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Influence of temperature on the structure of SiC coatings prepared by dynamic ion mixing
&WtOtUD RELATED MATERtAtS Diamond and Related Materials 4 (1995) 1340-1345
Influence of temperature on the structure of SIC coatings prepared by dynamic ion mixing M. Zaytouni, J.P. Rivikre, Ph. Goudeau Laboratoire de mttallurgie physique - URA 131 40, Avenue du Recteur Pineau, 86022 Poitiers, France Received
13 April 1995; accepted
in final form 19 July 1995
Abstract We deposited silicon carbide films, 0.5 pm and 0.86 pm thick at room temperature (RT) and 750 “C on (100) silicon wafers and TA6V substrates. An SIC target was sputtered with a 1.2 keV Ar+ ion beam delivered by a Kaufman-type ion source, and the growing films were continuously bombarded with a beam of 160 keV Ar ’ ions. The microstructural state of the films was investigated by complementary techniques: transmission electron microscopy (TEM), high-resolution TEM, glancing X-ray diffraction (GXRD) and Fourier transform infrared spectroscopy (FTIR). All these characterization methods show that the bombardment of the growing films induces important structural changes. The SIC films prepared at RT without mixing are amorphous, whereas those deposited by dynamic ion mixing (DIM) at RT exhibit the beginning of crystallization of the P-Sic phase. At 750 “C the films prepared by DIM are formed of nanocrystallized grains of the cubic I-Sic phase. Keywords:
Silicon carbide; Dynamic ion mixing; Coating; Phase identification
1. Introduction Sic is a covalently bonded material with tetrahedral coordination, and many polytypes have been identified [ 1,2]. However, the fl or cubic phase isomorphic to diamond is the most stable and most important one. A number of different techniques have been reported for the production of P-Sic, such as C+ implantation into Si [3,4], ion beam mixing [ 5,6] or more recently dynamic ion mixing [ 71 and chemical vapour deposition methods [S-11], which are the most developed ones for producing large areas of either polycrystalline or single crystalline [ 121 Sic. However, the major disadvantage of this last category of methods is to require high deposition temperatures (T > 1OOO’C) which are often incompatible with the structural stability of steel or Ti base alloys used in many tribological applications. The recently developed DIM technique [ 131 has been proposed as an alternative to synthesize P-Sic coatings at moderate temperature. The DIM method combines continuous deposition with a high-energy (100-300 keV) heavy ion beam. The high degree of control over the flux and energy of the incident ion beam allows the microstructural state to be modified in a controlled way. In addition, energetic ions induce dense displacement cascades and produce an effective collisional mixing at 0925-9635/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDI 0925-9635(95)00315-O
the coating-substrate interface in the first step of deposition, which markedly improves the coating adhesion [ 14,153. Another unique feature of ion bombardment is the possibility to induce crystallization at a much lower temperature than by a thermal treatment alone [ 161. In this paper we report on the structural characterization of Sic films, 0.5-0.86 pm thick, deposited by DIM at both room temperature and 750 “C. The objective of this study is to investigate the influence of both DIM and temperature on the structure of Sic coatings.
2. Experimental details 2.1. Coating deposition The Sic coatings were deposited at room temperature and at 750°C by a sputtering method using a broad beam ion source of Kaufman type with an apparatus described previously [ 13,143. The base pressure prior to deposition was better than 5 x 10Bs Pa and was maintained at 5 x 10e3 Pa during deposition. The energy of Ar+ sputtering ions was 1.2 keV and the beam intensity was 80 mA. A calibrated quartz crystal oscillator was used to monitor the deposition rate and the final thickness. Ion beam mixing during deposition was performed
1341
M. Zaytouni et al./Diamond and Related Materials 4 (199s) 1340-1345
with 160 keV Ar + ions accelerated by a 200 keV ion implanter operating in line with the deposition chamber. The ion per atom arrival rate ratio in these experiments was Q=7.5 x 10P3. The substrates used for this work were (lOO)-oriented silicon wafers and TA6V substrates. The wafers were etched in 10% HF immediately before deposition, cleaned in an ultrasonic bath, rinsed in deionized water and finally dried under a flux of argon. The TA6V substrates were polished with 0.02 l.trn alumina paste and rinsed with acetone and methanol. For the film elaborated at 750 “C we used a chromelalumel thermocouple welded to the substrate holder for the temperature measurement. 2.2. Methods of characterization Fourier transform infrared spectroscopy (FTIR) was used to study the structural evolution of the films deposited on the silicon substrate and to characterize the molecular bonding in the Sic coatings leading to short-range information on the structural state. The phase analysis of the deposited films was performed by glancing X-ray diffraction (GXRD) with CuK, radiation and by transmission electron microscopy (TEM). The characteristics of the specimens are given in Table 1. A quantitative chemical analysis was performed by X-ray photoelectron spectroscopy and Rutherford backscattering spectroscopy, indicating that the films are very close to the stoichiometric composition 42 at.% C-58 at.% Si.
