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Applied Surface Science 248 (2005) 113–117

Compositional, structural and optical properties of Si-rich

a-SiC:H thin films deposited by ArF-LCVD

E. Lopez a,*, S. Chiussi a, U. Kosch a, P. Gonzalez a, J. Serra a, C. Serra b, B. Leon a

a Dpto. Fısica Aplicada, Universidad de Vigo, Campus Lagoas-Marcosende, 36310 Vigo, Spainb CACTI, Universidad de Vigo, Campus Lagoas-Marcosende, 36310 Vigo, Spain

Available online 11 April 2005

Abstract

Silicon-rich amorphous hydrogenated silicon carbon (a-SiC:H) films with C content up to 23% have been grown on Si and

Corning glass substrates using ArF laser induced chemical vapor deposition (ArF-LCVD). This technique allows tailoring film

composition by controlling deposition parameters such as precursor gas mixture (disilane and ethylene diluted in helium) and

substrate temperature (180–400 8C). The influence of both parameters on composition and bonding were studied by Fourier

transform infrarred (FTIR) and X-Ray photoelectron spectroscopy (XPS). The optical gap of these semiconductors deposited at

250 8C varied from 1.6 to 2.4 eV and was determined by UV–vis spectroscopy. An additional analysis by profilometry and

atomic force microscopy (AFM) have been done for determining the deposition rate and the roughness (rms < 6 nm) of the films

as well as their surface morphology.

# 2005 Elsevier B.V. All rights reserved.

PACS: 42.70.H; 33.80.G; 81.15; 61.43 D; 78.30.E; 78.20; 61.16C; 33.60

Keywords: LCVD; a-SiC:H; Bandgap tailoring; Thin films; FTIR; XPS

1. Introduction

Hydrogenated amorphous silicon–carbon alloys

have gained technological importance as materials for

photovoltaics, thin film transistors and optoelectronic

devices due to the possibility of controlling their

bandgap and refractive index by changing the alloy

* Corresponding author. Tel.: +34 986 812216;

fax: +34 986 812201.

E-mail address: fani@uvigo.es (E. Lopez).

0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved

doi:10.1016/j.apsusc.2005.03.011

composition and the hydrogen content. Other promi-

nent properties such as mechanical strength, chemical

stability and irradiation resistance at high tempera-

tures make this material attractive for heat-resistant

coatings in metallurgy and as passivation layers for IC

devices [1].

Plasma enhanced chemical vapor deposition

(PECVD) [2–4], sputtering [5,6] and photo chemical

vapor deposition (Photo-CVD) [7–10] are low-

temperature processes used for producing SiC alloys.

Photo-CVD is characterized by its ability to excite

.

E. Lopez et al. / Applied Surface Science 248 (2005) 113–117114

precursor molecules very selectively and to introduce

fewer impurities into the films than sputtering or

plasma CVD. This applies in particular to laser

induced CVD (LCVD) because its monochromaticity

allows a selective excitation and better control by

products photolysis. Other advantages of LCVD are

depositing onto small or large selected regions using

masks or lens and the possibility of single chamber

processing. It is, thus, compatible with other processes

such as excimer laser crystallisation (ELC) for

obtaining nano-, micro- or polycrystalline coatings

[11] and pulsed laser induced epitaxy (PLIE) for

producing heteroepitaxial layers [12].

The aim of this work was to study the composi-

tional, structural and optical properties of carbon

deficient a-SiC:H films deposited by ArF-excimer

laser induced CVD and their dependence on both the

precursor gases flow and substrate temperature.

2. Experimental

Various Si-rich amorphous hydrogenated SiC (a-

SiC:H) films were deposited at a low substrate

temperature by ArF-LCVD in the parallel configura-

tion with the substrate above the beam. Simultaneous

deposition onto Corning (7059) and on Si (1 0 0) was

performed.

The experimental deposition set-up consisted of

a hybrid lab made stainless steel UHV-HV chamber

at a base pressure of 0.03 mPa connected to a gas

supply handling system described previously [13].

Different disilane (Si2H6) and ethylene (C2H4)

mixtures diluted in helium were introduced in the

near vicinity of the substrate at a constant total

pressure of 1.2 kPa and a substrate temperature of

250 8C. The precursor gases flow rates, regulated

with mass flow controllers, were varied from 0.25 to

2 sccm for Si2H6 and from 0.5 to 10 sccm for C2H4,

depending on the desired film composition. The

reactive gases were photolitically decomposed by

193 nm ArF-Excimer laser (Lambda Physik LPX

220i) radiation at constant laser power of 5 W and a

power density of 0.7 W/cm2.

An additional study of the influence of the substrate

temperature Ts (180, 250, 320 and 400 8C) on the film

properties was performed for samples deposited at

1.2 kPa using 1 sccm of Si2H6 and 2 sccm of C2H4.

The thickness of the coatings was characterised by

profilometry (Dektak3ST-Veeco) and the composition

of the films was determined by X-Ray Photoelectron

Spectroscopy (XPS; Escalab 250iXL-VG Scientific)

using monochromatic AlKa radiation at 1486.92 eV.

