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Synthesis and field emission properties of GaN nanowires
Applied Surface Science 257 (2011) 10850–10854
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Synthesis and field emission properties of GaN nanowires Enling Li ∗ , Zhen Cui, Yuanbin Dai, Danna Zhao, Tao Zhao Science School, Xi’an University of Technology, Xi’an 710048, China
a r t i c l e
i n f o
Article history: Received 18 April 2011 Received in revised form 24 July 2011 Accepted 25 July 2011 Available online 2 August 2011 Keywords: GaN nanowire Chemical vapor deposition (CVD) Field emission Open electric field
a b s t r a c t Gallium nitride (GaN) nanowires grown on nickel-coated n-type Si (1 0 0) substrates have been synthesized using chemical vapor deposition (CVD), and the field emission properties of GaN nanowires have been studied. The results show that (1) the grown GaN nanowires, which have diameters in the range of 50–100 nm and lengths of several micrometers, are uniformly distributed on Si substrates. The characteristics of the grown GaN nanowires have been investigated using X-ray diffraction (XRD) and transmission electron microscopy (TEM), and through these investigations it was found that the GaN nanowires are of a good crystalline quality (2) When the emission current density is 100 A/cm2 , the necessary electric field is an open electric field of around 9.1 V/m (at room temperature). The field enhancement factor is ∼730. The field emission properties of GaN nanowires films are related both to the surface roughness and the density of the nanowires in the film. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Field electron emission has attracted significant interest in recent years as a unique electron emission method. GaN semiconductor material, which has a small electron affinity (3.3 eV), stable physical and chemical properties and a higher melting point (1500 ◦ C), is a promising cathode material for field emission [1,2]. Research on the field emission properties of one-dimensional nanomaterials began in 1999. Fan et al. [3] studied the field emission properties of an array of carbon nanotubes. The electronic structure of carbon nanotubes limits their field emission application [4]; therefore, the field emission of other one-dimensional semiconductor nanomaterials (such as Si, SiC, ZnO and III-nitride) has also attracted the interest of researchers [5–7]. Kim et al. [8] have studied the field emission properties of GaN nanowires synthesized by chemical vapor deposition (CVD) with a nickel catalyst. Ha et al. [9] and Kim et al. [10] studied the field emission properties of GaN nanowires and nanorods arrays, and their results show that the field emission properties of one-dimensional GaN nanomaterials are better than those of carbon nanotubes. Ng et al. [11,12] have studied the field emission properties of GaN nanowires using the PLD method, and found that the field emission properties of nanowires depend not only on their crystal structure and electron affinity but also on the density of the nanowires and the film’s aspect ratio. Tang et al. [13] have investigated the field emission properties of individual GaN nanowires coated with Ga2 O3 ; they found that a coated layer is effective in improving the field emission
∗ Corresponding author. Tel.: +86 29 82066357; fax: +86 29 82066357. E-mail address: [email protected] (E. Li). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.07.120
properties of GaN nanowire. Dinh et al. [14] have also studied the properties of GaN nanowires, and their results show that the sharp ends and rough surfaces of the GaN nanowires are responsible for their good field emission properties. In our research, we synthesized high-quality single crystalline GaN nanowires on nickel-coated n-type Si (1 0 0) substrates using the CVD method. The GaN nanowires were grown via a vapor–liquid–solid mechanism. The shape and morphology of the GaN nanowires were controlled by changing the distribution of the nickel particles. The morphological, structural, and field emission properties of the GaN nanowires were investigated.
2. Synthesis and characterization GaN nanowires were synthesized using CVD. The Si (1 0 0) substrates were sequentially cleaned in trichloroethylene, acetone, and ethanol solutions using an ultrasonic cleaner. Ni thin films were deposited onto Si substrates by immersing them into the ethanol solution of nickel nitrate with a concentration of 2%. Ultrasonic oscillation was subsequently performed to obtain uniform catalyst particles distributed on the substrates. Next, the coated substrates were dried for 12 h in a muffle furnace; they were subsequently placed in a quartz boat, which was then transferred into the horizontal quartz tube of the CVD system. The chamber was heated and nitrogen was introduced for 30 min at 310 ◦ C. Ammonia replaced nitrogen at 800 ◦ C for another 30 min; the system was subsequently allowed to cool to room temperature. Finally, the size and the uniformity of the nickel particles were determined by ultrasonic oscillation. FESEM images of two nickel-coated substrates (Substrates 1 and 2) with different ultrasonic oscillation times of 30 min
E. Li et al. / Applied Surface Science 257 (2011) 10850–10854
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Fig. 1. FESEM image of nickel catalyst particles on Si (1 0 0) substrate. (1) Substrate 1 and (2) Substrate 2.
