**4. Summary**

In this chapter, the behavior of H atoms in poly‐Si was investigated in detail in order to evaluate and improve the quality of hydrogenated poly‐Si thin films. In the case of ELC, the plasma hydrogenation introduced as much as 1 at.% H into the film. Hydrogenation drasti‐ cally improves the Hall effect mobility, whereas excessive hydrogenation tends to degrade it. The catalytic method is useful to inhibit the excessive hydrogenation and damage suf‐ fered by the electric‐field acceleration of charged particle. The H‐termination of the dangling bonds at the grain boundaries can be observed indirectly or directly by chemical etching and Raman microscopy. Although preferential etching was found at the grain boundaries, hydrogenation interfered with it because of the electrochemical inactivation of the dangling bonds. This H‐termination appeared as the 2000 cm‐1 LVM in the Raman spectra. The break‐ ing of the Si–Si bonds by hydrogenation was detected as the 2100 cm‐1 LVM. In addition, the defects generated in the plasma process exhibit multiple fine LVMs after hydrogenation. These extremely weak LVMs were detected due to the enhancement of Raman scattering induced by the high‐density hillocks that are characteristic of ELC poly‐Si. In addition, we investigated the hydrogenation of LT poly‐Si TFTs from the perspective of the gettering phe‐ nomenon. The most important parameter for effective hydrogenation using forming gas an‐ nealing is the rate of cooling from 400°C, but the hydrogenation temperature is not important.