(b) Fig. 1. Cross-sectional TEM micrographs of Sic coating deposited on Si substrate without DIM (series Al). (a) Diffraction pattern and bright field image; (b) dark field image.
3. Results and discussion 3.1. Transmission electron microscopy Fig. 1 shows a bright field cross-sectional micrograph and the corresponding diffraction pattern of an Sic film deposited without DIM at room temperature. It is to be noted that a very sharp interface exists between the Sic coating and the TA6V substrate with a very uniform contrast. The diffraction pattern is formed of two very diffuse rings, indicating that coating Al is amorphous. For the film of series A2 (Fig. 2) produced by DIM,
the diffraction pattern consists of three rings, which are still a little diffuse, indicating that the partial crystallization of this coating has occurred. Moreover, the behaviour of the Sic/substrate interface indicates the presence of a large mixed area. This interfacial region is the consequence of the mixing effect induced by energetic Arf ions which penetrate deeply into the substrate in the first steps of deposition. We can also notice that the SiC/Si interface is relatively rough. The two Sic coatings Bl and B2 deposited at 750 “C produced either without DIM or with DIM had clearly
Table 1 Characteristics of the coatings deposited on Si (100) and TA6V substrates Series
Al A2 Bl B2
e (nm) 840 860 500 500
Temperature of deposition (“C)
Deposition conditions
A (%)
0.4 0.31 0.8 0.8
20 (RT) 20 (RT) 750 750
Without By DIM Without By DIM
Deposition rate
DIM with 160 keV Ari+ DIM with 160 keV Ar+
1342
M. Zaytouni et al./Diumond and Related Materials 4 (1995) 1340-1345
(‘4 Fig. 2. Cross-sectional TEM micrographs of Sic coating deposited on Si substrate with DIM (series A2). (a) Diffraction pattern and bright field image; (b) dark field image.
different morphologies, as shown in Figs. 3(a) and (b) (series Bl and B2). The electron diffraction pattern and the cross-sectional HRTEM of Fig. 3(b) indicate clearly that the coating is nanocrystallized in the P-Sic phase. The measured separation between parallel fringes in the crystalline grains of the Sic layer gives a value of 0.25 nm which corresponds very well to the (111) interplanar distance of /?-Sic. This result is confirmed by the measurements of the lattice spacing from the successive concentric rings on the diffraction pattern. The average size of the nanocrystallized grains is about 30-50 A. One can see at the substrate/coating interface the presence of a variable amorphous layer of about 2.5 nm, corresponding very likely to either a remaining SiOZ amorphous layer or to an amorphous Si layer produced by the ion bombardment in the first step of the deposition. 3.2. Fourier transform infrared spectroscopy Infrared spectroscopy is a powerful method for obtaining short-range order information on the structure of the Sic coating; however, FTIR alone cannot determine
whether the film is completely crystallized. In order to have information on the long-range ordered structure, X-ray diffraction experiments and electron diffraction are necessary. With the FTIR analysis technique the a and P-Sic phases give distinct and characteristic peaks. Based on previous work [5,8,17-J, the cubic silicon carbide has a transverse optical mode absorption peak at 794 cm- ‘. Silicon substrates can be used to investigate the 400-1300 cm-l region because of their transparency in this range of infrared (IR) frequencies. The infrared spectra were measured using a BOMEN MB 100 spectrometer. Fig. 4 illustrates the typical spectra obtained on the Sic films deposited at room temperature and at 750 “C. The spectra produced without and with DIM at room temperature (coatings Al and A2) exhibit a broad absorption band characteristic of a disordered material. Conversely, the coatings deposited at 750 “C (Bl and B2) give a prominent absorption peak centred around 800 cm-‘, which corresponds fairly well with the absorption peak produced by the Si-C stretching vibration mode of P-Sic. This result, together with the TED/TEM results presented in the previous paragraph, is a strong indication that the combination of an energetic ion bombardment and of a moderate substrate temperature during deposition increased the number of tetrahedral silicon-carbide bonds. This observation is in agreement with the high density of this coating which is close to the bulk value (d=3.21). The density determination is based on the precise measurement of the critical angle Bcfor the total reflection of X-rays. Indeed, the refractive index of a material is given by n= 1 - 6--i/?, where 1 -d characterizes the reflected part of the incident wave and the critical angle 8, for total reflection is given by qc= (2d)“2, where 6 is proportional to the film density 6 = Kd and K is a constant depending only on the X-ray wavelength and atomic parameters. The Bc values for films Bz and B1 are respectively 4.53 and 4.41 x 10e3 rad. giving density values of 3.20 f 0.03 and 3.18 + 0.03. Moreover, for the coating produced by DIM the peak became sharper and the intensity increased, which indicates improved crystallization. 