- Email: [email protected]
Field emission from electron-beam-irradiated bulk diamond
Applied Surface Science 146 Ž1999. 299–304
Field emission from electron-beam-irradiated bulk diamond Jun Taniguchi a
a,)
, Masanori Komuro
b,1
, Hiroshi Hiroshima b, Iwao Miyamoto
a
Department of Applied Electronics, Science UniÕersity of Tokyo, 2641 Yamazaki, Noda, Chiba 278, Japan b Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, Ibaraki 305, Japan
Abstract In order to extract electrons from bulk diamond, electron-beam ŽEB. irradiation that controls the surface resistance of synthetic diamond was developed. For slab-shaped bulk diamond which has various surface resistances, the characteristics of the emission current against the cathode voltage were fitted to a Fowler–Nordheim ŽF–N. plot, which suggests that field emission takes place. Judging from the gradient of the F–N plot before and after EB irradiation, this method produces a low-resistance and low-work function diamond surface. The surface conductive layer formed by electron irradiation contributes electron emission from bulk diamond. Four spot patterns were observed after EB irradiation with a large emission angle of about 458. Using a bulk diamond stylus, work functions of 0.17 eV were measured from F–N plot gradients. In the case of the diamond stylus, some spots were observed within the emission angle of 178. q 1999 Elsevier Science B.V. All rights reserved. PACS: 61.80.F; 79.70; 73.30 Keywords: Bulk diamond; Electron-beam irradiation; Work function; Field emission
1. Introduction The negative electron affinity of the diamond surface has been expected to enable realization of a stable and efficient electron emitter for microvacuum electronics devices and multiemitter display panels. There have been many reports concerning field emission from chemical vapor-deposited ŽCVD. diamond. For example, fabrications of a diamond field emitter array using CVD polycrystalline diamond w1x and low-threshold cold cathodes made of nitrogen-doped
)
Corresponding author. Tel.: q81-471-24-1501 ext. 4229; Fax: q81-471-22-9195; E-mail: [email protected] 1 Tel.: q81-298-54-5509; Fax: q81-298-54-5514; E-mail: [email protected].
CVD diamond w2x were reported. Many models about electron emission from CVD diamond film were proposed. For example, Xu et al. w3x reported that the grain boundary might form a conduction channel, which could transport the electrons to the surface. Fanciulli and Moustakas w4x reported that the lowfield emission from CVD polycrystalline diamond was due to electron transport through defect states. However, the electron emission mechanism of CVD diamond is not clearly understood yet. In order to examine the basic nature of electron emission from diamond, it is important to investigate characteristics of electron emission from bulk diamond. Ours is, to our knowledge, the first investigation of electron emission characteristics of bulk diamond. In this work, we develop surface treatment of bulk diamond to obtain a low-resistance surface, and investigate
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 0 3 5 - 5
300
J. Taniguchi et al.r Applied Surface Science 146 (1999) 299–304
characteristics of electron emission from bulk diamond modified by this method.
2. Experimental apparatus and procedure We used two kinds of diamonds as a specimen. One is synthetic single-crystal diamond surrounded with a Ž100.-oriented face and is 2.5 mm2 and 0.5 mm thick. The other is a diamond stylus, which has a 1 mm radius tip mounted on a steel rod. Slab-shaped diamond was put between two steel rods through a piece of pyrolytic graphite. Thus, we could measure the resistance of diamond sample from current–voltage characteristics between the two rods ŽFig. 1a.. Scanning electron microscopy ŽSEM. micrograph of the slab-shaped diamond, whose corner was a trigonal pyramid, is shown in Fig. 1b. The diamond
Fig. 2. Ža. Configuration of diamond stylus emitter; Žb. an enlarged SEM micrograph of the 1 mm tip radius.
Fig. 1. Ža. Configuration of emitter using slab-shaped diamond; Žb. an enlarged SEM micrograph of the diamond corner.
stylus was pasted on the steel rod, so that we could not measure resistance of the diamond surface. Fig. 2a shows the emitter configuration. An enlarged SEM micrograph of the emitter apex, whose tip radius is 1 mm, is shown in Fig. 2b. Such emitters were installed in a vacuum system less than 4 = 10y8 Torr with a spiral Ta wire, which acts as an anode, and thermal electron emitter for modification of the diamond surface. In the case of slab-shaped diamond, one corner was directed to a phosphorus screen collector. In order to decrease diamond surface resistance, we tried to make a hydrogen termination surface by the methods of Ta wire heating in H 2 gas atmosphere andror exposure to hydrogen DC plasma. However, these methods could not obtain sufficient low resistance Ž- 100 M V . and the repro-
J. Taniguchi et al.r Applied Surface Science 146 (1999) 299–304
Fig. 3. Ža. The EB irradiation system and Žb. electron current measurement system.
ducibility was not so good. Finally, we found out that electron-beam ŽEB. irradiation was very effective to get a low resistance surface ŽFig. 3a.. At first, to clean the contamination on the diamond surface, Ta wire heating was carried out for several hours. Then, thermal electrons were emitted from the Ta wire and irradiated on the diamond with an acceleration voltage of 300 V. After this treatment, negative voltage up to y10 kV was applied to the emitter Žcathode., and we measured electron current simultaneously with observation of emission patterns on phosphorus screen collector ŽFig. 3b..
301
out applied voltage of 300 V for 4 h. The irradiation conditions were 300 V acceleration voltage and 0.67 mA electron current in 4 = 10y8 Torr base pressure. The relationship between the irradiation time and conductance is shown in Fig. 4. After 90-min EB irradiation, diamond surface conductance rapidly increased. The reason for this will be discussed later. The characteristics of electron emission current against the cathode voltage are shown in Fig. 5a and Fowler–Nordheim ŽF–N. plots of these curves are shown in Fig. 5b. With decreasing surface resistance, emission currents increased. Thus, reducing surface resistance by EB irradiation is very effective for emission current enhancement. Since each curve in Fig. 5b is linear, it is suggested that field emission takes place. The gradients of F–N plots become gentle with the decrease in resistance, which suggests that the work function becomes smaller. EB irradiation seems to reduce the work function of the diamond surface. Fig. 6a shows the emission pattern in the case of no EB irradiation, where we cannot observe any spot patterns. The emission current of 430 nA at y10 kV was measured and this value is the same order as emission current without diamond. Fig. 6b shows the emission pattern in the case of 90-min EB irradiation time, where four major spots are observed and these emission patterns did not vary even if the resistance was changed by EB irradiation. The emission angle of each spot is estimated to be about 438 as indicated in Fig. 6b. We expected that the emission pattern was composed of two major spots, which corresponded to two trigonal
3. Results and discussion 3.1. Electron emission from slab-shaped diamond Electron-beam irradiation was performed to modify the diamond surface after Ta wire heating with-
Fig. 4. Relationship between EB irradiation time and conductance.
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J. Taniguchi et al.r Applied Surface Science 146 (1999) 299–304
kV. Fig. 7 shows the emission patterns at y8 kV. Electron emission is confined within the emission
Fig. 5. Ža. Emission current vs. cathode voltage characteristics of surface resistance diamond and Žb. Fowler–Nordheim plot of various surface resistance diamonds.
pyramid apexes directed to the phosphorus screen, but the observed emission patterns are different from this expectation. The reason for this will be discussed later. 3.2. Electron emission from diamond stylus Electron-beam irradiation was performed to modify the diamond surface after Ta wire heating without applied voltage at 300 V for 3 h. The irradiation conditions were 300 V acceleration voltage with 5.8 mA electron current. Several spots with an emission current of 36.3 nA at y10 kV were observed after Ta wire heating for 3 h. After EB irradiation, the emission current increased up to 3.48 mA at y10
Fig. 6. Ža. Emission pattern from bulk diamond at y10 kV cathode voltage in the case of no EB irradiation and Žb. emission patterns at y6 kV cathode voltage in the case of 90-min EB irradiation.
J. Taniguchi et al.r Applied Surface Science 146 (1999) 299–304
303
0.22 eV in the case of no EB irradiation and 0.17 eV in the case of EB irradiation. EB irradiation reduces the work function of the diamond surface. These results agree with previous works in which the diamond has negative or very low positive electron affinity w6–8x. 3.3. Model of electron emission from diamond surface As seen in the previous sections, the EB irradiation method is very effective to extract electron current from bulk diamond and to produce a low resistance surface. Fig. 9 shows a SEM micrograph of the diamond surface after this treatment and extracting electron current for several hours. It is seen that some deposited materials exist on the corner of the slab-shaped diamond. We are now speculating that EB irradiation breaks covalent bonds of the
Fig. 7. Emission pattern from 1 mm tip radius diamond stylus at y8 kV cathode voltage.
angle of 178. The characteristic of the emission current against the cathode voltage is shown in Fig. 8a and the F–N plot is shown in Fig. 8b. According to F–N theory, the gradient g in the F–N plot is expressed as:
ž
fs y
2
bg 6.489 = 10
7
/
3
,
Ž 1.
where f is the work function and b is the field enhancement factor. Field enhancement factor b was calculated using a sphere-on-orthogonal-cone ŽSOC. model w5x to be b s 105 cmy1 . Substituting this value and the gradients obtained from Fig. 8b into Eq. Ž1., the work function f is determined to be
Fig. 8. Ža. Emission current vs. cathode voltage characteristics of diamond stylus and Žb. Fowler–Nordheim plot of diamond stylus.
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J. Taniguchi et al.r Applied Surface Science 146 (1999) 299–304
4. Conclusion
Fig. 9. The SEM micrograph of diamond surface after EB irradiation and extracting electron current for several hours.
diamond and produces an amorphous carbon layer, which reduces surface resistance. Carbon atoms on the surface may migrate, probably through field-enhancement diffusion, and grow like the protrusion in Fig. 9 on the corner of the slab-shaped substrate. This protrusion may also act as field emission sites as seen in Fig. 7. In both cases of slab-shaped and stylus-shaped diamond, the low- resistance diamond surface shows higher emission current and lower work function than the high-resistance diamond surface. Thus, the major part of electron emission may come from field emission through the surface conductive layer. We consider this one model of electron emission. However, it seems to be difficult to explain why emission patterns have a large emission angle, especially in the case of the slab shape diamond ŽFig. 6b.. We surmise that: when electrons are emitted from protrusions apart from the two corners of the slab-shaped diamond, large angle emission patterns are obtained.
We developed an EB irradiation method for electron emission from bulk diamond, and investigated characteristics of electron emission from two types of bulk diamond. Using the slab-shaped diamond as an emitter, after 90-min EB irradiation, the diamond surface resistance was decreased and large angle emission patterns with a rather high emission current of 200 mA were observed. Judging from the fact that no emission pattern was observed before EB irradiation, the emission patterns of 90-min EB irradiation diamond indicate electron emission from bulk diamond. The EB irradiation diamond surface reduces work function. Using the diamond stylus, work function was estimated at 0.17 eV after EB irradiation. We surmise electron emission from bulk diamond as follows: electrons are emitted through the diamond surface conductive layer and protrusion emission sites. Acknowledgements We thank Dr. J. Itoh of the Electrotechnical Laboratory for his invaluable advice. References w1x K. Okano, K. Hoshina, M. Iida, S. Koizumi, T. Inuzuka, Appl. Phys. Lett. 64 Ž1994. 2742. w2x K. Okano, S. Koizumi, S. Ravi, P. Silva, G.A.J. Amaratunga, Nature 381 Ž1996. 140. w3x N.S. Xu, R.V. Latham, Y. Tzeng, Electron. Lett. 29 Ž1993. 1596. w4x M. Fanciulli, T.D. Moustakas, Phys. Rev. B 48 Ž1993. 14982. w5x W.P. Dyke, J.K. Trolan, W.W. Dolan, G. Barnes, J. Appl. Phys. 24 Ž5. Ž1953. 570. w6x N. Eimori, Y. Mori, A. Hata, T. Itoh, A. Hiraki, Jpn. J. Appl. Phys. 33 Ž1994. 6312. w7x M.W. Geis, A. Gregory, B.B. Pate, IEEE Trans. Electron. Devices 38 Ž1991. 619. w8x M. Aoki, H. Kawarada, Jpn. J. Appl. Phys. 33 Ž1994. L708.
Field emission from electron-beam-irradiated bulk diamond Jun Taniguchi a
a,)
, Masanori Komuro
b,1
, Hiroshi Hiroshima b, Iwao Miyamoto
a
Department of Applied Electronics, Science UniÕersity of Tokyo, 2641 Yamazaki, Noda, Chiba 278, Japan b Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, Ibaraki 305, Japan
Abstract In order to extract electrons from bulk diamond, electron-beam ŽEB. irradiation that controls the surface resistance of synthetic diamond was developed. For slab-shaped bulk diamond which has various surface resistances, the characteristics of the emission current against the cathode voltage were fitted to a Fowler–Nordheim ŽF–N. plot, which suggests that field emission takes place. Judging from the gradient of the F–N plot before and after EB irradiation, this method produces a low-resistance and low-work function diamond surface. The surface conductive layer formed by electron irradiation contributes electron emission from bulk diamond. Four spot patterns were observed after EB irradiation with a large emission angle of about 458. Using a bulk diamond stylus, work functions of 0.17 eV were measured from F–N plot gradients. In the case of the diamond stylus, some spots were observed within the emission angle of 178. q 1999 Elsevier Science B.V. All rights reserved. PACS: 61.80.F; 79.70; 73.30 Keywords: Bulk diamond; Electron-beam irradiation; Work function; Field emission
1. Introduction The negative electron affinity of the diamond surface has been expected to enable realization of a stable and efficient electron emitter for microvacuum electronics devices and multiemitter display panels. There have been many reports concerning field emission from chemical vapor-deposited ŽCVD. diamond. For example, fabrications of a diamond field emitter array using CVD polycrystalline diamond w1x and low-threshold cold cathodes made of nitrogen-doped
)
Corresponding author. Tel.: q81-471-24-1501 ext. 4229; Fax: q81-471-22-9195; E-mail: [email protected] 1 Tel.: q81-298-54-5509; Fax: q81-298-54-5514; E-mail: [email protected].
CVD diamond w2x were reported. Many models about electron emission from CVD diamond film were proposed. For example, Xu et al. w3x reported that the grain boundary might form a conduction channel, which could transport the electrons to the surface. Fanciulli and Moustakas w4x reported that the lowfield emission from CVD polycrystalline diamond was due to electron transport through defect states. However, the electron emission mechanism of CVD diamond is not clearly understood yet. In order to examine the basic nature of electron emission from diamond, it is important to investigate characteristics of electron emission from bulk diamond. Ours is, to our knowledge, the first investigation of electron emission characteristics of bulk diamond. In this work, we develop surface treatment of bulk diamond to obtain a low-resistance surface, and investigate
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 0 3 5 - 5
300
J. Taniguchi et al.r Applied Surface Science 146 (1999) 299–304
characteristics of electron emission from bulk diamond modified by this method.
2. Experimental apparatus and procedure We used two kinds of diamonds as a specimen. One is synthetic single-crystal diamond surrounded with a Ž100.-oriented face and is 2.5 mm2 and 0.5 mm thick. The other is a diamond stylus, which has a 1 mm radius tip mounted on a steel rod. Slab-shaped diamond was put between two steel rods through a piece of pyrolytic graphite. Thus, we could measure the resistance of diamond sample from current–voltage characteristics between the two rods ŽFig. 1a.. Scanning electron microscopy ŽSEM. micrograph of the slab-shaped diamond, whose corner was a trigonal pyramid, is shown in Fig. 1b. The diamond
Fig. 2. Ža. Configuration of diamond stylus emitter; Žb. an enlarged SEM micrograph of the 1 mm tip radius.
Fig. 1. Ža. Configuration of emitter using slab-shaped diamond; Žb. an enlarged SEM micrograph of the diamond corner.
stylus was pasted on the steel rod, so that we could not measure resistance of the diamond surface. Fig. 2a shows the emitter configuration. An enlarged SEM micrograph of the emitter apex, whose tip radius is 1 mm, is shown in Fig. 2b. Such emitters were installed in a vacuum system less than 4 = 10y8 Torr with a spiral Ta wire, which acts as an anode, and thermal electron emitter for modification of the diamond surface. In the case of slab-shaped diamond, one corner was directed to a phosphorus screen collector. In order to decrease diamond surface resistance, we tried to make a hydrogen termination surface by the methods of Ta wire heating in H 2 gas atmosphere andror exposure to hydrogen DC plasma. However, these methods could not obtain sufficient low resistance Ž- 100 M V . and the repro-
J. Taniguchi et al.r Applied Surface Science 146 (1999) 299–304
Fig. 3. Ža. The EB irradiation system and Žb. electron current measurement system.
ducibility was not so good. Finally, we found out that electron-beam ŽEB. irradiation was very effective to get a low resistance surface ŽFig. 3a.. At first, to clean the contamination on the diamond surface, Ta wire heating was carried out for several hours. Then, thermal electrons were emitted from the Ta wire and irradiated on the diamond with an acceleration voltage of 300 V. After this treatment, negative voltage up to y10 kV was applied to the emitter Žcathode., and we measured electron current simultaneously with observation of emission patterns on phosphorus screen collector ŽFig. 3b..
301
out applied voltage of 300 V for 4 h. The irradiation conditions were 300 V acceleration voltage and 0.67 mA electron current in 4 = 10y8 Torr base pressure. The relationship between the irradiation time and conductance is shown in Fig. 4. After 90-min EB irradiation, diamond surface conductance rapidly increased. The reason for this will be discussed later. The characteristics of electron emission current against the cathode voltage are shown in Fig. 5a and Fowler–Nordheim ŽF–N. plots of these curves are shown in Fig. 5b. With decreasing surface resistance, emission currents increased. Thus, reducing surface resistance by EB irradiation is very effective for emission current enhancement. Since each curve in Fig. 5b is linear, it is suggested that field emission takes place. The gradients of F–N plots become gentle with the decrease in resistance, which suggests that the work function becomes smaller. EB irradiation seems to reduce the work function of the diamond surface. Fig. 6a shows the emission pattern in the case of no EB irradiation, where we cannot observe any spot patterns. The emission current of 430 nA at y10 kV was measured and this value is the same order as emission current without diamond. Fig. 6b shows the emission pattern in the case of 90-min EB irradiation time, where four major spots are observed and these emission patterns did not vary even if the resistance was changed by EB irradiation. The emission angle of each spot is estimated to be about 438 as indicated in Fig. 6b. We expected that the emission pattern was composed of two major spots, which corresponded to two trigonal
3. Results and discussion 3.1. Electron emission from slab-shaped diamond Electron-beam irradiation was performed to modify the diamond surface after Ta wire heating with-
Fig. 4. Relationship between EB irradiation time and conductance.
302
J. Taniguchi et al.r Applied Surface Science 146 (1999) 299–304
kV. Fig. 7 shows the emission patterns at y8 kV. Electron emission is confined within the emission
Fig. 5. Ža. Emission current vs. cathode voltage characteristics of surface resistance diamond and Žb. Fowler–Nordheim plot of various surface resistance diamonds.
pyramid apexes directed to the phosphorus screen, but the observed emission patterns are different from this expectation. The reason for this will be discussed later. 3.2. Electron emission from diamond stylus Electron-beam irradiation was performed to modify the diamond surface after Ta wire heating without applied voltage at 300 V for 3 h. The irradiation conditions were 300 V acceleration voltage with 5.8 mA electron current. Several spots with an emission current of 36.3 nA at y10 kV were observed after Ta wire heating for 3 h. After EB irradiation, the emission current increased up to 3.48 mA at y10
Fig. 6. Ža. Emission pattern from bulk diamond at y10 kV cathode voltage in the case of no EB irradiation and Žb. emission patterns at y6 kV cathode voltage in the case of 90-min EB irradiation.
J. Taniguchi et al.r Applied Surface Science 146 (1999) 299–304
303
0.22 eV in the case of no EB irradiation and 0.17 eV in the case of EB irradiation. EB irradiation reduces the work function of the diamond surface. These results agree with previous works in which the diamond has negative or very low positive electron affinity w6–8x. 3.3. Model of electron emission from diamond surface As seen in the previous sections, the EB irradiation method is very effective to extract electron current from bulk diamond and to produce a low resistance surface. Fig. 9 shows a SEM micrograph of the diamond surface after this treatment and extracting electron current for several hours. It is seen that some deposited materials exist on the corner of the slab-shaped diamond. We are now speculating that EB irradiation breaks covalent bonds of the
Fig. 7. Emission pattern from 1 mm tip radius diamond stylus at y8 kV cathode voltage.
angle of 178. The characteristic of the emission current against the cathode voltage is shown in Fig. 8a and the F–N plot is shown in Fig. 8b. According to F–N theory, the gradient g in the F–N plot is expressed as:
ž
fs y
2
bg 6.489 = 10
7
/
3
,
Ž 1.
where f is the work function and b is the field enhancement factor. Field enhancement factor b was calculated using a sphere-on-orthogonal-cone ŽSOC. model w5x to be b s 105 cmy1 . Substituting this value and the gradients obtained from Fig. 8b into Eq. Ž1., the work function f is determined to be
Fig. 8. Ža. Emission current vs. cathode voltage characteristics of diamond stylus and Žb. Fowler–Nordheim plot of diamond stylus.
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J. Taniguchi et al.r Applied Surface Science 146 (1999) 299–304
4. Conclusion
Fig. 9. The SEM micrograph of diamond surface after EB irradiation and extracting electron current for several hours.
diamond and produces an amorphous carbon layer, which reduces surface resistance. Carbon atoms on the surface may migrate, probably through field-enhancement diffusion, and grow like the protrusion in Fig. 9 on the corner of the slab-shaped substrate. This protrusion may also act as field emission sites as seen in Fig. 7. In both cases of slab-shaped and stylus-shaped diamond, the low- resistance diamond surface shows higher emission current and lower work function than the high-resistance diamond surface. Thus, the major part of electron emission may come from field emission through the surface conductive layer. We consider this one model of electron emission. However, it seems to be difficult to explain why emission patterns have a large emission angle, especially in the case of the slab shape diamond ŽFig. 6b.. We surmise that: when electrons are emitted from protrusions apart from the two corners of the slab-shaped diamond, large angle emission patterns are obtained.
We developed an EB irradiation method for electron emission from bulk diamond, and investigated characteristics of electron emission from two types of bulk diamond. Using the slab-shaped diamond as an emitter, after 90-min EB irradiation, the diamond surface resistance was decreased and large angle emission patterns with a rather high emission current of 200 mA were observed. Judging from the fact that no emission pattern was observed before EB irradiation, the emission patterns of 90-min EB irradiation diamond indicate electron emission from bulk diamond. The EB irradiation diamond surface reduces work function. Using the diamond stylus, work function was estimated at 0.17 eV after EB irradiation. We surmise electron emission from bulk diamond as follows: electrons are emitted through the diamond surface conductive layer and protrusion emission sites. Acknowledgements We thank Dr. J. Itoh of the Electrotechnical Laboratory for his invaluable advice. References w1x K. Okano, K. Hoshina, M. Iida, S. Koizumi, T. Inuzuka, Appl. Phys. Lett. 64 Ž1994. 2742. w2x K. Okano, S. Koizumi, S. Ravi, P. Silva, G.A.J. Amaratunga, Nature 381 Ž1996. 140. w3x N.S. Xu, R.V. Latham, Y. Tzeng, Electron. Lett. 29 Ž1993. 1596. w4x M. Fanciulli, T.D. Moustakas, Phys. Rev. B 48 Ž1993. 14982. w5x W.P. Dyke, J.K. Trolan, W.W. Dolan, G. Barnes, J. Appl. Phys. 24 Ž5. Ž1953. 570. w6x N. Eimori, Y. Mori, A. Hata, T. Itoh, A. Hiraki, Jpn. J. Appl. Phys. 33 Ž1994. 6312. w7x M.W. Geis, A. Gregory, B.B. Pate, IEEE Trans. Electron. Devices 38 Ž1991. 619. w8x M. Aoki, H. Kawarada, Jpn. J. Appl. Phys. 33 Ž1994. L708.