**3.3 Fast deposition of highly crystallized μc-Si:H:Cl films with low defect density from SiH2Cl2 using low-pressure high-density microwave plasma**

#### **3.3.1 Fine structure of Si network of microcrystalline silicon thin-film fabricated from SiH2Cl2 and SiH4**

The typical FTIR spectra of 1-μm-thick μc-Si:H:Cl films fabricated from a SiH2Cl2-H2 mixture, compared with those of μc-Si:H films from SiH4 as shown in Fig. 17. Here, the peak assignments of SiH (bulk and surface stretching) and SiH2 bulk stretching are also shown in Table 1.

Novel Deposition Technique for Fast Growth

thickness of 1 μm.

films.

source.

of Hydrogenated Microcrystalline Silicon Thin-Film for Thin-Film Silicon Solar Cells 371

observed. Moreover, the inclusion Cl in the microcrystalline Si network produces a new absorption band, which is assigned to Si-Cl bonds centered at 530cm-1 as described in ref. These suggest that film structure is a continuous Si network including a mixture of amorphous and crystalline Si phase, although the crystalline size is smaller. The film deposition rate reached 20 Å/s for the film synthesized from 5sccm SiH2Cl2, which was almost same as that for the film synthesized from SiH4. Therefore, the fine structure of the

The spectroscopy ellipsometry (SE) characterization was performed for the μc-Si:H:Cl films fabricated from a SiH2Cl2-H2 mixture at different Tss. Figure 17 shows the imaginary part of pseudo-dielectric function <*ε*2> spectra of μc-Si:H:Cl films fabricated from a SiH2Cl2-H2

mixture at different Tss with that of μc-Si:H from SiH4 with a thickness of 1 μm.

Fig. 18. Imaginary part of pseudodielectric function <*ε*2> spectra of μc-Si:H:Cl films

fabricated from a SiH2Cl2-H2 mixture at different Tss with that of μc-Si:H from SiH4 with a

In both μc-Si:H:Cl and μc-Si:H films, the fine structures were observed clearly at 3.4 and 4.2 eV, which are attributed to the E1 and E2 optical band transitions, respectively, in the crystalline Si (c-Si) band structure. However, the magnitude of <*ε*2> was much smaller in the μc-Si:H films from SiH4 than in the μc-Si:H:Cl films from SiH2Cl2. Here, the magnitude of <*ε*2> presents qualitatively the degrees of homogeneity and the surface roughness of μc-Si

The <*ε*2> spectra were analyzed to understand the micro-structural properties of μc-Si:H:Cl films as described in section 3.2 above. The reflective index *n* at 2.2 eV determined by SE analysis also increased with Ts as determined using the reference poly-Si given by Jellison as shown in Fig. 19. It was higher for the μc-Si:H:Cl films in all Ts regions than that for μc-Si:H films. Thus, the rigidity of the Si-network is greater in the μc-Si:H:Cl films from SiH2Cl2 than in μc-Si:H films from SiH4 using the high-density microwave plasma

μc-Si network from SiH4 and SiH2Cl2 is different from each other.


Table 1. Assignment of SiH, SiH2 vibration modes

Fig. 17. FTIR spectra of µc-Si:H:Cl films at different Tss. The typical FTIR Spectrum for the SiHx stretching absorption region in the µc- Si:H film from SiH4 is also shown as a reference.

However, marked differences were observed in the fine structure between μc-Si:H:Cl and μc-Si:H. In the μc-Si:H films from SiH4, the absorption peaks at 2080 cm-1 and 2100 cm-1 attributable to surface and bulk SiHx stretching absorption modes, respectively, in the nanocrystalline Si phase were dominant with a negligibly small peak of SiH absorption in the bulk a-Si phase at 2000 cm-1. Thus, the film is highly crystallized with a negligibly small fraction of the amorphous Si phase. In addition, the IR absorption peak at 2080cm-1 corresponding to the surface SiH mode in the µc-Si phase appeared as a shoulder in the μc-Si:H film. These results suggest that the c-Si phase is isolated in a-Si network, which is not preferable for the Si thin-film solar cells. Moreover, because of excess dissociation of SiH4, the μc-Si:H network showed a porous structure, which resulted in a poor carrier transport property of photo-generated carriers.

