AVS1997 Session FP-TuM: Structures & Materials for Field Emission & Vacuum Microelectronics

Tuesday, October 21, 1997 8:20 AM in Room C1/2

Tuesday Morning

Start	Invited?	Item
8:20 AM	Invited	FP-TuM-1 Diamond Cold Cathodes* M.W. Geis, J.C. Twichell, T.M. Lyszczarz, K.E. Krohn, N.N. Efremow (MIT Lincoln Laboratory) Substantial progress has been made in understanding the physics of diamond cold cathodes. Four distinct regions: the metal contact, bulk diamond, vacuum interface, and free space, exhibit distinct physical processes affecting the behavior of the cathode. The contact region mediates the transfer of electrons and holes between the metal and the diamond. The bulk of the diamond limits the transport of carriers. Any dopants in this region will modify the fields throughout the diamond. The vacuum interface and the diamond termination at this interface control the emission into the vacuum, and impact the field in the diamond. Finally the electrons transit the vacuum region outside the diamond. Here, the electric field is set by the difference in potential between the anode and the diamond surface, which in turn depends on the charge distribution within the diamond. A number of diagnostics have been used to examine the characteristics of each region. *This work was sponsored by the Defense Advanced Research Projects Agency. Opinions, interpretations, conclusions, and recommendations are those of the author and are not necessarily endorsed by the United States Government.
9:20 AM		FP-TuM-4 Field Emission of Heavily Nitrogen Doped Diamond Films I.T. Han, S.H. Kim, N. Lee (Samsung Advanced Institute of Technology, Korea); S.W. Lee, D. Jeon (Myong-Ji University, Korea) We investigated field emission characteristics of nitrogen-doped diamond films, which were grown using MPECVD. N₂/CH₄ ratios for deposition varied between 1 and 0. Heavily nitrogen-doped films showed remarkably low turn-on voltages below 2 V/µm. Optical emission spectroscopy and FT-IR were used to compare nitrogen concentrations in the deposition gas mixture plasma and in the films, repectively. Using UV/VIS spectroscopy, we measured sub-band gap energies of free-standing CVD diamond films as a fuction of nitrogen concentrations after etching off the Si substrates. Surface morphologies, which were observed using atomic force microscopy and scanning electron microscopy, and raman spectra were correlated to field emission characteristics of the films. We will discuss the major origins of field emission of heavily nitrogen doped diamond films.
9:40 AM		FP-TuM-5 Electron Transport and Emission Properties of Diamond J.E. Yater, A. Shih, R. Abrams (Naval Research Laboratory) Cold electron emitters are needed for the development of flat panel displays and other advanced electron-based technology. Recently, the fabrication of wide bandgap emitters has attracted interest due to the low or negative electron affinity observed on some wide bandgap surfaces. Before reliable emitter structures can be developed, however, the electron transport properties of the material must be well understood, especially regarding limitations on the current density and energy spread of the transmitted beam. In this study, we examine the electron transport and emission properties of bare, hydrogenated, and cesiated single-crystal and CVD diamond using secondary electron emission spectroscopy. The strongest emission was observed from a cesiated C(100) surface, which produced extremely high yields (δ_max ~ 130) and sharply peaked energy distributions. A method was determined for identifying the conduction band minimum E_c in the measured spectra, thereby providing a reference point for measuring the kinetic energy of the emitted electrons and the height of the energy barrier at the surface. In spectra measured from the cesiated diamond surfaces, electron emission appears at energies E < E_c which gives direct evidence of a negative electron affinity. Analysis of the data indicates that the transport of low-energy electrons is very efficient in some of the diamond samples, especially the C(100) sample. An emission model for wide bandgap material is deduced that accounts for the observed energy spectra in terms of the surface properties and the internal electron energy distribution.
10:00 AM		FP-TuM-6 Demonstration of Low Work Function, Stable Li-Based Alloy Coatings for Field Emitter Displays O. Auciello, A.R. Krauss, T.G. McCauley, D. Zhou, T.D. Corrigan, D.M. Gruen (Argonne National Laboratory); R.P.H. Chang (Northwestern University) Low work function coatings are currently under investigation for application to field emission cathodes for many devices including field emission displays. Diamond is one of the most extensively investigated coatings for enhanced electron emission. Despite promising results, diamond needs to be synthesized at relatively high substrate temperatures (600-800 #188#C). This represents a compatibility issue for integration of diamond with glass substrates. We present here an alternative low work function coating technology based on Li-alloys. These materials can be used either as coatings on semiconductor or metallic field emitters, or as the field emitter structure itself. The alloy coatings have a number of advantageous properties for field emitter cathodes, namely: (a) they can be deposited at room temperature by a number of methods including ion beam and plasma sputter-deposition; (b) they are stable in vacuum and oxidizing atmospheres; (c) the alkali metal-based coatings form a self-sustained low work function layer on the surface, via Gibbsian segregation, such that the layer is stable against depletion by both ion and electron bombardment. The coatings have work functions in the range 2.2-2.5 eV (as compared with 4.2-4.8 eV for most transition metals and silicon). We have demonstrated that the coatings result in a three-fold decrease in threshold field for Si tips coated with Cu-Li alloys. The initial work reported here involved the sputter-synthesis and characterization of Cu-Li alloy integrated with Si plane surfaces and field emitters. Details of coating synthesis, electron emission measurements and surface segregation processes will be discussed in the paper.
10:20 AM		FP-TuM-7 Carbon Based Thin Film Cathodes for Field Emission Displays A. Weber, U. Hoffmann, T. Loehken, C.-P. Klages (Fraunhofer-Institut für Schicht- und Oberflächentechnik, Germany) Thin films of amorphous carbon and metal films containing nanocrystalline diamond powder were investigated for their application as field emission cathode materials. Field emitter films based on amorphous carbon (a-C) were deposited by sputtering of graphite employing an electron cyclotron resonance plasma as argon ion source. To investigate the vacuum electronic properties the a-C films were deposited on a chromium pattern on glass. In an UHV chamber the vacuum electronic properties of the films were checked by U/I and Fowler-Nordheim plots. To localize the emission sites the excitation of a low voltage phosphor (ZnO:Zn) was monitored by an CCD camera. Field emission of electrons occured at electrical fields as low as 3.2 V/µm and current densities up to 0.6 mA/mm² were achieved at a voltage of 350 V. The film resistivity of the a-C films is in the range of 0.1 - 1 ohm cm. As revealed by nanoindentation measurements the microhardness is 10-15 GPa indicating a relatively low sp3 content compared to tetrahedral amorphous carbon (ta-C) films. The a-C films were further investigated by SIMS, SEM, AFM, and TEM to obtain informations about the film composition and morphology. Electron emission was also detected employing a nanocrystalline (nc) diamond containing metal film deposited by electroplating using a nc diamond powder (particle size 4-6 nm) containing electrolyte. These films showed a threshold electrical field of 20 V/µm and in comparison to the a-C films a low current density of 0.02 mA/mm². However, this method will allow to produce large area FEDs without using an expensive vacuum process. An FED demonstrator with a 50x50 pixel array (2.5x2.5 cm²) was fabricated using an a-C thin film as electron emitter. The cathode plate consists of a-C coated chromium stripes prepared by conventional photolithography. The patterning of the phosphor was performed by laser ablation. This prototype can be operated with a driving voltage up to 1000 V.
10:40 AM		FP-TuM-8 Electron Emission Properties of Nanocrystalline Diamond- Coated Si Field Emitter Arrays* T.G. McCauley, D. Zhou, T.D. Corrigan, A.R. Krauss, O. Auciello, L.C. Qin, D.M. Gruen (Argonne National Laboratory); D. Temple, G.E. McGuire (Microelectronics Center of North Carolina); R.P.H. Chang (Northwestern University) Planar nanocrystalline diamond thin films on n-type Si have been shown to exhibit cold cathode electron emission at low threshold fields (as low as 2 V/µm)¹. In this paper, we report on the electron emission properties of Si field emitter arrays (FEA), which have been coated with a thin (~ 600 nm) layer of nanocrystalline diamond by microwave plasma-assisted chemical vapor deposition (MPCVD) in CH₄/Ar and C₆₀/Ar discharges. The conformality and local morphology of emitter arrays coated at different substrate temperatures and with different carbon precursors (i.e. CH₄ vs. C₆₀) are examined by field emission scanning electron microscopy (FESEM). The phase purity and degree of texturing are examined by micro-Raman spectroscopy and thin film x-ray diffraction (XRD). Possible correlations of local surface microstructures with electron emission are investigated by photoelectron emission microscopy (PEEM). The effects of partial preferential orientation and bulk microstructure, as determined by XRD and TEM studies, on electron injection and transport are discussed. *Work supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract W-31-109-ENG-38, and CRADA #9404900. ¹A. R. Krauss, D. M. Gruen. D. Zhou, T. G. McCauley, T. D. Corrigan, R. Fink, and R. P. H. Chang, submitted to J. Appl. Phys., 5/97.
11:00 AM		FP-TuM-9 Dynamics of Current-voltage Characteristics In Field Emission Cathodes Built from Various Types of Carbon Fibers A.G. Chakhovskoi (Univ. of California, Davis); A.L. Souvorov (Institute of Theoretical & Experimental Physics, Russia); C.E. Hunt (Univ. of California, Davis); E.P. Sheshin (Moscow Institute of Physics & Technology, Russia); A.B. Stolyarov (Institute of Theoretical & Experimental Physics, Russia); A.S. Baturin (Moscow Institute of Physics & Technology, Russia) Field emitters based on carbon fibers are known for more than a decade to yield significant emission currents in a relatively low vacuum environment (10-6 ...10-7 Torr). The field emission current, current-voltage (I/V) characteristics and stability of these cathodes are, to a large degree, determined by the type of the carbon fiber microstructure, by the features of the manufacturing process, and by the preliminary treatment of the emitters. In the present investigation, several types of carbon fibers manufactured under different conditions were first examined as field emission sources. Both single emitters and large area field emission "arrays" made from bundles of fibers grouped together by external metal conductors were analyzed. Prior to testing, the emitters were annealed while in vacuum in order to reduce outgassing and to remove organic binder material. The experiments were handled in a vacuum chamber with a base pressure around 10-6 Torr. The key factor of this study is the behavior of I/V characteristics of the cathodes. A special attention was given to the position and the slope of I/Vs. The initial I/V (the cathodes first turned on) was used as a reference to determine changes in the I/V characteristics after each 10 minutes of DC operation. A reduction of the current dispersion was observed over a 45-minute time interval. Statistical analysis of the changes in the I/V characteristics location, slope, and shape suggests a mechanism which can relate changes in these characteristics to the geometry of emission surface and to the number of working emission centers.
11:20 AM		FP-TuM-10 Field Emission from ZrC Films on Mo Field Emitters W.A. Mackie, T. Xie, P.R. Davis (Linfield Research Institute) We have been working for several years on field emission from hafnium carbide and zirconium carbide. These studies have covered emission from solid carbide emitters as well as thin film carbide overcoatings on single tip field emitters and field emitter arrays. These materials have work functions approximately 1 eV lower than Mo, making them attractive candidates for low voltage microelectronic field emitter arrays. Uses for arrays or single emitters of these carbides include video displays, microwave applications, high current, small spot electron sources, and cold cathodes for operation in poor vacuums. Since molybdenum is one common FEA emitter material we used it in this study. Our aim is to determine improvements in I-V characteristics, emission stability, capability of emission at high currents and in poor vacuum conditions, and the ability to withstand exposure to atmospheric pressure without degradation. Individually fabricated Mo field emitters were dosed via PVD from a high-purity ZrC source. The deposited film was subjected to a variety of heating treatments, followed by FEM examination and determination of I-V characteristics. The results of these experiments indicate that work function reductions of the order of 1 eV can be achieved. The observed FEM patterns indicate that the lowest film work functions occur on and around the (100) planes of the underlying Mo emitter. Stable high current emission has also been obtained in the 0.5 mA range with ZrC films on Mo. This represents a ten fold increase in the obtainable emission levels compared to emission from a clean Mo substrate. Emission of 10-25 µA in mTorr vacuum levels are also presented. The mechanism for these improvements are discussed. ¹Work supported by DARPA High Definition Systems Initiative under ONR Grant No. N00014-96-1-1011.
11:40 AM		FP-TuM-11 Low-Voltage Platinum Lateral-Edge Field Emitter Lamp B.R. Johnson, B.G. Heil, J.O. Holmen, D.A. Murphy (Honeywell Inc.); A.I. Akinwande (Massachusetts Institute of Technology) We have fabricated arrays of platinum lateral-edge field emitters that show stable electron emission current densities sufficient for a high-brightness (10,000 fL) cathodoluminescent avionics AMLCD backlight, at a DC gate voltage of only about 60 V. The arrays were tested in a vacuum chamber at 10^-8 to 10^-7 Torr, using a P45 (Y₂O₂S:Tb) white phosphor screen biased at 10 kV and located 7 mm above the arrays. The phosphor screen current from a 6 mm x 7 mm array of platinum emitters fluctuated only about 1% over times of order one minute. Similar arrays of molybdenum emitters showed phosphor screen fluctuations several times larger, which can be explained by the less reactive nature of the platinum surface. The molybdenum emitters required about the same gate voltage as the platinum emitters, to produce the same phosphor screen current, despite the higher workfunction of platinum. The superior performance of an emitter material with an inert surface agrees with the work of Ishikawa, et. al.¹ The uniformity of the image on the phosphor screen was improved, without changing the luminosity, by pulsing the gate voltage with 130 V at 1% duty cycle. The ratio of phosphor screen current to emitter current in pulsed operation was about 0.2. Most of the excess emitter current is believed to be due to electrical shorts in the array, rather than field emitted electrons captured by the gate. Lateral-edge field emitters may have longer emitter lifetime than conventional microtip field emitters, due to the upper gate electrode protecting the lateral-edge emitter from damage caused by flashover and ion sputtering, and due to the inherent robustness of the edge geometry compared to microtips, as discussed in previous work on smaller arrays of molybdenum emitters.² ¹J. