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Local field-emission characteristic of individual AlN cone fabricated by focused ion-beam etching method
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Applied Surface Science 254 (2008) 4840–4844 www.elsevier.com/locate/apsusc
Local field-emission characteristic of individual AlN cone fabricated by focused ion-beam etching method Y.L. Li, C.Y. Shi, J.J. Li *, C.Z. Gu Microfabrication Laboratory, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China Received 6 January 2008; accepted 23 January 2008 Available online 5 February 2008
Abstract Highly (0 0 2)-oriented AlN film was deposited on n-type (1 0 0)-oriented silicon substrates by the radio frequency magnetron sputtering method. An individual AlN cone with high aspect ratio was fabricated by the focused ion-beam (FIB) etching process in the surface of an as-formed AlN film. This etching method can easily control the tip radius and height to obtain AlN cones with different aspect ratios. The field-emission property of the individual AlN cone was measured in a scanning electron microscopy system equipped with a movable probe as the anode above the AlN tip. The results indicated that the as-formed single AlN cone with high aspect ratio possessed good field-emission ability although it only had a tiny emission area. Compared with a single Si tip fabricated by the same method, a single AlN cone exhibits better field-emission ability, and hence, has great potential as a promising candidate of point electron source for application in vacuum electronic devices. # 2008 Elsevier B.V. All rights reserved. PACS : 79.70+q; 81.16c Keywords: AlN cone; Field emission; Focused ion-beam etching
1. Introduction Aluminum nitride (AlN) has attracted significant interest because of its unique physical properties such as negative electron affinity, high thermal conductivity, high melting point, good chemical stability, and low dielectric loss, which make it a promising field-emission material for planar display devices and microelectronics field application [1–3]. To promote the field-emission performance of AlN, we desired to fabricate the AlN cone arrays because they have a large field enhancement factor that allow the electrons to tunnel into vacuum easily. To date, most studies have concentrated on AlN nanocone arrays and their field-emission properties [4–6]. However, the field shielding effect often occurs in the field-emission processing of cone arrays due to a short space between cones, which would make against the enhancement of field emission. Also, the field emission from the cone arrays cannot reflect an intrinsic fieldemission property of a single cone. On the other hand, an individual nanocone also has some important applications for
* Corresponding author. Tel.: +86 10 82648198; fax: +86 10 82648198. E-mail address: [email protected] (J.J. Li). 0169-4332/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.01.119
an electron source and atomic force microscopy (AFM) and scanning tunneling microscopy (STM) probes. Therefore, studying field emission of a single AlN cone is significant for understanding the field-emission process of whole cone arrays, but also, is very helpful in the application area. To investigate field emission from a single AlN cone, chief of all is to prepare AlN cone. But the reported AlN cone arrays were usually grown by the chemical vapor deposition (CVD) method [4–6]. With CVD, it is very difficult to control the cone density so that the field-emission property of an individual AlN cone is difficult to measure due to the limit of higher densities of the as-grown AlN cone. In this paper, an individual AlN cone was fabricated successfully by the focused ion-beam (FIB) etching method, and then its field-emission property was tested and discussed in detail. 2. Experimental details AlN films were deposited on n-type (1 0 0)-oriented silicon substrates in a radio frequency (RF) magnetron sputtering system. Aluminum with 99.99% purity was used as the sputtering target. Prior to loading into the sputtering chamber, the substrates were etched by dipping in a dilute solution of 4%
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HF for 60 s to remove the chemical surface oxide layer. Degreasing was carried out by ultrasonic-assisted cleaning in acetone and methanol. Once the base chamber gas pressure reached 1 104 Pa, a gas flow mixture N2 and Ar was maintained into the chamber with a ratio of about 60:40. The total gas pressure was kept at 0.75 Pa during the sputtering process. The sputtering power and substrate temperature were kept at 120 W and 700 8C, respectively. The surface morphology of the AlN film was characterized by scanning electron microscopy (SEM), and the X-ray diffraction (XRD) was used to prove that high-quality oriented AlN films are formed at the above-mentioned experimental conditions. An individual AlN cone with high aspect ratio was fabricated on the as-deposited AlN film by FIB system using 30 kV Ga+ ion source (produced by FEI Company, USA). We designed a simple doughnut pattern that had an inner and outer radius for the etching. Fig. 1(a) shows the etching profile sketch map. During the etching time, the annulus area was etched off, but the wafer and central area was protected so that a long conical structure was formed in a hole. Fig. 1(b) shows a typical SEM image of an as-formed single AlN cone, and its radius and height can easily be controlled by adjusting the etching parameter.
