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Features of diamond deposition on modified silica glass substrates
D IAMOND AND RELATED TERiALS ELSEVIER
Diamond and Related Materials 6 (1997) 444-449
Features of diamond deposition on modified silica glass substrates M . L . T e r r a n o v a a,,, M . R o s s i b, V. Sessa a "~Dipartimento di Scienze e Tecnologie Chhniche, Universit?t "Tot Vergata", Via della Ricerca Scientifica, 00133 Roma, Italy b Dipartimento di Energetica, Universith "La Sapienza" and Unit?t INFM-RM1, Via A. Scarpa, 00161 Roma, Italy
Abstract
The structural characteristics of diamond films grown by hot-filament CVD on silica glass plates, preliminarily modified with different surface treatment, have been investigated by RHEED, SEM, and Raman spectroscopy. The modifications of the silica glass surfaces were obtained in three different ways: ( 1) high-power sonication in diamond dust suspension; (2) e-beam irradiation for production of amorphous carbon surface layers; (3) exposition to glow-discharge for production of diamond-like surface layers. The analyses results are here discussed with reference to the structures produced at the substrate surface. © 1997 Elsevier Science S.A.
Keywords: diamond deposition; modified silica glass substrates; structural characteristics
1. Introduction A key issue in the technology of diamond synthesis using the CVD method is the so-called "selected-area" deposition of the films. Several techniques have been proposed in order to achieve selective growth of diamond crystallites at specific sites on the substrate [ I- 13]. The etticacy of the various procedures relies on the capability of thoroughly controlling the nucleation density by modifying surface and/or near-surface regions of the materials used as substrates ([13] and Refs. therein). The features of diamond nucleation on various substrates depend on composition and structure of the surface layer and on the specific chemical-physical processes that occur at the solid/gas interface. This means that, in order to meet the requirements for advanced applications of diamond films, for each substrate material the correlation between surface structure and diamond nucleation must be investigated. In this context we have undertaken a study of diamond deposition on pre-treated glassy materials [14,15]. In particular, the present paper deals with diamond coating of silica glass plates. Aiming to test the effects produced by different structural modifications of the surface layers, the silica glass substrates were previously submitted to mechanical treatments or were pre-coated with carbon layers. The aim was to identify and to reproduce in a controlled way the intermediate structures * Corresponding author. 0925-9635/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S0925-9635 (96) 00654-1
tailored to provide precise control of the diamond nucleation process.
2. Experimental
2. I. Preparation of the substrates The substrates were 10 × 10x0.5 mm vitreous silica plates with total impurities < 100 ppm. The substrates have been submitted to the following three different pre-treatments. (1) High-power (1 kW cm-2) sonication of the surface by using fine diamond powder of < 0.2 ~tm diameter in ethyl alcohol suspension. The treatment lasted 30 min. Before the CVD process, the substrates were thoroughly cleaned in an acetone ultrasonic bath. (2) Irradiation by electron b~,am under controlled conditions (1 keV, 10 mA, 1.3 x 10 -2 Pa) in an atmosphere enriched with hydrocarbon molecules. The runs lasted 150 and 750 s. (3) Exposure to pulsed d.c. glow discharge in Ar/butane atmosphere (2 kW, 100 Torr). The substrates were placed on the negatively biased ( - 120 V ) platinum electrode.
2.2. Diamond deposition In order to compare the features of diamond growth on the differently modified silica glass surfaces, the
M.L. Terranova etal. / Diamond and Related Materials 6 (1997) 444--449
445
deposition process was carried out under identical experimental conditions for all samples. Diamond depositions were performed in a hot filament CVD reactor employing a 1% CH4/H2 mixture, using runs lasting 120 rain. A detailed description of the apparatus and of the deposition procedures can be found in Ref. [ 16 ]. The chamber operating pressure was 36 Torr and the total flow rate of the gas was 200 sccm. The temperatures of the tantalum filament and of the substrate holder were kept at 2180_+ l0 and 750_+ 10°C, respectively. 2.3. Characterization techniques The structural characteristics of the substrate surfaces and those of the diamond films have been analyzed by reflection high energy electron diffraction (RHEED) using an AEI-EM6G electron microscope (acceleration voltage of 60 keV) equipped with a high-resolution diffraction stage. Some of the most significant results are reported here. Moreover, the morphology of the deposits has been studied by optical microscopy, SEM and TEM observations of evaporated carbon replicas; for the sake of brevity, a detailed description of the corresponding . . . . . . . . . . . . . . . reported ,~l~-,,,here Additional information regarding composition and structure of the substrate/film interface have been obtained by Raman spectroscopy in the standard backscattering configuration (Ar + laser, 488 nm, 180 mW ).
