- Email: [email protected]
Photoemission properties and surface structures of homoepitaxially grown CVD diamond(1 0 0) surfaces
Applied Surface Science 175±176 (2001) 474±479
Photoemission properties and surface structures of homoepitaxially grown CVD diamond(1 0 0) surfaces H. Murakami, M. Yokoyama, S.M. Lee, T. Ito* Department of Electrical Engineering, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan Accepted 15 November 2000
Abstract In the present study, the surface properties of homoepitaxially grown chemical vapor deposition (CVD) diamond have been investigated using total photoyield measurements, scanning tunneling microscope (STM) observations and scanning tunneling spectroscopy (STS). The STM results show that as-grown CVD diamond(1 0 0) surfaces are characterized mainly by two types of regions with 2 1/1 2 structures and much less structures. These well-ordered surface structures can be signi®cantly clearer after a low-temperature annealing at 150±2008C in ultra high vacuum. On the other hand, the STS results indicate the presence of band gap states, which agrees well with the fact that the observed photoemission threshold energy is located at 5 eV, well below the band gap energy of 5.5 eV, for the as-grown CVD diamond surface. The absolute value measurements of the photoemission total yields using a calibrated Si photodetector demonstrate substantially low photoemission ef®ciencies even for negative electron af®nity (NEA) diamond surfaces, being consistent with the STM/STS data. # 2001 Elsevier Science B.V. All rights reserved. Keywords: CVD diamond; Diamond(1 0 0) surface; Scanning tunneling microscope; Scanning tunneling spectroscopy; Quantum ef®ciency; Total photoelectron yield
1. Introduction In the case of diamond synthesized by means of chemical vapor deposition (CVD) methods, hydrogen atoms play important roles on the stability and electron af®nity of CVD diamond surfaces. The latter issue is often studied using ultraviolet photoelectron spectroscopy [1±6] or secondary electron spectroscopy [7±9]. At a negative electron af®nity (NEA) surface, electrons excited into the diamond conduction band can be emitted easily into vacuum without any energy barrier, and a sharp peak appears at low * Corresponding author. Fax: 81-6-879-7704. E-mail address: [email protected] (T. Ito).
kinetic energies in photoelectron spectra or secondary electron spectra. These remarkable features can make CVD diamond ®lms good candidates for new applications such as ultraviolet photon detectors and cold cathodes [10±12]. The origin of the diamond NEA is considered to be related to the surface dipole layer formed between surface C atoms and chemisorbed H atoms, which can lower the diamond work function [1±6,13±15]. On the other hand, the photoyield threshold energies observed for as-grown CVD diamond are well known to be always located around 5 eV, which is well below the energy gap of diamond (5.5 eV) [16]. This strongly suggests the presence of the gap states at the diamond surface or subsurface.
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 1 2 4 - 6
H. Murakami et al. / Applied Surface Science 175±176 (2001) 474±479
On the other hand, the atomic and electronic structures of H-terminated diamond(1 0 0) surfaces have been investigated using electron diffraction techniques [17], energy loss spectroscopy [18] and scanning tunneling microscope (STM) techniques [19,20]. Those studies mainly concentrate on natural singlecrystalline diamond. However, details of the atomic and electronic structures of as-grown CVD diamond(1 0 0) surfaces have seldom been reported so far. In this report we have investigated the relation between the atomic structure and the electronic states at the diamond(1 0 0) surface using a STM and scanning tunneling spectroscopy (STS). Furthermore, the absolute values of the total photoyield have been measured in order to discuss the electronic states in the subsurface region of (1 0 0) CVD diamond homoepitaxially grown. These results suggest that the gap states are present on or near the surface of as-grown diamond specimens with an NEA surface. 2. Experimental The substrates used in this study were (1 0 0)oriented diamond synthesized by a so-called high temperature and high pressure (HTHP) technique. Their size was 4 mm 4 mm 0:5 mm. Diamond ®lms were deposited using a conventional quartztube-type microwave±plasma±CVD system. In order to prevent the charging effect from total photoelectron yield measurements and STM/STS measurements, boron-doped (B-doped) (p-type) ®lms were employed in this study. The deposition conditions used for the Bdoped diamond growth are listed in Table 1 [16]. For STM/STS measurements, an UHV-compatible STM apparatus was used. A Si photodetector carefully calibrated for incident photons with known intensity
475
