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AES microstructural investigations of low-temperature, low-frequency plasma-deposited a-SixC1−x:H films
Applied Surface Science 64 (1993) 345-351 North-Holland
applii surface science
AES microstructural investigations of low-temperature, plasma-deposited a-Si,C,_,:H films
low-frequency
E. Gat a, B. Cros ‘,*, R. Berjoan ’ Laboratoire de Physicochimie des Mat&au&
b and J. Durand
a
URA 1312 CNRS, ENSCM, 8 Rue de I’Ecole Normale,
34053 Montpellier Cedex 01, France b Znstitut de Science et G&e des Mate%aux et Pro&d&
Odeillo, BP 5, 66120 Font Romey France
Received 10 June 1992; accepted for publication 17 November 1992
Amorphous a-Si,C, _,:H films have been prepared by low-temperature, low-frequency PECVD. Information on the microstructure was obtained from a fine study of the shape of the Si LW, Si KLL and C KVV peaks. Correlated with our previous IR, XPS and EXAPS studies, the results reveal the existence of three domains as a function of composition x, corresponding to three different types of materials. Structural models are proposed for each of them.
1. Introduction
still rarely used as a microstructural Auger electron spectroscopy (AES).
The development of the plasma-enhanced chemical vapor deposition (PECVD) technique has allowed the production of a large variety of novel materials with disordered structure. Amorphous hydrogenated silicon carbide a-SiC:H is one of these materials. It presents a current great interest, not only technologically because of its potential applications due to its singular properties, but also fundamentally in regard of its complex structure. Many structural studies have been realized on a-Si,C,_,:H in the last ten years; they have required the use of a variety of specific analysis techniques which can bring information on the microstructure and local bonding [l]. We have previously undertaken microstructural investigations of a-Si,C,_,:H PECVD films using Xray photoelectron spectroscopy (XPS), infrared absorption spectroscopy (IR) and extended X-ray absorption fine structure (EXAFS) [2,31. In the present work, we develop a fine study based on complementary analyses by a technique which is
* To whom correspondence 0169-4332/93/$06.00
should be addressed.
probe, the
2. previous works In the Auger relaxation process XYZ, where X, Y and 2 are electronic levels of an excited atom, the initial state is a hole in level X; one electron falls from level Y to X and the released energy is dissipated in the ejection of a so-called “Auger” electron from level Z. In the final state, there are two holes in levels Y and Z. The kinetic energy of the Auger electron is defined by the three electronic levels involved in the transition and can be given by [41 E XYZ
=E(X)
-E(Y)
-E(Z)
- U(Y,
Z),
where E(X), E(Y) and E(Z) are the binding energies of the X, Y and Z levels and U(Y, Z) is the effective hole-hole interaction energy. Transitions involving only core levels like the SiIUL transition are basically interpreted using atom physics. A change in the chemical environment of an atom is then associated to a charge
0 1993 - Elsevier Science Publishers B.V. All rights reserved
346
E. Gat et al. / AES microstructural
inuestigations
transfer and the resulting shift of the energy of the Auger electrons can be related to the electronegativity of the neighbouring atoms [5]. In the case of core-valence transitions like the Si LW or CKW transitions, the interpretation is more complex. These transitions present generally a wide line shape whose fine structure can be related to the self-convolution of the local density of the valence states (LDOS) adjacent to the silicon and carbon atoms, respectively. The chemical effects are then a shift and a change in the shape of the complex spectra. Previous works [6-91 on the C KW Auger line shapes for various gaseous carbon molecules have illustrated its sensitivity to local sp” hybridization. Six components (ss, sp,, p,p,, sp,, p,p, and p,p,) determine the Auger peak fine structure. They correspond to the self-convolution of the valence states with s- and p-character, and their relative intensities can be correlated to the carbon hybridization state. In a solid, the valence bands are much broader and tend to smooth out this fine structure. Moreover, matrix, core-hole screening and final-state hole-hole correlation (localization) effects have generally to be considered for a correct understanding of spectral features; this was demonstrated by studies on various solids such as polyethylene, graphite, diamond and transition-metal carbides [6]. A detailed line-shape interpretation requires an elaborate fitting procedure associated to a thorough understanding of the main factors contributing to the Auger process. In the case of a-Si,C,_,:H compounds, the possible occurrence of several types of carbon local environments, involving C-C, C-Si and C-H bonds in various proportions, makes a fine theoretical interpretation difficult. The SiL,,,W transition has also been extensively subject to theoretical interpretations based on the self-convolution of the local density of states (LDOS) of the silicon atoms. Studies on hydrogenated amorphous silicon (a-Si:H) have shown that the whole L,,,W signal covering the kinetic energy range 50-100 eV can be split into three components [lo]: . a main component at around 90 eV is attributed to the L,,M,,M,,, transition and cor1 1
of plasma-deposited
a-Si,C,
_ x:H
responds to the valence electron density with pp character; . a secondary component at around 80 eV is attributed to the L,,M,M, s transition and corresponds to the valence electron density with sp character; . a third component at around 70 eV arises from a final-state hole-hole correlation. The last component does not arise in this work since the background removal procedure involves the SiL,,,W “true” Auger line shape to be extracted over the kinetic energy range 70-100 eV. A theoretical interpretation based on a fitting procedure into Gaussian components relating to sp and pp partial densities of states of the silicon atoms seems not to be well adapted to the case of an a-Si,C,_,:H compound. Indeed, several types of local environment affecting Si-C, Si-Si and Si-H bonds in various proportions can occur and make difficult a theoretical study which moreover will not bring accurate information about the microstructure of the films. So we have chosen to restrict our investigations to simple overall lineshape considerations. Compared to the typical form of the Auger peak of some reference materials, these valence spectra act as a “fingerprint” of a particular chemical state.
