- Email: [email protected]
Spectral ellipsometric and compositional characterization of hydrogenated amorphous silicon carbide thin films
g!AhAOND RELATED MATERIALS Diamond
and Related
Materials
4 ( 1995) 702-705
Spectral ellipsometric and compositional characterization hydrogenated amorphous silicon carbide thin films E. Pascual, J.L. Andtijar, LPCF,
Departament
de Fisica
Aplicada
i Electrcinicu,
J.L. Fernimdez,
llniuersitat
de Barcelona,
of
E. Bertran Au. Diagonal,
647, EO8028 Burcelonu,
Spain
Abstract A spectroscopic (UV-visible) ellipsometric analysis of hydrogenated amorphous silicon carbide (a-Si, _.C, : H) thin films, grown by plasma-enhanced chemical vapour deposition, is presented. The films were analysed by X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) to determine their composition. In this study, a wide range of x values, from 0.16 to 0.59, was covered. These values depend mainly on the composition of the precursor gas. The spectroscopic ellipsometric analysis of the samples, by a multilayer model, shows that all the a-Si, __.C,: H films studied consist of a homogeneous layer. on a c-Si substrate, with an overlayer (effective medium mixture of the same material and 41% voids). This analysis also provides values of the dielectric function of the layer material. Ker~c~ords: Ellipsometry;
Optical
properties;
Silicon carbide
1. Introduction Thin
films of hydrogenated
amorphous
silicon
carbide
(a-Si, .C, : H) have been applied to solar cells, electrophotographic devices, light detectors and as protective coatings [ 1,2]. This material has received great interest due to the possible gradual modification of its properties from those of amorphous silicon to those of amorphous carbon [ 3,4]. Direct application in solar cells requires adaptation of the gap energy to the solar spectrum. Furthermore, plasma-enhanced chemical vapour deposition (PECVD) allows the combination of the easy deposition of film structures in a single run and the modulation of the properties of the film by controlling only the flow of the precursor gases. Compositional analysis of a-Si, .C, : H is required to establish connections between the technological parameters of plasma processing and the properties of the films. In particular, optical properties are a useful index of changes, since they reflect variations directly related to composition. However, the amorphous nature and usually complex structure of the films (interface, overlayer, roughness) make a fundamental interpretation of the properties observed difficult. A deeper optical analysis is necessary. In this paper, we propose the use of an ellipsometric analysis to establish the dependence of the optical prop0925-9635/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSL’I 0925-9635(94)05253-O
erties of a-Si, _,C,: H tilms on their composition, as determined by X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS). The results of this analysis give the optical parameter energy dependences of the material, isolated from the energy dependences of the whole sample structure. Moreover, they provide information on the film structure. roughness, substrate interface and thickness.
2. Experimental
details
2.1. Deposition
Thin films of a-Si,_&,: H were produced by r.f. ( 13.56 MHz) PECVD from a silaneemethane gas mixture in a plasma reactor described elsewhere [ 51. The films were deposited on crystalline silicon (c-Si), Ni+Zr and Corning 7059 glass substrates heated at 300 C and placed on the grounded electrode. In each deposition run, the flow rates of SiH, and CH, into the gas mixture were changed by means of mass flow controllers, while the total gas flow rate was kept constant at 40 standard cubic centimetres per minute (seem). Thus, in this experiment, the relative fraction of silane, defined as the silane flow rate divided by the total flow. was varied from 0.02 to 0.4. Other fixed deposition conditions were 20 Pa of
E. Puscual et al./Diamond
and Related Materials