The infrared spectra of samples deposited on Si

(1 0 0) were measured in the range 400–4000 cm�1 by

a FTIR spectrometer (MB100-Bomem) in order to

obtain information on silicon and carbon atoms

bonded to hydrogen and the dependence of these

bonds on the substrate temperature.

UV–vis absorption spectra (8452A Diode Array

Spectrophotometer, Hewlett Packard) of the samples

grown on Corning 7059 at 250 8C allowed us to

evaluate the optical gap.

3. Results and discussion

Experiments made at 250 8C and 1.2 kPa by

changing precursor gases concentrations led to

adherent smooth SiC films (rms < 3 nm) with

deposition rates ranging between 1.4 and 3.9 nm/

min (Fig. 1a). It was observed an enhancement of

deposition rate with increasing Si2H6 flow from 0 to

1 sccm for all C2H4 flows. However for 1.5 or 2 sccm

of Si2H6 lower deposition rates were obtained in

samples made with 1 or 2 sccm of C2H4.

XPS and FTIR spectroscopy have been performed

to find the composition and types of bonding existing

in the films. The intensities of the XPS peaks allowed

us to determine the carbon content in the coatings and

its flow rate dependence (Fig. 1b). The XPS analysis of

the film surfaces shows a peak with a binding energy

of 285 eV related to hydrocarbon due to air exposure.

This peak, that overlaps with the one corresponding to

graphite C1s (284.4–284.8 eV), disappears after

sputtering a 2 nm thick surface layer with Ar and

then the C1s signal is shifted approximately by 2 eV.

This binding energy at 283 eV corresponds to carbon

C1s bonded to silicon (Si–C).

The presence of Si–C bonds has been confirmed by

examining the Si2p transition too. The deconvolution

of the Si2p peak allowed us to identify the contribution

of Si–C bond at 100.5 eVand of Si–Si bond at 99.3 eV.

A small peak at 103 eV, which arises from silicon

bonded to oxygen in SiO at 101.0 eV, in Si2O3 at

102.1 eV and in SiO2 at 103.4 eV was observed. It

E. Lopez et al. / Applied Surface Science 248 (2005) 113–117 115

Fig. 1. Deposition rate (a) and C content (b) dependence on Si2H6

flow when the C2H4 flow is 1 sccm (&), 2 sccm (*) or 4 sccm (~),

for a-SiC:H films deposited at 250 8C and 1.2 kPa.

Fig. 2. FTIR absorption spectra of a-SiC:H samples deposited onto

silicon at 1.2 kPa and 250 8C with a fixed Si2H6 flow of 1 sccm and

different C2H4 flows: (a) 10 sccm, (b) 4 sccm (c) 2 sccm (d) 1 sccm

and (e) 0.5 sccm.

disappears after removing a 5 nm thick native oxide

layer.

The O1s signal around 531.6 eV for the sputtered

samples deposited at 180 8C evidences the presence of

O–H bonds.

FTIR studies confirmed the presence of Si, C and H

in the films. Typical infrared spectra of samples

prepared at 250 8C and 1.2 kPa with various ethylene

flows and, thus, with different carbon concentration

are shown in Fig. 2.

The weak absorption found in the 2900–3000 cm�1

range is assigned to the overlapping of the symme-

trical and asymmetrical stretching of the C–H bond in

the CH3 and CH2 groups [14–16]. The band at 1900–

2000 cm�1 is associated with the Si–H stretching

mode in a SiHx group [17] and the shoulder observed

at 2100 cm�1 can be attributed to the Si–H stretching

mode for SiH in voids in a (SiH2)n group [18].

Between 1250 and 1400 cm�1 a weak band corre-

sponding to the overlapping of the symmetrical and

asymmetrical bending of the CH3 group in Si–CH3

or C–CH3 was found. Near 1040 cm�1 the Si–O

stretching mode appears overlapped with the peak

corresponding to CHn rocking and/or wagging

vibrations in a Si–CHn group [14–16] (1000 cm�1).

The Si–C stretching mode is assigned to the peak at

680 cm�1 [2,14,15] and Si–CH3 rocking or wagging

vibrations appear at 780 cm�1 [14–16,18]. Si–H

related modes in the low wavenumber range appear

near 840 cm�1 for (SiH2)n or SiH2 bending vibrations

[17] and around 640 cm�1 for SiH or SiH2 group

rocking or wagging [15–16,18].

In Fig. 2 we noticed that, as the C2H4 flow increases

from 0.5 to 10 sccm and thus more C content occurs in

the films, the maxima of the Si–H absorption peaks

shift to higher wavenumber values, as observed

previously in a-SiC:H samples deposited by other

methods [6,19]. This shift can be provoked by the

bonding of Si to a more electronegative element like C

[15] or by the formation of more voids [6].