3. Experimental a-Si,C,_,:H films with various compositions and 200 nm thickness were prepared by a 110 kHz low-frequency glow discharge decomposition of a silane and methane mixture diluted by helium. The RF plasma system and the deposition conditions have been reported elsewhere [ll]. Auger measurements were performed under ultra-high vacuum with a Riber Mac 2 spectrometer using an incident electron beam with a diameter less than 1 pm. The spectra were scanned in the N(E) mode in two energy ranges: . a low-energy one between 25 and 550 eV in order to detect the Si LW, C KW and 0 KLL transitions; . a high-energy one between 1600 and 1650 eV in order to detect the SiKLL transition.
E. Gat et al. / AES microstructural
investigations
of plasma-deposited
a-Si,C,
347
_ .:H
The primary electron beam energy and spectrometer resolution were fixed respectively at 3 keV and 1 eV for the low-energy range and at 5 keV and 2 eV for the high-energy range. In order to remove surface contamination resulting from exposure to air, before the analyses the samples were submitted for 10 min to a bombardment with argon ions under an accelerating voltage of 3 kV.
4. Results and discussion Low-energy-range Auger spectra are shown in fig. 1 for a-Si,C, _,:H films with different compositions and for a polycrystalline P-Sic reference. The observed Auger features are the SiLW core-valence-valence transition around 90 eV and the C KW core-valence-valence transition around 270 eV. No detectable presence of oxygen is found on sputter-cleaned surfaces of the films as shown by the absence of the OKLL peak (energy 507 eV). The various a-Si,C,_,:H samples display a significant variation of the relative intensities of both Si LW and C KW peaks. The composition x has been determined from the relative peak height/background (P/B) ratios using the P-Sic sample as a reference 1121. In order to derive some microstructural information from the SiLW and CKW transitions, these peaks have been considered in detail after two kinds of background subtraction: the AES
100
500
300 Kinetic enwgy
@Iv)
Fig. 1. N(E) Auger spectra of a-Si,C,_,:H films: (a) p-Sic reference; (b) x = 0.25; (c) x = 0.50; (d) x = 0.70.
240
250
280
Fig. 2. CKW peak obtained for a-Si,C,_,:H films (x = 0, 0.25, 0.50 and 0.70) and two references, graphite and p-Sic, after removing primary and secondary electron background and Shirley subtraction of the energy-loss electron background.
data were primarily relieved of the background of primary and secondary electrons by a linear regression from the higher-energy side; then a Shirley subtraction of the energy-loss electron background led to the final peak [12]. Fig. 2 presents the final form of the CKW peak. The overall appearance of the Auger peak shape changes markedly on going from a carbonrich to a silicon-rich film and reveals a modification of the local density of the valence states adjacent to the carbon atoms. The main observations are as follows. The shape of the C KW peak of the silicon-rich film (composition x = 0.7) is typical of a carbide @i-C bonds) environment, illustrated by the P-Sic reference. The main peak centred at 267 eV is broadened at 253 and 248 eV. An accurate identification of these two shoulders is not easy though because plasmon losses obscure the fine details on the lower-energy side of the spectrum. This carbide character still exists even though slightly attenuated for the film with composition x = 0.5. The CKW peak of the SiO,,C,,,:H film displays a rather different line shape. Its broader form which does not present any visible fine
348
E. Gat et al. / AES microstrucrural investigations of plasma-deposited a-Si,C, _ ,:H
structure on the lower-energy side is characteristic of an alkane or alkane/diamond mixed type environment (C-C and C-H bonds in tetrahedral configuration) [6]. . The peak corresponding to the a-C:H film (x = 0) is far broader. It does not present the typical form obtained for the graphite with a marked shoulder located around 255 eV. Moreover, considering the broadness of the peak, we find no evidence to suggest a tetrahedral-coordinated alkane or diamond local type structure. The carbon local environment in the a-C:H film is probably of mixed alkane/diamond/graphite type. Fig. 3 shows the final form of the SiL,,,W peaks. The observed evolution of this Auger line shape interprets a strong modification of the local density of states (LDOS) of the silicon atoms. The peaks become narrower and their maximum shifts toward higher energies when the silicon content increases. The peak of the carbon-rich film (x = 0.25) presents the same characteristics as the P-Sic sample: it is relatively broad and its maximum occurs at 87 eV. The peak of the a-Si,,,C,,,:H film keeps similar characteristics.
