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Origin of the field-stimulated exoelectron emission from tungsten tip surfaces
Ultramicroscopy 73 (1998) 217—221
Origin of the field-stimulated exoelectron emission from tungsten tip surfaces Tadashi Shiota*, Masashi Morita, Masahito Tagawa, Nobuo Ohmae, Masataka Umeno Department of Materials and Life Science, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565, Japan Received 7 July 1997; received in revised form 10 November 1997
Abstract Emission characteristics of the field-stimulated exoelectron emission (FSEE) from tungsten surfaces and the effect of gas adsorption on the FSEE have been investigated. The clean tungsten tip surfaces were prepared using the field evaporation technique. The clean tungsten surfaces thus prepared did not liberate FSEE. Physical adsorption of the residual gas atoms at the tungsten surface resulted in FSEE. An obvious enhancement of the FSEE signal due to gas adsorption was observed in which no significant change of the field emission pattern was caused. Therefore, FSEE is thus revealed the high sensitivity to the surface conditions. ( 1998 Elsevier Science B.V. All rights reserved.
1. Introduction A transient electron emission phenomenon, which is categorized neither in photoemission, thermionic emission, secondary electron emission, nor field emission, is called exoelectron emission. Exoelectron emission has been studied for almost a century. It has also been found that not only electrons but ions, neutrals and photons are emitted from surface. The term ‘‘exoemission’’ includes all of these emission phenomena. Exoemission was first reported by Mclenann in 1902 [1], and has
* Corresponding author. Tel.: #81 6 879 7282; fax: #81 6 879 7282; e-mail: [email protected].
been investigated by many researchers [2—5]. Exoemission had been classified on the basis of its stimuli; photo-stimulated exoelectron emission (PSEE) [6,7], thermally stimulated exoelectron emission (TSEE) [8—11], and dark emission. However, exoemission phenomenon in the presence of high electric field was recently reported, and was included in the group of field stimulated exoelectron emission (FSEE). FSEE was theoretically predicted by Robertson in 1981 [12], and was experimentally verified by Tagawa et al. in 1988 [13]. Apart from PSEE or TSEE which have been widely studied, the specimen of FSEE must be finished into a sharp tip in order to apply high electric field to the specimen surface. Such a technique has been established to perform field ion
0304-3991/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S 0 3 0 4 - 3 9 9 1 ( 9 7 ) 0 0 1 5 9 - 9
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T. Shiota et al. / Ultramicroscopy 73 (1998) 217—221
microscopy (FIM) and field emission microscopy (FEM). FIM and FEM provide atomic arrangements and electronic states of the tip surface with an atomic resolution in a real space. Combination of FIM/FEM techniques with FSEE allows us to understand the relationship between emission characteristics of exoelectrons and atomic arrangements and/or electronic states of the surface. In this paper, a tungsten tip, which has been widely applied to FIM and FEM observations, was applied to FSEE study. Relationship between gas adsorption and emission properties of FSEE was investigated.
2. Storage effect of FSEE One of the characteristics of exoemission is that emission intensity depends on the cut-off period of stimulus. This phenomenon is called storage effect, and has been used as a fingerprint of exoemission. The storage effect was first discovered in PSEE by Shigekawa et al. [14], later by Tagawa et al. in FSEE [13]. Fig. 1 illustrates schematics of the storage effect of FSEE. The storage effect is a phenomenon in which the emission current increases temporally after the pause of stimulus. For example, PSEE intensity becomes high at the moment of illuminating the sample with ultraviolet light, and then decreases with time. The PSEE intensity at the beginning of illumination increases with the cut-off period of the illumination. The storage effect is explained by the two-process model which suggests that the electrons are pumped up to
Fig. 1. Schematics of the storage effect of FSEE.
the trap level during the pause of stimuli [14]. In the two-process model, it is assumed that exoelectron traps are located above the Fermi level. Electrons are excited to the trap level with a constant rate b. Exoelectron emission from occupied traps takes place with a constant rate a in the presence of stimulus. These two processes well explain the temporal emission increase after a pause of stimulus for TSEE and PSEE. An uncertainty is whether or not stimuli influence the excitation as well as the emission. Namely, the light or thermal energy introduced to the sample surface from the outside in order to emit exoelectrons may influence the excitation process of electrons. However, in the case of FSEE, exoelectrons are emitted from the sample surface in the presence of high electric field, and no external energies are applied for exciting electrons. Therefore, the two processes fit well to FSEE model, as these deal with the excitation and emission processes independently. The experimental data obtained with aluminum specimen could be explained very well with this model [13]. Therefore, we have applied this model to explain FSEE phenomenon at the tungsten surfaces in our work.
