Magnesium interlayered diamond coating on silicon

International Journal of Refractory Metals & Hard Materials 24 (2006) 418–426 www.elsevier.com/locate/ijrmhm

Magnesium interlayered diamond coating on silicon M.A. Dar a, S.G. Ansari b, Z.A. Ansari b, Hironobu Umemoto b, Young-Soon Kim a, Hyung-Kee Seo a, Gil-Sung Kim a, Eun-Kyung Suh c, Hyung-Shik Shin a,* a b

Thin Film Technology Laboratory, School of Chemical Engineering, Chonbuk National University, Chonju 561-756, South Korea School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan c Department of Semiconductor Science and Technology, Chonbuk National University, Chonju 561-756, South Korea Received 8 March 2005; accepted 30 June 2005

Abstract Diamond thin films have been deposited using hot filament chemical vapour deposition technique on manually scratched pSi(1 0 0) substrate, with and without magnesium interlayer. In spite of magnesium melting point being lower (Tm = 649 C) than the growth temperature of the substrate (Ts  750 C) used in these experiments, it was found that high quality diamond films could be grown on Mg covered substrate. A liquid substrate is probably generated during the diamond film growth. Raman spectroscopy analysis exhibited only the triply degenerate, zone centre optical phonon peak at 1333 cm1 indicating that nearly stress free crystallites were present. Broadening of the Raman peak (11.76 cm1) indicates that some small crystallites also are present. Scanning electron and atomic force microscopy accompanied by X-ray diffraction analysis where used to compare the details of diamond film growth directly on scratched Si(1 0 0) and Mg interlayered scratched Si(1 0 0) substrates.  2005 Elsevier Ltd. All rights reserved. Keywords: HFCVD; Diamond film; Buffer layer; Bias growth

1. Introduction Diamond thin films have been deposited on a variety of substrates in order to exploit the excellent mechanical, thermal, optical or electronic properties of diamond for a wide variety of applications. Properties and applications of diamond thin films have been discussed in some excellent reviews published earlier, e.g. Refs. [1– 3]. Due to their high wear resistance; diamond thin films coated tools are often used in industry. Protective, smooth coatings of diamond are deposited on magnetic disks. Chemical inertness and mechanical strength accompanied by their scratch resistance and large transmission coefficient in visible and infrared regions make *

Corresponding author. Tel.: +82 63 270 2438; fax: +82 63 270 2306. E-mail address: [email protected] (H.-S. Shin). 0263-4368/$ - see front matter  2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2005.06.009

them suitable as a window material for IR and other detectors. They are also investigated for their field emission properties and array of diamond coated metal tips are expected to be used in flat panel displays. Electronics industry finds diamond thin films also useful for highspeed devices, especially when they are to be deployed at high temperature. Their large thermal conductivity and large electrical resistance makes them suitable as heat sinks. When used in electronics, deposition of diamond film on silicon substrate becomes important in order to integrate it with current microelectronics technology. However, its nucleation density on polished silicon substrates is very low [4], in the range of 103–105 cm2. Therefore, several methods [4–7] have been used to increase the nucleation density and growth rate of diamond films on silicon. Success of Mitsuda et al. [8] to enhance the nucleation density of diamond film by scratching the