3.3. Glancing X-ray dij+Yaction The typical GXRD patterns corresponding to the coatings Al, A2, Bl and B2 are shown respectively in Figs. 5 and 6. It is to be noted that the films for which the cubic phase has been detected exhibit a peak at 20 = 35.6” characteristic of the (111) Bragg peaks of the /I-Sic phase. This diffraction peak was also observed for the coating Al deposited at room temperature and after a postannealing treatment up to 950°C (Fig. 5). When the film was prepared at room temperature without annealing,
1343
M. Zaytouni et al./Diamond and Related Materials 4 (1995) 1340-1345
04
(4 Fig. 3. Cross-sectional 7.50 “C with DIM.
HRTEM
micrographs
of SiC/Si sample:
(a) coating
the peak consist only of a broad background typical of an amorphous material. Fig. 6 shows the GXRD patterns (a=4.5”) for the films deposited at 750 “C. The diffraction peaks became higher and sharper with increasing temperature. Moreover, the DIM treatment leads to a reduction in the peak width, which is synonymous with an improvement of the crystallized B-Sic structure. The DIM technique seems to be useful for the formation of cubic silicon carbide films by assisting the growth process and reducing the temperature of crystallization. The diffractograms of the coatings show relatively broad peaks, which can be explained by the small grain sizes (Table 2). This observation was also confirmed by TEM. Indeed an average grain size Lhklof small particles can also be determined from the broadening of the Bragg peaks defined by Miller indices (hkl) using the Scherrer formula [ 181 Lhkl=
A Ae cos e
where A8 is the full width at half-maximum X-ray diffraction experiments are difficult formed of atoms of low atomic numbers as SIC films, for which carbon and silicon have
(1) intensity. for coatings for instance a very small
Bl deposited
at 750 “C without
mixing;
(b) coating
B2 deposited
at
cross-section for X-rays. Indeed for the diffractometer used in this study the power of the X-ray source is not sufficient to detect the other Bragg peaks because of their weak intensity with regard to the (111) Bragg peak. X-ray diffraction experiments using the intense synchrotron radiation of LURE (Laboratoire pour I’utilisation de rayonnement synchrotron ORSAY, France) have been performed in order to obtain more information on the average long-range ordered structure of our crystallized Sic coatings. Fig. 7 shows as an example the diffraction pattern for coating B2 obtained by this method. It can be seen clearly that many diffraction peaks appear on the spectrum which can be identified as the ( 11l), (220) and (311) peaks of B-Sic. The enhanced crystallization induced by ion bombardment during deposition has often been observed previously and can be understood on the basis of nucleation enhanced inside the displacement cascades and growth enhanced via the high defect concentrations which increases the diffusion rate. Computer simulations of growing films bombarded with ions [ 19,201 suggest also that the densification process is the consequence of atomic rearrangements induced by voids and atomic transport on the surface. Both experiments and collisional models indicate that a critical value of about 1
M. Zaytouni et aLlDiamond and Related Materials 4 (1995) 1340-1345
1344
a = 4.5”
25
30
35
40
4.5
20 (deg.) Fig. 6. GXRD spectra of Sic films deposited at 750 “C. Bl: deposited without DIM; B2: deposited with DIM.
800
1000
600
400
Table 2 Spectral widths of the different coatings of Figs. 5 and 6 and calculation of the particle size
WAVENUhlBER(emA) Series Fig. 4. FTIR spectra of selected Sic films. Al and A2 deposited at 20 “C respectively without and with mixing. Bl and B2 deposited at 750 “C respectively without and with mixing.
.i..> :.“; .-TY:. .,. .‘:. ‘.‘;.: ..&! .;<.-. , ..-.+<.. .? .:!A:.;., , .__L. .) -.<>.. . ‘-‘:.y:‘,.;. ‘.‘:7_ _ +J.” .:; :_.> . *_...:j Al
25
I
*
.
