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J. Phys. Chem. C 2007, 111, 12112-12115
Thermal Stability of Carbon-Nanotube-Based Field Emission Diodes Charan Masarapu,†,‡ Jeong Tae Ok,† and Bingqing Wei*,†,§ Department of Electrical & Computer Engineering, Louisiana State UniVersity, Baton Rouge, Louisiana 70803, and Departments of Electrical & Computer Engineering and of Mechanical Engineering, UniVersity of Delaware, Newark, Delaware 19716 ReceiVed: May 26, 2007; In Final Form: June 18, 2007
A field emission diode based on aligned multiwall carbon nanotube (MWNT) arrays was conceptually demonstrated, and its thermal stability from room temperature to 300 °C was investigated. Ideal diode behavior was observed with negligible change of the device parameters with the variation of temperature. The turn-on voltage and reverse leakage current of the MWNT diode can be readily controlled by simply varying the spacing between electrodes. Potential areas of application of the diode have been addressed.
Introduction Diodes are the basic building blocks of most current-day electronic devices. A traditional p-n junction diode is formed by joining a p-type semiconductor to an n-type semiconductor. An ideal diode shows zero conduction drop in the forward bias and zero current flow in the reverse bias. Diodes used in highpower devices must be able to withstand relatively high voltages with very low drop across them, and sometimes, they must be able to withstand relatively high temperature as well. The concept of a field emission diode was first introduced by Koyama and Kawai in 1966,1 where the field emission scenario is the quantum mechanical tunneling of electrons from a solid through the surface potential barrier by application of a strong electric field.2-4 Recently, field emission diodes utilizing diamond microstructures and silicon nanotips have also been reported.5,6 However, no attempts have been made in the open literature to analyze the performance of a field emission diode at elevated temperatures. Carbon nanotubes (CNTs) have many exceptional properties that make them attractive for a variety of applications.7-11 In particular, previous works have shown that CNTs can have outstanding electrical field emission properties, with high emission currents at low electric field strengths.12,13 The capability of the CNTs to carry high current densities14 in the range of 109 A/cm2 and their sharp tip structure a few nanometers in diameter15 makes them an ideal material for field emission when compared to other field emitters such as diamond5 and silicon.6 CNTs have shown good emission current stability and a long lifetime.16 Appreciable current densities can be readily achieved at electric fields as low as 1 V/µm.13 CNTs are hence attractive as cold-cathode field emission sources, especially for applications requiring high current densities and lightweight packages.17 A diode configuration of CNT electron field emission can be easily used for high-voltage applications due to its high-voltage-handling capability. In this study, a field emission diode based on aligned multiwall carbon nanotube (MWNT) arrays was conceptually * To whom correspondence should be addressed. E-mail:
[email protected]. † Louisian State University. ‡ Department of Electrical & Computer Engineering, University of Delaware. § Department of Mechanical Engineering, University of Delaware.
demonstrated and its thermal stability from room temperature to 300 °C was investigated. Ideal diode behavior was observed even at a temperature of 300 °C with very low turn-on voltages and negligible reverse leakage currents. No significant change in the diode behavior was observed with a change in the temperature. The advantage of using a CNT-based field emitter diode would be a high current density in the forward bias and an extremely low leakage current in the reverse bias if the spacing between electrodes is judiciously tailored. Moreover, it can be operated in elevated temperature environments without any noticeable deviation from ideal diode behavior. The threshold voltage and the breakdown voltage of the field emission diode can be suitably adjusted by varying the interelectrode spacing. This is an obviously great flexibility offered by this approach. A silicon substrate with an oxide layer of 100 nm in thickness was patterned as shown in Figure 1a. A chromium layer ∼1-2 nm thick acts as a bonding layer between the gold and SiO2 substrate and improves the adhesion of gold to the substrate. The gold layer serves two purposes in the experiment: (a) It acts as a mask for the selective growth of CNT on SiO2. (b) It provides a good conducting path for the electrons to flow in the forward bias. Aligned MWNT arrays were selectively synthesized on the SiO2 substrate in a thermal chemical vapor deposition furnace using a vaporized mixture of ferrocene and xylene.18 Ferrocene acts as the catalyst precursor and xylene as the carbon source. Figure 1b shows a scanning electron microscopy (SEM) image of the highly aligned MWNT patterns with an average nanotube height of ∼340 µm. An aluminum foil is placed between the copper electrode and the Si substrate. This provides a good path for the electrons to flow from copper to the nanotubes in the forward bias. This sample was utilized to evaluate the field emission diode behavior at different temperatures and with both the forward and the reverse bias. The patterns are mostly intact with negligible damage (Figure 1c) after all the field emission measurements in the forward and the reverse bias. The schematic representation of the field emission setup is presented in Figure 2. The experiments were carried out in a high-vacuum chamber equipped with a controlled heater stage. A base pressure of 10-6 Torr was maintained during all the emission measurements. In the forward bias setup (Figure 2a)
10.1021/jp074081y CCC: $37.00 © 2007 American Chemical Society Published on Web 07/26/2007
Stability of CNT-Based Field Emission Diodes
J. Phys. Chem. C, Vol. 111, No. 32, 2007 12113
Figure 1. (a) Schematic of the CNT array patterns on the Si substrate. (b) SEM image of the nanotube patterns before field emission measurements. Alignment of the individual nanotube pattern can be seen in the magnified image. (c) The patterns are mostly intact with negligible damage after all the field emission measurements in the forward and reverse bias.
