Thin Solid Films 430(2003) 33–36

0040-6090/03/$ - see front matter� 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0040-6090(03)00128-7

Effect of hydrogen radical on growth ofmc-Si in hetero-structured SiCxalloy films

Takashi Itoh *, Kazunori Fukunaga , Takao Fujiwara , Shuichi Nonomuraa, a a b

Department of Electrical Engineering, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japana

Environmental and Renewable Energy Systems, Graduate School of Engineering, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japanb

Abstract

The changes of the crystallinity ofmc-Si phase are studied in samples deposited with hydrogen dilution ratio, HySiH , from2 4

9.0 to 19.0 by hot-wire CVD(Cat-CVD). In the samples deposited at filament temperature,T , of 18508C, the crystalline fractionf

and the crystallite size ofmc-Si phase increased with increasing the HySiH . The carbon content, Cy(SiqC), was almost2 4

constant. In the XRD patterns, the intensity of Si(1 1 1) peak decreased and that of Si(2 2 0) peak increased with increasing theH ySiH . In the samples deposited atT of 2100 8C with H ySiH over 11.4, themc-Si phase was not formed and the Cy(Siq2 4 f 2 4

C) increased. The growth mechanism ofmc-Si in hetero-structured SiC alloy films is discussed.x

� 2003 Elsevier Science B.V. All rights reserved.

Keywords: Hetero-structured SiC alloy; Crystallinity ofmc-Si; Hydrogen radical; Etchingx

1. Introduction

In order to increase the efficiency of hydrogenatedamorphous silicon(a-Si:H) solar cells, B-doped hydro-genated amorphous silicon–carbon(a-SiC :H) alloy andx

B-doped hydrogenated microcrystalline silicon(mc-Si:H) are used as a p-type window layer material forp–i–n solar cellsw1,2x. However, the resistivity of a-SiC :H is large even if B-doped and the optical bandx

gap ofmc-Si:H is narrow. Hetero-structured SiC alloyx

films, which are deposited by hot-wire chemical vapordeposition (CVD), which is also called ‘Cat-CVD’,have heterogeneous structure composed of the a-SiC :Hx

phase as optical transport phase and themc-Si phase aselectrical transport phasew3,4x. Therefore, B-doped het-ero-structured SiC alloy films are wide optical bandx

gap and low resistive material. Using B-doped hetero-structured SiC alloy film as a p-type window layerx

material, therefore, it is expected that the efficiency ofa-Si:H p–i–n solar cell may improve. The crystallinityof the mc-Si phase would affect the resistivity and theoptical absorption of this material. Therefore, the growthcontrol of mc-Si phase in the hetero-structured SiCx

alloy films is important for the use as a window layer

*Corresponding author. Tel.:q81-58-293-2680; fax:q81-58-230-1894.

E-mail address: itoh@cc.gifu-u.ac.jp(T. Itoh).

material. The growth mechanism and the control ofcrystallinity of mc-Si phase in this material, however,have not been understood yet. The crystallinity ofmc-Si:H films depends on the hydrogen dilution ratiow5,6x.Therefore, the hydrogen dilution ratio would be one ofthe important deposition conditions for the growth con-trol of the mc-Si phase in the hetero-structured SiCx

alloy films.In this report, the hydrogen dilution effect on the

crytsallinity of themc-Si phase and the carbon contentin the hetero-structured SiC alloy films is studied. Thex

growth mechanism ofmc-Si in hetero-structured SiCxalloy films is also discussed.