and 60 min, respectively, are shown in Fig. 1. The nickel particles of substrate 2 are small and uniform. The source material was Ga2 O3, which was placed into a quartz boat. The Si substrate, which was dried after oscillation in the quartz boat, was placed 2 cm from the source. Next, the quartz boat was transferred to the center of the horizontal quartz tube of the CVD system. The chamber was heated, and nitrogen was introduced into the chamber. During the main growth period (at a temperature of 1050 ◦ C), high-purity NH3 (99.99%) gas was introduced into the system. The main growth time was 40 min. We synthesized two samples (Samples 1 and 2), which used the two nickel-coated substrates (1 and 2) that were prepared previously with the abovementioned experiment process. The X-ray diffraction (XRD) spectrum of the two samples are shown in Fig. 2; the diffraction peaks (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (1 1 2) and (2 0 1) in the figures are completely consistent with the standard card of hexagonal wurtzite GaN. This property means that the products of the ammoniated reaction displayed the hexagonal wurtzite structure of GaN crystals. FESEM images of the GaN nanowires films at (a) low magnification and (b) high magnification are shown in Fig. 3. The diameters of the GaN nanowires are in the range of 50–100 nm, and the lengths of the nanowires are equivalent to several micrometers. The thickness and smoothness of the GaN nanowires in Sample 2 are superior to the thickness and smoothness of the GaN nanowires in Sample
1, and the density of nanowires in Sample 2 is also greater than the density of the nanowires in Sample 1. This finding correlates with the nickel-coated substrates, which has different nickel catalyst particles that are shown in Fig. 1. In the high magnification of Fig. 3(b) it can be seen that there are Au catalysts at the ends of the nanowires, indicating that the growth mechanism of the GaN nanowires in the experiment is based on a vapor–liquid–solid (VLS) mechanism. For further structural information on the GaN nanowires, TEM and the corresponding selected-area electron diffraction (SAED) of an individual as-grown GaN nanowire were executed. TEM images of an individual as-grown GaN nanowire of the two samples are shown in Fig. 4. The diameter of the two individual nanowires is about 50 nm, and the surface of Sample 1 is rougher than that of Sample 2. The inset of Fig. 3 shows corresponding SAED patterns of the single nanowire, which show that the structures of the individual as-grown GaN nanowire are single-crystalline structures.
3. Field emission properties and analysis The field emission measurement systems of thin film materials consist of a bipolar structure and a three-pole structure. A plate bipolar structure was used in our measurement and is shown in Fig. 5; this structure is mainly composed of a power system
Fig. 2. XRD spectrum of the GaN nanowires. (1) Sample 1. (2) Sample 2.
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E. Li et al. / Applied Surface Science 257 (2011) 10850–10854
Fig. 3. FESEM images of the GaN nanowires films at (a) low magnification and (b) high magnification. (1) Sample 1. (2) Sample 2.
(including a DC power supply, voltmeter, ammeter, and protection resistors), a vacuum system (including diffusion pumps and mechanical pumps) and circuit components. The GaN nanowire film acts as the electron emission cathode, and the anode that receives electron emission is ITO conductive glass. The anode and
cathode are insulated using poly tetra fluoro ethylene (PTFE) such that the distance between the two electrodes is the thickness of PTFE (120 m). The vacuum is composed of 3.8 × 10−4 Pa. The field emission measurements (the J–E curve) are shown in Fig. 6; the current density is equal to the quotient of the current and the
Fig. 4. TEM image of individual GaN nanowires; the inset is the SAED pattern. (1) Sample 1. (2) Sample 2.
