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High rate homoepitaxial growth of diamond by microwave plasma CVD with nitrogen addition
Diamond & Related Materials 15 (2006) 455 – 459 www.elsevier.com/locate/diamond
High rate homoepitaxial growth of diamond by microwave plasma CVD with nitrogen addition Y. Mokuno ⁎, A. Chayahara, Y. Soda, H. Yamada, Y. Horino, N. Fujimori Diamond Research Center, National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan Available online 24 January 2006
Abstract Recent progress on high rate homoepitaxial growth of diamond by microwave plasma CVD with nitrogen addition is presented. Effects of process parameters, such as the holder diameter and the reactor pressure, on the growth rate is summarized. Nitrogen incorporation in the high rate grown films measured by secondary ion mass spectrometry (SIMS) was 3.9–39 ppm, which is comparable to that of HPHT Ib diamond. As a possible way to grow large diamond, thick diamond growth by the repetition of high rate growth and three-dimensional enlargement in crystal size by the growth on the side {100} surface is introduced. The repetition of high rate growth has been successfully applied to the growth of thick diamond as large as 10 mm. The crystallinity of a 2.1 mm thick diamond characterized by rocking curve of high resolution X-ray diffraction was 10 arcsec, which is also comparable to that of HPHT Ib diamond. © 2005 Elsevier B.V. All rights reserved. Keywords: High rate growth; Microwave plasma CVD; Homoepitaxial growth
1. Introduction Growth of large diamond is one of the key issues for diamond as a semiconductor material because a wafer like other semiconducting materials will be necessary for device fabrication. One of a possible approach is heteroepitaxial growth of diamond. If this approach is well working, a large size of diamond can be easily produced on existing wafers of other materials. However, further improvements in the crystallinity are required for high quality homoepitaxial growth, even for high quality heteroepitaxial diamond [1] grown on iridium layer using bias-enhanced nucleation procedure [2]. The other approach is to grow a bulk diamond by high-pressure/hightemperature synthesis (HPHT) method. However, it has also difficulty in growing single crystal larger than 10 mm (7–8 ct) due to the production cost. Microwave plasma chemical vapor deposition (CVD) has become potential candidate for the production of a large single crystal diamond, since total mass deposition rate increases by rapid development of high power systems and reaches 60 mg/ ⁎ Corresponding author. Tel.: +81 72 751 9531; fax: +81 72 751 9631. E-mail address: [email protected] (Y. Mokuno). 0925-9635/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.11.046
h for ASTeX type 5 kW reactor which is comparable to or exceeds that of HPHT growth. Therefore development of high rate growth technique has been expected to grow large diamond within a reasonable time. Recently, researchers from Carnegie Institution have succeeded in high rate growth of diamond with the growth rate of 50–150 μm/h [3]. This was achieved by modifying an microwave plasma CVD reactor to sustain a stable and energetic plasma and adding small amount of nitrogen which is known to promote (100) surface and enhance the growth rate. Since this report, making large diamond by microwave plasma CVD has become a possible alternative for HPHT growth. Recently, we have also succeeded in high rate growth using specially designed substrate holders to generate highdensity plasma and nitrogen addition [4], and have successfully grown a 10 mm thick diamond with the net weight of 4.65 ct by repetition of high rate growth under the optimized growth condition [5]. In addition, a method to enlarge the crystal size three-dimensionally has been also proposed and demonstrated [5]. This paper summarizes our recent progress on high rate homoepitaxial growth of diamond and the crystalline qualities such as nitrogen incorporation and crystallinity are investigated.
