possibility of producing thin diamond films by the so-called carbon-ion-implantation-out-diffusion method. The basic mechanisms of the carbon layer growth are ...
Thin Solid Films 263 (1995) 162-168
Carbon-based
hard films produced by high-temperature implantation T. Cabioc’h*,
Laboratoire
J.P. Rivikre, J. Delafond,
de Metallurgic Physique,
URA 131 CNRS,
Received 2 December
M. Jaouen,
40 avenue du Recteur
carbon-ion
M.F. Denanot
Pineau, F-86022 Poitiers Cedex,
France
1994; accepted 6 March 1995
Abstract
We have performed 120 keV carbon-ion implantations into copper at high doses (5 X 10” cm-*) and high temperatures (973 K G T c 1273 K) and characterized the microstructure of the surface layer formed by this process. This study reinvestigates the possibility of producing thin diamond films by the so-called carbon-ion-implantation-out-diffusion method. The basic mechanisms of the carbon layer growth are not well understood and we propose a model where the segregation of C atoms and the preferential sputtering of Cu atoms would play an important role. In order to examine the validity of such a hypothesis we have investigated the influence of both the ion flux and the temperature. For all experimental conditions we observe the formation of a uniform graphite layer (turbostratic graphite) with a more- or less-pronounced texture along the (0001) direction. We have identified in the graphite layer a high density of graphitic shelled micrograins (“fullerene onions”) but the presence of diamond has never been detected. Our results are in agreement with some previous studies on this subject that were not able to reproduce the first results claiming that diamond formation is possible by this method. It is suggested that a higher C ion flux or the presence of impurities in the copper substrate could be favourable for such a mechanism of diamond growth. Keywords:
Copper; Diamond;
Diffusion;
Segregation
1. Introduction
Diamond is not only the hardest known material, but it also exhibits a unique combination of optical, physical and electronic properties [ 1,4]. Therefore it is not surprising that the synthesis of diamond films is arousing great interest because of the wide range of promising industrial applications. A great variety of preparation methods have been tried, however the most successful ones so far are either the hot filament chemical vapour deposition technique or the plasmaassisted CVD [5,6]. Unfortunately, high-quality diamond formation on a metallic substrate (Steel, MO, Ni, Cu, etc.) is difficult to achieve without a preliminary treatment of the substrate. The low diamond nucleation rate is one of the main reasons for these difficulties. Techniques allowing both an increase and a high degree of control of the diamond nucleation rate are of great interest and become new fields of * Corresponding
author.
0040-6090/95/$09.50 0 SSDZ
1995 Elsevier Science S.A. All rights reserved
0040-6090195106588-l
research themselves. For example, one of the most simple solutions used for inducing a large enhancement of diamond nucleation consists of sowing the substrate with a diamond paste [7]. The simplicity of such a method is one of its advantages but a good reproducibility of the nucleation density seems difficult to obtain. Pretreatment techniques of the substrate using ion-implantation are, from the reproducibility and versatility point of view, more reliable because of the high degree of control of various parameters such as the dose or the ion depth distribution. In this way carbon-ion implantation in Ti-6Al-4V [S] or of Ti in an amorphous carbon thin film [9] have largely modified the diamond nucleation rate and the properties of the diamond thin film thus obtained. Among all the possible ion-substrate combinations, carbon-ion implantation in a copper substrate is one of the most promising. Originally proposed by Prins [lo], this technique was expected to be a powerful method for the heteroepitaxial growth of a single-crystalline diamond film. Even if some of the first reported
T. Cabioc’h et al. I Thin Solid Films 263 (1995)
