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Total photoyield experiments on hydrogen terminated n-type diamond
Diamond & Related Materials 14 (2005) 2019 – 2022 www.elsevier.com/locate/diamond
Total photoyield experiments on hydrogen terminated n-type diamond D. Takeuchi *, S.-G. Ri, H. Kato, C.E. Nebel, S. Yamasaki Diamond Research Center, National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba, 305-8568 Japan Available online 11 October 2005
Abstract Total photoyield spectroscopy (TPYS) is applied to characterize the surface electronic properties of n-type chemical-vapour-deposited (CVD) (111) homoepitaxial diamond with hydrogen termination (H-termination). To discuss the negative electron affinity of n-type diamond surfaces in a general way, TPYS is also applied on p-type diamond such as a high-pressure and high-temperature synthetic (HPHT) IIb (001) and a homoepitaxially grown CVD (111) film with H-termination. In case of IIb sample, a temperature dependent experiment was also applied. The onset of electron emission at 4.4 eV on p-, and n-type diamond indicates that Fermi levels do not affect the sub-band spectra. The sub-band spectra in the energy regime 4.5 – 5.2 eV shows nearly no temperature dependence between RT and 300 -C, than the TPY spectra above 5.2 and below 4.4 eV. Obviously, direct excitation of valence-band electrons into the vacuum takes place in the close vicinity of the surface with temperature independent optical cross sections. It is interesting to note that the expected rise of photoyield due to NEA in the regime hr > 5.47 eV is missing in n-type diamond. We attribute this to a surface space charge region arising from ionized phosphorous atoms. D 2005 Elsevier B.V. All rights reserved. Keywords: NEA; n-type; CVD diamond; (111) surface
1. Introduction Based on unique electronic properties of diamond, it is currently investigated for a variety of electronic applications [1,2]. Moreover, hydrogen terminated (H-terminated) diamond is expected to be one of the most promising materials for cold cathode emitters as the electron emissivity should be high due to the negative electron affinity (NEA) [3]. Hydrogen terminated carbon at the surface of diamond forms a dipole due to the larger electron negativity of carbon compared to hydrogen. This is considered to be the origin of the NEA of hydrogen terminated diamond [4,5]. In case of oxidized diamond surface the electron affinity will enlarge due to negative dipole effect of oxygen termination. Diamond therefore is a material which allows to tune the electron affinity from negative to positive and vice versa. Chemical-vapour-deposition (CVD) of homoepitaxial diamond films became an established technique to grow highquality diamond with respect to optical and electronic properties [6,7]. One of the most remarkable progress with respect to field emitter applications of diamond in the last ten * Corresponding author. Tel.: +81 29 861 5634; fax: +81 29 861 2773. E-mail address: [email protected] (D. Takeuchi). 0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.08.061
years was the establishment of n-type doping using phosphorus [8– 10]. Therefore, n-type diamond seems to be the material of choice for field emitters. Unfortunately, the phosphorous donor level is 600 meV below the conduction-band which limits the electron density to a small number. The electron affinity is one of the most important parameters for the performance of electron emission devices, however, the reported values are scattering and do not cover all possible terminations and lattice orientations of diamond. While most of the data has been detected on boron-doped and intrinsic diamond, only recently a first report about electron affinities of n-type diamond, doped with phosphorus is available in the literature [11]. Generally total photoyield spectroscopy (TPYS) is applied to measure the electron affinity of solids. Here the quantum efficiency of photoelectron emission is measured as a function of photon energy [12]. TPYS is able to reveal Fpositive_ electron affinities (PEA) of materials by detecting the onset of photoelectron emission, taking into account the band-gap of semiconductors under investigations [13,14]. For p-type Hterminated diamond with negative electron affinity, a strong onset of electron emission (TPY) at a band-gap excitation energy (E g) of hr = 5.5 eV has been reported [15,16]. This feature is used as indication for negative electron affinity where
