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Photoyield measurements of CVD diamond
ELAMOND RELATED MATERIALS ELSEVIER
Diamond and Related Materials 4 (1995) 806-80X
Photoyield
measurements
of CVD diamond
N. Eimori, Y. Mori, A. Hatta, T. Ito, A. Hiraki Depurtment
ofElectrical
Engineering,
Osaka
UnicersitJ;
Suita,
Osaka
565,
Japan
Abstract We investigated the electron affinity of the (lOO)-oriented single-crystal chemical vapor deposited (CVD) diamond surfaces. The photoyield spectra of (100) surfaces exposed to hydrogen plasma start from 5.4 eV incident photon energy, which is lower than the band gap of diamond (5.5 eV), using UV synchrotron radiation. This shows that the vacuum level at the (100) surface exists below the conduction band of the bulk. Also, the electron affinity of the (100) surface is found to be negative from changes in elastic and secondary electron emission peak intensities. Keywords: Surface; Characterization;
Optoelectronic
properties
1. Introduction A cold cathode made of a material with negative electron affinity would have wide applications in flat displays, microvacuum tubes which operate at a very high speed or which can operate correctly in radiomatric circumstances, etc. In general, negative electron affinity cannot be obsereved for any clean surfaces of semiconductors with mid gap energy, but can be observed after suitable treatments, e.g. Cs+0 treated Si [ 11. These treated surface are chemically too sensitive and not very stable. Although the large photothreshold energy (Eth = 5.5 f 0.05 eV) of the diamond (111) surface precludes its use as a photocathode in the visible and near-UV light regions, it still has potentially useful photoemission properties such as high photoyield efficiency in the vacuum UV region. For natural diamond, it is known that the quantum yield (electrons per photon) of the (111) surface increases nearly linearly with photon energies ranging from the photothreshold (5.5 eV) to 9 eV (20%), and then increases to 40YP70% throughout the energy range 12 < hp< 35 eV [2]. However, for silicon surfaces chemisorbed with small amounts of Cs and 0, a quantum yield of only 6% can be obtained at energies of 1
spectra using UV synchrotron radiation lights [3]. The photoemission yield from the (111) surface with hydrogen treatment is found to be higher than that of the (11 1)-oriented surface with oxygen treatment. This can be explained in terms of an increase in work function for the O-adsorbed surface. In the case of polycrystalline CVD diamond, it turns out that the photoemission yield spectrum is similar in shape to that of the single-crystal ( 111) surface. In this study, we investigated the electron affinity of the (lOO)-oriented single-crystalline-CVD diamond surfaces using low energy electron and UV synchrotron-radiation lights.
2. Experimental Diamond films were synthesized using a microwave plasma CVD on diamond (100) substrates. The conditions of diamond growth are shown in Table 1. A dilute B,H, gas was used for boron doping in order to prevent the specimens from charging during electron spectroscopic measurements. H-terminated surfaces were obtained for specimens exposed to hydrogen plasma using the mjciowave after diamond growth (H treatment). The experimental conditions for H treatment are shown in Table 1. The surface exposed to hydrogen plasma has diamond structure, same as the as-grown surface, and there are no peaks originating from surface states in electron energy losses (EEL) spectra for the hydrogen-plasma treated surface, while the surface-state-induced peaks appeared in EEL
807
N. Eimori et al/Diamond and Related Materials 4 (1995) 806-808 Table 1 Experimental
conditions
of diamond
Diamond deposition Source gas (standard cm3 min-‘) Pressure (Torr) Temperature (“C) Microwave power (W) Deposition time (h) Substrate
H treatment Gas (standard cm3 min-‘) Pressure (Torr) Temperature (“C) Microwave power (W) Treatment time (min)
deposition
and H treatment
CO( 10) + H,(90) 40 930 240 6 High pressure synthesis diamond (100)
H, 30 930 240 3
spectra when the specimen was heated to 900 “C [4]. This means that the diamond surface with hydrogen plasma treatment is covered with hydrogen atoms, and most of the hydrogen atoms exist at the specimen surface according to elastic recoil detection analysis [ 51. The Resistivity of H-treated specimens is low [6-91, and it is confirmed that charging during electron spectroscopic measurement did not occur. Photoyield measurements were carried out at room temperature in lo-* Torr using synchrotron radiation from the 750 MeV storage ring of the Institute for Molecular Science. Specimens were biased to -240 V to collect emitted electrons. A 5 mm thick quartz window was used to eliminate higher-order lights of hy > 8 eV. Photoyield spectra were corrected by the relative beam intensity of incident photons including the effects of wavelength-sensitive grating used and time-dependent storage ring current. Low energy electron scattering (LEES) measurements were performed using a double-pass cylindrical mirror analyzer (CMA) in an ultra-high vacuum chamber [3]. The -acceleration voltage of the electron beam was 100 V, while the bias voltage of the specimen was varied in the range 9OGlOO V.