results [lo, 1 l] appear successful, the reproducibility of the process is not demonstrated since controversial studies have been published [12,13] on this subject. The basic ideas of Prins are as follow: the miscibility of carbon into copper is very low even at high temperature (IO-’ at 1500 K); furthermore copper and carbon have a very close lattice constant (a,, = 0.361 nm, Qui;lmond= 0.357 nm) and structure (cubic face centred). Then during a carbon-ion implantation into copper at a high temperature, a solid state outdiffusion of the implanted carbon atoms occurs and copper- can be considered as a mould for diamond formation. The great interest of such a technique is two-fold: it can be used for direct diamond thin film formation or for increasing the diamond nucleation by producing a high density of nucleation sites on the substrate surface. With such a process, Prins and Gaigher [lo, 111 claimed that films of diamond are formed but Lee et al. [12] and Ong et al. [13] later concluded that one obtains only textured (0001) graphite formation rather than polycrystalline diamond. Hoff et al. [14] performed the same kind of experiment: their microRaman spectra and transmission electron microscopy (TEM) results allow one to conclude that a highquality single-crystalline diamond overgrowth layer has been formed, but only for 3 out of 25 implantations that have been performed. Despite this low reproducibility, their results are of great interest since a way to true heteroepitaxy between diamond and the copper substrate would be open. The use of copper single crystals as substrates under good implantation conditions, that are to be specified, could lead to a single-crystal diamond thin film on a substrate other than diamond or c-BN [15,16]. Furthermore, even if a diamond film cannot be obtained the process can be applied to enhance the diamond nucleation rate, as was suggested previously by different authors [13]. The solid-state diffusion of carbon in copper has also been successfully used by Narayan et al. [17], but in a two-step process. They made high-dose carbonion implantations in copper at room temperature and then irradiated the implanted copper substrates with laser pulses to induce the out-diffusion of carbon atoms: heteroepitaxial diamond thin films were then observed. Again a controversy appears since Lee et al. (181 have only synthesized graphite thin films for a priori identical experimental conditions. This brief discussion of the contradictory results obtained by the implantation out-diffusion method suggests that further investigations are needed. We think that a more detailed study of the mechanisms that governs the carbon thin film formation and therefore the solid-state diffusion of carbon in copper can provide new insights that are important for the application of the method. The objective of the present work is to investigate with various electron
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microscopy methods the fine structure of the layer grown on a copper substrate during high-dose carbonion implantation at high temperature ( 2 973 K) as a function of mainly two parameters: the substrate temperature and the ion flux. However this technique is very specific and we will try to identify and describe the various processes involved with this method since it has not been done previously.
2. Description of the method The implantation-out-diffusion method can be separated into four different steps. (a) Carbon ion implantation. When carbon ions penetrate into the copper substrate, they remove atoms from the surface by sputtering. In addition they produce many successive elastic collisions which displace target atoms from their lattice sites (displacement collision cascades) resulting in the formation of a high concentration of point defects (interstitials, vacancies). Finally the incident C ions stop when their energy is insufficient for displacing one more target atom. (b) Solid-state out-diffusion of C atoms. At high temperature, because of their immiscibility in copper. implanted carbon atoms will move toward the copper surface by solid-state diffusion due to the chemical gradient. This diffusion is easy due to the large number of point defects in the copper matrix created by thermal agitation and collisional cascades added to the small size of carbon atoms. Different processes can be considered for the next step which is the segregation of carbon. The most simple one ignores the segregation in the volume of the copper matrix: due to the chemical diffusion carbon atoms reach the surface where they will occupy the more energetically stable sites. If the copper surface energy was sufficient to maintain carbon atoms epitaxially, diamond formation on the surface could be possible. Unfortunately, the surface energy of copper is much lower than that of diamond and C-Cu bonds are very weak since carbon and copper are immiscible. So diamond formation cannot be possible with such a process. Attesting to this, carbon-ion implantation at room temperature followed by an annealing at 1173 K only yields graphite formation [13]. Nevertheless, in some cases diamond formation has been achieved by such a technique [14] and another kind of segregation has to be involved to explain diamond formation. A segregation of the carbon atoms inside the copper matrix could be more favourable. Let us consider the different sites that carbon atoms can occupy during the out-diffusion. Among them, copper vacancies are more stable than octahedral or tetrahedral sites. Irradiation with carbon ions and thermal agitation creates many vacancies that can therefore be occupied