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D. Takeuchi et al. / Diamond & Related Materials 14 (2005) 2019 – 2022
the vacuum level is equal to the conduction band (CB) minimum or even lower. Our recently published results of total photoyield spectroscopy (TPYS) at room temperature indicate that n-type diamond with H-termination shows also a NEA. As a strong rise of electron emission for photons of hr E g is missing, we introduced an energy barrier inside of diamond as reason [11]. In this paper, we characterize a H-terminated n-type single crystal diamond film by TPYS and compare the results with a p-type CVD homoepitaxial single crystalline diamond film and with a type-IIb synthetic single crystalline diamond. We discuss our model and the threshold energy of 4.4 eV taking into account results deduced on different doping and or application of different temperatures on IIb diamond. 2. Experiment In TPYS the quantum efficiency of photoelectron emission is measured as a function of photon energy. A Xe arc lamp and a D2 lamp are used as light sources, which cover photon energies from 2 to 7.75 eV. To characterize the NEA on n-type diamond with H-termination by means of TPYS, we used a high-pressure and high-temperature (HPHT) synthetic type IIb diamond with (001) crystal orientation (Sumitomo Co. Ltd.), a B-doped p-type CVD film (111) (CVD-B), and a P-doped ntype CVD film (111) (CVD-P). All are single crystalline. The thickness of IIb is 300 Am, of CVD-B 0.75 Am, and of CVD-P 2 Am, respectively. The CVD films were grown on synthetic type Ib (111) diamonds, which had misorientation angles of 2- – 3-. The electronic properties (p-type for CVD-B and ntype for CVD-P) were detected by Hall-effect measurements at RT. The samples were cleaned by boiling in a mixture of H2SO4 and HNO3 at 230 -C to remove non-diamond carbon and to oxidize the surfaces. After that, samples were Hterminated in a microwave hydrogen plasma at 800 -C. For TPYS measurements the samples were mounted on the same molybdenum (Mo) holder with a tantalum (Ta) cover, and introduced into a UHV system with a base pressure of 8 10 9 Pa. TPYS was carried out at room temperature without application of a pre-thermal annealing to remove adsorbates. Only the IIb sample has successfully been measured at 300 -C, and then measured again at room temperature after subsequent anneal at 300 -C for 24 h. 3. Results and discussions To investigate the effect of adsorbates and possible bandbending due to charge transfer on photoelectron emission we discuss the results of TPYS as detected on the ‘‘not annealed’’ CVD-B and CVD-P (111) layers, and on the IIb crystal which has been annealed at 300 -C for 24 h. Fig. 1 shows TPY results at room temperature for ashydrogenated CVD-B, CVD-P, and the annealed IIb diamond. The insert is the enlarged figure of the TPY spectra of the regime hr 5.2 eV. The data were normalized using the TPYSamplitude detected at 5.46 eV, which is just below the fundamental absorption edge of diamond at room temperature
Fig. 1. TPY spectra of plasma H-terminated p-type CVD-B (111) and n-type CVD-P (111) measured at RT, and of p-type IIb (001) at RT after 300 -C anneal for one-day, respectively. The insert is the enlarged picture of the band-gap excitation region of Fig. 1. The data were normalized at 5.46 eV. Solid line and open circles represent data of CVD-B, dashed line and open triangles of CVDP, and dotted line and filled circles of IIb, respectively. All samples show a threshold at around 4.4 eV.