pressure and 50 W of r.f. power level supplied to the plasma, corresponding to an r.f. power density of 125 mW cme2. The a-%, -$,: H thin films obtained were of high optical quality (homogeneous and flat). 2.2. Compositional analysis XPS analysis was performed on a-Si,_,C,: H/c-Si structures using a Perkin Elmer PHI 5500 ESCA system, with the Ka line of Mg (1253.6 eV) as X-ray source. The spectra were obtained at a detection angle of 45”, with density measurements of 0.125 eV per step and a pass energy of 29.35 eV. The energies analysed correspond to levels 1s for C (287 eV), 2~,,~-2p,,, doublet for Si (99.5 eV), 1s for 0 (536 eV) and 2p,,, for Ar (241 eV). SIMS is a very sensitive analysis technique that provides a verification of the purity of the films and the composition depth profile [6]. SIMS measurements were carried out using an Atomika 3000-30 ion microprobe operating with a 500 nA, 12 keV 02+ beam under normal incidence. The intensity of the positive ions was measured. The secondary ion current signal was electronically gated for 30% of the 0.16 mm2 rastered area to minimize crater edge effects. 2.3. Optical characterization The spectroscopic (1.5-4.5 eV) ellipsometric characterization of the a-Sir _,C, : H/c-Si structures was carried out using a rotating analyser ellipsometer (RAE) previously described [ 71. Ellipsometry measures the ellipsometric angles (Y, A) related to the complex reflectance ratio p by p”=rp/rs = tan Y
exp (iA)
From these, the complex dielectric function 5: is derived. The assumption of semi-infinite media (air and material) provides the pseudo-dielectric function (E). It corresponds mathematically to a function equivalent to the whole structure of the film (overlayer/film/interface/substrate). (Z) contains information on the roughness, thickness, composition and density, which were determined via an analysis of the (2) spectra, assuming multilayer plane-parallel structures and dielectric mixture models, using the Bruggeman effective medium approximation (EMA) for each layer [S]. The calculation of the optical parameters of the material is simplified for absorbing materials. Thus from the high absorption region, ~2 lo4 cm-‘, of the measured spectrum of the absorption coefficient c(, the gap energy E, can be calculated using the Taut expression [9]. For weakly absorbing materials, such as a-Si, -,C, : H/c-Si, the analysis becomes more laborious. Parameters such as the gap energy E, and dielectric function at long wavelengths E, cannot be determined directly from the pseudo- dielectric measured spectrum
703
4 (1995) 702-705
because the layer structure has strong effects on the spectrum. Furthermore, in the case of amorphous materials, the application of EMA theory is not always obvious because of the ignorance of the optical references of the appropriate phases forming the material or the absence of physical phases. Whatever is the case, the optical analysis must be carried out through the application of a dispersive model for the dielectric constant. In the case of an amorphous semiconductor, such as a-Si,_,C,: H, it is adequate to apply the following expressions [lo]
n(E)=& + E:~EB;“p, k(E) =
(la)
A(E - E,)2
(lb)
E2-BE+C
where n(E) and k(E) are the refractive index and the extinction coefficient, E is the radiation energy and A, B, B,, C and C, are constants. From this dispersive model, the spectral dependence of the dielectric function of the material was calculated and the physical constants E, and E, were obtained. Furthermore, other parameters of the layer structure, such as the roughness, porosity and thickness, were calculated using the multilayer analysis and the EMA.
3. Results and discussion The chemical compositions of the films were obtained from XPS measurements. The composition parameter x is defined as the relative atomic concentration of carbon ([Cl) referred to the total carbon plus silicon ([Cl + [Si]). Fig. 1 shows that x decreases from 0.59 to 0.16 when the relative fraction of SiH, in the r.f. plasma
,ZO
1.0 ,
0.8
0.6 x 0.4
0.2
I.
o-oo.o
I
I
I
I
0.1
I
I
I
0.2
I
I
I
I
0.3
I
L
I
I
0.4
I
I
I
I,
OX?
[SiH4l/([SiH,]+[CH,l> Fig. 1. Composition parameter x obtained from XPS measurements and deposition rate of the a-Si, _,C,: H samples as a function of the relative fraction of SiH, in the r.f. plasma process.
