- Email: [email protected]
Surface modification of diamond using low-energy nitrogen ions
Diamond and Related Materials 13 (2004) 736–739
Surface modification of diamond using low-energy nitrogen ions Kazuhiro Yamamotoa,*, Hiroaki Yoshidab a
National Institute of Advanced Industrial Science and Technology (AIST), Advanced Carbon Research Center, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan b Toshiba Corporation, Corporate Research and Development Center, 1 Komukai Toshiba-cho, Kawasaki 212-8582, Japan
Abstract Shallow nitrogen ion doping of synthetic type Ib diamond crystals with a hydrogen-terminated (1 0 0) surface was carried out with a 40-eV mass-separated 14 Nq ion beam at three ion dose strengths (3.3=1012 , 3.3=1013 and 4.8=1014 ionsycm2 ). The effects of the dose strength on the electrical state of the diamond surface were investigated. The diamond surface was characterized by X-ray photoelectron spectroscopy (XPS), and nitrogen was determined to be bonded to carbon in a sp3 manner. The work functions were determined from the XPS photoemission cutoff and valence band spectrum. The work function of the hydrogenterminated Ib (1 0 0) surface (4.5 eV before irradiation, 3.9 eV after irradiation at 3.3=1012 ionsycm2 ) decreased with increasing ion dose strength owing to a defect-related energy state. We believe that, at a dose strength of 3.3=1012 ionsycm2 , a new energy state is formed by displacement of nitrogen. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Diamond properties and applications; Nitrogen; Doping; XPS
1. Introduction Diamond is a wide-gap semiconductor with attractive properties such as high thermal conductivity, high electric breakdown field, low dielectric constant, and negative electron affinity. The electrical properties of diamond can be modified by introducing various dopants into its crystal lattice, and controlling the type of doping (p- or n-type) is important in electrical applications. The p-type diamond has been achieved by boron doping, and the activation energy for hole concentration in Bdoped diamond film has been measured as 0.36 eV w1x. Nitrogen is a dominant impurity in diamond and is a demonstrated dopant for n-type conduction. Interstitial nitrogen leads to a deep energy state 1.7 eV below the conduction band edge due to the threefold substitution of nitrogen and the distortion of the diamond lattice w2,3x. Ion implantation is a well-controlled method for introducing impurities into a solid; however, defects are also introduced, and the damage-related electrical conduction was observed w4x. The cold implantation and in situ rapid annealing method, which uses high-energy *Corresponding author. Tel.: q81-298-61-4503; fax: q81-298-614474. E-mail address: [email protected] (K. Yamamoto).
()10 keV) ion beams, has been developed to avoid implantation-related damage w5x. However, low-energy ion implantation below 100 eV, which is close to the displacement energy of the solid, has not been extensively investigated. Mass-separated ion beam deposition (MSIBD) is the most suitable method for low-energy ion irradiation. We have developed a high-ion-current, ultrahigh-vacuum (-8=10y7 Pa) MSIBD system w6x. In previous MSIBD studies w7,8x, the ion current was low, or the pressure during deposition was greater than 10y5 Pa. At pressures )10y5 Pa, high-speed neutral carbon species produced by inelastic collisions of the fast ions with residual gas molecules also impact and damage the target film. Our MSIBD system allows deposition without damage caused by the impact of high-speed neutral species on the film w6x. Because of the high vacuum during deposition, the target is less likely to be damaged. Low-energy nitrogen ion irradiation of the diamond crystal may lead to fourfold-coordinated nitrogen. Here, we report the surface modification of single-crystal (1 0 0) diamond with a 40-eV mass-separated 14Nq ion beam and discuss the nature of the chemical bonds formed by irradiation.
0925-9635/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2003.10.046
K. Yamamoto, H. Yoshida / Diamond and Related Materials 13 (2004) 736–739
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Fig. 1. XPS spectra of H-terminated diamonds before and after ion irradiation: (a) carbon 1s region; (b) nitrogen 1s region.