3. Experiment A high-purity tungsten wire (60 lm in diameter) was finished into a sharp tip by electropolishing using an electrolyte (3% KOH) at 0—5 V DC. Typical radius of curvature of a tip apex, thus prepared, was several tens of nanometers. The experimental apparatus used in this study is described elsewhere [15]. The vacuum chamber was evacuated by a sputter ion pump, a turbomolecular pump and a titanium getter pump, and the ultimate pressure was lower than 4.6]10~10 Torr. The tungsten tip was cooled to 185 K with the liquid-nitrogencooled cold finger to observe FIM images. Emission currents were measured with the ultrahigh resistancemeter, and spatial distribution of the emission was observed on a phosphorus screen intensified with a microchannel plate which was placed opposite to the tip. Exoemission images, as well as FIM/FEM images, were captured with a CCD camera directly connected to a PC. Experimental data including CCD images and integrated
T. Shiota et al. / Ultramicroscopy 73 (1998) 217—221
emission currents were stored in PCs. All of experiments reported here were carried out in darkness in order to avoid unexpected effect of light. Atomic arrangements and work function distribution over the tip surface were examined by FIM and FEM before the measurements of FSEE properties. Lattice imperfection, defects or cracks on the tip surface were carefully checked by the FIM images with helium as an image gas. The spatial distribution of the work function at the tungsten surface was observed by FEM pattern. Cleanliness of the tip surface was finally identified by the FEM pattern. The storage effect of FSEE was examined by a re-application of the tip voltage after 60 s of interruption.
219
Fig. 2. Emission property from the tungsten surface cleaned by field evaporation with the applied voltage of 8.5 kV.
4. Result and discussion The tungsten tip used in this study was carefully cleaned by using field evaporation technique. The cleanliness of the tip surface was confirmed from the FEM pattern. It has been known that a twofold symmetric pattern of FEM is evidence of clean tungsten surface. In order to identify whether or not FSEE was activated on such a clean surface of tungsten, FSEE component in the emission was detected using storage effect. The storage effect of FSEE was measured with the following procedures: (1) Cleaning of tungsten tip surface was carried out with field evaporation, (2) Polarity of the applied electric field was turned to negative and FEM pattern was observed, (3) If two-fold symmetric FEM pattern was not observed, processes (1) and (2) were repeated. After two-fold symmetric pattern of FEM was observed, the applied voltage was turned off for 60 s. (4) The same voltage as that in (3) was applied to the tip, and change in emission current was recorded. The results of the electric current measurements are shown in Fig. 2. As indicated in the figure, the emission current does not increase even after the re-application of high voltage. The FEM pattern observed before the pause of stimulus is also inserted in Fig. 3. This two-fold symmetric emission pattern shows the cleanliness of the tip surface at the experiment. This result suggests that FSEE does not occur from a clean tungsten surface. In
Fig. 3. FEM image of the clean tungsten tip, applied voltage !530 V.
order to clarify emission properties of FSEE, the storage effect of FSEE was measured at various experimental conditions with the same tungsten tip. Namely, the tip surface was cleaned by field evaporation with an applied voltage of 8.5 kV, and then the storage effect was examined. After the measurement, the tip was held for 180 s under the negative electric field in UHV in this case. Then the storage effect was measured with procedures (3) and (4). Fig. 4 shows the enhancement of FSEE due to the storage effect. The FSEE signal appeared was observed after keeping the tip for 180 s. The FEM images taken before the measurement of storage effect from the clean surface and after keeping
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T. Shiota et al. / Ultramicroscopy 73 (1998) 217—221
Fig. 4. Emission property from the clean tungsten tip and the effect of adsorption of residual gas atoms on emission property. Fig. 6. FEM image of the tungsten tip taken after storing in UHV for 180 s, applied voltage !530 V.