E. Pascual et al./Diamond
704
and Related Materials
process increases from 0.02 to 0.4. This decrease in the carbon incorporation corresponds to an increase in the deposition rate from 4 to 17.8 nm min ’ (Fig. 1). The uniformity in the composition of the films was checked by SIMS depth profiling, and the results indicate that the film composition was almost constant over the entire thickness. A SIMS depth profile corresponding to Si and C signals is presented in Fig. 2 for one sample of a-Si, _,C, : H on c-Si. XPS depth profiles also revealed a significant uniformity in depth for all the a-Si,_,C,: H/c-Si samples. In Fig. 3, we compare the XPS composition ratio between [C] and [Si] with the ratio of the SIMS intensity signals of the same elements (I, and I,J We can see the good correlation between the results of the two techniques. From the ellipsometric measurements, the experimental (Z) value of the whole structure of a-Si, _,C,: H on c-Si was fitted by a two-layer model. The first layer
6i
(29)
10 ’
4
( 1995) 702-705
follows the amorphous model of dispersion described in Section 2.3 and the top layer, or overlayer, corresponds to the same material with a void fraction of 41% which reproduces, by the EMA theory, the effect of roughness and porosity. Fig. 4 shows an example corresponding to the experimental and calculated (Q spectra assuming the model described. The results of the calculated optical parameters corresponding to each sample are listed in Table 1. a, and E, present wide variations, which agree with the variation induced in the carbon content of the films as shown in Fig. 1. The calculated spectral values for F~ and cz for the different films are plotted in Figs. 5 and 6. These spectra correspond to the dielectric function of a-Si,_,C,: H without the contribution of the sample structure. The c1 spectrum clearly changes its shape and decreases as the fraction of SiH, in the plasma decreases. The c2 maximum of the film with a higher Si content is located at 3.8 eV, near to the position for a-Si: H films (3.5 eV) [ 111; it shifts to higher energies and decreases when x increases.
4. Conclusions We have presented a spectroscopic ellipsometric analysis of a-Si, -,C,: H/c-Si samples which allows the determination of the optical properties of a-Si, _,C,: H isolated from the contribution of the whole sample structure. The layer structure model describing all the a-Sir _,C, : H/c-Si samples consists of a homogeneous a-Si,_,C,: H film on a c-Si substrate with a porous overlayer (41% void fraction). The results show a Fig. 2. SIMS depth profile of the Si and C signals for a sample obtained with a relative fraction of SiH, of 0.2 in the r.f. plasma process.
20
I
15 A
C&2>
-
-4
-
+o ^, Iz) v
5: O-
XPS
[C]/[Si]
Fig. 3. Intensity ratio of the SIMS signals I,/Isi vs. composition between C and Si obtained from XPS measurements.
ratio
Fig. 4. Spectra of the real and imaginary parts of the pseudo-dielectric function (2) of an a-Si,_,C,: H sample, obtained from ellipsometric results; full lines, fitted spectra. Layer measurements: *, experimental thickness is 896 k 3 nm and overlayer thickness is 6.6 f 0.1 nm with a void fraction of 41%.
E. Pascual et aLlDiamond Table 1 Calculated
optical
ISiHdCSiKJ
parameters
of the a-Sir -,C,:
H films deposited
+ KXJ)
and Related Materials
4 (1995) 702-705
with different
fractions
A
0.4 0.2 0.1 0.05 0.02
3.55 3.26 3.28 2.49 2.22
* k k k *
0.04 0.02 0.02 0.06 0.08
B
0.92 k 0.02 0.75 * 0.02 0.61 k 0.01 0.64 F 0.05 0.4*0.1
6.89 6.80 7.02 8.2 10.0
of SiH, in the r.f. plasma C
* 0.03 * 0.04 & 0.06 * 0.3 k 0.8
13.62 + 13.7 * 17.0 * 28.5 k 35*
process E, (ev)
0.09 0.1 0.2 0.8 1
1.64 1.85 1.95 2.04 2.1
f + + k *
0.01 0.02 0.02 0.05 0.1
gradual evolution of the optical properties of the samples with composition. Changes in compositional parameters lead to strong variations in E, and a,. The compositional analysis by XPS agrees with the evolution obtained by optical analysis, and gradual variations in the sample composition induce a gradual evolution of the optical parameters. The SIMS results agree with the relative composition [C]/[Si] obtained by XPS, and the depth profiles indicate a remarkable uniformity of composition.
15 -
-10 w
relative
705
-
-
5-
Acknowledgments
9”“““““,“,“,“,‘,,‘,,‘,’ 5
2.5
Energy
4.5
(%)
Fig. 5. Spectra of the real part aI of the dielectric function of the a-Si,_,C,: H films deposited with different relative fractions of SiH, in the r.f. plasma process.
