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Hydrogen analysis of CVD homoepitaxial diamond films by high-resolution elastic recoil detection
Nuclear Instruments and Methods in Physics Research B 190 (2002) 689–692 www.elsevier.com/locate/nimb
Hydrogen analysis of CVD homoepitaxial diamond films by high-resolution elastic recoil detection K. Kimura b
a,*
, K. Nakajima a, S. Yamanaka b, M. Hasegawa b, H. Okushi
b
a Department of Engineering Physics and Mechanics, Kyoto University, Kyoto 606-8501, Japan National Institute of Advanced Industrial Science and Technology, 1-1-1, Umezono, Tsukuba, Ibaraki 305-0045, Japan
Abstract We have measured hydrogen depth profiles in chemical vapor deposition (CVD) diamond films by high-resolution elastic recoil detection. The depth resolution of nearly 0.2 nm is achieved by means of a high-resolution magnetic spectrometer. The hydrogen profile in the as-grown sample shows a sharp peak at the surface. The peak has a small tail toward larger depth, showing that some hydrogen atoms are incorporated in the subsurface region. There is no difference in the hydrogen depth profiles between the undoped and B-doped diamond films. After surface hydrogen is removed by an acid solution, the CVD diamond is rehydrogenated using a hydrogen plasma at 800 °C. The rehydrogenated sample shows almost the same hydrogen profile as the as-grown sample. The hydrogen profile hardly changes by annealing at 400 °C, though a small change in the subsurface region cannot be excluded. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 82.82.Yc; 81.05.Tp; 81.15.Gh Keywords: Hydrogen; CVD diamond; High-resolution ERD
1. Introduction Diamond is a promising material for future electronic devices. A number of studies have been conducted on the growth of diamond. Recent investigation has shown that high-quality diamond films with atomically flat surfaces can be prepared by microwave-plasma chemical vapor deposition (CVD) under ultra-low CH4 /H2 conditions [1]. Diamond films prepared by CVD have a p-type semiconducting layer in the surface region [2]. This
*
Corresponding author. Tel.: +81-75-753-5253; fax: +81-757535253/7717286. E-mail address: [email protected] (K. Kimura).
semiconducting layer is considered to be related to the surface hydrogen because the surface conductivity can be removed by oxidation with acid solutions or by annealing at temperatures higher than 400 °C, and it is recovered by rehydrogenation in a hydrogen plasma [2–6]. Nevertheless, the origin of the surface conductivity induced by hydrogenation is still in debate [4–10]. Information about the hydrogen concentration in the surface region is of prime importance to understand the mechanism of the surface conductivity. However, few measurements on the hydrogen depth profiles in the CVD diamond films have been reported because quantitative analysis of hydrogen is very difficult. Recently we have demonstrated that high-resolution elastic recoil detection
0168-583X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 2 ) 0 0 4 5 0 - 0
690
K. Kimura et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 689–692
(ERD) can be successfully applied to measure hydrogen depth profiles in CVD diamond films [11]. The observed hydrogen profile shows a sharp peak at the surface. The hydrogen coverage is estimated to be 1 0:3 ML (1 ML ¼ 1:57 1015 cm2 ), indicating the formation of a monohydride structure. The surface peak has a small tail toward larger depth, indicating that a small amount of hydrogen is incorporated in the subsurface region. In the present paper, we extend the study to CVD diamond films which received various treatments, such as rehydrogenation in a hydrogen plasma after oxidization, annealing at 400 °C, and rehydrogenation after ion irradiation in an attempt to see if hydrogen content or profile are changed by these treatments.
2. Experimental Details of the preparation of the CVD diamond films are described elsewhere [6]. Briefly, undoped diamond films were grown on (0 0 1) surfaces of synthetic Ib diamond (4 4 0:3 mm3 ) by microwave-plasma CVD at 800 °C with 0.025% CH4 diluted by H2 gas. B-doped diamond films were prepared in the same way, but using 0.1% CH4 and 0.15–0.2 ppm B(CH3 )3 , diluted by H2 gas. One of the B-doped sample was oxidized using H2 SO4 and HNO3 solutions at 200 °C for 15 min in order to remove the hydrogen. The sample was then rehydrogenated in a hydrogen plasma at 800 °C for 15 min (it is being referred to as the ‘‘rehydrogenated sample’’). Another B-doped sample was annealed at 400 °C for 45 min in a N2 atmosphere. This sample is being referred to as the ‘‘annealed sample’’. A beam of 500 keV Cþ ions from a Van de Graaff accelerator was collimated to 2 mm (vertical) 1 mm (horizontal) by a series of 4-jaw slits. The beam current was monitored using a vibrating beam chopper. A typical beam current was 3 nA. The Cþ beam was incident on the CVD diamond films at an angle of 70°. The energy spectrum of Hþ ions recoiled at 25° was measured with a highresolution magnetic spectrometer. The spectrometer consisted of a 90° sector magnet and a one-dimensional position-sensitive detector (PSD) of 100 mm length. A quadrupole lens was installed be-
tween the target and the magnet for the correction of kinematic broadening. An electrostatic deflector, placed between the magnet and the PSD, was used to reject scattered carbon ions. The acceptance angle of the spectrometer was 0.2 msr.