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by neighbouring carbon atoms diffusing towards the surface. Due to the immiscibility, an isolated carbon atom will not stay on such a site in the copper matrix, but if a neighbouring tetrahedral site is occupied, a stable carbon nuclei can be obtained. The first soformed nearest-neighbour C-C distance will correspond to that of diamond. Many such nuclei can be created and other carbon atoms can precipitate on them to form sp3-bonded clusters. Furthermore, nonequilibrium conditions (high cooling rate of collision cascades, diffusivity) are fulfilled during irradiation. They can provide a significant contribution during diamond formation since the thermodynamically stable phase is graphite. (c) Preferential sputtering. During carbon-ion implantation, copper and carbon have very different sputtering yields. Using the TRIM code (~~1~91) [19], they are found to be 0.5 for copper and 0.012 for carbon respectively. This preferential sputtering effect is the second and an important mechanism involved in the process of carbon film growth since the copper surface is eroded during the implantation. Carbon clusters that have been formed in the volume will appear at the free surface once copper has been sputtered. As a result the concentration of carbon increases with respect to that of copper at the free surface. (d) Carbon film growth by inter-diffusion. As soon as the carbon concentration is higher than that of copper, one can expect an interdiffusion mechanism between carbon and copper. A continuous carbon thin film will then be formed. According to our model, we see that the mechanisms of the carbon-atom segregation are the main parameters to control. For a high-temperature carbonion implantation into copper, the four parameters to consider are: - the substrate temperature that controls the diffusivity of carbon atoms towards the surface, since diffusion is a thermally activated process; - the ion energy that imposes the depth distribution of implanted atoms and the sputtering yield; - the flux that modifies the enhanced diffusion and the number of carbon atoms for a given time at a given depth, and thus the probability of volume segregation; - the dose that gives the final film thickness. The sputtering yield varies slowly with energy, so we focus our study on the parameters of substrate temperature and ion flux to try to determine optimal conditions leading to diamond thin film formation.
km) for plan-view observations, and rectangularshaped samples (10 x 20 X 1 mm3) for cross-sectional observations. After a mechanical polishing, the substrates were annealed for 6 h at 1253 K in vacuum (10m4 Pa) to produce grain growth (= 200 pm) and allow an easier observation of the relationships between the thin film and the Cu grain orientation. Substrates were then positioned in a little furnace and maintained with a graphite mask having 2.8 mm diameter holes (Fig. 1). This furnace, designed and built in our laboratory, can reach temperatures as high as 1300 K. A cryogenic vacuum better than lo-” Pa is obtained during temperature rise and carbon-ion implantation. The carbon-ion energy (120 keV) and the dose (5 x 10” cm-‘) are kept constant for all experiments. They have been chosen by comparison with those that have given the best results reported in previous works [lo] [ 11,141. The substrate temperature (973 K < T G 1273 K) and the carbon ion-flux (0.95 X 10’” cm-* s-l G ion flux G 3.3 X lOI cm-* s-‘) varied from experiment to experiment. The overgrown carbon layers were then characterized by scanning electron microscopy (SEM), TEM (JEOL 2OOCX), high-resolution TEM (HRTEM) (JEOL 3010) and energy dispersive X-ray spectrometry (EDS).
3. Experimental procedures
4. Results
Two types of copper substrate (purity better than 99.99%) have been used to carry out the TEM experiments: discs (diameter, 3 mm, thickness, 100
After implantation, a thin film can be clearly observed on copper samples. SEM micrographs (Fig. 2) reveal a uniform wavy surface layer. The important
xouple
-
---* --_--)
-
---*
I -
120_!!!! L ---*
C+
--_--)
I Vacyum ( 10-3
Pa )
n7 Fig. 1. Experimental system for high-temperature implantations. The same energy (120 keV) and dose (5 x IO” cm-‘) have been used in all the experiments.