3. Results
3. I. Ultrasonically treated,vuhstrate,s' The high-power ultrasonic bombardment produced severe surface roughening of the samples. Optical microscopy observations evide:aced the presence of overlapping craters on the sonicated circular area. Using such samples as substrate for CVD deposition, remarkable differences have been revealed for the deposits obtained on and out of the ultrasonically modified region. Fig. l(a) shows, at low magnification, the boundary region of the sonicated area. Successive SEM and RHEED (Fig. l(b)) analyses allowed to determine thai homogeneous and continuous deposits of (110) preferentially oriented diamond crystallites (with a mean size of about 300 nm) ha,,~ been synthesized on the sonicated area. Imme~,i,ate,ly out of this area (right side of Fi~:. l (a), region B), larger (d~2 lam) and randomly oriented diamond cvystallites have been revealed (Fig. l(c)). On the undamaged areas (i.e., on the regions relatively far from the bombarded one) only sparsely distributed crystallite~, of mean dian~,::~r ~5 p.m have b:en generated ( Fig. 1(d)).
Fig. l. CVD deposit obtained on partially sonicated silica glass plate. (a) Optical micrograph (plane view} showing tile morphological differences between the deposits obtained in (region A) and out of (region B) the sonicated area. (b,c} RHEED patterns corresponding to region A (b) and B (c), respectively. (d) Optical micrograph (plane view) of the diamond deposit obtained relatively far fi'om the sonicated region.
3.2. Carbon-coated subso'ates using electron beam irradiation Electron beam irradiations produced amorphous carbon broad signature peaking at Raman spectrum reported
of the silica glass surface layers, as revealed by the about 1570cm -t in the in Fig. 2(a). On these
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mixture in a d.c. glow discharge. The two bands at about 1350 and 1580 cm- ~ [ 18-20] are consistent with a diamond-like structure lacking long-range order and characterized by a predominance of sp2-bonded microcrystalline graphitic domains cross-linked by a small fraction of sp3-bonded carbon sites [21,22]. The diamond films produced by the successive CVD process were carefully detached from the substrate. This procedure allowed us to analyze by RHEED the top and bottom sides of each film, as well as the substrate surface after CVD deposition: the corresponding diffraction patterns are reported in Figs. 3(b)-(d), respectively. The substrate surface (Fig. 3(d)) still retains a diamond-like structure. The Debye rings obtained from the bottom side of the film ( Fig. 3(c)) evidence the structural properties of a disordered graphitic lattice [17], whereas the pattern taken from the top of the film (Fig. 3(b)) is characteristics of a polycrystalline diamond phase. To rule out the possibility that delamination could disrupt the interface and produce inhomogeneity in the location of the various phases, a RHEED investigation was performed in SA (selected area) conditions across the stepped edges of a CVD deposit obtained following the procedure described in Ref. [23]. These additional measurements confirmed the above results.
4. Discussion and conclusions
Fig. 2. (a) Raman spectrum of amorphous carbon deposit obtained by e-beam irradiation. (b,c) RHEED patterns from CVD films obtained on the carbon layers produced by e-beam irradiation of silica glass plates tbr 150 {b} and 750 s (c).
pre-coated samples the CVD process yielded continuous polycrystalline diamond films [17]. Figs. 2(b) and 2(c) show the RHEED patterns taken from two diamond films grown under the same conditions on silica glass substrates irradiated tbr 150 (Fig. 2(b)) and 750s ( Fig. 2(c)).