Fig. 5. FEM image of the clean tungsten tip taken before the measurement of the storage effect, applied voltage !530 V.
the tip for 180 s are shown in Figs. 5 and 6, respectively. The FEM image taken after 180 s shown in Fig. 6 indicates a little difference from the original symmetrical pattern in Fig. 5. It is thought that this change of FEM pattern is assigned to the adsorption of residual gas atoms on the tip surface. Therefore, residual gas atoms adsorbed on the tip contributes to FSEE. After the FSEE measurement, the tip was cleaned by the field evaporation with the same voltage of 8.5 kV, and procedures (3) and (4) were again applied to the same tip. Very good reproducibility was obtained. It is concluded that a clean tungsten tip surface does not liberate FSEE and
Fig. 7. FEM image of the cleaned tungsten surface taken by applying !530 V.
that adsorbates at a tungsten tip surface contribute to FSEE. The tip cleaned by the same procedure was held in UHV for 15 h without the negative electric field and the change of the emission characteristics was examined. Experimental procedure is as follows: firstly, the storage effect from the clean tungsten tip surface was investigated. Then, the applied voltage was reduced, and the tip was kept in UHV for 15 h. After that, a field desorption with the applied voltage of 8.5 kV was initiated to reduce the adsorbed gas atoms, and the storage effect was checked
T. Shiota et al. / Ultramicroscopy 73 (1998) 217—221
221
Table 1 Comparison of FSEE property before and after keeping the tip in UHV for 15 h without applying negative electric field Emission current (nA)
After field evaporation After storing for 15 h in UHV
Before intermission
After intermission
44.2 40.0
43.4 43.2
5. Conclusions The tungsten tip surface which is carefully cleaned by field evaporation does not liberate FSEE. However, after adsorption of residual gas atoms on the tip surface, FSEE signal arises. Even though the FEM pattern still keeps the two-fold pattern of clean tungsten surface after gas adsorption, FSEE signal becomes obvious. This observation suggests the possibility of using FSEE as a sensitive tool for the investigation of surface gas adsorption. References Fig. 8. FEM image of the tungsten tip after keeping in UHV for 15 h without negative electric field but with a field desorption at an applied voltage of 8.5 kV, applied voltage !530 V.
again. The results are summarized in Table 1. FSEE did not occur from the clean tungsten surface, while FSEE appeared obviously for the surface stored in UHV after 15 h as exhibited by the increase in emission current after 15 h in spite of removing the adsorbed atoms by the field desorption. The FEM images taken before and after keeping the tip in UHV for 15 h are shown in Figs. 7 and 8. As for the FEM images no remarkable change was observed, but notable changes were observed in FSEE properties. Therefore, FSEE has a higher sensitivity to surface characteristics than the FEM image.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
J. McLenann, Philos. Mag. 3 (1902) 105. J. Kramer, Z. Phys. 125 (1949) 739. J. Kramer, Z. Phys. 128 (1950) 538. I.V. Krylova, Russian Chem. Rev. 45 (12) (1976) 1101. J.A. Ramsey, Jpn. J. Appl. Phys. (Suppl.) 24 (1985) 32. M. Kamada, K. Yoshihara, K. Tsutsumi, Jpn. J. Appl. Phys. 23 (1984) 286. W. Kreigseis, A. Scharman, Phys. Stat. Sol. A 29 (1975) 407. H. Shigekawa, S. Hyodo, Jpn. J. Appl. Phys. 21 (1982) 1278. J.A. Ramsey, Surf. Sci. 8 (1967) 313. D.R. Aromtt, J.A. Ramsey, Surf. Sci. 28 (1977) 1. E. Linke, K. Meyer, Surf. Sci. 20 (1970) 304. A.J.B. Robertson, Int. J. Electron. 51 (1981) 607. M. Tagawa, S. Takenobu, N. Ohmae, M. Umeno, Appl. Phys. Lett. 53 (7) (1988) 626. H. Shigekawa, S. Hyodo, Appl. Surf. Sci. 22/23 (1985) 361. N. Ohmae, M. Yamamoto, M. Umeno, M. Tagawa, Proc. 11th Int. Symp. on Exoelectron Emission and Applications, Glucholazy, Poland, 11—17 September 1994, p. 245.
