**2.1. Crystallization of poly‐Si thin films**

The fabrication of LT poly‐Si was enabled by development of laser crystallization technology, where amorphous Si (a‐Si) thin films on low alkali glass substrate were used as precursor. The details of the laser crystallization are provided in [8]. Here, we used quartz glass as a substrate, which was available for high‐temperature heat treatment to investigate the defects in poly‐Si. The thicknesses of the a‐Si layers were 50–150 nm. Two techniques were employed for laser crystallization, excimer laser crystallization (ELC) [9] and continuous wave laser lateral crystallization (CLC) [10].

The poly‐Si fabricated by ELC (ELC poly‐Si) is already used in the industry, and the process is performed primarily by using KrF or XeCl excimer lasers, which supply intense pulsed light with durations of approximately 30 ns. A crystalline orientation map of ELC poly‐Si is shown in **Figure 1(a)**, which is obtained by electron backscattering diffraction in terms of the surface normal direction. Grain growth with an average grain size of approximately 300 nm was observed. While the surface orientations of the individual grains were scattered over a wide range, they exhibited a maximum value at {001}. Most of the grain boundaries were high‐ energy random boundaries. The field‐effect mobility µFE obtained for n‐channel (n‐ch) TFT increased with increase in grain size and reached 320 cm2 /Vs for an average grain size of 700 nm, where the dominant factor varied from grain boundary scattering to lattice scattering [11].

The poly‐Si fabricated by CLC (CLC poly‐Si) was developed by one of the authors [10]. This technique uses diode‐pumped solid‐state laser with a wavelength of 532 nm as the heat source. CLC poly‐Si exhibits flow‐shape growth as shown in **Figure 1(b)** by adjusting the scanning velocity and output power of the laser. Aligning the TFT channel in parallel with the flow effectively prevents grain boundary scattering. This alignment led to a µFE of 566 cm2 /Vs which is comparable with the µFE of 670 cm2 /Vs obtained for a MOSFET made from a single‐crystalline Si‐channel layer separated by ion‐implanted oxygen [11].

#### **2.2. Hydrogenation techniques of poly‐Si thin films**

Semiconductor devices were improved by an annealing process in a forming gas (mixture of inert gas and H2) and a plasma hydrogenation technique [1, 12, 13]. These effects were understood as the passivation of defects with hydrogen atoms (H). However, in the case of the forming gas, something catalytic is believed to have acted in the device structure because the dissociation of H2 is extremely small at the device process temperature. For instance, the efficiency of a multicrystalline sheet Si solar cell reportedly improved by the combination of aluminum evaporation on the back face and the forming gas annealing [14]. In this section, we will deal with plain poly‐Si thin films on glass substrates and hydrogen radical (H\*) inten‐ tionally generated by using plasma or hot catalyzer, which is useful for verifying the hydro‐ genation effects under simplified conditions.

such as hydrogen plasma [1–3], covering with a hydrogen‐containing film [4], and hydrogen ion implantation [5–7] were evaluated. However, the behavior of hydrogen atoms and their effects on the electronic and electrochemical properties of poly‐Si films are not clear yet. In this chapter, the behavior of hydrogen atoms in poly‐Si film is investigated in detail. In addition, we investigated the hydrogenation of LT poly‐Si TFTs from the perspective of the gettering

The fabrication of LT poly‐Si was enabled by development of laser crystallization technology, where amorphous Si (a‐Si) thin films on low alkali glass substrate were used as precursor. The details of the laser crystallization are provided in [8]. Here, we used quartz glass as a substrate, which was available for high‐temperature heat treatment to investigate the defects in poly‐Si. The thicknesses of the a‐Si layers were 50–150 nm. Two techniques were employed for laser crystallization, excimer laser crystallization (ELC) [9] and continuous wave laser lateral

The poly‐Si fabricated by ELC (ELC poly‐Si) is already used in the industry, and the process is performed primarily by using KrF or XeCl excimer lasers, which supply intense pulsed light with durations of approximately 30 ns. A crystalline orientation map of ELC poly‐Si is shown in **Figure 1(a)**, which is obtained by electron backscattering diffraction in terms of the surface normal direction. Grain growth with an average grain size of approximately 300 nm was observed. While the surface orientations of the individual grains were scattered over a wide range, they exhibited a maximum value at {001}. Most of the grain boundaries were high‐ energy random boundaries. The field‐effect mobility µFE obtained for n‐channel (n‐ch) TFT

nm, where the dominant factor varied from grain boundary scattering to lattice scattering [11].