From the FTIR results the structure of silicon rich

a-SiC:H films deposited with LCVD at 250 8C can be

described as a disordered a-Si network in which H

atoms are mainly forming voids as well as Si–H or

E. Lopez et al. / Applied Surface Science 248 (2005) 113–117116

Table 1

Deposition rate (nm/min), C content (%at.) and surface roughness

(nm) of some a-SiC:H films deposited with 1 sccm of Si2H6 and

2 sccm of C2H4 at 1.2 kPa for different substrate temperatures

Ts (8C) Deposition rate

(nm/min)

C content (%at.) rms (nm)

180 6.2 6.1 5.4

250 3.6 9.3 2.3

320 6.6 1.5 3.9

400 12.6 1.1 0.8

SiH2 species. This is not consistent with the expected

H preferential bonding to C due to the larger bonding

energy for C–H than for Si–H [5].

The absorption coefficient a has been evaluated as a

function of photon energy in the range of 190–820 nm

for the samples deposited at 1.2 kPa and 250 8C. From

these data the optical bandgap EG can be determined by

extrapolating linearly, according to the Tauc formula,

(ahn)1/2 = B (hn – Eg). We also measured the E04 value

defined as the energy at which the absorption co-

efficient a is 104 cm�1. The values for EG are 1.6–

2.2 eVand for E04 1.6–2.4 eV. A theoretical work made

by Soref [20], using an interpolation technique,

established that the energy gap of crystalline SiC

alloys increases with the C concentration since both,

ordered SiC and pure carbon (diamond) have larger

band gaps than Si. However, the bandgap of our

amorphous samples deposited with 1 sccm of C2H4

seems to be influenced by the precursor gas flows

(Fig. 3). In fact, as the Si2H6 gas flow increases, higher

EG values were obtained in spite of the fact the C

content of the film diminishes. Taking into account that

the substrate temperature (250 8C) and the ethylene

flow (1 sccm) are low, more SiH and SiH2 species or

voids can be formed, favouring the hydrogenation of

the film and thus increasing the optical bandgap.

The influence of the substrate temperature was also

studied in samples deposited at 1.2 kPa with using

1 sccm of Si2H6 and 2 sccm of C2H4 (Table 1). An

increase of the Ts from 250 to 400 8C, at 1.2 kPa and

Fig. 3. Optical band gap dependence on Si2H6 flow for a-Si1�xCx:H

films deposited at 250 8C and 1.2 kPa with 1 sccm (~) or 4 sccm

(&) of C2H4. C content is also shown (%at.).

constant gas flow ratio, led to higher deposition rates.

However, the sample deposited at 180 8C shows an

abnormal high growth rate. This effect, often observed

in CVD processes at low substrate temperatures, is

attributed to the formation of more porous hydrogen

rich material with a large amount of voids which

increases the film thickness. AFM analysis of the films

surface shows that the sample deposited at 180 8C is

the roughest one.

XPS analysis reveals a diminishing of the carbon

concentration in the film as the substrate temperature

increases, hinting that C incorporation is favoured at

Ts � 250 8C.

A variation of the Ts for a particular gas flow

mixture affected also the FTIR spectrum (Fig. 4),

especially the Si–H vibration modes. The fast decrease

Fig. 4. FTIR absorption spectra of a-SiC:H samples deposited with

1 sccm of Si2H6 and 2 sccm of C2H4 at 1.2 kPa and different

substrates temperatures (a) 180 8C, (b) 250 8C, (c) 320 8C and (d)

400 8C.

E. Lopez et al. / Applied Surface Science 248 (2005) 113–117 117

in intensity of the 1040 and 2100 cm�1 peaks is

consistent with the lower oxygenation of the films

when Ts increases as well as the preferential formation

of voids and ‘‘dihydride bonding’’ (SiH2) at

Ts � 250 8C. The slightly increase in the intensity

of the Si–C stretching mode suggests the presence of

more unhydrogenated Si–C bonds as Ts increases from

180 to 400 8C.

4. Conclusions

Smooth amorphous hydrogenated SiC films with

carbon content up to 23% were produced by LCVD

using an ArF laser irradiating parallel to Corning

(7059) and Si (1 0 0) substrates at 250 8C and 1.2 kPa.

Both, the uniform composition of the films (Si, C and

H) and the presence of Si–C, C–H and various types of

Si–H bonds were found by XPS and FTIR for different

gas flow mixtures and substrates temperatures.

Although C incorporation is favoured at Ts � 250 8C,

8C, more unhydrogenated Si–C bonds were detected at

Ts > 250 8C. Optical absorption studies of amorphous

samples deposited at 250 8C revealed the influence of

the precursor gases flow mixture on the bandgap,

showing that ArF-LCVD gives wide-bandgap semi-

conductors at low substrate temperature.

Acknowledgements

This work has been partially supported by EU as

well as by Spanish contracts and grants: HA1999-

0106, MAT2000-1050, MAT2003-04908, XUGA32

107BB92DOG211, UV62903I5F4, PGIDT01PX130

301PN and PR405A2001/35-0.

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