On the other hand, both SiH and SiH2 absorption peaks were observed at 2000 and 2100 cm-1, respectively, in the μc-Si:H:Cl films, were similar as to the μc-Si:H films fabricated using conventional rf and VHF PE-CVDs of SiH4. No SiH at the surface of μc-Si phase was

Fig. 17. FTIR spectra of µc-Si:H:Cl films at different Tss. The typical FTIR Spectrum for the SiHx stretching absorption region in the µc- Si:H film from SiH4 is also shown as a reference. However, marked differences were observed in the fine structure between μc-Si:H:Cl and μc-Si:H. In the μc-Si:H films from SiH4, the absorption peaks at 2080 cm-1 and 2100 cm-1 attributable to surface and bulk SiHx stretching absorption modes, respectively, in the nanocrystalline Si phase were dominant with a negligibly small peak of SiH absorption in the bulk a-Si phase at 2000 cm-1. Thus, the film is highly crystallized with a negligibly small fraction of the amorphous Si phase. In addition, the IR absorption peak at 2080cm-1 corresponding to the surface SiH mode in the µc-Si phase appeared as a shoulder in the μc-Si:H film. These results suggest that the c-Si phase is isolated in a-Si network, which is not preferable for the Si thin-film solar cells. Moreover, because of excess dissociation of SiH4, the μc-Si:H network showed a porous structure, which resulted in a poor carrier transport

2200 2100 2000 1900

**Wave number (cm-1)** 

On the other hand, both SiH and SiH2 absorption peaks were observed at 2000 and 2100 cm-1, respectively, in the μc-Si:H:Cl films, were similar as to the μc-Si:H films fabricated using conventional rf and VHF PE-CVDs of SiH4. No SiH at the surface of μc-Si phase was

Table 1. Assignment of SiH, SiH2 vibration modes

**Transmittance (%)** 

property of photo-generated carriers.

**Si-H (bulk stretching ) 2000 cm-1 Si-H (surface stretching) 2080 cm-1 SiH2 (bulk stretching) 2100 cm-1**

**SiH4/H2** 

**250<sup>0</sup> C** 

**SiH2Cl2/H2** 

 **100<sup>0</sup>**

**250<sup>0</sup> C** 

**400<sup>0</sup> C**

**C** 

observed. Moreover, the inclusion Cl in the microcrystalline Si network produces a new absorption band, which is assigned to Si-Cl bonds centered at 530cm-1 as described in ref. These suggest that film structure is a continuous Si network including a mixture of amorphous and crystalline Si phase, although the crystalline size is smaller. The film deposition rate reached 20 Å/s for the film synthesized from 5sccm SiH2Cl2, which was almost same as that for the film synthesized from SiH4. Therefore, the fine structure of the μc-Si network from SiH4 and SiH2Cl2 is different from each other.

The spectroscopy ellipsometry (SE) characterization was performed for the μc-Si:H:Cl films fabricated from a SiH2Cl2-H2 mixture at different Tss. Figure 17 shows the imaginary part of pseudo-dielectric function <*ε*2> spectra of μc-Si:H:Cl films fabricated from a SiH2Cl2-H2 mixture at different Tss with that of μc-Si:H from SiH4 with a thickness of 1 μm.

Fig. 18. Imaginary part of pseudodielectric function <*ε*2> spectra of μc-Si:H:Cl films fabricated from a SiH2Cl2-H2 mixture at different Tss with that of μc-Si:H from SiH4 with a thickness of 1 μm.

