Thin Solid Films, 173 (1989) 253-262

PREPARATION AND CHARACTERIZATION 253

PHYSICAL PROPERTIES AND STRUCTURE OF CARBON-RICH

a-SiC:H FILMS PREPARED BY r.f. GLOW DISCHARGE

DECOMPOSITION

KENJI YAMAMOTO*, YOSUKE ICHIKAWA, NOBORU FUKADA, TAKEHISA NAKAYAMA AND

YOSHIHISA TAWADA

Central Research Laboratories. Kanegajiichi Chemical Industry Co. Ltd.. Yoshidacho. Hyogo-ku. Kobe 652 (Japan)

(Received July 26, 1988; revised January 17, 1989; accepted March 8, 1989)

Carbon-rich a-SiC:H films were prepared by glow discharge decomposition from SiH, and CH,. The deposition rate, hydrogen content, optical band gap and microhardness were investigated for various carbon contents. In particular, carbon atom coordination was investigated by solid-state r3C magic angle spinning nuclear magnetic resonance measurements of the sp’ and sp3 bonding sites. Unsaturated character (sp* carbon) appears clearly in the film of composition a-S&,,&,,, at about 130 ppm. Film hardness and optical band gap correlate with both the hydrogen content and the fraction of tetrahedral(sp3) vs. graphitic(sp’) bonding. It was also observed from annealing treatment experiments that a change of network structure takes place, from high density to low density, at a carbon content of about 0.63.

1. INTRODUCTION

Hydrogenated amorphous silicon carbide (a-SiC:H) thin films are receiving increasing attention for electric and photovoltaic applications. Tawada et al.’ tried to improve the efficiency of p-i-n a-Si solar cells and found that a-SiC:H films have good valence-electron controllability.

There are many reports on a-SiC:H films prepared by glow discharge or sputtering methods ~ . ’ I1 However, most of the reports are concerned with a-SiC:H of low carbon content and there are few reports on carbon-rich a-SiC:H films. Anderson and Spear2 showed that the optical band gap (E,,,) of carbon-rich a-SiC:H would be altered by changing the film composition and showed a maximum at a composition of Si0,32C0,68. A similar tendency was reported by Katayama et al.’ 2. They showed that the curve of binding energy of Cls us. carbon content x has a kink at a carbon content of 0.550.6 and at this content, the chemical bonding states of the carbon atoms change. Recently there have been many reports on a-C:H films because of their unique combination of useful properties such as extreme hardness, chemical inertness, and optical transparency (especially in the IR region).

* Present address: Center of Materials Research, Stanford University, Stanford, CA 94305-4045, U.S.A.

0040-6090/89/$3.50 0 Elsevier Sequoia/Printed in The Netherlands

254 K. YAMAMOTO et d.

In the present study we have prepared carbon-rich a-SiC:H films by r.f. glow discharge decomposition and investigated their physical structure and properties. In particular, in order to determine the structure of a-SiC:H, solid r3C nuclear magnetic resonance (NMR) was used to investigate the carbon atom coordination.

2. EXPERIMENTAL PROCEDURE

Deposition of a-SiC:H films from pure SiH,, H, and CH, glow discharge was performed in a conventional chemical vapour deposition (CVD) apparatus shown in Fig. 1. The r.f. power at a frequency of 13.56 MHz was applied through a matching network. The total reactant gas flow rate (Q(SiH,)+Q(CH,)) into the discharge chamber was maintained at 90 standard cm3 min-‘. H, gas was always maintained at 100 standard cm3 min- 1 and was introduced to stabilize the discharge. The gas

composition is represented by the carbon volume fraction X = (Q(CH,)/Q(SiH,) + Q(CH,)), where Q represents the flow rate. Total gas pressure was set at 1.5 Torr. The r.f. power density was 0.1 W cm -* throughout the study. The d.c. self-bias

voltage on the r.f. electrode was measured and these values were almost constant (0 V) throughout the study. We used single-crystal silicon and Corning 7059 glass as substrates.