Fig. 3. SiL,,,W peak obtained for a-Si,C,_,:H films (x = 0.25, 0.50, 0.70 and 1) and two references, B-Sic and Si(lOO), after the background subtraction procedure described in fig. 2.
This reveals that, for films with composition equal to, or lower than 0.5, the local environment of the silicon atoms is mainly composed of carbon atoms (carbide type arrangement). The much narrower peak obtained for the silicon-rich film (x = 0.7) reveals a local environment mainly composed of silicon atoms. The narrowest peak is obtained for the a-Si:H film (x = 1). Its line shape and posi-
b
1620
1615 Ktnatlc energy
1
1615
162P
(ev)
1615 Kinetic energy
1620 (ev)
$U
1615 Kinetic energy
1820 WI
Fig. 4. Fitting of the SiKL,,,L,,, p eaks using Gaussian components (table 1): (a) a-Sio~zCo~,s:H film and p-Sic reference; (b) a-Si:H film and c-Si(100) reference; (c) a-Si,C1 _,:H films (x = 0.5); (d) a-Si,C,_,:H films (x = 0.7).
349
E. Gat et al. / AES microstructural investigations of plasma-deposited a-Si,C, _ .:H
tion (89 eV> are quite similar to the ones of a c-Si sample. It is characteristic of tetrahedral environments such as Si[Si,l, Si[Si,Hl and even Si[Si,H2]. In order to obtain more precise information about the local environment of the silicon atoms, we studied the evolution of the SiKL,,,L%, peak. This transition, scanned in the high kinetic energy range (1600-1650 eV>, involves only the discrete core electronic K and L,,, levels. Therefore, any shift or change in line shape can be more easily interpreted; depending on charge transfers with surrounding atoms, they can be directly related to the local environment of the silicon atoms. Figs. 4a and 4b show, on a magnified scale, the peaks obtained for the crystalline references pSic and c-Si after background subtraction. They present the following characteristics: their fullwidth at half-maximum (FWHM) and energy position are respectively 3.4 and 1615.7 eV for P-Sic, 3.0 eV and 1617.5 eV for c-Si. The higher FWHM and the shifted position toward lower kinetic energies obtained for P-Sic are due to an increased charge transfer between the silicon atoms and their nearest neighbours when the latter are carbon atoms. We have also reported in figs. 4a and 4b the peaks obtained for the a-Si,,C,,,,:H and a-Si:H (x = 1) films in order to underline their respective remarkable similarity in line shape and position with the p-Sic and c-Si references. These similarities which have also been found for the SiL,,,W peaks denote two microstructural pieces of information: . in carbon-rich films, the silicon atoms are mainly bonded to carbon atoms in a P-Sic type local configuration (Si[C,] tetrahedral; . in the a-Si:H film, the local environment of the silicon atoms is mainly composed of Si[Si,l and Si[Si,H] tetrahedral units. In the case of a-Si,C,_,:H films with composition x = 0.5 and 0.7, microstructural information requires a fine study of the SiKL,,,L,, peaks. They have an energy position intermediate between those obtained for p-Sic and c-Si and slightly broader line shapes (FWHM = 3.5 eV> than that measured for P-Sic. In order to investigate the local environment of the silicon atoms
Table 1 Characteristics of the Gaussian components used in the fitting procedure of the SiKL,,,La,s peaks Component
Environments SK,1 Si[Si,C,H,] a Si[Si,], Si[Si,H]
1 2 3
Energy
FWHM
(eV)
(eV)
1615.7 1616-1617 1617.5
3.4 3-3.4 3
al n + m + I= 4 with n and m different from zero.
(no chemical ordering, chemical ordering with homogeneous dipersion or chemical ordering with phase separation [13,14]), we have fitted the SiKL 2,3L 2,3 peaks with Gaussian components whose characteristics are reported in table 1. Components 1 and 3 correspond to well defined Si[C,] and Si[Si,]/Si[Si,Hl local environments, respectively. Component 2 corresponds to all the possible tetrahedral local environments which are not taken into account by the two others. The results obtained for films with composition c = 0.5 and 0.7 are shown in figs. 4c and 4d. The peak of the a-Si,,C,,,:H film fits into two components: the stronger one (1) reveals the predominance of a chemical ordering of Si[C,] type and the weaker one (2) indicates the appearance of other tetrahedral local environments. For the a-Si,&,,:H film, the three components presented in table 1 had to be used in order to realize a suitable
loo-
8,O
;/,
02
034
0,a
/ 0,6
COMPOSITION
D
X
Fig. 5. Evolution of SiKL,,,L,,s fitting Gaussian components (table 1) as a function of the composition x of the a-Si,C, _,:H films.