In both μc-Si:H:Cl and μc-Si:H films, the fine structures were observed clearly at 3.4 and 4.2 eV, which are attributed to the E1 and E2 optical band transitions, respectively, in the crystalline Si (c-Si) band structure. However, the magnitude of <*ε*2> was much smaller in the μc-Si:H films from SiH4 than in the μc-Si:H:Cl films from SiH2Cl2. Here, the magnitude of <*ε*2> presents qualitatively the degrees of homogeneity and the surface roughness of μc-Si films.

The <*ε*2> spectra were analyzed to understand the micro-structural properties of μc-Si:H:Cl films as described in section 3.2 above. The reflective index *n* at 2.2 eV determined by SE analysis also increased with Ts as determined using the reference poly-Si given by Jellison as shown in Fig. 19. It was higher for the μc-Si:H:Cl films in all Ts regions than that for μc-Si:H films. Thus, the rigidity of the Si-network is greater in the μc-Si:H:Cl films from SiH2Cl2 than in μc-Si:H films from SiH4 using the high-density microwave plasma source.

Novel Deposition Technique for Fast Growth

of Hydrogenated Microcrystalline Silicon Thin-Film for Thin-Film Silicon Solar Cells 373

Fig. 20. The fc-Si, fa-Si, fvoids in the bulk (layer 3) of μc-Si:H:Cl films plotted as a function of

Tss. The results of μc-Si:H are also shown as a triangles symbol

Fig. 19. The refractive index at 2.2eV in the bulk layer for μc-Si:H:Cl films plotted as a function of Ts

*f*c-Si , *fa-S*i and *fv*oid in the μc-Si:H:Cl films, corresponding to the bulk component, are shown in Fig.20 as a function of Ts together with those in the films synthesized from SiH4 . Notably, *fvoid* in the μc-Si:H:Cl films is less than 5% despite that being 10-15% in μc-Si:H.

The differences in the fine structure of the μc-Si network between μc-Si:H:Cl films and μc-Si:H films is shown in Fig.21. The degree of the excess dissociation of SiH2Cl2 is considered to be suppressed rather than that of SiH4, because the threshold energy of SiH2Cl2 is higher than that of SiH4, although the high energy part of electron energy distribution (EED) also depends on the feed gas. Film crystallization was promoted efficiently in the high-density and low-temperature microwave plasma of SiH4. However, the resulting Si film structure was still porous with much *fvoid,* although *fc-Si* was over 80%. These findings originated from the excessive dissociation of both SiH4 and H2 in the plasma, which promoted the generation rate of not only of SiH3 but also short life-time radicals, i.e., SiH and Si. On the other hand, *fa-Si* was still more in the μc-Si:H:Cl films than in the μc-Si:H films with less *f*void.

#### **3.4 Defect density of microcrystalline silicon thin-film fabricated from SiH2Cl2 and SiH4**

In the case of MW SiH4 plasma, film deposition rate 65 Å/s was achieved while maintaining the low defect density but that μc-Si:H film was not available for solar cell application because of film structure was porous as described above. Similar study was performed using SiH2Cl2 to realize the fast deposition of μc-Si:H:Cl films with no creating additional defects, higher flux of SiHxCly generated by the primary reaction in the gas phase was supplied to the depleted growing surface by increasing flow rate of SiH2Cl2 at a constant pressure of 120 mTorr and Ts of 250C. Here, the deposition precursor SiHxCly generated by the primary reaction in the plasma is expected to be supplied at the growing surface efficiently by increasing a flux of SiH2Cl2 at a constant pressure. Thus, the fast deposition of highly crystallized μc-Si:H:Cl film with lower defect density is expected because the efficient

Fig. 19. The refractive index at 2.2eV in the bulk layer for μc-Si:H:Cl films plotted as a

*fvoid* in the μc-Si:H:Cl films is less than 5% despite that being 10-15% in μc-Si:H.

*fa-Si* was still more in the μc-Si:H:Cl films than in the μc-Si:H films with less *f*void.