Field-emission property of an individual AlN cone has been studied using a customized double-probe SEM system at a chamber pressure of about 106 torr, as illustrated in Fig. 2(a). Fig. 2(b) displays the SEM image for the fieldemission measurements of an individual AlN cone. The SEM system is equipped with a microsized anode probe that is more efficient at collecting locally emitted electrons than conventional anode panel systems. The anode probe has a tip radius of about 1 mm formed by chemical etching of a tungsten wire, and the distance between the anode probe tip and a AlN cone top can be adjusted to as little as 0.1 mm by the probe controlled system for all field-emission measurement. The anode probes used in our work can be adjusted in three-dimensional while we can image the sample and probe using the scanning electron microscopy. Owing to the lowest step size of probe moving in our probe controlled system is about 5 nm, we can firstly touch the cone top by moving the probe and then leave along cone top, and the elevated spacing from the cone top can be adjusted to 100 nm (0.1 mm) by probe controlled system. For the as-formed tips with different radii, the same probe controlled process was carried out to assure a similar spacing from cone tip to the probe for the measurement of their field-emission current–voltage (I–V) property. The emission current was measured by a picoampere meter upon applying a dc voltage between the anode probe and the silicon substrate. The measurement was performed at room temperature.
Fig. 1. (a) Sketch map of AlN cone fabrication. (b) SEM images of individual AlN tip.
Fig. 2. Typical experimental configuration for field-emission measurements: (a) sketch map, (b) SEM image.
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3. Results and discussion 3.1. Morphological and structural characterization Fig. 3(a) shows the surface morphology of the AlN film deposited on the (1 0 0)-oriented silicon substrates by the RF magnetron sputtering method. It can be seen that the pebblelike grains are formed uniformly on the surface of the AlN film, which reflects the surface morphology character of (0 0 2)oriented the AlN films. But this characteristic still needs to be further proven by XRD. The corresponding XRD results are shown in Fig. 3(b), in which a strong diffraction peak appears at about 368 of 2u, indicating a highly (0 0 2)-oriented character for the as-formed AlN film. Thus, we concluded that the highquality AlN film with (0 0 2) orientation was formed at the above-mentioned experimental conditions. Fig. 4 shows SEM images of a single AlN cone on the surface of an AlN film fabricated by the FIB etching method. An as-formed single AlN cone has different tip radii, which is 200, 80, and 40 nm, respectively. All the cones are kept at the same height of 4 mm by controlling FIB etching parameters. The formation of these different tip radii is due to changing the inner radius of the doughnut pattern during FIB milling; thus, AlN tips with high aspect ratio can be achieved. These as-formed AlN cones with different tip radii are used for field-emission measurement. 3.2. Field-emission property Fig. 5(a) shows a typical field-emission current–voltage curve of an individual AlN cone with a tip radius of 40 nm and a
Fig. 4. SEM images of individual AlN tips with different tip radii and same height (a) 200 nm, (b) 80 nm, (c) 40 nm.