E. Gat et al. / AES microstmctural
350
investigations
of plasma-deposited
a-Si,C,
_ .:H
3. Conclusion
161 8’ 90
092
0,4
‘396
COMPOSITION
Fig. 6. Evolution of SiKL,,,L,,, transition composition x of the a-Si,C,_,:H
036
1,o
X
energy films.
with
the
deconvolution which reveals an absence of chemical ordering. We have reported in fig. 5 the evolution with the silicon content of the relative contributions of each component. Some drastic change appears in the local environment of the silicon atoms, when the silicon content increases in the films. For x < 0.5, the predominance of component 1 indicates a /?-Sic type local chemical ordering in the form of Si[C,] units. Above x = 0.5, the coexistence of the three components defined in table 1 and the increased contribution of component 2 reveal the heterogeneous character of the microstructure increasingly disordered. Fig. 6 reports the evolution of the energy of the Si KLa,L2 3 Auger transition with the composition of the films. We do not observe a linear increase as it would be the case if a random replacement occurred of the carbon atoms by silicon atoms in a tetrahedral network. The weaker increase of the transition energy in the carbon-rich composition range can be explained as follows: the remaining Si[C,] local environment attenuates the increased shift of the Auger transition towards higher energies, which is correlated to the progressive replacement of carbon atoms by silicon atoms in the nearest neighbouring of silicon.
The AES technique has been succesfully used as a microstructural probe for amorphous aSi,C,_,:H PECVD films. Information on the local environment of the carbon and silicon atoms has been derived from a fine study of the CKW, SiLW and SiKLL Auger transitions including line shape and peak shift considerations. This information completes the results previously obtained by XPS, IR and EXAFS (Si K-edge) analyses [2,31 and allows the proposition of a structural model which accounts for a marked evolution of the microstructure of the films with their composition. Three composition ranges are distinguished: Carbon-rich films. Both examinations of the Si LW and SiKLL peaks show, in agreement with the XPS and EXAFS analyses, that the silicon atoms are mainly bonded to four carbon atoms. The EXAFS analyses [3], moreover, have revealed the existence of second-nearest neighbours of a P-Sic structure, which proves that the local environment of the silicon atoms is ordered. This ordering has been reported by other authors in recent works on a-Si,C,_,:H films [14-171, but the degree of chemical order is not clearly defined. The typical line shape of the CKW peak is characteristic of a carbon local environment of an alkane or alkane/diamond type. This alkane character is related to the existence of alkyl groups as brought to light by IR analyses [2]. In a NMR study of a-Si,C,_,:H PECVD films Petrich et al. [18] explained the presence of these groups by the incorporation of hydrocarbon species in the films during the deposition process. The assumption of a presence of graphitic sp2 type local environments based on XPS analyses [2] is not supported by the AES results except for a-C:H films. The global structure of the carbon-rich films cannot be accurately defined. It is probably close to the one of an almost fully cross-linked polymer (hydrogenated polycarbon) incorporating silicon atoms in a P-Sic type local configuration. Films with composition
close to stoichiometry.
The persistence of a P-Sic type local ordering around the silicon atoms, highlighted by EXAFS, is confirmed by the fine study of the SiKLL
E. Cat et al. / AES microstructural investigations of plasma-deposited a-Si,C, _ .:H
Auger transition. On the other hand, a carbide character (Sic) of the carbon environment, not disclosed by XPS and IR studies, is demonstrated by a simple overall line-shape observation of the CKVV peak. The structure of these films, in which the amount of Si-C bonds is maximum, as revealed by IR analyses, can be considered to be close to the one of a locally ordered a-Sic tetrahedral network incorporating some hydrogen atoms bonded to carbon rather than to silicon. Silicon-rich films. The SiKLL Auger peak fine study reveals, in agreement with the XPS, IR and EXAPS results, that the local environment of the silicon atoms presents a heterogeneous character and an absence of local ordering. The local environment of the carbon atoms (which are minor species in the silicon-rich films) still has a carbide Sic character as displayed by the film of the CKVV peak. The structure of these films can be described by a disordered tetrahedral a-Si network incorporating some hydrogen atoms in the form of SiH and SiH, groups and carbon atoms in a sp3 carbide type configuration.
References [l]