T. Cabioc’h et al. I Thin Solid Films 263 (199-f) 162-168
165
Fig. 2. Selected area electron diffraction pattern of carbon-layer areas free of micrograins. Influence of the substrate temperature and of the carbon ion flux: (a) T= 973 K. ion flux =0.95 x 10” cm ’ hi’; (b) T= 1173 K, ion flux =0.95 x 10” cm-’ s -‘; (c) T = 11-3 K. ion flux = 3.3 x 10” cm ’ s ‘_
thermal expansion difference between all the carbon phases and copper allows one to explain these observations easily. Thin TEM observable areas are obtained by electrolytic polishing or by ion-milling for plan or cross-sectional views respectively. In all cases bright-field observations show a carbon film formed from a uniform layer sealed with micrograins that are often circular (Figs. 3(a) and 3(c). EDS analysis of the observed areas reveal only the presence of carbon with a very little amount of copper that can come either from the thin film or also from some sublayer of non-polished copper. The uniform layer, as well as the micrograins, are polycrystalline as can be seen in the related selected area electron diffraction patterns (SAED). Two different kinds of SAED have been observed. The first (Fig. 3(d) is characteristic of regions where both the uniform layer and the micrograins coexist. The second, corresponding to areas free of micrograins, exhibits the kind of pattern shown in Fig. 3(b). Here a more- or less-pronounced texture is clearly visible that is both flux and temperature dependent: for a given carbon ion flux the texture of the uniform layer increases with the temperature (Figs. 4(a) and 4(b). The same behaviour holds for a constant temperature when the ion flux decreases (Figs. 4(b) and 4(c). Indexing the diffraction rings identifies the micrograins as being graphitic. The diffraction ring patterns related to the uniform layer areas are indexed either with a graphite structure-textured (0001) or with a polycrystalline diamond pattern (Table 1). Prins [lo], who obtained very similar diffraction patterns, first concluded that they were due to the presence of polycrystalline diamond whereas Lee et al. [12] attributed them to textured graphite. The origin of such a
Fig. 3. (a) Plan-view TEM micrograph (bright field) of a uniform carbon layer area formed by carbon-ion implantation into copper at 1073 K and a flux = 1.87 x 10” cm ’ s- ’ (b) Corresponding selected area electron diffraction pattern. (c) Plan-view TEM micrograph (bright field) of the same sample for an area where the uniform layer and micrograins coexist. area electron diffraction pattern.
(d) Corresponding
selected
controversial interpretation can be understood from the comparison between textured graphite (0001) and polycrystalline diamond interplanar distances. Because of the experimental uncertainties, an unambiguous identification of the phase is not possible. However a cross-sectional observation brings us a clear answer about the crystallographic nature of the uniform layer since the two large spots shown in Fig. 5 are without doubt indexable as (0002) graphite planes. This means that the c axis of the hexagonal structure is perpendicular to the substrate surface. The cross-sectional observations give us an approximation of the carbon thin film thickness (= 40 nm). With a dose of 5 x 10” cm-*, if we consider that there is only formation of graphite due to a surface segregation, the theoretical thickness is about 44 nm. This quite good correspondence between the experimental and the calcu-
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lated thicknesses enables us to assume that all, or nearly all, the carbon atoms have reached the copper surface. All plan-view and cross-sectional observations confirm this result since carbon clusters have never been found inside the copper matrix. We observed the same texture systematically for all the experiments we performed. The diamond phase was never detected. Furthermore, the crystallographic relationships between the substrate and the film do not correspond to the heteroepitaxial ones, the observed texture being independent of the copper orientation (loo), (IlO), (ill), etc. The presence of graphitic circular micrograins has never been quoted in papers devoted to identical experiments. Fig. 6 shows an example of a typical HRTEM micrograph that exhibits a spherical micrograin formed of curved planes of graphite. The basal plane (0002) rolls up around the centre of the micrograin. The external interface can be constituted either of well-crystallized planes of carbon or of a narrow amorphous ribbon. The structure of this amorphous ribbon is more organized on the micrograin side, and it indicates that the formation of these
Fig. 4. Selected area electron diffraction pattern of carbon-layer areas free of micrograins. Influence of the substrate temperature and of the carbon ion flux: (a) T = 973 K, ion flux = 0.95 x 10” cm-’ s-‘; (b) T= 1173 K, ion flux=O.95 X 10” cm-’ s-l; (c) T = 1173 K, ion flux = 3.3 x 1013 cm-* s-l.
