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Characterization of planar-diode bias-treatment in DC-plasma hetero-epitaxial diamond growth on Ir(001)
Diamond & Related Materials 16 (2007) 594 – 599 www.elsevier.com/locate/diamond
Characterization of planar-diode bias-treatment in DC-plasma hetero-epitaxial diamond growth on Ir(001) T. Aoyama a,1 , N. Amano a , T. Goto a , T. Abukawa a , S. Kono a,⁎, Y. Ando b , A. Sawabe b a
b
IMRAM, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan Department of Electrical Engineering, Aoyama-Gakuin University, Sagamihara 229-8558, Japan
Received 16 May 2006; received in revised form 23 September 2006; accepted 13 November 2006 Available online 4 January 2007
Abstract The effect of bias-treatment (BT) on Ir(001)/MgO(001) substrates in a newly invented planar-diode DC-plasma system has been characterized in-situ and ex-situ by X-ray photoelectron diffraction (XPD), XPS, LEED and SEM. Features of XPD patterns of C 1s core levels were in good agreement with those of three-electrode BT [Diamond Relat. Mater. 13 (2004) 2081], although the degrees of anisotropy of C 1s XPD were smaller. Thicknesses of carbon films estimated from intensity ratios of C 1s/Ir 4d5/2 (or 4f) XPS peaks were about 2 times larger than those of three-electrode BT. LEED patterns showed no diffraction spots after BT. As a result, we conclude that epitaxial diamond crystallites with the size of a few nm or so are embedded in a non-oriented carbon layer. In the cases where no finite anisotropy of C 1s XPD was observed, no epitaxial diamond grains were grown in post-CVD as revealed by ex-situ SEM. Thus, it is concluded that the anisotropy of C 1s XPD can be a useful measure of diamond nucleation by BT on Ir(001) substrates. © 2006 Elsevier B.V. All rights reserved. Keywords: Diamond growth and characterization; CVD hetero-epitaxial diamond; X-ray photoelectron diffraction; Ir(001) substrate
1. Introduction Since the first report on hetero-epitaxy of diamond on Ir(001) [1], so far the best quality chemical vapour deposition (CVD) hetero-epitaxial diamond (001) films have been grown on Ir(001) substrates. A process referred to as bias-treatment (BT) or biasenhanced nucleation (BEN) or biasing [2] is performed prior to a CVD diamond growth process, which plays an essential role in the hetero-epitaxial diamond growth. Therefore, it is important to understand the role of BTat an atomic level for the application and improvement of hetero-epitaxial diamond growth. The effect of BT on Ir(001) substrates has been in fact extensively studied by various techniques although the kinds of plasma and the conditions of BT are different. In an experiment of microwave plasma BEN, it was reported that diamond crystallites of 10– 20 nm size were observed on a bias-treated Ir(001)/MgO(001) ⁎ Corresponding author. Fax: +81 22 217 5405. E-mail address: [email protected] (S. Kono). 1 Present address: Steel Research Lab., JFE Steel Corporation, Kawasaki, 2100855 Japan. 0925-9635/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2006.11.045
substrate by scanning electron microscopy (SEM) and reflection high energy electron diffraction (RHEED) [3]. On the other hand, “domains” were observed as bright areas in SEM images with several μm lateral sizes and negligible topographic contrast in Kelvin probe force microscopy on Ir(001)/SrTiO3(001) substrate after BT in microwave plasma CVD [4]. Recently, an in-situ study by X-ray photoelectron diffraction (XPD), X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED) has been performed on the effect of three-electrode BT in DCplasma diamond growth on Ir(001)/MgO(001) [5]. It was found that BT creates hetero-epitaxial diamond crystallites a few nm or less in size [5]. Meanwhile, in experiments of abrupt quenching BT with microwave plasma CVD of Ir(001) on SrTiO3(001) or a-plane Al2O3, uniformly distributed islands with a lateral size of 7–8 nm separated by 12–14 nm were observed by SEM and were considered to be diamond nanocrystallites [6,7]. Recently, an XPD study combined with SEM, atomic force microscopy (AFM), XPS and LEED strongly indicated that the 7–8 nm islands observed by SEM are indeed diamond nanocrystallites and embedded in a disordered carbon matrix [8]. Gsell et al. [9] also studied the nature of the nucleation centers formed during
T. Aoyama et al. / Diamond & Related Materials 16 (2007) 594–599 Table 1 Typical BT conditions used in this work Sample
Analysis
Bias voltage (V)
BT current (A)
BT duration (s)
A B C D
Ex-situ In-situ In-situ In-situ
390 360 400 400
0.17 0.16 0.20 0.20
90 60 60 180
BEN in a microwave plasma CVD setup by subjecting the asbiased Ir(001)/SrTiO3(001) samples to different treatments. They reported that the nucleation centers behaved similarly to diamond situated at the surface of the iridium buffer layer possibly covered by a thin stable amorphous carbon layer. Therefore, it is rather clear that well-oriented diamond (001) crystallites of different sizes are grown during BT on Ir(001) substrates. Recently, an important development has been made in the hetero-epitaxial CVD diamond growth on Ir(001); a patterned growth of epitaxial diamond films of an area of 10 × 10 mm2 or more has been achieved by DC-plasma CVD on Ir(001)/MgO (001) substrates which are bias-treated using a planar-diode system [10]. It is therefore of interest to examine whether and what kind of diamond crystallites are grown by the BT using a planar-diode system in DC-plasma CVD. In the present study, we extend the atomic level characterization by XPD, XPS and LEED to this planar-diode BT.