I
30
,
.
,
.
I
3.5 28 (deg.)
,
.
.
.
I
40
.
,
.
8.03 7.34 3.4 2.9
1154 1354 27+4 3254
dpa (displacement per target atom) is in general necessary for complete densification and crystallization as long as the defect mobility is high enough [ 16,191. We have calculated the damage profile in the growing Sic film using the TRIM Monte Carlo computer simulation program [ 211. The displacement threshold energy E. used in the calculations was Ed = 40 eV [22]. We can see on Fig. 8 that a uniform damage of about 8 dpa is generated in the growing film during ion bombardment, which is well above the critical value of 1 dpa. These calculations seem to support that both densification and crystallization during bombardment of growing film are correlated with displacement cascade effects.
,:.Y a = 4.5” . i.,.*: ..-.. A’ ‘.. .;: c. . _.:? ‘.. ..,,“’ ‘;C.‘: “i;.: ,. .; ‘. ;:.*.z;...... .-;x. :..*: ’ .-.<.,, .*“..:y.. %,
.
Al +950 “C A2 + 950 “C Bl B2
4. Conclusion
,
4s
Fig. 5. GXRD spectra of Sic films after a post-annealing treatment up to 950 “C. Al: deposited without DIM; A2: deposited with DIM.
The DIM technique was applied to produce cubic silicon carbide coatings at a moderate temperature. The results indicate that the films produced without DIM at room temperature are amorphous; conversely, the films produced by DIM at room temperature exhibit the beginning of crystallization and the coatings deposited
1345
M. Zaytouni et al./Diamond and Related Materials 4 (1995) 1340-1345
70
75
Fig. 7. Synchrotron GXRD spectrum of B2 coating deposited at 750 “C with DIM.
Cl1 L.K. Frevel, D.R. Peterson
and C.K. Saka, J. Mater. Sci., 27 (1992) 1913. PI H. Matsunami, Diamond Relat. Mater., 2 (1993) 1043. c31 T. Kimura, S. Kagiyama and S. Yugo, Thin Solid Films, 94 (1982) 191. c41 K.Kh. Nussupov, V.O. Sigle and N.B. Bysenkhanov, Nucl. Instrum. Methods Phys. Res. B, 82 (1993) 69. CSI T. Kimura, Y. Tatebe, A. Kawamura, S. Yugo and Y. Adachi, Jpn. J. Appl. Phys., 24 (1985) 1712. C61 J.P. Riviere, M. Zaytouni, J. Delafond and J. Allain, Mater. Sci. Eng., 829 (1995) 105. c71 J.P. Riviere, M. Zaytouni and J. Delafond, Surf Coat. Technol., 67 (1994) 43-49.
0
1000
2000
3000
4000
5000
dep~(h
Fig. 8. Calculated damage depth profile in a 500 nm thick SIC coating deposited by DIM with 160 keV Ar+ ions (DPA=displacement per target atom).
at 750 “C with DIM exhibit crystallized structure in the cubic P-Sic phase. It is known that ion irradiation may be used for lowtemperature crystallization of amorphous surface layers in silicon. It appears from this study that the collisional damage produced by high-energy heavy ions also has crucial importance for the crystallization of Sic.
Acknowledgements
The authors wish to thank M.F. Denanot and C. Fayoux for their effective assistance in TEM observations and mixing experiments.