3.3. Carbon layers bj' d.c. glow d&charge Fig. 3(a) shows the Raman spectrum taken from the carbon layers produced by cracking of a Ar/butane
Structural properties and crystallographic features of the diamond grown by CVD are controlled by the relative rates of several concomitant and concurrent processes, such as: (i) transport o1" gaseous species; (it) surface diffusion of C and H: {iii) adsorption/desorption of carbon spccics~ (iv) formation of carbon clusters; (v) selective removal of carbonaceous materials by H etching. In the course of diamond synthesis, the interactions between the impinging gaseous species and the underlying solid carbon are expected to modify the chemistry of the gas phase in the vicinity of the substrate, as well as the structure of the surface. In this context structure and etching rate of solid carbon present at the substrate surface at the onset of the CVD process can dramatically influence the characteristics of diamond deposits by affecting both structure and composition of the interfacial reaction layer. Regarding the ultrasonically bombarded glasses, one observes differences which are thought to be related to the formation of different carbon concentrations at specific locations on the damaged material. It must be considered indeed that diamond nucleation from the gas phase is found to be hampered, not only when no surface carbon is present, but also when carbon is present in excess [24]. In the latter case, hydrogen etching of such solid carbon is expected to produce localised carbon
M.L. Terranova et ai. / Diamond and Related Materials 6 (1997) 444-449
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Fig. 3. (a) Raman spectrum obtained from the diamond-like layer produced by cracking of an Ar/butane mixture in a d.c. glow discharge. ( b d ) RHEED patterns after CVD deposition process carried out on substrate pre-coated with a diamond-like layer produced by cracking of a At/butane mixture in a d.c. glow discharge. (b) Top side of the CVD film before its detachment from the substrate. (c) Bottom side of the CVD tiim after its detachment from the substrate. Jd) Surface substrate after CVD film detachment.
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M.L. Terranova et al. / Diamond and Related Materials 6 (1997) 444-449
supersaturation in the near-surface region. As a consequence, the situation of the gas phase contacting the substrate can result in strong displacement from the critical diamond-growth domain in Bachmann's phase diagram [25]. For our samples, on the regions surrounding the sonicated area, the rather high concentration of active species impinging on the substcate can likely give rise to a relevant clustering of carbon atoms via surface diffusion. Under these conditions [13], the synthesis process is driven towards the preferential growth of non-diamond carbon. In these areas, indeed, only sparse diamond crystallites have been detected on continuous carbonaceous layers. Conversely, continuous diamond films are produced inside the ultrasonically modified area, where the reduced concentration of carbon species which reach the convex internal surface prevents formation of thick carbon clusters and subsequent near-surface supersaturation. On the amorphous carbon deposits produced by e-beam irradiations we obtained, in any case, polycrystalline diamond films. However, as revealed by our previous experiments [14], the crystallographic properties of the diamond film depend on the thickness of the underlying carbon layers. For diamond deposited on the 750-s irradiated surface, the d spacings obtained from the well-defined diffraction lines (Fig. 2(c)) are found to match with those of natural diamond to within 1% uncertainty. In the RHEED pattern taken from the diamond film grown on the thinner interlayer ( Fig. 2(b)) the remarkable broadening of the Debye rings in the radial direction reveals, conversely, the presence of a less-ordered diamond phase, i.e., of a structure with a certain indeterminate lattice parameter. For the films deposited onto carbon layers with diamond-like properties, the analysis of the R HEED signals taken from the delaminated substrate and from the freestanding film allowed us to deduce the structure of the interfacial layer at the end of the CVD process. The stratification of the various phases was found to be as follows: diamond-like, graphite, and diamond. The SA measurements confirmed that the substrate/film interface is a complex structure arising from the growth sequence of diamond-like contacting the silica glass substrate, disordered graphite as intermediate structure, and finally diamond. The evidence that diamond nucleates on disordered graphitic layers confirms the hypothesis [26,14] that carbon materials, characterized by structural disorder and high resistance to H etching, are required as precursors for diamond nucleation. Overall, the features of our diamond films correlate with the presence at the substrate surface of specific carbon structures, either formed during the early stage of the CVD process or ad hoc generated by two different pre-t:eatments. The findings of the present experiments are consistent with the results obtained in other laboratories by using silica glass substrates [27,28]. Moreover,
the above issues conform to previous indications that the characteristics of the diamond films deposited on a substrate depend on both structure and thickness of the carbon layers gener~Lted at the surface of that material [13]. Based on the present observations, it is thought that the combined use of techniques for local modification of the substrate topography and for deposition of selected carbon stractures would represent a successful approach to achieve deposition of diamond structures on well-defined areas of silica glass plates.
Acknowledgement This work was partially supported by INCM (Unit:~ di Roma - - Tor Vergata).