Fig. 3. (a) SEM image of as-deposited AlN films. (b) Corresponding XRD result.
total height of 4 mm. The turn-on voltage is defined as 2 V, which is corresponding a current of 4 nA. As we can see, when a voltage of 54 V was applied, the emission current is increased to 40 nA; meanwhile, the corresponding emission current density is estimated as near 800 A/cm2. A maximum current of
Y.L. Li et al. / Applied Surface Science 254 (2008) 4840–4844
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Fig. 6. Field-emission properties of AlN tips with different tip radii.
Fig. 5. (a) Typical field-emission I–V curve of the individual AlN tip. (b) Corresponding FN plot.
560 nA is obtained at an applied voltage of 100 V, which corresponds to a large emission current density although the AlN cone only has a tiny emission area. It indicates that the individual AlN tip has good field-emission property, with the ability to be a point electron source. The corresponding Fowler–Nordheim (FN) plot was also shown in Fig. 5(b). At low applied field, the FN plot is linear, which accords with the conventional FN theory, and indicates that the electron emission is indeed caused by vacuum tunneling [7]. But at high applied field, the bending trend of the FN plot occurs due to a different field enchantment factor from that at low field, which may be related to the surface desorption of the AlN cone top and Ga+ injected into the surface layer of AlN cone during the FIB milling process by high-energy Ga ions. When a high field is applied, emission current is increased and simultaneously the local temperature of the cone top is also enhanced; thus, the surface adsorbent was removed by the surface desorption so that the surface effective work function was changed. In addition, the effect of the Ga ions remained in the top of AlN cone may be another reason why the surface work function change with the increase of applied field. At high electric field, these injected Ga ions may be reduced or removed by the electron emission process, which will influence the effective work function of the AlN cone top, and hence changes the FN plot. The above field-emission results reflect the
intrinsic field-emission property of the AlN cone without any field shielding effect that often occurs with nanocone or nanotube arrays. Thus, the intrinsic field-emission property of a single AlN cone is very helpful for its potential application in the microelectronic field. In addition, the field-emission properties of an individual AlN cone with different tip radii and the same cone height were also measured, and the effect of tip radius on field-emission ability of the AlN cone was discussed. The field-emission results are shown in Fig. 6. Among these individual AlN cones, the total cone height was kept at 4 mm, but the tip radius is changed to 200, 80 and 40 nm, respectively. From Fig. 6, we can see that the AlN cone with a tip radius of 40 nm has better field-emission ability than other AlN cones with a tip radius of 80 and 200 nm. When the applied voltage is increased to 50 V, the corresponding emission current is 60 nA for a tip radius of 40 nm, 45 nA for a tip radius of 80 nm, and 31 nA for a tip radius of 200 nm, respectively. Further, the maximum emission current of the AlN tip with a tip radius of 40 nm at 100 V is more than 500 nA, which is about twice as lager as the AlN cones with a tip radius of 200 nm. But the emission area of the AlN tip with a tip radius of 40 nm is much less than that with a tip radius of 200 nm, and thus, the emission current density of the AlN tip with a tip radius of 40 nm is much greater than that of one with a tip radius of 200 nm. Therefore, the AlN tip with a radius of 40 nm has the best field-emission ability among asformed AlN cones with different tip radii. The reason why the individual AlN cone with a smaller tip radius has better field emission is that this AlN tip has a greater field enhancement factor b, which is very favorable to enhancing field emission of the AlN cone. In conventional FN theory, the relationship between the field-emission current (I) and the applied voltage (V) can be described as follows [8]: b I ¼ aV 2 exp V a ¼ 1:4 106 ’1 b2 expð9:89’1=2 Þa b ¼ 6:53 107 ’3=2 b1 ;
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composite structure of AlN cone and the silicon substrate, including the upper part of AlN cone and trapeziform lower part of Si substrate, as shown in Fig. 2(a). This composite structure is favorable for increasing the field-emission property of the AlN cone because it can effectively decrease the effective work function [9] compared to the individual silicon cone. 4. Conclusions
Fig. 7. Field-emission properties of AlN tip and individual silicon tip with the same aspect ratio.