E. Li et al. / Applied Surface Science 257 (2011) 10850–10854
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Fig. 5. Schematic representation of the test system of the field emission properties.
area of the sample, and the electric field intensity is equal to the quotient of the voltage between the poles and the electrode spacing. The Fowler–Nordheim (F–N) formula is generally used to describe the field emission properties of the materials, as shown by the following equation:
J=
Aˇ2 E 2
exp
−B3/2 ˇE
(1)
where A, B are constants, J is the current density, ˇ is the geometric enhancement factor, and is the work function. The abovementioned equation can also be rewritten as: ln
J −B3/2 1 = − ln 2 2 E ˇ E Aˇ
(2)
Taking 1/E as the abscissa and ln J/E2 as the vertical axis, we obtained the F–N curve shown in Fig. 7. From Eq. (2), we can see that 1/E and ln J/E2 should satisfy a linear relationship and that the intercept reflects the relationship of the emitter geometry enhancement factor ˇ, the work function , and the constant A; the slope reflects the relationship of ˇ, , and the constant B. From = 4.1 eV, B = 6.83 × 103 (V eV−3/2 V m−1 ), and the F–N line slope of Samples 1 and 2, we obtain the ˇ of Sample 1 and 2 as 730 and 551, respectively. In Fig. 7, we can see that the curves of the two samples can be divided into two parts. The first part is approximately linear, indicating that the electron field emission properties of the two samples are close to the F–N emission. The second parts of the curves of the
Fig. 7. Corresponding F–N curves of the GaN nanowires films; line 1 represents Sample 1 and line 2 represents Sample 2.
two samples drift off the F–N line, and we suspect that this finding is because of the effect of the space charge and the absorbent. The space charge can lead to a drifting of the F–N line; there may be H2 O in the remaining gas in the vacuum, and the samples may absorb H2 O from the atmosphere that can decrease the surface work function of the GaN nanowires [15,16] also leading to a drifting of the F–N line. However, along with the field emission current density enlarge, a number of nanowires were flattened and shortened when they were bombarded by the remained gas particles. The electric field is defined as an open electric field when the value of the emission’s current density is at 100 A/cm2 . In Fig. 6, we can see that the open electric field of Sample 1 is ∼9.1 V/m, which is smaller than that of Sample 2, and that ˇ1 (∼730) > ˇ2 (∼551). Therefore, we can conclude that the field emission property of Sample 1 is superior to the field emission property of sample 2. We believe that this result is due to (1) the density of the nanowires of Sample 2 being greater than that of Sample 1 such that the nanowires’ film shielding effect in Sample 2 is stronger than that of Sample 1, leading to a worse field emission property in Sample 2. (2) In Fig. 4, we can see that the surface of Sample 2 is smooth but that there are nanometer protrusions on the surface of Sample 1. In the case of high voltage, the electric field at these small protrusions will be greatly enhanced, and these small protrusions become emission centers that could involve electron emission. In other words, the electron emission of GaN nanowires would occur not only at the edge but also at the surface.
4. Conclusion
Fig. 6. Field emission J–E curves of the GaN nanowires films; line 1 represents Sample 1 and line 2 represents Sample 2.
We synthesized two GaN nanowires film samples using the CVD method and studied their field emission properties. The asgrown GaN nanowires, which are of a good crystalline quality and have diameters in the range of 50–100 nm and lengths of several micrometers, are uniformly distributed on Si substrates. The open electric field of Sample 1 is approximately 9.1 V/m, and the field enhancement factor is ∼730. Compared with Sample 1, the field emission properties of Sample 2 are poor. According to our research, a rough surface and a low density are favorable for field emission properties. The study of the field emission properties of GaN nanowires in our paper is propitious for understanding the theory of nanomaterials, and the study indicates great prospects in plane display.
