Novel Deposition Technique for Fast Growth of Hydrogenated Microcrystalline Silicon Thin-Film for Thin-Film Silicon Solar Cells

The microcrystalline silicon material is reported to be a quite complex material consisting of an amorphous matrix with embedded crystallites plus grain boundaries. Although this material has a complex microstructure, its optical properties have a marked crystalline characteristic: an optical gap at 1.12 eV like c-Si. This implies the spectral absorption of μcSi:H covers a much larger range than a-Si:H which posses an optical gap between 1.6 and 1.75eVi. Compared to a-Si:H that absorbs light up to 800 nm, μc-Si:H absorbs light coming from a wider spectral range, extending up to 1100 nm . On the other hand, within its range of absorption, the absorption of a-Si:H is higher than that of μc-Si:H –due to the indirect gap of the latter. Therefore, the optical combination of these two materials takes advantage of a larger part of the solar spectrum (compared to a single-junction cell) and the conversion efficiency of the incident light into electricity can be consequently improved. Furthermore, the μc-Si:H solar cell is reported to be largely stable against light induced degradation and enhanced carrier mobility in contrast to amorphous silicon films counterpart. Consequently hydrogenated microcrystalline silicon is one of the promising materials for application to thin-film silicon solar cells.


Introduction
The microcrystalline silicon material is reported to be a quite complex material consisting of an amorphous matrix with embedded crystallites plus grain boundaries.Although this material has a complex microstructure, its optical properties have a marked crystalline characteristic: an optical gap at 1.12 eV like c-Si.This implies the spectral absorption of µc-Si:H covers a much larger range than a-Si:H which posses an optical gap between 1.6 and 1.75eV i .Compared to a-Si:H that absorbs light up to 800 nm, µc-Si:H absorbs light coming from a wider spectral range, extending up to 1100 nm .On the other hand, within its range of absorption, the absorption of a-Si:H is higher than that of µc-Si:H -due to the indirect gap of the latter.Therefore, the optical combination of these two materials takes advantage of a larger part of the solar spectrum (compared to a single-junction cell) and the conversion efficiency of the incident light into electricity can be consequently improved.Furthermore, the µc-Si:H solar cell is reported to be largely stable against light induced degradation and enhanced carrier mobility in contrast to amorphous silicon films counterpart.Consequently hydrogenated microcrystalline silicon is one of the promising materials for application to thin-film silicon solar cells.

Growth techniques of hydrogenated microcrystalline silicon
The growth of µc-Si:H material uses silane (SiH 4 ) and hydrogen as source-gas.It is currently admitted that free radical precursors (SiH x )-SiH 3 is suspected to favor the µc-Si:H growthand H-enhances crystalline growth by etching of looser a-Si:H tissue-were needed to attain microcrystalline growth.In order to obtain such reactive species, decomposition of the source-gases is necessary.At first, this was obtained by using PE-CVD at high temperatures (600 °C).The use of low deposition temperatures of 200-300 °C with a plasma present in the deposition chamber, the so called Plasma-Enhanced Chemical Vapor Deposition technique (PE-CVD) was developed later on and allowed the low-temperature deposition of µc-Si:H films, and rapid progresses have been achieved.Unfortunately, "state-of-the-art" microcrystalline silicon solar cells consist of intrinsic µc-Si:H layers that are deposited by rf and VHF PE-CVD at deposition rates of only 1-5 Å/s.On the other hand, a µc-Si:H film with a 2-µm-thickness intrinsic absorption layer is required for application to Si thin-film solar cells because of the low optical absorption in the visible region.The µc-Si:H i-layer deposition step is the most time consuming step in the deposition sequence of the solar cell.Therefore, a novel fast deposition technique of µc-Si:H is required.

Novel fast deposition techniques of microcrystalline silicon
Now-a-days, for the high throughput of high-efficiency µc-Si solar cells in PV industry, one of the most crucial requirements is fast deposition of µc-Si without deteriorating the optical, structural and electronic properties of the film.To overcome the difficulty, several highdensity plasma sources have been developed, such as very high frequency (VHF) plasma, inductive coupling plasma (ICP) and surface wave plasma (SWP).As it has been reported, the excitation frequency of a plasma source has an important effect on the electron acceleration in the plasma, and a high excitation frequency is expected to result in a high electron density and a low electron temperature.Therefore two new microwave plasma sources have been developed i.e.Low-pressure high-density microwave plasma source utilizing the spoke antenna and the remote-type high-pressure microwave plasma using a quartz tube having an inner diameter of 10 mm and applied those for the fast deposition of µc-Si films for Si thin-film solar cells.The remote-type high-pressure microwave plasma will be discussed in elsewhere.