where a is the field-emission effective area and b is the field enhancement factor. Assuming that h is the tip height and r is the tip radius, b can be described as h/r, which is adaptable to tip structures. The value of a can also be described simply as pr2. Based on this assumption, it can be seen that the coefficient a in the conventional FN equation is the same for AlN tips with different tip radii but with the same tip height. So the fieldemission current is mainly dependent on the coefficient b, which is primarily related to the field enhancement factor b. When the tip height h is kept constant, the smaller the tip radius r, the greater the field enhancement factor b. This can well explain that the AlN cone with a tip radius of 40 nm has better field emission than AlN cones with a bigger tip radius. According to the above discussion, emission current density of the AlN cone with 40 nm tip radius should be 50 times larger than that of the cone with 200 nm tip radius at 100 V. As a comparison, a single silicon cone can also be fabricated by the same method as the AlN cone. Fig. 7 shows the typical field-emission I–V curves of an individual AlN cone and individual Si cone with the same aspect ratio. Both cones have the same height of 4 mm and a tip radius of 40 nm. As seen in Fig. 7, the threshold voltage of the individual AlN is below 3 V, which is much lower than that of Si cone (12 V). When a voltage of 50 V was applied, the emission current was near 60 nA for the AlN cone and 23 nA for silicon cone. It is obvious that the individual AlN cone has better field-emission ability than the individual Si tip at the same aspect ratio. On the one hand, this may be attributed mostly to AlN’s negative electron affinity. On the other hand, for the individual AlN cone fabricated by FIB etching method in our work, it is a
In summary, an individual AlN cone with high aspect ratio was fabricated by FIB technology. The intrinsic field-emission characteristic of the individual tip was measured in a dualprobe SEM system. The results indicated that an as-formed single AlN cone with high aspect ratio exhibited good fieldemission ability without any field shielding effect, although it only had a tiny emission area. The field-emission property of a single AlN cone depends on its tip radius, and a smaller tip radius is very favorable to enhancing field-emission property. In addition, compared with a single Si cone fabricated by the same method, a single AlN cone has better electron emission ability due to AlN’s negative electron affinity and the composite structure of the AlN and Si substrate. Therefore, an individual AlN cone fabricated by the FIB milling method has great potential as point electron source in future vacuum electronic devices. Acknowledgement This work was supported by the National Natural Science Foundation of China (Grant No. 60671048). References [1] A.F. Belyanin, L.L. Bouilov, V.V. Zhirnov, A.I. Kamenev, K.A. Kovalskij, B.V. Spitsyn, Diamond Relat. Mater. 8 (1999) 369. [2] C.I. Wu, A. Kahn, E.S. Hellman, D.N.E. Buchanan, Appl. Phys. Lett. 73 (1998) 1346. [3] A.P. Huang, P.K. Chu, X.L. Wu, Appl. Phys. Lett. 88 (2006) 251103. [4] Y.B. Tang, H.T. Cong, Z.G. Chen, H.M. Cheng, Appl. Phys. Lett. 86 (2005) 233104. [5] Y.B. Tang, H.T. Cong, Z.G. Zhao, H.M. Cheng, Appl. Phys. Lett. 86 (2005) 153104. [6] S.C. Shi, C.F. Chen, S. Chattopadhyay, K.H. Chen, L.C. Chen, Appl. Phys. Lett. 87 (2005) 073109. [7] J.J. Li, C.Z. Gu, Q. Wang, P. Xu, Z.L. Wang, Z. Xu, X.D. Bai, Appl. Phys. Lett. 87 (2005) 143107. [8] R.H. Fowler, L. Nordheim, Proc. R. Soc. Lond. Ser. A 119 (1928) 173. [9] V.V. Zhirnov, G.J. Wojak, W.B. Choi, J.J. Cuomo, J.J. Hren, J. Vac. Sci. Technol. A 15 (3) (1997) 1733.