Figure 2. Schematic of the field emission setup in the (a) forward bias with CNTs as the cathode and copper as the anode and (b) reverse bias with copper as the cathode and CNTs as the anode. Corresponding diode equivalent circuits are shown in (c) the forward bias and (d) the reverse bias. PA is the picoammeter, and HV is the dc high-voltage supply.
the Si substrate with the MWNT patterned arrays was kept on the heater stage and acts as a cathode. A polished copper electrode was used as the counter electrode, i.e., an anode. The anode and the cathode were separated using alumina spacers
with a thickness of 530 µm. The distance between the anode and the tips of the nanotubes was therefore about 190 µm. A positive voltage was applied to the anode using the dc highvoltage supply, and the field emission current was measured using the picoammeter connected to the cathode. The forward bias field emission measurements were done at various temperatures. An equilibration time of 1 h was given after each temperature set point was reached to ensure that the cathode and the heater stage are at an identical temperature. The diode equivalent circuit in the forward bias is illustrated in Figure 2c. Since in a field emission device the cathode governs the emitted current, in the reverse bias setup the copper electrode was placed on the heater stage and acted as the cathode (see Figure 2b). The CNT patterns were the current collectors as the anode. The spacing between the nanotubes and the cathode was maintained the same as in the forward bias setup. The emission area of the copper cathode is considered the same as that of the area of the nanotube patterns. Figure 2d shows the diode equivalent circuit in the reverse bias. First, the forward bias experiments were conducted with the MWNT pattern substrate on the heater stage. Field emission currents were measured at room temperature and 100, 200, and 300 °C by varying the voltage of the copper anode from 0 to 490 V. In the reverse bias, a positive voltage up to 1100 V was applied to the electrode with the MWNT patterns, and the field emission measurements were performed at the same temperature set points as done in the forward bias. Two rounds of measurements were conducted at each temperature, and a similar kind of emission behavior was observed and recorded. The applied voltage versus current density characteristics of the field emission diode at different temperatures in the forward and reverse bias is shown in Figure 3. The turn-on voltage (VTO)
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Figure 3. Characteristics of the voltage vs current density plot of the CNT-based field emission diode in the forward and reverse bias at different temperatures.
TABLE 1: Comparison of the Turn-On Voltages (VTO) in the Forward and Reverse Bias along with the Calculated Work Function of Carbon Nanotubes and Copper at Different Temperaturesa forward bias
reverse bias
operation temp (°C)
VTO (V)
nanotube work function (eV)
VTO (V)
copper work function (eV)
room temp 100 200 300
250 240 200 202
5 4.95 4.89 4.91
1100 953 804 710
4.5 3.6 3.48 3.47
a
The interelectrode spacing is 190 µm.
TABLE 2: Emitted Current Density in the Forward and Reverse Bias with Fixed Applied Voltages at Various Temperaturesa forward bias
reverse bias
operation temp (°C)
VF (V)
emitted current density (mA/cm2)
VR (V)
leakage current density (mA/cm2)
room temp 100 200 300
490 490 490 490
7.1 7.1 7.2 7.2
1100 1100 1100 1100
0.010 0.144 0.180 0.188
a
The interelectrode spacing is 190 µm.
of a diode is defined as the voltage required to achieve an emission current density of 10 µA/cm2. Table 1 compares the VTO in the forward and reverse bias at different temperatures. In the forward bias VTO values of 250 V (turn-on electric field 1.32 V/µm), 240 V (1.26 V/µm), 200 V (1.05 V/µm), and 202 V (1.06 V/µm) were noticed at room temperature and 100, 200, and 300 °C, respectively. VTO values as high as 1100 V (5.79 V/µm), 953 V (5.02 V/µm), 804 V (4.23 V/µm), and 710 V (3.74 V/µm) were observed at room temperature and 100, 200, and 300 °C, respectively, in the reverse bias. The diode forward voltage (VF) and the reverse voltage (VR) are defined as the voltages applied to the field emission diode in the forward bias and the reverse bias. At a VF of 490 V the current density was 7.2 mA/cm2, whereas a reverse leakage current of only 188 µA/cm2 was observed for a reverse voltage VR of 1100 V even at 300 °C. Table 2 summarizes the emission current density at fixed diode voltages VF and VR at different temperatures.