Low-pressure high-density microwave plasma source utilizing the spoke antenna
The microwave plasma source is shown schematically in Fig. 1, which is composed of the combination of a conventional microwave discharge and a spoke antenna.Its chamber size is 22 cm in diameter, which enables large-scale film processing.The spoke antenna is located on a 15 mm-thick quartz plate, which is not inside of the vacuum chamber.The antenna system is shown in Fig. 2 more in detail.The length of each spoke is 4 cm, which is about 1/4 of the wavelength of a 2.45 GHz wave.The design of the spoke antenna assembly is based on an inter-digital filter composed of parallel cylindrical rods (spokes) arranged between parallel-grounded plates.The spokes are resonantly coupled by the stray capacitance between adjacent spokes and the inductance of the spokes themselves.The resonance condition of an introduced angular frequency is given by ω=2πf=1/(C×L) 1/2 , where f is the introduced frequency, C is the array capacitance, and L is the antenna inductance.Thus, the antenna operates as a band-pass filter.The spokes are arranged like those in a wheel, and the plasma serves as one of the grounded plates.The electromagnetic wave propagates through the spokes consecutively with a phase difference of 90°, and microwave current flows in every spoke.The current in the spokes couples inductively and capacitively to the plasma ("CM coupling"), and the induction current in the plasma accelerates the electrons to sustain the plasma, as shown in Fig. 2 & 3.The power is supplied from the center of the antenna, and the plasma under the spoke antenna is radially discharged because induction current flows near every spoke.As a result, uniform microwave plasma over an area of diameter greater than 20 cm can be generated efficiently.As well, since no magnetic field is required to generate the high-density microwave plasma, it is possible to design a simple source yielding high-density and lowtemperature plasma.From a material processing standpoint, large-area microwave plasmas (MWPs) have several advantages in comparison with other types of high-density sources.First, MWPs, being no magnetized sources, are free from such magnetic field induced problems as inhomogeneous density profile and charge-up damage, which is often, experienced in electron cyclotron resonance (ECR) or helicon plasma sources.Second, MWPs can be enlarged to diameters longer than 1 m more easily than inductively coupled plasmas (ICPs).Thus, the application of MWPs to giant electronic devices such as solar cells is promising.Third, MWPs have lower bulk-electron temperature.Fourth, MWPs can be operated stably from atomic pressure down to below 10 mTorr.A uniform, high-density (electron density, n e : >10 11 cm -3 ) and low-temperature (electron temperature, T e :1~2 eV) plasma can be generated by the microwave plasma source utilizing a spoke antenna without using complex components such as magnetic coil as shown in Figures 6 & 7 .The T e is almost independent of working pressure up to ~150 mTorr as shown Figure 8, which is suitable for the large area thin film processing.

Fast deposition of highly crystallized μc-Si:H films with low defect density from SiH 4 using low-pressure high-density microwave plasma
In this study, a new source gas supply method was introduced, i.e., the SiH 4 was introduced using a shower head placed 2 cm above the substrate holder under a steady flow of the H 2 plasma supplied by the ring.The results from these gas supply method were compared with the results from the another gas supply method, i.e. a SiH 4 -H 2 mixture was fed into the chamber using a ring just beneath the quartz plate.Figure 9 shows the schematic of the two different gas supply methods.The film deposition parameters were included the SiH 4 concentration R=Fr(SiH 4 )/[Fr(SiH 4 )+Fr(H 2 )] (Fr is the flow rate).The SiH 4 concentration was varied in a range from 5% to 67% by increasing Fr(SiH 4 ) from 3 to 30 sccm with a constant H 2 flow rate of 15 sccm.The film depositions were performed at the distance (Z) between the quartz plate and the substrate holder of 6 cm and the working pressure of 80 mTorr.The microwave power was fixed at 700W.Here, the film deposition rate was controlled by varying Fr(SiH 4 ) from 3 to 30 sccm under constant Fr(H 2 ) of 15 sccm and working pressure of 80 mTorr.For all samples, the film thickness was ~ 1.5 μm and the ESR measurements were performed directly on these films.It is to be noted that N s was decreased by about one order of magnitude when the shower head was used for both T s conditions despite the other deposition conditions being the same.However, N s was almost independent of Fr(SiH 4 ) on the order of (3-4)×10 16 cm -3 , which was still one order of magnitude larger than that of high quality μc-Si films reported elsewhere.A very fast deposition rate of 65Å/s has been realized for µc-Si:H films with a Raman crystallinity ratio of Ic/Ia of about 3.5 under very low H 2 dilution (i.e. with high SiH4 concentration of 67%) as shown in Fig. 12 and low defect density of (1-2) ×10 16 cm -3 using high-density and low-temperature microwave plasma.The imaginary part of the dielectric function < 2 > spectra of µc-Si:H films fabricated from SiH 4 using high-density and lowtemperature microwave plasma is shown in Fig. 13 along with that using rf PE-CVD methods.Using the optical model the best fitted volume fraction of c-Si and void i.e. fc-Si and fvoid in the bulk layer and void in surface layer, f void , with SiH 4 concentration R for the corresponding μc-Si films is shown in Fig. 14.Photon Energy (eV)