Applied Surface Science 254 (2008) 4840–4844 www.elsevier.com/locate/apsusc
Local field-emission characteristic of individual AlN cone fabricated by focused ion-beam etching method Y.L. Li, C.Y. Shi, J.J. Li *, C.Z. Gu Microfabrication Laboratory, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China Received 6 January 2008; accepted 23 January 2008 Available online 5 February 2008
Abstract Highly (0 0 2)-oriented AlN film was deposited on n-type (1 0 0)-oriented silicon substrates by the radio frequency magnetron sputtering method. An individual AlN cone with high aspect ratio was fabricated by the focused ion-beam (FIB) etching process in the surface of an as-formed AlN film. This etching method can easily control the tip radius and height to obtain AlN cones with different aspect ratios. The field-emission property of the individual AlN cone was measured in a scanning electron microscopy system equipped with a movable probe as the anode above the AlN tip. The results indicated that the as-formed single AlN cone with high aspect ratio possessed good field-emission ability although it only had a tiny emission area. Compared with a single Si tip fabricated by the same method, a single AlN cone exhibits better field-emission ability, and hence, has great potential as a promising candidate of point electron source for application in vacuum electronic devices. # 2008 Elsevier B.V. All rights reserved. PACS : 79.70+q; 81.16c Keywords: AlN cone; Field emission; Focused ion-beam etching
1. Introduction Aluminum nitride (AlN) has attracted significant interest because of its unique physical properties such as negative electron affinity, high thermal conductivity, high melting point, good chemical stability, and low dielectric loss, which make it a promising field-emission material for planar display devices and microelectronics field application [1–3]. To promote the field-emission performance of AlN, we desired to fabricate the AlN cone arrays because they have a large field enhancement factor that allow the electrons to tunnel into vacuum easily. To date, most studies have concentrated on AlN nanocone arrays and their field-emission properties [4–6]. However, the field shielding effect often occurs in the field-emission processing of cone arrays due to a short space between cones, which would make against the enhancement of field emission. Also, the field emission from the cone arrays cannot reflect an intrinsic fieldemission property of a single cone. On the other hand, an individual nanocone also has some important applications for
* Corresponding author. Tel.: +86 10 82648198; fax: +86 10 82648198. E-mail address: [email protected] (J.J. Li). 0169-4332/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.01.119
an electron source and atomic force microscopy (AFM) and scanning tunneling microscopy (STM) probes. Therefore, studying field emission of a single AlN cone is significant for understanding the field-emission process of whole cone arrays, but also, is very helpful in the application area. To investigate field emission from a single AlN cone, chief of all is to prepare AlN cone. But the reported AlN cone arrays were usually grown by the chemical vapor deposition (CVD) method [4–6]. With CVD, it is very difficult to control the cone density so that the field-emission property of an individual AlN cone is difficult to measure due to the limit of higher densities of the as-grown AlN cone. In this paper, an individual AlN cone was fabricated successfully by the focused ion-beam (FIB) etching method, and then its field-emission property was tested and discussed in detail. 2. Experimental details AlN films were deposited on n-type (1 0 0)-oriented silicon substrates in a radio frequency (RF) magnetron sputtering system. Aluminum with 99.99% purity was used as the sputtering target. Prior to loading into the sputtering chamber, the substrates were etched by dipping in a dilute solution of 4%
Y.L. Li et al. / Applied Surface Science 254 (2008) 4840–4844
4841
HF for 60 s to remove the chemical surface oxide layer. Degreasing was carried out by ultrasonic-assisted cleaning in acetone and methanol. Once the base chamber gas pressure reached 1 104 Pa, a gas flow mixture N2 and Ar was maintained into the chamber with a ratio of about 60:40. The total gas pressure was kept at 0.75 Pa during the sputtering process. The sputtering power and substrate temperature were kept at 120 W and 700 8C, respectively. The surface morphology of the AlN film was characterized by scanning electron microscopy (SEM), and the X-ray diffraction (XRD) was used to prove that high-quality oriented AlN films are formed at the above-mentioned experimental conditions. An individual AlN cone with high aspect ratio was fabricated on the as-deposited AlN film by FIB system using 30 kV Ga+ ion source (produced by FEI Company, USA). We designed a simple doughnut pattern that had an inner and outer radius for the etching. Fig. 1(a) shows the etching profile sketch map. During the etching time, the annulus area was etched off, but the wafer and central area was protected so that a long conical structure was formed in a hole. Fig. 1(b) shows a typical SEM image of an as-formed single AlN cone, and its radius and height can easily be controlled by adjusting the etching parameter.
