**3. Properties and stabilities of the triode-deposited a-Si:H**

### **3.1 Properties of the a-Si:H films prepared by the triode system 3.1.1 Hydrogen concentration**

The hydrogen concentrations of the a-Si:H films prepared by the triode system were measured by FTIR. Figure 2 (a) shows the spectrum of the film prepared at 250 oC with the

Fabrication of the Hydrogenated

350 oC, respectively.

**3.1.3 Microscopic structure** 

Amorphous Silicon Films Exhibiting High Stability Against Light Soaking 307

Fig. 3. Si-H and Si-H2 bond densities in the a-Si:H films fabricated with the triode deposition system (triode) under the various distances between the mesh and the substrate (*d*ms). As a comparison, those of the conventionally prepared films without the mesh are also shown (diode, *d*ms = 0 cm). The films were prepared at the substrate temperatures of 200, 250 and

cases, but the growth rates are different each other where very low growth rate is observed with the double mesh. The growth rate with the double mesh at 10 W is c.a. 0.1 Å/s which is close to the value observed at the VHF power of 2 W with the single mesh. However, the observed Si-H and Si-H2 bond densities are lower in the case of 2 W with the single mesh.

input power (W) mesh growth rate (Å/s) Si-H (at.%) Si-H2 (at.%) 2 single 0.18 4.0 < 0.1 10 double 0.12 6.1 0.9 10 single 0.80 6.6 1.0

Table 1. Si-H and Si-H2 bond densities and the observed growth rate of the films prepared under the several conditions with fixing *d*ms (= 4 cm) and the substrate temperature (= 250 oC).

In figures 5 (a) and (b), the FWHM of the Si-H and Si-H2 stretching mode peaks in the FTIR spectra are platted against the density of Si-H and Si-H2, respectively. The films were prepared at the VHF input power of 2 or 10 W using the each electrode configuration i.e., triode or diode system as indicated in the figure. The substrate temperature is 250 oC in every case. While the scattered relation is observed in the Si-H bond case, one can see the good correlation between the Si-H2 bond densities and their FWHMs. Moreover, while

The similar trend is observed under the different conditions as shown in figure 4.

distance between the mesh and the substrate, *d*ms, of 3 cm [Shimizu et al., 2005]. As a comparison, that of the conventionally prepared a-Si:H film at the same substrate temperature is shown in figure 2 (b) [Shimizu et al., 2005]. One can see that the Si-H2 bond density is low in the case of the triode deposition.

Fig. 2. Si-H and Si-H2 stretching mode absorption spectra obtained in the FTIR measurement. The films were prepared by the: (a) triode system at the *d*ms of 3 cm, and the (b) conventional diode system. In both cases, the substrate temperatures are 250 oC. [Shimizu et al., 2005]

Furthermore, the a-Si:H films were fabricated with changing *d*ms, and the results are summarized in figure 3. One can see that, as *d*ms is increased, both Si-H and Si-H2 densities decrease. The a-Si:H film prepared at *d*ms = 4 cm contains the Si-H bond density of 4.0 at.% and less than 0.1 at.% (close to the detection limit of FTIR) of Si-H2 bond density. On the other hand, the film prepared by the conventional diode method contains 9.0 at.% of Si-H bonds and 1.5 at.% of Si-H2 bonds at the same substrate temperature. The similar reductions of Si-H and Si-H2 bond densities with the triode system are observed in the films prepared under the several substrate temperatures as shown in figure 3.

#### **3.1.2 Growth of a-Si:H with double mesh**

With installing a mesh and increasing *d*ms, the growth rate is reduced. To see the effect of growth rate on the resulting hydrogen concentration, the films were prepared with installing a second mesh at a fixed VHF input power and *d*ms. With such a configuration, one can control the growth rate without changing the gas phase conditions, whereas it is not the case if the VHF power or *d*ms is changed to control the growth rate, because the generation rate of precursors changes with the input power, and as discussed later, *d*ms affects the flux of the precursors reaching to the substrate. Thus, to see the effect of the growth rate, the double mesh configuration was used.

Here, the films were prepared with or without the second mesh, which is represented as double or a single mesh, respectively. All the films were prepared at 250 oC. The results are summarized in table 1 and figure 4 [Shimizu et al., 2007]. At the VHF power of 10 W, almost the same hydrogen concentrations are observed both in the single and the double mesh

distance between the mesh and the substrate, *d*ms, of 3 cm [Shimizu et al., 2005]. As a comparison, that of the conventionally prepared a-Si:H film at the same substrate temperature is shown in figure 2 (b) [Shimizu et al., 2005]. One can see that the Si-H2 bond

Fig. 2. Si-H and Si-H2 stretching mode absorption spectra obtained in the FTIR

under the several substrate temperatures as shown in figure 3.

**3.1.2 Growth of a-Si:H with double mesh** 

double mesh configuration was used.

measurement. The films were prepared by the: (a) triode system at the *d*ms of 3 cm, and the (b) conventional diode system. In both cases, the substrate temperatures are 250 oC.

Furthermore, the a-Si:H films were fabricated with changing *d*ms, and the results are summarized in figure 3. One can see that, as *d*ms is increased, both Si-H and Si-H2 densities decrease. The a-Si:H film prepared at *d*ms = 4 cm contains the Si-H bond density of 4.0 at.% and less than 0.1 at.% (close to the detection limit of FTIR) of Si-H2 bond density. On the other hand, the film prepared by the conventional diode method contains 9.0 at.% of Si-H bonds and 1.5 at.% of Si-H2 bonds at the same substrate temperature. The similar reductions of Si-H and Si-H2 bond densities with the triode system are observed in the films prepared

With installing a mesh and increasing *d*ms, the growth rate is reduced. To see the effect of growth rate on the resulting hydrogen concentration, the films were prepared with installing a second mesh at a fixed VHF input power and *d*ms. With such a configuration, one can control the growth rate without changing the gas phase conditions, whereas it is not the case if the VHF power or *d*ms is changed to control the growth rate, because the generation rate of precursors changes with the input power, and as discussed later, *d*ms affects the flux of the precursors reaching to the substrate. Thus, to see the effect of the growth rate, the

Here, the films were prepared with or without the second mesh, which is represented as double or a single mesh, respectively. All the films were prepared at 250 oC. The results are summarized in table 1 and figure 4 [Shimizu et al., 2007]. At the VHF power of 10 W, almost the same hydrogen concentrations are observed both in the single and the double mesh

density is low in the case of the triode deposition.

[Shimizu et al., 2005]

Fig. 3. Si-H and Si-H2 bond densities in the a-Si:H films fabricated with the triode deposition system (triode) under the various distances between the mesh and the substrate (*d*ms). As a comparison, those of the conventionally prepared films without the mesh are also shown (diode, *d*ms = 0 cm). The films were prepared at the substrate temperatures of 200, 250 and 350 oC, respectively.

cases, but the growth rates are different each other where very low growth rate is observed with the double mesh. The growth rate with the double mesh at 10 W is c.a. 0.1 Å/s which is close to the value observed at the VHF power of 2 W with the single mesh. However, the observed Si-H and Si-H2 bond densities are lower in the case of 2 W with the single mesh. The similar trend is observed under the different conditions as shown in figure 4.


Table 1. Si-H and Si-H2 bond densities and the observed growth rate of the films prepared under the several conditions with fixing *d*ms (= 4 cm) and the substrate temperature (= 250 oC).
