**2.5. Si–H bonds in ELC poly‐Si observed via local vibrational modes**

at the gate. The mobility of plain poly‐Si is expressed by the value µ0 obtained under thermal equilibrium by Hall effect. In both cases, the mobility is limited by the trapping states at the

The poly‐Si layer for TFT was almost depleted because of the small thickness and high purity. Therefore, phosphorous (P) ions were doped into poly‐Si followed by annealing at 600°C for 2 min in N2 for activation. The averaged density of P obtained by SIMS was 3 × 1018 cm‐3. The obtained µ0 values are treated as relative values because of the uncertainty about the effects of the charged state on the free surface and at the Si/under‐layer interface in addition to the uncertain barrier height at grain boundaries depending on the impurity density. The variation in µ0 with the hydrogenation time is shown in **Figure 5**, where the hydrogenation is performed by using plasma or catalyzer. The plasma hydrogenation enhanced the value of µ0 in a short time. A larger value of µ0 was obtained by catalytic hydrogenation, whereas longer time was required to reach the maximum, which was because no damage was caused by the charged particles. In both cases, excess hydrogenation decreased µ0. The increase in defect density over

**Figure 5.** Variation in Hall effect mobility with hydrogenation times for ELC poly‐Si hydrogenated in PE‐CVD and cat‐

alytic reactors, where the broken line indicates the value at the stage of non‐hydrogenation.

a long hydrogenation time was also reported for CVD poly‐Si [26].

88 New Advances in Hydrogenation Processes - Fundamentals and Applications

grain boundaries [24, 25].

The bonding states of H in Si were investigated via local vibrational mode (LVM) by using infrared absorption, attenuated total reflection and Raman scattering. Many literatures indi‐ cate the appearance of the LVM at 2000 and/or 2100 cm‐1 for various crystalline phases, which were summarized in [18] and [27]. The difference in frequency was attributed to the orders of hydride, that is Si–H*<sup>x</sup>* (*x* is a positive integer), and structures around the hydride [28]. The 2000 cm‐1 and the 2100 cm‐1 bands are related to the isolated SiH*x* (principally Si–H) and clustered Si–H*x* group (principally Si–H2), respectively.

Raman microscopy with backscattering geometry is useful for Si thin films on glass sub‐ strates [27]. The intensity of the LVM is 10‐3 of that of the optical phonon mode (OPM). However, it is known that Raman scattering is enhanced by roughening the surface or fabri‐ cating nanostructures, which is analyzed by using the electromagnetic model [29, 30]. High‐ density hillocks were generated on the surface of ELC poly‐Si after crystallization, which was determined by the interference between the multiple‐shot laser and the surface rough‐ ness [31]. The 20 times of enhancement was exhibited for ELC poly‐Si leading to a remarka‐ ble improvement in sensitivity for LVMs [32].

In general, the defects in Si exhibited no characteristic band in the Raman scattering spectra. However, the dangling bonds at the defects were easily terminated by H. The coupling of the hydrogenation and observation of Si–H*x* LVMs are expected to be useful for investigating defects in Si. The LVM spectra obtained for various H‐containing Si thin films are summar‐ ized in **Figure 6**. LVMs at 2000 and 2100 cm‐1 were observed for all the films. The device grade a‐Si:H clearly exhibits a 2000 cm‐1 band with Gaussian shape as shown in **Figure 6(a)**. Thus, the 2000 cm‐1 band is attributed to the hydride characterizing the amorphous phase. **Figure 6(b)** shows the LVMs observed for the Si thin film deposited by catalytic CVD on glass. At least two additional LVMs are found at 2170 and 2260 cm‐1. The OPM spectrum indicates that the film consists of amorphous, nanocrystal, and crystal components; therefore, it is deduced that the difference in the frequency of LVM bands corresponds to the different lattice structures around the hydrides.

The LVMs for ELC poly‐Si are summarized in **Figure 6(c**–**f)**. **Figure 6(c)** and **(d)** shows the LVMs for catalytic hydrogenated ELC poly‐Si. The dominance of 2000 or 2100 cm‐1 varies with crystallization and hydrogenation conditions. It is notable that the 2100 cm‐1 band exhibits Lorentzian shape in some cases, which suggests that the spectral shape reflects the structural order around the hydride. The LVM bands obtained after intensive plasma hydrogenation are shown in **Figure 6(e)**. At least four additional bands are found at 2030, 2130, 2140, and 2200 cm‐1. The best fit was obtained when three of these bands were set to Lorentzian. These Lorentzian bands can be related to the hydrides in ordered structures such as platelets. **Figure 6(f)** shows the spectrum for oxygen plasma‐irradiated ELC poly‐Si followed by catalytic hydrogenation. Multiple fine bands were observed there, in which meaningful fitting is not easy. Such fine LVMs were also found for H ion‐implanted Si [33]. The correlation of frequen‐ cies of multiple LVMs between the experimental and theoretical values was demonstrated for H‐passivated defects arising in ion implantation and particle radiation [34]. Thus, the fine bands observed in **Figure 6(e)** and **(f)** are attributed to the damage caused by the charged particles in plasma.

**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 2100 cm‐1.

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 roughening of surface.

**Figure 7.** Variation in LVM intensity with Secco etching time, where the intensities of ∼2000 and ∼2100 cm‐1 bands are normalized with the values at the surface.

The relationship between the intensity of the two dominant LVMs and hydrogenation time indicates that the 2000 cm‐1 band intensity saturates with time, whereas the 2100 cm‐1 band intensity monotonically increases. This implies that hydrogenation generates defects in the grains corresponding to the 2100 cm‐1 band. A model of breaking weak bonds by H was proposed for a‐Si:H for solar cells [37].