was used to measure absolute quantum ef®ciencies of the photoelectron yield [21]. 3. Results and discussion 3.1. Total photoyield measurements Fig. 1 shows typical photoyield data as a function of incident photon energy, which were taken for an asgrown (a) and a partially oxidized (b) CVD diamond(1 0 0) surface. The vertical scale denotes the calibrated quantum ef®ciency. The observed absolute ef®ciencies of the 10 3 order at the low photon energies are roughly two-order smaller than the values reported for a natural diamond(1 1 1) surface. This fact may be related to either an upward band-bending effect or possible presence of considerable amounts of energy gap states. Since cathodoluminescence spectra taken from B-doped specimens are characterized by band-A emission, a substantial amount of gap states is concluded to exist in B-doped single-crystalline specimens. Therefore, the gap states are considered as a possible main reason for the poor quantum ef®ciencies observed. This consideration is consistent with the fact that our quantum ef®ciencies are still one-order larger than those reported for B-doped polycrystalline CVD diamond [21] since the crystalline quality of
Table 1 Deposition conditions B-doped diamond Source gases Reaction pressure Microwave power Substrate temperature Thickness
B/C 100 ppm H2: 85 sccm, CO: 10 sccm, B2H6 (100 ppm in H2): 5 sccm 5 kPa 300 W 880±9008C 0.6 mm
Fig. 1. Photoemission quantum ef®ciency, YQE, as a function of incident photon energy, Eph, for (a) an as-grown diamond(1 0 0) specimen and (b) a partially-oxidized diamond(1 0 0) specimen. For the latter, the vertical scale was ®ve times enlarged since the ef®ciencies obtained were considerably smaller than the former. The inset shows plots of sin Y QE Y QE 1=2 vs. Eph to more clearly estimate the threshold energies.
476
H. Murakami et al. / Applied Surface Science 175±176 (2001) 474±479
polycrystalline CVD diamonds is substantially poorer than that of single-crystalline CVD diamond. In order to analyze the quantum ef®ciencies obtained in more details, the three-step model may be employed for the photoelectron emission process concerned [21,22]. In this case, the quantum ef®ciency YQE may be represented as a function of the incident photon energy Eph by the following equation. YQE
1
R Eph B Eph aph Eph L Eph f1 aph Eph L Eph g
(1)
Here, R(Eph) is the optical re¯ection coef®cient, aph(Eph) the absorption coef®cient for electronic transition above the vacuum level and L(Eph) the escape depth of the photoelectron concerned. B(Eph) usually ranges from a few tenths and unity. Using Eq. (1) with R 0, B 0:1 and aph 104 cm 1 [21], one can obtain L 10 nm for Y QE 10 3 at Eph above and near the threshold energy of 5 eV. The deduced order of L values satis®es the relation that L ! 1=aph , meaning that in the indirect transition Eph region, L(Eph) is much smaller than the absorption depth of the incident light, 1/aph. Therefore, it is concluded that a substantial amount of gap states exist on the specimen surface and in the subsurface to signi®cantly kill photoexcited electrons. This is consistent with the fact that the threshold energy is located around 5 eV, which is well below the band gap energy of 5.5 eV (see Fig. 1) [16]. On the other hand, after partial oxidation the photoyield curves of B-doped specimens shift to the higher energy side by some energy dependent on the amount of oxygen atoms on the specimen surfaces, indicating that the electron af®nity changes from negative to positive values after the oxidation. The details of the dependence of the threshold energy on the amount of surface oxygen atoms have been reported elsewhere [16]. The observed YQE values are still considerably low, meaning that the escape depth L are substantially short, or that the bulk properties in the subsurface region substantially limit the escape depth of the photoexcited electrons.
Fig. 2. Typical STM image observed from an as-grown diamond(1 0 0) specimen. The surface is characterized by two types of regions with different structures, typically shown as A (wellordered) and B (structureless). The image was taken with a sample voltage of 2 V and a sample current of 1.3 nA for a scanning area of 580 nm 580 nm.
large area clear dimer rows related to the 2 1 superstructure [19,23] while in the other one denoted by B such clear structure cannot be observed. In order to observe them in more details, further enlarged images were taken. A typical image obtained for the latter region B is demonstrated in Fig. 3. One can recognize
3.2. STM measurements Fig. 2 shows a typical STM image taken from an asgrown diamond(1 0 0) surface. One can clearly recognize two types of surface regions with different structures. In the one region denoted by A, there are in a
Fig. 3. Typical STM image taken for the region B shown in Fig. 2. This image also contains well-ordered regions and much lessordered regions in a scanned area of 9:5 nm 9:5 nm. The ordered patterns correlate to 2 1 or 1 2 dimer rows with characteristic distances of 0.25 and 0.50 nm.