C81 M.P. Delplancke, J.M. Powers, G.J. Vandentop, M. Salmeron and G.A. Somojai, J. Vat. Sci. Technol. A, 9 (3) (1991) 450. c91 H.J. Kim, R.F. Davis, X.B. Cost and R.W. Linton, J. Electrochem. Sot., 134 (9) (1987) 2269. Cl01 M.A. Pickering, R.L. Taylor and J.T. Keeley, Nucl. Instrum. Methods Phys. Res. A, 291 (1990) 95. Cl11 A. Adamiano and J.A. Sprague, Appl. Phys. Lett., 44 (5) (1984) 525. Cl21 M.I. Chaudhry and R.L. Wright, J. Mater. Res., 5 (8) (1990) 1595. Cl31 M. Jaulin, G. Laplanche, J. Delafond and S. Pimbert, Surf. Coat. Technol., 37 (1989) 225. Cl41 J.P. Rivitre, P. Guesdon, J. Delafond, M.F. Denanot, G. Farges and D. Degout, Surf. Coat. Technol., 45 (1991) 83. Cl51 J. Von Stebut, J.P. Riviere, J. Delafond, C. Sarrazin and S. Michaux, Mater. Sci. Eng. A, 115 (1989) 267. Cl61 J.P. Riviere, P. Guesdon, J. Delafond, M.F. Denanot, G. Farges and D. Degout, Thin Solid Films, 204 (1991) 151. Cl71 S.J. Ting, C.J. Chu, J.D. Mackenzie, J. Mater. Res., 7 (1992) 164. Cl81 H.P. Klug and L.E. Alexander, in X-Ray Difraction Procedures,
John Wiley, New York, 1974. Cl91 K.H. Muller, Phys. Rev. B, 28 (1987) 527. c201 D.R. Brighton and G.K. Hubler, Nucl. Instrum. Methods Phys. Res. B, 28 (1987) 527. c211 J.P. Biersack and L.G. Haggmark, Nucl. Instrum. Methods, 174 (1980) 257. c221 F.W. Clinard, in R.A. Johnson et al. (eds.), Physics of Radiation Ejects in Crystals, Elsevier, 1988, p. 392.
Influence of temperature on the structure of SIC coatings prepared by dynamic ion mixing M. Zaytouni, J.P. Rivikre, Ph. Goudeau Laboratoire de mttallurgie physique - URA 131 40, Avenue du Recteur Pineau, 86022 Poitiers, France Received
13 April 1995; accepted
in final form 19 July 1995
Abstract We deposited silicon carbide films, 0.5 pm and 0.86 pm thick at room temperature (RT) and 750 “C on (100) silicon wafers and TA6V substrates. An SIC target was sputtered with a 1.2 keV Ar+ ion beam delivered by a Kaufman-type ion source, and the growing films were continuously bombarded with a beam of 160 keV Ar ’ ions. The microstructural state of the films was investigated by complementary techniques: transmission electron microscopy (TEM), high-resolution TEM, glancing X-ray diffraction (GXRD) and Fourier transform infrared spectroscopy (FTIR). All these characterization methods show that the bombardment of the growing films induces important structural changes. The SIC films prepared at RT without mixing are amorphous, whereas those deposited by dynamic ion mixing (DIM) at RT exhibit the beginning of crystallization of the P-Sic phase. At 750 “C the films prepared by DIM are formed of nanocrystallized grains of the cubic I-Sic phase. Keywords:
Silicon carbide; Dynamic ion mixing; Coating; Phase identification
1. Introduction Sic is a covalently bonded material with tetrahedral coordination, and many polytypes have been identified [ 1,2]. However, the fl or cubic phase isomorphic to diamond is the most stable and most important one. A number of different techniques have been reported for the production of P-Sic, such as C+ implantation into Si [3,4], ion beam mixing [ 5,6] or more recently dynamic ion mixing [ 71 and chemical vapour deposition methods [S-11], which are the most developed ones for producing large areas of either polycrystalline or single crystalline [ 121 Sic. However, the major disadvantage of this last category of methods is to require high deposition temperatures (T > 1OOO’C) which are often incompatible with the structural stability of steel or Ti base alloys used in many tribological applications. The recently developed DIM technique [ 131 has been proposed as an alternative to synthesize P-Sic coatings at moderate temperature. The DIM method combines continuous deposition with a high-energy (100-300 keV) heavy ion beam. The high degree of control over the flux and energy of the incident ion beam allows the microstructural state to be modified in a controlled way. In addition, energetic ions induce dense displacement cascades and produce an effective collisional mixing at 0925-9635/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDI 0925-9635(95)00315-O
the coating-substrate interface in the first step of deposition, which markedly improves the coating adhesion [ 14,153. Another unique feature of ion bombardment is the possibility to induce crystallization at a much lower temperature than by a thermal treatment alone [ 161. In this paper we report on the structural characterization of Sic films, 0.5-0.86 pm thick, deposited by DIM at both room temperature and 750 “C. The objective of this study is to investigate the influence of both DIM and temperature on the structure of Sic coatings.