References [ 1] T. Yoneara, Y. Nishigaki, H. Mizutani, S. Kondoh, K. Yamagata, T. Noma and T. lchikawa, Appi. Phys. Lett., 52 (1988) 1231. [2] K. Hirabayashi, Y. Taniguchi, O. Takamatsu, T. lkeda, K. Ikoma and N. lwasaki-Kurihara, Appl. Phys. Lett., 53 (1988) 1815. [3] A.R. Kirkpatrick, B.W. Ward and N.P. Economou, J. Vat'. Sc. Technol. B, 7 ( 1989 ) 1947. [4] T. inoue. H. Tachibana, K. Kumagai, K. Miyata, K. Nishimura, K. Kobashi and A. Nakaue, J. Appl. Phys., 67 (1990) 7329. [5] S. '~ u, Z. Yin, X. Lu and G. Zou, Mater. Lett., 10 (1990) 375. [6] F.G. Celii, H.H. Nelson and P.R. Pehrsson, J. Mater. Res., 5 (1991)) 2337. [7] J.S. Ma, H. Yagyu, A. Hiraki, H. Kawarada and T. Yoneara. Th#t Solid l'7h,s, 206 ( 1991 ) 192. [8] S.J. Lin, J. Hwang, C.S. Chang and H.Y. Wen, Appl. Phys. Lett., 60 ( 1992 ) 1559. [9] R. Ramesham and C. Ellis, in T.D. Moustakas, J.i. Pankove and Y. Hamakawa (eds.), WMe Band Gap Semicomhu'tm" $:vmposium, Pittsburgh, PA MRS, 1992, p. 69. [10] V.G. Rachelko, A.A. Smolin, K.G. Korotushenko, M.S. Nounoupatov, S.M. Pimenov and B.K. Vodolaga, in M. Yoshikawa, M. Murakawa, Y. Tzeng and W.A. Yarborough (eds.), 2ml hit. Col!f on AppI. of Dktmomt Films and Rehtted Materials, M v u , Tokyo, 1993, p. 475. [ 1 ! ] V.Yu. Armeyev, VG. Rachelko, T.V. Kononenko, S.M. Pimrnov, N.V. Chernenko, E.N. Loubnin and A.Yu. Zlobin, Dktmond Relat. Mater., 2 ( 1993 ) 904. [12] C.-F. Chen and S.-H. Chen, Thin Solid Fib,s, 253 (1994) 168. [13] H. Li,A and D.S. Dandy, Dhmumd Chemical I'apor Deposithm, Noyes Publications, Park Ridge, N J, USA. 1995, and references thereto. [14] M.L. Terranova. M. Rossi, V. Sessa and G. Vitali, Phys. Star. Sol., 154 (1996) 127. [151 M.L. Terranova, M. Rossi, V. Sessa and G. Vitali, Mater. Sci. Forum, 203 (1996) 291. [16] M.L. Terranova, V. Sessa, M. Rossi, G. Vitali, G. Cappuccio and C. Veroli, J. Phys., C5 (1995) 879. [17] ICCD Diffraction Database, International Centre for Diffraction Dart:, Sandia National Laboratories, Swarthmore. PA. [181 N. Wada, P.J. Gaczi and S.A. Solin, J. Non-Cryst. Soli~b, 35,36 (1980) 543.
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[19] R.E. Shroder, R.J. Nemani,zh and J.T. Glass, Phys. Rev. B, 41 (1990) 3738. [20] D.S. Knight and W.B. White, J. Mater Res., 4 (1989) 385. [21] J. Robertson and E.P. O'Reilly, Phys. Rev. B, 35 (1987)2946. [22] J. Robertson, Philos. Mag. B, 66 (1992) 199. [23] M.L. Terranova, M. Rossi and G. Vitali, J. Appi. Phys., 80 (1006) 3552. [24] Z. Feng, K. Komvopoulos, I.G. Brown and D.B. Bogy, J. Appl. Phys.. 74 (1993) 2841.
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[25] P.K. Bachmann, D. Leers and H. Lydtin, Diamond Relat. Mater., 1 (1991] 1. [26] Z. Feng, M.A. Brewer, K. Komvopoulos, I.G. Brown and D.B. Bogy, J. Mater. Res., 10 (1995) 165. [27] D.J. Pickrell, W. Zhu, A.R. Badzian, R.E. Newnham and R. Messier, J. Mater Res., 6 (1991) 1266. [28] J. Rankin, R.E. Boekenhauer, R. Csencsits, Y. Shigesato, M.W. Jacobson and B.W. Sheldon, J. Mater. Res., 9 (1994) 2164.