H. Murakami et al. / Applied Surface Science 175±176 (2001) 474±479
477
Fig. 4. STM image observed for B-doped homoepitaxial diamond(1 0 0) after annealing at 150±2008C for 30 min in a UHV chamber. The scanning area was 8 nm 8 nm. The image of a smaller size in the right is enlarged one for the lower-left corner region. In this case the row separation is only 0.25 nm, which is one-half of that of the dimer atomic rows.
two types of features in the region B: the one is the substantial structure related to 2 1 and 1 2 dimer rows (the distance between the adjacent rows is 0.5 nm) while no obvious surface structure is observed in the regions between them. In such structureless regions, the observed data were not very stable nor reproducible during successive STM observations. When the as-grown specimen was heated in a UHV chamber at a temperature of 150±2008C, the atomic structure observed became signi®cantly clearer. Almost all the images obtained are characterized by either 2 1 or 1 2 dimer rows with a row separation of 0.50 nm although there still exists a different wellordered structure with a row separation of 0.25 nm in limited regions near the boundaries between the two types of the dimer rows described above, as shown in Fig. 4. The details of the new structure are unknown although a possible structure model is proposed in such limited narrow areas for a hydrogen-induced 1 1 structure with H-terminated dangling bonds and without paired dangling bonds [24]. This may be consistent with the amount of H atoms (about two C(1 0 0) monolayers) near the top surface of diamond(1 0 0) layer epitaxially grown [25]. Fig. 5 shows
the STM image observed after further UHV annealing at a much higher temperature of 600±7008C. This annealing brought substantial reduction of the structureless regions as shown in Fig. 3, without any substantial change in the surface structure with the
Fig. 5. Enlarged STM image observed for homoepitaxial diamond(1 0 0) after 30 min UHV annealing at 600±7008C. Vacancy type defects appeared between 2 1 or 1 2 dimer rows as the trace of the structureless regions.
478
H. Murakami et al. / Applied Surface Science 175±176 (2001) 474±479
discussion cannot be performed on the STS spectra under the present precision of the STS data. 4. Conclusions
Fig. 6. STS spectra observed for region A and B of B-doped specimens (denoted in Fig. 2). In (a) sample currents I were shown while in (b) dI/dV data were shown as a function of sample voltage V. The tip conditions employed were the sample voltage of 3 V and the sample current of 0.05 nA.
2 1 rows. It is interesting that many vacancy type defects appeared between 2 1 or 1 2 dimer rows as the trace of the structureless regions, as shown here. 3.3. STS measurements For the as-grown specimen, STS spectra were measured, and I±V curves obtained as well as their derivatives were typically shown in Fig. 6. The reasonably steep rises of the tunnel current were observed below 1 V and near 3.5 V in the region B (structureless) while those were observed near 3 V and below 4 V in the region A (well-ordered). Here, a positive bias corresponds to an energy state above the Fermi level of the specimen. Since the derivatives of the tunneling current dI/dV are more directly related to the densityof-states of the system concerned than the tunneling current I, the derivatives may be helpful to discuss the electronic states. Therefore, the data taken from the region B indicate the presence of the gap states or a reduced energy gap in the case of the as-grown specimens since the potential window is well smaller than the energy gap of 5.5 eV. This is consistent with the total photoyield data discussed above. On the other hand, the data obtained for the well-ordered structure regions may suggest possible band-bending in the top surface region of the specimen since the apparent potential window in this vertical scale of the ®gure is larger than the energy gap while weak current rises were observed near 2 and 3.5 V. At present, however, more detailed