Origin of the field-stimulated exoelectron emission from tungsten tip surfaces Tadashi Shiota*, Masashi Morita, Masahito Tagawa, Nobuo Ohmae, Masataka Umeno Department of Materials and Life Science, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565, Japan Received 7 July 1997; received in revised form 10 November 1997
Abstract Emission characteristics of the field-stimulated exoelectron emission (FSEE) from tungsten surfaces and the effect of gas adsorption on the FSEE have been investigated. The clean tungsten tip surfaces were prepared using the field evaporation technique. The clean tungsten surfaces thus prepared did not liberate FSEE. Physical adsorption of the residual gas atoms at the tungsten surface resulted in FSEE. An obvious enhancement of the FSEE signal due to gas adsorption was observed in which no significant change of the field emission pattern was caused. Therefore, FSEE is thus revealed the high sensitivity to the surface conditions. ( 1998 Elsevier Science B.V. All rights reserved.
1. Introduction A transient electron emission phenomenon, which is categorized neither in photoemission, thermionic emission, secondary electron emission, nor field emission, is called exoelectron emission. Exoelectron emission has been studied for almost a century. It has also been found that not only electrons but ions, neutrals and photons are emitted from surface. The term ‘‘exoemission’’ includes all of these emission phenomena. Exoemission was first reported by Mclenann in 1902 [1], and has
* Corresponding author. Tel.: #81 6 879 7282; fax: #81 6 879 7282; e-mail: [email protected].
been investigated by many researchers [2—5]. Exoemission had been classified on the basis of its stimuli; photo-stimulated exoelectron emission (PSEE) [6,7], thermally stimulated exoelectron emission (TSEE) [8—11], and dark emission. However, exoemission phenomenon in the presence of high electric field was recently reported, and was included in the group of field stimulated exoelectron emission (FSEE). FSEE was theoretically predicted by Robertson in 1981 [12], and was experimentally verified by Tagawa et al. in 1988 [13]. Apart from PSEE or TSEE which have been widely studied, the specimen of FSEE must be finished into a sharp tip in order to apply high electric field to the specimen surface. Such a technique has been established to perform field ion
0304-3991/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S 0 3 0 4 - 3 9 9 1 ( 9 7 ) 0 0 1 5 9 - 9
218
T. Shiota et al. / Ultramicroscopy 73 (1998) 217—221
microscopy (FIM) and field emission microscopy (FEM). FIM and FEM provide atomic arrangements and electronic states of the tip surface with an atomic resolution in a real space. Combination of FIM/FEM techniques with FSEE allows us to understand the relationship between emission characteristics of exoelectrons and atomic arrangements and/or electronic states of the surface. In this paper, a tungsten tip, which has been widely applied to FIM and FEM observations, was applied to FSEE study. Relationship between gas adsorption and emission properties of FSEE was investigated.
2. Storage effect of FSEE One of the characteristics of exoemission is that emission intensity depends on the cut-off period of stimulus. This phenomenon is called storage effect, and has been used as a fingerprint of exoemission. The storage effect was first discovered in PSEE by Shigekawa et al. [14], later by Tagawa et al. in FSEE [13]. Fig. 1 illustrates schematics of the storage effect of FSEE. The storage effect is a phenomenon in which the emission current increases temporally after the pause of stimulus. For example, PSEE intensity becomes high at the moment of illuminating the sample with ultraviolet light, and then decreases with time. The PSEE intensity at the beginning of illumination increases with the cut-off period of the illumination. The storage effect is explained by the two-process model which suggests that the electrons are pumped up to
Fig. 1. Schematics of the storage effect of FSEE.
the trap level during the pause of stimuli [14]. In the two-process model, it is assumed that exoelectron traps are located above the Fermi level. Electrons are excited to the trap level with a constant rate b. Exoelectron emission from occupied traps takes place with a constant rate a in the presence of stimulus. These two processes well explain the temporal emission increase after a pause of stimulus for TSEE and PSEE. An uncertainty is whether or not stimuli influence the excitation as well as the emission. Namely, the light or thermal energy introduced to the sample surface from the outside in order to emit exoelectrons may influence the excitation process of electrons. However, in the case of FSEE, exoelectrons are emitted from the sample surface in the presence of high electric field, and no external energies are applied for exciting electrons. Therefore, the two processes fit well to FSEE model, as these deal with the excitation and emission processes independently. The experimental data obtained with aluminum specimen could be explained very well with this model [13]. Therefore, we have applied this model to explain FSEE phenomenon at the tungsten surfaces in our work.