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substrate with diamond grit has lead many groups to use this procedure as a common practice. The substrates have also been scratched using c-BN [9], Cu [10], silicon carbide [11], etc. Manual as well as ultrasonic scratching has been employed to increase the nucleation density by nearly five orders of magnitude. Scratching creates defects, steps or dislocations, which can act as nucleation centres. Sometimes the residual particles of scratching material left behind also initiate the growth. However, it is rather difficult to reproduce or control the surface uniformity by scratching. Nonetheless systematic efforts were made by Ali et al. [12] to investigate the effect of particle size of the scratching material and scratching time on diamond film growth on the substrate. They showed that better results could be obtained when using particle size less than 3 lm and scratching time 10 min or above produced similar results. It has also been shown in the past that seeding or coating [13,14] enhances the nucleation centres on the substrate. A recent work using SnO2 overlayers [15] scratching also was essential to enhance the diamond thin film growth. Yugo et al. [16] and many others later (e.g. [17]) adopted a different approach to achieve as high as 1010 cm2 nucleation density without scratching or using any other surface pre-treatment. They simply biased the substrate to enhance the nucleation density. Combination of bias enhanced nucleation (BEN) method with diamond grit scratched silicon surface produces good quality diamond thin films as demonstrated in our laboratory [18] and some others. Another approach bearing similarity to the traditional growth technique of high temperature high-pressure growth of diamonds is to use some catalyst. Carbide forming catalysts like Ti, W, Fe, Co, B, etc. and non-carbidic catalysts like Au, Ag, Cu, etc. also appear to have been widely used [4]. In certain cases, metals, metal oxides, metal carbides as catalysts or their overlayers [4,7,15,19–21] have been utilized in order to understand the growth rates of diamond films on these substrates. In general the growth of diamond thin films on carbidic surfaces is better. Besides the substrate materials and their pre-treatment, the methods of deposition and conditions are very crucial in the growth of diamond films [2,4]. In most of the depositions, growth of diamond films is observed at relatively high substrate temperatures, viz. above 600 C. At such temperatures surface diffusion would also be quite important in determining the growth of diamond film on any surface. In this context it is interesting to note that diamonds have been grown [22] with Cu, Zn and Ge at high temperature and pressure much above their melting point. Magnesium melting temperature is 649 C, lower than that used in hot filament chemical vapour deposition (HFCVD). It was therefore found interesting to use magnesium interlayer to grow diamond thin film on silicon substrate, using HFCVD

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technique. To the best of our knowledge, so far there are no reports about diamond film growth either on Mg or its thin film. Besides, there are certain advantages of using Mg as an interlayer. Magnesium is also a light element, useful as a structural material. However it can easily get oxidized and scratched. Therefore protective coating like diamond thin film on Mg is desired. Amongst the various chemical vapour deposition (CVD) techniques like DC, RF, microwave, electron cyclotron resonance (ECR) or laser ablation, hot filament chemical vapour deposition HFCVD has been one of the most attractive methods due to its relatively simple instrumentation and high deposition rate capability. In this communication, we show that high quality diamond thin films comparable or even better than those on scratched silicon substrate can be grown on Mg interlayered scratched Si(1 0 0) using HFCVD method.

2. Experimental 2.1. Substrate preparation In these experiments p-Si(1 0 0) substrates (boron doped) cut from a single wafer to 15 · 7 mm2 size were employed. Wafers of 650-lm thickness were obtained, purchased from Siltron Inc. (LG) Korea. Substrates were degreased in boiling acetone for 20 min and then washed ultrasonically in de-ionized water. Many groups have adopted this cleaning procedure for diamond film deposition by HFCVD. Substrates were scratched manually using 1 lm diamond paste (mono-crystalline diamond, commercially available for grinding and polishing). This was followed by cleaning in acetone and ultrasonic bath as before. 2.2. Thin films depositions Deposition of magnesium overlayers on scratched Si(1 0 0) substrates was carried out in a physical vapour deposition system pumped to a base vacuum 106 Torr. Ten samples were deposited simultaneously by evaporating high purity Mg(99.9%) from Aldrich, held in a tungsten filament. Post deposition analysis of thin films using stylus technique showed that 0.05 ± 0.002 lm thick films were deposited. Deposition of diamond thin films on scratched Si(1 0 0) and Mg covered scratched Si(1 0 0) substrate was carried out in an HFCVD reported elsewhere [18]. Briefly, the HFCVD reactor is an ultra high vacuum stainless steel chamber equipped with a filament and substrate manipulators, gas inlets and view ports. The filament temperature is monitored using an optical pyrometer (Minolta TR-630, Japan) and substrate

