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XPS study of diamond surface after mass-separated low-energy phosphorus ion irradiation
Diamond & Related Materials 14 (2005) 389 – 392 www.elsevier.com/locate/diamond
XPS study of diamond surface after mass-separated low-energy phosphorus ion irradiation Kazuhiro Yamamotoa,T, Hideyuki Watanabeb, Masahiko Ogurab a
Research Institute of Instrumental Frontier, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan b Diamond Research Center, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan Available online 12 February 2005
Abstract Phosphorus-ion irradiation of synthetic type II a diamond crystals with a hydrogen-terminated (100) surface was carried out by using a 40eV mass-separated 31P+ ion beam at ion doses from 4.41011 to 3.41014 ions/cm2. The effects of the dose strength on the electrical state of the diamond surface were investigated. The diamond surface was characterized by using X-ray photoelectron spectroscopy (XPS). Work functions were determined from the XPS photoemission cutoff and valence band spectrum. The hydrogen-terminated II a (100) surface demonstrated the negative electron affinity (NEA) property, and the work function was found to be 3.9 eV. The NEA property disappeared after phosphorus-ion irradiation higher than 2.31012 ions/cm2, and the work function increased to 4.7 eV at 1.31013 ions/cm2. We believe that a new energy level is formed due to the displacement of phosphorus. Defects were introduced at ion irradiation doses greater than 3.01013 ions/cm2, and a defect-related energy state was formed. D 2005 Elsevier B.V. All rights reserved. Keywords: Diamond properties and applications; Phosphorus; Surface; XPS
1. Introduction Diamond is a wide-band-gap semiconductor with attractive properties such as high thermal conductivity, high electric breakdown field and low dielectric constant. Diamond after the hydrogenation of its surface also shows a negative electron affinity. Controlling the type of doping (p- or n-type) of diamond is important in electrical applications. A p-type diamond has been achieved by boron doping, and the activation energy for hole concentration in B-doped diamond film has been measured as 0.36 eV [1]. Nitrogen is a dominant impurity in diamond and is a demonstrated dopant for n-type conduction. Substitutional 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 [2,3]. Another dopant for n-type conduction with a more shallow energy state than nitrogen is phosphorus. The donor level in T Corresponding author. Tel.: +81 298 61 9414; fax: +81 298 61 5882. E-mail address: [email protected] (K. Yamamoto). 0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.01.001
P-doped diamond film with (111) orientation produced by using chemical vapor deposition (CVD) was reported as 0.6 eV below the conduction band edge [4]. However, phosphorus doping of (100) orientated diamond film by CVD has not been achieved. Ion implantation using high-energy ion beams is a method for the controlled introduction of impurities into a solid; however, defects are also introduced, and damagerelated electrical conduction has been observed [5]. In silicon semiconductor technology, thermal or laser annealing after ion implantation recovers the crystallization of silicon. However, the crystallite of diamond after ion implantation is not recovered by thermal annealing. For diamond, the method of cold implantation and in situ rapid annealing (CIRA), which uses high-energy (N10 keV) ion beams, was developed to avoid implantation-related damage [6]. However, low-energy ion implantation below 100 eV, which is close to the displacement energy of the solid, has not been extensively investigated. The use of a mass-separated ion beam (MSIB) is the most suitable method for low-energy ion irradiation. We
390
K. Yamamoto et al. / Diamond & Related Materials 14 (2005) 389–392
have developed a high-ion-current, ultrahigh-vacuum (b810 7 Pa) MSIB system [7]. In previous MSIB studies [8,9], the ion current was low, or the pressure during irradiation was greater than 10 5 Pa. At pressures N10 5 Pa, high-speed neutral species produced by inelastic collisions of the fast ions with residual gas molecules also impact and damage the target. Our MSIB system allows irradiation without damage caused by the impact of highspeed neutral species on the target [7]. Because of the high vacuum during ion irradiation, the target is less likely to be damaged. Low-energy phosphorus-ion irradiation may make the doping of diamond with (100) orientation possible. Here, we report the ion irradiation of single crystals (100) diamond surface with a 40-eV mass-separated 31P+ ion beam to attempt the ion doping without damage and discuss the electrical state and the work function of diamond surface.