595
A BT was made as follows. The substrate was pre-heated to ∼500 °C in order to avert a breakage of substrate by a sudden temperature-rise when BT started. For BT, a negative voltage was supplied to the Ir(001) substrate while the planar electrode was at the earth potential. The conditions for BT are shown in Table 1: Other common conditions were CH4/H2 ratio, 2%; total pressure, 115 Torr; H2 flow rate, 200 sccm; substrate temperature, ∼ 900 °C. For in-situ analyses, the CVD chamber was evacuated after BT by a turbo-molecular pump and samples B, C and D were transferred to the UHV analysis chamber when the pressure of the CVD chamber reached ∼ 10− 7 mbar. As a comparison to the in-situ analysis, an ex-situ analysis was also carried out by air-transferring sample A which was bias-treated as in [10]. After surface analyses, the sample was brought out from the chamber and cleaved into four square pieces. Each of the samples was then sent to the CVD chamber and an equivalent diamond growth was performed in the conventional threeelectrode CVD system [5]. The conditions of CVD growth are as follows: CH4/H2 ratio, 2%; total pressure, 150 Torr; H2 flow rate, 100 sccm; cathode–anode voltage (current), 890 V (320 mA); duration, 10 min; substrate temperature, ∼ 900 °C. The experiments were carried out several times for each BT condition on different substrates and the results showed almost
2. Experimental A DC-plasma CVD chamber used was similar in design to a previous study [10]. It consisted of a circular and planar Mo electrode and a Mo substrate holder. The planar electrode was set ∼ 5 mm above and parallel to the substrate surface. Ir(001) substrates films (∼ 500 nm) were made by sputtering or MBE onto cleaved MgO(001) wafer (10 × 10 × 0.5 mm3). The Ir(001)/ MgO substrate was masked by a Mo sheet with a 7 × 7 mm2 window in order to cover MgO edges which cause a chargingup problem during BT. The substrate was then held by a Mo holder keeping an electric contact between the Ir surface and the Mo holder. The UHV analysis chamber used consisted of an LEED optic, a twin-anode soft X-ray source (Mg and Al Kα lines), a five-axis sample manipulator, and an angle-resolved electron energy analyzer on a two-axis manipulator. A transfer chamber/ system was connected between the analysis chamber and the CVD growth chamber through gate valves and transfer rods. The base pressure of the analysis chamber was ∼3 × 1011 mbar. A collimator was placed at the front end of the X-ray source so that a region of diameter of ∼ 1.2 mm of the sample was examined by XPS and XPD. For XPD measurements, photoelectron intensities were measured as a function of azimuthal angle of emission at a fixed emission polar angle by rotating the sample about the surface normal. The region of XPS and XPD measurements on the sample was changed by shifting the sample holder with respect to the plate of azimuthal rotation. Samples were examined by XPS, XPD and LEED in the analysis chamber both before and after BT. Samples after diamond growth were examined ex-situ by SEM.
Fig. 1. LEED patterns (a) before and (b) after BT of sample C at an electron energy of 190 eV. Mirror indexes of spots of 10, 01 and 11 are those for Ir(001) 1 × 1.
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the same results. For quantitative comparison of XPD patterns in different BT conditions, an anisotropy of XPD patterns is used. The anisotropy is expressed as (I − Iave) / Iave where I is the electron intensity at a given azimuth and Iave is the azimuthally averaged intensity at a fixed polar angle. 3. Results Fig. 1 shows LEED patterns of sample C (a) before and (b) after BT. No in-situ cleaning of the sample was performed for the LEED observation. Fig. 1(a) shows weak Ir(001)1 × 1 spots for the sample before BT. After BT, LEED pattern in Fig. 1(b) shows only background intensities. It is worth noting here that no diffraction spots of diamond (001) or Ir(001) were found for all the samples after BT at the measured electron energies of 140–200 eV. Fig. 2 shows wide energy range XPS spectra before (for the sample B) and after BT for all the four samples as excited by Mg Kα line at an emission polar angle (θ) of 45°. The inset shows an expanded view of Ir 4d and C 1s XPS peaks for the sample B before and after BT: A second peak at a lower binding energy side of C 1 s after BT is due to C 1s excited by Mg Kα satellite lines. Before BT, strong Ir core-level peaks and weak O 1s and C 1s peaks, derived from contaminations, are observed in curve (e). After BT, the C 1s peak becomes larger and Ir core-level peaks become much smaller. It is noticeable that O 1s peak and other contaminant peaks are not discernible for the in-situ analysis of samples B, C and D. For the ex-situ analysis of sample A, O 1s XPS peak is observed, which must be due to contamination during the air-transfer. The amount of oxygen is roughly estimated to be 3–4 layers in the unit of Ir(001) layer, which is reasonable for a usual air-transfer. From peak intensity ratio of C 1s to Ir 4d5/2 (or Ir 4f) and assuming a continuous carbon layer over Ir substrate, the thicknesses of carbon layers of samples A, B, C and D are roughly estimated to be ∼36 ML, ∼ 37 ML, ∼ 45 ML and ∼40 ML, respectively (1 ML being the
Fig. 2. a)–d) Wide energy range XPS spectra after BT for samples A–D, respectively. e) Wide XPS spectrum before BT for sample B. The inset is an expanded view of Ir 4d and C 1s XPS peaks for sample B. The abscissa binding energy is not calibrated.
Fig. 3. Azimuthal angle XPD patterns of C 1s core level from the bias-treated samples A–D. The azimuthal orientations are denoted for those of Ir(001). The lines of XPD patterns are classified as in Fig. 2.