OES

Substrate

Fig. 1. Schematic diagram of plasma CVD apparatus.

An optical spectrometer was attached to the reactor to measure the emission intensities of species in the glow discharge.

The microhardness of films of more than 2 urn thickness was measured using a micro-Vickers-hardness apparatus. We used mainly 20 g load for the measurements.

CARBON-RICH a-Sic : H FILMS 255

Determination of the relative hydrogen content of the films was carried out by Fourier transform IR spectrometry. ’ % NMR data were obtained at 75 MHz on a Varian spectrometer. Cross-polarization was achieved with a contact time of 2 ms and magic angle spinning was used to achieve resolved carbon spectra. Sample spinning speeds in excess of 3 kHz were used. The substrate used to collect samples for NMR measurements was SUS 304 plate bonded to the r.f. electrode and the sample weight was about 300 mg. Some flakes of a-SiC:H were broken down in the tube to improve the NMR filling factor. The carbon content in the films was determined by X-ray photoelectron spectrometry (XPS).

3. RESULTS AND DISCUSSION

3.1. Deposition rate and deposition environment

The relationship between the deposition rate and the CH, gas fraction is shown in Fig. 2 for a substrate temperature of 300 “C. With increasing CH, fraction, the deposition rate decreases slightly and finally approaches zero at the CH, fraction approaching 1 .O.

In order to investigate the deposition process of a-Sic: H films from SiH,-CH,, optical emission spectroscopy (OES) was carried out. The emission intensity of CH(4314), SiH(4140) and Hcr(6500) and the emission intensity ratio HP:Ha are shown as functions of the CH, fraction in Fig. 3. With increasing CH, gas fraction, the emission intensity of Ha and SiH decreases, while the emission intensity of CH remains constant. The decrease in the emission intensity of SiH corresponds to the

0.6 0.7 0.8 0.9 1.0

CH4 fraction

-:

L

0.2

0.6 0.8 1.0

I I

0.6 0.8 1.0

CH4/( CH4+ SiH4)

Fig. 2. Relationship between film deposition rate and CH, gas fraction for a-SiC:H films deposited at

300 “C.

Fig. 3. Emission intensity of CH(4314), SiH(4140), Hu(6500) and emission intensity ratio of Hu:HP as a

function of CHI gas fraction.

256 K. YAMAMOTO et al.

decrease of SiH, gas in the glow discharge. The constant emission intensity means that the emission intensity of CH is not restricted to the flow rate of CH, but to the r.f. power (at high CH, fraction, the flow rate of CH, in the glow discharge is sufficient to allow for a high concentration of emitting species).

An important result obtained from Fig. 3 is a decrease in emission intensity of Ha. This change of emission intensity of Her with CH, fraction is ascribed to the dissociation ratio difference between CH, and SiH,. The CH, gas is more difficult to decompose in the glow discharge and this leads to a decrease in the deposition rate with increasing CH, fraction. (Eventually, excited hydrogen atoms H* are produced by several processes. Among these, excitation from H and H,, which are produced from the decomposition of H,, CH, and SiH,, is probable when energy is considered.) This result corresponds with that obtained by a previous worker’.

Another possible reason for the deposition rate dependence on CH, fraction is the low sticking coefficient of the CH, radical forming the a-SiC:H films at our deposition temperature. (Tachibana’ 3 and Perrinr4 showed that the sticking coefficient of CH, radicals decreases with increasing substrate temperature. In contrast, for the a-SiC:H deposition, Matsudars reported that the deposition rate does not depend on the substrate temperature and that the sticking coefficient of SiH, radicals is constant.) Figure 4 shows the relationship between the substrate temperature and deposition rate for a CH, fraction of 0.83. It is shown that the deposition rate decreases with increasing substrate temperature. From these results, it is concluded that both a low decomposition ratio of CH, and a low sticking coefficient of CH, radicals lead to the deposition dependence on the CH, fraction.