3. Experiment A high-purity tungsten wire (60 lm in diameter) was finished into a sharp tip by electropolishing using an electrolyte (3% KOH) at 0—5 V DC. Typical radius of curvature of a tip apex, thus prepared, was several tens of nanometers. The experimental apparatus used in this study is described elsewhere [15]. The vacuum chamber was evacuated by a sputter ion pump, a turbomolecular pump and a titanium getter pump, and the ultimate pressure was lower than 4.6]10~10 Torr. The tungsten tip was cooled to 185 K with the liquid-nitrogencooled cold finger to observe FIM images. Emission currents were measured with the ultrahigh resistancemeter, and spatial distribution of the emission was observed on a phosphorus screen intensified with a microchannel plate which was placed opposite to the tip. Exoemission images, as well as FIM/FEM images, were captured with a CCD camera directly connected to a PC. Experimental data including CCD images and integrated
T. Shiota et al. / Ultramicroscopy 73 (1998) 217—221
emission currents were stored in PCs. All of experiments reported here were carried out in darkness in order to avoid unexpected effect of light. Atomic arrangements and work function distribution over the tip surface were examined by FIM and FEM before the measurements of FSEE properties. Lattice imperfection, defects or cracks on the tip surface were carefully checked by the FIM images with helium as an image gas. The spatial distribution of the work function at the tungsten surface was observed by FEM pattern. Cleanliness of the tip surface was finally identified by the FEM pattern. The storage effect of FSEE was examined by a re-application of the tip voltage after 60 s of interruption.
219
Fig. 2. Emission property from the tungsten surface cleaned by field evaporation with the applied voltage of 8.5 kV.
4. Result and discussion The tungsten tip used in this study was carefully cleaned by using field evaporation technique. The cleanliness of the tip surface was confirmed from the FEM pattern. It has been known that a twofold symmetric pattern of FEM is evidence of clean tungsten surface. In order to identify whether or not FSEE was activated on such a clean surface of tungsten, FSEE component in the emission was detected using storage effect. The storage effect of FSEE was measured with the following procedures: (1) Cleaning of tungsten tip surface was carried out with field evaporation, (2) Polarity of the applied electric field was turned to negative and FEM pattern was observed, (3) If two-fold symmetric FEM pattern was not observed, processes (1) and (2) were repeated. After two-fold symmetric pattern of FEM was observed, the applied voltage was turned off for 60 s. (4) The same voltage as that in (3) was applied to the tip, and change in emission current was recorded. The results of the electric current measurements are shown in Fig. 2. As indicated in the figure, the emission current does not increase even after the re-application of high voltage. The FEM pattern observed before the pause of stimulus is also inserted in Fig. 3. This two-fold symmetric emission pattern shows the cleanliness of the tip surface at the experiment. This result suggests that FSEE does not occur from a clean tungsten surface. In
Fig. 3. FEM image of the clean tungsten tip, applied voltage !530 V.
order to clarify emission properties of FSEE, the storage effect of FSEE was measured at various experimental conditions with the same tungsten tip. Namely, the tip surface was cleaned by field evaporation with an applied voltage of 8.5 kV, and then the storage effect was examined. After the measurement, the tip was held for 180 s under the negative electric field in UHV in this case. Then the storage effect was measured with procedures (3) and (4). Fig. 4 shows the enhancement of FSEE due to the storage effect. The FSEE signal appeared was observed after keeping the tip for 180 s. The FEM images taken before the measurement of storage effect from the clean surface and after keeping
220
T. Shiota et al. / Ultramicroscopy 73 (1998) 217—221
Fig. 4. Emission property from the clean tungsten tip and the effect of adsorption of residual gas atoms on emission property. Fig. 6. FEM image of the tungsten tip taken after storing in UHV for 180 s, applied voltage !530 V.