2. Experimental High-pressure, high-temperature synthetic (1 0 0) type Ib single-crystal diamond plates (4=4=0.5 mm3) with polished surfaces were used as irradiation targets. The diamond plates were cleaned by sonication in ethanol and then in HF acid. These samples were oxidized by exposing them to an H2SO4 –HNO3 mixture at 473 K for 15 min to remove the graphitic component at the surface. After oxidization, the diamond surface was terminated with hydrogen by immersion in a microwaveexcited hydrogen plasma for 30 min. Samples were irradiated with a mass-separated 14Nq ion beam. The MSIBD system consisted of an ion source, a mass-separation magnet, a transport tube with magnetic quadrupole lenses to condense the ion beam, a deflection magnet to separate the ions and the fast neutrals, deceleration electrodes, and an irradiation chamber with a sample-exchange transporter. Details of the system are given elsewhere w6x. N2 was used as the source gas for the nitrogen ions. The energy of the nitrogen ions was 40 eV, which is the displacement energy of diamond at room temperature w9x. The diamond plates were not heated during irradiation. The base pressure in the irradiation chamber was 1.0=10y8 Pa, and the pressure during irradiation was 2=10y7 Pa. Three nitrogen ion dose strengths (3.3=1012, 3.3=1013 and 4.8=1014 ionsycm2) were used. The core-level spectra, valence band spectra, and photoelectron emission of the diamond were determined by ex situ X-ray photoelectron spectroscopy (XPS) using monochromated Al Ka X-ray radiation. The spectrometer was calibrated by using Au(4f7y2)s84.0 eV, Cu(2p3y2)s932.7 eV, Cu(LMM)s567.97 eV, and Cu(3p)s75.14 eV. The analysis area was 800 mm in diameter. The take-off angle was 458 for the core-level spectra and valence band spectra, and 908 for the
photoemission spectra. To prevent surface damage of the samples, Ar ion etching was not performed prior to XPS measurements. The work functions of the samples were determined from the XPS photoemission cutoff and valence band spectrum. 3. Results and discussion The diamond specimens showed electrical conductivity before and after irradiation at 3.3=1012 and 3.3=1013 ionsycm2, and no insulating behavior was observed. However, the specimens were insulating after irradiation at 4.8=1014 ionsycm2, and the charging effect was compensated by electron irradiation. This result is probably due to the removal of hydrogen from the diamond surface upon nitrogen ion irradiation and due to the disappearance of the surface conductive layer. Fig. 1a presents the C 1s XPS spectra of a diamond specimen at room temperature before and after irradiation. Owing to the hydrogen-terminated surface of the specimen, oxygen was not detected before irradiation. The binding energy (BE) of the hydrogen-terminated specimen before irradiation was 284.2 eV. The peaks in the C 1s spectrum shifted to slightly higher energy with increasing ion dose. The BE after irradiation at 4.8=1014 ionsycm2 was 284.4 eV, which was almost the same as the BE before irradiation. The BE of carbon in the C–N bond is 287.7 eV for sp3 hybridization and 286.1 eV for sp2 hybridization w10x. However, subpeaks attributable to sp2- or sp3-hybridized nitrogen were not detected. Fig. 1b presents the N 1s XPS spectra for a diamond specimen at room temperature. Nitrogen could not be detected after irradiation at 3.3=1012 ionsycm2. The implantation depth of nitrogen ions with an energy of 40 eV was calculated to be 1 nm by TRIM calculation w11x. For an irradiation dose of 3.3=1012 ionsycm2, the
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K. Yamamoto, H. Yoshida / Diamond and Related Materials 13 (2004) 736–739
Fig. 2. Carbon 1s and characteristic energy loss spectra of H-terminated diamonds before and after ion irradiation. The carbon 1s peaks have been aligned.