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High rate growth of diamond was performed using a 2.45 GHz, 5 kW microwave plasma CVD reactor (Seki Technotron Corp. AX-5250). In order to obtain high growth rate by enhancing the plasma density, the substrate holder was replaced by two types of specially designed one which are made of molybdenum [4,5]. Both of the holders have a conical shape with a flat top and the top surfaces are designed to have a smaller diameter than the ASTeX's original one. These holders have a small difference in the way of supporting a substrate. The “enclosed” type holder supports a substrate inside a drilled hole of the Mo rod in which the surface of the substrate is beneath the top surface of the substrate holder. The “open” type holder supports a substrate on the Mo rod. In this case, the surface of the substrate is above the top surface of the substrate holder. The growth temperature was measured by an optical pyrometer (Chino IR-U) and monitored by a 2-color infrared radiation thermometer (Chino IR-C) through viewing ports of the reactor. The thermometer is used only for monitoring purpose due to the strong optical emission from the plasma during the deposition. The growth temperature was controlled by input microwave power ranging from 1 to 3.7 kW. Seed crystal was a (100) oriented, type Ib HPHT synthetic single crystal diamond. Substrate was cleaned with isopropanol in ultrasonic bath and put into the reactor with the substrate holder. After the substrate temperature reached growth temperature under the H2 plasma with the H2 flow rate of 500 sccm, 1.8 sccm N2 was added to the flow. The substrate was immersed in the plasma for 30 min for etching purpose. The typical etching depth by this process is estimated to be 1.4 μm. After the etching, crystal growth was started by the flow of methane. The process gas was high purity hydrogen (6 N), methane (6 N) and nitrogen (4 N). The pressure of the reactor during the etching and the growth was kept at 17–29 kPa and the gas flow rate was 500 sccm for H2, 60 sccm for CH4 and 0.6–1.8 sccm for N2. The growth rate was estimated from the net weight gain measured by analytical balance with the minimum readability of 0.01 mg or directly measured by a micrometer gauge. The crystallinity was characterized by the diamond (400) rocking curve of high resolution X-ray diffraction (XRD) using a
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monochromized X-ray source of Cu-Kα1 with a Ge (440) fourcrystal monochromator. The depth profile of nitrogen concentration in the high rate grown diamond films was measured by high mass resolution secondary ion mass spectrometry (SIMS) using a 14.5 keV Cs+ primary beam. 3. Results and discussion 3.1. High rate growth with nitrogen addition High growth rate (N 100 μm/h) has been achieved by enhancing the plasma density using specially designed substrate holder and adding small amount of nitrogen in the gas phase [4]. In the paper, the effect of nitrogen addition on the growth rate at the constant growth temperature (1155 °C for open type holder and 1080 °C for enclosed type holder) has been reported. Growth rate enhancement by addition of nitrogen is about a factor of 2 and the growth rate increases with increasing N2 flow rate, that is, N/C ratio in the gas phase. The growth rate is much higher for open type holder and exceeds 100 μm/h, and the growth rate for the enclosed type holder is about half of the open type holder.
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Fig. 3. Effect of growth temperature on the growth rate for open type holder. Optical microscope image shows CVD diamond with the thickness of about 100 μm grown on a 3 × 3 × 0.5 mm3 HPHT Ib substrate. Growth temperature corresponding to each image is indicated in the plot.
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Depth (μm) Fig. 4. Nitrogen concentration in high rate grown diamond films with the nitrogen flow of 0.6, 1.2 and 1.8 sccm measured by SIMS depth profiling.
Fig. 1 shows influence of holder diameter of enclosed type holder on the growth rate. The growth temperature was 1130 °C and the reactor pressure was 24 kPa with the nitrogen flow rate of 0.6 sccm. The growth rate linearly increases with decreasing holder diameter which is defined as a diameter of the top surface of the substrate holder. However, the average input microwave power during the growth, which was required to maintain constant growth temperature, was linearly decreased from 3.6 to 1.2 kW with decreasing holder diameter. These results suggest that the effect of plasma concentration using a holder of smaller diameter is enhancing the growth rate. This supports recent observation in high-quality diamond growth in which an increase of plasma or power density achieved by high microwave power [6] or increasing the reactor pressure [7] leads to an increase in growth rate due to an increase of atomic hydrogen and methyl radicals concentration. Though the roll of nitrogen in the gas phase is not clear at this stage, the shape of the substrate holder is one of the important factors to achieve high rate growth. The growth rate can be further increased by increasing the reactor pressure both for the open and the enclosed type holders as shown in Fig. 2. A maximum growth rate of about 150 μm/h has been obtained at the reactor pressure of 30 kPa. Using the open type holder, temperature dependence of growth rate was investigated. Diamond films were grown for about 1 h at the growth temperature range of 1060 to 1250 °C. Reactor pressure was 21 kPa and the flow rate of nitrogen was 1.8 sccm. As shown in Fig. 3, high growth rate was obtained at the measured temperature range. It seems there is no temperature dependence on the growth rate at 1100–1200 °C. The color of grown diamond films grown at higher than 1100 °C reflects the color of Ib seed crystal indicating that the films are transparent. However, the color becomes dark for the film grown at 1060 °C. This may be attributed to incorporation of higher amount of nitrogen or increasing the number of defects at the lower temperature region with high N/C ratio in the gas phase. Nitrogen concentration in the high rate grown films with nitrogen addition is an interesting subject to understand the high rate growth process or the effects of nitrogen on optical or electrical properties. However, total amount of nitrogen in the
Fig. 5. A 2.1 mm thick diamond grown by repetition of high rate growth. This diamond was grown by 8 times repetition of high rate growth.