3.2. Film structure The carbon content in the films was determined by XPS using an Sic single

crystal as a standard specimenlO. The dependence of carbon content on the CH, fraction is shown in Fig. 5. The carbon content in the film increases gradually with increasing CH, fraction, as observed by previous workers”2v5,6*8*9.

0 100 200 300 400 0.7 0.8 0.9 1:O Substrate Temcwature (‘C ) CH4 fraction

Fig. 4. Relationship between substrate temperature and deposition rate for a CH, gas fraction of 0.83.

Fig. 5. Relationship between carbon content and CH, gas fraction for a-SiC:H films deposited at 300 “C.

The carbon content in the film was determined by XPS using an Sic single crystal as a standard specimen.

CARBON-RICH a-Sic : H FILMS 257

In the IR spectra there are no clear peaks attributed to the sp2 C-H stretching mode even for a high carbon content.

The concentrations of hydrogen atoms bonded to carbon and silicon atoms, (C-H) and (Si-H) respectively, were evaluated using the expression

CH=A&(K)/KdK

where U(K) is the absorption coefficient at a wave number K and A, is the value of the inverse absorption cross section. The A, value used here is 1.0 x 1021 cmm2 and 1.4 x 102r cm- 2 for C-H,, and Si-H, respectively”. The relative hydrogen contents of Si-H,, C-H,, and the total values are shown as a function of carbon content in Fig. 6. In our experimental conditions, the total hydrogen content in the films, which at high values of x consists entirely of C-H hydrogen, is almost constant until x z 0.63. It increases sharply at higher values of x, but is much smaller than those observed by previous workers 2,18 In general, the hydrogen content in . a-SiC:H strongly depends on the deposition conditions, especially on both substrate temperature and deposition rate.

A 20 - A

A q n

A

. At 10 -

n q q

; ‘I . n

n m 0 0.2 0.4 0.6 0.8 1.0

Carbon content

Fig. 6. Relative hydrogen contents of Si-H,, C-H, and total hydrogen content for a-SiC:H films deposited at 300°C. The inverse absorption cross sections used here are 1.0x 10zl cm-’ and

1.4 x 10” cm-’ for C-H, and Si-H, respectively.

3.3. Physicalproperties ofjilm The optical band gap (E,,,) was estimated from the intercept of a plot of (cl.hv)1’2

us. hv, where tl is the absorption coefficient and hv is the photon energy. Figure 7 shows the relationship between the optical band gap and the carbon content, before and after an anneal at 600 “C. At higher carbon content, Eop, decreases with increasing carbon content. This trend is similar to those observed by previous workers 2,5*11*17*18. The maximum value of Eopt obtained (CH, fraction at 0.83 and carbon content at 0.63) is as high as 2.4 eV. In Fig 7 the data of Anderson and Spear’ are also written. The shape of our data is seen to be close to that of Anderson and Spear2, especially for their films which were prepared at a substrate temperature of 800 K, even though our a-SiC:H was prepared at a substrate temperature of 300 “C.

Figure 8 shows the relationship between the square root of the B value, which is

258 K. YAMAMOTO et ai.

2.6 -

2.4 -

s 2.2-

1.4 . *erumcd6aPC .

1.2

o 600 2

t

. 2

.

m z . . s 400 - . e 9. z m $

1.

* 200 -

0. 0.2 0.4 0.6 0.6 1.0 0.0 0.2 0.4 0.6 0.8 1.0

Carbon content Carbon content

Fig. 7. Relationship between the optical band gap (E,,,) and carbon content for a-SiC:H films deposited

at 300 “C, before (0) and after (U) an anneal at 600°C. The solid and dashed lines show the data of

Anderson and Spear’ for films prepared at 500 K and 800 K respectively.