3. Results and discussion Fig. 1 shows examples of the observed Hþ spectra for the as-grown, undoped (closed circles), and B-doped (open circles) CVD diamond films. The conversion from the recoil energy to depth was done using stopping powers estimated with the TRIM code [12]. The obtained depth scale is shown on the upper abscissa. The observed spectra are essentially the same, showing that the hydrogen depth profile is not affected by the boron doping. The spectra have a sharp peak at the surface (at 117.4 keV). The peak width is about 0.9 keV, which corresponds to a depth resolution of 0.23 nm. The peak is slightly asymmetric having a small tail toward lower energies. This small tail can be ascribed to hydrogen atoms in the subsurface region [11]. Except for the peak region the yield is at the same level as the dark noise of the PSD, indicating that hydrogen concentration in
Fig. 1. Examples of the observed energy spectra of recoiled Hþ ions for as-grown undoped and B-doped CVD diamond films. The spectrum has a sharp peak at the surface. The depth resolution estimated from the peak width is 0.2 nm. There is no difference between the undoped and B-doped samples.
K. Kimura et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 689–692
Fig. 2. ERD spectra for the as-grown and rehydrogenated Bdoped samples. The rehydrogenation recovers the surface hydrogen completely.
the bulk region is smaller than the detection limit of the present measurement (0:05 at:% ¼ 9 1019 cm3 ). Fig. 2 shows the comparison of the ERD spectra for the as-grown and rehydrogenated samples. It is known that the surface conductivity is removed by oxidation with acid solutions, and the conductivity can be recovered by rehydrogenation. There is no difference between the asgrown and rehydrogenated samples, showing that surface hydrogen is completely recovered by the present rehydrogenation process. Fig. 3 shows the comparison between the asgrown and annealed samples. It is known that the surface conductivity disappears after annealing at temperatures higher than 400 °C, suggesting that the surface hydrogen is removed by the annealing. Surprisingly, the present result indicates that the hydrogen depth profile is not changed by the annealing at 400 °C. This suggests that the surface conductivity is not related to the surface hydrogen but related to the subsurface hydrogen. Further study on this subject is in progress. Finally, the effect of ion irradiation on the hydrogen profile has been investigated. The hydrogen is desorbed from the CVD diamond films during the ERD measurement probably due to desorption induced by electronic excitation [11].
691
Fig. 3. Comparison between the as-grown and annealed samples. The annealing was done at 400 °C in a N2 atmosphere. The hydrogen surface peak is not changed by the annealing.
Fig. 4. Observed ERD spectrum for the Cþ -irradiated CVD diamond film followed by rehydrogenation. The spectrum for an as-grown film is shown for comparison. The hydrogen concentration in the subsurface region is larger than in the asgrown sample possibly due to the hydrogen trapping by radiation damages.
After an ERD measurement with a total fluence of 4 1015 cm2 , 90% of the hydrogen atoms were desorbed. The irradiated sample was rehydrogenated in a hydrogen plasma at 800 °C for 15 min, and an ERD measurement was performed. The observed ERD spectrum is shown in Fig. 4
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K. Kimura et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 689–692
together with the spectrum for the as-grown sample. It is evident that the hydrogen concentration in the subsurface region is increased by this treatment. The hydrogen concentration is of the order of 1 at.% at a depth of 1 nm. This is attributed to hydrogen trapping by lattice defects created by the Cþ ion irradiation. Besides the change in the subsurface region, the peak becomes slightly broader. The peak broadening might be attributed to surface roughening caused by ion irradiation.
Acknowledgements We are grateful to the members of the Quantum Science and Engineering Center at Kyoto University for the use of the Van de Graaff accelerator. This work was supported in part by a Grant-in-Aid for Scientific Research from the ministry of Education, Culture, Sports, Science and Technology.