Photon
energy (eV)
Fig. 1. Photoyield spectra of (100) surfaces of CVD diamond. TW threshold of photoyield is 5.4 eV, which is lower than the indirect band gap of diamond (5.5 eV). This indicate that the vacuum level lies below the conduction band minimum at the (lOO)-oriented surface.
the conduction band. Such surface states, which are assumed to be located in the band gap near the valence band maximum, may be formed by exposure to H plasma [6,9]. It is reported that the hole concentration formed at the surface by exposure to hydrogen plasma is estimated to be 3.0 x 1018 cme3 [9]. The other possibility is that the electrons at the acceptor level originating from doped boron are excited to the conduction band and diffuse to the surface. Thus electron emission with sub-band-gap energy suggests that the vacuum level at the (lOO)-oriented surface is not located above the conduction band minimum. Intensities of the elastic peak and secondary emission peak in LEES spectra depend strongly on the incident electron energy for diamond specimens, as shown in Fig. 2. When the incident energy is decreased, the elastic peak intensity begins to increase around incident energy of 5.0 eV. This is due to the presence of the forbidden energy band. The slow increase in the elastic peak intensity with decreasing incident electron energy may be related to the broadening of incident electron energy
8001
1
-
3. Results and discussion Fig. 1 shows photoemission yields taken in the energy range 4.5 dhp<7 eV from a (100) single-crystal CVD diamond surface treated with hydrogen. In Fig. 1, the photoemission yield from the H-treated (100) surface appears just above an energy lower than the band gap energy of diamond. The photoemission peak around 5.35 eV was observed as shown in Fig. 1. Two possible reasons can be considered for the origin of electron emission at photon energies lower than the band gap energy for the H-treated surface. One possibility is that the electrons of possible surface states may be excited to
incident electron energy (eV) ‘” 8 Fig. 2. Changes in the elastic (0) and secondary electron (0) peak intensities as a function of incident energy ranging from 1.2 to 10 eV. The specimen was (100) single crystal CVD diamond.
808
N. Eimori et cll.;Diunwnd und Related Mrrteriu1.r 4 ( lYY5j 806-808
and to a low incident beam current due to the prevention of charging. However, the secondary electron emission disappears at an incident energy of 3.5 eV. The dependence of secondary electron emission on incident energy has two gradients. One is at energy higher than 5.0 eV, and the other is energy lower than 5.0 eV. When the incident energy is higher than 5.0 eV, it is considered that electrons in the valence band or energy level located in the band gap are excited to the conduction band. and scattered to the surface and emitted to vacuum. However electron emission around a lower incident energy than 5.0 eV seems to originate from the energy level due to damage by H plasma without the diamond structure or boron doped in diamond, located in the band gap near the surface, because the incident electron energy lower than 5.0 eV corresponds to the energy level in the band gap of diamond and cannot excite the electrons in the valence band. We should note that the incident electron energy with almost constant beam current is affected by the work functions of both electron-emitting filament and analyzer. The incident electron energy E,, is expressed by
where V, and ~~/bias are the voltage biased to the filament and specimen, #f and 4, are the work functions of filament and analyzer. The work function of the electron-emitting filament tif is approximately 4.6 eV because it is made of W, while the work function of the analyzer $a is estimated to be 4.65 eV because the analyzer consists of Cu [lo]. Thus, the incident electron energy is approximated as V, - I/bias. Even if the absolute value of work function of the specimen cannot be discussed in more detail, the relationship between the conduction band minimum and vacuum level can be deduced from the changes of elastic and emission peak intensities. In addition, these results are the same as those for ( 111 )-oriented surface exposed to H plasma [ 31. Thus, it is concluded from the above that electrons excited to the conduction band can be emitted to vacuum. and
that the vacuum minimum.
level lies below the conduction
band
4. Conclusion We investigated the electron affinity of single-crystal CVD diamond surfaces. The photoyield from (lOO)oriented surface with H treatment starts from photon energies below the band gap energy (5.5 eV). This indicates that the electron affinity is negative for H-treated surfaces.