To understand the experimental emission results in the forward bias, by assuming the room temperature work function of the nanotube to be 5 eV,19 the work function of the nanotube at higher temperatures was calculated using the method employed by Tan et al.20 The results are also tabulated in Table 1. Unlike the results of Tan et al., no significant trend in the variation of the work function was noticed with an increase of temperature. Moreover, the relative change in the work function at high temperatures is about 0.1 eV from the room-temperature value. This remarkably negligible change suggests that the emission current is independent of temperature (at least within the temperature limit applied in our experiments) in the forward bias as supported by our experimental results from Tables 1 and 2. This thermal stability could be understood as follows: The nanotube tips reach temperatures in excess of 2000 K during field emission.21 As the nanotubes have excellent thermal conductivity,22 the heat generated at the tip tends to distribute quickly and uniformly along the length of the nanotube during the continued field emission process. A change in the external temperature applied from room temperature to 300 °C may not have a significant effect on the work function of the nanotube. Further work is in progress to study the emission characteristics of the nanotube at much higher temperatures. According to Fowler and Nordheim,3,4 the electron tunneling from the emitter is determined by the applied voltage, the spacing between the cathode and anode, and the geometry of the emitter surface. In general, the sharper the emitter, the stronger the electric field for a given voltage and the higher the emission current. This is the operating principle of a field emission diode. For a fixed distance between electrodes, nanotubes with their sharp tips emit electrons at much lower applied electric fields when compared to the field required to extract the same amount of electrons from the polished copper surface, as can be observed in the reverse bias. Assuming the work function of copper at room temperature as 4.5 eV,23 the approximate change in the work function with temperature could also be calculated as done for the nanotubes. The results can be read from Table 1. Compared to that of nanotubes, a relatively larger decrease in the work function with an increase in the temperature was observed. However, the maximum change was about 1 eV from room temperature to 300 °C so that an ideal diode behavior was retained with negligible leakage current density even at 300 °C (Figure 3). It should be mentioned that the surface morphology of the copper electrode plays an important role in the emission of the reverse leakage current.24 The copper electrode used in our experiments was just polished with sandpaper and washed with distilled water before each experiment. The leakage current density observed from the copper electrode, however, was only 10 µA/cm2 for a VR higher than 1 kV at room temperature. By employing special surface preparation techniques,25 this leakage current can be reduced by several orders of magnitude. The advantages of a CNT field emission diode over a conventional diode are obvious. The turn-on and breakdown voltages of the diode can be readily adjustable by means of changing the interelectrode distance or changing the length of the CNTs. In addition, a judicious choice of metal material can be made to get the desired very low reverse breakdown voltage as the reverse breakdown voltage depends on the electron work function of the metal used. Ideally, a zero leakage current should be achievable under low-voltage operation (for instance, hundreds of volts). Minimization of the reverse leakage current will greatly reduce the overall power loss in the diode device. Therefore, relatively higher switching speeds are reasonably
Stability of CNT-Based Field Emission Diodes expected because of the lack of any charge accumulation process in the nanotube field emission diode. Depending on the requirements such as the diode size and emission current density for a specific application, the size of the nanotube pattern and the number of patterns can be easily optimized by the fabrication process. Such a kind of high-temperature, high-voltage diode is a promising candidate for circuits used in down-hole drilling, seismographic, and space applications. While the operating temperature window of the most commercially available diodes for such applications is 30-200 °C,26,27 by using a CNT-based field emission diode the upper operating range can be extended to around 300 °C. In conclusion, a field emission diode based on aligned MWNT arrays is demonstrated. The diode exhibits a remarkably stable operation from room temperature to 300 °C. In the forward bias the emission from the CNT emitters is almost independent of temperature due to a very meager change in the work function of the nanotube with temperature. The breakdown voltage of the diode depends on the type of selected material used in the reverse bias. The diode is more flexible compared to the normal p-n diodes as the diode parameters such as the turn-on and breakdown voltages can be readily controlled by just tailoring the distance between the electrodes. Acknowledgment. We gratefully acknowledge financial support from the Louisiana Board of Regents under Awards LEQSF(2005-08)-RD-B-05 and LEQSF(2005-08)-RD-A-13, Schlumberger Technology Corp., and the University of Delaware. References and Notes (1) Koyama, M.; Kawai, H. ReV. Sci. Instrum. 1966, 37, 1159. (2) Schottky, W. Z. Phys. 1928, 14, 63. (3) Folwer, R. H.; Nordheim, L. W. Proc. R. Soc. London, Ser. A 1928, 119, 173. (4) Nordheim, L. W. Proc. R. Soc. London, Ser. A 1928, 121, 626.
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