References
4. Conclusions It is demonstrated that hydrogen depth profiling in CVD diamond films can be performed by high-resolution ERD with a depth resolution of nearly 0.2 nm. There is no difference between the hydrogen depth profiles in undoped and B-doped CVD diamond films. The effects of different treatments on the hydrogen depth profile have been examined. It has been shown that rehydrogenation in a hydrogen plasma recovers the surface hydrogen almost completely. Annealing at 400 °C in a N2 atmosphere hardly changes the surface hydrogen peak. Rehydrogenation after irradiation of 500 keV Cþ ions at a fluence of 4 1015 cm2 leads to the incorporation of a substantial amount of hydrogen in the subsurface region, which may be due to hydrogen trapping caused by radiation damage.
[1] H. Watanabe, D. Takeuchi, S. Yamanaka, H. Okushi, K. Kajimura, T. Sekiguchi, Diam. Rel. Mater. 8 (1999) 1272. [2] M.I. Landstrass, K.V. Ravi, Appl. Phys. Lett. 55 (1989) 975. [3] S.A. Grot, G.Sh. Gildenblat, C.W. Hatfield, C.W. Wronski, A.R. Badzian, T. Badzian, R. Messier, IEEE Electron Device Lett. 11 (1990) 100. [4] Y. Mori, A. Hatta, T. Ito, A. Hiraki, Jpn. J. Appl. Phys. 31 (1992) L1718. [5] H. Kiyota, E. Matsushima, K. Sato, H. Okushi, T. Ando, M. Kamo, Y. Sato, M. Iida, Appl. Phys. Lett. 67 (1995) 3596. [6] K. Hayashi, S. Yamanaka, H. Okushi, K. Kajimura, Appl. Phys. Lett. 68 (1996) 376. [7] H. Kawarada, H. Sasaki, A. Sato, Phys. Rev. B 52 (1995) 11351. [8] S. Ri, T. Mizumasa, Y. Akiba, Y. Hirose, T. Kurosu, M. Iida, Jpn. J. Appl. Phys. 34 (1995) 5550. [9] J. Shirafuji, T. Sugino, Diam. Rel. Mater. 5 (1996) 706. [10] F. Maier, M. Riedel, B. Mantel, J. Ristein, L. Ley, Phys. Rev. Lett. 85 (2000) 3472. [11] K. Kimura, K. Nakajima, S. Yamanaka, M. Hasegawa, H. Okushi, Appl. Phys. Lett. 78 (2001) 1679. [12] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1985.
Hydrogen analysis of CVD homoepitaxial diamond films by high-resolution elastic recoil detection K. Kimura b
a,*
, K. Nakajima a, S. Yamanaka b, M. Hasegawa b, H. Okushi
b
a Department of Engineering Physics and Mechanics, Kyoto University, Kyoto 606-8501, Japan National Institute of Advanced Industrial Science and Technology, 1-1-1, Umezono, Tsukuba, Ibaraki 305-0045, Japan
Abstract We have measured hydrogen depth profiles in chemical vapor deposition (CVD) diamond films by high-resolution elastic recoil detection. The depth resolution of nearly 0.2 nm is achieved by means of a high-resolution magnetic spectrometer. The hydrogen profile in the as-grown sample shows a sharp peak at the surface. The peak has a small tail toward larger depth, showing that some hydrogen atoms are incorporated in the subsurface region. There is no difference in the hydrogen depth profiles between the undoped and B-doped diamond films. After surface hydrogen is removed by an acid solution, the CVD diamond is rehydrogenated using a hydrogen plasma at 800 °C. The rehydrogenated sample shows almost the same hydrogen profile as the as-grown sample. The hydrogen profile hardly changes by annealing at 400 °C, though a small change in the subsurface region cannot be excluded. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 82.82.Yc; 81.05.Tp; 81.15.Gh Keywords: Hydrogen; CVD diamond; High-resolution ERD
1. Introduction Diamond is a promising material for future electronic devices. A number of studies have been conducted on the growth of diamond. Recent investigation has shown that high-quality diamond films with atomically flat surfaces can be prepared by microwave-plasma chemical vapor deposition (CVD) under ultra-low CH4 /H2 conditions [1]. Diamond films prepared by CVD have a p-type semiconducting layer in the surface region [2]. This
*
Corresponding author. Tel.: +81-75-753-5253; fax: +81-757535253/7717286. E-mail address: [email protected] (K. Kimura).