Field-emission property of an individual AlN cone has been studied using a customized double-probe SEM system at a chamber pressure of about 106 torr, as illustrated in Fig. 2(a). Fig. 2(b) displays the SEM image for the fieldemission measurements of an individual AlN cone. The SEM system is equipped with a microsized anode probe that is more efficient at collecting locally emitted electrons than conventional anode panel systems. The anode probe has a tip radius of about 1 mm formed by chemical etching of a tungsten wire, and the distance between the anode probe tip and a AlN cone top can be adjusted to as little as 0.1 mm by the probe controlled system for all field-emission measurement. The anode probes used in our work can be adjusted in three-dimensional while we can image the sample and probe using the scanning electron microscopy. Owing to the lowest step size of probe moving in our probe controlled system is about 5 nm, we can firstly touch the cone top by moving the probe and then leave along cone top, and the elevated spacing from the cone top can be adjusted to 100 nm (0.1 mm) by probe controlled system. For the as-formed tips with different radii, the same probe controlled process was carried out to assure a similar spacing from cone tip to the probe for the measurement of their field-emission current–voltage (I–V) property. The emission current was measured by a picoampere meter upon applying a dc voltage between the anode probe and the silicon substrate. The measurement was performed at room temperature.
Fig. 1. (a) Sketch map of AlN cone fabrication. (b) SEM images of individual AlN tip.
Fig. 2. Typical experimental configuration for field-emission measurements: (a) sketch map, (b) SEM image.
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3. Results and discussion 3.1. Morphological and structural characterization Fig. 3(a) shows the surface morphology of the AlN film deposited on the (1 0 0)-oriented silicon substrates by the RF magnetron sputtering method. It can be seen that the pebblelike grains are formed uniformly on the surface of the AlN film, which reflects the surface morphology character of (0 0 2)oriented the AlN films. But this characteristic still needs to be further proven by XRD. The corresponding XRD results are shown in Fig. 3(b), in which a strong diffraction peak appears at about 368 of 2u, indicating a highly (0 0 2)-oriented character for the as-formed AlN film. Thus, we concluded that the highquality AlN film with (0 0 2) orientation was formed at the above-mentioned experimental conditions. Fig. 4 shows SEM images of a single AlN cone on the surface of an AlN film fabricated by the FIB etching method. An as-formed single AlN cone has different tip radii, which is 200, 80, and 40 nm, respectively. All the cones are kept at the same height of 4 mm by controlling FIB etching parameters. The formation of these different tip radii is due to changing the inner radius of the doughnut pattern during FIB milling; thus, AlN tips with high aspect ratio can be achieved. These as-formed AlN cones with different tip radii are used for field-emission measurement. 3.2. Field-emission property Fig. 5(a) shows a typical field-emission current–voltage curve of an individual AlN cone with a tip radius of 40 nm and a
Fig. 4. SEM images of individual AlN tips with different tip radii and same height (a) 200 nm, (b) 80 nm, (c) 40 nm.