high rate grown diamond has not been directly measured. To get the information of nitrogen concentration, SIMS measurements were performed. Films were grown using enclosed type holder at typical growth conditions. The growth temperature of the film was 1130 °C and the reactor pressure was 24 kPa. The flow rate of nitrogen was 0.6, 1.2 and 1.8 sccm respectively. The thickness of these films was 220–290 μm. In order to get higher sensitivity by separating the signal of 12C14N from other signals such as 13C2 or 12C2H2, high mass resolution measurements were performed. The depth profiles of nitrogen concentration in Fig. 4 show that the nitrogen is homogeneously distributed in the depth direction except the surface region. The amount of nitrogen incorporated in the films increases with increasing nitrogen flow rate. The nitrogen concentrations obtained from the plateau are 8.9–39 ppm, which are comparable to the typical value of the HPHT synthetic Ib diamond. The incorporation probability is calculated to be 3–7 × 10− 4 which is the same order as the measured value for the diamond with conventional growth rate [8]. 3.2. Growth of thick diamond by repetition of high rate growth Growth of thick diamond should be the next step for high rate growth. Generally, longer growth time result in thick diamond, however, it is difficult to prevent deposition of polycrystalline diamond on the substrate holder, because similar growth condition for diamond growth is partially satisfied at the
HTHP Ib substrate 2.1 mm thick CVD diamond
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Δω (deg) Fig. 6. Rocking curve of diamond (400) high resolution XRD for HPHT synthetic Ib diamond (FWHM = 7 arcsec) and thick diamond of Fig. 5 (FWHM = 10 arcsec).
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Fig. 7. Large CVD diamonds grown by repetition of high rate growth. (a) A 10 mm, 4.7 ct CVD diamond, (b) a 9.6 mm, 3.5 ct CVD diamond, (c) an 8.7 mm, 4.4 ct CVD diamond.
holder. As the amount of the polycrystalline diamonds increases, these sometimes peeled off the holder or overheated, and incorporated into the grown films or disturb the growth temperature measurement. To avoid this, it is necessary to interrupt the growth within a limited time and clean the substrate holder. Therefore, repetition of growth is inevitable to grow thick diamond by high rate growth. It was found that smooth and flat surface morphology provided by the use of enclosed type holder is beneficial for the repetition of high rate growth [3]. This indicates control of edge growth is important for the stable growth. Detail analysis by numerical simulation is under way to understand the effects of the process parameters on the improved macroscale surface morphology [9]. Fig. 5 shows optical microscope images of as-grown diamond by repetition of high rate growth using the enclosed type holder. The film thickness is 2.1 mm. The growth process was repeated using as-grown surfaces, because the use of asgrown surface eliminates polishing process which may introduce damaged layer on the surface. As the thickness of diamond increased, the substrate holder was modified to have deeper hole. Each growth was interrupted so that the surface
of grown diamond is below the top surface of the substrate holder in order to keep flat surface morphology. The growth temperature was 1130 °C and the reactor pressure was 24 kPa. The flow rate of nitrogen was 0.6 sccm. The surface morphology is much improved and smooth and flat surface morphology without growth hillocks or nonepitaxial crystallites has been obtained even after 8 times repetition of high rate growth. Fig. 6 shows a high resolution XRD rocking curve of the diamond film. The FWHM was 10 arcsec, while the FWHM for HPHT Ib diamond using the same apparatus was 7 arcsec. This value is comparable to the typical value of HPHT synthetic Ib diamond (6–20 arcsec) [10], indicating the crystallinity of our thick diamond is comparable to the HPHT Ib diamond extensively used for the growth of a high quality homoepitaxial layer. The repetition of high rate growth using enclosed type holder was further used to grow very thick diamonds. As a result, as shown in Fig. 7, large diamonds with the thickness of 8.7–10 mm and the weight of 3.5–4.65 ct has been successfully grown on a 27–37 mm2 seed after 24–31 times growth. The growth temperature was 1130 °C and the reactor pressure was 21 or 24 kPa. The flow rate of nitrogen was 0.6 sccm. The average growth rate was 52–68 μm/h. Though the side surface is covered with black polycrystalline diamond, the top surface maintains the smooth and flat surface morphology. Further evaluation on the crystallinity is in progress. 3.3. Three-dimensional enlargement of CVD diamond Due to the difficulty in three-dimensional enlargement of the crystal size using the growth condition mentioned above, a method of enlarging crystal size by growing on side {100} surface has been developed [5]. The concept of the enlarging process is schematically shown in Fig. 8. In step 1, a thick diamond is grown by repetition of high rate growth. In the next step 2, at least one of the side {100} surfaces normal to the previously grown surface is cut and polished. This surface
Step1 Thick CVD layers grown on (100) seed by repetition of high rate growth
CVD layers
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quality diamond by this method, further improvements in crystallinity of the grown diamond on the side {100} surface will be important. 4. Summary
Fig. 9. Enlargement of crystal size by growing one of the side {100} surface of cut and polished diamond surrounded by six {100} surfaces. The diamond with the thickness of 1 mm was grown by 3 times repetition of high rate growth.