nitrogen atom concentration is estimated at 200 ppm, which is distributed over a 1-nm depth. This value is comparable to the initial nitrogen concentration in type Ib diamond. Thus, the nitrogen was too dilute in the specimen irradiated at 3.3=1012 ionsycm2 to be detected by XPS. A nitrogen peak was detected after irradiation at 3.3=1013 ionsycm2, and the intensity of the peak increased after irradiation at 4.8=1014 ionsycm2. The BE of nitrogen in the C–N bond is 398.3 eV for sp3 hybridization and 400.9 eV for sp2 hybridization w12x. For our specimen, the BE of nitrogen was 398.3 eV. Thus, we conclude that nitrogen is bonded to carbon in a sp3 manner. Fig. 2 presents the C 1s and associated characteristic energy loss spectra of the H-terminated sample before and after irradiation (wide-range spectra of the C 1s line). The x-axis in Fig. 2 is the photoelectron loss energy relative to the carbon 1s line. The broad resonance with a maximum at 35 eV is due to plasmon excitation of the diamond specimen. The intensity of the peak at 23 eV increased with increasing nitrogen ion dose. It has been reported that the peak at 23 eV might be due to an interband transition w13x. The increasing intensity at 23 eV suggests that some of the electrical state is formed by the nitrogen irradiation. The valence band XPS spectrum of the H-terminated diamond before ion irradiation has maxima at 13.2 and 17.1 eV and is nicely related to the calculated density of state of diamond w14x (Fig. 3). The valence band spectra after irradiation at 3.3=1012, 3.3=1013, and 4.8=1014 ionsycm2 were not significantly different (Fig. 3).
The photoemission spectra were measured by using XPS. A bias was applied to the specimens to overcome the work function of the analyzer, and the energy scale was aligned to the kinetic energy relative to the Fermi edge. The Fermi edge was determined by using the sharp middle peak at 13.1 eV in the valence band spectrum of each specimen. The photoemission spectrum of the specimen before irradiation shows a single peak with a sharp edge in the lower-energy region (Fig. 4). This peak shifts to lower energy after ion irradiation at 3.3=1012 ionsycm2. The work function of the hydrogen-terminated Ib (1 0 0) surface was 4.5 eV. After nitrogen ion irradiation at 3.3=1012 ionsycm2, the value of the work function was 3.9 eV. The nitrogen content after irradiation at 3.3=1012 ionsycm2 is comparable with the initial nitrogen content in type Ib diamond. The initial nitrogen in diamond exists in a deep energy state. The energy of the nitrogen ions in this study was 40 eV, which is comparable to the displacement energy of nitrogen in diamond. Nitrogen ions might occupy interstitial sites without the vacancy. We believe that a new energy level may be formed as a result of the displacement of nitrogen and the reduction of the work function from 4.5 to 3.9 eV. A small shoulder appeared at 3.8 eV after irradiation at 3.3=1013 ionsycm2, and the intensity of this shoulder increased after irradiation at 4.8=1014 ionsycm2. The
Fig. 3. Valence band spectra of H-terminated diamonds before and after ion irradiation, and the calculated density of state w14x.
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4. Conclusion Type Ib (1 0 0) diamond was irradiated with a 40-eV mass-separated 14Nq ion beam at three ion dose strengths, and the effects of the ion dose strength on the surface electrical state of the diamond were investigated. Nitrogen was determined to be bonded to carbon in a sp3 manner. The work function of the hydrogen-terminated Ib (1 0 0) surface was 4.5 eV. After nitrogen ion irradiation at 3.3=1012 ionsycm2, the work function was 3.9 eV and the nitrogen content of the irradiated sample was comparable with the initial nitrogen content in type Ib diamond. A new energy level may be formed due to displacement of nitrogen. Defects are introduced at ion irradiation doses greater than 1013 ionsycm2, and a defect-related energy state is formed, as reflected by the photoemission spectra and the characteristic energy losses of the C 1s line. References
Fig. 4. Photoemission spectra of H-terminated diamonds before and after ion irradiation. The energy scale is relative to the Fermi edge.
