International Journal of Physics and Research (IJPR) ISSN 2250-0030 Vol.3, Issue 1, Mar 2013, 17-22 TJPRC Pvt. Ltd. COMPARISON OF J-E CHARACTERISTICS OF A CNT BASED COLD CATHODE GROWN BY CVD AND PECVD G SUDHEER KUMAR 1 & JAGIRDAR V RAMANA RAO 2 1 Defence Institute of Advanced Technology, Pune, Maharashtra, India 2 Jawaharlal Nehru Technological University, Hyderabad, Andhra Pradesh, India ABSTRACT The high aspect ratio of the CNT (of the order of 10,000) coupled with novel electrical, mechanical, and thermal properties of the CNT make them very attractive candidate for cold cathode application in high power vacuum devices. Such CNT based cold cathode(ccc) can provide large and stable emission current densities at reasonably low field and can be switched on and off instantaneously with no limit on grid modulation. This CCC based vacuum devices can also be effectively operated under harsh ambient conditions and therefore, ideally suited for airborne and space applications. In this paper, the development of CCC using vertically-aligned CNTs grown by CVD & PECVD technique on silicon substrate has been reported. The J-E characteristics of a typical CNT based cold cathode grown via CVD & PECVD were compared. The field emission current density obtained in case of CVD sample is 7mA/cm 2 at a field of 3.9V/µm while it was 37mA/cm 2 at the same field in case of PECVD grown CNT based cold cathode. The emission current in most of the samples is found to be stable over a long period of time. The initial measured data suggest great promise for achieving high current densities at practical electric fields. KEYWORDS: Carbon Nanotubes, Field Emission, Cold Cathode INTRODUCTION The vacuum electronic devices with hot filament electron emitter replaced with cold cathode will have the best features of both, vacuum tubes and solid-state devices. These cold cathode microwave devices can be turned on and off instantaneously and will have improved efficiency. In addition, elimination of hot cathode also removes the restriction on placement of the control grid close to the cathode thereby making the high frequency and low grid voltage operation possible while simultaneously reducing the size. Such power efficient, reduced size, radiation resistant microwave device is expected to be very attractive for airborne and space communication applications. Previously good amount of efforts have been made to fabricate cold cathode microwave device with spindt type field emitter using molybdenum tips. Traveling wave tubes with such type of field emitters operating at 10 GHz and emission current densities of 50 A/cm2 at high field has been demonstrated. The large aspect ratio along with chemical stability, mechanical strength and extraordinary electronic properties suggest the use of carbon nanotubes as an ideal electron field emitter in variety of vacuum electronic devices. These properties show great potential for development of CNT based cold cathode, which can provide high current density at relatively low turn on voltage. The efforts made in the development of flat panel displays using CNT based field emitter has shown that individual CNT can carry current up to 1 ma or current density of about 105 A/cm2. Recently, current density of 4 A/cm2 at an applied electric field of 60 V/µm has been reported. In order to achieve best emission current density, emitters are required to have optimum tip-to-tip spacing for simultaneous reduction of screening effectiveness and enhancement of emitter tip density. We report the efforts made to optimize the growth of vertically aligned CNTs on
18 G Sudheer Kumar & Jagirdar V Ramana Rao silicon substrate using chemical vapour deposition and plasma enhanced chemical vapour deposition technique for CCC to be used in microwave tubes. The field emission characteristic of CCC grown via CVD and PECVD were compared The significant improvement achieved in emission current density obtained in PECVD samples in comparison to the CVD samples reported earlier has also been explained. Field Emission Properties of CNTs Field emission relates to the extraction of electrons from a solid material by tunneling through the surface potential barrier. The emitted current depends directly on the local electric field at the emitting surface and on its work function. The Fowler Nordheim model shows an exponential dependence of the emitted current on the local electric field and the work function. Given that the emitted current is strongly influenced by the emitter shape (geometric field enhancement) and by the chemical state of the surface, the small diameter and elongated shape of nanotubes lead to a high geometrical field enhancement making them ideal candidates for field emission applications such as displays or triodes. EXPERIMENTAL Growth of CNTs A silicon substrate was deposited with a 10nm Ni metal which act as a catalyst for CNT growth using thermal evaporation method. The deposited samples are placed onto a heating plate in the center of the PECVD reactor, which is then pumped down to a low base pressure (~1mTorr) to evacuate atmospheric gasses. Then the substrate is heated to a temperature shown to produce carbon nanotubes (450 to 700 C depending on process and chemistry). Field Emission Measurements The schematic of the measurement setup used for field emission measurements is shown in Fig 1. The Verticallyaligned CNT sample to be used as cathode was pasted over a copper plate with conducting epoxy. An indium tin oxide (ITO) coated glass was used as the anode. Both the cathode and anode were mounted on a Teflon plate with double stick as a guide to keep both the plates parallel to each other. The entire sample assembly was kept inside a vacuum chamber evacuated to vacuum better than 10-6 torr. An over current protection circuit was also designed and added to the measurement setup so that the current can be maintained within the safe limits. The emission current measurement as a function of electric field was carried out under the dynamic vacuum condition over a variety of samples grown by two techniques and having a different catalyst thickness, dot size and spacing. The current density (J) versus electric field (E) plots for one typical sample each of two techniques is shown in Figure 3.a & 3.b. The emission current measurements were also carried out at different anode to cathode spacing while maintaining the electric field constant. Measurements at still higher fields could not be carried out due to the power supply limitations. However, the extrapolation of the current field data at higher fields was carried out by extracting the Fowler-Nordheim (F-N) parameters from the experimentally obtained Fowler-Nordheim curves and calculation of emission current at higher fields. J (E) = (ηa/φ) (E 1 ) 2 exp(-(bφ 3/2 )/E 1 ) Where the local electric field E 1 is connected with the external macroscopic electric field E 1 as E 1 =βe, Φ is the work function, and β is the field enhancement factor (likely related to the geometry); a =1.54x10-6 AeVV -2 and b = 6.83x109 ev -3/2 Vm -1 are universal constants. The factor η describes the effective emitting area.