Table I Comparison
of experimental
interplanar
distances
d,,, with theoretical
diamond and graphite ones
Exp.
Graphite
d,,, (A)
d,,,(A)
Sd,,, (%)
Plan (Ml)
3.44” 2.127 1.72” 1.233 1.069 0.805 0.705 0.610 0.586
3.355 2.130 1.678 1.230 1.065 0.813 0.710 0.615 0.591
2.38 0.16 2.5 0.24 0.35 0.2 0.78 0.81 0.82
wy
’ Interplanar The uniform
Diamond
distances do not appear for SAED on carbon-layer layer can be identified as textured (0001) graphite
d,.,, (A)
6d,.,, (%)
Plan (hkl)
(1010) (0004)
2.061
3.2
(111)
(1120) (2020) (3120) (3030) (2240) (4150)
1.262 1.076 0.819 0.729 0.631 0.603
2.24 0.70 1.88 2.55 3.34 2.89
(220) (311) (331) (422) (333) (531)
areas rather
free of micrograins. than polycrystalline
diamond.
Fig. 5. (a) Cross-sectional TEM micrograph (dark field) of a carbon layer formed by carbon-ion implantation into copper at 1173 K with an ion flux = 3.3 X lOI cmm2 s-l (b) Corresponding selected area electron diffraction pattern of the carbon layer showing (0002) graphite spots.
T. Cabioc’h et al. I Thin Solid Films 263 (19%)
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167
5. Discussion 5.1.
Fig. 6. HRTEM image of the shelled graphitic micrograins synthesized by carbon-ion implantation (ion flux = 1.87 x 10” cm ~’ SC’) into a copper substrate held at 1073 K. The distance between the planes is 0.34 nm.
micrograins occurs via an external atomic transport or rearrangement. The spherical shape of these micrograins is deduced experimentally by tilting the samples in the TEM since the SAED and the bright-field contrast remain unchanged when tilting. This unexpected result is confirmed by the cross-sectional view shown in Fig. 5 where one micrograin can be observed. These micrograins are present in all samples, their number varying from area to area. The highest micrograin density was found for the experiment performed at 1173 K for a flux of 3.3 X 10’” cm-? s-’ (Fig. 7). It must be pointed out that this flux value is the highest we have used.
Fig. 7. Plan-view TEM micrograph (dark field) of a carbon layer formed by carbon-ion implantation into copper at 1173 K, ion flux = 3.3 X 10” cm-’ SC’. An important density of spherical micrograins can be seen.