is used as a substrate for the next growth. Finally in step 3, step 2 is repeated. By repeating this procedure for many times, three-dimensional enlargement of crystal size will be possible. By integrating the thick diamond growth and this method, diamond with the size of larger than 10 × 10 mm2 which is difficult to produce by HPHT growth may be produced. Fig. 9 shows an example of this method. A thick diamond was prepared by 9 times repetition of high rate growth on 3 × 3 × 0.5 mm3 seed using an enclosed type holder. The growth temperature was 1130 °C and the reactor pressure was 21 or 24 kPa. The flow rate of nitrogen was 0.6 sccm. It was cut and polished into cubic shape surrounded by six {100} surfaces (left part of Fig. 9). Though the side surface of as-grown crystal was covered with polycrystalline diamond like the crystals shown in Fig. 7, the inside is a single crystal diamond which has some degree of optical transparency. The HPHT Ib seed can be clearly identified by the difference in color, and in CVD layers, fluctuation in optical transparency and interface between each growth run was clearly observed. These are probably due to the fluctuation of growth condition such as substrate temperature and gas composition. After that, diamond with the thickness of 1 mm has been successfully grown on one of the side {100} surfaces by 3 times repetition of high rate growth using the same condition (right part of Fig. 9). The grown diamond is confirmed to be a single crystal diamond by XRD measurement. The FWHM of XRD rocking curve after 9th growth with the thickness of 2.6 mm was 37 arcsec. In order to obtain high
The holder diameter and the reactor pressure have strong effects on the growth rate as well as N/C ratio in the gas phase. The color of diamond becomes dark at the temperature of 1060 °C. Nitrogen incorporation in the high rate grown films at typical growth condition was comparable to that of HPHT Ib diamond and the incorporation ratio was comparable to the previously reported value for the films grown at conventional growth rate. A possible way to grow large diamond; thick diamond growth by repetition of high rate growth and three-dimensional enlargement in crystal size by growth on side {100} surface, was introduced. The repetition of high rate growth was successfully applied to growth of thick diamond and the crystallinity of the diamond characterized by rocking curve of X-ray diffraction (XRD) is comparable to that of HPHT Ib diamond. This technique was used to grow very thick diamond with the thickness of 8.7–10 mm. Though the crystallinity of the diamond grown on the side {100} surface requires further improvements, the combination of these two methods will be promising for producing large diamond which exceeds the crystal size of 10 × 10 mm2. References [1] M. Schreck, H. Roll, B. Stritzker, Appl. Phys. Lett. 74 (1999) 650. [2] K. Ohtsuka, H. Fukuda, K. Suzuki, A. Sawabe, Jpn. J. Appl. Phys. 36 (1997) 1214. [3] C.S. Yan, Y.K. Vohra, H.K. Mao, R.J. Hemley, Proc. Natl. Acad. Sci. U. S. A. 99 (2002) 12523. [4] A. Chayahara, Y. Mokuno, Y. Horino, Y. Takasu, H. Kato, N. Fujimori, Diamond Relat. Mater. 13 (2004) 1954. [5] Y. Mokuno, A. Chayahara, Y. Soda, Y. Horino, N. Fujimori, Diamond Relat. Mater. 14 (2005) 1743. [6] T. Teraji, T. Ito, J. Cryst. Growth 271 (2004) 409. [7] J. Achard, A. Tallaire, R. Sussmann, F. Silva, A. Gicquel, J. Cryst. Growth 284 (2005) 396. [8] R. Samlenski, C. Haug, R. Brenn, C. Wild, R. Locher, P. Koidl, Appl. Phys. Lett. 67 (1995) 2798. [9] H. Yamada, A. Chayahara, Y. Mokuno, Y. Horino, S. Shikata, Diamond Relat. Mater. (in press). [10] H. Sumiya, N. Toda, Y. Nishibayashi, S. Satoh, J. Cryst. Growth 178 (1997) 485.