work function decreased with increasing ion dose; the work function was 3.4 eV after irradiation at 3.3=1013 ionsycm2 and 3.1 eV after irradiation at 4.8=1014 ionsy cm2. We believe that the shoulder at 3.8 eV in the photoemission spectra is caused by defects, which were introduced at heavy ion doses, and that a defect-related energy state was formed. The interband transition increases with increasing ion dose, as reflected by the characteristic energy losses of the C 1s line. This interband transition is due to a defect-related energy state.
w1x A.T. Collins, A.W.S. Williams, J. Phys. C 4 (1971) 1789. w2x R.G. Farer, Solid State Commun. 7 (1969) 685. w3x S.A. Kajihara, A. Antonelli, J. Bernholc, Phys. Rev. Lett. 66 (1991) 2010. w4x A. Reznik, C. Uzan-Saguy, R. Kalish, Diamond Relat. Mater. 9 (2000) 1051. w5x J.F. Prins, Phys. Rev. B 38 (1988) 5576. w6x K. Yamamoto, T. Watanabe, K. Wazumi, F. Kokai, Y. Koga, S. Fujiwara, Diamond Relat. Mater. 10 (2000) 895. w7x A.H. Al-bayati, D. Marton, S.S. Todorov, et al., Rev. Sci. Instrum. 65 (1994) 2680. w8x H. Hofsass, ¨ H. Binder, T. Klumpp, E. Recknagel, Diamond Relat. Mater. 3 (1994) 137. w9x J.W. Rabalais, Low Energy Ion–Surface Interactions, Wiley, West Sussex, England, 1994. w10x K. Yamamoto, Y. Koga, S. Fujiwara, Jap. J. Appl. Phys. 40 (2001) L123. w11x J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon Press, New York, 1985. w12x K. Yamamoto, T. Watanabe, K. Wazumi, Y. Koga, Diamond Relat. Mater. 12 (2003) 1061. w13x N.R. Whetten, Appl. Phys. Lett. 8 (1966) 135. w14x G.S. Painter, D.E. Ellis, A.R. Lubinsky, Phys. Rev. B 4 (1971) 3610.
Surface modification of diamond using low-energy nitrogen ions Kazuhiro Yamamotoa,*, Hiroaki Yoshidab a
National Institute of Advanced Industrial Science and Technology (AIST), Advanced Carbon Research Center, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan b Toshiba Corporation, Corporate Research and Development Center, 1 Komukai Toshiba-cho, Kawasaki 212-8582, Japan
Abstract Shallow nitrogen ion doping of synthetic type Ib diamond crystals with a hydrogen-terminated (1 0 0) surface was carried out with a 40-eV mass-separated 14 Nq ion beam at three ion dose strengths (3.3=1012 , 3.3=1013 and 4.8=1014 ionsycm2 ). The effects of the dose strength on the electrical state of the diamond surface were investigated. The diamond surface was characterized by X-ray photoelectron spectroscopy (XPS), and nitrogen was determined to be bonded to carbon in a sp3 manner. The work functions were determined from the XPS photoemission cutoff and valence band spectrum. The work function of the hydrogenterminated Ib (1 0 0) surface (4.5 eV before irradiation, 3.9 eV after irradiation at 3.3=1012 ionsycm2 ) decreased with increasing ion dose strength owing to a defect-related energy state. We believe that, at a dose strength of 3.3=1012 ionsycm2 , a new energy state is formed by displacement of nitrogen. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Diamond properties and applications; Nitrogen; Doping; XPS
1. Introduction Diamond is a wide-gap semiconductor with attractive properties such as high thermal conductivity, high electric breakdown field, low dielectric constant, and negative electron affinity. The electrical properties of diamond can be modified by introducing various dopants into its crystal lattice, and controlling the type of doping (p- or n-type) is important in electrical applications. The p-type diamond has been achieved by boron doping, and the activation energy for hole concentration in Bdoped diamond film has been measured as 0.36 eV w1x. Nitrogen is a dominant impurity in diamond and is a demonstrated dopant for n-type conduction. Interstitial nitrogen leads to a deep energy state 1.7 eV below the conduction band edge due to the threefold substitution of nitrogen and the distortion of the diamond lattice w2,3x. Ion implantation is a well-controlled method for introducing impurities into a solid; however, defects are also introduced, and the damage-related electrical conduction was observed w4x. The cold implantation and in situ rapid annealing method, which uses high-energy *Corresponding author. Tel.: q81-298-61-4503; fax: q81-298-614474. E-mail address: [email protected] (K. Yamamoto).