semiconducting layer is considered to be related to the surface hydrogen because the surface conductivity can be removed by oxidation with acid solutions or by annealing at temperatures higher than 400 °C, and it is recovered by rehydrogenation in a hydrogen plasma [2–6]. Nevertheless, the origin of the surface conductivity induced by hydrogenation is still in debate [4–10]. Information about the hydrogen concentration in the surface region is of prime importance to understand the mechanism of the surface conductivity. However, few measurements on the hydrogen depth profiles in the CVD diamond films have been reported because quantitative analysis of hydrogen is very difficult. Recently we have demonstrated that high-resolution elastic recoil detection
0168-583X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 2 ) 0 0 4 5 0 - 0
690
K. Kimura et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 689–692
(ERD) can be successfully applied to measure hydrogen depth profiles in CVD diamond films [11]. The observed hydrogen profile shows a sharp peak at the surface. The hydrogen coverage is estimated to be 1 0:3 ML (1 ML ¼ 1:57 1015 cm2 ), indicating the formation of a monohydride structure. The surface peak has a small tail toward larger depth, indicating that a small amount of hydrogen is incorporated in the subsurface region. In the present paper, we extend the study to CVD diamond films which received various treatments, such as rehydrogenation in a hydrogen plasma after oxidization, annealing at 400 °C, and rehydrogenation after ion irradiation in an attempt to see if hydrogen content or profile are changed by these treatments.
2. Experimental Details of the preparation of the CVD diamond films are described elsewhere [6]. Briefly, undoped diamond films were grown on (0 0 1) surfaces of synthetic Ib diamond (4 4 0:3 mm3 ) by microwave-plasma CVD at 800 °C with 0.025% CH4 diluted by H2 gas. B-doped diamond films were prepared in the same way, but using 0.1% CH4 and 0.15–0.2 ppm B(CH3 )3 , diluted by H2 gas. One of the B-doped sample was oxidized using H2 SO4 and HNO3 solutions at 200 °C for 15 min in order to remove the hydrogen. The sample was then rehydrogenated in a hydrogen plasma at 800 °C for 15 min (it is being referred to as the ‘‘rehydrogenated sample’’). Another B-doped sample was annealed at 400 °C for 45 min in a N2 atmosphere. This sample is being referred to as the ‘‘annealed sample’’. A beam of 500 keV Cþ ions from a Van de Graaff accelerator was collimated to 2 mm (vertical) 1 mm (horizontal) by a series of 4-jaw slits. The beam current was monitored using a vibrating beam chopper. A typical beam current was 3 nA. The Cþ beam was incident on the CVD diamond films at an angle of 70°. The energy spectrum of Hþ ions recoiled at 25° was measured with a highresolution magnetic spectrometer. The spectrometer consisted of a 90° sector magnet and a one-dimensional position-sensitive detector (PSD) of 100 mm length. A quadrupole lens was installed be-
tween the target and the magnet for the correction of kinematic broadening. An electrostatic deflector, placed between the magnet and the PSD, was used to reject scattered carbon ions. The acceptance angle of the spectrometer was 0.2 msr.
3. Results and discussion Fig. 1 shows examples of the observed Hþ spectra for the as-grown, undoped (closed circles), and B-doped (open circles) CVD diamond films. The conversion from the recoil energy to depth was done using stopping powers estimated with the TRIM code [12]. The obtained depth scale is shown on the upper abscissa. The observed spectra are essentially the same, showing that the hydrogen depth profile is not affected by the boron doping. The spectra have a sharp peak at the surface (at 117.4 keV). The peak width is about 0.9 keV, which corresponds to a depth resolution of 0.23 nm. The peak is slightly asymmetric having a small tail toward lower energies. This small tail can be ascribed to hydrogen atoms in the subsurface region [11]. Except for the peak region the yield is at the same level as the dark noise of the PSD, indicating that hydrogen concentration in
Fig. 1. Examples of the observed energy spectra of recoiled Hþ ions for as-grown undoped and B-doped CVD diamond films. The spectrum has a sharp peak at the surface. The depth resolution estimated from the peak width is 0.2 nm. There is no difference between the undoped and B-doped samples.
K. Kimura et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 689–692
Fig. 2. ERD spectra for the as-grown and rehydrogenated Bdoped samples. The rehydrogenation recovers the surface hydrogen completely.
the bulk region is smaller than the detection limit of the present measurement (0:05 at:% ¼ 9 1019 cm3 ). Fig. 2 shows the comparison of the ERD spectra for the as-grown and rehydrogenated samples. It is known that the surface conductivity is removed by oxidation with acid solutions, and the conductivity can be recovered by rehydrogenation. There is no difference between the asgrown and rehydrogenated samples, showing that surface hydrogen is completely recovered by the present rehydrogenation process. Fig. 3 shows the comparison between the asgrown and annealed samples. It is known that the surface conductivity disappears after annealing at temperatures higher than 400 °C, suggesting that the surface hydrogen is removed by the annealing. Surprisingly, the present result indicates that the hydrogen depth profile is not changed by the annealing at 400 °C. This suggests that the surface conductivity is not related to the surface hydrogen but related to the subsurface hydrogen. Further study on this subject is in progress. Finally, the effect of ion irradiation on the hydrogen profile has been investigated. The hydrogen is desorbed from the CVD diamond films during the ERD measurement probably due to desorption induced by electronic excitation [11].