surface atomic density of diamond (001) in this case). These amounts of carbon coverage are about twice as large as those of the bias-treated surfaces by DC-plasma in the three-electrode system [5] or microwave plasma [8]. Among the in-situ samples, it is found that the samples with a higher bias voltage (samples C and D) have thicker carbon coverage than that of the sample with a lower bias voltage (sample B). Among samples C and D with the same bias voltage of 400 V, the carbon coverage of the sample with shorter BT duration (sample C) is thicker than that of the sample with a longer BT duration (sample D). For sample A, BT was performed in a similar but different planar-diode system [10], thus the carbon coverage may not be compared quantitatively. XPS measurements were also carried out at other places of all the sample surfaces; 2 mm up, down, left and right from the center. There were little changes in XPS spectra at these off-center regions, indicating that the effect of BT is rather uniform over the surface. Fig. 3 shows C 1s XPD patterns taken at the central region of the samples after BT for the four samples A–D. In evaluation of C 1s XPD patterns, the XPD effect of Ir 4d5/2 satellite peaks was estimated to be small and subtracted from a known intensity ratio of the satellite peaks to the main peaks (∼10% with ∼10 eV separation [11]). The features of C 1s XPD patterns of samples A–D are similar to each other although the anisotropies are different. The C 1s XPD patterns at θ = 45° show maximum anisotropies of ∼16%, ∼22%, 9% and 9% for samples A–D, respectively. XPD patterns were also measured at 2 mm up, down, left and right regions of the samples and there were only little differences. The features of these patterns are in very good agreement with those of the sample bias-treated with DCplasma by the three-electrode system [5]. However, the anisotropies of the present samples are much smaller than that of the three-electrode system of ∼ 46%. After LEED, XPS and XPD analyses, the samples were taken out in air and cleaved into four equal-size square pieces
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anisotropy values of ∼16 and ∼9% show large (111) facets resulting in a “pyramidal” shape of the grains (Fig. 4a and c), while sample B with anisotropy of ∼ 22% shows grains with (001) faces resulting in a “tiled” shape of the diamond grains (Fig. 4b). It is also seen in Fig. 4 that the uniformity in grain size distribution improves as the XPD anisotropy increases, i.e., in an order from sample C to A and to B. It is worth mentioning here that no finite (larger that a few %) anisotropy was found in cases where the BT conditions were changed largely from those of Table 1 and no epitaxial diamond grains were found after the CVD growth process in those cases. The thickness of carbon layers found in these non-epitaxial cases tended to be higher than ∼ 40 ML. 4. Discussion and conclusions
Fig. 4. SEM images after diamond growth for 10 min as described in Section 2 following BT in various conditions. a) For samples A, b) B and c) C.
and then each piece was subjected to the same CVD diamond growth process in the three-electrode CVD system as described in Section 2. Fig. 4 shows SEM images of samples A–C after growth. Grains of epitaxial diamonds of sub-micron meter sizes are seen in the SEM images for all the samples. Although the density of grains is rather constant (roughly estimated in Fig. 4 as ∼ 2.4, ∼ 2.2, ∼ 3.1 × 109/cm2 for samples A, B and C, respectively), there exist some tendencies that the diamond grains have larger (111) facets for samples with smaller anisotropies of C 1s XPD. Namely, samples A and C with
The features of C 1s XPD patterns for the present samples are very similar to those for the samples bias-treated by the DCplasma three-electrode system [5] and microwave plasma [8]. This proves the presence of hetero-epitaxial diamond crystallites on the substrate after BT. However, the anisotropies of C 1s XPD patterns of present cases are about half or less than those of the bias-treated sample in three-electrode system and homoepitaxial diamond (001) [5]. This means that hetero-epitaxial diamond crystallites and non-oriented (either poly-crystalline or amorphous) carbon matrix coexist on the Ir substrate after BT since non-oriented carbon gives no diffraction features thus the anisotropy decreases. Another important fact is that no LEED diffraction spots are seen after BT. There are basically two possibilities to explain this. The first one is that epitaxial diamond crystallites are large enough to show LEED diffraction spots but are covered with non-oriented carbon since a difference in probing depth for LEED (∼ 0.8 nm) and XPD (∼2.7 nm) [12] makes XPD patterns “visible” while LEED spots “invisible”. The second one is that although epitaxial diamond crystallites are not completely covered by nonoriented carbon, the sizes of epitaxial diamond crystallites are too small (a few nanometers or less) to show LEED diffraction spots since LEED diffraction spots require crystal sizes larger than a typical coherence length (∼10 nm) of LEED electron beam [13]. There is also a possibility that the sizes of the epitaxial diamond crystallites are small and they are covered by non-oriented carbon. Although we cannot distinguish from the present experiment alone which possibility is real. However, in a previous case [5] where essentially no non-oriented carbon atoms cover the diamond crystallites, the sizes of the epitaxial diamond crystallites were found to be a few nanometers since no LEED spots were visible. In the other case [8] where a nonoriented carbon matrix is present and no LEED spots are visible, the surfaces of epitaxial diamond crystallites are not covered by the non-oriented carbon matrix, since SEM images show the presence of crystallites of 7–8 nm. Therefore, it is likely in the present case that the hetero-epitaxial diamond crystallites exist in the non-oriented carbon matrix with their surfaces nearly exposed. The fractions of hetero-epitaxial diamond in the deposited carbon layers can be roughly estimated from the ratio of
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maximum anisotropies of C 1s XPD patterns to that for ideal diamond (001) [5]. The fractions are 1/3, 1/2, 1/5 and 1/5 for samples A–D, respectively and are quite small as compared with the case of three-electrode system [5], where almost all carbon atoms deposited are in the framework of epitaxial diamond crystallites. Although the fraction of hetero-epitaxial diamond in the deposited carbon layers is different after BT, growth of epitaxial diamond films is observed on all the samples. Fig. 4 shows that the shape and uniformity of diamond grains improve as the fraction increases. Thus, the fraction, and thus XPD anisotropy, can be a useful measure of the effect of BT. However, details of this measure are not yet clear. The grain density as found in Fig. 4 is an order of 109/cm2. If we assume a mean size of ∼ 5 nm for nanometer diamond crystallites and the fraction of 1/2 after BT, the density of nanometer diamonds is an order of 1012/cm2. Thus, one out of ∼ 1000 diamond crystallites remains after the post-CVD growth. This may be related to “over-growth” in which larger-sized diamond crystallites “overgrow” smaller-sized diamond crystallites or extinction of smaller-sized diamond crystallites during CVD. This is an important subject in diamond hetero-epitaxy but beyond the scope of the present study. The mechanism of hetero-nucleation of diamond during BT has been studied extensively [14], in particular, for diamond growth on silicon [15,16]. Lee et al. [15] have observed TEM images after BT by hot filament CVD on a Si single crystal. They showed that diamond crystallites of size 2–6 nm are embedded in an amorphous carbon matrix and they are randomly oriented in most cases. When the diamond crystallites are in contact with Si steps, some of them are epitaxially aligned. The hetero-epitaxial growth of diamond in CVD process is then suggested [15] that the etching of the amorphous carbon during CVD leaves only those epitaxially aligned diamond crystallites directly nucleated on the Si substrate. In a recent study of the effect of DC glow-discharge plasma BT on a Si substrate using TEM and other spectroscopies, a little different observation is reported [16]. As BT proceeds, hydrogenated amorphous carbon (a-C:H) layers are formed over the Si substrate. The a-C:H layers consist of oriented graphitic planes perpendicular to the substrate. According to the proposed mechanism of diamond nucleation [14], 100% sp3 coordinated carbon clusters are precipitated in the a-C:H layers and then grow into diamond crystallites. 2–5 nm diamond crystallites embedded in a graphitic matrix were seen in TEM images which showed slightly preferred orientation with respect to the substrate [16]. However, a good epitaxial growth of diamond may not be expected in this case, since epitaxially aligned diamond crystallites directly nucleated on the Si substrate are missing in this case. As mentioned already, diamond crystallites created during BT on Ir(001) substrates are reported to be seen only in two cases [3,8]. It is reported that a SEM observation combined with RHEED of an Ir(001)/MgO substrate after a BT in a microwave CVD chamber, revealed 10–20 nm size diamond crystallites aligned along the b110N directions [3]. No amorphous carbon layers were identified in this case. SEM and AFM observations combined with XPD of an Ir(001)/SrTiO3 substrate after a BT in
a microwave CVD chamber, strongly indicated that 7–8 nm size particles are diamond crystallites embedded in a non-oriented carbon matrix [8]. The seemingly diamond crystallites are arranged in a manner that indicates a strong correlation among the crystallites. In the two cases of “direct” visualization of diamond crystallites nucleated during BT on Ir substrate, different kinds of distribution of different sized crystallites are seen. It is, therefore, too early to draw a conclusion about the nucleation mechanism of hetero-epitaxy on Ir(001). In conclusion, the effect of BT in DC-plasma with planardiode system on Ir(001)/MgO(001) substrates has been studied in-situ and ex-situ by XPD together with XPS and LEED. The features of XPD patterns of C 1s core levels are in good agreement with those of homoepitaxial diamond (001) and those of three-electrode BT, although degrees of anisotropy of C 1s XPD are smaller. Thicknesses of carbon films estimated from intensity ratios of C 1s/Ir 4d5/2 (or 4f) XPS peaks are about 2 times larger than those of three-electrode BT. On the other hand, LEED patterns show no diffraction spots, implying either the size of diamond crystallites to be smaller than the LEED coherence length of ∼ 10 nm or the presence of non-oriented carbon layers over diamond crystallites. From comparison to similar cases of BT where diamond crystallites are not covered by non-oriented carbon layers, we conclude that epitaxial diamond crystallites with the size of a few nanometers or so are embedded in a non-oriented carbon layer. In the cases where no anisotropy of C 1s XPD was found, no epitaxial diamond grains were grown after post-CVD. In the cases where a finite anisotropy of C 1s XDP is found, the shapes of epitaxial diamond grains grown during a post-CVD seem to depend on the anisotropy, thus it is considered that the anisotropy of C 1s XPD can be a useful measure of diamond nucleation by BT on Ir (001) substrates. Acknowledgements This work is supported in part by the Advanced Diamond Device Project administered by NEDO, Japan and by a Grant-inAid for Young Scientists (B) (No. 17710089) from the Ministry of Education, Culture, Sports, Science and Technology, Japan and by CREST of Japan Science and Technology Corporation. References [1] K. Ohtsuka, K. Suzuki, A. Sawabe, T. Inuzuka, Jpn. J. Appl. Phys. 35 (1996) L1072. [2] S. Yugo, T. Kanai, T. Kimura, T. Muto, Appl. Phys. Lett. 58 (1991) 1036. [3] T. Fujisaki, M. Tachiki, N. Taniyama, M. Kudo, H. Kawarada, Diamond Relat. Mater. 12 (2003) 246. [4] M. Schreck, Th. Bauer, S. Gsell, F. Hörmann, H. Bielefeldt, B. Stritzker, Diamond Relat. Mater. 12 (2003) 262. [5] S. Kono, T. Takano, T. Goto, Y. Ikejima, M. Shiraishi, T. Abukawa, T. Yamada, A. Sawabe, Diamond Relat. Mater. 13 (2004) 2081. [6] C. Bednarski, Z. Dai, A.-P. Li, B. Golding, Diamond Relat. Mater. 12 (2003) 241. [7] B. Golding, C. Bednarski-Meinke, Z. Dai, Diamond Relat. Mater. 13 (2004) 545. [8] S. Kono, M. Shiraishi, N.I. Plusnin, T. Goto, Y. Ikejima, T. Abukawa, M. Shimomura, Z. Dai, C. Bednarski-Meinke, B. Golding, New Diam. Front. Carbon Technol. 15 (2005) 363.
T. Aoyama et al. / Diamond & Related Materials 16 (2007) 594–599 [9] S. Gsell, M. Schreck, T. Bauer, H. Karl, G. Thorwarth, A. Bergmaier, G. Dollinger, B. Stritzker, Diamond Relat. Mater. 14 (2005) 328. [10] Y. Ando, J. Kuwabara, K. Suzuki, A. Sawabe, Diamond Relat. Mater. 13 (2004) 1975. [11] M.O. Krause, J.G. Ferreira, J. Phys. B 8 (1975) 2007. [12] S. Tanuma, C.J. Powell, D.R. Penn, Surf. Interface Anal. 17 (1988) 911. [13] For example, M. Prutton, Surface Physics, Oxford University Press, Oxford, 1983, p. 57.