The electronic and atomic structures of B-doped ptype diamond(1 0 0) surfaces epitaxially grown on HTHP substrates have been investigated using total photoyield and STM/STS measurements. (1) On one hand, relatively low quantum ef®ciency of the photoemission process is recognized in the case of the NEA while the photoemission threshold energy is well below the energy gap for as-grown specimens. These indicate the presence of a substantial amount of gap states in the top surface and subsurface regions. (2) On the other hand, STM measurements reveals the presence of well-ordered 2 1 superstructures with a row separation of 0.50 nm even for the as-grown specimen. This well corresponds to those reported for the Hterminated C(1 0 0) surface. After UHV vacuum annealing at 150±2008C, the superstructure images became clearer with appearance of well-order structure with a row separation of 0.25 nm, which is an half of that of the 2 1 dimer rows observed. (3) Furthermore, STS data demonstrate that a considerable amount of gap states exist on the top surface and in the subsurface region, or that the energy gap is slightly reduced in that region. Acknowledgements This work was supported by the ``Research for the Future'' Program (No. 96R15401) from the Japan Society for the Promotion of Science. The authors would like to thank Profs. A. Hiraki and A. Hatta of Koch University of Technology for their helpful discussion. References [1] B.B. Pate, in: L.S. Pan, D.R. Kania (Eds.), Diamond: Electronic Properties and Applications, Kluwer Academic Publishers, Boston, USA, 1994, pp. 31. [2] N. Eimori, Y. Mori, A. Hatta, T. Ito, A. Hiraki, Jpn. J. Appl. Phys. 33 (1994) 6312. [3] N. Eimori, Y. Mori, A. Hatta, T. Ito, A. Hiraki, Diam. Relat. Mater. 4 (1995) 806.
H. Murakami et al. / Applied Surface Science 175±176 (2001) 474±479 [4] F.J. Himpel, J.A. Knapp, J.A. van Vechten, D.E. Eastman, Phys. Rev. 20 (1979) 624. [5] J. van der Weide, Z. Zhang, P.K. Baumann, M.G. Wensell, J. Bernholc, R.J. Nemanich, Phys. Rev. B 50 (1994) 5803. [6] C. Bandis, B.B. Pate, Phys. Rev. B 52 (1995) 12056. [7] I.L. Krainsky, V.M. Asnin, G.T. Mearini, J.A. Dayton Jr., Phys. Rev. B 53 (1996) R7650. [8] I.L. Krainsky, V.M. Asnin, J.A. Dayron Jr., Appl. Surf. Sci. 111 (1997) 265. [9] A. Shih, J. Yater, P. Pehrsson, J. Bulter, C. Hor, R. Abrams, J. Appl. Phys. 82 (1997) 1860. [10] M.W. Geis, J.C. Twichell, J. Macaulay, K. Okano, Appl. Phys. Lett. 67 (1995) 9. [11] A. Hatta, K. Ogawa, N. Eimori, M. Deguchi, M. Kitabatake, T. Ito, A. Hiraki, Appl. Surf. Sci. 117/118 (1997) 592. [12] T. Ito, M. Nishimura, A. Hatta, Appl. Phys. Lett. 73 (1998) 3739. [13] P.K. Baumann, R.J. Nemanich, J. Appl. Phys. 83 (1998) 2072, and references therein.
[14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
479
J. Robertson, Diam. Relat. Mater. 7 (1998) 620. J. Ristein, W. Stein, L. Ley, Diam. Relat. Mater. 7 (1998) 626. M. Yokoyama, T. Ito, Appl. Surf. Sci. 162/163 (2000) 457. H. Shiomi, K. Tanabe, Y. Nishibayashi, N. Fujimori, Jpn. J. Appl. Phys. 29 (1990) 3. T. Aizawa, T. Ando, M. Kamo, Y. Sato, Phys. Rev. B48 (1993) 18348. T. Tsuno, T. Imai, Y. Nishibayashi, K. Hamada, N. Fujimori, Jpn. J. Appl. Phys. 30 (1991) 1063. H. Kawarada, H. Sasaki, A. Sato, Phys. Rev. B52 (1995) 11351. A. Laikhtman, A. Hoffman, R. Kalish, Y. Avigal, A. Breskin, R. Chechik, E. Shefer, Y. Lifshitz, Appl. Phys. Lett. 73 (1998) 1433. R.U. Martinelli, D.G. Fisher, Proc. IEEE 62 (1974) 1339. A.V. Hamza, D.G. Kubiak, R.H. Stulen, Surf. Sci. 237 (1990) 35. S.H. Yang, D.A. Drabold, J.B. Adams, Phys. Rev. B48 (1993) 5261. H. Yagi, A. Hatta, T. Ito, Appl. Surf. Sci. 137 (1999) 50.