()10 keV) ion beams, has been developed to avoid implantation-related damage w5x. However, low-energy ion implantation below 100 eV, which is close to the displacement energy of the solid, has not been extensively investigated. Mass-separated ion beam deposition (MSIBD) is the most suitable method for low-energy ion irradiation. We have developed a high-ion-current, ultrahigh-vacuum (-8=10y7 Pa) MSIBD system w6x. In previous MSIBD studies w7,8x, the ion current was low, or the pressure during deposition was greater than 10y5 Pa. At pressures )10y5 Pa, high-speed neutral carbon species produced by inelastic collisions of the fast ions with residual gas molecules also impact and damage the target film. Our MSIBD system allows deposition without damage caused by the impact of high-speed neutral species on the film w6x. Because of the high vacuum during deposition, the target is less likely to be damaged. Low-energy nitrogen ion irradiation of the diamond crystal may lead to fourfold-coordinated nitrogen. Here, we report the surface modification of single-crystal (1 0 0) diamond with a 40-eV mass-separated 14Nq ion beam and discuss the nature of the chemical bonds formed by irradiation.
0925-9635/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2003.10.046
K. Yamamoto, H. Yoshida / Diamond and Related Materials 13 (2004) 736–739
737
Fig. 1. XPS spectra of H-terminated diamonds before and after ion irradiation: (a) carbon 1s region; (b) nitrogen 1s region.
2. Experimental High-pressure, high-temperature synthetic (1 0 0) type Ib single-crystal diamond plates (4=4=0.5 mm3) with polished surfaces were used as irradiation targets. The diamond plates were cleaned by sonication in ethanol and then in HF acid. These samples were oxidized by exposing them to an H2SO4 –HNO3 mixture at 473 K for 15 min to remove the graphitic component at the surface. After oxidization, the diamond surface was terminated with hydrogen by immersion in a microwaveexcited hydrogen plasma for 30 min. Samples were irradiated with a mass-separated 14Nq ion beam. The MSIBD system consisted of an ion source, a mass-separation magnet, a transport tube with magnetic quadrupole lenses to condense the ion beam, a deflection magnet to separate the ions and the fast neutrals, deceleration electrodes, and an irradiation chamber with a sample-exchange transporter. Details of the system are given elsewhere w6x. N2 was used as the source gas for the nitrogen ions. The energy of the nitrogen ions was 40 eV, which is the displacement energy of diamond at room temperature w9x. The diamond plates were not heated during irradiation. The base pressure in the irradiation chamber was 1.0=10y8 Pa, and the pressure during irradiation was 2=10y7 Pa. Three nitrogen ion dose strengths (3.3=1012, 3.3=1013 and 4.8=1014 ionsycm2) were used. The core-level spectra, valence band spectra, and photoelectron emission of the diamond were determined by ex situ X-ray photoelectron spectroscopy (XPS) using monochromated Al Ka X-ray radiation. The spectrometer was calibrated by using Au(4f7y2)s84.0 eV, Cu(2p3y2)s932.7 eV, Cu(LMM)s567.97 eV, and Cu(3p)s75.14 eV. The analysis area was 800 mm in diameter. The take-off angle was 458 for the core-level spectra and valence band spectra, and 908 for the