Acknowledgements The authors wish to thank H. Makita, T. Yara, H. Yagyu, H. Yagi, K. Nishimura, K. Mishuku and J. Tanaka for helpful discussions, and T. Okada, M. Shimizu, K. Bekku and K. Kadota for their assistance. This work was partly supported by the Joint Studies Program ( 1994-1995) of the Institute for Molecular Science and by a Grant-in-Aid for Scientific Research (05555087) from the Ministry of Education, Science and Culture of Japan.
References [II R.U. Martineli, Appl. Phys. Lat., 16 (1970) 261. Cl1 F.J. Himpsel, J.A. Knapp. J.A. Van Vechten and D.E. Eastman, Phys. Rec. B, 20 (1979) 624. 131 N. Eimori, Y. Mori, A. Hatta, T. Ito and A. Hirakl, Proc. Firsr ht. Symp. on Control of Semiconductor Inte$aces. Elsevier. Amsterdam, 1994, p. 149. 141 Y. Mori, N. Eimori, J.S. Ma. T. Ito and A. Hiraki. Appl. Surf: Sci.. 56 (1992) 89. A. Hatta, T. Ito and A. Hirakl, Proc. 151 H. Yagi. K. Nishimura, ht. Conf. on rhe New Diamond Scientc~ und Teclznolog~ 4. in press. C61 Y. Mori, N. Eimori, A. Hatta, T. Ito and A. Hiraki, Jpn. .I. Appl. Phys., 31 (1992) L1718. and K.V. Ravi, Appl. Phys. Lett., 35 (1989) 975. [71 M.I. Landstrass rs1 S. Albin, L. Watkins, IEEE Electron Deices Lert.. 11 (1990) 159. 191 T. Maki, S. Shikama, M. Komori, Y. Sakabuchi, K. Sakuta and T. Kobayashi, Jpn. J. Appl. Phn, 31 ( 1992) Ll446. to Solid State Physks, New York. 1986. llO1 C. Kittel. Introduction p. 537.
Diamond and Related Materials 4 (1995) 806-80X
Photoyield
measurements
of CVD diamond
N. Eimori, Y. Mori, A. Hatta, T. Ito, A. Hiraki Depurtment
ofElectrical
Engineering,
Osaka
UnicersitJ;
Suita,
Osaka
565,
Japan
Abstract We investigated the electron affinity of the (lOO)-oriented single-crystal chemical vapor deposited (CVD) diamond surfaces. The photoyield spectra of (100) surfaces exposed to hydrogen plasma start from 5.4 eV incident photon energy, which is lower than the band gap of diamond (5.5 eV), using UV synchrotron radiation. This shows that the vacuum level at the (100) surface exists below the conduction band of the bulk. Also, the electron affinity of the (100) surface is found to be negative from changes in elastic and secondary electron emission peak intensities. Keywords: Surface; Characterization;
Optoelectronic
properties
1. Introduction A cold cathode made of a material with negative electron affinity would have wide applications in flat displays, microvacuum tubes which operate at a very high speed or which can operate correctly in radiomatric circumstances, etc. In general, negative electron affinity cannot be obsereved for any clean surfaces of semiconductors with mid gap energy, but can be observed after suitable treatments, e.g. Cs+0 treated Si [ 11. These treated surface are chemically too sensitive and not very stable. Although the large photothreshold energy (Eth = 5.5 f 0.05 eV) of the diamond (111) surface precludes its use as a photocathode in the visible and near-UV light regions, it still has potentially useful photoemission properties such as high photoyield efficiency in the vacuum UV region. For natural diamond, it is known that the quantum yield (electrons per photon) of the (111) surface increases nearly linearly with photon energies ranging from the photothreshold (5.5 eV) to 9 eV (20%), and then increases to 40YP70% throughout the energy range 12 < hp< 35 eV [2]. However, for silicon surfaces chemisorbed with small amounts of Cs and 0, a quantum yield of only 6% can be obtained at energies of 1
spectra using UV synchrotron radiation lights [3]. The photoemission yield from the (111) surface with hydrogen treatment is found to be higher than that of the (11 1)-oriented surface with oxygen treatment. This can be explained in terms of an increase in work function for the O-adsorbed surface. In the case of polycrystalline CVD diamond, it turns out that the photoemission yield spectrum is similar in shape to that of the single-crystal ( 111) surface. In this study, we investigated the electron affinity of the (lOO)-oriented single-crystalline-CVD diamond surfaces using low energy electron and UV synchrotron-radiation lights.