[1,15,16]. The result of IIb in Fig. 1 shows a strong rise and no oscillatory TPY spectrum in the regime hr 5.48 eV, which indicates that the adsorbates layer induced high surface conductivity was removed as discussed in Ref. [17]. It is known that a thermal annealing at 300 -C in UHV removes adsorbates from the surface, therefore the result on IIb diamond is attributed to an ‘‘adsorbate free’’ TPYS spectrum. It is interesting to note that the spectra in the sub-band-gap regime between 4.8 to 5.5 eV are not very different. However, around the onset regime a clear difference is detected. The annealed sample shows an onset at 4.2 eV and the adsorbatecovered films one at 4.4 eV [11]. We expect such a difference if a surface-hole accumulation layer is generated due to charge transfer. The difference in energies of 200 meV implies that the Fermi-level is about 200 meV deep in the valence band [5]. A change of occupation of surface defects can also generate such a threshold difference. Further experiments are required to confirm these results as these TPYS experiments have been performed also on diamond samples with different crystal orientation. In case of n-type CVD-P, a rise for hr > 5.5 eV could not been detected, although the thickness of 2 Am is larger than that
D. Takeuchi et al. / Diamond & Related Materials 14 (2005) 2019 – 2022
of CVD-B. As electrons are majority carriers in phosphorus doped diamond, the life time spof ffiffiffiffiffiffi electrons is large and therefore the diffusion length L ¼ Ds should be large enough to detect a rise for hr > 5.5 eV, where D is the diffusion coefficient of electrons. This is however not detected. As the surface is H-terminated, as detected by wetting angle experiments (95-), we assume that C – H dipoles generate a negative electron affinity. Our sub-band-gap TPY indicates indeed a negative electron affinity of 1.1 eV. The missing electron emission is attributed to an energy barrier in the vicinity of the surface inside of diamond. The energy barrier most likely arises by ionized phosphorus donors [11]. Fig. 2 shows a result of temperature dependent experiments on H-terminated IIb (001) sample. Fig. 3 shows the spectra of Fig. 2 normalized using the TPYS-signal detected at 5.15 eV, just below a remarkable rise at hr > 5.2 eV in case of TPY experiments at 300 -C. The inserts in Fig. 2 and Fig. 3 are the enlarged figures of the TPY spectra of the regime hr 5.3 and hr 5.0 eV, respectively. Firstly, the intensity of the TPY spectrum in the sub-band regime which has been measured at room temperature after anneal at 300 -C for 24 h is larger than that measured at 300 -C (see Fig. 2). The data at 300 -C was taken immediately after reaching 300 -C. The reason for the intensity change can be attributed to adsorbates (water layer), which are removed after annealing for 24 h, as discussed in Ref. [17].
Fig. 2. A result of temperature dependent experiments on H-terminated IIb (001) sample. The insert is the enlarged figure of the TPY spectra of the regime hr 5.3 eV. The shift of the onset at 5.48 eV detected at 300 -C and at 5.54 eV detected at room temperature are marked with two vertical lines in the inserted figure.
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Fig. 3. The normalized spectra of Fig. 2 using TPYS-signals detected at 5.15 eV, just below a remarkable rise at hr > 5.2 eV in case of TPY at 300 -C. The insert is the enlarged figure of the TPY spectra of the regime hr 5.0 eV. Shifts, the rise of photoelectron emission at 5.48 eV detected at 300 -C and at 5.54 eV detected at room temperature are marked with two vertical lines in the inserted figure.
Second, a large onset of electron emission at 5.54 eV at room temperature is detected at 300 -C at 5.48 eV, which is a shift for 60 meV. The variations are marked with two vertical lines in both inserts of Fig. 2 and Fig. 3. This shift is in very good agreement with the temperature dependent shift of the band-gap of diamond, which is 59.536 meV [18]. Details have to be discussed taking into account exciton-derived photoyield. The exciton-related photoexcitation (absorption) threshold is also temperature dependent due to band-gap variation, which therefore is no contradiction. Third, a new remarkable rise from about 5.2 eV appears at 300 -C. There are two possibilities to explain the rise, taking into account the band-gap energy of 5.42 eV at 300 -C. The threshold energy at 5.2 eV is close to the energy of excitonrelated photoexcitations with transverse optical phonons, of 140 – 160 meV energy. This results in 5.16 – 5.20 eV transition energy. In addition we have to take into account the binding energy of excitons, of 80 –100 meV, and the increased phonon density at 300 -C. Taking into account the curvature in the semi-logarithmic scale, the electron photoexcitation from the ionized boron acceptor level (VBM + 360 meV) can also be considered, which results in a threshold energy of 5.06 eV. According to the literature [17], the excitation from acceptors is the most promising explanation, however, further investigations are required to solve this problem.