The poly‐Si fabricated by CLC (CLC poly‐Si) was developed by one of the authors [10]. This technique uses diode‐pumped solid‐state laser with a wavelength of 532 nm as the heat source. CLC poly‐Si exhibits flow‐shape growth as shown in **Figure 1(b)** by adjusting the scanning velocity and output power of the laser. Aligning the TFT channel in parallel with the flow

Semiconductor devices were improved by an annealing process in a forming gas (mixture of inert gas and H2) and a plasma hydrogenation technique [1, 12, 13]. These effects were understood as the passivation of defects with hydrogen atoms (H). However, in the case of the forming gas, something catalytic is believed to have acted in the device structure because the

effectively prevents grain boundary scattering. This alignment led to a µFE of 566 cm2

/Vs for an average grain size of 700

/Vs obtained for a MOSFET made from a single‐crystalline

/Vs which

phenomenon.

**2. Hydrogenation of poly‐Si thin films**

84 New Advances in Hydrogenation Processes - Fundamentals and Applications

increased with increase in grain size and reached 320 cm2

Si‐channel layer separated by ion‐implanted oxygen [11].

**2.2. Hydrogenation techniques of poly‐Si thin films**

is comparable with the µFE of 670 cm2

**2.1. Crystallization of poly‐Si thin films**

crystallization (CLC) [10].

**Figure 1.** Crystalline orientation maps in terms of the surface normal direction for (a) ELC poly‐Si and (b) CLC poly‐Si observed by electron beam backscattering diffraction.

Three types of hydrogenation setups are shown in **Figure 2**. **Figure 2(a)** shows a reactor with parallel plate electrodes supplying a radio frequency of 13.56 MHz which is widely used for plasma‐enhanced chemical vapor deposition (PE‐CVD). Although the degree of perfection of the setup is high, electric‐field‐accelerated charged particles can cause damage to the semi‐ conductors. Plasma hydrogenation was performed in a PE‐CVD reactor with a power of 30 W at a substrate temperature of 350°C for 1–25 min. **Figure 2(b)** shows a simple remote plasma reactor. This setup was convenient to maintain an effective supply of H\* under no electric‐field acceleration in which H\* was generated in a cavity supplying 2.45 GHz microwave. **Figure 2(c)** shows a reactor using an electrically heated tungsten (W) wire as the catalyzer for the disso‐ ciation of H2. This setup was convenient for hydrogenation under no electric field. The principle was known for a long time, and catalytic CVD reactor was developed for industry [15, 16]. The W filament was mesh or coil shaped and was heated to approximately 1300°C under an H2 pressure of 0.1–0.7 Torr [17, 18].

**Figure 2.** Setups for hydrogenation (a) PE‐CVD, (b) remote plasma, and (c) catalytic reactors.

#### **2.3. Hydrogen and grain boundaries in ELC poly‐Si**

The depth profile of H concentration in ELC poly‐Si obtained by secondary ion mass spectro‐ scopy (SIMS) is shown in **Figure 3**, where hydrogenation is performed in the PE‐CVD reactor. The enhancement in the concentration near the surface and the Si/SiO2 interface is apparent values. While the 1‐min treatment produced a profile of H diffusing from the surface to the interface, the 10‐min treatment led to an almost saturated profile, in which the average density reached was 5 × 1020 cm‐3 that is 1 atomic percent (at.%).