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[14] Y. Lifshitz, Th. Köhler, Th. Frauenheim, I. Guzmann, A. Hoffman, R.Q. Zhang, X.T. Zhou, S.T. Lee, Science 297 (2002) 1531. [15] S.T. Lee, H.Y. Peng, X.T. Zhou, N. Wang, C.S. Lee, L. Bello, Y. Lifshitz, Science 287 (2000) 104. [16] Y. Lifshitz, X.M. Meng, S.T. Lee, R. Akhveldiany, A. Hoffman, Phys. Rev. Lett. 93 (2004) 056101.
Characterization of planar-diode bias-treatment in DC-plasma hetero-epitaxial diamond growth on Ir(001) T. Aoyama a,1 , N. Amano a , T. Goto a , T. Abukawa a , S. Kono a,⁎, Y. Ando b , A. Sawabe b a
b
IMRAM, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan Department of Electrical Engineering, Aoyama-Gakuin University, Sagamihara 229-8558, Japan
Received 16 May 2006; received in revised form 23 September 2006; accepted 13 November 2006 Available online 4 January 2007
Abstract The effect of bias-treatment (BT) on Ir(001)/MgO(001) substrates in a newly invented planar-diode DC-plasma system has been characterized in-situ and ex-situ by X-ray photoelectron diffraction (XPD), XPS, LEED and SEM. Features of XPD patterns of C 1s core levels were in good agreement with those of three-electrode BT [Diamond Relat. Mater. 13 (2004) 2081], although the degrees of anisotropy of C 1s XPD were smaller. Thicknesses of carbon films estimated from intensity ratios of C 1s/Ir 4d5/2 (or 4f) XPS peaks were about 2 times larger than those of three-electrode BT. LEED patterns showed no diffraction spots after BT. As a result, we conclude that epitaxial diamond crystallites with the size of a few nm or so are embedded in a non-oriented carbon layer. In the cases where no finite anisotropy of C 1s XPD was observed, no epitaxial diamond grains were grown in post-CVD as revealed by ex-situ SEM. Thus, it is concluded that the anisotropy of C 1s XPD can be a useful measure of diamond nucleation by BT on Ir(001) substrates. © 2006 Elsevier B.V. All rights reserved. Keywords: Diamond growth and characterization; CVD hetero-epitaxial diamond; X-ray photoelectron diffraction; Ir(001) substrate
1. Introduction Since the first report on hetero-epitaxy of diamond on Ir(001) [1], so far the best quality chemical vapour deposition (CVD) hetero-epitaxial diamond (001) films have been grown on Ir(001) substrates. A process referred to as bias-treatment (BT) or biasenhanced nucleation (BEN) or biasing [2] is performed prior to a CVD diamond growth process, which plays an essential role in the hetero-epitaxial diamond growth. Therefore, it is important to understand the role of BTat an atomic level for the application and improvement of hetero-epitaxial diamond growth. The effect of BT on Ir(001) substrates has been in fact extensively studied by various techniques although the kinds of plasma and the conditions of BT are different. In an experiment of microwave plasma BEN, it was reported that diamond crystallites of 10– 20 nm size were observed on a bias-treated Ir(001)/MgO(001) ⁎ Corresponding author. Fax: +81 22 217 5405. E-mail address: [email protected] (S. Kono). 1 Present address: Steel Research Lab., JFE Steel Corporation, Kawasaki, 2100855 Japan. 0925-9635/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2006.11.045
substrate by scanning electron microscopy (SEM) and reflection high energy electron diffraction (RHEED) [3]. On the other hand, “domains” were observed as bright areas in SEM images with several μm lateral sizes and negligible topographic contrast in Kelvin probe force microscopy on Ir(001)/SrTiO3(001) substrate after BT in microwave plasma CVD [4]. Recently, an in-situ study by X-ray photoelectron diffraction (XPD), X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED) has been performed on the effect of three-electrode BT in DCplasma diamond growth on Ir(001)/MgO(001) [5]. It was found that BT creates hetero-epitaxial diamond crystallites a few nm or less in size [5]. Meanwhile, in experiments of abrupt quenching BT with microwave plasma CVD of Ir(001) on SrTiO3(001) or a-plane Al2O3, uniformly distributed islands with a lateral size of 7–8 nm separated by 12–14 nm were observed by SEM and were considered to be diamond nanocrystallites [6,7]. Recently, an XPD study combined with SEM, atomic force microscopy (AFM), XPS and LEED strongly indicated that the 7–8 nm islands observed by SEM are indeed diamond nanocrystallites and embedded in a disordered carbon matrix [8]. Gsell et al. [9] also studied the nature of the nucleation centers formed during
T. Aoyama et al. / Diamond & Related Materials 16 (2007) 594–599 Table 1 Typical BT conditions used in this work Sample
Analysis
Bias voltage (V)
BT current (A)
BT duration (s)
A B C D
Ex-situ In-situ In-situ In-situ
390 360 400 400
0.17 0.16 0.20 0.20
90 60 60 180
BEN in a microwave plasma CVD setup by subjecting the asbiased Ir(001)/SrTiO3(001) samples to different treatments. They reported that the nucleation centers behaved similarly to diamond situated at the surface of the iridium buffer layer possibly covered by a thin stable amorphous carbon layer. Therefore, it is rather clear that well-oriented diamond (001) crystallites of different sizes are grown during BT on Ir(001) substrates. Recently, an important development has been made in the hetero-epitaxial CVD diamond growth on Ir(001); a patterned growth of epitaxial diamond films of an area of 10 × 10 mm2 or more has been achieved by DC-plasma CVD on Ir(001)/MgO (001) substrates which are bias-treated using a planar-diode system [10]. It is therefore of interest to examine whether and what kind of diamond crystallites are grown by the BT using a planar-diode system in DC-plasma CVD. In the present study, we extend the atomic level characterization by XPD, XPS and LEED to this planar-diode BT.