High rate homoepitaxial growth of diamond by microwave plasma CVD with nitrogen addition Y. Mokuno ⁎, A. Chayahara, Y. Soda, H. Yamada, Y. Horino, N. Fujimori Diamond Research Center, National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan Available online 24 January 2006
Abstract Recent progress on high rate homoepitaxial growth of diamond by microwave plasma CVD with nitrogen addition is presented. Effects of process parameters, such as the holder diameter and the reactor pressure, on the growth rate is summarized. Nitrogen incorporation in the high rate grown films measured by secondary ion mass spectrometry (SIMS) was 3.9–39 ppm, which is comparable to that of HPHT Ib diamond. As a possible way to grow large diamond, thick diamond growth by the repetition of high rate growth and three-dimensional enlargement in crystal size by the growth on the side {100} surface is introduced. The repetition of high rate growth has been successfully applied to the growth of thick diamond as large as 10 mm. The crystallinity of a 2.1 mm thick diamond characterized by rocking curve of high resolution X-ray diffraction was 10 arcsec, which is also comparable to that of HPHT Ib diamond. © 2005 Elsevier B.V. All rights reserved. Keywords: High rate growth; Microwave plasma CVD; Homoepitaxial growth
1. Introduction Growth of large diamond is one of the key issues for diamond as a semiconductor material because a wafer like other semiconducting materials will be necessary for device fabrication. One of a possible approach is heteroepitaxial growth of diamond. If this approach is well working, a large size of diamond can be easily produced on existing wafers of other materials. However, further improvements in the crystallinity are required for high quality homoepitaxial growth, even for high quality heteroepitaxial diamond [1] grown on iridium layer using bias-enhanced nucleation procedure [2]. The other approach is to grow a bulk diamond by high-pressure/hightemperature synthesis (HPHT) method. However, it has also difficulty in growing single crystal larger than 10 mm (7–8 ct) due to the production cost. Microwave plasma chemical vapor deposition (CVD) has become potential candidate for the production of a large single crystal diamond, since total mass deposition rate increases by rapid development of high power systems and reaches 60 mg/ ⁎ Corresponding author. Tel.: +81 72 751 9531; fax: +81 72 751 9631. E-mail address: [email protected] (Y. Mokuno). 0925-9635/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.11.046
h for ASTeX type 5 kW reactor which is comparable to or exceeds that of HPHT growth. Therefore development of high rate growth technique has been expected to grow large diamond within a reasonable time. Recently, researchers from Carnegie Institution have succeeded in high rate growth of diamond with the growth rate of 50–150 μm/h [3]. This was achieved by modifying an microwave plasma CVD reactor to sustain a stable and energetic plasma and adding small amount of nitrogen which is known to promote (100) surface and enhance the growth rate. Since this report, making large diamond by microwave plasma CVD has become a possible alternative for HPHT growth. Recently, we have also succeeded in high rate growth using specially designed substrate holders to generate highdensity plasma and nitrogen addition [4], and have successfully grown a 10 mm thick diamond with the net weight of 4.65 ct by repetition of high rate growth under the optimized growth condition [5]. In addition, a method to enlarge the crystal size three-dimensionally has been also proposed and demonstrated [5]. This paper summarizes our recent progress on high rate homoepitaxial growth of diamond and the crystalline qualities such as nitrogen incorporation and crystallinity are investigated.