2. Experimental Diamond films were synthesized using a microwave plasma CVD on diamond (100) substrates. The conditions of diamond growth are shown in Table 1. A dilute B,H, gas was used for boron doping in order to prevent the specimens from charging during electron spectroscopic measurements. H-terminated surfaces were obtained for specimens exposed to hydrogen plasma using the mjciowave after diamond growth (H treatment). The experimental conditions for H treatment are shown in Table 1. The surface exposed to hydrogen plasma has diamond structure, same as the as-grown surface, and there are no peaks originating from surface states in electron energy losses (EEL) spectra for the hydrogen-plasma treated surface, while the surface-state-induced peaks appeared in EEL
807
N. Eimori et al/Diamond and Related Materials 4 (1995) 806-808 Table 1 Experimental
conditions
of diamond
Diamond deposition Source gas (standard cm3 min-‘) Pressure (Torr) Temperature (“C) Microwave power (W) Deposition time (h) Substrate
H treatment Gas (standard cm3 min-‘) Pressure (Torr) Temperature (“C) Microwave power (W) Treatment time (min)
deposition
and H treatment
CO( 10) + H,(90) 40 930 240 6 High pressure synthesis diamond (100)
H, 30 930 240 3
spectra when the specimen was heated to 900 “C [4]. This means that the diamond surface with hydrogen plasma treatment is covered with hydrogen atoms, and most of the hydrogen atoms exist at the specimen surface according to elastic recoil detection analysis [ 51. The Resistivity of H-treated specimens is low [6-91, and it is confirmed that charging during electron spectroscopic measurement did not occur. Photoyield measurements were carried out at room temperature in lo-* Torr using synchrotron radiation from the 750 MeV storage ring of the Institute for Molecular Science. Specimens were biased to -240 V to collect emitted electrons. A 5 mm thick quartz window was used to eliminate higher-order lights of hy > 8 eV. Photoyield spectra were corrected by the relative beam intensity of incident photons including the effects of wavelength-sensitive grating used and time-dependent storage ring current. Low energy electron scattering (LEES) measurements were performed using a double-pass cylindrical mirror analyzer (CMA) in an ultra-high vacuum chamber [3]. The -acceleration voltage of the electron beam was 100 V, while the bias voltage of the specimen was varied in the range 9OGlOO V.
Photon
energy (eV)
Fig. 1. Photoyield spectra of (100) surfaces of CVD diamond. TW threshold of photoyield is 5.4 eV, which is lower than the indirect band gap of diamond (5.5 eV). This indicate that the vacuum level lies below the conduction band minimum at the (lOO)-oriented surface.
the conduction band. Such surface states, which are assumed to be located in the band gap near the valence band maximum, may be formed by exposure to H plasma [6,9]. It is reported that the hole concentration formed at the surface by exposure to hydrogen plasma is estimated to be 3.0 x 1018 cme3 [9]. The other possibility is that the electrons at the acceptor level originating from doped boron are excited to the conduction band and diffuse to the surface. Thus electron emission with sub-band-gap energy suggests that the vacuum level at the (lOO)-oriented surface is not located above the conduction band minimum. Intensities of the elastic peak and secondary emission peak in LEES spectra depend strongly on the incident electron energy for diamond specimens, as shown in Fig. 2. When the incident energy is decreased, the elastic peak intensity begins to increase around incident energy of 5.0 eV. This is due to the presence of the forbidden energy band. The slow increase in the elastic peak intensity with decreasing incident electron energy may be related to the broadening of incident electron energy
8001
1
-
3. Results and discussion Fig. 1 shows photoemission yields taken in the energy range 4.5 dhp<7 eV from a (100) single-crystal CVD diamond surface treated with hydrogen. In Fig. 1, the photoemission yield from the H-treated (100) surface appears just above an energy lower than the band gap energy of diamond. The photoemission peak around 5.35 eV was observed as shown in Fig. 1. Two possible reasons can be considered for the origin of electron emission at photon energies lower than the band gap energy for the H-treated surface. One possibility is that the electrons of possible surface states may be excited to
incident electron energy (eV) ‘” 8 Fig. 2. Changes in the elastic (0) and secondary electron (0) peak intensities as a function of incident energy ranging from 1.2 to 10 eV. The specimen was (100) single crystal CVD diamond.