Z (cm
SiH 4 concentration (%) www.intechopen.comHighly crystallized µc-Si:H film was synthesized despite low H 2 dilution ratio rather than the conventional rf and VHF plasmas, because of high generation efficiency of atomic hydrogen.FTIR spectra and microstructure of µc-Si:H film and of SiH n absorption region are shown in Fig. 11 & 15 for the corresponding µc-Si film.Generally, two IR absorption peaks are observed at 2000 and 2100 cm -1 , which are attributed to the bulk SiH in a-Si and SiH 2 in µc-Si phase, respectively, in the film fabricated by the rf plasma CVD.However, no SiH absorption peak at 2000 cm -1 is observed in the film fabricated by high-density microwave plasma.These imply that the film crystallization is promoted extremely in the high-density plasma with negligibly small fraction of amorphous Si phase.In addition, the

Transmittance (%T) 2300 1800
Wave number (cm -1 ) www.intechopen.com IR absorption peak at 2090 cm -1 corresponding to the surface SiH mode in the µc-Si phase appeared as a shoulder in the high-density film.These results suggest that the c-Si phase is isolated in a-Si network as shown in Fig. 11 & 16, which is not preferable for the Si thin-film solar cells.Therefore, the suppression of the excess film crystallization is required by the selection of deposition precursor.The typical FTIR spectra of 1-μm-thick μc-Si:H:Cl films fabricated from a SiH 2 Cl 2 -H 2 mixture, compared with those of μc-Si:H films from SiH 4 as shown in Fig. 17.Here, the peak assignments of SiH (bulk and surface stretching) and SiH 2 bulk stretching are also shown in Table 1.

µc-Si phase SiH (µc-Si phase)
Si-H (bulk stretching ) 2000 cm -1 Si-H (surface stretching) 2080 cm -1 SiH 2 (bulk stretching) 2100 cm -1 Table 1.Assignment of SiH, SiH 2 vibration modes Fig. 17.FTIR spectra of µc-Si:H:Cl films at different T s s.The typical FTIR Spectrum for the SiH x stretching absorption region in the µc-Si:H film from SiH 4 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 SiH 4 , the absorption peaks at 2080 cm -1 and 2100 cm -1 attributable to surface and bulk SiH x 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 SiH 4 , 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 SiH 2 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 SiH 4 .No SiH at the surface of μc-Si phase was Wave number (cm -1 ) www.intechopen.comobserved.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 SiH 2 Cl 2 , which was almost same as that for the film synthesized from SiH 4 .Therefore, the fine structure of the μc-Si network from SiH 4 and SiH 2 Cl 2 is different from each other.
The spectroscopy ellipsometry (SE) characterization was performed for the μc-Si:H:Cl films fabricated from a SiH 2 Cl 2 -H 2 mixture at different T s s. Figure 17 shows the imaginary part of pseudo-dielectric function <ε 2 > spectra of μc-Si:H:Cl films fabricated from a SiH 2 Cl 2 -H 2 mixture at different T s s with that of μc-Si:H from SiH 4 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 SiH 2 Cl 2 -H 2 mixture at different T s s with that of μc-Si:H from SiH 4 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 E 1 and E 2 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 SiH 4 than in the μc-Si:H:Cl films from SiH 2 Cl 2 .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 T s 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 T s 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 SiH 2 Cl 2 than in μc-Si:H films from SiH 4 using the high-density microwave plasma source.20 as a function of T s together with those in the films synthesized from SiH 4 .Notably, f void 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 SiH 2 Cl 2 is considered to be suppressed rather than that of SiH 4 , because the threshold energy of SiH 2 Cl 2 is higher than that of SiH 4 , 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 SiH 4 .However, the resulting Si film structure was still porous with much f void , although f c-Si was over 80%.These findings originated from the excessive dissociation of both SiH 4 and H 2 in the plasma, which promoted the generation rate of not only of SiH 3 but also short life-time radicals, i.e., SiH and Si.On the other hand, f a-Si was still more in the μc-Si:H:Cl films than in the μc-Si:H films with less f void .

Defect density of microcrystalline silicon thin-film fabricated from SiH 2 Cl 2 and SiH 4
In the case of MW SiH 4 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 SiH 2 Cl 2 to realize the fast deposition of μc-Si:H:Cl films with no creating additional defects, higher flux of SiH x Cl y generated by the primary reaction in the gas phase was supplied to the depleted growing surface by increasing flow rate of SiH 2 Cl 2 at a constant pressure of 120 mTorr and T s of 250˚C.Here, the deposition precursor SiH x Cl y generated by the primary reaction in the plasma is expected to be supplied at the growing surface efficiently by increasing a flux of SiH 2 Cl 2 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. 20.The f c-Si , f a-Si , f voids in the bulk (layer 3) of μc-Si:H:Cl films plotted as a function of T s s.The results of μc-Si:H are also shown as a triangles symbol By supplying the sufficient flux of SiH x Cl y at a high T s of 400˚C, 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 N s was almost constant of 4-5×10 16 cm -3 at T s up to 250˚C, whereas it decreased markedly to 3-4x10 15 cm -3 with Fr(SiH 2 Cl 2 ) at T s of 400˚C.Therefore, highly crystallized μc-Si:H:Cl film with low defect density was formed from a SiH 2 Cl 2 -H 2 mixture.As a consequence, highly crystallized μc-Si:H:Cl film with low defect densities of 3-4x10 15 cm -3 was fabricated at fast deposition rate of 27Å/s.These findings suggest that the efficient abstraction of H-and Cl-terminated growing surface.The level of photoconductivity was 10 -5 S/cm under 100 mW/cm 2 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 SiH 2 Cl 2 flow rate at the substrate temperatures of 250 °C and 400 °C The activation energies of electrical conductivity were 0.40-0.80eV, suggesting that both a-Si:H:Cl and µc-Si:H:Cl films were intrinsic semiconductor films.deposition of ZnO:Al and p-type Si layers on SnO 2 coated glass, respectively by rf magnetron sputtering and plasma CVD methods, μc-Si:H:Cl film with a 2-µm-thickness is fabricated using a high-density microwave plasma as a photovoltaic layer and n-type Si layer was fabricated using conventional rf plasma CVD method.When the samples were being transported between the rf chamber and MWP chamber, they were exposed to air.Subsequently, ZnO:Al and Ag layers were deposited as a top electrode using a shadow mask with a 5×5 mm 2 holes.Table 3 shows the typical deposition conditions for p, i and n layers, respectively.Table 4 shows the typical deposition conditions for ZnO:Al, Ag layers fabricated by rf magnetron sputtering.The photocurrent-voltage, I-V characteristics under AM 1.5, 100mW/cm 2 exposure condition are measured and the performance of solar cell is characterized with open circuit voltage, V oc , short circuit current, I sc , fill factor, FF and conversion efficiency, η.The collection efficiency was also measured from 300 to 1200 nm under bias light conditions.SiH 2 Cl 2 .The 5-6% efficiencies have been achieved for the cells fabricated by the conventional rf plasma-CVD method.However, the performance is still poor and the open circuit voltage, (Voc):0.54V, short circuit density, (Jsc):2.15mA/cm 2 , Fill Factor, FF:0.5236 and the conversion efficiency was 0.5236% in the cell made by the high-density microwave plasma from SiH 2 Cl 2 but solar cell performance is confirmed by the high-density microwave plasma from SiH 2 Cl 2 for the first time .The diffusion of Boron and Chlorine happens easily in i-layer by the high-density microwave plasma.Moreover, the etching reaction of p layer has occurred because of the hydrogen plasma.It is required to evaluate not only a single film but it is also necessary to evaluate the each interface i.e.AZO/p, p/i and i/n in order to improve the solar cell performance.More over precise control of p/i, i/n, AZO/p interface formation is needed for obtaining the further high performance.