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D. Takeuchi et al. / Diamond & Related Materials 14 (2005) 2019 – 2022
Fourth, both the normalized sub-band yield at 300 -C and at room temperature after anneal shown in Fig. 3 are comparable in the energy regime 4.5 – 5.15 eV. This is attributed to temperature independent optical cross section of transitions involved. Photoexcitation from surface defects to the vacuum level should show a temperature dependence based on the temperature induced change of occupation. This is not seen here. Therefore, this result strengthens our recent model of direct photoexcitation from the valence-band to the vacuum level in the close vicinity of the surface, due to the negative electron affinity with temperature independent optical cross sections [11]. Finally, a further shift to lower threshold energy in the TPY spectrum is observed at 300 -C compared to that measured at room temperature after annealing. As there maybe some defect states involved we need further experiments to elucidate the reason for the shifted threshold [17]. 4. Conclusions In summary, total photoyield spectroscopy (TPYS) was applied on hydrogen terminated p-, and n-type diamond samples. Below 5.5 eV, an unique threshold at 4.4 eV is observed from both p-type and n-type samples with adsorbates. After removing the adsorbates on p-type IIb crystal by 300 -C annealing, the threshold is shifted to 4.2 eV. In addition, the sub-band photoyield spectra of p-type IIb crystal are almost identical in the energy regime 4.5– 5.15 eV measured at room temperature and at 300 -C. Therefore adsorbates, which affect the surface conductivity, do not significantly change the optical cross-section of the sub-band yield. This feature is attributed to the direct photoexcitation of electrons from valence band states in or close to the surface to the vacuum level due to the negative electron affinity. In phosphorous doped diamond the bulk-derived photoyield rise in the regime hr > 5.5 eV is missing. The most plausible model is an energy barrier inside of diamond due to ionized phosphorus atoms in the vicinity of the negative electron affinity surface.
Acknowledgements The authors would like to thank Dr. Okushi (Diamond Research Center in AIST) for fruitful discussions about TPYS results. The authors also would like to thank Dr. Watanabe (Diamond Research Center in AIST) for H-termination experiments. The authors gratefully acknowledged financial support by the NEDO (ADD project) and the JST (CREST project). References [1] P. Bergonzo, R.B. Jackman, in: C.E. Nebel, J. Ristein (Eds.), ThinFilm Diamond II, Semiconductors and Semimetals, vol. 77, Elsevier, 2004, p. 197; H. Kawarada, in: E. Nebel, J. Ristein (Eds.), Thin-Film Diamond II, Semiconductors and Semimetals, vol. 77, Elsevier, 2004, p. 311. [2] E. Kohn, M. Adamschik, P. Schmid, A. Denisenko, A. Aleksov, W. Ebert, J. Phys., D, Appl. Phys. 34 (2001) R77. [3] F.J. Himpsel, J.A. Knapp, J.A. Van Vechten, D.E. Eastman, Phys. Rev., B 20 (1979) 624. [4] J.B. Cui, J. Ristein, L. Ley, Phys. Rev. Lett. 81 (1998) 429. [5] F. Maier, J. Ristein, L. Ley, Phys. Rev., B 64 (2001) 165411. [6] H. Okushi, H. Watanabe, G.S. Ri, S. Yamanaka, D. Takeuchi, J. Cryst. Growth 2237 (2002) 1269. [7] D. Takeuchi, S. Yamanaka, H. Watanabe, H. Okushi, Phys. Status Solidi, A Appl. Res. 186 (2001) 269. [8] S. Koizumi, M. Kamo, Y. Sato, H. Ozaki, T. Inuzuka, Appl. Phys. Lett. 71 (1997) 1065. [9] M. Nesla´dek, K. Haenen, J. D’Haen, S. Koizumi, H. Kanda, Phys. Status Solidi, A Appl. Res. 199 (2003) 77. [10] H. Kato, W. Futako, S. Yamasaki, H. Okushi, Diamond Relat. Mater. 13 (2004) 2117. [11] D. Takeuchi, H. Kato, G.S. Ri, T. Yamada, P.R. Vinod, D. Hwang, C.E. Nebel, H. Okushi, S. Yamasaki, Appl. Phys. Lett. 86 (2005) 152103. [12] J. Scha¨fer, J. Ristein, L. Ley, H. Ibach, Rev. Sci. Instrum. 64 (1993) 653. [13] E.O. Kane, Phys. Rev. 127 (1962) 131. [14] J.M. Ballantyne, Phys. Rev., B 6 (1972) 1436. [15] C. Bandis, B.B. Pate, Phys. Rev., B 52 (1995) 12056. [16] J. Ristein, W. Stein, L. Ley, Phys. Rev. Lett. 78 (1997) 1803. [17] D. Takeuchi, M. Riedel, J. Ristein, L. Ley, Phys. Rev., B 68 (2003) 41304(R). [18] Y.P. Varshni, Physica 34 (1967) 149.