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A BT was made as follows. The substrate was pre-heated to ∼500 °C in order to avert a breakage of substrate by a sudden temperature-rise when BT started. For BT, a negative voltage was supplied to the Ir(001) substrate while the planar electrode was at the earth potential. The conditions for BT are shown in Table 1: Other common conditions were CH4/H2 ratio, 2%; total pressure, 115 Torr; H2 flow rate, 200 sccm; substrate temperature, ∼ 900 °C. For in-situ analyses, the CVD chamber was evacuated after BT by a turbo-molecular pump and samples B, C and D were transferred to the UHV analysis chamber when the pressure of the CVD chamber reached ∼ 10− 7 mbar. As a comparison to the in-situ analysis, an ex-situ analysis was also carried out by air-transferring sample A which was bias-treated as in [10]. After surface analyses, the sample was brought out from the chamber and cleaved into four square pieces. Each of the samples was then sent to the CVD chamber and an equivalent diamond growth was performed in the conventional threeelectrode CVD system [5]. The conditions of CVD growth are as follows: CH4/H2 ratio, 2%; total pressure, 150 Torr; H2 flow rate, 100 sccm; cathode–anode voltage (current), 890 V (320 mA); duration, 10 min; substrate temperature, ∼ 900 °C. The experiments were carried out several times for each BT condition on different substrates and the results showed almost
2. Experimental A DC-plasma CVD chamber used was similar in design to a previous study [10]. It consisted of a circular and planar Mo electrode and a Mo substrate holder. The planar electrode was set ∼ 5 mm above and parallel to the substrate surface. Ir(001) substrates films (∼ 500 nm) were made by sputtering or MBE onto cleaved MgO(001) wafer (10 × 10 × 0.5 mm3). The Ir(001)/ MgO substrate was masked by a Mo sheet with a 7 × 7 mm2 window in order to cover MgO edges which cause a chargingup problem during BT. The substrate was then held by a Mo holder keeping an electric contact between the Ir surface and the Mo holder. The UHV analysis chamber used consisted of an LEED optic, a twin-anode soft X-ray source (Mg and Al Kα lines), a five-axis sample manipulator, and an angle-resolved electron energy analyzer on a two-axis manipulator. A transfer chamber/ system was connected between the analysis chamber and the CVD growth chamber through gate valves and transfer rods. The base pressure of the analysis chamber was ∼3 × 1011 mbar. A collimator was placed at the front end of the X-ray source so that a region of diameter of ∼ 1.2 mm of the sample was examined by XPS and XPD. For XPD measurements, photoelectron intensities were measured as a function of azimuthal angle of emission at a fixed emission polar angle by rotating the sample about the surface normal. The region of XPS and XPD measurements on the sample was changed by shifting the sample holder with respect to the plate of azimuthal rotation. Samples were examined by XPS, XPD and LEED in the analysis chamber both before and after BT. Samples after diamond growth were examined ex-situ by SEM.
Fig. 1. LEED patterns (a) before and (b) after BT of sample C at an electron energy of 190 eV. Mirror indexes of spots of 10, 01 and 11 are those for Ir(001) 1 × 1.
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the same results. For quantitative comparison of XPD patterns in different BT conditions, an anisotropy of XPD patterns is used. The anisotropy is expressed as (I − Iave) / Iave where I is the electron intensity at a given azimuth and Iave is the azimuthally averaged intensity at a fixed polar angle. 3. Results Fig. 1 shows LEED patterns of sample C (a) before and (b) after BT. No in-situ cleaning of the sample was performed for the LEED observation. Fig. 1(a) shows weak Ir(001)1 × 1 spots for the sample before BT. After BT, LEED pattern in Fig. 1(b) shows only background intensities. It is worth noting here that no diffraction spots of diamond (001) or Ir(001) were found for all the samples after BT at the measured electron energies of 140–200 eV. Fig. 2 shows wide energy range XPS spectra before (for the sample B) and after BT for all the four samples as excited by Mg Kα line at an emission polar angle (θ) of 45°. The inset shows an expanded view of Ir 4d and C 1s XPS peaks for the sample B before and after BT: A second peak at a lower binding energy side of C 1 s after BT is due to C 1s excited by Mg Kα satellite lines. Before BT, strong Ir core-level peaks and weak O 1s and C 1s peaks, derived from contaminations, are observed in curve (e). After BT, the C 1s peak becomes larger and Ir core-level peaks become much smaller. It is noticeable that O 1s peak and other contaminant peaks are not discernible for the in-situ analysis of samples B, C and D. For the ex-situ analysis of sample A, O 1s XPS peak is observed, which must be due to contamination during the air-transfer. The amount of oxygen is roughly estimated to be 3–4 layers in the unit of Ir(001) layer, which is reasonable for a usual air-transfer. From peak intensity ratio of C 1s to Ir 4d5/2 (or Ir 4f) and assuming a continuous carbon layer over Ir substrate, the thicknesses of carbon layers of samples A, B, C and D are roughly estimated to be ∼36 ML, ∼ 37 ML, ∼ 45 ML and ∼40 ML, respectively (1 ML being the
Fig. 2. a)–d) Wide energy range XPS spectra after BT for samples A–D, respectively. e) Wide XPS spectrum before BT for sample B. The inset is an expanded view of Ir 4d and C 1s XPS peaks for sample B. The abscissa binding energy is not calibrated.