2. Experimental method High-pressure, high-temperature synthetic (100) type II a single-crystal diamond plates (2.5 mm2.5 mm0.3 mm) 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 30 min to remove the graphitic component of the diamond surface. After oxidization, the diamond surface was terminated with hydrogen by immersion in a microwave-excited hydrogen
plasma. The plasma reactor was made by Astex Inc. The substrate temperature was 1073 K, the input power of the microwave was 750 W, the hydrogen gas pressure was 25 Torr, and the treatment time was 5 min. Samples were irradiated with a mass-separated 31P+ ion beam. The MSIB system consisted of an ion source, a massseparation 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 [7]. Trimethylphosphine (TMP) was used as the source gas for the phosphorus ions. The energy of the phosphorus ions was 40 eV, which is the displacement energy of the carbon atoms out of their lattice site in diamond at room temperature [10]. The phosphorus ion dose strengths ranged from 4.41011 to 3.41014 ions/cm2. The same diamond specimen irradiated repeatedly. The diamond plates were not heated during ion irradiation. The base pressure in the irradiation chamber was 1.010 8 Pa, and the pressure during irradiation was 310 7 Pa. The core-level spectra, valence band spectra, and photoelectron emission of the diamond were determined by ex situ XPS using monochromated Al Ka X-ray radiation (hm= 1486.6 eV). The XPS measurement was carried out at room temperature. The spectrometer was calibrated by using Au(4f7/2)=84.0 eV, Cu(2p3/2)=932.7 eV, Cu(LMM)=567.97 eV, and Cu(3p)=75.14 eV. The analysis area was 800 Am in diameter. The take-off angle was 458 for the core-level spectra and valence band spectra, and 908 for the photoemission spectra. A bias was applied to the specimens to
(b) C1s
Intensity (arb. units)
Intensity (arb. units)
(a) P2p
Before 140
135
130
125
Binding energy (eV)
120
290
288
286
284
282
280
Binding energy (eV)
Fig. 1. XPS spectra of H-terminated diamonds; (a) after ion irradiation in phosphorus 2p region and (b) before and after ion irradiation in carbon 1s region.
K. Yamamoto et al. / Diamond & Related Materials 14 (2005) 389–392
overcome the work function of the analyzer for the measurement of the photoemission spectra. The work functions of the samples were determined from the XPS photoemission cutoff. To prevent surface damage of the samples, Ar-ion etching was not performed prior to XPS measurements.
391
4.5 eV
3. Results and discussion
Intensity (arb. units)
Before
DOS
28
24
20
16
12
8
4
0
Binding energy (eV) Fig. 2. Valence band spectra of H-terminated diamonds before and after ion irradiation and the calculated density of state (DOS) [12].
3.9 eV Before 5
3.9 eV 4.2 eV
4
Intensity (counts/s)
In the XPS measurement, the diamond specimens showed electrical conductivity even after ion irradiation from doses of 4.41011 to 3.41014 ions/cm2, and no insulating behavior was observed. This result is probably due to the hydrogen termination of the diamond surface, which kept the surface layer conductive following the ion irradiation. Moreover, because of the hydrogen-terminated surface of the specimen, oxygen was not detected. The P2p XPS spectra for a diamond specimen after ion irradiation of various ion doses are shown in Fig. 1a. The implantation depth of phosphorus ions with an energy of 40 eV was calculated to be 1 nm by TRIM calculation [11]. For irradiation doses from 4.41011 to 3.41014 ions/cm2, the phosphorus atom concentrations were estimated from 4.41018 to 3.41021 ions/cm3, distributed over a 1-nm depth. Thus, the phosphorus was too dilute in the specimens irradiated below 3.01013 ions/cm2 to be detected by XPS. Phosphorus can be detected at a binding energy (BE) of 132.8 eV after irradiation at 1.61014 ions/cm2, and the intensity of the peak increased up to an ion dose of
4.5 eV 3
2
4.7 eV
5
4.5 eV
10 20
4.2 eV 1
20 4.2 eV 0 0 EF
2
4
6
8
10
Kinetic energy rel. to EF (eV)
Fig. 3. Photoemission spectra of H-terminated diamonds before and after ion irradiation. The energy scale is relative to the Fermi edge. The spectra after ion irradiation between 1.31013 and 3.41014 ions/cm2 are shown magnified by the factor indicated. The cutoff of each spectrum is also shown.