Photoemission properties and surface structures of homoepitaxially grown CVD diamond(1 0 0) surfaces H. Murakami, M. Yokoyama, S.M. Lee, T. Ito* Department of Electrical Engineering, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan Accepted 15 November 2000
Abstract In the present study, the surface properties of homoepitaxially grown chemical vapor deposition (CVD) diamond have been investigated using total photoyield measurements, scanning tunneling microscope (STM) observations and scanning tunneling spectroscopy (STS). The STM results show that as-grown CVD diamond(1 0 0) surfaces are characterized mainly by two types of regions with 2 1/1 2 structures and much less structures. These well-ordered surface structures can be signi®cantly clearer after a low-temperature annealing at 150±2008C in ultra high vacuum. On the other hand, the STS results indicate the presence of band gap states, which agrees well with the fact that the observed photoemission threshold energy is located at 5 eV, well below the band gap energy of 5.5 eV, for the as-grown CVD diamond surface. The absolute value measurements of the photoemission total yields using a calibrated Si photodetector demonstrate substantially low photoemission ef®ciencies even for negative electron af®nity (NEA) diamond surfaces, being consistent with the STM/STS data. # 2001 Elsevier Science B.V. All rights reserved. Keywords: CVD diamond; Diamond(1 0 0) surface; Scanning tunneling microscope; Scanning tunneling spectroscopy; Quantum ef®ciency; Total photoelectron yield
1. Introduction In the case of diamond synthesized by means of chemical vapor deposition (CVD) methods, hydrogen atoms play important roles on the stability and electron af®nity of CVD diamond surfaces. The latter issue is often studied using ultraviolet photoelectron spectroscopy [1±6] or secondary electron spectroscopy [7±9]. At a negative electron af®nity (NEA) surface, electrons excited into the diamond conduction band can be emitted easily into vacuum without any energy barrier, and a sharp peak appears at low * Corresponding author. Fax: 81-6-879-7704. E-mail address: [email protected] (T. Ito).
kinetic energies in photoelectron spectra or secondary electron spectra. These remarkable features can make CVD diamond ®lms good candidates for new applications such as ultraviolet photon detectors and cold cathodes [10±12]. The origin of the diamond NEA is considered to be related to the surface dipole layer formed between surface C atoms and chemisorbed H atoms, which can lower the diamond work function [1±6,13±15]. On the other hand, the photoyield threshold energies observed for as-grown CVD diamond are well known to be always located around 5 eV, which is well below the energy gap of diamond (5.5 eV) [16]. This strongly suggests the presence of the gap states at the diamond surface or subsurface.
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 1 2 4 - 6
H. Murakami et al. / Applied Surface Science 175±176 (2001) 474±479
On the other hand, the atomic and electronic structures of H-terminated diamond(1 0 0) surfaces have been investigated using electron diffraction techniques [17], energy loss spectroscopy [18] and scanning tunneling microscope (STM) techniques [19,20]. Those studies mainly concentrate on natural singlecrystalline diamond. However, details of the atomic and electronic structures of as-grown CVD diamond(1 0 0) surfaces have seldom been reported so far. In this report we have investigated the relation between the atomic structure and the electronic states at the diamond(1 0 0) surface using a STM and scanning tunneling spectroscopy (STS). Furthermore, the absolute values of the total photoyield have been measured in order to discuss the electronic states in the subsurface region of (1 0 0) CVD diamond homoepitaxially grown. These results suggest that the gap states are present on or near the surface of as-grown diamond specimens with an NEA surface. 2. Experimental The substrates used in this study were (1 0 0)oriented diamond synthesized by a so-called high temperature and high pressure (HTHP) technique. Their size was 4 mm 4 mm 0:5 mm. Diamond ®lms were deposited using a conventional quartztube-type microwave±plasma±CVD system. In order to prevent the charging effect from total photoelectron yield measurements and STM/STS measurements, boron-doped (B-doped) (p-type) ®lms were employed in this study. The deposition conditions used for the Bdoped diamond growth are listed in Table 1 [16]. For STM/STS measurements, an UHV-compatible STM apparatus was used. A Si photodetector carefully calibrated for incident photons with known intensity
475