**3.4 Defect density of microcrystalline silicon thin-film fabricated from SiH2Cl2 and** 

In the case of MW SiH4 plasma, film deposition rate 65 Å/s was achieved while maintaining the low defect density but that μc-Si:H film was not available for solar cell application because of film structure was porous as described above. Similar study was performed using SiH2Cl2 to realize the fast deposition of μc-Si:H:Cl films with no creating additional defects, higher flux of SiHxCly generated by the primary reaction in the gas phase was supplied to the depleted growing surface by increasing flow rate of SiH2Cl2 at a constant pressure of 120 mTorr and Ts of 250C. Here, the deposition precursor SiHxCly generated by the primary reaction in the plasma is expected to be supplied at the growing surface efficiently by increasing a flux of SiH2Cl2 at a constant pressure. Thus, the fast deposition of highly crystallized μc-Si:H:Cl film with lower defect density is expected because the efficient

*f*c-Si , *fa-S*i and *fv*oid in the μc-Si:H:Cl films, corresponding to the bulk component, are shown in Fig.20 as a function of Ts together with those in the films synthesized from SiH4 . Notably,

The differences in the fine structure of the μc-Si network between μc-Si:H:Cl films and μc-Si:H films is shown in Fig.21. The degree of the excess dissociation of SiH2Cl2 is considered to be suppressed rather than that of SiH4, because the threshold energy of SiH2Cl2 is higher than that of SiH4, although the high energy part of electron energy distribution (EED) also depends on the feed gas. Film crystallization was promoted efficiently in the high-density and low-temperature microwave plasma of SiH4. However, the resulting Si film structure was still porous with much *fvoid,* although *fc-Si* was over 80%. These findings originated from the excessive dissociation of both SiH4 and H2 in the plasma, which promoted the generation rate of not only of SiH3 but also short life-time radicals, i.e., SiH and Si. On the other hand,

function of Ts

**SiH4** 

Fig. 20. The fc-Si, fa-Si, fvoids in the bulk (layer 3) of μc-Si:H:Cl films plotted as a function of Tss. The results of μc-Si:H are also shown as a triangles symbol

Novel Deposition Technique for Fast Growth

density was formed from a SiH2Cl2-H2 mixture.

abstraction of H- and Cl- terminated growing surface.

crystallinity of Ic/Ia:5-6 were obtained.

**SiH2Cl2 and SiH4** 

SiH2Cl2 at Ts of 250C.

of Hydrogenated Microcrystalline Silicon Thin-Film for Thin-Film Silicon Solar Cells 375

By supplying the sufficient flux of SiHxCly at a high Ts of 400C, the termination of dangling bond is accelerated with enhancing the abstraction of H and Cl. These findings are effective to form a rigid Si network with less void fractions. In fact, the defect density Ns was almost constant of 4-5×1016 cm-3 at Ts up to 250C, whereas it decreased markedly to 3-4x1015 cm-3 with Fr(SiH2Cl2) at Ts of 400C. Therefore, highly crystallized μc-Si:H:Cl film with low defect

**3.5 XRD and Raman spectra of microcrystalline silicon thin-film fabricated from** 

The XRD diffraction patterns and the Raman spectrum of the μc-Si:H:Cl film fabricated at Ts of 250°C and 400°C with increasing SiH2Cl2 flow rates are shown in Fig 23 and Fig. 24. The XRD and Raman study of μc-Si:H:Cl films fabricated at Ts of 400°C revealed that high film crystallinities with diffraction intensities ratio of I(220)/I(111) of 1.5-8.75 and with Raman

As a consequence, highly crystallized μc-Si:H:Cl film with low defect densities of 3-4x1015 cm-3 was fabricated at fast deposition rate of 27Å/s. These findings suggest that the efficient

Fig. 23. XRD and Raman spectra of the μc-Si:H:Cl films fabricated at different flow rate of