Total photoyield experiments on hydrogen terminated n-type diamond D. Takeuchi *, S.-G. Ri, H. Kato, C.E. Nebel, S. Yamasaki Diamond Research Center, National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba, 305-8568 Japan Available online 11 October 2005
Abstract Total photoyield spectroscopy (TPYS) is applied to characterize the surface electronic properties of n-type chemical-vapour-deposited (CVD) (111) homoepitaxial diamond with hydrogen termination (H-termination). To discuss the negative electron affinity of n-type diamond surfaces in a general way, TPYS is also applied on p-type diamond such as a high-pressure and high-temperature synthetic (HPHT) IIb (001) and a homoepitaxially grown CVD (111) film with H-termination. In case of IIb sample, a temperature dependent experiment was also applied. The onset of electron emission at 4.4 eV on p-, and n-type diamond indicates that Fermi levels do not affect the sub-band spectra. The sub-band spectra in the energy regime 4.5 – 5.2 eV shows nearly no temperature dependence between RT and 300 -C, than the TPY spectra above 5.2 and below 4.4 eV. Obviously, direct excitation of valence-band electrons into the vacuum takes place in the close vicinity of the surface with temperature independent optical cross sections. It is interesting to note that the expected rise of photoyield due to NEA in the regime hr > 5.47 eV is missing in n-type diamond. We attribute this to a surface space charge region arising from ionized phosphorous atoms. D 2005 Elsevier B.V. All rights reserved. Keywords: NEA; n-type; CVD diamond; (111) surface
1. Introduction Based on unique electronic properties of diamond, it is currently investigated for a variety of electronic applications [1,2]. Moreover, hydrogen terminated (H-terminated) diamond is expected to be one of the most promising materials for cold cathode emitters as the electron emissivity should be high due to the negative electron affinity (NEA) [3]. Hydrogen terminated carbon at the surface of diamond forms a dipole due to the larger electron negativity of carbon compared to hydrogen. This is considered to be the origin of the NEA of hydrogen terminated diamond [4,5]. In case of oxidized diamond surface the electron affinity will enlarge due to negative dipole effect of oxygen termination. Diamond therefore is a material which allows to tune the electron affinity from negative to positive and vice versa. Chemical-vapour-deposition (CVD) of homoepitaxial diamond films became an established technique to grow highquality diamond with respect to optical and electronic properties [6,7]. One of the most remarkable progress with respect to field emitter applications of diamond in the last ten * Corresponding author. Tel.: +81 29 861 5634; fax: +81 29 861 2773. E-mail address: [email protected] (D. Takeuchi). 0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.08.061
years was the establishment of n-type doping using phosphorus [8– 10]. Therefore, n-type diamond seems to be the material of choice for field emitters. Unfortunately, the phosphorous donor level is 600 meV below the conduction-band which limits the electron density to a small number. The electron affinity is one of the most important parameters for the performance of electron emission devices, however, the reported values are scattering and do not cover all possible terminations and lattice orientations of diamond. While most of the data has been detected on boron-doped and intrinsic diamond, only recently a first report about electron affinities of n-type diamond, doped with phosphorus is available in the literature [11]. Generally total photoyield spectroscopy (TPYS) is applied to measure the electron affinity of solids. Here the quantum efficiency of photoelectron emission is measured as a function of photon energy [12]. TPYS is able to reveal Fpositive_ electron affinities (PEA) of materials by detecting the onset of photoelectron emission, taking into account the band-gap of semiconductors under investigations [13,14]. For p-type Hterminated diamond with negative electron affinity, a strong onset of electron emission (TPY) at a band-gap excitation energy (E g) of hr = 5.5 eV has been reported [15,16]. This feature is used as indication for negative electron affinity where