3.41014 ions/cm2. The phosphorus content of diamond after ion irradiation at 3.41014 ions/cm2 was estimated to be 0.3 at.% from XPS measurement. The C1s XPS spectra of a diamond specimen before and after ion irradiation of various ion doses are shown in Fig. 1b. The BE of the hydrogen-terminated specimen before irradiation was 284.3 eV. The peak in the C1s spectrum shifted to a slightly higher energy with each increase in ion dose. The BE after irradiation at 3.41014 ions/cm2 was 284.9 eV. This shift of BE is due to a change in the surface band bending and/or to movement of the Fermi level of the specimen by the phosphorus ion irradiation. The valence band XPS spectra for a diamond specimen before and after ion irradiation of various ion doses were shown in Fig. 2. The valence band spectrum of the Hterminated diamond before ion irradiation, which is the bottom-most spectrum in the figure, had local maxima at 13.2 and 17.1 eV and related well to the calculated density of state of diamond [12], which is also shown in the figure. The valence band spectra after irradiation from 4.41011 to 3.41014 ions/cm2 were not significantly different. The photoemission spectra were measured by using XPS. A bias was applied to the specimens to overcome the
392
K. Yamamoto et al. / Diamond & Related Materials 14 (2005) 389–392
substitutional sites without vacancy. We believe that a new energy level may be formed as a result of the displacement of phosphorus. The position of the Fermi level and the surface band bending of the specimen seemed to change, which led to an increase in the work function from 3.9 to 4.7 eV. We think that the electrical transport measurement is necessary to judge the doping successful. A small shoulder appeared in the lower energy region at 3.01013 ions/cm2, and the intensity of the photoemission became very low. This shoulder clearly appeared at 4.5 eV at an increased ion dose. The work function decreased from 4.7 to 4.2 eV. We believe that the shoulder at 4.5 eV was caused by defects that were introduced at heavy ion doses, and that a defect-related energy state was formed.
5
Intensity (counts-eV/s)
Before 4
3
2
1
0 1011
1012
1013
1014
1015
Ion dose (ions/cm2) Fig. 4. Dependence of the integrated intensities of the photoemission spectra shown in Fig. 3 on the ion dose.
work function of the analyzer, and the energy scale was aligned to the kinetic energy relative to the Fermi edge. The valence band spectrum of biasing samples also measured. The Fermi edge was determined by using the sharp middle peak of 13.1 eV in the valence band spectrum as reference. The photoemission spectra of the II a (100) diamond before and after ion irradiation are shown in Fig. 3. The intensity of the excited X-ray was the same for all measurements. The integrated intensities of the photoemission spectra are shown in Fig. 4. Following ion irradiation, the intensity decreased rapidly to small values of less than 105 counts eV/s. The photoemission spectrum of H-terminated II a (100) diamond is very strong at 4.9 eV (topmost spectrum in Fig. 3). The peak at a lower kinetic energy below this strong peak is the spectrum cutoff, which is similar to the spectrum obtained from B-doped diamond with (100) orientation [13]. We think this strong peak is caused by the negative electron affinity (NEA) characteristic of H-terminated II a (100) diamond. The cutoff of the NEA peak is 4.5 eV, and the lower kinetic energy cutoff is 3.9 eV, which is the work function of this specimen. Therefore, the electron affinity of this specimen is 0.6 eV below the conduction band minimum (CBM). The NEA peak at 4.9 eV weakened after ion irradiation of 4.41011 ions/cm2 and disappeared after ion irradiation higher than 2.31012 ions/cm2. The work functions of the specimens were obtained from the cutoff of each spectrum and are shown in Fig. 3. The photoemission spectrum of the specimen shifted to higher energy following ion irradiation as the ion dose increased from 4.41011 to 1.31013 to ions/cm2. The work function increased from 3.9 to 4.7 eV with the increasing ion dose. The energy of the phosphorus ions in this study was 40 eV, which is comparable to the displacement energy of phosphorus in diamond. Phosphorus ions might occupy
4. Conclusion Type II a (100) diamond specimens were irradiated with 40-eV mass-separated 31P+ ions at ion dose strengths from 4.41011 to 3.41014 ions/cm2, and the effects of the ion dose strength on the surface electrical state of the diamond were investigated. The hydrogen-terminated II a (100) surface showed the NEA property, and the work function was 3.9 eV. The NEA property disappeared after phosphorus ion irradiation higher than 2.31012 ions/cm2, and the work function increased with increasing ion dose. The value of the work function after irradiation at 1.31013 ions/cm2 was 4.7 eV. The position of the Fermi level and the surface band bending of the specimen seemed to change, and a new energy level seemed to be formed due to the displacement of phosphorus. Defects were introduced at ion irradiation doses greater than 3.01013 ions/cm2, and the work function decreased due to the forming of a defect-related energy state.