J. Bullot and M.P. Schmidt, (1987) 345.
Phys. Status
Solidi fb) 143
351
[2] E. Gat, M.A. El Khakani, M. Chaker, A. Jean, S. Boily, H. Pepin, J.C. Kieffer, J. Durand, B. Cros, F. Rousseaux and S. Gujrathi, J. Mater. Res. 4 (1992) 2478. [3] A.M. Haghiri-Gosnet, F. Rousseaux, E. Gat, J. Durand and A.M. Flank, Microelectron. Eng. 17 (1992) 215. [4] G.A. Sawatzky and D. Port, Phys. Rev. B 20 (1979) 1546. [5] T.W. Haas, J.T. Grant and G.J. Dooley III, J. Appl. Phys. 43 (1972) 1853. 161D.E. Ramaker, Appl. Surf. Sci. 21 (1985) 243. 171 R.R. Rye, T.E. Madey, J.E. Houston and P.H. Holloway, J. Chem. Phys. 69 (1978) 1504. k31 R.R. Rye, D.R. Jennison and J.E. Houston, J. Chem. Phys. 73 (19801 4867. [91 J.E. Houston and R.R. Rye, J. Chem. Phys. 74 (1981) 71. A.J. Nelson and L.L. 1101 N.A. Burnham, A.B. Swartzlander, Kazmerski, Sol. Cells 21 (1987) 135. Mater. 1111 E. Gat, B. Cros, R. Berjoan and J. Durand, Manuf. Processes 7 (1992) 345. WI B. Cros, R. Berjoan, C. Monteil, E. Gat, N. Azema, D. Perarnau and J. Durand, in: Proc. EURO CVD 8 [J. Phys. III 2 (1992) 13731. D31 K. Mui and F.W. Smith, Phys. Rev. B 35 (19871 8080. 1141D.R. McKenzie, G.B. Smith and Z.Q. Liu, Phys. Rev. B 37 (19881 8875. [I51 K. Mui, D.K. Basa, F.W. Smith and R. Corderman, Phys. Rev. B 35 (1987) 8089. 1161 S. Pascarelli, F. Boscherini, S. Mobilio and F. Evangelisti, Phys. Rev. B 45 (1992) 1650. 1171 C. Meneghini, S. Pascarelli, F. Bosherini, S. Mobolio and F. Evangelisti, J. Non-Cryst. Solids 137/138 (1991) 75. 1181 M.A. Petrich, K.K. Gleason and J.A. Reimer, Phys. Rev. B 36 (1987) 9722.
applii surface science
AES microstructural investigations of low-temperature, plasma-deposited a-Si,C,_,:H films
low-frequency
E. Gat a, B. Cros ‘,*, R. Berjoan ’ Laboratoire de Physicochimie des Mat&au&
b and J. Durand
a
URA 1312 CNRS, ENSCM, 8 Rue de I’Ecole Normale,
34053 Montpellier Cedex 01, France b Znstitut de Science et G&e des Mate%aux et Pro&d&
Odeillo, BP 5, 66120 Font Romey France
Received 10 June 1992; accepted for publication 17 November 1992
Amorphous a-Si,C, _,:H films have been prepared by low-temperature, low-frequency PECVD. Information on the microstructure was obtained from a fine study of the shape of the Si LW, Si KLL and C KVV peaks. Correlated with our previous IR, XPS and EXAPS studies, the results reveal the existence of three domains as a function of composition x, corresponding to three different types of materials. Structural models are proposed for each of them.
1. Introduction
still rarely used as a microstructural Auger electron spectroscopy (AES).
The development of the plasma-enhanced chemical vapor deposition (PECVD) technique has allowed the production of a large variety of novel materials with disordered structure. Amorphous hydrogenated silicon carbide a-SiC:H is one of these materials. It presents a current great interest, not only technologically because of its potential applications due to its singular properties, but also fundamentally in regard of its complex structure. Many structural studies have been realized on a-Si,C,_,:H in the last ten years; they have required the use of a variety of specific analysis techniques which can bring information on the microstructure and local bonding [l]. We have previously undertaken microstructural investigations of a-Si,C,_,:H PECVD films using Xray photoelectron spectroscopy (XPS), infrared absorption spectroscopy (IR) and extended X-ray absorption fine structure (EXAFS) [2,31. In the present work, we develop a fine study based on complementary analyses by a technique which is
* To whom correspondence 0169-4332/93/$06.00
should be addressed.
probe, the
2. previous works In the Auger relaxation process XYZ, where X, Y and 2 are electronic levels of an excited atom, the initial state is a hole in level X; one electron falls from level Y to X and the released energy is dissipated in the ejection of a so-called “Auger” electron from level Z. In the final state, there are two holes in levels Y and Z. The kinetic energy of the Auger electron is defined by the three electronic levels involved in the transition and can be given by [41 E XYZ
=E(X)
-E(Y)
-E(Z)
- U(Y,
Z),
where E(X), E(Y) and E(Z) are the binding energies of the X, Y and Z levels and U(Y, Z) is the effective hole-hole interaction energy. Transitions involving only core levels like the SiIUL transition are basically interpreted using atom physics. A change in the chemical environment of an atom is then associated to a charge
0 1993 - Elsevier Science Publishers B.V. All rights reserved
346
E. Gat et al. / AES microstructural
inuestigations
