Field Emission from Hydrogenated Amorphous Carbon ... - Core

Journal of The Electrochemical Society, 152 共6兲 C366-C369 共2005兲

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0013-4651/2005/152共6兲/C366/4/$7.00 © The Electrochemical Society, Inc.

Field Emission from Hydrogenated Amorphous Carbon Nanotips Grown on Cu/Ti/Si„100… C. H. Wei,a C. H. Chen,a C.-M. Yeh,a M. Y. Chen,a J. Hwang,a,z A. P. Lee,b and C. S. Koub a

Department of Materials Science and Engineering, and bDepartment of Physics, National Tsing Hua University, Hsin-Chu City, Taiwan

Hydrogenated amorphous carbon 共a-C:H兲 nanotips have been successfully grown on Cu/Ti/Si共100兲 by microwave plasmaenhanced chemical vapor deposition. A Cu etching process occurs simultaneously during the growth of the a-C:H nanotips. Both the Cu etching and a-C:H nanotips growth rates continuously increase with time. In the beginning, the Cu etching rate is approximately 3 nm/min and the upward growth rate of the a-C:H nanotips is approximately 1.2 nm/min. At the end of the growth, the Cu etching rate reaches 9 nm/min and the upward growth rate of the a-C:H nanotips reaches 5 nm/min. An etchinggrowth mechanism has been proposed to explain the formation of a-C:H nanotips on Cu/Ti/Si共100兲. The structure of the a-C:H nanotips exhibits very good field-emission characteristics where a low turn-on field of 3.2 V/␮m at 10 ␮A/cm2 is achieved. © 2005 The Electrochemical Society. 关DOI: 10.1149/1.1899264兴 All rights reserved. Manuscript submitted June 7, 2004; revised manuscript received December 8, 2004. Available electronically April 22, 2005.

The search for excellent field emitters has been an active research topic in the field of cold-cathode field-emission displays.1-4 A low turn-on field and a high current density can be achieved if a field emitter exhibits a high local field at its emission sites and channels of high conductivity for electrons to flow into vacuum. The carbon nanotube 共CNT兲 has been recognized as the best material to date for field emitters because of the high local field at its extremely sharp tip5 and its high conductivity, enabling it to carry a very large current density.6 Amorphous carbon 共a-C兲 is another promising material for field emitters because it exhibits good conductivity and a high local field at its internal tip inside the a-C film.7 Two types of a-C materials, hydrogenated amorphous carbon 共a-C:H兲 and diamond-like carbon, are usually used to fabricate flat a-C field emitters.7-10 However, the field-emission characteristics of flat a-C field emitters are worse than those of CNTs due to the lack of the nanotip structure. Very recently, well-aligned a-C:H nanotips have been successfully grown on a copper substrate by microwave plasma-enhanced chemical vapor deposition 共PECVD兲 at low temperature.11 The turn-on field and field-emission properties of a-C:H nanotips are close to those of CNTs because of the nanoscale tip structure. However, copper substrates are soft and easy to bend, which is not appropriate for flat panel displays. This inspires us to grow similar a-C:H nanotips on a Cu layer deposited on a flat substrate. Si共100兲 has been chosen as the flat substrate because it is well characterized and commercially available. A thin Ti interlayer layer is useful to improve the adhesion between Cu and Si共100兲. The growth of a-C:H nanotips on Cu/Ti/Si共100兲 was considered the same as that on Cu in the beginning of our work. However, no a-C:H nanotips were grown on Cu/Ti/Si共100兲 in our first trial. Further investigations have led us to discover the existence of a concurrent Cu etching process during the growth of a-C:H nanotips. In this article, we present the growth and field-emission characteristics of a-C:H nanotips grown on Cu/Ti/Si共100兲. Experimental A copper layer 1 ␮m thick deposited on p-Si共100兲 with a Ti interlayer of 300 nm to improve adhesion was cleaned with acetone and deionized water before being put on a 4 in. substrate holder in a planer microwave PECVD system. Details about the planar microwave PECVD system can be found in previous publications.12,13 A mixed gas of CH4 /H2 /N2 = 14:47:2 standard cubic centimeters per minute was fed into the chamber to ignite the CH4 /H2 /N2 mixed plasma operated at 3000 W and 0.14 Torr. The substrate tempera-