Fig. 5. FEM image of the clean tungsten tip taken before the measurement of the storage effect, applied voltage !530 V.
the tip for 180 s are shown in Figs. 5 and 6, respectively. The FEM image taken after 180 s shown in Fig. 6 indicates a little difference from the original symmetrical pattern in Fig. 5. It is thought that this change of FEM pattern is assigned to the adsorption of residual gas atoms on the tip surface. Therefore, residual gas atoms adsorbed on the tip contributes to FSEE. After the FSEE measurement, the tip was cleaned by the field evaporation with the same voltage of 8.5 kV, and procedures (3) and (4) were again applied to the same tip. Very good reproducibility was obtained. It is concluded that a clean tungsten tip surface does not liberate FSEE and
Fig. 7. FEM image of the cleaned tungsten surface taken by applying !530 V.
that adsorbates at a tungsten tip surface contribute to FSEE. The tip cleaned by the same procedure was held in UHV for 15 h without the negative electric field and the change of the emission characteristics was examined. Experimental procedure is as follows: firstly, the storage effect from the clean tungsten tip surface was investigated. Then, the applied voltage was reduced, and the tip was kept in UHV for 15 h. After that, a field desorption with the applied voltage of 8.5 kV was initiated to reduce the adsorbed gas atoms, and the storage effect was checked
T. Shiota et al. / Ultramicroscopy 73 (1998) 217—221
221
Table 1 Comparison of FSEE property before and after keeping the tip in UHV for 15 h without applying negative electric field Emission current (nA)
After field evaporation After storing for 15 h in UHV
Before intermission
After intermission
44.2 40.0
43.4 43.2
5. Conclusions The tungsten tip surface which is carefully cleaned by field evaporation does not liberate FSEE. However, after adsorption of residual gas atoms on the tip surface, FSEE signal arises. Even though the FEM pattern still keeps the two-fold pattern of clean tungsten surface after gas adsorption, FSEE signal becomes obvious. This observation suggests the possibility of using FSEE as a sensitive tool for the investigation of surface gas adsorption. References Fig. 8. FEM image of the tungsten tip after keeping in UHV for 15 h without negative electric field but with a field desorption at an applied voltage of 8.5 kV, applied voltage !530 V.
again. The results are summarized in Table 1. FSEE did not occur from the clean tungsten surface, while FSEE appeared obviously for the surface stored in UHV after 15 h as exhibited by the increase in emission current after 15 h in spite of removing the adsorbed atoms by the field desorption. The FEM images taken before and after keeping the tip in UHV for 15 h are shown in Figs. 7 and 8. As for the FEM images no remarkable change was observed, but notable changes were observed in FSEE properties. Therefore, FSEE has a higher sensitivity to surface characteristics than the FEM image.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
J. McLenann, Philos. Mag. 3 (1902) 105. J. Kramer, Z. Phys. 125 (1949) 739. J. Kramer, Z. Phys. 128 (1950) 538. I.V. Krylova, Russian Chem. Rev. 45 (12) (1976) 1101. J.A. Ramsey, Jpn. J. Appl. Phys. (Suppl.) 24 (1985) 32. M. Kamada, K. Yoshihara, K. Tsutsumi, Jpn. J. Appl. Phys. 23 (1984) 286. W. Kreigseis, A. Scharman, Phys. Stat. Sol. A 29 (1975) 407. H. Shigekawa, S. Hyodo, Jpn. J. Appl. Phys. 21 (1982) 1278. J.A. Ramsey, Surf. Sci. 8 (1967) 313. D.R. Aromtt, J.A. Ramsey, Surf. Sci. 28 (1977) 1. E. Linke, K. Meyer, Surf. Sci. 20 (1970) 304. A.J.B. Robertson, Int. J. Electron. 51 (1981) 607. M. Tagawa, S. Takenobu, N. Ohmae, M. Umeno, Appl. Phys. Lett. 53 (7) (1988) 626. H. Shigekawa, S. Hyodo, Appl. Surf. Sci. 22/23 (1985) 361. N. Ohmae, M. Yamamoto, M. Umeno, M. Tagawa, Proc. 11th Int. Symp. on Exoelectron Emission and Applications, Glucholazy, Poland, 11—17 September 1994, p. 245.