This work was supported financially by the C.I.C.Y.T. of Spain under contracts MAT 0955190, MAT 1127/92-CE and MAT 1.511/94-CE. The authors are grateful to the Serveis Cientifico-Tecnics of the Universitat de Barcelona for the SIMS and XPS measurements.
References [II
121 [31 c41
c51 C61 Fig. 6. Spectra of the imaginary part aZ of the dielectric the a-Si,_,C,: H films deposited with different relative SiH, in the r.f. plasma process.
function fractions
of of
c71 CSI c91 Cl01 Cl11
Y. Hatanaka, M. Suzuki, K. Watanabe and Y. Nakanishi, Appl. Surf. Sci., 65166 (1993) 532. F. Demichelis, G. Crovini, C.F. Pirri and E. Tresso, Phlox Mag. B, 68 (1993) 329. G. Krotz, G. Muller, G. Derst, Ch. Wilbertz and S. Kalbitzer, Diamond R&t. Muter., 3 (1994) 917. H. Herremans, W. Grevendonk, R.A.C.M.M. van Swaaij, W.G.J.H.M. van Sark, A.J.M. Berntsen, W.M. Arnold Bik and J. Bezemer, Philos. Mug. B, 66 (1992) 787. J.L. Andtijar, E. Bertran, A. Canillas, J. Esteve, J. Andreu and J.L. Morenza, Vacuum, 39 (1989) 795. F. Lopez, M.V. Garcia-Cuenca, C. Serra and J.L. Morenza, Diamond Relat. Mater., 2 (1993) 229. E. Pascual, C. Serra, J. Esteve and E. Bertran, Surf. Coat. Technol., 47 (1991) 263. D.A.G. Bruggeman, Ann. Phys. (Leipzig), 24 (1935) 636. E.A. Davis and N.F. Mott, Philos. Msg., 22 (1970) 903. E.K. Palik (ed.), Handbook of Optical Constants of Solids, Vol. II, Academic Press, Boston, MA, 1991, p. 154. A. Canillas, E. Bertran, J.L. Andujar and B. Drtvillon, J. Appl. Phys., 68 (1990) 2752.
and Related
Materials
4 ( 1995) 702-705
Spectral ellipsometric and compositional characterization hydrogenated amorphous silicon carbide thin films E. Pascual, J.L. Andtijar, LPCF,
Departament
de Fisica
Aplicada
i Electrcinicu,
J.L. Fernimdez,
llniuersitat
de Barcelona,
of
E. Bertran Au. Diagonal,
647, EO8028 Burcelonu,
Spain
Abstract A spectroscopic (UV-visible) ellipsometric analysis of hydrogenated amorphous silicon carbide (a-Si, _.C, : H) thin films, grown by plasma-enhanced chemical vapour deposition, is presented. The films were analysed by X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) to determine their composition. In this study, a wide range of x values, from 0.16 to 0.59, was covered. These values depend mainly on the composition of the precursor gas. The spectroscopic ellipsometric analysis of the samples, by a multilayer model, shows that all the a-Si, __.C,: H films studied consist of a homogeneous layer. on a c-Si substrate, with an overlayer (effective medium mixture of the same material and 41% voids). This analysis also provides values of the dielectric function of the layer material. Ker~c~ords: Ellipsometry;
Optical
properties;
Silicon carbide
1. Introduction Thin
films of hydrogenated
amorphous
silicon
carbide
(a-Si, .C, : H) have been applied to solar cells, electrophotographic devices, light detectors and as protective coatings [ 1,2]. This material has received great interest due to the possible gradual modification of its properties from those of amorphous silicon to those of amorphous carbon [ 3,4]. Direct application in solar cells requires adaptation of the gap energy to the solar spectrum. Furthermore, plasma-enhanced chemical vapour deposition (PECVD) allows the combination of the easy deposition of film structures in a single run and the modulation of the properties of the film by controlling only the flow of the precursor gases. Compositional analysis of a-Si, .C, : H is required to establish connections between the technological parameters of plasma processing and the properties of the films. In particular, optical properties are a useful index of changes, since they reflect variations directly related to composition. However, the amorphous nature and usually complex structure of the films (interface, overlayer, roughness) make a fundamental interpretation of the properties observed difficult. A deeper optical analysis is necessary. In this paper, we propose the use of an ellipsometric analysis to establish the dependence of the optical prop0925-9635/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSL’I 0925-9635(94)05253-O
erties of a-Si, _,C,: H tilms on their composition, as determined by X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS). The results of this analysis give the optical parameter energy dependences of the material, isolated from the energy dependences of the whole sample structure. Moreover, they provide information on the film structure. roughness, substrate interface and thickness.