Diamond and Related Materials 6 (1997) 444-449
Features of diamond deposition on modified silica glass substrates M . L . T e r r a n o v a a,,, M . R o s s i b, V. Sessa a "~Dipartimento di Scienze e Tecnologie Chhniche, Universit?t "Tot Vergata", Via della Ricerca Scientifica, 00133 Roma, Italy b Dipartimento di Energetica, Universith "La Sapienza" and Unit?t INFM-RM1, Via A. Scarpa, 00161 Roma, Italy
Abstract
The structural characteristics of diamond films grown by hot-filament CVD on silica glass plates, preliminarily modified with different surface treatment, have been investigated by RHEED, SEM, and Raman spectroscopy. The modifications of the silica glass surfaces were obtained in three different ways: ( 1) high-power sonication in diamond dust suspension; (2) e-beam irradiation for production of amorphous carbon surface layers; (3) exposition to glow-discharge for production of diamond-like surface layers. The analyses results are here discussed with reference to the structures produced at the substrate surface. © 1997 Elsevier Science S.A.
Keywords: diamond deposition; modified silica glass substrates; structural characteristics
1. Introduction A key issue in the technology of diamond synthesis using the CVD method is the so-called "selected-area" deposition of the films. Several techniques have been proposed in order to achieve selective growth of diamond crystallites at specific sites on the substrate [ I- 13]. The etticacy of the various procedures relies on the capability of thoroughly controlling the nucleation density by modifying surface and/or near-surface regions of the materials used as substrates ([13] and Refs. therein). The features of diamond nucleation on various substrates depend on composition and structure of the surface layer and on the specific chemical-physical processes that occur at the solid/gas interface. This means that, in order to meet the requirements for advanced applications of diamond films, for each substrate material the correlation between surface structure and diamond nucleation must be investigated. In this context we have undertaken a study of diamond deposition on pre-treated glassy materials [14,15]. In particular, the present paper deals with diamond coating of silica glass plates. Aiming to test the effects produced by different structural modifications of the surface layers, the silica glass substrates were previously submitted to mechanical treatments or were pre-coated with carbon layers. The aim was to identify and to reproduce in a controlled way the intermediate structures * Corresponding author. 0925-9635/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S0925-9635 (96) 00654-1
tailored to provide precise control of the diamond nucleation process.
2. Experimental
2. I. Preparation of the substrates The substrates were 10 × 10x0.5 mm vitreous silica plates with total impurities < 100 ppm. The substrates have been submitted to the following three different pre-treatments. (1) High-power (1 kW cm-2) sonication of the surface by using fine diamond powder of < 0.2 ~tm diameter in ethyl alcohol suspension. The treatment lasted 30 min. Before the CVD process, the substrates were thoroughly cleaned in an acetone ultrasonic bath. (2) Irradiation by electron b~,am under controlled conditions (1 keV, 10 mA, 1.3 x 10 -2 Pa) in an atmosphere enriched with hydrocarbon molecules. The runs lasted 150 and 750 s. (3) Exposure to pulsed d.c. glow discharge in Ar/butane atmosphere (2 kW, 100 Torr). The substrates were placed on the negatively biased ( - 120 V ) platinum electrode.
2.2. Diamond deposition In order to compare the features of diamond growth on the differently modified silica glass surfaces, the
M.L. Terranova etal. / Diamond and Related Materials 6 (1997) 444--449
445
deposition process was carried out under identical experimental conditions for all samples. Diamond depositions were performed in a hot filament CVD reactor employing a 1% CH4/H2 mixture, using runs lasting 120 rain. A detailed description of the apparatus and of the deposition procedures can be found in Ref. [ 16 ]. The chamber operating pressure was 36 Torr and the total flow rate of the gas was 200 sccm. The temperatures of the tantalum filament and of the substrate holder were kept at 2180_+ l0 and 750_+ 10°C, respectively. 2.3. Characterization techniques The structural characteristics of the substrate surfaces and those of the diamond films have been analyzed by reflection high energy electron diffraction (RHEED) using an AEI-EM6G electron microscope (acceleration voltage of 60 keV) equipped with a high-resolution diffraction stage. Some of the most significant results are reported here. Moreover, the morphology of the deposits has been studied by optical microscopy, SEM and TEM observations of evaporated carbon replicas; for the sake of brevity, a detailed description of the corresponding . . . . . . . . . . . . . . . reported ,~l~-,,,here Additional information regarding composition and structure of the substrate/film interface have been obtained by Raman spectroscopy in the standard backscattering configuration (Ar + laser, 488 nm, 180 mW ).
