melting. Their properties were investigated and compared with those obtained by the one-step method. We obtained the following results:
The crystallization growth from the YSZ interface is more enhanced by the two-step method than by the one-two-step (or conventional) method. It was found that a higher crystallization degree, better crystalline quality, and a larger grain size were obtained by the two-step method, compared with the one-step method at the same total annealing time and lower total irradiation energy density.
Comparing the two structures of Si/YSZ/glass and Si/glass at their own optimized irradiation conditions, we obtained a better crystalline quality in the former.
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Since the crystalline quality of Si films is much improved by the two-step method, we also proposed a three-step method for further improving crystalline quality of the Si film. Although crystalline fraction of the Si films obtained by the three-step method is a little higher than that of the two-step method, the film quality of the former seems to be inferior in comparison with the latter. Also, considering mass production, the three-step method is not suitable. Therefore, we decided to use the two-step method for next investigations.
By applying the two-step method, we succeeded in crystallization of a-Si films on YSZ-CI layers under the crystallization condition for area expansion. Their film crystalline quality was investigated and compared with those of the one-step method. It was revealed that the crystallinity of the Si films is improved by the two-step method in the case of the sample moving during the irradiation as well as the static case. The (111)-preferential orientation of the YSZ was found to be transferred into the crystallized Si film, thanks to the CI effect of the YSZ. The smooth surface of the crystallized Si film and no incubation layer at the interface was obtained. The diffusion of Zr and Y from the YSZ layer into the crystallized Si film was found to be as small as or lower than the order of 1017 atoms/cm3. However, these impurities should be further suppressed for device application.
We also measured the carrier concentration, Hall mobility, and conductivity of the crystallized Si films by using the resistivity and AC Hall effect measurements. The results for both the undoped and doped Si films revealed that the higher mobility and carrier concentration (therefore higher conductivities) are obtained for the Si/YSZ/glass structure compared with those of the Si/glass.
8.1.3 Effect of metal films on enhancing crystalline quality of Si films
On the basis of the results obtained for the Si/glass and Si/YSZ/glass, we investigated crystalline quality of the Si/YSZ/metal/glass structure crystallized by both the one-step and two-step methods. It was found that higher crystallization degree and better crystalline quality were obtained by the two-step method compared with the one-step method at the same annealing time and lower total irradiation energy density. This indicates that the two-step method is also effective in improving crystalline quality of Si films on YSZ/metal/glass. Moreover, the metal layer has a small heating effect on enhancing crystalline quality of the Si film. Irradiation condition was optimized for bottom-gate
poly-119 Si TFTs fabrication. However, considering the occurrence of bubbles or holes on surface of the crystallized Si film, the Si/YSZ/metal/glass structure is not suitable for bottom-gate TFT fabrication.
8.1.4 Performance of the fabricated top-gate TFTs
The investigation of film crystalline quality reveals that Si films crystallized on the YSZ layers by the two-step method are more suitable for application in electronic devices.
Moreover, the Si/YSZ/metal/glass structure is not suitable for bottom-gate TFT fabrication. Therefore, we proceeded the fabrication of top-gate TFTs on two structures of Si/glass and Si/YSZ/glass, in which Si films were crystallized by the two-step method with pulsed laser. The device parameters of field-effect mobility µeff, threshold voltage Vth, subthreshold voltage S.S, and ON/OFF current ratio of the fabricated TFTs were estimated as well as their uniformity. It was found that the TFTs fabricated on the Si/YSZ/glass exhibit a relatively better performance and superior device-to-device uniformity than those on the Si/glass. This result is considered owing to the better crystalline quality of the Si film on the YSZ/glass and uniform distribution of grains as well as crystalline defects, which indicating effectiveness of the crystallization-induction effect of the YSZ layer.
However, a hysteresis phenomenon was observed in the TFTs characteristics. Although the hysteresis mechanism is not well known at present, it might be related to the presence of mobile ions in the gate oxide.