Comparison of J-E Characteristics of a CNT Based Cold Cathode Grown by CVD and PECVD 19 Figure 1: Field Emission Measurement Setup RESULTS AND DISCUSSIONS Structure Characterization The samples were characterized by using Scanning Electron Microscopy (SEM). SEM micrographs have a characteristic three-dimensional appearance and are useful for judging the surface structure of the sample. Fig. 2.a. Shows the SEM image of CNTs after CVD-Process using a mixture of NH3 (15sccm) and C2H2 (15sccm) at Reaction temperature of 7300C. And Fig.2.b. Is the Top view SEM image of vertically aligned CNTs after PECVD with RF-Plasma using a mixture of NH3 (10sccm) and C2H2 (10sccm) at a Plasma power of 5W and Reaction temperature of 7000C. (a) (b) Figure 2: a. SEM Image of CNTs after CVD b. Top View SEM Image of Vertically Aligned CNTs after PECVD with RF-Plasma From the above two SEM images it is clear that the growth of CNTs produced is tip-growth, where the nanotube lifts the catalyst from the substrate during the growth, as the nanotube nucleates and grows below the catalyst. And the CNTs grown by PECVD technique are smaller in dia. The PECVD grown sample is also observed to have less dense growth in comparison to the nanotubes grown by CVD technique for the samples having the same catalyst condition. J-E Characteristics The field emission current Measurement results of two typical CNT sample each having grown on Ni catalyst of thickness 10 nm is reported. In the CVD sample an emission current density of 7mA/cm2 at a field of 3.9V/µm was measured where as an emission current density of 37mA/cm2 at same field was obtained in the PECVD grown sample [Fig. 3.a & 3.b].
20 G Sudheer Kumar & Jagirdar V Ramana Rao CURRENT DENSITY (ma/cm2) 35 30 25 20 15 10 5 0 0 1 2 3 4 5 6-5 FIELD (Volts/micrometer) CURRENT DENSITY (ma/cm2) 40 35 30 25 20 15 10 5 0-5 0 1 2 3 4 5 FIELD (Volts/micrometer) (a) (b) Figure 3: a. JE-Curve of CVD Grown Sample, b. JE-Curve of PECVD Grown Sample X-axis: field (V/µm) Y-axis: current density (ma/cm 2 ) This observed increase in the emission current density in the case of the PECVD sample could be attributed to much less dense growth of CNTs of smaller diameter. These geometrical factors are expected to improve the geometrical field enhancement factor by way of increased aspect ratio and reduced shielding effect of one emitter (CNT) over another. The increased geometrical field enhancement factor ultimately results in lowering of the threshold field and improved field emission current. Efforts are being made to optimize the size of the Ni dots and spacing in between them to get an emission current density of about 1 A/cm2 at practical field, which is evident from the estimated current density with field curve. It was also observed that initially during I-V measurements of field emitters some non-uniformity in the emission current was observed which make the device unstable. But this issue was resolved by giving a high field treatment to device for longer duration. At a fixed electric field the measured emission current densities were found to be independent of the spacing between cathode and anode. The stability of the emission current was also found to depend upon the vacuum conditions during the measurement. This could be because of the degradation of the CNT emitter tips due to ion bombardments at poor vacuum conditions. CONCLUSIONS CNTs were synthesized using PECVD technique. A thin layer of Nickel metal was deposited on silicon substrate using thermal evaporation technique. The CNTs was grown on the same film by PECVD method at different conditions and samples characterized by SEM to see the surface topography. Initial measurement results of emission current data obtained on selectively grown CNT cathode samples by PECVD technique in diode configuration, show great potential for cold cathode application in Microwave tube. With fine refinement of the growth process leading to the most optimum nanotip density it should be possible to get a stable current density in excess of 1 A/cm 2 at reasonable field. Further improvement in current densities at the much lower field is expected to be made possible by using triode configuration wherein a control grid placed much closer to the emitter tips shall be able to extract electrons at much lower gate voltage. Established semiconductor processing techniques can be exploited to fabricate triode type of structure. Hence this system directly can apply in various futuristic electron emission applications.
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