References [1] A.T. Collins, A.W.S. Williams, J. Phys. C 4 (1971) 1789. [2] R.G. Farer, Solid State Commun. 7 (1969) 685. [3] S.A. Kajihara, A. Antonelli, J. Bernholc, Phys. Rev. Lett. 66 (1991) 2010. [4] S. Koizumi, T. Teraji, H. Kanda, Diamond Relat. Mater. 9 (2000) 935. [5] A. Reznik, C. Uzan-Saguy, R. Kalish, Diamond Relat. Mater. 9 (2000) 1051. [6] J.F. Prins, Phys. Rev. B 38 (1988) 5576. [7] K. Yamamoto, T. Watanabe, K. Wazumi, F. Kokai, Y. Koga, S. Fujiwara, Diamond Relat. Mater. 10 (2000) 895. [8] A.H. Al-bayati, D. Marton, S.S. Todorov, K.J. Boyd, J.W. Rabalais, D.G. Armour, J.S. Gordon, G. Duller, Rev. Sci. Instrum. 65 (1994) 2680. [9] H. Hofs7ss, H. Binder, T. Klumpp, E. Recknagel, Diamond Relat. Mater. 3 (1994) 137. [10] J.W. Rabalais, Low Energy Ion-Surface Interactions, John Wiley and Sons, West Sussex, England, 1994. [11] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon Press, New York, 1985. [12] G.S. Painter, D.E. Ellis, A.R. Lubinsky, Phys. Rev. B 4 (1971) 3610. [13] L. Diederich, O.M. Kqttel, P. Ruffieux, Th. Pillo, P. Aebi, L. Schlapbach, Surf. Sci. 417 (1998) 41.
XPS study of diamond surface after mass-separated low-energy phosphorus ion irradiation Kazuhiro Yamamotoa,T, Hideyuki Watanabeb, Masahiko Ogurab a
Research Institute of Instrumental Frontier, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan b Diamond Research Center, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan Available online 12 February 2005
Abstract Phosphorus-ion irradiation of synthetic type II a diamond crystals with a hydrogen-terminated (100) surface was carried out by using a 40eV mass-separated 31P+ ion beam at ion doses from 4.41011 to 3.41014 ions/cm2. The effects of the dose strength on the electrical state of the diamond surface were investigated. The diamond surface was characterized by using X-ray photoelectron spectroscopy (XPS). Work functions were determined from the XPS photoemission cutoff and valence band spectrum. The hydrogen-terminated II a (100) surface demonstrated the negative electron affinity (NEA) property, and the work function was found to be 3.9 eV. The NEA property disappeared after phosphorus-ion irradiation higher than 2.31012 ions/cm2, and the work function increased to 4.7 eV at 1.31013 ions/cm2. We believe that a new energy level is formed due to the displacement of phosphorus. Defects were introduced at ion irradiation doses greater than 3.01013 ions/cm2, and a defect-related energy state was formed. D 2005 Elsevier B.V. All rights reserved. Keywords: Diamond properties and applications; Phosphorus; Surface; XPS
1. Introduction Diamond is a wide-band-gap semiconductor with attractive properties such as high thermal conductivity, high electric breakdown field and low dielectric constant. Diamond after the hydrogenation of its surface also shows a negative electron affinity. Controlling the type of doping (p- or n-type) of diamond is important in electrical applications. A p-type diamond has been achieved by boron doping, and the activation energy for hole concentration in B-doped diamond film has been measured as 0.36 eV [1]. Nitrogen is a dominant impurity in diamond and is a demonstrated dopant for n-type conduction. Substitutional 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 [2,3]. Another dopant for n-type conduction with a more shallow energy state than nitrogen is phosphorus. The donor level in T Corresponding author. Tel.: +81 298 61 9414; fax: +81 298 61 5882. E-mail address: [email protected] (K. Yamamoto). 0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.01.001