Fig. 3. Azimuthal angle XPD patterns of C 1s core level from the bias-treated samples A–D. The azimuthal orientations are denoted for those of Ir(001). The lines of XPD patterns are classified as in Fig. 2.
surface atomic density of diamond (001) in this case). These amounts of carbon coverage are about twice as large as those of the bias-treated surfaces by DC-plasma in the three-electrode system [5] or microwave plasma [8]. Among the in-situ samples, it is found that the samples with a higher bias voltage (samples C and D) have thicker carbon coverage than that of the sample with a lower bias voltage (sample B). Among samples C and D with the same bias voltage of 400 V, the carbon coverage of the sample with shorter BT duration (sample C) is thicker than that of the sample with a longer BT duration (sample D). For sample A, BT was performed in a similar but different planar-diode system [10], thus the carbon coverage may not be compared quantitatively. XPS measurements were also carried out at other places of all the sample surfaces; 2 mm up, down, left and right from the center. There were little changes in XPS spectra at these off-center regions, indicating that the effect of BT is rather uniform over the surface. Fig. 3 shows C 1s XPD patterns taken at the central region of the samples after BT for the four samples A–D. In evaluation of C 1s XPD patterns, the XPD effect of Ir 4d5/2 satellite peaks was estimated to be small and subtracted from a known intensity ratio of the satellite peaks to the main peaks (∼10% with ∼10 eV separation [11]). The features of C 1s XPD patterns of samples A–D are similar to each other although the anisotropies are different. The C 1s XPD patterns at θ = 45° show maximum anisotropies of ∼16%, ∼22%, 9% and 9% for samples A–D, respectively. XPD patterns were also measured at 2 mm up, down, left and right regions of the samples and there were only little differences. The features of these patterns are in very good agreement with those of the sample bias-treated with DCplasma by the three-electrode system [5]. However, the anisotropies of the present samples are much smaller than that of the three-electrode system of ∼ 46%. After LEED, XPS and XPD analyses, the samples were taken out in air and cleaved into four equal-size square pieces
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anisotropy values of ∼16 and ∼9% show large (111) facets resulting in a “pyramidal” shape of the grains (Fig. 4a and c), while sample B with anisotropy of ∼ 22% shows grains with (001) faces resulting in a “tiled” shape of the diamond grains (Fig. 4b). It is also seen in Fig. 4 that the uniformity in grain size distribution improves as the XPD anisotropy increases, i.e., in an order from sample C to A and to B. It is worth mentioning here that no finite (larger that a few %) anisotropy was found in cases where the BT conditions were changed largely from those of Table 1 and no epitaxial diamond grains were found after the CVD growth process in those cases. The thickness of carbon layers found in these non-epitaxial cases tended to be higher than ∼ 40 ML. 4. Discussion and conclusions
Fig. 4. SEM images after diamond growth for 10 min as described in Section 2 following BT in various conditions. a) For samples A, b) B and c) C.
and then each piece was subjected to the same CVD diamond growth process in the three-electrode CVD system as described in Section 2. Fig. 4 shows SEM images of samples A–C after growth. Grains of epitaxial diamonds of sub-micron meter sizes are seen in the SEM images for all the samples. Although the density of grains is rather constant (roughly estimated in Fig. 4 as ∼ 2.4, ∼ 2.2, ∼ 3.1 × 109/cm2 for samples A, B and C, respectively), there exist some tendencies that the diamond grains have larger (111) facets for samples with smaller anisotropies of C 1s XPD. Namely, samples A and C with
The features of C 1s XPD patterns for the present samples are very similar to those for the samples bias-treated by the DCplasma three-electrode system [5] and microwave plasma [8]. This proves the presence of hetero-epitaxial diamond crystallites on the substrate after BT. However, the anisotropies of C 1s XPD patterns of present cases are about half or less than those of the bias-treated sample in three-electrode system and homoepitaxial diamond (001) [5]. This means that hetero-epitaxial diamond crystallites and non-oriented (either poly-crystalline or amorphous) carbon matrix coexist on the Ir substrate after BT since non-oriented carbon gives no diffraction features thus the anisotropy decreases. Another important fact is that no LEED diffraction spots are seen after BT. There are basically two possibilities to explain this. The first one is that epitaxial diamond crystallites are large enough to show LEED diffraction spots but are covered with non-oriented carbon since a difference in probing depth for LEED (∼ 0.8 nm) and XPD (∼2.7 nm) [12] makes XPD patterns “visible” while LEED spots “invisible”. The second one is that although epitaxial diamond crystallites are not completely covered by nonoriented carbon, the sizes of epitaxial diamond crystallites are too small (a few nanometers or less) to show LEED diffraction spots since LEED diffraction spots require crystal sizes larger than a typical coherence length (∼10 nm) of LEED electron beam [13]. There is also a possibility that the sizes of the epitaxial diamond crystallites are small and they are covered by non-oriented carbon. Although we cannot distinguish from the present experiment alone which possibility is real. However, in a previous case [5] where essentially no non-oriented carbon atoms cover the diamond crystallites, the sizes of the epitaxial diamond crystallites were found to be a few nanometers since no LEED spots were visible. In the other case [8] where a nonoriented carbon matrix is present and no LEED spots are visible, the surfaces of epitaxial diamond crystallites are not covered by the non-oriented carbon matrix, since SEM images show the presence of crystallites of 7–8 nm. Therefore, it is likely in the present case that the hetero-epitaxial diamond crystallites exist in the non-oriented carbon matrix with their surfaces nearly exposed. The fractions of hetero-epitaxial diamond in the deposited carbon layers can be roughly estimated from the ratio of