z

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ture was fixed at 200°C by monitoring an SiC heater. The a-C:H nanotips were then deposited on the Cu/Ti/Si共100兲 substrates at a bias voltage of −250 V for different time periods. The morphologies of the a-C:H nanotips were characterized by using a JEOL JSM6500F field-emission-scanning electron microscope 共FE-SEM兲. The Raman spectra were examined using a Renishaw 2000 Raman microscope 共Nd-YAG 532 nm兲. The field-emission characteristics were measured at 10−5 Torr in a planar diode configuration at room temperature. The interelectrode spacing was 70 ␮m defined by a spacer located outside the emission area and the area of indium-tin oxide collecting emission current was about 0.4 ⫻ 0.4 cm. Results and Discussion As mentioned, a-C:H nanotips have been successfully grown on a copper substrate.11 In the beginning of the work, a 1500 Å copper layer was considered thick enough for the growth of a-C:H nanotips on Cu/Ti/Si共100兲. However, no a-C:H nanotips appeared after a regular growth process under FE-SEM inspection. The 1500 Å Cu layer also disappeared after the growth, as shown by FE-SEM inspection. This suggests that a Cu etching process occurs during the growth process inside the CH4 /H2 /N2 mixed plasma. To investigate the Cu etching effect, a-C:H nanotips are grown on a copper layer 1 ␮m thick predeposited on Ti/Si共100兲. Figure 1 shows the sequential FE-SEM images of the a-C:H nanotips grown on the Cu/Ti/Si共100兲 substrate at different growth times. Initially, Cu/Ti/Si共100兲 exhibits a very rough surface with a peak-to-valley distance of about 40 nm, as shown in Fig. 1a. After 10 min growth, no nanotips were observed on Cu/Ti/Si共100兲. The thickness of Cu stays the same, and the rough surface becomes smoother, as shown in Fig. 1b. Figure 1c shows small nanotips grown around the valley of hills after 60 min growth. The surface roughness of Cu/Ti/Si共100兲 greatly increases and the peak-to-valley distance ranges approximately from 150 to 300 nm. The nanotips are determined to be 73 ± 17 nm high and 36 ± 8 nm diam, where the errors are the standard deviations of length and diameter. After 120 min growth, a distinct nanotip array structure appears on Cu/Ti/Si共100兲, as shown in Fig. 1d. The variations of the lengths of the nanotips in height and diameter become larger. The nanotips are about 315 ± 53 nm high and 119 ± 17 diam. Evidently, the growth rate of the nanotip becomes faster as the growth time increases beyond 60 min. The chemical information of the nanotips can be determined by the Raman spectrum shown in Fig. 2. The Raman peak consists of D and G band contributions as a typical a-C:H should have. The nanotip is determined to be a-C:H by comparing the Raman spectrum with the published data by Robertson.14 The growth rate of a-C:H nanotips is determined by measuring the averaged lengths of a-C:H nanotips in height and diameter at

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Figure 1. FE-SEM images of a-C:H nanotips at different growth times: 共a兲 0, 共b兲 10, 共c兲 60, and 共d兲 120 min.

different growth times, as shown in Fig. 3a. The upward growth rate of a-C:H nanotips is slightly faster than the lateral growth rate in the beginning and becomes faster at longer growth times. This gives the a-C:H nanotips a better tip shape, especially at 120 min growth time, as shown in Fig. 1d and 3a. Note that the upward growth rate continuously varies with growth time and is roughly divided into two stages for discussion. At the first stage, i.e., less than 60 min, the upward growth rate is approximately 1.2 nm/min, and it becomes 5 nm/min at the second stage, i.e., longer than 60 min. The time dependence of the upward growth rate is similar to what has been observed for Cu etching. Figure 3b shows the reduction of the Cu thickness at different growth times, where the thickness of Cu is determined from the cross-sectional view FE-SEM images. The error bars in Fig. 3b are the standard deiviation of the surface roughness of Cu. At growth times of less than 60 min, the Cu etching rate is about 3 nm/min on average, and it becomes 9 nm/min at growth times longer than 60 min. Note that the surface roughness of Cu varies with the growth time. It decreases at growth times of less than

Figure 2. Raman spectrum of a-C:H nanotips after 120 min growth.