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temperature is recorded using K-type thermocouple. For diamond deposition the substrate and filament distance was kept as 5 mm. The tungsten filament (coil shaped, 0.5 mm diameters, 150 mm length) was resistively heated to 2150 ± 50 C temperature resulting into the substrate temperature equal to 750 ± 50 C. The substrate surface temperature must be higher than this temperature, as it was measured by touching thermocouple at the backside of the substrate. In the present setup, it was not possible to measure the surface temperature. A mixture of high purity H2(99.999%) and CH4(99.5%) was introduced from the bottom of the reaction chamber. The gas flow rates were controlled using mass flow controllers in such a way that volume % of CH4:H2 was 1:100. The total flow rate was 100 sccm and the total pressure inside the chamber was maintained at 30 ± 2 Torr during the deposition. The substrates were negatively biased to 280 V for first 30 min and further deposition was carried out for 90 min. The entire sample (scratched Si and Mg coated Si) was deposited under similar conditions. As the only difference between the two substrates is the Mg overlayer, which covers the scratches on Si(1 0 0) we expect any change in the diamond film growth to be due to the presence of Mg layers exclusively. In case of substrates coated with Mg, the bias current was slightly less (5–7 mA) than the Si without Mg overlayer. 2.3. Analysis of samples Surface topography/morphology of the samples was observed using atomic force microscope (ATM) and scanning electron microscope (SEM). The AFM Nanoscope R IV from Digital Instruments Metrology Group was employed in tapping mode. A silicon probe with 10 nm diameters was used in the analysis. SEM analysis was performed using an ISM-6400, JEOL, Japan microscope. Structure analysis of the samples was carried out using X-ray diffraction technique (XRD). For this purpose, a Rigaku III/A, Japan, powder diffractometer with ˚ ) and nickel filter was used. copper target (Cu Ka-1.54 A Raman spectra were recorded using a Micro Raman ˚ ). Spec(Reinshaw) with He–Ne laser source (k-6328 A tral resolution 2 cm1 could be achieved with this instrument. The X-ray photoelectron spectroscopic study was carried out using S-Probe ESCA model 2803 (Fision Instrument, 10 kV, 20 mA) with Al Ka as the source of X-rays.

3. Results and discussion Nucleation and growth on transition metals, metal oxides and carbides have been widely studied and also reviewed [4]. In most of the cases they are used as cata-

lysts. As well as the structural defects like terraces, steps, and pores along with chemical impurities, seeds and overlayers greatly affect the quality and crystallinity of the diamond film. Therefore growth directly on defected/over layers deposited on Si has been a subject of research and is being investigated. Here we discuss the effect of a Mg layer (0.05 lm), deposited on scratched Si(1 0 0), on the nucleation and growth of diamond film, in comparison with that of diamond film grown on scratched Si(1 0 0) substrate, an often used substrate. Magnesium has a hexagonal close pack ˚ and c = 5.21 A ˚ as lattice (hcp) structure with a = 3.21 A parameters. Therefore the lattice mismatch of diamond with magnesium can be 6.6% or 33.6% depending upon the growth. However as mentioned earlier, magnesium has a melting point of 649 C, which is substantially lower than the substrate heating performed in this experiment (750 ± 50 C). It should be mentioned here that magnesium thin films were deposited in a relatively poor vacuum 106 Torr and were transported in laboratory environment from one system to another for HFCVD growth, therefore inclusion of some carbon and oxygen is inevitable. However as discussed in some reviews earlier [1,4], small amounts of oxygen and inclusion may not adversely affect the nucleation and growth but even assist in some cases. During the electrical biasing, it was noticed that the bias current, with Mg layer was slight less (5–7 mA) than Si substrate. This might be due the inclusion of oxygen while sample transfer. SEM images of diamond film on scratched Si(1 0 0) and that with Mg interlayer on scratched Si(1 0 0) are illustrated in Fig. 1(a) and (b), respectively. Here the immediate impression is that the particles grown on scratched Si(1 0 0) are larger than those with Mg interlayer. A careful examination of Fig. 1(a) shows that there are many triangular platelets on top of bigger crystallites. These triangles are (1 1 1) oriented crystallites and have 0.5–1.5 lm sides. Fig. 1(b) too has similar sized triangular platelets but they are not grown on larger crystallites, and they appear to be smaller, denser and uniform. A rough estimate, from SEM image, of the surface nucleation density is of the order of 0.3 · 1010 cm2, 0.8 · 1010 cm2 whereas growth rate is 0.2 · 1010 cm2 h1, 0.5 · 1010 cm2 h1, for the scratched and Mg coated substrate. As AFM is an extremely surface sensitive technique with zoom-in capability, more insight about the differences between the two films was possible from the following analysis. Fig. 2 illustrate the AFM (planar and 3D) images of (a) scratched Si(1 0 0) surface, (b) Mg thin film grown on scratched Si(1 0 0) surface (before diamond deposition), (c) diamond thin film grown on Si(1 0 0) surface and (d) that grown on Mg covered Si(1 0 0) surface. In Fig. 2(a) random but nearly parallel 50 nm width lines, broken at various points are observed. There are also few other bigger particles, proba-