Fig. 3. (a) SEM image of as-deposited AlN films. (b) Corresponding XRD result.
total height of 4 mm. The turn-on voltage is defined as 2 V, which is corresponding a current of 4 nA. As we can see, when a voltage of 54 V was applied, the emission current is increased to 40 nA; meanwhile, the corresponding emission current density is estimated as near 800 A/cm2. A maximum current of
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Fig. 6. Field-emission properties of AlN tips with different tip radii.
Fig. 5. (a) Typical field-emission I–V curve of the individual AlN tip. (b) Corresponding FN plot.
560 nA is obtained at an applied voltage of 100 V, which corresponds to a large emission current density although the AlN cone only has a tiny emission area. It indicates that the individual AlN tip has good field-emission property, with the ability to be a point electron source. The corresponding Fowler–Nordheim (FN) plot was also shown in Fig. 5(b). At low applied field, the FN plot is linear, which accords with the conventional FN theory, and indicates that the electron emission is indeed caused by vacuum tunneling [7]. But at high applied field, the bending trend of the FN plot occurs due to a different field enchantment factor from that at low field, which may be related to the surface desorption of the AlN cone top and Ga+ injected into the surface layer of AlN cone during the FIB milling process by high-energy Ga ions. When a high field is applied, emission current is increased and simultaneously the local temperature of the cone top is also enhanced; thus, the surface adsorbent was removed by the surface desorption so that the surface effective work function was changed. In addition, the effect of the Ga ions remained in the top of AlN cone may be another reason why the surface work function change with the increase of applied field. At high electric field, these injected Ga ions may be reduced or removed by the electron emission process, which will influence the effective work function of the AlN cone top, and hence changes the FN plot. The above field-emission results reflect the
intrinsic field-emission property of the AlN cone without any field shielding effect that often occurs with nanocone or nanotube arrays. Thus, the intrinsic field-emission property of a single AlN cone is very helpful for its potential application in the microelectronic field. In addition, the field-emission properties of an individual AlN cone with different tip radii and the same cone height were also measured, and the effect of tip radius on field-emission ability of the AlN cone was discussed. The field-emission results are shown in Fig. 6. Among these individual AlN cones, the total cone height was kept at 4 mm, but the tip radius is changed to 200, 80 and 40 nm, respectively. From Fig. 6, we can see that the AlN cone with a tip radius of 40 nm has better field-emission ability than other AlN cones with a tip radius of 80 and 200 nm. When the applied voltage is increased to 50 V, the corresponding emission current is 60 nA for a tip radius of 40 nm, 45 nA for a tip radius of 80 nm, and 31 nA for a tip radius of 200 nm, respectively. Further, the maximum emission current of the AlN tip with a tip radius of 40 nm at 100 V is more than 500 nA, which is about twice as lager as the AlN cones with a tip radius of 200 nm. But the emission area of the AlN tip with a tip radius of 40 nm is much less than that with a tip radius of 200 nm, and thus, the emission current density of the AlN tip with a tip radius of 40 nm is much greater than that of one with a tip radius of 200 nm. Therefore, the AlN tip with a radius of 40 nm has the best field-emission ability among asformed AlN cones with different tip radii. The reason why the individual AlN cone with a smaller tip radius has better field emission is that this AlN tip has a greater field enhancement factor b, which is very favorable to enhancing field emission of the AlN cone. In conventional FN theory, the relationship between the field-emission current (I) and the applied voltage (V) can be described as follows [8]: b I ¼ aV 2 exp V a ¼ 1:4 106 ’1 b2 expð9:89’1=2 Þa b ¼ 6:53 107 ’3=2 b1 ;
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composite structure of AlN cone and the silicon substrate, including the upper part of AlN cone and trapeziform lower part of Si substrate, as shown in Fig. 2(a). This composite structure is favorable for increasing the field-emission property of the AlN cone because it can effectively decrease the effective work function [9] compared to the individual silicon cone. 4. Conclusions
Fig. 7. Field-emission properties of AlN tip and individual silicon tip with the same aspect ratio.