Fig. 25 shows the relation between the dark and photo conductivities for the μc-Si:H:Cl films fabricated by increasing the SiH2Cl2 flow rate at the substrate temperatures of 250°C and 400°C. The photosensitivity reached at 5-6 orders of magnitude at room temperature. The level of photoconductivity was 10-5 S/cm under 100 mW/cm2 white light exposure. The dark and photo-conductivities were the order of 10-12 and 10-5 S/cm, respectively, which shows highly photosensitive films. Fig.26 shows the activation energies for the μc-Si:H:Cl films fabricated by increasing the SiH2Cl2 flow rate at the substrate temperatures of 250°C and 400°C The activation energies of electrical conductivity were 0.40-0.80 eV, suggesting

**3.6 Photoelectrical properties of a-Si:H:Cl and μc-Si:H:Cl films** 

that both a-Si:H:Cl and µc-Si:H:Cl films were intrinsic semiconductor films.

Fig. 21. Schematic of μc-Si:H and μc-Si:H:Cl network

termination of dangling bond by efficient supply of SiHxCly is accelerated with the abstraction of H and Cl as HCl at the depleted growing surface. The deposition study was performed at two different Tss of 250 and 400C. Figure 22 demonstrates Nss and deposition rates of μc-Si:H:Cl films fabricated at different Fr(SiH2Cl2) at Tss of 250 and 400C, respectively. The high deposition rate of 40 Å/s has been achieved with increasing Fr(SiH2Cl2) up to 20 sccm at Tss of 400C and 2500C respectively. The Ns was almost independent of Fr(SiH2Cl2) at Ts of 250C, whereas the Ns was markedly decreased at Ts of 400C. These are considered because of the efficient abstraction of H and Cl at the growing surface. The film crystallization was enhanced up to flow rate of 20 sccm of SiH2Cl2 at Ts of 400C. The Nss were decreased systematically with increasing SiH2Cl2 at both Tss to 4x1015 cm-3 at 15 sccm of SiH2Cl2.

Fig. 22. ESR spin density and deposition rates of the μc-Si:H:Cl films fabricated at different flow rates of SiH2Cl2 at Ts of 250 and 400C.

termination of dangling bond by efficient supply of SiHxCly is accelerated with the abstraction of H and Cl as HCl at the depleted growing surface. The deposition study was performed at two different Tss of 250 and 400C. Figure 22 demonstrates Nss and deposition rates of μc-Si:H:Cl films fabricated at different Fr(SiH2Cl2) at Tss of 250 and 400C, respectively. The high deposition rate of 40 Å/s has been achieved with increasing Fr(SiH2Cl2) up to 20 sccm at Tss of 400C and 2500C respectively. The Ns was almost independent of Fr(SiH2Cl2) at Ts of 250C, whereas the Ns was markedly decreased at Ts of 400C. These are considered because of the efficient abstraction of H and Cl at the growing surface. The film crystallization was enhanced up to flow rate of 20 sccm of SiH2Cl2 at Ts of 400C. The Nss were decreased systematically with

Fig. 22. ESR spin density and deposition rates of the μc-Si:H:Cl films fabricated at different

Fig. 21. Schematic of μc-Si:H and μc-Si:H:Cl network

increasing SiH2Cl2 at both Tss to 4x1015 cm-3 at 15 sccm of SiH2Cl2.

flow rates of SiH2Cl2 at Ts of 250 and 400C.

By supplying the sufficient flux of SiHxCly at a high Ts of 400C, the termination of dangling bond is accelerated with enhancing the abstraction of H and Cl. These findings are effective to form a rigid Si network with less void fractions. In fact, the defect density Ns was almost constant of 4-5×1016 cm-3 at Ts up to 250C, whereas it decreased markedly to 3-4x1015 cm-3 with Fr(SiH2Cl2) at Ts of 400C. Therefore, highly crystallized μc-Si:H:Cl film with low defect density was formed from a SiH2Cl2-H2 mixture.