photoemission spectra. To prevent surface damage of the samples, Ar ion etching was not performed prior to XPS measurements. The work functions of the samples were determined from the XPS photoemission cutoff and valence band spectrum. 3. Results and discussion The diamond specimens showed electrical conductivity before and after irradiation at 3.3=1012 and 3.3=1013 ionsycm2, and no insulating behavior was observed. However, the specimens were insulating after irradiation at 4.8=1014 ionsycm2, and the charging effect was compensated by electron irradiation. This result is probably due to the removal of hydrogen from the diamond surface upon nitrogen ion irradiation and due to the disappearance of the surface conductive layer. Fig. 1a presents the C 1s XPS spectra of a diamond specimen at room temperature before and after irradiation. Owing to the hydrogen-terminated surface of the specimen, oxygen was not detected before irradiation. The binding energy (BE) of the hydrogen-terminated specimen before irradiation was 284.2 eV. The peaks in the C 1s spectrum shifted to slightly higher energy with increasing ion dose. The BE after irradiation at 4.8=1014 ionsycm2 was 284.4 eV, which was almost the same as the BE before irradiation. The BE of carbon in the C–N bond is 287.7 eV for sp3 hybridization and 286.1 eV for sp2 hybridization w10x. However, subpeaks attributable to sp2- or sp3-hybridized nitrogen were not detected. Fig. 1b presents the N 1s XPS spectra for a diamond specimen at room temperature. Nitrogen could not be detected after irradiation at 3.3=1012 ionsycm2. The implantation depth of nitrogen ions with an energy of 40 eV was calculated to be 1 nm by TRIM calculation w11x. For an irradiation dose of 3.3=1012 ionsycm2, the
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K. Yamamoto, H. Yoshida / Diamond and Related Materials 13 (2004) 736–739
Fig. 2. Carbon 1s and characteristic energy loss spectra of H-terminated diamonds before and after ion irradiation. The carbon 1s peaks have been aligned.
nitrogen atom concentration is estimated at 200 ppm, which is distributed over a 1-nm depth. This value is comparable to the initial nitrogen concentration in type Ib diamond. Thus, the nitrogen was too dilute in the specimen irradiated at 3.3=1012 ionsycm2 to be detected by XPS. A nitrogen peak was detected after irradiation at 3.3=1013 ionsycm2, and the intensity of the peak increased after irradiation at 4.8=1014 ionsycm2. The BE of nitrogen in the C–N bond is 398.3 eV for sp3 hybridization and 400.9 eV for sp2 hybridization w12x. For our specimen, the BE of nitrogen was 398.3 eV. Thus, we conclude that nitrogen is bonded to carbon in a sp3 manner. Fig. 2 presents the C 1s and associated characteristic energy loss spectra of the H-terminated sample before and after irradiation (wide-range spectra of the C 1s line). The x-axis in Fig. 2 is the photoelectron loss energy relative to the carbon 1s line. The broad resonance with a maximum at 35 eV is due to plasmon excitation of the diamond specimen. The intensity of the peak at 23 eV increased with increasing nitrogen ion dose. It has been reported that the peak at 23 eV might be due to an interband transition w13x. The increasing intensity at 23 eV suggests that some of the electrical state is formed by the nitrogen irradiation. The valence band XPS spectrum of the H-terminated diamond before ion irradiation has maxima at 13.2 and 17.1 eV and is nicely related to the calculated density of state of diamond w14x (Fig. 3). The valence band spectra after irradiation at 3.3=1012, 3.3=1013, and 4.8=1014 ionsycm2 were not significantly different (Fig. 3).