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High rate growth of diamond was performed using a 2.45 GHz, 5 kW microwave plasma CVD reactor (Seki Technotron Corp. AX-5250). In order to obtain high growth rate by enhancing the plasma density, the substrate holder was replaced by two types of specially designed one which are made of molybdenum [4,5]. Both of the holders have a conical shape with a flat top and the top surfaces are designed to have a smaller diameter than the ASTeX's original one. These holders have a small difference in the way of supporting a substrate. The “enclosed” type holder supports a substrate inside a drilled hole of the Mo rod in which the surface of the substrate is beneath the top surface of the substrate holder. The “open” type holder supports a substrate on the Mo rod. In this case, the surface of the substrate is above the top surface of the substrate holder. The growth temperature was measured by an optical pyrometer (Chino IR-U) and monitored by a 2-color infrared radiation thermometer (Chino IR-C) through viewing ports of the reactor. The thermometer is used only for monitoring purpose due to the strong optical emission from the plasma during the deposition. The growth temperature was controlled by input microwave power ranging from 1 to 3.7 kW. Seed crystal was a (100) oriented, type Ib HPHT synthetic single crystal diamond. Substrate was cleaned with isopropanol in ultrasonic bath and put into the reactor with the substrate holder. After the substrate temperature reached growth temperature under the H2 plasma with the H2 flow rate of 500 sccm, 1.8 sccm N2 was added to the flow. The substrate was immersed in the plasma for 30 min for etching purpose. The typical etching depth by this process is estimated to be 1.4 μm. After the etching, crystal growth was started by the flow of methane. The process gas was high purity hydrogen (6 N), methane (6 N) and nitrogen (4 N). The pressure of the reactor during the etching and the growth was kept at 17–29 kPa and the gas flow rate was 500 sccm for H2, 60 sccm for CH4 and 0.6–1.8 sccm for N2. The growth rate was estimated from the net weight gain measured by analytical balance with the minimum readability of 0.01 mg or directly measured by a micrometer gauge. The crystallinity was characterized by the diamond (400) rocking curve of high resolution X-ray diffraction (XRD) using a
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monochromized X-ray source of Cu-Kα1 with a Ge (440) fourcrystal monochromator. The depth profile of nitrogen concentration in the high rate grown diamond films was measured by high mass resolution secondary ion mass spectrometry (SIMS) using a 14.5 keV Cs+ primary beam. 3. Results and discussion 3.1. High rate growth with nitrogen addition High growth rate (N 100 μm/h) has been achieved by enhancing the plasma density using specially designed substrate holder and adding small amount of nitrogen in the gas phase [4]. In the paper, the effect of nitrogen addition on the growth rate at the constant growth temperature (1155 °C for open type holder and 1080 °C for enclosed type holder) has been reported. Growth rate enhancement by addition of nitrogen is about a factor of 2 and the growth rate increases with increasing N2 flow rate, that is, N/C ratio in the gas phase. The growth rate is much higher for open type holder and exceeds 100 μm/h, and the growth rate for the enclosed type holder is about half of the open type holder.
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Fig. 3. Effect of growth temperature on the growth rate for open type holder. Optical microscope image shows CVD diamond with the thickness of about 100 μm grown on a 3 × 3 × 0.5 mm3 HPHT Ib substrate. Growth temperature corresponding to each image is indicated in the plot.
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Depth (μm) Fig. 4. Nitrogen concentration in high rate grown diamond films with the nitrogen flow of 0.6, 1.2 and 1.8 sccm measured by SIMS depth profiling.
Fig. 1 shows influence of holder diameter of enclosed type holder on the growth rate. The growth temperature was 1130 °C and the reactor pressure was 24 kPa with the nitrogen flow rate of 0.6 sccm. The growth rate linearly increases with decreasing holder diameter which is defined as a diameter of the top surface of the substrate holder. However, the average input microwave power during the growth, which was required to maintain constant growth temperature, was linearly decreased from 3.6 to 1.2 kW with decreasing holder diameter. These results suggest that the effect of plasma concentration using a holder of smaller diameter is enhancing the growth rate. This supports recent observation in high-quality diamond growth in which an increase of plasma or power density achieved by high microwave power [6] or increasing the reactor pressure [7] leads to an increase in growth rate due to an increase of atomic hydrogen and methyl radicals concentration. Though the roll of nitrogen in the gas phase is not clear at this stage, the shape of the substrate holder is one of the important factors to achieve high rate growth. The growth rate can be further increased by increasing the reactor pressure both for the open and the enclosed type holders as shown in Fig. 2. A maximum growth rate of about 150 μm/h has been obtained at the reactor pressure of 30 kPa. Using the open type holder, temperature dependence of growth rate was investigated. Diamond films were grown for about 1 h at the growth temperature range of 1060 to 1250 °C. Reactor pressure was 21 kPa and the flow rate of nitrogen was 1.8 sccm. As shown in Fig. 3, high growth rate was obtained at the measured temperature range. It seems there is no temperature dependence on the growth rate at 1100–1200 °C. The color of grown diamond films grown at higher than 1100 °C reflects the color of Ib seed crystal indicating that the films are transparent. However, the color becomes dark for the film grown at 1060 °C. This may be attributed to incorporation of higher amount of nitrogen or increasing the number of defects at the lower temperature region with high N/C ratio in the gas phase. Nitrogen concentration in the high rate grown films with nitrogen addition is an interesting subject to understand the high rate growth process or the effects of nitrogen on optical or electrical properties. However, total amount of nitrogen in the
Fig. 5. A 2.1 mm thick diamond grown by repetition of high rate growth. This diamond was grown by 8 times repetition of high rate growth.