10854
E. Li et al. / Applied Surface Science 257 (2011) 10850–10854
Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51042010) and the Science and Technology Project Foundation of Shaanxi Province, China (2011K07-09). References [1] H. Yoshida, T. Urushido, H. Miyake, K. Hiramatsu, Formation of GaN selforganized nanotips by reactive ion etching, Jpn. J. Appl. Phys. Part 2: Lett. 40 (2001) 1301–1304. [2] D.K.T. Ng, M.H. Hong, L.S. Tan, Y.W. Zhu, C.H. Sow, Field emission enhancement from patterned gallium nitride nanowires, Nanotechnology 18 (2007) 375707 (1–5). [3] S.S. Fan, M.G. Chapline, N.R. Franklin, T.W. Tombler, A.M. Cassell, H.J. Dai, Selforiented regular arrays of carbon nanotubes and their field emission properties, Science 283 (1999) 512–514. [4] H.M. Kim, T.W. Kang, K.S. Chung, J.P. Hong, W.B. Choi, Field emission displays of wide-bandgap gallium nitride nanorod arrays grown by hydride vapor phase epitaxy, Chem. Phys. Lett. 77 (2003) 491–494. [5] Z.S. Wu, S.Z. Deng, N.S. Xu, J. Chen, J. Zhou, J. Chen, Needle-shaped silicon carbide nanowires: synthesis and field electron emission properties, Appl. Phys. Lett. 80 (2002) 3829–3831. [6] C.Y. Lee, T.Y. Tseng, S.Y. Li, P. Lin, Effect of phosphorus dopant on photoluminescence and field-emission properties of Mg0.1 Zn0.9 O nanowires, J. Appl. Phys. 99 (2006) 024303 (1–6).
[7] S.Q. Li, Y.X. Liang, T.H. Wang, Nonlinear properties of the Fowler–Nordheim plot for field emission from In2 O3 nanowires grown on InAs substrate, Appl. Phys. Lett. 88 (2006) 053107. [8] T.Y. Kim, S.H. Lee, Y.H. Mo, H.W. Shim, K.S. Nahm, E.K. Suh, J.W. Yang, K.Y. Lim, G.S. Park, Growth of GaN nanowires on Si substrate using Ni catalyst in vertical chemical vapor deposition reactor, J. Cryst. Growth 257 (2003) 815–818. [9] B. Ha, S.H. Seo, J.H. Cho, C.S. Yoon, J. Yoo, G.C. Yi, C.Y. Park, C.J. Lee, Optical field emission properties of thin single crystalline GaN nanowires, J. Phys. Chem. B 109 (2005) 11095–11099. [10] H.M. Kim, T.W. Kang, K.S. Chung, J.P. Hong, W.B. Choi, Field emission displays of wide-bandgap gallium nitride nanorod arrays grown by hydride vapor phase epitaxy, Chem. Phys. Lett. 377 (2003) 491–494. [11] D.K.T. Ng, M.H. Hong, L.S. Tan, Y.W. Zhu, C.H. Sow, Field emission enhancement from patterned gallium nitride nanowires Nanotechnology, Nanotechnology 18 (2007) 375707 (1–5). [12] D.K.T. Ng, M.H. Hong, L.S. Tan, Y.W. Zhu, C.H. Sow, PLD synthesis of GaN nanowires and nanodots on patterned catalyst surface for field emission study, Appl. Phys. A 93 (2008) 685–689. [13] C.C. Tang, X.W. Xu, L. Hu, Y.X. Li, Improving field emission properties of GaN nanowires by oxide coating, Appl. Phys. Lett. 94 (2009) 243105 (1–3). [14] D.V. Dinh, S.M. Kang, J.H. Yang, S.W. Kim, D.H. Yoon, Synthesis and field emission properties of triangular-shaped GaN nanowires on Si (1 0 0) substrates, J. Cryst. Growth 311 (2009) 495–499. [15] A. Wadhawan, R.E. Stallcup, K.F. Stephens, J.M. Perez, I.A. Akwani, Effects of O2 , Ar and H2 gases on the field-emission properties of single walled and multiwalled carbon nanotubes, Appl. Phys. Lett. 79 (2001) 1867–1869. [16] A. Maiti, J. Andzelm, N. Tanpipat, P. Allmen, Effect of adsorbates on field emission from carbon nanotubes, Phys. Rev. Lett. 87 (2001) 155502.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Synthesis and field emission properties of GaN nanowires Enling Li ∗ , Zhen Cui, Yuanbin Dai, Danna Zhao, Tao Zhao Science School, Xi’an University of Technology, Xi’an 710048, China
a r t i c l e
i n f o
Article history: Received 18 April 2011 Received in revised form 24 July 2011 Accepted 25 July 2011 Available online 2 August 2011 Keywords: GaN nanowire Chemical vapor deposition (CVD) Field emission Open electric field