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maximum anisotropies of C 1s XPD patterns to that for ideal diamond (001) [5]. The fractions are 1/3, 1/2, 1/5 and 1/5 for samples A–D, respectively and are quite small as compared with the case of three-electrode system [5], where almost all carbon atoms deposited are in the framework of epitaxial diamond crystallites. Although the fraction of hetero-epitaxial diamond in the deposited carbon layers is different after BT, growth of epitaxial diamond films is observed on all the samples. Fig. 4 shows that the shape and uniformity of diamond grains improve as the fraction increases. Thus, the fraction, and thus XPD anisotropy, can be a useful measure of the effect of BT. However, details of this measure are not yet clear. The grain density as found in Fig. 4 is an order of 109/cm2. If we assume a mean size of ∼ 5 nm for nanometer diamond crystallites and the fraction of 1/2 after BT, the density of nanometer diamonds is an order of 1012/cm2. Thus, one out of ∼ 1000 diamond crystallites remains after the post-CVD growth. This may be related to “over-growth” in which larger-sized diamond crystallites “overgrow” smaller-sized diamond crystallites or extinction of smaller-sized diamond crystallites during CVD. This is an important subject in diamond hetero-epitaxy but beyond the scope of the present study. The mechanism of hetero-nucleation of diamond during BT has been studied extensively [14], in particular, for diamond growth on silicon [15,16]. Lee et al. [15] have observed TEM images after BT by hot filament CVD on a Si single crystal. They showed that diamond crystallites of size 2–6 nm are embedded in an amorphous carbon matrix and they are randomly oriented in most cases. When the diamond crystallites are in contact with Si steps, some of them are epitaxially aligned. The hetero-epitaxial growth of diamond in CVD process is then suggested [15] that the etching of the amorphous carbon during CVD leaves only those epitaxially aligned diamond crystallites directly nucleated on the Si substrate. In a recent study of the effect of DC glow-discharge plasma BT on a Si substrate using TEM and other spectroscopies, a little different observation is reported [16]. As BT proceeds, hydrogenated amorphous carbon (a-C:H) layers are formed over the Si substrate. The a-C:H layers consist of oriented graphitic planes perpendicular to the substrate. According to the proposed mechanism of diamond nucleation [14], 100% sp3 coordinated carbon clusters are precipitated in the a-C:H layers and then grow into diamond crystallites. 2–5 nm diamond crystallites embedded in a graphitic matrix were seen in TEM images which showed slightly preferred orientation with respect to the substrate [16]. However, a good epitaxial growth of diamond may not be expected in this case, since epitaxially aligned diamond crystallites directly nucleated on the Si substrate are missing in this case. As mentioned already, diamond crystallites created during BT on Ir(001) substrates are reported to be seen only in two cases [3,8]. It is reported that a SEM observation combined with RHEED of an Ir(001)/MgO substrate after a BT in a microwave CVD chamber, revealed 10–20 nm size diamond crystallites aligned along the b110N directions [3]. No amorphous carbon layers were identified in this case. SEM and AFM observations combined with XPD of an Ir(001)/SrTiO3 substrate after a BT in
a microwave CVD chamber, strongly indicated that 7–8 nm size particles are diamond crystallites embedded in a non-oriented carbon matrix [8]. The seemingly diamond crystallites are arranged in a manner that indicates a strong correlation among the crystallites. In the two cases of “direct” visualization of diamond crystallites nucleated during BT on Ir substrate, different kinds of distribution of different sized crystallites are seen. It is, therefore, too early to draw a conclusion about the nucleation mechanism of hetero-epitaxy on Ir(001). In conclusion, the effect of BT in DC-plasma with planardiode system on Ir(001)/MgO(001) substrates has been studied in-situ and ex-situ by XPD together with XPS and LEED. The features of XPD patterns of C 1s core levels are in good agreement with those of homoepitaxial diamond (001) and those of three-electrode BT, although degrees of anisotropy of C 1s XPD are smaller. Thicknesses of carbon films estimated from intensity ratios of C 1s/Ir 4d5/2 (or 4f) XPS peaks are about 2 times larger than those of three-electrode BT. On the other hand, LEED patterns show no diffraction spots, implying either the size of diamond crystallites to be smaller than the LEED coherence length of ∼ 10 nm or the presence of non-oriented carbon layers over diamond crystallites. From comparison to similar cases of BT where diamond crystallites are not covered by non-oriented carbon layers, we conclude that epitaxial diamond crystallites with the size of a few nanometers or so are embedded in a non-oriented carbon layer. In the cases where no anisotropy of C 1s XPD was found, no epitaxial diamond grains were grown after post-CVD. In the cases where a finite anisotropy of C 1s XDP is found, the shapes of epitaxial diamond grains grown during a post-CVD seem to depend on the anisotropy, thus it is considered that the anisotropy of C 1s XPD can be a useful measure of diamond nucleation by BT on Ir (001) substrates. Acknowledgements This work is supported in part by the Advanced Diamond Device Project administered by NEDO, Japan and by a Grant-inAid for Young Scientists (B) (No. 17710089) from the Ministry of Education, Culture, Sports, Science and Technology, Japan and by CREST of Japan Science and Technology Corporation. References [1] K. Ohtsuka, K. Suzuki, A. Sawabe, T. Inuzuka, Jpn. J. Appl. Phys. 35 (1996) L1072. [2] S. Yugo, T. Kanai, T. Kimura, T. Muto, Appl. Phys. Lett. 58 (1991) 1036. [3] T. Fujisaki, M. Tachiki, N. Taniyama, M. Kudo, H. Kawarada, Diamond Relat. Mater. 12 (2003) 246. [4] M. Schreck, Th. Bauer, S. Gsell, F. Hörmann, H. Bielefeldt, B. Stritzker, Diamond Relat. Mater. 12 (2003) 262. [5] S. Kono, T. Takano, T. Goto, Y. Ikejima, M. Shiraishi, T. Abukawa, T. Yamada, A. Sawabe, Diamond Relat. Mater. 13 (2004) 2081. [6] C. Bednarski, Z. Dai, A.-P. Li, B. Golding, Diamond Relat. Mater. 12 (2003) 241. [7] B. Golding, C. Bednarski-Meinke, Z. Dai, Diamond Relat. Mater. 13 (2004) 545. [8] S. Kono, M. Shiraishi, N.I. Plusnin, T. Goto, Y. Ikejima, T. Abukawa, M. Shimomura, Z. Dai, C. Bednarski-Meinke, B. Golding, New Diam. Front. Carbon Technol. 15 (2005) 363.
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