high rate grown diamond has not been directly measured. To get the information of nitrogen concentration, SIMS measurements were performed. Films were grown using enclosed type holder at typical growth conditions. The growth temperature of the film was 1130 °C and the reactor pressure was 24 kPa. The flow rate of nitrogen was 0.6, 1.2 and 1.8 sccm respectively. The thickness of these films was 220–290 μm. In order to get higher sensitivity by separating the signal of 12C14N from other signals such as 13C2 or 12C2H2, high mass resolution measurements were performed. The depth profiles of nitrogen concentration in Fig. 4 show that the nitrogen is homogeneously distributed in the depth direction except the surface region. The amount of nitrogen incorporated in the films increases with increasing nitrogen flow rate. The nitrogen concentrations obtained from the plateau are 8.9–39 ppm, which are comparable to the typical value of the HPHT synthetic Ib diamond. The incorporation probability is calculated to be 3–7 × 10− 4 which is the same order as the measured value for the diamond with conventional growth rate [8]. 3.2. Growth of thick diamond by repetition of high rate growth Growth of thick diamond should be the next step for high rate growth. Generally, longer growth time result in thick diamond, however, it is difficult to prevent deposition of polycrystalline diamond on the substrate holder, because similar growth condition for diamond growth is partially satisfied at the
HTHP Ib substrate 2.1 mm thick CVD diamond
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Δω (deg) Fig. 6. Rocking curve of diamond (400) high resolution XRD for HPHT synthetic Ib diamond (FWHM = 7 arcsec) and thick diamond of Fig. 5 (FWHM = 10 arcsec).
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Fig. 7. Large CVD diamonds grown by repetition of high rate growth. (a) A 10 mm, 4.7 ct CVD diamond, (b) a 9.6 mm, 3.5 ct CVD diamond, (c) an 8.7 mm, 4.4 ct CVD diamond.
holder. As the amount of the polycrystalline diamonds increases, these sometimes peeled off the holder or overheated, and incorporated into the grown films or disturb the growth temperature measurement. To avoid this, it is necessary to interrupt the growth within a limited time and clean the substrate holder. Therefore, repetition of growth is inevitable to grow thick diamond by high rate growth. It was found that smooth and flat surface morphology provided by the use of enclosed type holder is beneficial for the repetition of high rate growth [3]. This indicates control of edge growth is important for the stable growth. Detail analysis by numerical simulation is under way to understand the effects of the process parameters on the improved macroscale surface morphology [9]. Fig. 5 shows optical microscope images of as-grown diamond by repetition of high rate growth using the enclosed type holder. The film thickness is 2.1 mm. The growth process was repeated using as-grown surfaces, because the use of asgrown surface eliminates polishing process which may introduce damaged layer on the surface. As the thickness of diamond increased, the substrate holder was modified to have deeper hole. Each growth was interrupted so that the surface
of grown diamond is below the top surface of the substrate holder in order to keep flat surface morphology. The growth temperature was 1130 °C and the reactor pressure was 24 kPa. The flow rate of nitrogen was 0.6 sccm. The surface morphology is much improved and smooth and flat surface morphology without growth hillocks or nonepitaxial crystallites has been obtained even after 8 times repetition of high rate growth. Fig. 6 shows a high resolution XRD rocking curve of the diamond film. The FWHM was 10 arcsec, while the FWHM for HPHT Ib diamond using the same apparatus was 7 arcsec. This value is comparable to the typical value of HPHT synthetic Ib diamond (6–20 arcsec) [10], indicating the crystallinity of our thick diamond is comparable to the HPHT Ib diamond extensively used for the growth of a high quality homoepitaxial layer. The repetition of high rate growth using enclosed type holder was further used to grow very thick diamonds. As a result, as shown in Fig. 7, large diamonds with the thickness of 8.7–10 mm and the weight of 3.5–4.65 ct has been successfully grown on a 27–37 mm2 seed after 24–31 times growth. The growth temperature was 1130 °C and the reactor pressure was 21 or 24 kPa. The flow rate of nitrogen was 0.6 sccm. The average growth rate was 52–68 μm/h. Though the side surface is covered with black polycrystalline diamond, the top surface maintains the smooth and flat surface morphology. Further evaluation on the crystallinity is in progress. 3.3. Three-dimensional enlargement of CVD diamond Due to the difficulty in three-dimensional enlargement of the crystal size using the growth condition mentioned above, a method of enlarging crystal size by growing on side {100} surface has been developed [5]. The concept of the enlarging process is schematically shown in Fig. 8. In step 1, a thick diamond is grown by repetition of high rate growth. In the next step 2, at least one of the side {100} surfaces normal to the previously grown surface is cut and polished. This surface
Step1 Thick CVD layers grown on (100) seed by repetition of high rate growth
CVD layers
Step 2 CVD growth on the side {100} surface
Seed
Step 3 Repeat of step 2
Seed
Seed Fig. 8. Concept of three-dimensional enlarging process of crystal size by growing on side {100} surface.
