ARTICLE IN PRESS

Solar Energy Materials & Solar Cells 87 (2005) 343–348

0927-0248/$ -

doi:10.1016/j

�CorrespoUniversidade

Portugal. Tel

E-mail ad

www.elsevier.com/locate/solmat

Characterization of silicon carbide thin films andtheir use in colour sensor

S. Zhanga,b,�, L. Ranieroa, E. Fortunatoa, X. Liaob, Z. Hub,I. Ferreiraa, H. Aguasa, A.R. Ramosc,d, E. Alvesc,d, R. Martinsa

aDepartamento de Ciencia dos Materiais, Faculdade de Ciencias e Tecnologia, Universidade Nova de Lisboa

and CEMOP/UNINOVA, Campus da Caparica, 2829-516 Caparica, PortugalbState Key Laboratory for Surface Physics, Institute of Semiconductors & Center for Condensed Mater

Physics, Chinese Academy of Sciences, Beijing 100083, ChinacITN – Nuclear and Technological Institute, EN 10, 2686-953 Sacavem

dCFN da Universidade de Lisboa, Av. Prof. Gama Pinto 2, 1649-003 Lisboa, Portugal

Received 15 May 2004; received in revised form 16 June 2004; accepted 21 June 2004

Available online 2 December 2004

Abstract

A series of hydrogenated amorphous silicon carbide (a-Si1�xCx:H) films were prepared by

plasma-enhanced chemical vapour deposition (PECVD) using a gas mixture of silane,

methane, and hydrogen as the reactive source. The previous results show that a high excitation

frequency, together with a high hydrogen dilution ratio of the reactive gases, allow an easier

incorporation of the carbon atoms into the silicon-rich a-Si1�xCx:H film, widen the valence

controllability. The data show that films with optical gaps ranging from about 1.9 to 3.6 eV

could be produced. In this work the influence of the hydrogen dilution ratio of the reactive

gases on the a-Si1�xCx:H film properties was investigated. The microstuctural and

photoelectronic properties of the silicon carbide films were characterized by Rutherford

backscattering spectrometry (RBS), elastic recoil detection analysis (ERDA), and FT-IR

spectrometry. The results show that a higher hydrogen dilution ratio enhances the

incorporation of silicon atoms in the amorphous carbon matrix for carbon-rich a-Si1�xCx:H

see front matter r 2004 Elsevier B.V. All rights reserved.

.solmat.2004.06.019

nding author. Departamento de Ciencia dos Materiais, Faculdade de Ciencias e Tecnologia,

Nova de Lisboa and CEMOP/UNINOVA, Campus da Caparica, 2829-516 Caparica,

.: +351 212948525; fax: +351 212941365.

dress: sz@uninova.pt (S. Zhang).

ARTICLE IN PRESS

S. Zhang et al. / Solar Energy Materials & Solar Cells 87 (2005) 343–348344

films. One pin structure was prepared by using the a-Si1�xCx:H film as the intrinsic layer. The

light spectral response shows that this structure fits the requirement for the top junction of

colour sensor.

r 2004 Elsevier B.V. All rights reserved.

Keywords: Thin film materials & devices; Silicon carbide; Thin film; Sensor

1. Introduction

Due to its adjustable band gap in the visible range of the energy spectrum, a-Si1�xCx:H films are widely used in optoelectronics, such as a p-type window layer insolar cells and other light emitting and detecting devices [1-6]. In our previous work, a-Si1�xCx:H films with different optical band gaps from 1.83 to 3.64 eV were obtainedby adjusting the plasma parameters using the plasma enhanced chemical vapordeposition (PECVD) technology. It has been noticed that the hydrogen dilution ratioplays a very important role in the deposition conditions [7]. Here, a series of a-Si1�xCx:H films with different hydrogen dilution ratios were prepared to investigate itsinfluence on the microstructral and photoelectronic properties of the film.

2. Experimental details

The a-Si1�xCx:H films were deposited by PECVD on glass and monocrystallinesilicon wafer. The substrate temperature, radio frequency power density, and growthpressure were 473K, 50mW/cm2, and 0.75Torr, respectively. The hydrogen flowrate varied from 0 to 50 sccm, and the flow rates of methane and silane were kept at10 and 1 sccm. The IR analysis of the a-Si1�xCx:H films was performed using an ATIMattson FTIR in the 400–4000 cm�1 range. Rutherford backscattering spectrometry(RBS) and elastic recoil detection analysis (ERDA) were used to investigate the filmcomposition. He–RBS spectra were obtained with 2 silicon surface barrier (SSB)detectors placed in CORNELL geometry at 1601 and 1801 scattering angles, withresolutions of 15 and 20 keV, respectively, using a 2.0MeV He+ beam. Analyseswere performed with samples tilted at 451 and 781 (angle between beam direction andsample normal). ERD spectra were simultaneously obtained with an SSB detectorplaced at a 241 scattering angle in IBM geometry, with a resolution of 20 keV. Toprevent He+ backscattered particles from hitting the ERD detector, a Kapton filterwith 8.2� 1019 at/cm2 was placed in front of it. H-BS spectra were obtained with 2SSB detectors placed in IBM geometry at 1401 and 1801 scattering angles, withresolutions of 15 and 25 keV, respectively, using a 1.5MeV H+ beam. Analyses wereperformed with samples tilted at 301. All spectra obtained for the same sample weresimultaneously analyzed with NDF [8] and a unique solution was found. In asimplified manner, one can say that from the ERD spectra one gets the H depthdistribution and from the combined H-BS and He-RBS analysis the distribution of

ARTICLE IN PRESS

S. Zhang et al. / Solar Energy Materials & Solar Cells 87 (2005) 343–348 345

all other elements; H-BS spectra allow increased sensitivity to light elements—like Cand O—, while He-RBS allows better mass and depth resolution.

3. Results and discussion

3.1. RBS and ERDA

The actual amount of carbon and silicon in the a-Si1�xCx:H was determined byRBS. The hydrogen concentration was determined by ERDA. The results are shownin Table 1. The Si/C atom ratio varies from 0.56 to 0.71, showing that all the filmsare carbon rich phases. The amount of incorporated hydrogen detected by ERDAwas approximately 40 a%. It can be noted that with higher hydrogen dilution ratios,there are more silicon atoms incorporated into the a-Si1�xCx:H film. The a-Si1�xCx:H film prepared with a 20 sccm H2 flow rate has the maximum hydrogenincorporated into the film (48 at%).

3.2. FT-IR spectra

The Fourier transformed infrared spectra is a powerful tool to investigate thebonding structure in a-Si1�xCx:H films. Fig. 1 shows the infrared absorptionspectrum of a-Si1�xCx:H films with different hydrogen dilution ratio in the range of400–1400 cm�1. The vibration modes around 750–790, 960, and 1250 cm�1 arerelated with the Si–C stretching mode, the Si–CH3/CHn wagging/rocking modes, andthe SiCH3 scissoring modes, respectively [9-11]. There are another three absorptionpeaks in the long-wavelength range, located around 2080, 2300, 2880 cm�1,respectively, referred to the Si–H/Si–H2 stretching mode, OQSi–H stretchingmode, and CH2/CH3 stretching mode. It can be noted that the absorption intensityof the peak near 770 cm�1 increases and the absorption intensity of the peak near960 cm�1 decreases, with increasing hydrogen flow rate. This indicates that theamount of Si-C bonding in the films increases with increasing hydrogen dilutionratio, which is consistent with the RBS result. There is an asymmetric widening forthe absorption peaks around 770 cm�1 with increasing hydrogen flow rate, which canbe attributed to the increase of the absorption intensity of Si–H/Si–H2 bonding dueto the greater number of silicon atoms incorporated into the a-Si1�xCx:H film. Theabsorption peak of the wagging mode of the Si–H bond is located around 640 cm�1.The a-Si1�xCx:H film prepared with 20 sccm H2 flow rate shows a higher infrared

Table 1

Composition of a-Si1�xCx:H films with different hydrogen dilution ratios obtained from RBS

H2 flow rate (sccm) Si (at%) C (at%) H (at%) Thickness (� 1015 at/cm2)

10 23 39 37 2520

20 19 33 48 1230

50 25 35 40 2050

ARTICLE IN PRESS

glass TCO(ZnO:Ga)

p-a-SiC:H i-a-Si1-xCx:H

n-a-Si:H Al

Fig. 2. Pin structure using the a-Si1�xCx:H film as the intrinsic layer.

400 600 800 1000 1200 1400

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Abs

orpt

ion

coef

ficie

nt (

a.u.

)

Wave number (cm-1)

From bottom H

2 0 sccm

H2 10 sccm

H2 20 sccm

H2 50 sccm

Fig. 1. FT-IR spectra of a-Si1�xCx:H films with different hydrogen dilution ratios.

S. Zhang et al. / Solar Energy Materials & Solar Cells 87 (2005) 343–348346

absorption peak near 2300 cm�1, indicating that some oxygen atoms areincorporated. This may explain why so many hydrogen atoms are incorporatedinto this film.

3.3. Light spectrum response of the pin structure

By using the a-Si1�xC:H film as the intrinsic layer, a pin structure was prepared ontransparent conducting oxides (TCO) coated glass by the PECVD technology. Thedevice structure is shown in Fig. 2. The metal contacts were made by electron beamevaporation. The light spectrum response of the device was obtained using a JobinYvon monochromator with a 75W xenon lamp as light source and the current wasmeasured using a Keithley 238. Fig. 3 shows the spectral response of the pin device inthe range of 400–800 nm. The response peak is located around 460 nm.Comparingwith the hydrogenated amorphous silicon (a-Si:H)-based pin structure, the lightresponse of this device has a narrow absorption spectrum and the absorption peakdecreases very fast as the wavelength exceeds 530 nm. This is very important for thecolour sensor to get a good recognition between blue and red colours.

ARTICLE IN PRESS

400 500 600 700 800

0

10

20

30

40

50

60

70

QE

(%

)

Wave length (nm)

Fig. 3. Light response of the pin structure using the a-Si1�xCx:H film as the intrinsic layer.

S. Zhang et al. / Solar Energy Materials & Solar Cells 87 (2005) 343–348 347

4. Conclusion

A series of carbon-rich a-Si1�xCx:H films with different hydrogen dilution ratioswere prepared. The results of FT-IR and RBS show that higher hydrogen dilutionratios enhance the incorporation of silicon atoms in the amorphous carbon network.By using the a-Si1�xCx:H film as the intrinsic layer, one pin structure was prepared.The light response indicates that this structure is good for the colour detector torecognize the blue light as the top junction.

Acknowledgement

This research was supported by the Fundac- ao para a Ciencia e TecnologiaPortugal through the pluriannual contract of CENIMAT and project POCTI/CTM/37344/2001. Thanks are also due to UE Asinet network for the support given.

References

[1] J. Koh, Y. Lu, S. Kim, J.S. Burnham, C.R. Wronski, R.W. Collins, Appl. Phys. Lett. 67 (18) (1995)

2669.

[2] A. Cabrita, L. Pereira, D. Brida, A. Lopes, A. Marques, I. Ferreira, E. Fortunato, R. Martins, Appl.

Surf. Sci. 184 (2001) 437.

[3] F. Irrera, F. Lemmi, F. Palma, IEEE Trans. Electron Dev. 44 (9) (1997) 1410.

[4] D. Knipp, H. Stiebig, J. Folsch, F. Finger, H. Wagner, J. Appl. Phys. 83 (3) (1998) 1463.

[5] M. Topic, F. Smole, J. Furlan, W. Kusian, J. Non-Crystal. Solids 198–200 (1996) 1180.

[6] Q. Zhu, S. Coors, B. Schneider, P. Rieve, M. Bohm, IEEE Trans. Electron. Dev. 45 (7) (1998) 1393.

[7] S. Zhang, L. Raniero, E. Fortunato, L. Pereira, N. Martins, P. Canhola, I. Ferreira, N. Nedev,

H. Aguas, R. Martins, J. Non-Crystal Solids 338–340 (2004) 530.

ARTICLE IN PRESS

S. Zhang et al. / Solar Energy Materials & Solar Cells 87 (2005) 343–348348

[8] C. Jeynes, N.P. Barradas, P.K. Marriott, M. Jenkin, E. Wendler, G. Boudreault, R.P. Webb, J. Phys.

D: Appl. Phys. 36 (2003) R97.

[9] R.A.C.M.M. van Swaaij, A.J.M. Berntsen, W.G.J.H.M. van-Sark, H. Herremans, J. Bezemer,

W.F. van der Weg, J. Appl. Phys. 76 (1994) 251.

[10] H. Wieder, M. Cardona, C.R. Guarnieri, Phys. Stat. Sol. (b) 92 (1979) 99.

[11] G. Ambrosone, U. Coscia, S. Lettieri, P. Maddalena, C. Privato, S. Ferrero, Thin Solid Films 403

(2002) 349.