The photoemission spectra were measured by using XPS. A bias was applied to the specimens to overcome the work function of the analyzer, and the energy scale was aligned to the kinetic energy relative to the Fermi edge. The Fermi edge was determined by using the sharp middle peak at 13.1 eV in the valence band spectrum of each specimen. The photoemission spectrum of the specimen before irradiation shows a single peak with a sharp edge in the lower-energy region (Fig. 4). This peak shifts to lower energy after ion irradiation at 3.3=1012 ionsycm2. The work function of the hydrogen-terminated Ib (1 0 0) surface was 4.5 eV. After nitrogen ion irradiation at 3.3=1012 ionsycm2, the value of the work function was 3.9 eV. The nitrogen content after irradiation at 3.3=1012 ionsycm2 is comparable with the initial nitrogen content in type Ib diamond. The initial nitrogen in diamond exists in a deep energy state. The energy of the nitrogen ions in this study was 40 eV, which is comparable to the displacement energy of nitrogen in diamond. Nitrogen ions might occupy interstitial sites without the vacancy. We believe that a new energy level may be formed as a result of the displacement of nitrogen and the reduction of the work function from 4.5 to 3.9 eV. A small shoulder appeared at 3.8 eV after irradiation at 3.3=1013 ionsycm2, and the intensity of this shoulder increased after irradiation at 4.8=1014 ionsycm2. The
Fig. 3. Valence band spectra of H-terminated diamonds before and after ion irradiation, and the calculated density of state w14x.
K. Yamamoto, H. Yoshida / Diamond and Related Materials 13 (2004) 736–739
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4. Conclusion Type Ib (1 0 0) diamond was irradiated with a 40-eV mass-separated 14Nq ion beam at three ion dose strengths, and the effects of the ion dose strength on the surface electrical state of the diamond were investigated. Nitrogen was determined to be bonded to carbon in a sp3 manner. The work function of the hydrogen-terminated Ib (1 0 0) surface was 4.5 eV. After nitrogen ion irradiation at 3.3=1012 ionsycm2, the work function was 3.9 eV and the nitrogen content of the irradiated sample was comparable with the initial nitrogen content in type Ib diamond. A new energy level may be formed due to displacement of nitrogen. Defects are introduced at ion irradiation doses greater than 1013 ionsycm2, and a defect-related energy state is formed, as reflected by the photoemission spectra and the characteristic energy losses of the C 1s line. References
Fig. 4. Photoemission spectra of H-terminated diamonds before and after ion irradiation. The energy scale is relative to the Fermi edge.
work function decreased with increasing ion dose; the work function was 3.4 eV after irradiation at 3.3=1013 ionsycm2 and 3.1 eV after irradiation at 4.8=1014 ionsy cm2. We believe that the shoulder at 3.8 eV in the photoemission spectra is caused by defects, which were introduced at heavy ion doses, and that a defect-related energy state was formed. The interband transition increases with increasing ion dose, as reflected by the characteristic energy losses of the C 1s line. This interband transition is due to a defect-related energy state.
w1x A.T. Collins, A.W.S. Williams, J. Phys. C 4 (1971) 1789. w2x R.G. Farer, Solid State Commun. 7 (1969) 685. w3x S.A. Kajihara, A. Antonelli, J. Bernholc, Phys. Rev. Lett. 66 (1991) 2010. w4x A. Reznik, C. Uzan-Saguy, R. Kalish, Diamond Relat. Mater. 9 (2000) 1051. w5x J.F. Prins, Phys. Rev. B 38 (1988) 5576. w6x K. Yamamoto, T. Watanabe, K. Wazumi, F. Kokai, Y. Koga, S. Fujiwara, Diamond Relat. Mater. 10 (2000) 895. w7x A.H. Al-bayati, D. Marton, S.S. Todorov, et al., Rev. Sci. Instrum. 65 (1994) 2680. w8x H. Hofsass, ¨ H. Binder, T. Klumpp, E. Recknagel, Diamond Relat. Mater. 3 (1994) 137. w9x J.W. Rabalais, Low Energy Ion–Surface Interactions, Wiley, West Sussex, England, 1994. w10x K. Yamamoto, Y. Koga, S. Fujiwara, Jap. J. Appl. Phys. 40 (2001) L123. w11x J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon Press, New York, 1985. w12x K. Yamamoto, T. Watanabe, K. Wazumi, Y. Koga, Diamond Relat. Mater. 12 (2003) 1061. w13x N.R. Whetten, Appl. Phys. Lett. 8 (1966) 135. w14x G.S. Painter, D.E. Ellis, A.R. Lubinsky, Phys. Rev. B 4 (1971) 3610.