a b s t r a c t Gallium nitride (GaN) nanowires grown on nickel-coated n-type Si (1 0 0) substrates have been synthesized using chemical vapor deposition (CVD), and the field emission properties of GaN nanowires have been studied. The results show that (1) the grown GaN nanowires, which have diameters in the range of 50–100 nm and lengths of several micrometers, are uniformly distributed on Si substrates. The characteristics of the grown GaN nanowires have been investigated using X-ray diffraction (XRD) and transmission electron microscopy (TEM), and through these investigations it was found that the GaN nanowires are of a good crystalline quality (2) When the emission current density is 100 A/cm2 , the necessary electric field is an open electric field of around 9.1 V/m (at room temperature). The field enhancement factor is ∼730. The field emission properties of GaN nanowires films are related both to the surface roughness and the density of the nanowires in the film. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Field electron emission has attracted significant interest in recent years as a unique electron emission method. GaN semiconductor material, which has a small electron affinity (3.3 eV), stable physical and chemical properties and a higher melting point (1500 ◦ C), is a promising cathode material for field emission [1,2]. Research on the field emission properties of one-dimensional nanomaterials began in 1999. Fan et al. [3] studied the field emission properties of an array of carbon nanotubes. The electronic structure of carbon nanotubes limits their field emission application [4]; therefore, the field emission of other one-dimensional semiconductor nanomaterials (such as Si, SiC, ZnO and III-nitride) has also attracted the interest of researchers [5–7]. Kim et al. [8] have studied the field emission properties of GaN nanowires synthesized by chemical vapor deposition (CVD) with a nickel catalyst. Ha et al. [9] and Kim et al. [10] studied the field emission properties of GaN nanowires and nanorods arrays, and their results show that the field emission properties of one-dimensional GaN nanomaterials are better than those of carbon nanotubes. Ng et al. [11,12] have studied the field emission properties of GaN nanowires using the PLD method, and found that the field emission properties of nanowires depend not only on their crystal structure and electron affinity but also on the density of the nanowires and the film’s aspect ratio. Tang et al. [13] have investigated the field emission properties of individual GaN nanowires coated with Ga2 O3 ; they found that a coated layer is effective in improving the field emission
∗ Corresponding author. Tel.: +86 29 82066357; fax: +86 29 82066357. E-mail address: [email protected] (E. Li). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.07.120
properties of GaN nanowire. Dinh et al. [14] have also studied the properties of GaN nanowires, and their results show that the sharp ends and rough surfaces of the GaN nanowires are responsible for their good field emission properties. In our research, we synthesized high-quality single crystalline GaN nanowires on nickel-coated n-type Si (1 0 0) substrates using the CVD method. The GaN nanowires were grown via a vapor–liquid–solid mechanism. The shape and morphology of the GaN nanowires were controlled by changing the distribution of the nickel particles. The morphological, structural, and field emission properties of the GaN nanowires were investigated.
2. Synthesis and characterization GaN nanowires were synthesized using CVD. The Si (1 0 0) substrates were sequentially cleaned in trichloroethylene, acetone, and ethanol solutions using an ultrasonic cleaner. Ni thin films were deposited onto Si substrates by immersing them into the ethanol solution of nickel nitrate with a concentration of 2%. Ultrasonic oscillation was subsequently performed to obtain uniform catalyst particles distributed on the substrates. Next, the coated substrates were dried for 12 h in a muffle furnace; they were subsequently placed in a quartz boat, which was then transferred into the horizontal quartz tube of the CVD system. The chamber was heated and nitrogen was introduced for 30 min at 310 ◦ C. Ammonia replaced nitrogen at 800 ◦ C for another 30 min; the system was subsequently allowed to cool to room temperature. Finally, the size and the uniformity of the nickel particles were determined by ultrasonic oscillation. FESEM images of two nickel-coated substrates (Substrates 1 and 2) with different ultrasonic oscillation times of 30 min
E. Li et al. / Applied Surface Science 257 (2011) 10850–10854
10851
Fig. 1. FESEM image of nickel catalyst particles on Si (1 0 0) substrate. (1) Substrate 1 and (2) Substrate 2.