2020
D. Takeuchi et al. / Diamond & Related Materials 14 (2005) 2019 – 2022
the vacuum level is equal to the conduction band (CB) minimum or even lower. Our recently published results of total photoyield spectroscopy (TPYS) at room temperature indicate that n-type diamond with H-termination shows also a NEA. As a strong rise of electron emission for photons of hr E g is missing, we introduced an energy barrier inside of diamond as reason [11]. In this paper, we characterize a H-terminated n-type single crystal diamond film by TPYS and compare the results with a p-type CVD homoepitaxial single crystalline diamond film and with a type-IIb synthetic single crystalline diamond. We discuss our model and the threshold energy of 4.4 eV taking into account results deduced on different doping and or application of different temperatures on IIb diamond. 2. Experiment In TPYS the quantum efficiency of photoelectron emission is measured as a function of photon energy. A Xe arc lamp and a D2 lamp are used as light sources, which cover photon energies from 2 to 7.75 eV. To characterize the NEA on n-type diamond with H-termination by means of TPYS, we used a high-pressure and high-temperature (HPHT) synthetic type IIb diamond with (001) crystal orientation (Sumitomo Co. Ltd.), a B-doped p-type CVD film (111) (CVD-B), and a P-doped ntype CVD film (111) (CVD-P). All are single crystalline. The thickness of IIb is 300 Am, of CVD-B 0.75 Am, and of CVD-P 2 Am, respectively. The CVD films were grown on synthetic type Ib (111) diamonds, which had misorientation angles of 2- – 3-. The electronic properties (p-type for CVD-B and ntype for CVD-P) were detected by Hall-effect measurements at RT. The samples were cleaned by boiling in a mixture of H2SO4 and HNO3 at 230 -C to remove non-diamond carbon and to oxidize the surfaces. After that, samples were Hterminated in a microwave hydrogen plasma at 800 -C. For TPYS measurements the samples were mounted on the same molybdenum (Mo) holder with a tantalum (Ta) cover, and introduced into a UHV system with a base pressure of 8 10 9 Pa. TPYS was carried out at room temperature without application of a pre-thermal annealing to remove adsorbates. Only the IIb sample has successfully been measured at 300 -C, and then measured again at room temperature after subsequent anneal at 300 -C for 24 h. 3. Results and discussions To investigate the effect of adsorbates and possible bandbending due to charge transfer on photoelectron emission we discuss the results of TPYS as detected on the ‘‘not annealed’’ CVD-B and CVD-P (111) layers, and on the IIb crystal which has been annealed at 300 -C for 24 h. Fig. 1 shows TPY results at room temperature for ashydrogenated CVD-B, CVD-P, and the annealed IIb diamond. The insert is the enlarged figure of the TPY spectra of the regime hr 5.2 eV. The data were normalized using the TPYSamplitude detected at 5.46 eV, which is just below the fundamental absorption edge of diamond at room temperature
Fig. 1. TPY spectra of plasma H-terminated p-type CVD-B (111) and n-type CVD-P (111) measured at RT, and of p-type IIb (001) at RT after 300 -C anneal for one-day, respectively. The insert is the enlarged picture of the band-gap excitation region of Fig. 1. The data were normalized at 5.46 eV. Solid line and open circles represent data of CVD-B, dashed line and open triangles of CVDP, and dotted line and filled circles of IIb, respectively. All samples show a threshold at around 4.4 eV.