2. Experimental
details
2.1. Deposition
Thin films of a-Si,_&,: H were produced by r.f. ( 13.56 MHz) PECVD from a silaneemethane gas mixture in a plasma reactor described elsewhere [ 51. The films were deposited on crystalline silicon (c-Si), Ni+Zr and Corning 7059 glass substrates heated at 300 C and placed on the grounded electrode. In each deposition run, the flow rates of SiH, and CH, into the gas mixture were changed by means of mass flow controllers, while the total gas flow rate was kept constant at 40 standard cubic centimetres per minute (seem). Thus, in this experiment, the relative fraction of silane, defined as the silane flow rate divided by the total flow. was varied from 0.02 to 0.4. Other fixed deposition conditions were 20 Pa of
E. Puscual et al./Diamond
and Related Materials
pressure and 50 W of r.f. power level supplied to the plasma, corresponding to an r.f. power density of 125 mW cme2. The a-%, -$,: H thin films obtained were of high optical quality (homogeneous and flat). 2.2. Compositional analysis XPS analysis was performed on a-Si,_,C,: H/c-Si structures using a Perkin Elmer PHI 5500 ESCA system, with the Ka line of Mg (1253.6 eV) as X-ray source. The spectra were obtained at a detection angle of 45”, with density measurements of 0.125 eV per step and a pass energy of 29.35 eV. The energies analysed correspond to levels 1s for C (287 eV), 2~,,~-2p,,, doublet for Si (99.5 eV), 1s for 0 (536 eV) and 2p,,, for Ar (241 eV). SIMS is a very sensitive analysis technique that provides a verification of the purity of the films and the composition depth profile [6]. SIMS measurements were carried out using an Atomika 3000-30 ion microprobe operating with a 500 nA, 12 keV 02+ beam under normal incidence. The intensity of the positive ions was measured. The secondary ion current signal was electronically gated for 30% of the 0.16 mm2 rastered area to minimize crater edge effects. 2.3. Optical characterization The spectroscopic (1.5-4.5 eV) ellipsometric characterization of the a-Sir _,C, : H/c-Si structures was carried out using a rotating analyser ellipsometer (RAE) previously described [ 71. Ellipsometry measures the ellipsometric angles (Y, A) related to the complex reflectance ratio p by p”=rp/rs = tan Y
exp (iA)
From these, the complex dielectric function 5: is derived. The assumption of semi-infinite media (air and material) provides the pseudo-dielectric function (E). It corresponds mathematically to a function equivalent to the whole structure of the film (overlayer/film/interface/substrate). (Z) contains information on the roughness, thickness, composition and density, which were determined via an analysis of the (2) spectra, assuming multilayer plane-parallel structures and dielectric mixture models, using the Bruggeman effective medium approximation (EMA) for each layer [S]. The calculation of the optical parameters of the material is simplified for absorbing materials. Thus from the high absorption region, ~2 lo4 cm-‘, of the measured spectrum of the absorption coefficient c(, the gap energy E, can be calculated using the Taut expression [9]. For weakly absorbing materials, such as a-Si, -,C, : H/c-Si, the analysis becomes more laborious. Parameters such as the gap energy E, and dielectric function at long wavelengths E, cannot be determined directly from the pseudo- dielectric measured spectrum
703
4 (1995) 702-705
because the layer structure has strong effects on the spectrum. Furthermore, in the case of amorphous materials, the application of EMA theory is not always obvious because of the ignorance of the optical references of the appropriate phases forming the material or the absence of physical phases. Whatever is the case, the optical analysis must be carried out through the application of a dispersive model for the dielectric constant. In the case of an amorphous semiconductor, such as a-Si,_,C,: H, it is adequate to apply the following expressions [lo]
n(E)=& + E:~EB;“p, k(E) =
(la)
A(E - E,)2
(lb)
E2-BE+C
where n(E) and k(E) are the refractive index and the extinction coefficient, E is the radiation energy and A, B, B,, C and C, are constants. From this dispersive model, the spectral dependence of the dielectric function of the material was calculated and the physical constants E, and E, were obtained. Furthermore, other parameters of the layer structure, such as the roughness, porosity and thickness, were calculated using the multilayer analysis and the EMA.
3. Results and discussion The chemical compositions of the films were obtained from XPS measurements. The composition parameter x is defined as the relative atomic concentration of carbon ([Cl) referred to the total carbon plus silicon ([Cl + [Si]). Fig. 1 shows that x decreases from 0.59 to 0.16 when the relative fraction of SiH, in the r.f. plasma
,ZO
1.0 ,
0.8
0.6 x 0.4
0.2
I.
o-oo.o
I
I
I
I
0.1
I
I
I
0.2
I
I
I
I
0.3
I
L
I
I
0.4
I
I
I
I,
OX?
[SiH4l/([SiH,]+[CH,l> Fig. 1. Composition parameter x obtained from XPS measurements and deposition rate of the a-Si, _,C,: H samples as a function of the relative fraction of SiH, in the r.f. plasma process.
E. Pascual et al./Diamond
704
and Related Materials
process increases from 0.02 to 0.4. This decrease in the carbon incorporation corresponds to an increase in the deposition rate from 4 to 17.8 nm min ’ (Fig. 1). The uniformity in the composition of the films was checked by SIMS depth profiling, and the results indicate that the film composition was almost constant over the entire thickness. A SIMS depth profile corresponding to Si and C signals is presented in Fig. 2 for one sample of a-Si, _,C, : H on c-Si. XPS depth profiles also revealed a significant uniformity in depth for all the a-Si,_,C,: H/c-Si samples. In Fig. 3, we compare the XPS composition ratio between [C] and [Si] with the ratio of the SIMS intensity signals of the same elements (I, and I,J We can see the good correlation between the results of the two techniques. From the ellipsometric measurements, the experimental (Z) value of the whole structure of a-Si, _,C,: H on c-Si was fitted by a two-layer model. The first layer
6i
(29)
10 ’
4
( 1995) 702-705
follows the amorphous model of dispersion described in Section 2.3 and the top layer, or overlayer, corresponds to the same material with a void fraction of 41% which reproduces, by the EMA theory, the effect of roughness and porosity. Fig. 4 shows an example corresponding to the experimental and calculated (Q spectra assuming the model described. The results of the calculated optical parameters corresponding to each sample are listed in Table 1. a, and E, present wide variations, which agree with the variation induced in the carbon content of the films as shown in Fig. 1. The calculated spectral values for F~ and cz for the different films are plotted in Figs. 5 and 6. These spectra correspond to the dielectric function of a-Si,_,C,: H without the contribution of the sample structure. The c1 spectrum clearly changes its shape and decreases as the fraction of SiH, in the plasma decreases. The c2 maximum of the film with a higher Si content is located at 3.8 eV, near to the position for a-Si: H films (3.5 eV) [ 111; it shifts to higher energies and decreases when x increases.
4. Conclusions We have presented a spectroscopic ellipsometric analysis of a-Si, -,C,: H/c-Si samples which allows the determination of the optical properties of a-Si, _,C,: H isolated from the contribution of the whole sample structure. The layer structure model describing all the a-Sir _,C, : H/c-Si samples consists of a homogeneous a-Si,_,C,: H film on a c-Si substrate with a porous overlayer (41% void fraction). The results show a Fig. 2. SIMS depth profile of the Si and C signals for a sample obtained with a relative fraction of SiH, of 0.2 in the r.f. plasma process.
20
I
15 A
C&2>
-
-4
-
+o ^, Iz) v
5: O-
XPS
[C]/[Si]
Fig. 3. Intensity ratio of the SIMS signals I,/Isi vs. composition between C and Si obtained from XPS measurements.
ratio
Fig. 4. Spectra of the real and imaginary parts of the pseudo-dielectric function (2) of an a-Si,_,C,: H sample, obtained from ellipsometric results; full lines, fitted spectra. Layer measurements: *, experimental thickness is 896 k 3 nm and overlayer thickness is 6.6 f 0.1 nm with a void fraction of 41%.
E. Pascual et aLlDiamond Table 1 Calculated
optical
ISiHdCSiKJ
parameters
of the a-Sir -,C,:
H films deposited
+ KXJ)
and Related Materials
4 (1995) 702-705
with different
fractions
A
0.4 0.2 0.1 0.05 0.02
3.55 3.26 3.28 2.49 2.22
* k k k *
0.04 0.02 0.02 0.06 0.08
B
0.92 k 0.02 0.75 * 0.02 0.61 k 0.01 0.64 F 0.05 0.4*0.1
6.89 6.80 7.02 8.2 10.0
of SiH, in the r.f. plasma C
* 0.03 * 0.04 & 0.06 * 0.3 k 0.8
13.62 + 13.7 * 17.0 * 28.5 k 35*
process E, (ev)
0.09 0.1 0.2 0.8 1
1.64 1.85 1.95 2.04 2.1
f + + k *
0.01 0.02 0.02 0.05 0.1
gradual evolution of the optical properties of the samples with composition. Changes in compositional parameters lead to strong variations in E, and a,. The compositional analysis by XPS agrees with the evolution obtained by optical analysis, and gradual variations in the sample composition induce a gradual evolution of the optical parameters. The SIMS results agree with the relative composition [C]/[Si] obtained by XPS, and the depth profiles indicate a remarkable uniformity of composition.
15 -
-10 w
relative
705
-
-
5-
Acknowledgments
9”“““““,“,“,“,‘,,‘,,‘,’ 5
2.5
Energy
4.5
(%)
Fig. 5. Spectra of the real part aI of the dielectric function of the a-Si,_,C,: H films deposited with different relative fractions of SiH, in the r.f. plasma process.
This work was supported financially by the C.I.C.Y.T. of Spain under contracts MAT 0955190, MAT 1127/92-CE and MAT 1.511/94-CE. The authors are grateful to the Serveis Cientifico-Tecnics of the Universitat de Barcelona for the SIMS and XPS measurements.
References [II
121 [31 c41
c51 C61 Fig. 6. Spectra of the imaginary part aZ of the dielectric the a-Si,_,C,: H films deposited with different relative SiH, in the r.f. plasma process.
function fractions
of of
c71 CSI c91 Cl01 Cl11
Y. Hatanaka, M. Suzuki, K. Watanabe and Y. Nakanishi, Appl. Surf. Sci., 65166 (1993) 532. F. Demichelis, G. Crovini, C.F. Pirri and E. Tresso, Phlox Mag. B, 68 (1993) 329. G. Krotz, G. Muller, G. Derst, Ch. Wilbertz and S. Kalbitzer, Diamond R&t. Muter., 3 (1994) 917. H. Herremans, W. Grevendonk, R.A.C.M.M. van Swaaij, W.G.J.H.M. van Sark, A.J.M. Berntsen, W.M. Arnold Bik and J. Bezemer, Philos. Mug. B, 66 (1992) 787. J.L. Andtijar, E. Bertran, A. Canillas, J. Esteve, J. Andreu and J.L. Morenza, Vacuum, 39 (1989) 795. F. Lopez, M.V. Garcia-Cuenca, C. Serra and J.L. Morenza, Diamond Relat. Mater., 2 (1993) 229. E. Pascual, C. Serra, J. Esteve and E. Bertran, Surf. Coat. Technol., 47 (1991) 263. D.A.G. Bruggeman, Ann. Phys. (Leipzig), 24 (1935) 636. E.A. Davis and N.F. Mott, Philos. Msg., 22 (1970) 903. E.K. Palik (ed.), Handbook of Optical Constants of Solids, Vol. II, Academic Press, Boston, MA, 1991, p. 154. A. Canillas, E. Bertran, J.L. Andujar and B. Drtvillon, J. Appl. Phys., 68 (1990) 2752.