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Fig. 1. SEM images of (a) diamond film grown on scratched Si(1 0 0) surface, (b) diamond film grown on Mg interlayered scratched Si(1 0 0) surface.

bly from the diamond grit, randomly distributed on the surface. Small particles usually are active centres for further growth due to their large surface area and dangling bonds in case of semiconductors. As high is the density of such particles as better it is for the film growth. Average root mean square (RMS) roughness of scratched Si(1 0 0) surface as estimated from AFM is 2.6 nm. The roughened surface of the substrate is considered to be responsible for the enhanced growth and adhesion of thin film. Thus growth of Mg thin film, as seen in Fig. 2(b), also would be favoured by a silicon-roughened surface. As can be seen from Fig. 2(b), Mg film has nearly spherical particles 0.05–0.1 lm size, randomly distributed over the surface. However they are quite dense. Film RMS roughness is 10 nm as obtained by AFM. In this film Si scratches are no more observed, except few regions, which appear like reminiscences of scratching. Such regions are marked with arrows and dotted lines in Fig. 2(b). In bulk XRD very weak signal

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(not showed here) due to Mg could be detected at 2h = 34.4 and 36.6 corresponding to hcp Mg(0 0 2) and (1 0 1) planes. The signal is very weak because Mg overlayer thickness (0.05 lm) is small and bulk XRD is employed here. However, the effect of Mg over layer on diamond film deposition could be observed. In Fig. 2(c) and (d) morphology of diamond thin film on silicon scratched and that on Mg covered scratched silicon surface can be seen. These images grabbed by AFM are shown on a much wider scale (30 · 30 lm2) in each case compared to Fig. 2(a) and (b) because the size of the diamond particles are 0.5–1.5 lm (as in SEM) in both the cases. Images in Fig. 2(c) and (d) appear to be similar, except that the particle density is higher and height shorter in Fig. 2(d) as compared to that in Fig. 2(c). Thus by observing Fig. 2(c) and (d), one gets an impression that morphology and perhaps the quality of films grown on scratched Si(1 0 0) and Mg covered scratched Si(1 0 0) are nearly the same. However further analysis of the films reveals some marked differences. Fig. 2(e) and (f), respectively show the magnified top view of the smaller regions on typical particles for diamond film on scratched Si and that with Mg overlayer. Interestingly, these images of single particles indeed show morphological differences. In Fig. 2(e), for diamond film on scratched Si, two faceted particles with base of about 400 nm are visible. Whereas on Mg covered scratched Si substrate there are numerous rather flat but triangular particles with varied sizes. Most of these particles have sides smaller than 100 nm. On these smaller particles there can be even smaller particles, however they could not be observed with the AFM used in these experiments. In any case, the surface of Mg interlayered diamond film has much smaller and denser particles (surface nucleation density 0.8 · 1010 cm2) as compared to film grown on scratched Si(0.3 · 1010 cm2). Also in Fig. 2(e) and (f) the RMS roughness are 17 and 6 nm, respectively. Thus, much smother films are formed on Mg interlayered surface. Therefore denser diamond film is expected to grow on Mg interlayered substrate. Further differences in the qualities of diamond films on scratched Si(1 0 0) and Mg covered scratched Si(1 0 0) are discerned by X-ray diffraction analysis. On scratched Si(1 0 0) diffraction peak at 2h = 69.04 was observed due to Si(4 0 0). However in Fig. 3 only a limited range has been shown as the intensity of Si peak is too high compared to that due to (1 1 1) peaks of diamond for the same reasons discussed earlier for Mg film, viz. XRD instrument is for bulk analysis and the diamond films are thin. Fig. 3(a)–(d) shows the XRD for all the four samples under consideration. There are no peaks (curve a and b) on scratched Si or Mg covered Si scratched substrates in 2h = 38–54 range. In Fig. 3(c) the peak at 43.89 is due to the diamond (1 1 1) plane [Ref. JCPDS-06-0675] and broad peak at

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47.84 is identified with a-SiC [23]. For magnesium interlayered diamond film there is a single peak at 2h = 43.85 suggesting that purity of diamond film is better when using an Mg overlayer. Raman spectroscopy data enables to evaluate the quality of the diamond films grown on two substrates.