[1,15,16]. The result of IIb in Fig. 1 shows a strong rise and no oscillatory TPY spectrum in the regime hr 5.48 eV, which indicates that the adsorbates layer induced high surface conductivity was removed as discussed in Ref. [17]. It is known that a thermal annealing at 300 -C in UHV removes adsorbates from the surface, therefore the result on IIb diamond is attributed to an ‘‘adsorbate free’’ TPYS spectrum. It is interesting to note that the spectra in the sub-band-gap regime between 4.8 to 5.5 eV are not very different. However, around the onset regime a clear difference is detected. The annealed sample shows an onset at 4.2 eV and the adsorbatecovered films one at 4.4 eV [11]. We expect such a difference if a surface-hole accumulation layer is generated due to charge transfer. The difference in energies of 200 meV implies that the Fermi-level is about 200 meV deep in the valence band [5]. A change of occupation of surface defects can also generate such a threshold difference. Further experiments are required to confirm these results as these TPYS experiments have been performed also on diamond samples with different crystal orientation. In case of n-type CVD-P, a rise for hr > 5.5 eV could not been detected, although the thickness of 2 Am is larger than that
D. Takeuchi et al. / Diamond & Related Materials 14 (2005) 2019 – 2022
of CVD-B. As electrons are majority carriers in phosphorus doped diamond, the life time spof ffiffiffiffiffiffi electrons is large and therefore the diffusion length L ¼ Ds should be large enough to detect a rise for hr > 5.5 eV, where D is the diffusion coefficient of electrons. This is however not detected. As the surface is H-terminated, as detected by wetting angle experiments (95-), we assume that C – H dipoles generate a negative electron affinity. Our sub-band-gap TPY indicates indeed a negative electron affinity of 1.1 eV. The missing electron emission is attributed to an energy barrier in the vicinity of the surface inside of diamond. The energy barrier most likely arises by ionized phosphorus donors [11]. Fig. 2 shows a result of temperature dependent experiments on H-terminated IIb (001) sample. Fig. 3 shows the spectra of Fig. 2 normalized using the TPYS-signal detected at 5.15 eV, just below a remarkable rise at hr > 5.2 eV in case of TPY experiments at 300 -C. The inserts in Fig. 2 and Fig. 3 are the enlarged figures of the TPY spectra of the regime hr 5.3 and hr 5.0 eV, respectively. Firstly, the intensity of the TPY spectrum in the sub-band regime which has been measured at room temperature after anneal at 300 -C for 24 h is larger than that measured at 300 -C (see Fig. 2). The data at 300 -C was taken immediately after reaching 300 -C. The reason for the intensity change can be attributed to adsorbates (water layer), which are removed after annealing for 24 h, as discussed in Ref. [17].
Fig. 2. A result of temperature dependent experiments on H-terminated IIb (001) sample. The insert is the enlarged figure of the TPY spectra of the regime hr 5.3 eV. The shift of the onset at 5.48 eV detected at 300 -C and at 5.54 eV detected at room temperature are marked with two vertical lines in the inserted figure.
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Fig. 3. The normalized spectra of Fig. 2 using TPYS-signals detected at 5.15 eV, just below a remarkable rise at hr > 5.2 eV in case of TPY at 300 -C. The insert is the enlarged figure of the TPY spectra of the regime hr 5.0 eV. Shifts, the rise of photoelectron emission at 5.48 eV detected at 300 -C and at 5.54 eV detected at room temperature are marked with two vertical lines in the inserted figure.
Second, a large onset of electron emission at 5.54 eV at room temperature is detected at 300 -C at 5.48 eV, which is a shift for 60 meV. The variations are marked with two vertical lines in both inserts of Fig. 2 and Fig. 3. This shift is in very good agreement with the temperature dependent shift of the band-gap of diamond, which is 59.536 meV [18]. Details have to be discussed taking into account exciton-derived photoyield. The exciton-related photoexcitation (absorption) threshold is also temperature dependent due to band-gap variation, which therefore is no contradiction. Third, a new remarkable rise from about 5.2 eV appears at 300 -C. There are two possibilities to explain the rise, taking into account the band-gap energy of 5.42 eV at 300 -C. The threshold energy at 5.2 eV is close to the energy of excitonrelated photoexcitations with transverse optical phonons, of 140 – 160 meV energy. This results in 5.16 – 5.20 eV transition energy. In addition we have to take into account the binding energy of excitons, of 80 –100 meV, and the increased phonon density at 300 -C. Taking into account the curvature in the semi-logarithmic scale, the electron photoexcitation from the ionized boron acceptor level (VBM + 360 meV) can also be considered, which results in a threshold energy of 5.06 eV. According to the literature [17], the excitation from acceptors is the most promising explanation, however, further investigations are required to solve this problem.
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D. Takeuchi et al. / Diamond & Related Materials 14 (2005) 2019 – 2022
Fourth, both the normalized sub-band yield at 300 -C and at room temperature after anneal shown in Fig. 3 are comparable in the energy regime 4.5 – 5.15 eV. This is attributed to temperature independent optical cross section of transitions involved. Photoexcitation from surface defects to the vacuum level should show a temperature dependence based on the temperature induced change of occupation. This is not seen here. Therefore, this result strengthens our recent model of direct photoexcitation from the valence-band to the vacuum level in the close vicinity of the surface, due to the negative electron affinity with temperature independent optical cross sections [11]. Finally, a further shift to lower threshold energy in the TPY spectrum is observed at 300 -C compared to that measured at room temperature after annealing. As there maybe some defect states involved we need further experiments to elucidate the reason for the shifted threshold [17]. 4. Conclusions In summary, total photoyield spectroscopy (TPYS) was applied on hydrogen terminated p-, and n-type diamond samples. Below 5.5 eV, an unique threshold at 4.4 eV is observed from both p-type and n-type samples with adsorbates. After removing the adsorbates on p-type IIb crystal by 300 -C annealing, the threshold is shifted to 4.2 eV. In addition, the sub-band photoyield spectra of p-type IIb crystal are almost identical in the energy regime 4.5– 5.15 eV measured at room temperature and at 300 -C. Therefore adsorbates, which affect the surface conductivity, do not significantly change the optical cross-section of the sub-band yield. This feature is attributed to the direct photoexcitation of electrons from valence band states in or close to the surface to the vacuum level due to the negative electron affinity. In phosphorous doped diamond the bulk-derived photoyield rise in the regime hr > 5.5 eV is missing. The most plausible model is an energy barrier inside of diamond due to ionized phosphorus atoms in the vicinity of the negative electron affinity surface.
Acknowledgements The authors would like to thank Dr. Okushi (Diamond Research Center in AIST) for fruitful discussions about TPYS results. The authors also would like to thank Dr. Watanabe (Diamond Research Center in AIST) for H-termination experiments. The authors gratefully acknowledged financial support by the NEDO (ADD project) and the JST (CREST project). References [1] P. Bergonzo, R.B. Jackman, in: C.E. Nebel, J. Ristein (Eds.), ThinFilm Diamond II, Semiconductors and Semimetals, vol. 77, Elsevier, 2004, p. 197; H. Kawarada, in: E. Nebel, J. Ristein (Eds.), Thin-Film Diamond II, Semiconductors and Semimetals, vol. 77, Elsevier, 2004, p. 311. [2] E. Kohn, M. Adamschik, P. Schmid, A. Denisenko, A. Aleksov, W. Ebert, J. Phys., D, Appl. Phys. 34 (2001) R77. [3] F.J. Himpsel, J.A. Knapp, J.A. Van Vechten, D.E. Eastman, Phys. Rev., B 20 (1979) 624. [4] J.B. Cui, J. Ristein, L. Ley, Phys. Rev. Lett. 81 (1998) 429. [5] F. Maier, J. Ristein, L. Ley, Phys. Rev., B 64 (2001) 165411. [6] H. Okushi, H. Watanabe, G.S. Ri, S. Yamanaka, D. Takeuchi, J. Cryst. Growth 2237 (2002) 1269. [7] D. Takeuchi, S. Yamanaka, H. Watanabe, H. Okushi, Phys. Status Solidi, A Appl. Res. 186 (2001) 269. [8] S. Koizumi, M. Kamo, Y. Sato, H. Ozaki, T. Inuzuka, Appl. Phys. Lett. 71 (1997) 1065. [9] M. Nesla´dek, K. Haenen, J. D’Haen, S. Koizumi, H. Kanda, Phys. Status Solidi, A Appl. Res. 199 (2003) 77. [10] H. Kato, W. Futako, S. Yamasaki, H. Okushi, Diamond Relat. Mater. 13 (2004) 2117. [11] D. Takeuchi, H. Kato, G.S. Ri, T. Yamada, P.R. Vinod, D. Hwang, C.E. Nebel, H. Okushi, S. Yamasaki, Appl. Phys. Lett. 86 (2005) 152103. [12] J. Scha¨fer, J. Ristein, L. Ley, H. Ibach, Rev. Sci. Instrum. 64 (1993) 653. [13] E.O. Kane, Phys. Rev. 127 (1962) 131. [14] J.M. Ballantyne, Phys. Rev., B 6 (1972) 1436. [15] C. Bandis, B.B. Pate, Phys. Rev., B 52 (1995) 12056. [16] J. Ristein, W. Stein, L. Ley, Phys. Rev. Lett. 78 (1997) 1803. [17] D. Takeuchi, M. Riedel, J. Ristein, L. Ley, Phys. Rev., B 68 (2003) 41304(R). [18] Y.P. Varshni, Physica 34 (1967) 149.