**Figure 3.** Depth profiles of H concentration in ELC poly‐Si hydrogenated for 1 and 10 min in the PE‐CVD reactor.

H in poly‐Si was often related to the crystalline defects. A representative defect in poly‐Si is the high‐energy grain boundary. The quantitative detection of the dangling bonds at the grain boundaries was performed by using electron paramagnetic resonance (EPR), which was also used to observe effects of hydrogenation on them. It was observed that the isolated dangling bonds aggregated at the grain boundaries for ELC poly‐Si. The detected electron spin density was in the order of 1018 cm‐3 [19]. These dangling bonds were effectively passi‐ vated by hydrogenation. However, considering the large density of H shown in **Figure 3**, it is assumed that a majority of the H existed in the film such as interstitial instead of at the termination of dangling bonds.

**2.3. Hydrogen and grain boundaries in ELC poly‐Si**

86 New Advances in Hydrogenation Processes - Fundamentals and Applications

reached was 5 × 1020 cm‐3 that is 1 atomic percent (at.%).

The depth profile of H concentration in ELC poly‐Si obtained by secondary ion mass spectro‐ scopy (SIMS) is shown in **Figure 3**, where hydrogenation is performed in the PE‐CVD reactor. The enhancement in the concentration near the surface and the Si/SiO2 interface is apparent values. While the 1‐min treatment produced a profile of H diffusing from the surface to the interface, the 10‐min treatment led to an almost saturated profile, in which the average density

**Figure 3.** Depth profiles of H concentration in ELC poly‐Si hydrogenated for 1 and 10 min in the PE‐CVD reactor.

H in poly‐Si was often related to the crystalline defects. A representative defect in poly‐Si is the high‐energy grain boundary. The quantitative detection of the dangling bonds at the grain boundaries was performed by using electron paramagnetic resonance (EPR), which was also used to observe effects of hydrogenation on them. It was observed that the isolated dangling bonds aggregated at the grain boundaries for ELC poly‐Si. The detected electron spin density was in the order of 1018 cm‐3 [19]. These dangling bonds were effectively passi‐ Grain boundaries in poly‐Si can be revealed by Secco etching, which was developed to detect dislocations in single‐crystalline Si [20]. **Figure 4(a)** shows a scanning electron microscopy (SEM) image for ELC poly‐Si after the etching. Grain boundaries appear as evident lines. On the other hand, almost no grain boundaries are revealed for the hydrogenated film, whereas faint twin boundaries are observed as shown in **Figure 4(b)**. This effect of hydrogenation can be understood by the electrochemical model as follows. The chemical etching proceeds with the electron transfer from the conduction band to proton in solution, which leads to production of an intermediate with a higher oxidization state. Although the electron transfer is intercepted by the energy barrier, the transfer can occur via an electron‐hole recombination centers in the band gap [21]. It was shown that the localized states acting as the recombination centers are formed at the high‐energy grain boundaries, and it is followed by enhancement of the etching. On the other hand, the hydrogenation relaxes the metastable states with extrinsic H‐termina‐ tion, which leads to suppression of the electrochemical reaction at grain boundaries.

**Figure 4.** SEM images of ELC poly‐Si after chemical etching for 40 s (a) as‐crystallized and (b) hydrogenated films.

Next, hydrogenation effects on the ingrain defects were examined. The presence of defects in the grains is attributed to the large cooling velocity after the laser irradiation. The cooling velocity was estimated to be as large as ∼1010 K/s [22]. The thermal equilibrium defects at the high temperature partially aggregate with each other during the rapid cooling and reside in the grains even at room temperature. In fact, the chemical etching rate decreased to 2/3 by reannealing at 1000°C for 10 min even with no hydrogenation, which was attributed to the annealing out of ingrain defect [23].

#### **2.4. Hydrogenation effects on electron mobility**

The mobility of poly‐Si for TFT is generally expressed by µFE, which depends not only on the Si film characteristics but also on the device structures and performance of the Si/SiO2 interface 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 grain boundaries [24, 25].

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

**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.