was used to measure absolute quantum ef®ciencies of the photoelectron yield [21]. 3. Results and discussion 3.1. Total photoyield measurements Fig. 1 shows typical photoyield data as a function of incident photon energy, which were taken for an asgrown (a) and a partially oxidized (b) CVD diamond(1 0 0) surface. The vertical scale denotes the calibrated quantum ef®ciency. The observed absolute ef®ciencies of the 10 3 order at the low photon energies are roughly two-order smaller than the values reported for a natural diamond(1 1 1) surface. This fact may be related to either an upward band-bending effect or possible presence of considerable amounts of energy gap states. Since cathodoluminescence spectra taken from B-doped specimens are characterized by band-A emission, a substantial amount of gap states is concluded to exist in B-doped single-crystalline specimens. Therefore, the gap states are considered as a possible main reason for the poor quantum ef®ciencies observed. This consideration is consistent with the fact that our quantum ef®ciencies are still one-order larger than those reported for B-doped polycrystalline CVD diamond [21] since the crystalline quality of
Table 1 Deposition conditions B-doped diamond Source gases Reaction pressure Microwave power Substrate temperature Thickness
B/C 100 ppm H2: 85 sccm, CO: 10 sccm, B2H6 (100 ppm in H2): 5 sccm 5 kPa 300 W 880±9008C 0.6 mm
Fig. 1. Photoemission quantum ef®ciency, YQE, as a function of incident photon energy, Eph, for (a) an as-grown diamond(1 0 0) specimen and (b) a partially-oxidized diamond(1 0 0) specimen. For the latter, the vertical scale was ®ve times enlarged since the ef®ciencies obtained were considerably smaller than the former. The inset shows plots of sin Y QE Y QE 1=2 vs. Eph to more clearly estimate the threshold energies.
476
H. Murakami et al. / Applied Surface Science 175±176 (2001) 474±479
polycrystalline CVD diamonds is substantially poorer than that of single-crystalline CVD diamond. In order to analyze the quantum ef®ciencies obtained in more details, the three-step model may be employed for the photoelectron emission process concerned [21,22]. In this case, the quantum ef®ciency YQE may be represented as a function of the incident photon energy Eph by the following equation. YQE
1
R Eph B Eph aph Eph L Eph f1 aph Eph L Eph g
(1)
Here, R(Eph) is the optical re¯ection coef®cient, aph(Eph) the absorption coef®cient for electronic transition above the vacuum level and L(Eph) the escape depth of the photoelectron concerned. B(Eph) usually ranges from a few tenths and unity. Using Eq. (1) with R 0, B 0:1 and aph 104 cm 1 [21], one can obtain L 10 nm for Y QE 10 3 at Eph above and near the threshold energy of 5 eV. The deduced order of L values satis®es the relation that L ! 1=aph , meaning that in the indirect transition Eph region, L(Eph) is much smaller than the absorption depth of the incident light, 1/aph. Therefore, it is concluded that a substantial amount of gap states exist on the specimen surface and in the subsurface to signi®cantly kill photoexcited electrons. This is consistent with the fact that the threshold energy is located around 5 eV, which is well below the band gap energy of 5.5 eV (see Fig. 1) [16]. On the other hand, after partial oxidation the photoyield curves of B-doped specimens shift to the higher energy side by some energy dependent on the amount of oxygen atoms on the specimen surfaces, indicating that the electron af®nity changes from negative to positive values after the oxidation. The details of the dependence of the threshold energy on the amount of surface oxygen atoms have been reported elsewhere [16]. The observed YQE values are still considerably low, meaning that the escape depth L are substantially short, or that the bulk properties in the subsurface region substantially limit the escape depth of the photoexcited electrons.
Fig. 2. Typical STM image observed from an as-grown diamond(1 0 0) specimen. The surface is characterized by two types of regions with different structures, typically shown as A (wellordered) and B (structureless). The image was taken with a sample voltage of 2 V and a sample current of 1.3 nA for a scanning area of 580 nm 580 nm.
large area clear dimer rows related to the 2 1 superstructure [19,23] while in the other one denoted by B such clear structure cannot be observed. In order to observe them in more details, further enlarged images were taken. A typical image obtained for the latter region B is demonstrated in Fig. 3. One can recognize
3.2. STM measurements Fig. 2 shows a typical STM image taken from an asgrown diamond(1 0 0) surface. One can clearly recognize two types of surface regions with different structures. In the one region denoted by A, there are in a
Fig. 3. Typical STM image taken for the region B shown in Fig. 2. This image also contains well-ordered regions and much lessordered regions in a scanned area of 9:5 nm 9:5 nm. The ordered patterns correlate to 2 1 or 1 2 dimer rows with characteristic distances of 0.25 and 0.50 nm.