2. Experimental details 2.1. Coating deposition The Sic coatings were deposited at room temperature and at 750°C by a sputtering method using a broad beam ion source of Kaufman type with an apparatus described previously [ 13,143. The base pressure prior to deposition was better than 5 x 10Bs Pa and was maintained at 5 x 10e3 Pa during deposition. The energy of Ar+ sputtering ions was 1.2 keV and the beam intensity was 80 mA. A calibrated quartz crystal oscillator was used to monitor the deposition rate and the final thickness. Ion beam mixing during deposition was performed
1341
M. Zaytouni et al./Diamond and Related Materials 4 (199s) 1340-1345
with 160 keV Ar + ions accelerated by a 200 keV ion implanter operating in line with the deposition chamber. The ion per atom arrival rate ratio in these experiments was Q=7.5 x 10P3. The substrates used for this work were (lOO)-oriented silicon wafers and TA6V substrates. The wafers were etched in 10% HF immediately before deposition, cleaned in an ultrasonic bath, rinsed in deionized water and finally dried under a flux of argon. The TA6V substrates were polished with 0.02 l.trn alumina paste and rinsed with acetone and methanol. For the film elaborated at 750 “C we used a chromelalumel thermocouple welded to the substrate holder for the temperature measurement. 2.2. Methods of characterization Fourier transform infrared spectroscopy (FTIR) was used to study the structural evolution of the films deposited on the silicon substrate and to characterize the molecular bonding in the Sic coatings leading to short-range information on the structural state. The phase analysis of the deposited films was performed by glancing X-ray diffraction (GXRD) with CuK, radiation and by transmission electron microscopy (TEM). The characteristics of the specimens are given in Table 1. A quantitative chemical analysis was performed by X-ray photoelectron spectroscopy and Rutherford backscattering spectroscopy, indicating that the films are very close to the stoichiometric composition 42 at.% C-58 at.% Si.
(b) Fig. 1. Cross-sectional TEM micrographs of Sic coating deposited on Si substrate without DIM (series Al). (a) Diffraction pattern and bright field image; (b) dark field image.
3. Results and discussion 3.1. Transmission electron microscopy Fig. 1 shows a bright field cross-sectional micrograph and the corresponding diffraction pattern of an Sic film deposited without DIM at room temperature. It is to be noted that a very sharp interface exists between the Sic coating and the TA6V substrate with a very uniform contrast. The diffraction pattern is formed of two very diffuse rings, indicating that coating Al is amorphous. For the film of series A2 (Fig. 2) produced by DIM,
the diffraction pattern consists of three rings, which are still a little diffuse, indicating that the partial crystallization of this coating has occurred. Moreover, the behaviour of the Sic/substrate interface indicates the presence of a large mixed area. This interfacial region is the consequence of the mixing effect induced by energetic Arf ions which penetrate deeply into the substrate in the first steps of deposition. We can also notice that the SiC/Si interface is relatively rough. The two Sic coatings Bl and B2 deposited at 750 “C produced either without DIM or with DIM had clearly
Table 1 Characteristics of the coatings deposited on Si (100) and TA6V substrates Series
Al A2 Bl B2
e (nm) 840 860 500 500
Temperature of deposition (“C)
Deposition conditions
A (%)
0.4 0.31 0.8 0.8
20 (RT) 20 (RT) 750 750
Without By DIM Without By DIM
Deposition rate
DIM with 160 keV Ari+ DIM with 160 keV Ar+
1342
M. Zaytouni et al./Diumond and Related Materials 4 (1995) 1340-1345
(‘4 Fig. 2. Cross-sectional TEM micrographs of Sic coating deposited on Si substrate with DIM (series A2). (a) Diffraction pattern and bright field image; (b) dark field image.