808
N. Eimori et cll.;Diunwnd und Related Mrrteriu1.r 4 ( lYY5j 806-808
and to a low incident beam current due to the prevention of charging. However, the secondary electron emission disappears at an incident energy of 3.5 eV. The dependence of secondary electron emission on incident energy has two gradients. One is at energy higher than 5.0 eV, and the other is energy lower than 5.0 eV. When the incident energy is higher than 5.0 eV, it is considered that electrons in the valence band or energy level located in the band gap are excited to the conduction band. and scattered to the surface and emitted to vacuum. However electron emission around a lower incident energy than 5.0 eV seems to originate from the energy level due to damage by H plasma without the diamond structure or boron doped in diamond, located in the band gap near the surface, because the incident electron energy lower than 5.0 eV corresponds to the energy level in the band gap of diamond and cannot excite the electrons in the valence band. We should note that the incident electron energy with almost constant beam current is affected by the work functions of both electron-emitting filament and analyzer. The incident electron energy E,, is expressed by
where V, and ~~/bias are the voltage biased to the filament and specimen, #f and 4, are the work functions of filament and analyzer. The work function of the electron-emitting filament tif is approximately 4.6 eV because it is made of W, while the work function of the analyzer $a is estimated to be 4.65 eV because the analyzer consists of Cu [lo]. Thus, the incident electron energy is approximated as V, - I/bias. Even if the absolute value of work function of the specimen cannot be discussed in more detail, the relationship between the conduction band minimum and vacuum level can be deduced from the changes of elastic and emission peak intensities. In addition, these results are the same as those for ( 111 )-oriented surface exposed to H plasma [ 31. Thus, it is concluded from the above that electrons excited to the conduction band can be emitted to vacuum. and
that the vacuum minimum.
level lies below the conduction
band
4. Conclusion We investigated the electron affinity of single-crystal CVD diamond surfaces. The photoyield from (lOO)oriented surface with H treatment starts from photon energies below the band gap energy (5.5 eV). This indicates that the electron affinity is negative for H-treated surfaces.
Acknowledgements The authors wish to thank H. Makita, T. Yara, H. Yagyu, H. Yagi, K. Nishimura, K. Mishuku and J. Tanaka for helpful discussions, and T. Okada, M. Shimizu, K. Bekku and K. Kadota for their assistance. This work was partly supported by the Joint Studies Program ( 1994-1995) of the Institute for Molecular Science and by a Grant-in-Aid for Scientific Research (05555087) from the Ministry of Education, Science and Culture of Japan.
References [II R.U. Martineli, Appl. Phys. Lat., 16 (1970) 261. Cl1 F.J. Himpsel, J.A. Knapp. J.A. Van Vechten and D.E. Eastman, Phys. Rec. B, 20 (1979) 624. 131 N. Eimori, Y. Mori, A. Hatta, T. Ito and A. Hirakl, Proc. Firsr ht. Symp. on Control of Semiconductor Inte$aces. Elsevier. Amsterdam, 1994, p. 149. 141 Y. Mori, N. Eimori, J.S. Ma. T. Ito and A. Hiraki. Appl. Surf: Sci.. 56 (1992) 89. A. Hatta, T. Ito and A. Hirakl, Proc. 151 H. Yagi. K. Nishimura, ht. Conf. on rhe New Diamond Scientc~ und Teclznolog~ 4. in press. C61 Y. Mori, N. Eimori, A. Hatta, T. Ito and A. Hiraki, Jpn. .I. Appl. Phys., 31 (1992) L1718. and K.V. Ravi, Appl. Phys. Lett., 35 (1989) 975. [71 M.I. Landstrass rs1 S. Albin, L. Watkins, IEEE Electron Deices Lert.. 11 (1990) 159. 191 T. Maki, S. Shikama, M. Komori, Y. Sakabuchi, K. Sakuta and T. Kobayashi, Jpn. J. Appl. Phn, 31 ( 1992) Ll446. to Solid State Physks, New York. 1986. llO1 C. Kittel. Introduction p. 537.