Conclusion
The highly photoconductive and crystallized μc-Si:H:Cl films with less volume fraction of void and defect density were synthesized using the high-density and low-temperature microwave plasma source of a SiH 2 Cl 2 -H 2 mixture rather than those from SiH 4 while maintaining a high deposition rate of 27 Å/s.The μc-Si:H:Cl film possesses a μc-Si and a-Si mixture structure with less volume fraction of voids.The role of chlorine in the growth of μc-Si:H:Cl films is the suppression of the excess film crystallization at the growing surface.H termination of growing surface is more effective to suppress the defect density rather than that of Cl termination.The fast deposition of the μc-Si:H:Cl film with low defect density of 3-4 ×10 15 cm -3 is achieved with reducing Cl concentration during the film growth.Both a-Si:H:Cl and µc-Si:H:Cl films show

Fig. 1 .
Fig. 1.A schematic illustration of the microwave plasma source

Fig. 2 .
Fig. 2. The newly developed spoke antenna for introduction of microwave power (a) Microwave current, (b) Electric field.

Fig. 4 .
Fig. 3.The coupling of the spoke antenna with microwave plasma [x] Fig. 4. demonstrates that Ar plasma maintains a uniform state over 22 cm in diameter up to 20 Torr.The schematic diagram of the low-pressure highdensity microwave plasma utilizing the spoke antenna is shown in Fig. 5.

Fig. 5 .Fig. 6 .
Fig. 5.The schematic diagram of the low-pressure high-density microwave plasma utilizing the spoke antenna

Fig. 9 .
Fig.9.Schematic of the two different gas supply methods used in this study.The distance (Z) between the quartz plate and substrate holder was 6 cm.Fig.10shows the deposition rate dependence of ESR spin density, N s for the corresponding μc-Si films fabricated using two different SiH 4 gas supply methods at T s of 150 and 250°C.

Fig. 10 .
Fig.10.ESR spin density, N s for corresponding μc-Si films fabricated using different gas supply method as well as that for samples prepared at T s =150˚C are plotted as a function of film deposition rate R d .

Fig. 13 .
Fig. 13.Imaginary part of the pseudo dielectric function< 2 > spectra for the µc-Si films fabricated from SiH 4 using MW Plasma along with that using rf PECVD methods and five layers optical model.

Fig. 14 .
Fig. 14.Changes in f c-Si and f void in the bulk layer and surface layer, with SiH 4 concentration R for the corresponding μc-Si films shown in Fig. 11.

Fig. 16 .
Fig. 16.µc-Si:H films fabricated from SiH 4 using MW plasmaHighly crystallized μc-Si:H films with a preferred (220) crystal orientation at a high deposition rate of 65 Å/s were fabricated from SiH 4 with a negligibly small volume fraction of amorphous Si but μc-Si network included high volume fraction of voids as shown in Fig.15.which was hardly compatible with a device quality material.To overcome this problems, the fast deposition of highly photoconductive hydrogenated chlorinated microcrystalline Si (μc-Si:H:Cl) films with amorphous Si phase and with less volume fraction of void have been fabricated from SiH 2 Cl 2 with higher threshold energy for the dissociation instead of SiH 4 .3.3 Fast deposition of highly crystallized μc-Si:H:Cl films with low defect density from SiH 2 Cl 2 using low-pressure high-density microwave plasma 3.3.1 Fine structure of Si network of microcrystalline silicon thin-film fabricated from SiH 2 Cl 2 and SiH 4The typical FTIR spectra of 1-μm-thick μc-Si:H:Cl films fabricated from a SiH 2 Cl 2 -H 2 mixture, compared with those of μc-Si:H films from SiH 4 as shown in Fig.17.Here, the peak assignments of SiH (bulk and surface stretching) and SiH 2 bulk stretching are also shown in Table1.

Fig. 19 .
Fig. 19.The refractive index at 2.2eV in the bulk layer for μc-Si:H:Cl films plotted as a function of T s f c-Si , f a-Si and f void in the μc-Si:H:Cl films, corresponding to the bulk component, are shown in Fig.20as a function of T s together with those in the films synthesized from SiH 4 .Notably, f void 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 SiH 2 Cl 2 is considered to be suppressed rather than that of SiH 4 , because the threshold energy of SiH 2 Cl 2 is higher than that of SiH 4 , 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 SiH 4 .However, the resulting Si film structure was still porous with much f void , although f c-Si was over 80%.These findings originated from the excessive dissociation of both SiH 4 and H 2 in the plasma, which promoted the generation rate of not only of SiH 3 but also short life-time radicals, i.e., SiH and Si.On the other hand, f a-Si was still more in the μc-Si:H:Cl films than in the μc-Si:H films with less f void .