P-doped diamond film with (111) orientation produced by using chemical vapor deposition (CVD) was reported as 0.6 eV below the conduction band edge [4]. However, phosphorus doping of (100) orientated diamond film by CVD has not been achieved. Ion implantation using high-energy ion beams is a method for the controlled introduction of impurities into a solid; however, defects are also introduced, and damagerelated electrical conduction has been observed [5]. In silicon semiconductor technology, thermal or laser annealing after ion implantation recovers the crystallization of silicon. However, the crystallite of diamond after ion implantation is not recovered by thermal annealing. For diamond, the method of cold implantation and in situ rapid annealing (CIRA), which uses high-energy (N10 keV) ion beams, was developed to avoid implantation-related damage [6]. However, low-energy ion implantation below 100 eV, which is close to the displacement energy of the solid, has not been extensively investigated. The use of a mass-separated ion beam (MSIB) is the most suitable method for low-energy ion irradiation. We
390
K. Yamamoto et al. / Diamond & Related Materials 14 (2005) 389–392
have developed a high-ion-current, ultrahigh-vacuum (b810 7 Pa) MSIB system [7]. In previous MSIB studies [8,9], the ion current was low, or the pressure during irradiation was greater than 10 5 Pa. At pressures N10 5 Pa, high-speed neutral species produced by inelastic collisions of the fast ions with residual gas molecules also impact and damage the target. Our MSIB system allows irradiation without damage caused by the impact of highspeed neutral species on the target [7]. Because of the high vacuum during ion irradiation, the target is less likely to be damaged. Low-energy phosphorus-ion irradiation may make the doping of diamond with (100) orientation possible. Here, we report the ion irradiation of single crystals (100) diamond surface with a 40-eV mass-separated 31P+ ion beam to attempt the ion doping without damage and discuss the electrical state and the work function of diamond surface.
2. Experimental method High-pressure, high-temperature synthetic (100) type II a single-crystal diamond plates (2.5 mm2.5 mm0.3 mm) 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 30 min to remove the graphitic component of the diamond surface. After oxidization, the diamond surface was terminated with hydrogen by immersion in a microwave-excited hydrogen
plasma. The plasma reactor was made by Astex Inc. The substrate temperature was 1073 K, the input power of the microwave was 750 W, the hydrogen gas pressure was 25 Torr, and the treatment time was 5 min. Samples were irradiated with a mass-separated 31P+ ion beam. The MSIB system consisted of an ion source, a massseparation 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 [7]. Trimethylphosphine (TMP) was used as the source gas for the phosphorus ions. The energy of the phosphorus ions was 40 eV, which is the displacement energy of the carbon atoms out of their lattice site in diamond at room temperature [10]. The phosphorus ion dose strengths ranged from 4.41011 to 3.41014 ions/cm2. The same diamond specimen irradiated repeatedly. The diamond plates were not heated during ion irradiation. The base pressure in the irradiation chamber was 1.010 8 Pa, and the pressure during irradiation was 310 7 Pa. The core-level spectra, valence band spectra, and photoelectron emission of the diamond were determined by ex situ XPS using monochromated Al Ka X-ray radiation (hm= 1486.6 eV). The XPS measurement was carried out at room temperature. The spectrometer was calibrated by using Au(4f7/2)=84.0 eV, Cu(2p3/2)=932.7 eV, Cu(LMM)=567.97 eV, and Cu(3p)=75.14 eV. The analysis area was 800 Am in diameter. The take-off angle was 458 for the core-level spectra and valence band spectra, and 908 for the photoemission spectra. A bias was applied to the specimens to
(b) C1s
Intensity (arb. units)
Intensity (arb. units)
(a) P2p
Before 140
135
130
125
Binding energy (eV)
120
290
288
286
284
282
280
Binding energy (eV)
Fig. 1. XPS spectra of H-terminated diamonds; (a) after ion irradiation in phosphorus 2p region and (b) before and after ion irradiation in carbon 1s region.
K. Yamamoto et al. / Diamond & Related Materials 14 (2005) 389–392
overcome the work function of the analyzer for the measurement of the photoemission spectra. The work functions of the samples were determined from the XPS photoemission cutoff. To prevent surface damage of the samples, Ar-ion etching was not performed prior to XPS measurements.