different morphologies, as shown in Figs. 3(a) and (b) (series Bl and B2). The electron diffraction pattern and the cross-sectional HRTEM of Fig. 3(b) indicate clearly that the coating is nanocrystallized in the P-Sic phase. The measured separation between parallel fringes in the crystalline grains of the Sic layer gives a value of 0.25 nm which corresponds very well to the (111) interplanar distance of /?-Sic. This result is confirmed by the measurements of the lattice spacing from the successive concentric rings on the diffraction pattern. The average size of the nanocrystallized grains is about 30-50 A. One can see at the substrate/coating interface the presence of a variable amorphous layer of about 2.5 nm, corresponding very likely to either a remaining SiOZ amorphous layer or to an amorphous Si layer produced by the ion bombardment in the first step of the deposition. 3.2. Fourier transform infrared spectroscopy Infrared spectroscopy is a powerful method for obtaining short-range order information on the structure of the Sic coating; however, FTIR alone cannot determine
whether the film is completely crystallized. In order to have information on the long-range ordered structure, X-ray diffraction experiments and electron diffraction are necessary. With the FTIR analysis technique the a and P-Sic phases give distinct and characteristic peaks. Based on previous work [5,8,17-J, the cubic silicon carbide has a transverse optical mode absorption peak at 794 cm- ‘. Silicon substrates can be used to investigate the 400-1300 cm-l region because of their transparency in this range of infrared (IR) frequencies. The infrared spectra were measured using a BOMEN MB 100 spectrometer. Fig. 4 illustrates the typical spectra obtained on the Sic films deposited at room temperature and at 750 “C. The spectra produced without and with DIM at room temperature (coatings Al and A2) exhibit a broad absorption band characteristic of a disordered material. Conversely, the coatings deposited at 750 “C (Bl and B2) give a prominent absorption peak centred around 800 cm-‘, which corresponds fairly well with the absorption peak produced by the Si-C stretching vibration mode of P-Sic. This result, together with the TED/TEM results presented in the previous paragraph, is a strong indication that the combination of an energetic ion bombardment and of a moderate substrate temperature during deposition increased the number of tetrahedral silicon-carbide bonds. This observation is in agreement with the high density of this coating which is close to the bulk value (d=3.21). The density determination is based on the precise measurement of the critical angle Bcfor the total reflection of X-rays. Indeed, the refractive index of a material is given by n= 1 - 6--i/?, where 1 -d characterizes the reflected part of the incident wave and the critical angle 8, for total reflection is given by qc= (2d)“2, where 6 is proportional to the film density 6 = Kd and K is a constant depending only on the X-ray wavelength and atomic parameters. The Bc values for films Bz and B1 are respectively 4.53 and 4.41 x 10e3 rad. giving density values of 3.20 f 0.03 and 3.18 + 0.03. Moreover, for the coating produced by DIM the peak became sharper and the intensity increased, which indicates improved crystallization. 3.3. Glancing X-ray dij+Yaction The typical GXRD patterns corresponding to the coatings Al, A2, Bl and B2 are shown respectively in Figs. 5 and 6. It is to be noted that the films for which the cubic phase has been detected exhibit a peak at 20 = 35.6” characteristic of the (111) Bragg peaks of the /I-Sic phase. This diffraction peak was also observed for the coating Al deposited at room temperature and after a postannealing treatment up to 950°C (Fig. 5). When the film was prepared at room temperature without annealing,
1343
M. Zaytouni et al./Diamond and Related Materials 4 (1995) 1340-1345
04
(4 Fig. 3. Cross-sectional 7.50 “C with DIM.
HRTEM
micrographs
of SiC/Si sample:
(a) coating
the peak consist only of a broad background typical of an amorphous material. Fig. 6 shows the GXRD patterns (a=4.5”) for the films deposited at 750 “C. The diffraction peaks became higher and sharper with increasing temperature. Moreover, the DIM treatment leads to a reduction in the peak width, which is synonymous with an improvement of the crystallized B-Sic structure. The DIM technique seems to be useful for the formation of cubic silicon carbide films by assisting the growth process and reducing the temperature of crystallization. The diffractograms of the coatings show relatively broad peaks, which can be explained by the small grain sizes (Table 2). This observation was also confirmed by TEM. Indeed an average grain size Lhklof small particles can also be determined from the broadening of the Bragg peaks defined by Miller indices (hkl) using the Scherrer formula [ 181 Lhkl=
A Ae cos e
where A8 is the full width at half-maximum X-ray diffraction experiments are difficult formed of atoms of low atomic numbers as SIC films, for which carbon and silicon have
(1) intensity. for coatings for instance a very small
Bl deposited
at 750 “C without
mixing;
(b) coating
B2 deposited
at
cross-section for X-rays. Indeed for the diffractometer used in this study the power of the X-ray source is not sufficient to detect the other Bragg peaks because of their weak intensity with regard to the (111) Bragg peak. X-ray diffraction experiments using the intense synchrotron radiation of LURE (Laboratoire pour I’utilisation de rayonnement synchrotron ORSAY, France) have been performed in order to obtain more information on the average long-range ordered structure of our crystallized Sic coatings. Fig. 7 shows as an example the diffraction pattern for coating B2 obtained by this method. It can be seen clearly that many diffraction peaks appear on the spectrum which can be identified as the ( 11l), (220) and (311) peaks of B-Sic. The enhanced crystallization induced by ion bombardment during deposition has often been observed previously and can be understood on the basis of nucleation enhanced inside the displacement cascades and growth enhanced via the high defect concentrations which increases the diffusion rate. Computer simulations of growing films bombarded with ions [ 19,201 suggest also that the densification process is the consequence of atomic rearrangements induced by voids and atomic transport on the surface. Both experiments and collisional models indicate that a critical value of about 1
M. Zaytouni et aLlDiamond and Related Materials 4 (1995) 1340-1345
1344
a = 4.5”
25
30
35
40
4.5
20 (deg.) Fig. 6. GXRD spectra of Sic films deposited at 750 “C. Bl: deposited without DIM; B2: deposited with DIM.