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quality diamond by this method, further improvements in crystallinity of the grown diamond on the side {100} surface will be important. 4. Summary
Fig. 9. Enlargement of crystal size by growing one of the side {100} surface of cut and polished diamond surrounded by six {100} surfaces. The diamond with the thickness of 1 mm was grown by 3 times repetition of high rate growth.
is used as a substrate for the next growth. Finally in step 3, step 2 is repeated. By repeating this procedure for many times, three-dimensional enlargement of crystal size will be possible. By integrating the thick diamond growth and this method, diamond with the size of larger than 10 × 10 mm2 which is difficult to produce by HPHT growth may be produced. Fig. 9 shows an example of this method. A thick diamond was prepared by 9 times repetition of high rate growth on 3 × 3 × 0.5 mm3 seed using an enclosed type holder. The growth temperature was 1130 °C and the reactor pressure was 21 or 24 kPa. The flow rate of nitrogen was 0.6 sccm. It was cut and polished into cubic shape surrounded by six {100} surfaces (left part of Fig. 9). Though the side surface of as-grown crystal was covered with polycrystalline diamond like the crystals shown in Fig. 7, the inside is a single crystal diamond which has some degree of optical transparency. The HPHT Ib seed can be clearly identified by the difference in color, and in CVD layers, fluctuation in optical transparency and interface between each growth run was clearly observed. These are probably due to the fluctuation of growth condition such as substrate temperature and gas composition. After that, diamond with the thickness of 1 mm has been successfully grown on one of the side {100} surfaces by 3 times repetition of high rate growth using the same condition (right part of Fig. 9). The grown diamond is confirmed to be a single crystal diamond by XRD measurement. The FWHM of XRD rocking curve after 9th growth with the thickness of 2.6 mm was 37 arcsec. In order to obtain high
The holder diameter and the reactor pressure have strong effects on the growth rate as well as N/C ratio in the gas phase. The color of diamond becomes dark at the temperature of 1060 °C. Nitrogen incorporation in the high rate grown films at typical growth condition was comparable to that of HPHT Ib diamond and the incorporation ratio was comparable to the previously reported value for the films grown at conventional growth rate. A possible way to grow large diamond; thick diamond growth by repetition of high rate growth and three-dimensional enlargement in crystal size by growth on side {100} surface, was introduced. The repetition of high rate growth was successfully applied to growth of thick diamond and the crystallinity of the diamond characterized by rocking curve of X-ray diffraction (XRD) is comparable to that of HPHT Ib diamond. This technique was used to grow very thick diamond with the thickness of 8.7–10 mm. Though the crystallinity of the diamond grown on the side {100} surface requires further improvements, the combination of these two methods will be promising for producing large diamond which exceeds the crystal size of 10 × 10 mm2. References [1] M. Schreck, H. Roll, B. Stritzker, Appl. Phys. Lett. 74 (1999) 650. [2] K. Ohtsuka, H. Fukuda, K. Suzuki, A. Sawabe, Jpn. J. Appl. Phys. 36 (1997) 1214. [3] C.S. Yan, Y.K. Vohra, H.K. Mao, R.J. Hemley, Proc. Natl. Acad. Sci. U. S. A. 99 (2002) 12523. [4] A. Chayahara, Y. Mokuno, Y. Horino, Y. Takasu, H. Kato, N. Fujimori, Diamond Relat. Mater. 13 (2004) 1954. [5] Y. Mokuno, A. Chayahara, Y. Soda, Y. Horino, N. Fujimori, Diamond Relat. Mater. 14 (2005) 1743. [6] T. Teraji, T. Ito, J. Cryst. Growth 271 (2004) 409. [7] J. Achard, A. Tallaire, R. Sussmann, F. Silva, A. Gicquel, J. Cryst. Growth 284 (2005) 396. [8] R. Samlenski, C. Haug, R. Brenn, C. Wild, R. Locher, P. Koidl, Appl. Phys. Lett. 67 (1995) 2798. [9] H. Yamada, A. Chayahara, Y. Mokuno, Y. Horino, S. Shikata, Diamond Relat. Mater. (in press). [10] H. Sumiya, N. Toda, Y. Nishibayashi, S. Satoh, J. Cryst. Growth 178 (1997) 485.