and 60 min, respectively, are shown in Fig. 1. The nickel particles of substrate 2 are small and uniform. The source material was Ga2 O3, which was placed into a quartz boat. The Si substrate, which was dried after oscillation in the quartz boat, was placed 2 cm from the source. Next, the quartz boat was transferred to the center of the horizontal quartz tube of the CVD system. The chamber was heated, and nitrogen was introduced into the chamber. During the main growth period (at a temperature of 1050 ◦ C), high-purity NH3 (99.99%) gas was introduced into the system. The main growth time was 40 min. We synthesized two samples (Samples 1 and 2), which used the two nickel-coated substrates (1 and 2) that were prepared previously with the abovementioned experiment process. The X-ray diffraction (XRD) spectrum of the two samples are shown in Fig. 2; the diffraction peaks (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (1 1 2) and (2 0 1) in the figures are completely consistent with the standard card of hexagonal wurtzite GaN. This property means that the products of the ammoniated reaction displayed the hexagonal wurtzite structure of GaN crystals. FESEM images of the GaN nanowires films at (a) low magnification and (b) high magnification are shown in Fig. 3. The diameters of the GaN nanowires are in the range of 50–100 nm, and the lengths of the nanowires are equivalent to several micrometers. The thickness and smoothness of the GaN nanowires in Sample 2 are superior to the thickness and smoothness of the GaN nanowires in Sample
1, and the density of nanowires in Sample 2 is also greater than the density of the nanowires in Sample 1. This finding correlates with the nickel-coated substrates, which has different nickel catalyst particles that are shown in Fig. 1. In the high magnification of Fig. 3(b) it can be seen that there are Au catalysts at the ends of the nanowires, indicating that the growth mechanism of the GaN nanowires in the experiment is based on a vapor–liquid–solid (VLS) mechanism. For further structural information on the GaN nanowires, TEM and the corresponding selected-area electron diffraction (SAED) of an individual as-grown GaN nanowire were executed. TEM images of an individual as-grown GaN nanowire of the two samples are shown in Fig. 4. The diameter of the two individual nanowires is about 50 nm, and the surface of Sample 1 is rougher than that of Sample 2. The inset of Fig. 3 shows corresponding SAED patterns of the single nanowire, which show that the structures of the individual as-grown GaN nanowire are single-crystalline structures.
3. Field emission properties and analysis The field emission measurement systems of thin film materials consist of a bipolar structure and a three-pole structure. A plate bipolar structure was used in our measurement and is shown in Fig. 5; this structure is mainly composed of a power system
Fig. 2. XRD spectrum of the GaN nanowires. (1) Sample 1. (2) Sample 2.
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E. Li et al. / Applied Surface Science 257 (2011) 10850–10854
Fig. 3. FESEM images of the GaN nanowires films at (a) low magnification and (b) high magnification. (1) Sample 1. (2) Sample 2.
(including a DC power supply, voltmeter, ammeter, and protection resistors), a vacuum system (including diffusion pumps and mechanical pumps) and circuit components. The GaN nanowire film acts as the electron emission cathode, and the anode that receives electron emission is ITO conductive glass. The anode and
cathode are insulated using poly tetra fluoro ethylene (PTFE) such that the distance between the two electrodes is the thickness of PTFE (120 m). The vacuum is composed of 3.8 × 10−4 Pa. The field emission measurements (the J–E curve) are shown in Fig. 6; the current density is equal to the quotient of the current and the
Fig. 4. TEM image of individual GaN nanowires; the inset is the SAED pattern. (1) Sample 1. (2) Sample 2.
E. Li et al. / Applied Surface Science 257 (2011) 10850–10854
10853
Fig. 5. Schematic representation of the test system of the field emission properties.
area of the sample, and the electric field intensity is equal to the quotient of the voltage between the poles and the electrode spacing. The Fowler–Nordheim (F–N) formula is generally used to describe the field emission properties of the materials, as shown by the following equation:
J=
Aˇ2 E 2
exp
−B3/2 ˇE
(1)
where A, B are constants, J is the current density, ˇ is the geometric enhancement factor, and is the work function. The abovementioned equation can also be rewritten as: ln
J −B3/2 1 = − ln 2 2 E ˇ E Aˇ
(2)
Taking 1/E as the abscissa and ln J/E2 as the vertical axis, we obtained the F–N curve shown in Fig. 7. From Eq. (2), we can see that 1/E and ln J/E2 should satisfy a linear relationship and that the intercept reflects the relationship of the emitter geometry enhancement factor ˇ, the work function , and the constant A; the slope reflects the relationship of ˇ, , and the constant B. From = 4.1 eV, B = 6.83 × 103 (V eV−3/2 V m−1 ), and the F–N line slope of Samples 1 and 2, we obtain the ˇ of Sample 1 and 2 as 730 and 551, respectively. In Fig. 7, we can see that the curves of the two samples can be divided into two parts. The first part is approximately linear, indicating that the electron field emission properties of the two samples are close to the F–N emission. The second parts of the curves of the
Fig. 7. Corresponding F–N curves of the GaN nanowires films; line 1 represents Sample 1 and line 2 represents Sample 2.