It can be seen from Fig. 4(a) that diamond film deposited on scratched Si has a sharp peak at 1333.0 cm1 and another well-resolved peak at 1605.0 cm1. For natural diamond the position of Raman peak is at 1332.6 cm1. Diamond with cubic crystal structure has a triply degenerate zone centre optical phonon peak at

Fig. 2. AFM images of (a) scratched Si(1 0 0), (b) Mg thin film (50 nm) on scratched Si(1 0 0), (c) diamond film on scratched Si(1 0 0), (d) diamond film on Mg interlayered scratched Si(1 0 0), (e) magnified image of a typical particle in (c), (f) magnified image of a typical particle in (d), the images on left side of the panel are plane views and those on the right hand side are 3-D views of respective plane views.

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Fig. 2 (continued )

this position. Sharp peak at 1333 cm1is very slightly shifted to higher side. Shifts to higher or lower frequency are expected [24–26] when compressive or hydrostatic pressure is applied. Uniaxial stress removes the degeneracy and splits the peak at 1332.6 cm1, as observed in many instances. See for example, Refs. [26,28]. Besides the stress this Raman peak is also influenced by temperature and domain size within the

particles [26]. A highly symmetric peak with negligible shift (0.4 cm1) in Fig. 4(a) shows that good quality, nearly stress free, diamond crystallites are present. However presence of some background signal is an indication of some inclusion of sp2 bonded non-diamond phases [28]. Especially the peak at 1605.0 cm1 for diamond film directly on silicon substrate is an indication of substantial amount of graphitic inclusion. McCulloch et al.

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SiC

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Bragg angle (2θ, degrees) Fig. 3. X-ray diffraction of (a) scratched Si(1 0 0), (b) Mg covered scratched Si(1 0 0), (c) diamond film on scratched Si(1 0 0) and (d) ˚ ) is diamond film on Mg covered scratched Si(1 0 0), Cu Ka (k = 1.54 A used for XRD.

[29] showed that due to small graphitic crystallites, a Raman peak (D 0 ) arises at 1615 cm1 along with peaks (G and D) at 1580 cm1 and 1355 cm1, respectively. As a result of increase in disorder D 0 and G peaks convolute to form a single peak whose position is somewhere between their original position and shifting to lower frequency with increasing disorder. Thus peak in curve Fig. 4(a) at 1605 cm1 can be explained. Besides, small variations in substrate conditions, specially uncontrolled manual scratching of the substrates may be responsible for this change. Mg covered scratched Si (Fig. 4(b)) shows only a single, symmetric peak at 1333.0 cm1. There could be small component of amorphous diamond (relating to sp2 carbon) as indicated by increase in background from 1400 to 1600 cm1. However there is no specific peak and background is less compared to that in curve of Fig. 4(a). Thus the diamond film on Mg overlayer is better as far as inclusions of non-diamond film in agreement with XRD results. The FWHM of Raman peaks observed at 1333.0 cm1 for diamond film grown on scratched Si and Mg coated Si is 13.16 cm1 and 11.76 cm1, respectively. The standard FWHM for pure diamond is 2.5 cm1. The additional width can arise due to domain size effect. In fact, the smaller domains lead to the break down in the wave vector (k) conservation rule and thus, states with nonzero k can also contribute in the Raman scattering process, which leads to peak broadening [26,27]. Therefore other non-k zero states also may contribute and increase the Raman peak width. It is thus possible that smaller grains as observed from AFM may be contributing to the observed Raman peak width in this case. Quantitative

Fig. 4. Raman spectra of (a) diamond film on scratched Si(1 0 0), (b) diamond film on Mg interlayered scratched Si(1 0 0). Spectra are normalized w.r.t. height of the Raman peak at 1333 cm1. Spectra are ˚ ). recorded using He–Ne laser (k = 6328 A