800
1000
600
400
Table 2 Spectral widths of the different coatings of Figs. 5 and 6 and calculation of the particle size
WAVENUhlBER(emA) Series Fig. 4. FTIR spectra of selected Sic films. Al and A2 deposited at 20 “C respectively without and with mixing. Bl and B2 deposited at 750 “C respectively without and with mixing.
.i..> :.“; .-TY:. .,. .‘:. ‘.‘;.: ..&! .;<.-. , ..-.+<.. .? .:!A:.;., , .__L. .) -.<>.. . ‘-‘:.y:‘,.;. ‘.‘:7_ _ +J.” .:; :_.> . *_...:j Al
25
I
*
.
I
30
,
.
,
.
I
3.5 28 (deg.)
,
.
.
.
I
40
.
,
.
8.03 7.34 3.4 2.9
1154 1354 27+4 3254
dpa (displacement per target atom) is in general necessary for complete densification and crystallization as long as the defect mobility is high enough [ 16,191. We have calculated the damage profile in the growing Sic film using the TRIM Monte Carlo computer simulation program [ 211. The displacement threshold energy E. used in the calculations was Ed = 40 eV [22]. We can see on Fig. 8 that a uniform damage of about 8 dpa is generated in the growing film during ion bombardment, which is well above the critical value of 1 dpa. These calculations seem to support that both densification and crystallization during bombardment of growing film are correlated with displacement cascade effects.
,:.Y a = 4.5” . i.,.*: ..-.. A’ ‘.. .;: c. . _.:? ‘.. ..,,“’ ‘;C.‘: “i;.: ,. .; ‘. ;:.*.z;...... .-;x. :..*: ’ .-.<.,, .*“..:y.. %,
.
Al +950 “C A2 + 950 “C Bl B2
4. Conclusion
,
4s
Fig. 5. GXRD spectra of Sic films after a post-annealing treatment up to 950 “C. Al: deposited without DIM; A2: deposited with DIM.
The DIM technique was applied to produce cubic silicon carbide coatings at a moderate temperature. The results indicate that the films produced without DIM at room temperature are amorphous; conversely, the films produced by DIM at room temperature exhibit the beginning of crystallization and the coatings deposited
1345
M. Zaytouni et al./Diamond and Related Materials 4 (1995) 1340-1345
70
75
Fig. 7. Synchrotron GXRD spectrum of B2 coating deposited at 750 “C with DIM.
Cl1 L.K. Frevel, D.R. Peterson
and C.K. Saka, J. Mater. Sci., 27 (1992) 1913. PI H. Matsunami, Diamond Relat. Mater., 2 (1993) 1043. c31 T. Kimura, S. Kagiyama and S. Yugo, Thin Solid Films, 94 (1982) 191. c41 K.Kh. Nussupov, V.O. Sigle and N.B. Bysenkhanov, Nucl. Instrum. Methods Phys. Res. B, 82 (1993) 69. CSI T. Kimura, Y. Tatebe, A. Kawamura, S. Yugo and Y. Adachi, Jpn. J. Appl. Phys., 24 (1985) 1712. C61 J.P. Riviere, M. Zaytouni, J. Delafond and J. Allain, Mater. Sci. Eng., 829 (1995) 105. c71 J.P. Riviere, M. Zaytouni and J. Delafond, Surf Coat. Technol., 67 (1994) 43-49.
0
1000
2000
3000
4000
5000
dep~(h
Fig. 8. Calculated damage depth profile in a 500 nm thick SIC coating deposited by DIM with 160 keV Ar+ ions (DPA=displacement per target atom).
at 750 “C with DIM exhibit crystallized structure in the cubic P-Sic phase. It is known that ion irradiation may be used for lowtemperature crystallization of amorphous surface layers in silicon. It appears from this study that the collisional damage produced by high-energy heavy ions also has crucial importance for the crystallization of Sic.
Acknowledgements
The authors wish to thank M.F. Denanot and C. Fayoux for their effective assistance in TEM observations and mixing experiments.
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