two samples drift off the F–N line, and we suspect that this finding is because of the effect of the space charge and the absorbent. The space charge can lead to a drifting of the F–N line; there may be H2 O in the remaining gas in the vacuum, and the samples may absorb H2 O from the atmosphere that can decrease the surface work function of the GaN nanowires [15,16] also leading to a drifting of the F–N line. However, along with the field emission current density enlarge, a number of nanowires were flattened and shortened when they were bombarded by the remained gas particles. The electric field is defined as an open electric field when the value of the emission’s current density is at 100 A/cm2 . In Fig. 6, we can see that the open electric field of Sample 1 is ∼9.1 V/m, which is smaller than that of Sample 2, and that ˇ1 (∼730) > ˇ2 (∼551). Therefore, we can conclude that the field emission property of Sample 1 is superior to the field emission property of sample 2. We believe that this result is due to (1) the density of the nanowires of Sample 2 being greater than that of Sample 1 such that the nanowires’ film shielding effect in Sample 2 is stronger than that of Sample 1, leading to a worse field emission property in Sample 2. (2) In Fig. 4, we can see that the surface of Sample 2 is smooth but that there are nanometer protrusions on the surface of Sample 1. In the case of high voltage, the electric field at these small protrusions will be greatly enhanced, and these small protrusions become emission centers that could involve electron emission. In other words, the electron emission of GaN nanowires would occur not only at the edge but also at the surface.
4. Conclusion
Fig. 6. Field emission J–E curves of the GaN nanowires films; line 1 represents Sample 1 and line 2 represents Sample 2.
We synthesized two GaN nanowires film samples using the CVD method and studied their field emission properties. The asgrown GaN nanowires, which are of a good crystalline quality and have diameters in the range of 50–100 nm and lengths of several micrometers, are uniformly distributed on Si substrates. The open electric field of Sample 1 is approximately 9.1 V/m, and the field enhancement factor is ∼730. Compared with Sample 1, the field emission properties of Sample 2 are poor. According to our research, a rough surface and a low density are favorable for field emission properties. The study of the field emission properties of GaN nanowires in our paper is propitious for understanding the theory of nanomaterials, and the study indicates great prospects in plane display.
10854
E. Li et al. / Applied Surface Science 257 (2011) 10850–10854
Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51042010) and the Science and Technology Project Foundation of Shaanxi Province, China (2011K07-09). References [1] H. Yoshida, T. Urushido, H. Miyake, K. Hiramatsu, Formation of GaN selforganized nanotips by reactive ion etching, Jpn. J. Appl. Phys. Part 2: Lett. 40 (2001) 1301–1304. [2] D.K.T. Ng, M.H. Hong, L.S. Tan, Y.W. Zhu, C.H. Sow, Field emission enhancement from patterned gallium nitride nanowires, Nanotechnology 18 (2007) 375707 (1–5). [3] S.S. Fan, M.G. Chapline, N.R. Franklin, T.W. Tombler, A.M. Cassell, H.J. Dai, Selforiented regular arrays of carbon nanotubes and their field emission properties, Science 283 (1999) 512–514. [4] H.M. Kim, T.W. Kang, K.S. Chung, J.P. Hong, W.B. Choi, Field emission displays of wide-bandgap gallium nitride nanorod arrays grown by hydride vapor phase epitaxy, Chem. Phys. Lett. 77 (2003) 491–494. [5] Z.S. Wu, S.Z. Deng, N.S. Xu, J. Chen, J. Zhou, J. Chen, Needle-shaped silicon carbide nanowires: synthesis and field electron emission properties, Appl. Phys. Lett. 80 (2002) 3829–3831. [6] C.Y. Lee, T.Y. Tseng, S.Y. Li, P. Lin, Effect of phosphorus dopant on photoluminescence and field-emission properties of Mg0.1 Zn0.9 O nanowires, J. Appl. Phys. 99 (2006) 024303 (1–6).
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