transfer and the resulting shift of the energy of the Auger electrons can be related to the electronegativity of the neighbouring atoms [5]. In the case of core-valence transitions like the Si LW or CKW transitions, the interpretation is more complex. These transitions present generally a wide line shape whose fine structure can be related to the self-convolution of the local density of the valence states (LDOS) adjacent to the silicon and carbon atoms, respectively. The chemical effects are then a shift and a change in the shape of the complex spectra. Previous works [6-91 on the C KW Auger line shapes for various gaseous carbon molecules have illustrated its sensitivity to local sp” hybridization. Six components (ss, sp,, p,p,, sp,, p,p, and p,p,) determine the Auger peak fine structure. They correspond to the self-convolution of the valence states with s- and p-character, and their relative intensities can be correlated to the carbon hybridization state. In a solid, the valence bands are much broader and tend to smooth out this fine structure. Moreover, matrix, core-hole screening and final-state hole-hole correlation (localization) effects have generally to be considered for a correct understanding of spectral features; this was demonstrated by studies on various solids such as polyethylene, graphite, diamond and transition-metal carbides [6]. A detailed line-shape interpretation requires an elaborate fitting procedure associated to a thorough understanding of the main factors contributing to the Auger process. In the case of a-Si,C,_,:H compounds, the possible occurrence of several types of carbon local environments, involving C-C, C-Si and C-H bonds in various proportions, makes a fine theoretical interpretation difficult. The SiL,,,W transition has also been extensively subject to theoretical interpretations based on the self-convolution of the local density of states (LDOS) of the silicon atoms. Studies on hydrogenated amorphous silicon (a-Si:H) have shown that the whole L,,,W signal covering the kinetic energy range 50-100 eV can be split into three components [lo]: . a main component at around 90 eV is attributed to the L,,M,,M,,, transition and cor1 1
of plasma-deposited
a-Si,C,
_ x:H
responds to the valence electron density with pp character; . a secondary component at around 80 eV is attributed to the L,,M,M, s transition and corresponds to the valence electron density with sp character; . a third component at around 70 eV arises from a final-state hole-hole correlation. The last component does not arise in this work since the background removal procedure involves the SiL,,,W “true” Auger line shape to be extracted over the kinetic energy range 70-100 eV. A theoretical interpretation based on a fitting procedure into Gaussian components relating to sp and pp partial densities of states of the silicon atoms seems not to be well adapted to the case of an a-Si,C,_,:H compound. Indeed, several types of local environment affecting Si-C, Si-Si and Si-H bonds in various proportions can occur and make difficult a theoretical study which moreover will not bring accurate information about the microstructure of the films. So we have chosen to restrict our investigations to simple overall lineshape considerations. Compared to the typical form of the Auger peak of some reference materials, these valence spectra act as a “fingerprint” of a particular chemical state.
3. Experimental a-Si,C,_,:H films with various compositions and 200 nm thickness were prepared by a 110 kHz low-frequency glow discharge decomposition of a silane and methane mixture diluted by helium. The RF plasma system and the deposition conditions have been reported elsewhere [ll]. Auger measurements were performed under ultra-high vacuum with a Riber Mac 2 spectrometer using an incident electron beam with a diameter less than 1 pm. The spectra were scanned in the N(E) mode in two energy ranges: . a low-energy one between 25 and 550 eV in order to detect the Si LW, C KW and 0 KLL transitions; . a high-energy one between 1600 and 1650 eV in order to detect the SiKLL transition.
E. Gat et al. / AES microstructural
investigations
of plasma-deposited
a-Si,C,
347
_ .:H
The primary electron beam energy and spectrometer resolution were fixed respectively at 3 keV and 1 eV for the low-energy range and at 5 keV and 2 eV for the high-energy range. In order to remove surface contamination resulting from exposure to air, before the analyses the samples were submitted for 10 min to a bombardment with argon ions under an accelerating voltage of 3 kV.
4. Results and discussion Low-energy-range Auger spectra are shown in fig. 1 for a-Si,C, _,:H films with different compositions and for a polycrystalline P-Sic reference. The observed Auger features are the SiLW core-valence-valence transition around 90 eV and the C KW core-valence-valence transition around 270 eV. No detectable presence of oxygen is found on sputter-cleaned surfaces of the films as shown by the absence of the OKLL peak (energy 507 eV). The various a-Si,C,_,:H samples display a significant variation of the relative intensities of both Si LW and C KW peaks. The composition x has been determined from the relative peak height/background (P/B) ratios using the P-Sic sample as a reference 1121. In order to derive some microstructural information from the SiLW and CKW transitions, these peaks have been considered in detail after two kinds of background subtraction: the AES
100
500
300 Kinetic enwgy
@Iv)
Fig. 1. N(E) Auger spectra of a-Si,C,_,:H films: (a) p-Sic reference; (b) x = 0.25; (c) x = 0.50; (d) x = 0.70.
240
250
280
Fig. 2. CKW peak obtained for a-Si,C,_,:H films (x = 0, 0.25, 0.50 and 0.70) and two references, graphite and p-Sic, after removing primary and secondary electron background and Shirley subtraction of the energy-loss electron background.