2. Experimental aspects

Samples were deposited by hot-wire CVD using 10%SiH diluted in H , 100% CH and 100% H . The4 2 4 2

hydrogen dilution ratio, HySiH , was varied from 9.02 4

to 19.0. The SiHyCH gas ratio was 0.15 and the total4 4

pressure was 1 Torr. The tungsten(W) or carbon(C)filament was located 1.5 cm from the substrate holder.The filament temperature,T , was 1850 or 21008C andf

was measured by a radiation thermometer. The substratetemperature was 2508C and it increased when thesubstrates were exposed to the hot filament. Corning7059 glass for Raman spectroscopy and X-ray diffrac-

34 T. Itoh et al. / Thin Solid Films 430 (2003) 33–36

Fig. 1. Raman spectra for SiC alloy films with various HySiH .x 2 4

Upper four curves withT s18508C and lower three curves withT sf f

2100 8C. Fig. 3. Dependence of crystalline fraction ofmc-Si phase onH ySiH .2 4

Fig. 2. XRD patterns for SiC alloy films with various HySiH . Upperx 2 4

four curves withT s1850 8C and lower three curves withT s2100f f

8C. Fig. 4. Dependence of crystallite size ofmc-Si phase on HySiH .2 4

tion (XRD), and crystalline Si for XPS were used assubstrates. The film thickness of the samples was from0.89 to 1.30mm.The crystallinity ofmc-Si phase in the samples was

investigated by Raman spectroscopy and XRD. Ramanspectra were measured using an Ar ion laser at aq

wavelength of 514.5 nm(RENISHAW RAMASCOPE-2000). The crystalline fraction ofmc-Si phase wasestimated as the ratio(I qI )y(I qI qI ), whereI , Ic i c i a a i

and I correspond to the integrated intensities of Ramanc

peaks near 480, 510 and 520 cm , respectivelyw5x.y1

The crystallite size ofmc-Si phase was estimated fromthe Si(1 1 1) XRD peak using Scherrer’s formula. Thecarbon content, Cy(SiqC), was obtained from theintegrated intensity of C-1s and Si-2p XPS peaks usinga SHIMADZU ESCA-850 system.

3. Results and discussion

Figs. 1 and 2 show the Raman spectra and XRDpatterns for the samples deposited with various Hy2

SiH , respectively. In the samples deposited atT s18504 f

8C and those deposited with the HySiH below 11.4 at2 4

T s2100 8C, the Raman and XRD peaks of crystallinef

Si (c-Si) were observed. In those deposited with theH ySiH 011.4, however, the peaks of c-Si were not2 4

observed. Based on above results, every sample includedthe mc-Si phase except those deposited with Hy2SiH 011.4 atT s21008C. In the XRD patterns for the4 f

samples deposited atT s1850 8C, the intensity of thef

Si(1 1 1) peak decreased and that of the Si(2 2 0) peakincreased with increasing the HySiH . Inmc-Si:H films,2 4

this tendency was also observedw5x. This result showsthat themc-Si phase can have(2 2 0) preferential ori-entation in hetero-structured SiC alloy films depositedx

with large H ySiH . Figs. 3 and 4 show the dependence2 4

of the crystalline fraction and the crystallite size of themc-Si phase on the HySiH , respectively. In the samples2 4

deposited atT s1850 8C, the crystalline fraction andf

the crystallite size ofmc-Si phase increased with increas-ing the H ySiH . These results indicate that the crystal-2 4

35T. Itoh et al. / Thin Solid Films 430 (2003) 33–36

Fig. 5. Dependence of Cy(SiqC) on H ySiH .2 4

Fig. 6. Dependence of deposition rates ofmc-Si:H and SiC alloy filmsx

on H ySiH .2 4

linity of mc-Si phase is controlled by hydrogen dilutionratio. For mc-Si:H films, the crystallinity depends onthe hydrogen dilution ratio and the hydrogen radicalplays an important rolew5,6x. Based on the above results,the role of the hydrogen radical may also be importantin the growth of themc-Si phase in the hetero-structuredSiC alloy films. Fig. 5 shows the dependence of thex

Cy(SiqC) on the HySiH . In the samples deposited at2 4

T s1850 8C, the Cy(SiqC) was almost constant. Inf

the samples deposited atT s2100 8C, the Cy(SiqC)f

increased with the HySiH , particularly above 10.11.2 4

The growth mechanism ofmc-Si phase in hetero-structured SiC alloy films is now considered. Inmc-x

Si:H films, mc-Si nucleation occurs in the a-Si phase(Si–Si network) w7x. In SiC alloy films, therefore,mc-x

Si phase would grow after a Si–Si network is formed.Fig. 6 shows the dependence of deposition rate ofmc-Si:H films on the HySiH . The mc-Si:H films were2 4

deposited with 10% SiH diluted in H and 100% H4 2 2

by hot-wire CVD. The HySiH was varied from 9.0 to2 4

32.3. The other deposition conditions were the same asthose for hetero-structured SiC alloy films. For com-x

parison, the deposition rates of the hetero-structuredSiC alloy films are also plotted. In the hot-wire CVDx

process atT s2100 8C with H ySiH 011.4, mc-Si:Hf 2 4

films were not deposited. The etching rate of the Si–Sinetwork increases with increasing the density of thehydrogen radical. The density of hydrogen radicals islarger in the hot-wire CVD process at higherT withf

larger H ySiH w8,9x. When the etching rate is larger2 4

than the deposition rate of the Si–Si network,mc-Si:Hfilms are not deposited. In the process atT s2100 8Cf

with the H ySiH 011.4, therefore, the etching rate2 4

would be larger than the deposition rate of the Si–Sinetwork. The deposition rate of the hetero-structuredSiC alloy films also shows the same tendency(Fig. 6).x

In the process in whichmc-Si:H films were not depos-ited, the SiC alloy films also did not include themc-Six

phase. Therefore, this would also be caused by theetching effect of the hydrogen radical. a-SiC :H is alsox

etched by the hydrogen radical. However, the etchingrate of 4.4 Ays formc-Si:H film with crystalline fraction˚of approximately 65% was larger than that of 1.6 Ays˚for a-SiC :H alloy films with Cy(SiqC) of approxi-x

mately 0.5. Here, the etching rates were determinedusing the decrease of the film thickness by the hydrogenradical treatment. The samples were treated by hydrogenradicals for 4 min using hot-wire CVD system. Theconditions of the hydrogen radical treatment were asfollows: T s2100 8C, H gas pressures1 Torr. Thef 2

other conditions were the same as those used in thedeposition. The etching rate of c-Si is much smallerthan that of a-Si:Hw10x. Themc-Si:H film is composedof c-Si and a-Si phases. Therefore, the above resultmeans that the etching rate of a-Si is larger than that ofa-SiC :H. This result agrees with the binding energy,x

where the binding energies of Si–Si and Si–C bonds are326.8 and 451.5 kJymol, respectivelyw11x. Based onthe above results, the growth ofmc-Si phase can beexplained as follows. In deposition with a suitabledensity of the hydrogen radical, the deposition rate islarger than the etching rate of Si–Si network and themc-Si phase grows. As the result, the sample includesthe mc-Si phase. In the deposition with a much largerdensity of the hydrogen radical, the deposition rate issmaller than the etching rate of Si–Si network. Then,Si–Si network is not formed andmc-Si phase does notgrow. However, a Si–C network is formed, since theetching rate of a-SiC :H is smaller than that of a-Si:H.x

As the result, the sample does not include themc-Siphase and would show a larger carbon content.

4. Summary

The hydrogen dilution effect on the crystallinity ofthemc-Si phase in the hetero-structured SiC alloy filmsx

36 T. Itoh et al. / Thin Solid Films 430 (2003) 33–36

has been studied. The role of the hydrogen radical isimportant in the growth ofmc-Si phase in hetero-structured SiC alloy films. We demonstrate the controlx

of the crystallinity ofmc-Si phase in hetero-structuredSiC alloy films. The crystallinity of themc-Si phasex

increased with increasing hydrogen dilution ratio. How-ever, the growth ofmc-Si phase is prevented in thedeposition with much larger hydrogen dilution ratio.Then, the carbon content was large. These effects canbe explained by the etching effect of the hydrogenradical.

Acknowledgments

The authors thank Prof. H. Matsumura, Dr A. Masudaand Prof. S. Nitta for their advice and discussions andDr N. Yoshida and Mr H. Kondo for their help. Thiswork was partially funded by NEDO as part of the NewSunshine Program of MITI, the Naito Research Grant,Research Foundation for the Electrotechnology of Chubuand the Mazda Foundation Research Grant.

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