Uniform layer
In agreement with previous results [12, 131, the presently studied carbon-ion implantations into copper at high temperature only yield turbostratic graphite thin film formation. The absence of diamond indicates that the chosen parameters are not pertinent for its formation. Among the different assumptions discussed in Section 2, some of them have to be reconsidered. - We proposed that carbon segregation inside copper could be favourable for diamond formation and that the probability of such a segregation is related to the ion flux. In our experiments we can note that in a steady-state out-diffusion regime there is less than one carbon atom per one hundred copper atoms. Thus, the average distance between two neighbouring carbon atoms can be too large during the out-diffusion process to allow C-C bond formation. Then the rate of formation of carbon clusters is too low to lead to diamond thin film formation. A possible way to increase this rate will be to work with higher ion fluxes. - A carbon atom located on a vacancy occupies the most stable site in the copper lattice. Owing to the immiscibility its lifetime in this vacancy is short and thus it will diffuse. From this point of view, the presence of impurities could be favourable since they can trap carbon atoms. Hoff et al. [14] indicated that an ultrapure clean surface seems to inhibit diamond growth. If we consider that the segregation of the carbon atoms in the copper matrix is the key process for diamond formation, the presence of impurities in the nearby surface region could yield sp’ hybridized carbon clusters. - Without volume segregation, the carbon atoms reach the surface. The copper surface energy is too low compared with that of diamond to maintain carbon atoms heteroepitaxially on the copper lattice. If nonequilibrium conditions are not fulfilled when the carbon atoms reach the surface, graphite is formed. Then the highly anisotropic nature of graphite, the lack of reactivity of carbon with the copper surface, as shown by the absence of orientation relationships between the carbon thin film layer and the substrate, combined with a flat substrate surface are sufficient to explain the turbostratic graphite formation [ 121. The influence of both substrate temperature and ion flux have been clearly observed on the uniform layer texture. An increase of the temperature allows a better crystallization since it is a thermally activated process. This explains the higher texture of the thin film obtained at high temperature. Ion-flux variations modify the number of atoms that are introduced in the near-surface region per unit of time. The higher this
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number of atoms for a given temperature, the lower the degree of crystallization. However the presence of a turbostratic carbon layer on a copper substrate is known to increase the diamond nucleation rate [13,20,21]. Even if more simple techniques can be used for turbostratic carbon thin film formation, the present process offers the possibility of controlling the crystallization of a carbon layer. A nearly single crystalline layer of textured (0001) graphite has been achieved by an implantation at high temperature (1273 K) with a low ion flux. By decreasing the substrate temperature or increasing the ion flux, less organized layers can be obtained and such a control of the crystallinity of a carbon layer may be interesting for a good control of the nucleation rate of diamond. 5.2. Micrograins The presence of spherical micrograins, also called fullerene onions, has never been reported before in papers related to carbon implantation into copper. Some spherical graphitic structures have already been synthesized [21,22] by very different experimental methods. Furthermore they were never as large as those observed here, which are the largest observed to our knowledge. Their properties were not previously well known and their influence on the diamond nucleation has not yet been studied, but some processes using fullerenes as diamond precursors seem to yield interesting results [23].
6. Conclusion In this study, we characterized carbon thin films produced by high-dose carbon-ion implantation at high temperature into copper substrates and we proposed a model for the thin film formation. We studied both the influence of the carbon ion flux and of the substrate temperature on the thin film microstructure to try to optimize diamond formation. Whatever the experimental conditions were (973 KG T c 1273 K, 0.95 X 1013 cmm2 s-’ 6 ion flux < 3.3 X 1013 cmm2 ssl), we only observed a uniform textured (0001) graphite layer (turbostratic graphite) and graphitic shelled micrograins. Our results tend to indicate that diamond formation is not possible on a pure copper substrate in the range of ion flux and temperature used. Nevertheless we still believe in the potentiality of the carboncopper system since some previous results [14,17] show unambiguously the presence of single-crystalline diamond. We can point out here the fact that a carbon segregation inside the copper matrix seems to be one of the crucial points to fulfil for diamond film formation since carbon segregation on the surface only yields graphite. Studies concerning the out-diffusion
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under highly non-equilibrium conditions such as those that are obtained with heavy ion irradiation or laserpulse irradiation will also be interesting since conditions (high temperature, high cooling rate) favourable to diamond formation can then be expected. Nevertheless the control of the crystallinity of the turbostratic layer, as well as the formation of the graphitic shelled micrograins, are interesting phenomena for further experiments devoted to controlled diamond nucleation.
Acknowledgments
The authors thank Mr C. Fayoux for building the furnace mounted in line into the implantor and for performing the implantations at high temperature.
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