3. Results
3. I. Ultrasonically treated,vuhstrate,s' The high-power ultrasonic bombardment produced severe surface roughening of the samples. Optical microscopy observations evide:aced the presence of overlapping craters on the sonicated circular area. Using such samples as substrate for CVD deposition, remarkable differences have been revealed for the deposits obtained on and out of the ultrasonically modified region. Fig. l(a) shows, at low magnification, the boundary region of the sonicated area. Successive SEM and RHEED (Fig. l(b)) analyses allowed to determine thai homogeneous and continuous deposits of (110) preferentially oriented diamond crystallites (with a mean size of about 300 nm) ha,,~ been synthesized on the sonicated area. Imme~,i,ate,ly out of this area (right side of Fi~:. l (a), region B), larger (d~2 lam) and randomly oriented diamond cvystallites have been revealed (Fig. l(c)). On the undamaged areas (i.e., on the regions relatively far from the bombarded one) only sparsely distributed crystallite~, of mean dian~,::~r ~5 p.m have b:en generated ( Fig. 1(d)).
Fig. l. CVD deposit obtained on partially sonicated silica glass plate. (a) Optical micrograph (plane view} showing tile morphological differences between the deposits obtained in (region A) and out of (region B) the sonicated area. (b,c} RHEED patterns corresponding to region A (b) and B (c), respectively. (d) Optical micrograph (plane view) of the diamond deposit obtained relatively far fi'om the sonicated region.
3.2. Carbon-coated subso'ates using electron beam irradiation Electron beam irradiations produced amorphous carbon broad signature peaking at Raman spectrum reported
of the silica glass surface layers, as revealed by the about 1570cm -t in the in Fig. 2(a). On these
446
31. L. TerramJva et aL/ Diamomi aml Rchtted Materials 6 (1997) 444-449
A
!,ffJ Z m
Z I
Z
1000
I
I
I
1200
1400
1600
RAMAN SHIFT (cm "~)
mixture in a d.c. glow discharge. The two bands at about 1350 and 1580 cm- ~ [ 18-20] are consistent with a diamond-like structure lacking long-range order and characterized by a predominance of sp2-bonded microcrystalline graphitic domains cross-linked by a small fraction of sp3-bonded carbon sites [21,22]. The diamond films produced by the successive CVD process were carefully detached from the substrate. This procedure allowed us to analyze by RHEED the top and bottom sides of each film, as well as the substrate surface after CVD deposition: the corresponding diffraction patterns are reported in Figs. 3(b)-(d), respectively. The substrate surface (Fig. 3(d)) still retains a diamond-like structure. The Debye rings obtained from the bottom side of the film ( Fig. 3(c)) evidence the structural properties of a disordered graphitic lattice [17], whereas the pattern taken from the top of the film (Fig. 3(b)) is characteristics of a polycrystalline diamond phase. To rule out the possibility that delamination could disrupt the interface and produce inhomogeneity in the location of the various phases, a RHEED investigation was performed in SA (selected area) conditions across the stepped edges of a CVD deposit obtained following the procedure described in Ref. [23]. These additional measurements confirmed the above results.
4. Discussion and conclusions
Fig. 2. (a) Raman spectrum of amorphous carbon deposit obtained by e-beam irradiation. (b,c) RHEED patterns from CVD films obtained on the carbon layers produced by e-beam irradiation of silica glass plates tbr 150 {b} and 750 s (c).
pre-coated samples the CVD process yielded continuous polycrystalline diamond films [17]. Figs. 2(b) and 2(c) show the RHEED patterns taken from two diamond films grown under the same conditions on silica glass substrates irradiated tbr 150 (Fig. 2(b)) and 750s ( Fig. 2(c)).