Figure 3. 共a兲 Lengths of a-C:H nanotips in height and diameter and 共b兲 Cu thickness at different growth times.


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Figure 5. Field emission from a-C:H nanotips on Cu/Ti/Si. Turn-on field is 3.2 V/␮m. Current density of 1 mA/cm2 is reached at an electric field of 12 V/␮m.

turn-on field is 3.2 V/␮m, which is defined as the electric field at a current density of 10 ␮A/cm2. The current density of 1 mA/cm2 is reached at an electric field of 12 V/␮m. The enhance factor ␤ can be derived from the slope of the F-N plot. ␤ is estimated to be 8484 from the equation ␤ = −B⌽3/2 /m, where B = 6.83 ± 109 and ⌽ = 5 eV, and m is the slope of the curve after turn-on.16 The fieldemission characteristics of the a-C:H nanotips are comparable to those of other a-C:H field emitters.17-19 Conclusions

Figure 4. Etching-growth mechanism of a-C:H nanotips: rough surface of as-cleaned Cu/Ti/Si共100兲, smoothing by Cu etching process in CH4 /H2 /N2 mixed plasma, etching-growth mechanism during growth of a-C:H nanotips, and final shape of a-C:H nanotips.

30 min and then greatly increases at growth times larger than 60 min, which is evidence of Cu etching concurrent with the growth process of a-C:H nanotips. Both the upward growth rate and Cu etching rate are strongly correlated, which can be explained by the proposed etching-growth mechanism shown in Fig. 4 and is discussed as follows. In the beginning of the growth of the a-C:H nanotips, the as-cleaned Cu/Ti/Si共100兲 exhibits a rough surface. The roughed Cu/Ti/Si共100兲 surface is smoothed by a Cu etching process in a CH4 /H2 /N2 mixed plasma. The Cu etching rate is about 3 nm/min, which is probably due to the attack of positive ions, such as H+ and N+ ions.15 As long as the a-C:H nanotips start to grow on the valley area of the hills, the Cu underneath the nanotips is protected from the attack of positive ions. This causes more positive ions to flow over the uncovered Cu surface and results in a higher Cu etching rate. As the growth of the a-C:H nanotips continues, the Cu etching rate becomes faster and reaches 9 nm/min at growth times longer than 60 min. The a-C:H nanotips of high density stand upwards on a smooth Cu surface, as shown in Fig. 1d and 4. The increase of the upward growth rate of the a-C:H nanotips at growth times longer than 60 min is mainly due to the contribution of Cu etching around the bottom part of the a-C:H nanotips. Figure 5 shows the field-emission properties of the a-C:H nanotips, plotted as current density 共A/cm2兲 vs. applied field V/␮m. A Fowler-Nordheim 共F-N兲 plot is inserted in the upper left. The

Hydrogenated amorphous carbon nanotips have been successfully grown on Cu/Ti/Si共100兲 substrates. A Cu etching process simultaneously occurs in the CH4 /H2 /N2 mixed plasma during the growth of a-C:H nanotips. Both the Cu etching rate and the upward growth rate of a-C:H nanotips increase with growth time. An etching-growth mechanism is proposed to explain the formation of a-C:H nanotips on Cu/Ti/Si共100兲. In the beginning of the growth, the Cu etching and upward growth rates of the a-C:H nanotips are slow. Small a-C:H nanotips start to form around valleys of hills. At growth times longer than 60 min, both the Cu etching rate and upward growth rate become much faster. A higher density of a-C:H nanotips appears after 120 min growth. The field-emission characteristics of a-C:H nanotips are reasonably good. The turn-on field of a-C:H nanotips is about 3.2 V/␮m, and the field enhance factor is ␤ = 8484. Acknowledgments The authors thank Dr. G.-C. Chen for his technical assistance. This work was sponsored by the National Science Council under grant NSC 92-2216-E-007-048. Professor Jennchang Hwang assisted in meeting the publication costs of this article.

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