Fig. 8. Relationship between the square root of the B value and carbon content for a-SiC:H films

deposited at 300 “C.

the slope of the (c&v) us. hv plot, and the carbon content. A monotonic decrease in B value with carbon content can be seen. The low square root of the B value at high CH, fraction implies a strongly disordered amorphous structure and the existence of wide tail states in the energy band structures, which has also been mentioned by Saraie et al.“.

An accurate measurement of the hardness of a thin film can only be obtained if the depth of indent is appreciably less than the thickness of the film4. Generally, small loads are required to measure accurately the hardness of films but for a hard material, such as Sic, it can be obtained using a 20g load. Figure 9 shows the relationship between carbon content and hardness. With increasing carbon content, the hardness increases, passes through a maximum and finally decreases at higher carbon content. The hardness of single-crystal Sic with a 20 g load is also shown in

u) 4000

%

6 % I

2 2000

x 0

j_

0 I

0.5 1

Carbon Content

Fig. 9. Relationship between carbon content and hardness for a-SiC:H films deposited at 300°C. The

dotted area shows the hardness of single-crystal Sic measured with a load of 20 g.

CARBON-RICH a-Sic : H FILMS 259

Fig. 9. The maximum hardness of an a-SiC:H film (a-Si,,37C,-,63) is about 3000, similar to that of single-crystal Sic and diamond-like carbon films. The carbon content of a-SiC:H with maximum hardness coincides with that of the maximum value of E,,,.

3.4. Further investigation of$lm structure In order to explain the above results, further investigation of the film structure

was undertaken. The important problem in a quantitative understanding of the microstructure of carbon-rich a-Sic: H is the ratio of carbon atoms with tetrahedral coordination (sp3) to carbon atoms with trigonal coordination (sp2). Recently Kaplan et aLI9 reported an investigation of the fraction of sp3 vs. sp2 bonding in a-C:H films, prepared by both ion beam sputtering and glow discharge techniques, using solid state 13C NMR magic angle spinning nuclear resonance measurements. We measured some a-Sic: H films using solid state ’ 3C NMR magic angle spinning nuclear resonance and the result are shown in Fig. 10. The NMR spectrum of a-C:H film2’ deposited on the r.f. electrode is also shown in Fig. 10. The chemical shifts are centered at 120-140 ppm (sp2) and O-40 ppm (sp3) relative to the carbon signal from

I I I I I I I 200 100 0 -100

PPM

Fig. 10. 13C magic angle spectra of a-SiC:H films deposited with several carbon contents. Typical

chemical shift ranges of the various carbon types are also represented. The down-field peak at 130ppm is due to graphic carbon atoms while the up-field peak centered at 40ppm is due to tetrahedral carbon

atoms.

260 K. YAMAMOTO et ai.

tetramethylsilane. From the intensities of the two peaks in the NMR spectra it is possible to compare the relative contribution of threefold and fourfold carbon sites in the films. Inspection of Fig. 10 reveals that the unsaturated character (sp’ carbon) of a-SiC:H films causes a peak to appear in the film of composition a-Si,,25C0,75 at about 140ppm. In the film of a-Si,,,,C,,,, a broad sp2 carbon peak is observed while in the a-Si,,,,C,,,, film no sp2 peak is observed.

We believe that this change of coordination from fourfold to threefold causes the decrease of both E,,, and hardness.

A chemical shift to higher parts per million is observed in the sp3 carbon site (&30ppm) with increasing carbon content. This chemical shift is attributed to a dependence on the environment of the carbon. Typical chemical shift ranges of the various carbon types observed are represented by the horizontal bars in Fig. 11, where we show carbon bonding configurations of sp3 ((l)C (quaternary), (2) CH(methine) and CH,(methylene), (3) CH, (methyl), (4) Si,CH and Si,C) and sp2 ((5) C=, (6) CH=, CH2=, (7) Si-C=C). A chemical shift to higher parts per million at higher carbon content is observed, since the concentration of Si-C bonds decreases with increasing carbon content.

r Si-CH 1

I Si-C=C

200 100 0 -100 PPM From TMS

Fig. 11. Typical chemical shift ranges of the various carbon types relative to the carbon signal from

tetramethylsilane.