691
Fig. 3. Comparison between the as-grown and annealed samples. The annealing was done at 400 °C in a N2 atmosphere. The hydrogen surface peak is not changed by the annealing.
Fig. 4. Observed ERD spectrum for the Cþ -irradiated CVD diamond film followed by rehydrogenation. The spectrum for an as-grown film is shown for comparison. The hydrogen concentration in the subsurface region is larger than in the asgrown sample possibly due to the hydrogen trapping by radiation damages.
After an ERD measurement with a total fluence of 4 1015 cm2 , 90% of the hydrogen atoms were desorbed. The irradiated sample was rehydrogenated in a hydrogen plasma at 800 °C for 15 min, and an ERD measurement was performed. The observed ERD spectrum is shown in Fig. 4
692
K. Kimura et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 689–692
together with the spectrum for the as-grown sample. It is evident that the hydrogen concentration in the subsurface region is increased by this treatment. The hydrogen concentration is of the order of 1 at.% at a depth of 1 nm. This is attributed to hydrogen trapping by lattice defects created by the Cþ ion irradiation. Besides the change in the subsurface region, the peak becomes slightly broader. The peak broadening might be attributed to surface roughening caused by ion irradiation.
Acknowledgements We are grateful to the members of the Quantum Science and Engineering Center at Kyoto University for the use of the Van de Graaff accelerator. This work was supported in part by a Grant-in-Aid for Scientific Research from the ministry of Education, Culture, Sports, Science and Technology.
References
4. Conclusions It is demonstrated that hydrogen depth profiling in CVD diamond films can be performed by high-resolution ERD with a depth resolution of nearly 0.2 nm. There is no difference between the hydrogen depth profiles in undoped and B-doped CVD diamond films. The effects of different treatments on the hydrogen depth profile have been examined. It has been shown that rehydrogenation in a hydrogen plasma recovers the surface hydrogen almost completely. Annealing at 400 °C in a N2 atmosphere hardly changes the surface hydrogen peak. Rehydrogenation after irradiation of 500 keV Cþ ions at a fluence of 4 1015 cm2 leads to the incorporation of a substantial amount of hydrogen in the subsurface region, which may be due to hydrogen trapping caused by radiation damage.
[1] H. Watanabe, D. Takeuchi, S. Yamanaka, H. Okushi, K. Kajimura, T. Sekiguchi, Diam. Rel. Mater. 8 (1999) 1272. [2] M.I. Landstrass, K.V. Ravi, Appl. Phys. Lett. 55 (1989) 975. [3] S.A. Grot, G.Sh. Gildenblat, C.W. Hatfield, C.W. Wronski, A.R. Badzian, T. Badzian, R. Messier, IEEE Electron Device Lett. 11 (1990) 100. [4] Y. Mori, A. Hatta, T. Ito, A. Hiraki, Jpn. J. Appl. Phys. 31 (1992) L1718. [5] H. Kiyota, E. Matsushima, K. Sato, H. Okushi, T. Ando, M. Kamo, Y. Sato, M. Iida, Appl. Phys. Lett. 67 (1995) 3596. [6] K. Hayashi, S. Yamanaka, H. Okushi, K. Kajimura, Appl. Phys. Lett. 68 (1996) 376. [7] H. Kawarada, H. Sasaki, A. Sato, Phys. Rev. B 52 (1995) 11351. [8] S. Ri, T. Mizumasa, Y. Akiba, Y. Hirose, T. Kurosu, M. Iida, Jpn. J. Appl. Phys. 34 (1995) 5550. [9] J. Shirafuji, T. Sugino, Diam. Rel. Mater. 5 (1996) 706. [10] F. Maier, M. Riedel, B. Mantel, J. Ristein, L. Ley, Phys. Rev. Lett. 85 (2000) 3472. [11] K. Kimura, K. Nakajima, S. Yamanaka, M. Hasegawa, H. Okushi, Appl. Phys. Lett. 78 (2001) 1679. [12] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1985.