**2.6. Hydrogen and defects in CLC poly‐Si**

bands observed in **Figure 6(e)** and **(f)** are attributed to the damage caused by the charged

90 New Advances in Hydrogenation Processes - Fundamentals and Applications

**Figure 6.** LVMs observed for various Si films: (a) a‐Si:H, (b) catalytic CVD Si, and (c–f) hydrogenated ELC poly‐Si. ELC poly‐Si films were treated as follows: (c) and (d) 3 h catalytic hydrogenation, (e) 10 min plasma hydrogenation, and (f) 1 h catalytic hydrogenation after oxygen plasma exposure, where the broken lines indicate the positions of 2000 and

The location of Si–H*x* bonds corresponding to the individual bands, that is, grain boundary or ingrain, was examined. The variation in LVM intensity with the etching time is shown in **Figure 7**, where hydrogenation is performed after the etching of individual time. The inten‐ sities are normalized by that at non‐etching time. The 2000 cm‐1 band disappeared in the early stage, whereas the 2100 cm‐1 band was continuously detected until the diminishing of the Si layer. Molecular dynamics simulation predicted that a few atomic layers of amor‐ phous components reside at the grain boundary in covalent materials [35, 36]. Thus, the 2000 cm‐1 band was attributed to the hydride in amorphous‐like structure at the grain boun‐ dary. Furthermore, the 2100 cm‐1 band is attributed to the hydride mainly located at the in‐ grains. The enhancement of the 2100 cm‐1 band in the long etching time region is due to the

particles in plasma.

2100 cm‐1.

roughening of surface.

CLC poly‐Si is essentially different from ELC poly‐Si in terms of the directionality and solidification velocity during recrystallization. Almost no LVMs were detected for CLC poly‐ Si even after hydrogenation, which is due to low density of H‐terminated defects and/or relatively smooth surface leading to the lack of enhancement of Raman scattering.

The H‐termination state in the CLC poly‐Si was examined by the chemical etching in the same manner as the case of ELC poly‐Si. The precursor was 150‐nm‐thick a‐Si. At non‐hydrogenation stage, high‐energy grain boundaries were revealed as flow‐shaped lines that extend along the laser scanning direction. The growth proceeds by repeating the generation in grains and coalescence with each other as shown by the SEM image in **Figure 8(a)**. Almost no grain boundaries were revealed in the hydrogenated film by the etching as shown in **Figure 8(b)**. This implies that the grain boundaries were inactivated by the hydrogenation as well as the case of the ELC.

#### **2.7. Summary on the hydrogenation of poly‐Si thin films**

Hydrogenation has two opposite influences on the performance of poly‐Si: improvement caused by the passivation of dangling bonds at the defects and degradation caused by the breaking of the Si–Si bond. Here, the hydrogenation of plane poly‐Si thin films on glass with H\* generated by plasma or catalyzer was described.

**Figure 8.** SEM images of CLC poly‐Si after chemical etching for 25 s (a) as‐crystallized and (b) hydrogenated films.

The plasma hydrogenation process introduced as much as 1 at.% H into the ELC film. Although hydrogenation drastically improves the Hall effect mobility, excessive hydrogenation tends to degrade it. The catalytic method is useful for preventing excessive hydrogenation and damage caused by the electric‐field acceleration of charged particle.

The H‐termination of 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 the process because of the electro‐ chemical inactivation of dangling bond. This H‐termination appeared as 2000 cm‐1 LVM in the Raman spectra. The breaking of Si–Si bonds by hydrogenation was determined from the appearance of the 2100 cm‐1 LVM. In addition, the defects generated in the plasma process exhibited multiple fine LVMs after hydrogenation. The detection of extremely weak LVMs was caused by the enhancement of Raman scattering induced by the high‐density hillocks that are the characteristics of ELC poly‐Si.

In the case of CLC, almost no LVM was detected because of the low defect density and relatively smooth surface. Although flow‐shaped grain boundaries were revealed by the chemical etching, they were protected from the etching by hydrogenation as well as ELC poly‐Si. The density of defects residing in the grains was estimated to be considerably smaller than that observed for ELC, which was due to the unidirectional solidification and low cooling velocity during recrystallization.