estimation of these contributions is rather difficult to make at this stage in presence of small particles on large particles. Further insight may be obtained in future, using AFM and with Raman spectroscopy. The most important question however is, what is the mechanism for the observed growth of diamond film on the Mg film with melting point lower than the substrate temperature? The concept of Ôliquid substrateÕ for growth of diamond films has been proposed by Roy et al. [30] and proved in a few cases. Recently Kao et al. [31] have shown the diamond formation in case of gold thin film deposited on amorphous silicon layers. They showed that Au–Si alloying takes place and diamond formation occurs at a temperature much above the melting point of Au–Si, suggesting the diamond growth on a ÔliquidÕ substrate. As mentioned before, the substrate temperature is higher than the melting temperature of Mg; therefore, it can be possible that Mg can evaporate. An attempt is made to understand what happens with Mg while the early nucleation of diamond by studying the samples with X-ray photoelectron spectroscopic technique. For this study, three types of samples are used, viz. (a) Mg deposited on Si, (b) after 20 min exposure to diamond growth (with BEN) and (c) diamond grown on Mg interlayer Si. The corresponding wide scan spectra are shown in Fig. 5(A). In case of sample type (a), peaks related to Mg, C and O are seen. The C-contamination is due to the ambient exposure. It is worth noticing that Si-substrate is fully coated with Mg. When the sample was exposed just for 20 min to the initial diamond growth (with BEN), the Mg peaks disappeared from the XPS spectra, instead Si peaks with a minor W (5p) peaks, can be seen. The C-peak height has increased and

Mg (2s)

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Mg (Auger ) Mg (Auger )

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An attempt is made to fit the core level C1s XPS curves for all three samples using Shirley-type base line with mixed Gaussian (30%) and Lorentzian (70%) profile and controlled FWHM. The entire curve fitting was carried out with chi-squared value less than 2 eV, which is reported to be good value for best fitting. It can be seen that it consist of only one peak at 284.16 eV relating to crystalline carbon [31]. This indicates that the grown films are fully crystallized as observed by Raman spectroscopic studies. It is well known that carbide formation (e.g. SiC or WC) helps in nucleation and growth of diamond on non-carbide substrate. In present study, it is difficult to comment on the formation of carbide with Mg as we could not obtained any information from spectroscopic studies. A detailed study is required to confirm the formation of such phases. There can also be the possibility of the formation of a complex Mg–Si–C–H phase during the nucleation of diamond particles. As Mg film has eliminated the effect of scratching by completely covering it (Fig. 2(b)), the effect on the nucleation/growth can be due to the overlayer only. The biasing condition, which is quite important and responsible for the enhancement of nucleation density, has been applied exactly in the same way to both the substrates. Further experiment to elucidate the nucleation and growth of diamond film on magnesium coated Si substrate with in situ growth study is required.

4. Conclusions

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Fig. 5. (A) XPS spectra: (a) Mg film on scratched Si(1 0 0), (b) after 20 min exposure to diamond growth (with BEN), (c) diamond film on Mg interlayered scratched Si(1 0 0). (B) Core level C (1s) XPS spectra of diamond film on Mg interlayed Si (100) with Shirley type curve fitting.

O-peak height has reduced. This indicates that during initial nucleation, Mg evaporates and helps in nucleation of diamond, which is clearly evident by the increase in the C-peak height. This means that a liquid Mg film helps in the nucleation of diamond, the phenomenon same as reported by Roy et al. [30]. The XPS measurements after diamond growth shows an intense, symmetric clear C (1s) peak with a minor oxygen peak. The corresponding core level (C1s) spectrum is presented in Fig. 5(B), which clearly confirms that the film composition is basically carbon.

We have showed here that it is feasible to grow high quality, dense diamond films on scratched Si(1 0 0) with Mg interlayer. It is confirmed that while initial nucleation, Mg evaporates thereby enhancing diamond nucleation. This shows that growth of diamond film takes place on liquid substrate of magnesium or its alloy with underlying silicon. A complex metal–carbon–hydrogen– silicon phase may be formed supporting the film growth. The diamond films are nearly free from amorphous carbon or graphite. Quality of these films is better as compared to that on scratched Si(1 0 0) deposited under similar conditions. Therefore it would be possible to deposit diamond film on Mg as a protective coating and growing media. Alternatively in order to obtain smooth, compact, dense diamond film, Mg can be considered as an alternative interlayer.

Acknowledgements S.G. Ansari would like to acknowledge the financial support received from Chonbuk National University postdoc program and JSPS fellowship program. M.A. Dar acknowledges the KOSEF graduate student

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fellowship program. KMOST (2004-01352) and KOSEF (R01-2004-000-10792-0) supported this work. The authors gratefully acknowledge J.P. Jeong for his assistance in Raman analysis.

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