391
4.5 eV
3. Results and discussion
Intensity (arb. units)
Before
DOS
28
24
20
16
12
8
4
0
Binding energy (eV) Fig. 2. Valence band spectra of H-terminated diamonds before and after ion irradiation and the calculated density of state (DOS) [12].
3.9 eV Before 5
3.9 eV 4.2 eV
4
Intensity (counts/s)
In the XPS measurement, the diamond specimens showed electrical conductivity even after ion irradiation from doses of 4.41011 to 3.41014 ions/cm2, and no insulating behavior was observed. This result is probably due to the hydrogen termination of the diamond surface, which kept the surface layer conductive following the ion irradiation. Moreover, because of the hydrogen-terminated surface of the specimen, oxygen was not detected. The P2p XPS spectra for a diamond specimen after ion irradiation of various ion doses are shown in Fig. 1a. The implantation depth of phosphorus ions with an energy of 40 eV was calculated to be 1 nm by TRIM calculation [11]. For irradiation doses from 4.41011 to 3.41014 ions/cm2, the phosphorus atom concentrations were estimated from 4.41018 to 3.41021 ions/cm3, distributed over a 1-nm depth. Thus, the phosphorus was too dilute in the specimens irradiated below 3.01013 ions/cm2 to be detected by XPS. Phosphorus can be detected at a binding energy (BE) of 132.8 eV after irradiation at 1.61014 ions/cm2, and the intensity of the peak increased up to an ion dose of
4.5 eV 3
2
4.7 eV
5
4.5 eV
10 20
4.2 eV 1
20 4.2 eV 0 0 EF
2
4
6
8
10
Kinetic energy rel. to EF (eV)
Fig. 3. Photoemission spectra of H-terminated diamonds before and after ion irradiation. The energy scale is relative to the Fermi edge. The spectra after ion irradiation between 1.31013 and 3.41014 ions/cm2 are shown magnified by the factor indicated. The cutoff of each spectrum is also shown.
3.41014 ions/cm2. The phosphorus content of diamond after ion irradiation at 3.41014 ions/cm2 was estimated to be 0.3 at.% from XPS measurement. The C1s XPS spectra of a diamond specimen before and after ion irradiation of various ion doses are shown in Fig. 1b. The BE of the hydrogen-terminated specimen before irradiation was 284.3 eV. The peak in the C1s spectrum shifted to a slightly higher energy with each increase in ion dose. The BE after irradiation at 3.41014 ions/cm2 was 284.9 eV. This shift of BE is due to a change in the surface band bending and/or to movement of the Fermi level of the specimen by the phosphorus ion irradiation. The valence band XPS spectra for a diamond specimen before and after ion irradiation of various ion doses were shown in Fig. 2. The valence band spectrum of the Hterminated diamond before ion irradiation, which is the bottom-most spectrum in the figure, had local maxima at 13.2 and 17.1 eV and related well to the calculated density of state of diamond [12], which is also shown in the figure. The valence band spectra after irradiation from 4.41011 to 3.41014 ions/cm2 were not significantly different. The photoemission spectra were measured by using XPS. A bias was applied to the specimens to overcome the
392
K. Yamamoto et al. / Diamond & Related Materials 14 (2005) 389–392
substitutional sites without vacancy. We believe that a new energy level may be formed as a result of the displacement of phosphorus. The position of the Fermi level and the surface band bending of the specimen seemed to change, which led to an increase in the work function from 3.9 to 4.7 eV. We think that the electrical transport measurement is necessary to judge the doping successful. A small shoulder appeared in the lower energy region at 3.01013 ions/cm2, and the intensity of the photoemission became very low. This shoulder clearly appeared at 4.5 eV at an increased ion dose. The work function decreased from 4.7 to 4.2 eV. We believe that the shoulder at 4.5 eV was caused by defects that were introduced at heavy ion doses, and that a defect-related energy state was formed.