Even though an sp2 carbon site is clearly seen in the film with a carbon content of 0.75 (a-Si0,2&0,7J, the sp2 C-H stretching (3000 cm- ‘-3050 cm- ‘) mode was not observed in the IR spectra, as already mentioned. This result leads us to the conclusion that little sp* C-H exists in the film and the C=C bonds mainly exist as species bonded to silicon or carbon. (Since the sp2 C-H stretching absorption in the IR spectra was clearly observed in a-C:H films prepared by CsH6 glow discharge2’, there is no possibility that A, for sp* C-H is too weak to detect.)

Figures 12 (a) and(b) show the variation of XPS spectra of a-SiC:H films having various carbon compositions x. The C 1s and Si2p peak energies show similar trends. In our experiment, we could not observe a clear kink at around x = 0.550.6, as was observed by Katayama eta1.12 even though the sp2 carbon site is clearly observed in the NMR spectrum of a-Si,,2,C0,75 film.

The IR absorption changes of Si-H (stretching) and C-H (stretching) as a result of heat treatment were also examined in order to investigate the network

CARBON-RICH a-SiC:H FILMS

n Si 2p

90 130 270 320

(a) B.E. (eV)

(b) B.E. (eV)

Fig. 12. Variation of XPS spectra for a-SiC:H films having several carbon contents: (a) Si 2p spectra, (b)

C Is spectra.

structure in more detail. Figures 13 (a) and (b) show the relationship between carbon content and normalized hydrogen content after heat treatment at 500 “C or 600°C. The a-Si 0,3,C0,63:H film prepared with a CH, fraction of0.83, which possesses both maximum hardness and maximum Eopt, is the most stable film during the annealing treatment. This result may be explained by considering the network structure of the a-SiC:H film. When dense a-SiC:H films are formed, less hydrogen effusion can be observed. A change in the network structure takes place, from a high density to low

1.1 , I

5 0.9

= 8 0.8

5 0.7

% 0.6 P

1

0.0’ I I I ’ . ’ 0.0 - 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0

(4 Carbon content (‘4 Carbon content

Fig. 13. Relationship between carbon content and normalized hydrogen content after heat treatment at

500°C (m) and 600°C (0): (a) normalized hydrogen content bonded to Si (Si-H); (b) normalized

hydrogen content bonded to C (C-H); the hydrogen content is normalized to the value before the anneal.

262 K. YAMAMOTO et d.

density network, with increasing CH, fraction. The network structure of carbon- rich a-SiC:H (carbon content greater than 0.7) is a low density network. The changes in the E,,, after heat treatment shown in Fig. 7 correlate well with the above results.

4. CONCLUSION

Carbon-rich a-SiC:H films were prepared by glow discharge decomposition from SiH, and CH, at a substrate temperature of 300 “C. The main conclusion of this study may be summarized as follows.

(1) A maximum was observed in both the optical band gap and the hardness as a function ofcarbon content. The carbon content which gives the maximum value in each case coincides with a film composition of Si,,,,C,,,,. The value of maximum hardness is 3000 kg mm-2 which is almost the same as that of single-crystal Sic.

(2) The existence of sp2 bonding in a-SiC:H films has been investigated by solid state 14C NMR and the unsaturated character (sp2 bond) of carbon-rich a-SiC:H films was clearly observed for a film composition of Si0,25C,,75 at about 130 ppm in the NMR spectra.

(3) Annealing treatments show that a change in the network structure takes place, where the network goes from a high to a low density, in the region of carbon content of 0.63.

(4) A decrease in both Eopt and hardness in high carbon content films is caused by a changes from fourfold (sp3) to threefold (sp2) coordination and a change of network structure.

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