data were primarily relieved of the background of primary and secondary electrons by a linear regression from the higher-energy side; then a Shirley subtraction of the energy-loss electron background led to the final peak [12]. Fig. 2 presents the final form of the CKW peak. The overall appearance of the Auger peak shape changes markedly on going from a carbonrich to a silicon-rich film and reveals a modification of the local density of the valence states adjacent to the carbon atoms. The main observations are as follows. The shape of the C KW peak of the silicon-rich film (composition x = 0.7) is typical of a carbide @i-C bonds) environment, illustrated by the P-Sic reference. The main peak centred at 267 eV is broadened at 253 and 248 eV. An accurate identification of these two shoulders is not easy though because plasmon losses obscure the fine details on the lower-energy side of the spectrum. This carbide character still exists even though slightly attenuated for the film with composition x = 0.5. The CKW peak of the SiO,,C,,,:H film displays a rather different line shape. Its broader form which does not present any visible fine
348
E. Gat et al. / AES microstrucrural investigations of plasma-deposited a-Si,C, _ ,:H
structure on the lower-energy side is characteristic of an alkane or alkane/diamond mixed type environment (C-C and C-H bonds in tetrahedral configuration) [6]. . The peak corresponding to the a-C:H film (x = 0) is far broader. It does not present the typical form obtained for the graphite with a marked shoulder located around 255 eV. Moreover, considering the broadness of the peak, we find no evidence to suggest a tetrahedral-coordinated alkane or diamond local type structure. The carbon local environment in the a-C:H film is probably of mixed alkane/diamond/graphite type. Fig. 3 shows the final form of the SiL,,,W peaks. The observed evolution of this Auger line shape interprets a strong modification of the local density of states (LDOS) of the silicon atoms. The peaks become narrower and their maximum shifts toward higher energies when the silicon content increases. The peak of the carbon-rich film (x = 0.25) presents the same characteristics as the P-Sic sample: it is relatively broad and its maximum occurs at 87 eV. The peak of the a-Si,,,C,,,:H film keeps similar characteristics.
Fig. 3. SiL,,,W peak obtained for a-Si,C,_,:H films (x = 0.25, 0.50, 0.70 and 1) and two references, B-Sic and Si(lOO), after the background subtraction procedure described in fig. 2.
This reveals that, for films with composition equal to, or lower than 0.5, the local environment of the silicon atoms is mainly composed of carbon atoms (carbide type arrangement). The much narrower peak obtained for the silicon-rich film (x = 0.7) reveals a local environment mainly composed of silicon atoms. The narrowest peak is obtained for the a-Si:H film (x = 1). Its line shape and posi-
b
1620
1615 Ktnatlc energy
1
1615
162P
(ev)
1615 Kinetic energy
1620 (ev)
$U
1615 Kinetic energy
1820 WI
Fig. 4. Fitting of the SiKL,,,L,,, p eaks using Gaussian components (table 1): (a) a-Sio~zCo~,s:H film and p-Sic reference; (b) a-Si:H film and c-Si(100) reference; (c) a-Si,C1 _,:H films (x = 0.5); (d) a-Si,C,_,:H films (x = 0.7).
349
E. Gat et al. / AES microstructural investigations of plasma-deposited a-Si,C, _ .:H
tion (89 eV> are quite similar to the ones of a c-Si sample. It is characteristic of tetrahedral environments such as Si[Si,l, Si[Si,Hl and even Si[Si,H2]. In order to obtain more precise information about the local environment of the silicon atoms, we studied the evolution of the SiKL,,,L%, peak. This transition, scanned in the high kinetic energy range (1600-1650 eV>, involves only the discrete core electronic K and L,,, levels. Therefore, any shift or change in line shape can be more easily interpreted; depending on charge transfers with surrounding atoms, they can be directly related to the local environment of the silicon atoms. Figs. 4a and 4b show, on a magnified scale, the peaks obtained for the crystalline references pSic and c-Si after background subtraction. They present the following characteristics: their fullwidth at half-maximum (FWHM) and energy position are respectively 3.4 and 1615.7 eV for P-Sic, 3.0 eV and 1617.5 eV for c-Si. The higher FWHM and the shifted position toward lower kinetic energies obtained for P-Sic are due to an increased charge transfer between the silicon atoms and their nearest neighbours when the latter are carbon atoms. We have also reported in figs. 4a and 4b the peaks obtained for the a-Si,,C,,,,:H and a-Si:H (x = 1) films in order to underline their respective remarkable similarity in line shape and position with the p-Sic and c-Si references. These similarities which have also been found for the SiL,,,W peaks denote two microstructural pieces of information: . in carbon-rich films, the silicon atoms are mainly bonded to carbon atoms in a P-Sic type local configuration (Si[C,] tetrahedral; . in the a-Si:H film, the local environment of the silicon atoms is mainly composed of Si[Si,l and Si[Si,H] tetrahedral units. In the case of a-Si,C,_,:H films with composition x = 0.5 and 0.7, microstructural information requires a fine study of the SiKL,,,L,, peaks. They have an energy position intermediate between those obtained for p-Sic and c-Si and slightly broader line shapes (FWHM = 3.5 eV> than that measured for P-Sic. In order to investigate the local environment of the silicon atoms
Table 1 Characteristics of the Gaussian components used in the fitting procedure of the SiKL,,,La,s peaks Component
Environments SK,1 Si[Si,C,H,] a Si[Si,], Si[Si,H]
1 2 3
Energy
FWHM
(eV)
(eV)
1615.7 1616-1617 1617.5
3.4 3-3.4 3
al n + m + I= 4 with n and m different from zero.