where a is the field-emission effective area and b is the field enhancement factor. Assuming that h is the tip height and r is the tip radius, b can be described as h/r, which is adaptable to tip structures. The value of a can also be described simply as pr2. Based on this assumption, it can be seen that the coefficient a in the conventional FN equation is the same for AlN tips with different tip radii but with the same tip height. So the fieldemission current is mainly dependent on the coefficient b, which is primarily related to the field enhancement factor b. When the tip height h is kept constant, the smaller the tip radius r, the greater the field enhancement factor b. This can well explain that the AlN cone with a tip radius of 40 nm has better field emission than AlN cones with a bigger tip radius. According to the above discussion, emission current density of the AlN cone with 40 nm tip radius should be 50 times larger than that of the cone with 200 nm tip radius at 100 V. As a comparison, a single silicon cone can also be fabricated by the same method as the AlN cone. Fig. 7 shows the typical field-emission I–V curves of an individual AlN cone and individual Si cone with the same aspect ratio. Both cones have the same height of 4 mm and a tip radius of 40 nm. As seen in Fig. 7, the threshold voltage of the individual AlN is below 3 V, which is much lower than that of Si cone (12 V). When a voltage of 50 V was applied, the emission current was near 60 nA for the AlN cone and 23 nA for silicon cone. It is obvious that the individual AlN cone has better field-emission ability than the individual Si tip at the same aspect ratio. On the one hand, this may be attributed mostly to AlN’s negative electron affinity. On the other hand, for the individual AlN cone fabricated by FIB etching method in our work, it is a
In summary, an individual AlN cone with high aspect ratio was fabricated by FIB technology. The intrinsic field-emission characteristic of the individual tip was measured in a dualprobe SEM system. The results indicated that an as-formed single AlN cone with high aspect ratio exhibited good fieldemission ability without any field shielding effect, although it only had a tiny emission area. The field-emission property of a single AlN cone depends on its tip radius, and a smaller tip radius is very favorable to enhancing field-emission property. In addition, compared with a single Si cone fabricated by the same method, a single AlN cone has better electron emission ability due to AlN’s negative electron affinity and the composite structure of the AlN and Si substrate. Therefore, an individual AlN cone fabricated by the FIB milling method has great potential as point electron source in future vacuum electronic devices. Acknowledgement This work was supported by the National Natural Science Foundation of China (Grant No. 60671048). References [1] A.F. Belyanin, L.L. Bouilov, V.V. Zhirnov, A.I. Kamenev, K.A. Kovalskij, B.V. Spitsyn, Diamond Relat. Mater. 8 (1999) 369. [2] C.I. Wu, A. Kahn, E.S. Hellman, D.N.E. Buchanan, Appl. Phys. Lett. 73 (1998) 1346. [3] A.P. Huang, P.K. Chu, X.L. Wu, Appl. Phys. Lett. 88 (2006) 251103. [4] Y.B. Tang, H.T. Cong, Z.G. Chen, H.M. Cheng, Appl. Phys. Lett. 86 (2005) 233104. [5] Y.B. Tang, H.T. Cong, Z.G. Zhao, H.M. Cheng, Appl. Phys. Lett. 86 (2005) 153104. [6] S.C. Shi, C.F. Chen, S. Chattopadhyay, K.H. Chen, L.C. Chen, Appl. Phys. Lett. 87 (2005) 073109. [7] J.J. Li, C.Z. Gu, Q. Wang, P. Xu, Z.L. Wang, Z. Xu, X.D. Bai, Appl. Phys. Lett. 87 (2005) 143107. [8] R.H. Fowler, L. Nordheim, Proc. R. Soc. Lond. Ser. A 119 (1928) 173. [9] V.V. Zhirnov, G.J. Wojak, W.B. Choi, J.J. Cuomo, J.J. Hren, J. Vac. Sci. Technol. A 15 (3) (1997) 1733.