5
Intensity (counts-eV/s)
Before 4
3
2
1
0 1011
1012
1013
1014
1015
Ion dose (ions/cm2) Fig. 4. Dependence of the integrated intensities of the photoemission spectra shown in Fig. 3 on the ion dose.
work function of the analyzer, and the energy scale was aligned to the kinetic energy relative to the Fermi edge. The valence band spectrum of biasing samples also measured. The Fermi edge was determined by using the sharp middle peak of 13.1 eV in the valence band spectrum as reference. The photoemission spectra of the II a (100) diamond before and after ion irradiation are shown in Fig. 3. The intensity of the excited X-ray was the same for all measurements. The integrated intensities of the photoemission spectra are shown in Fig. 4. Following ion irradiation, the intensity decreased rapidly to small values of less than 105 counts eV/s. The photoemission spectrum of H-terminated II a (100) diamond is very strong at 4.9 eV (topmost spectrum in Fig. 3). The peak at a lower kinetic energy below this strong peak is the spectrum cutoff, which is similar to the spectrum obtained from B-doped diamond with (100) orientation [13]. We think this strong peak is caused by the negative electron affinity (NEA) characteristic of H-terminated II a (100) diamond. The cutoff of the NEA peak is 4.5 eV, and the lower kinetic energy cutoff is 3.9 eV, which is the work function of this specimen. Therefore, the electron affinity of this specimen is 0.6 eV below the conduction band minimum (CBM). The NEA peak at 4.9 eV weakened after ion irradiation of 4.41011 ions/cm2 and disappeared after ion irradiation higher than 2.31012 ions/cm2. The work functions of the specimens were obtained from the cutoff of each spectrum and are shown in Fig. 3. The photoemission spectrum of the specimen shifted to higher energy following ion irradiation as the ion dose increased from 4.41011 to 1.31013 to ions/cm2. The work function increased from 3.9 to 4.7 eV with the increasing ion dose. The energy of the phosphorus ions in this study was 40 eV, which is comparable to the displacement energy of phosphorus in diamond. Phosphorus ions might occupy
4. Conclusion Type II a (100) diamond specimens were irradiated with 40-eV mass-separated 31P+ ions at ion dose strengths from 4.41011 to 3.41014 ions/cm2, and the effects of the ion dose strength on the surface electrical state of the diamond were investigated. The hydrogen-terminated II a (100) surface showed the NEA property, and the work function was 3.9 eV. The NEA property disappeared after phosphorus ion irradiation higher than 2.31012 ions/cm2, and the work function increased with increasing ion dose. The value of the work function after irradiation at 1.31013 ions/cm2 was 4.7 eV. The position of the Fermi level and the surface band bending of the specimen seemed to change, and a new energy level seemed to be formed due to the displacement of phosphorus. Defects were introduced at ion irradiation doses greater than 3.01013 ions/cm2, and the work function decreased due to the forming of a defect-related energy state.
References [1] A.T. Collins, A.W.S. Williams, J. Phys. C 4 (1971) 1789. [2] R.G. Farer, Solid State Commun. 7 (1969) 685. [3] S.A. Kajihara, A. Antonelli, J. Bernholc, Phys. Rev. Lett. 66 (1991) 2010. [4] S. Koizumi, T. Teraji, H. Kanda, Diamond Relat. Mater. 9 (2000) 935. [5] A. Reznik, C. Uzan-Saguy, R. Kalish, Diamond Relat. Mater. 9 (2000) 1051. [6] J.F. Prins, Phys. Rev. B 38 (1988) 5576. [7] K. Yamamoto, T. Watanabe, K. Wazumi, F. Kokai, Y. Koga, S. Fujiwara, Diamond Relat. Mater. 10 (2000) 895. [8] A.H. Al-bayati, D. Marton, S.S. Todorov, K.J. Boyd, J.W. Rabalais, D.G. Armour, J.S. Gordon, G. Duller, Rev. Sci. Instrum. 65 (1994) 2680. [9] H. Hofs7ss, H. Binder, T. Klumpp, E. Recknagel, Diamond Relat. Mater. 3 (1994) 137. [10] J.W. Rabalais, Low Energy Ion-Surface Interactions, John Wiley and Sons, West Sussex, England, 1994. [11] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon Press, New York, 1985. [12] G.S. Painter, D.E. Ellis, A.R. Lubinsky, Phys. Rev. B 4 (1971) 3610. [13] L. Diederich, O.M. Kqttel, P. Ruffieux, Th. Pillo, P. Aebi, L. Schlapbach, Surf. Sci. 417 (1998) 41.