(no chemical ordering, chemical ordering with homogeneous dipersion or chemical ordering with phase separation [13,14]), we have fitted the SiKL 2,3L 2,3 peaks with Gaussian components whose characteristics are reported in table 1. Components 1 and 3 correspond to well defined Si[C,] and Si[Si,]/Si[Si,Hl local environments, respectively. Component 2 corresponds to all the possible tetrahedral local environments which are not taken into account by the two others. The results obtained for films with composition c = 0.5 and 0.7 are shown in figs. 4c and 4d. The peak of the a-Si,,C,,,:H film fits into two components: the stronger one (1) reveals the predominance of a chemical ordering of Si[C,] type and the weaker one (2) indicates the appearance of other tetrahedral local environments. For the a-Si,&,,:H film, the three components presented in table 1 had to be used in order to realize a suitable
loo-
8,O
;/,
02
034
0,a
/ 0,6
COMPOSITION
D
X
Fig. 5. Evolution of SiKL,,,L,,s fitting Gaussian components (table 1) as a function of the composition x of the a-Si,C, _,:H films.
E. Gat et al. / AES microstmctural
350
investigations
of plasma-deposited
a-Si,C,
_ .:H
3. Conclusion
161 8’ 90
092
0,4
‘396
COMPOSITION
Fig. 6. Evolution of SiKL,,,L,,, transition composition x of the a-Si,C,_,:H
036
1,o
X
energy films.
with
the
deconvolution which reveals an absence of chemical ordering. We have reported in fig. 5 the evolution with the silicon content of the relative contributions of each component. Some drastic change appears in the local environment of the silicon atoms, when the silicon content increases in the films. For x < 0.5, the predominance of component 1 indicates a /?-Sic type local chemical ordering in the form of Si[C,] units. Above x = 0.5, the coexistence of the three components defined in table 1 and the increased contribution of component 2 reveal the heterogeneous character of the microstructure increasingly disordered. Fig. 6 reports the evolution of the energy of the Si KLa,L2 3 Auger transition with the composition of the films. We do not observe a linear increase as it would be the case if a random replacement occurred of the carbon atoms by silicon atoms in a tetrahedral network. The weaker increase of the transition energy in the carbon-rich composition range can be explained as follows: the remaining Si[C,] local environment attenuates the increased shift of the Auger transition towards higher energies, which is correlated to the progressive replacement of carbon atoms by silicon atoms in the nearest neighbouring of silicon.
The AES technique has been succesfully used as a microstructural probe for amorphous aSi,C,_,:H PECVD films. Information on the local environment of the carbon and silicon atoms has been derived from a fine study of the CKW, SiLW and SiKLL Auger transitions including line shape and peak shift considerations. This information completes the results previously obtained by XPS, IR and EXAFS (Si K-edge) analyses [2,31 and allows the proposition of a structural model which accounts for a marked evolution of the microstructure of the films with their composition. Three composition ranges are distinguished: Carbon-rich films. Both examinations of the Si LW and SiKLL peaks show, in agreement with the XPS and EXAFS analyses, that the silicon atoms are mainly bonded to four carbon atoms. The EXAFS analyses [3], moreover, have revealed the existence of second-nearest neighbours of a P-Sic structure, which proves that the local environment of the silicon atoms is ordered. This ordering has been reported by other authors in recent works on a-Si,C,_,:H films [14-171, but the degree of chemical order is not clearly defined. The typical line shape of the CKW peak is characteristic of a carbon local environment of an alkane or alkane/diamond type. This alkane character is related to the existence of alkyl groups as brought to light by IR analyses [2]. In a NMR study of a-Si,C,_,:H PECVD films Petrich et al. [18] explained the presence of these groups by the incorporation of hydrocarbon species in the films during the deposition process. The assumption of a presence of graphitic sp2 type local environments based on XPS analyses [2] is not supported by the AES results except for a-C:H films. The global structure of the carbon-rich films cannot be accurately defined. It is probably close to the one of an almost fully cross-linked polymer (hydrogenated polycarbon) incorporating silicon atoms in a P-Sic type local configuration. Films with composition
close to stoichiometry.
The persistence of a P-Sic type local ordering around the silicon atoms, highlighted by EXAFS, is confirmed by the fine study of the SiKLL
E. Cat et al. / AES microstructural investigations of plasma-deposited a-Si,C, _ .:H
Auger transition. On the other hand, a carbide character (Sic) of the carbon environment, not disclosed by XPS and IR studies, is demonstrated by a simple overall line-shape observation of the CKVV peak. The structure of these films, in which the amount of Si-C bonds is maximum, as revealed by IR analyses, can be considered to be close to the one of a locally ordered a-Sic tetrahedral network incorporating some hydrogen atoms bonded to carbon rather than to silicon. Silicon-rich films. The SiKLL Auger peak fine study reveals, in agreement with the XPS, IR and EXAPS results, that the local environment of the silicon atoms presents a heterogeneous character and an absence of local ordering. The local environment of the carbon atoms (which are minor species in the silicon-rich films) still has a carbide Sic character as displayed by the film of the CKVV peak. The structure of these films can be described by a disordered tetrahedral a-Si network incorporating some hydrogen atoms in the form of SiH and SiH, groups and carbon atoms in a sp3 carbide type configuration.
References [l]
J. Bullot and M.P. Schmidt, (1987) 345.
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