Fig. 21 .
Fig. 21.Schematic of μc-Si:H and μc-Si:H:Cl network termination of dangling bond by efficient supply of SiH x Cl y is accelerated with the abstraction of H and Cl as HCl at the depleted growing surface.The deposition study was performed at two different T s s of 250 and 400˚C.Figure 22 demonstrates N s s and deposition rates of μc-Si:H:Cl films fabricated at different Fr(SiH 2 Cl 2 ) at T s s of 250 and 400˚C, respectively.The high deposition rate of 40 Å/s has been achieved with increasing Fr(SiH 2 Cl 2 ) up to 20 sccm at T s s of 400˚C and 250 0 C respectively.The N s was almost independent of Fr(SiH 2 Cl 2 ) at T s of 250˚C, whereas the N s was markedly decreased at T s of 400˚C.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 SiH 2 Cl 2 at T s of 400˚C.The N s s were decreased systematically with increasing SiH 2 Cl 2 at both T s s to 4x10 15 cm -3 at 15 sccm of SiH 2 Cl 2 .

3. 5
XRD and Raman spectra of microcrystalline silicon thin-film fabricated from SiH 2 Cl 2 and SiH 4 The XRD diffraction patterns and the Raman spectrum of the μc-Si:H:Cl film fabricated at T s of 250 °C and 400 °C with increasing SiH 2 Cl 2 flow rates are shown in Fig 23 and Fig. 24.The XRD and Raman study of μc-Si:H:Cl films fabricated at T s 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 crystallinity of I c /I a :5-6 were obtained.

Fig. 23 .
Fig. 23.XRD and Raman spectra of the μc-Si:H:Cl films fabricated at different flow rate of SiH 2 Cl 2 at T s of 250˚C.3.6 Photoelectrical properties of a-Si:H:Cl and μc-Si:H:Cl films Fig. 25 shows the relation between the dark and photo conductivities for the μc-Si:H:Cl films fabricated by increasing the SiH 2 Cl 2 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/cm 2 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.26shows the activation energies for the μc-Si:H:Cl films fabricated by increasing the SiH 2 Cl 2 flow rate at the substrate temperatures of 250 °C and 400 °C The activation energies of electrical conductivity were 0.40-0.80eV, suggesting that both a-Si:H:Cl and µc-Si:H:Cl films were intrinsic semiconductor films.

Fig. 24 .
Fig. 24.XRD and Raman spectra of the μc-Si:H:Cl films fabricated at different flow rate of SiH 2 Cl 2 at T s of 250 and 400˚C.

Fig. 25 .
Fig. 25.Dark and photo conductivities for the μc-Si:H:Cl films as a function of SiH 2 Cl 2 flow rate at the substrate temperatures of 250 °Cand 400 °C4.Preliminary results of p-i-n structure μc-Si:H:Cl thin-film solar cellsThe preliminary result of Si thin-film solar cells using μc-Si:H:Cl thin-film fabricated by the high-density microwave plasma (MWP) of a SiH 2 Cl 2 -H 2 mixture are shown here.High-rate grown μc-Si:H:Cl thin-films were applied to p-i-n structure Si thin-film solar cells as intrinsic absorption layer.The solar cell was fabricated using a single chamber system.The structure of the solar cell TCO/ZnO:Al/p-i-n/ZnO:Al/Ag is as shown in Fig.27.After the

Fig. 27 .
Fig. 27.The structure of the p-i-n solar cell

Fig. 28 .
Fig. 28.(a) Photocurrent-voltage characteristics under AM 1.5 exposure condition and (b) QE spectra under -1V biased conditions of Si thin-film solar cells using a 1µm-thick μc-Si:H:Cl layer by high-density microwave plasma source.

Fig. 28
Fig.28illustrates photocurrent -voltage characteristics for Si:H:Cl thin-film solar cells under 100 mW/cm 2 white light exposure.Fig.28ashows the I-V characteristics for the cell using μc-Si:H:Cl films fabricated at 20 Å/s by the high-density microwave plasma CVD of

Table 3 .
Typical deposition conditions for p, i and n layers

Table 4 .
Typical deposition condition for ZnO:Al and Ag layers