3.3. Carbon layers bj' d.c. glow d&charge Fig. 3(a) shows the Raman spectrum taken from the carbon layers produced by cracking of a Ar/butane
Structural properties and crystallographic features of the diamond grown by CVD are controlled by the relative rates of several concomitant and concurrent processes, such as: (i) transport o1" gaseous species; (it) surface diffusion of C and H: {iii) adsorption/desorption of carbon spccics~ (iv) formation of carbon clusters; (v) selective removal of carbonaceous materials by H etching. In the course of diamond synthesis, the interactions between the impinging gaseous species and the underlying solid carbon are expected to modify the chemistry of the gas phase in the vicinity of the substrate, as well as the structure of the surface. In this context structure and etching rate of solid carbon present at the substrate surface at the onset of the CVD process can dramatically influence the characteristics of diamond deposits by affecting both structure and composition of the interfacial reaction layer. Regarding the ultrasonically bombarded glasses, one observes differences which are thought to be related to the formation of different carbon concentrations at specific locations on the damaged material. It must be considered indeed that diamond nucleation from the gas phase is found to be hampered, not only when no surface carbon is present, but also when carbon is present in excess [24]. In the latter case, hydrogen etching of such solid carbon is expected to produce localised carbon
M.L. Terranova et ai. / Diamond and Related Materials 6 (1997) 444-449
447
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z
n,-
I
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1200
I
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Fig. 3. (a) Raman spectrum obtained from the diamond-like layer produced by cracking of an Ar/butane mixture in a d.c. glow discharge. ( b d ) RHEED patterns after CVD deposition process carried out on substrate pre-coated with a diamond-like layer produced by cracking of a At/butane mixture in a d.c. glow discharge. (b) Top side of the CVD film before its detachment from the substrate. (c) Bottom side of the CVD tiim after its detachment from the substrate. Jd) Surface substrate after CVD film detachment.
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M.L. Terranova et al. / Diamond and Related Materials 6 (1997) 444-449
supersaturation in the near-surface region. As a consequence, the situation of the gas phase contacting the substrate can result in strong displacement from the critical diamond-growth domain in Bachmann's phase diagram [25]. For our samples, on the regions surrounding the sonicated area, the rather high concentration of active species impinging on the substcate can likely give rise to a relevant clustering of carbon atoms via surface diffusion. Under these conditions [13], the synthesis process is driven towards the preferential growth of non-diamond carbon. In these areas, indeed, only sparse diamond crystallites have been detected on continuous carbonaceous layers. Conversely, continuous diamond films are produced inside the ultrasonically modified area, where the reduced concentration of carbon species which reach the convex internal surface prevents formation of thick carbon clusters and subsequent near-surface supersaturation. On the amorphous carbon deposits produced by e-beam irradiations we obtained, in any case, polycrystalline diamond films. However, as revealed by our previous experiments [14], the crystallographic properties of the diamond film depend on the thickness of the underlying carbon layers. For diamond deposited on the 750-s irradiated surface, the d spacings obtained from the well-defined diffraction lines (Fig. 2(c)) are found to match with those of natural diamond to within 1% uncertainty. In the RHEED pattern taken from the diamond film grown on the thinner interlayer ( Fig. 2(b)) the remarkable broadening of the Debye rings in the radial direction reveals, conversely, the presence of a less-ordered diamond phase, i.e., of a structure with a certain indeterminate lattice parameter. For the films deposited onto carbon layers with diamond-like properties, the analysis of the R HEED signals taken from the delaminated substrate and from the freestanding film allowed us to deduce the structure of the interfacial layer at the end of the CVD process. The stratification of the various phases was found to be as follows: diamond-like, graphite, and diamond. The SA measurements confirmed that the substrate/film interface is a complex structure arising from the growth sequence of diamond-like contacting the silica glass substrate, disordered graphite as intermediate structure, and finally diamond. The evidence that diamond nucleates on disordered graphitic layers confirms the hypothesis [26,14] that carbon materials, characterized by structural disorder and high resistance to H etching, are required as precursors for diamond nucleation. Overall, the features of our diamond films correlate with the presence at the substrate surface of specific carbon structures, either formed during the early stage of the CVD process or ad hoc generated by two different pre-t:eatments. The findings of the present experiments are consistent with the results obtained in other laboratories by using silica glass substrates [27,28]. Moreover,
the above issues conform to previous indications that the characteristics of the diamond films deposited on a substrate depend on both structure and thickness of the carbon layers gener~Lted at the surface of that material [13]. Based on the present observations, it is thought that the combined use of techniques for local modification of the substrate topography and for deposition of selected carbon stractures would represent a successful approach to achieve deposition of diamond structures on well-defined areas of silica glass plates.
Acknowledgement This work was partially supported by INCM (Unit:~ di Roma - - Tor Vergata).
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