H. Murakami et al. / Applied Surface Science 175±176 (2001) 474±479
477
Fig. 4. STM image observed for B-doped homoepitaxial diamond(1 0 0) after annealing at 150±2008C for 30 min in a UHV chamber. The scanning area was 8 nm 8 nm. The image of a smaller size in the right is enlarged one for the lower-left corner region. In this case the row separation is only 0.25 nm, which is one-half of that of the dimer atomic rows.
two types of features in the region B: the one is the substantial structure related to 2 1 and 1 2 dimer rows (the distance between the adjacent rows is 0.5 nm) while no obvious surface structure is observed in the regions between them. In such structureless regions, the observed data were not very stable nor reproducible during successive STM observations. When the as-grown specimen was heated in a UHV chamber at a temperature of 150±2008C, the atomic structure observed became signi®cantly clearer. Almost all the images obtained are characterized by either 2 1 or 1 2 dimer rows with a row separation of 0.50 nm although there still exists a different wellordered structure with a row separation of 0.25 nm in limited regions near the boundaries between the two types of the dimer rows described above, as shown in Fig. 4. The details of the new structure are unknown although a possible structure model is proposed in such limited narrow areas for a hydrogen-induced 1 1 structure with H-terminated dangling bonds and without paired dangling bonds [24]. This may be consistent with the amount of H atoms (about two C(1 0 0) monolayers) near the top surface of diamond(1 0 0) layer epitaxially grown [25]. Fig. 5 shows
the STM image observed after further UHV annealing at a much higher temperature of 600±7008C. This annealing brought substantial reduction of the structureless regions as shown in Fig. 3, without any substantial change in the surface structure with the
Fig. 5. Enlarged STM image observed for homoepitaxial diamond(1 0 0) after 30 min UHV annealing at 600±7008C. Vacancy type defects appeared between 2 1 or 1 2 dimer rows as the trace of the structureless regions.
478
H. Murakami et al. / Applied Surface Science 175±176 (2001) 474±479
discussion cannot be performed on the STS spectra under the present precision of the STS data. 4. Conclusions
Fig. 6. STS spectra observed for region A and B of B-doped specimens (denoted in Fig. 2). In (a) sample currents I were shown while in (b) dI/dV data were shown as a function of sample voltage V. The tip conditions employed were the sample voltage of 3 V and the sample current of 0.05 nA.
2 1 rows. It is interesting that many vacancy type defects appeared between 2 1 or 1 2 dimer rows as the trace of the structureless regions, as shown here. 3.3. STS measurements For the as-grown specimen, STS spectra were measured, and I±V curves obtained as well as their derivatives were typically shown in Fig. 6. The reasonably steep rises of the tunnel current were observed below 1 V and near 3.5 V in the region B (structureless) while those were observed near 3 V and below 4 V in the region A (well-ordered). Here, a positive bias corresponds to an energy state above the Fermi level of the specimen. Since the derivatives of the tunneling current dI/dV are more directly related to the densityof-states of the system concerned than the tunneling current I, the derivatives may be helpful to discuss the electronic states. Therefore, the data taken from the region B indicate the presence of the gap states or a reduced energy gap in the case of the as-grown specimens since the potential window is well smaller than the energy gap of 5.5 eV. This is consistent with the total photoyield data discussed above. On the other hand, the data obtained for the well-ordered structure regions may suggest possible band-bending in the top surface region of the specimen since the apparent potential window in this vertical scale of the ®gure is larger than the energy gap while weak current rises were observed near 2 and 3.5 V. At present, however, more detailed
The electronic and atomic structures of B-doped ptype diamond(1 0 0) surfaces epitaxially grown on HTHP substrates have been investigated using total photoyield and STM/STS measurements. (1) On one hand, relatively low quantum ef®ciency of the photoemission process is recognized in the case of the NEA while the photoemission threshold energy is well below the energy gap for as-grown specimens. These indicate the presence of a substantial amount of gap states in the top surface and subsurface regions. (2) On the other hand, STM measurements reveals the presence of well-ordered 2 1 superstructures with a row separation of 0.50 nm even for the as-grown specimen. This well corresponds to those reported for the Hterminated C(1 0 0) surface. After UHV vacuum annealing at 150±2008C, the superstructure images became clearer with appearance of well-order structure with a row separation of 0.25 nm, which is an half of that of the 2 1 dimer rows observed. (3) Furthermore, STS data demonstrate that a considerable amount of gap states exist on the top surface and in the subsurface region, or that the energy gap is slightly reduced in that region. Acknowledgements This work was supported by the ``Research for the Future'' Program (No. 96R15401) from the Japan Society for the Promotion of Science. The authors would like to thank Profs. A. Hiraki and A. Hatta of Koch University of Technology for their helpful discussion. References [1] B.B. Pate, in: L.S. Pan, D.R. Kania (Eds.), Diamond: Electronic Properties and Applications, Kluwer Academic Publishers, Boston, USA, 1994, pp. 31. [2] N. Eimori, Y. Mori, A. Hatta, T. Ito, A. Hiraki, Jpn. J. Appl. Phys. 33 (1994) 6312. [3] N. Eimori, Y. Mori, A. Hatta, T. Ito, A. Hiraki, Diam. Relat. Mater. 4 (1995) 806.
H. Murakami et al. / Applied Surface Science 175±176 (2001) 474±479 [4] F.J. Himpel, J.A. Knapp, J.A. van Vechten, D.E. Eastman, Phys. Rev. 20 (1979) 624. [5] J. van der Weide, Z. Zhang, P.K. Baumann, M.G. Wensell, J. Bernholc, R.J. Nemanich, Phys. Rev. B 50 (1994) 5803. [6] C. Bandis, B.B. Pate, Phys. Rev. B 52 (1995) 12056. [7] I.L. Krainsky, V.M. Asnin, G.T. Mearini, J.A. Dayton Jr., Phys. Rev. B 53 (1996) R7650. [8] I.L. Krainsky, V.M. Asnin, J.A. Dayron Jr., Appl. Surf. Sci. 111 (1997) 265. [9] A. Shih, J. Yater, P. Pehrsson, J. Bulter, C. Hor, R. Abrams, J. Appl. Phys. 82 (1997) 1860. [10] M.W. Geis, J.C. Twichell, J. Macaulay, K. Okano, Appl. Phys. Lett. 67 (1995) 9. [11] A. Hatta, K. Ogawa, N. Eimori, M. Deguchi, M. Kitabatake, T. Ito, A. Hiraki, Appl. Surf. Sci. 117/118 (1997) 592. [12] T. Ito, M. Nishimura, A. Hatta, Appl. Phys. Lett. 73 (1998) 3739. [13] P.K. Baumann, R.J. Nemanich, J. Appl. Phys. 83 (1998) 2072, and references therein.
[14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
479
J. Robertson, Diam. Relat. Mater. 7 (1998) 620. J. Ristein, W. Stein, L. Ley, Diam. Relat. Mater. 7 (1998) 626. M. Yokoyama, T. Ito, Appl. Surf. Sci. 162/163 (2000) 457. H. Shiomi, K. Tanabe, Y. Nishibayashi, N. Fujimori, Jpn. J. Appl. Phys. 29 (1990) 3. T. Aizawa, T. Ando, M. Kamo, Y. Sato, Phys. Rev. B48 (1993) 18348. T. Tsuno, T. Imai, Y. Nishibayashi, K. Hamada, N. Fujimori, Jpn. J. Appl. Phys. 30 (1991) 1063. H. Kawarada, H. Sasaki, A. Sato, Phys. Rev. B52 (1995) 11351. A. Laikhtman, A. Hoffman, R. Kalish, Y. Avigal, A. Breskin, R. Chechik, E. Shefer, Y. Lifshitz, Appl. Phys. Lett. 73 (1998) 1433. R.U. Martinelli, D.G. Fisher, Proc. IEEE 62 (1974) 1339. A.V. Hamza, D.G. Kubiak, R.H. Stulen, Surf. Sci. 237 (1990) 35. S.H. Yang, D.A. Drabold, J.B. Adams, Phys. Rev. B48 (1993) 5261. H. Yagi, A. Hatta, T. Ito, Appl. Surf. Sci. 137 (1999) 50.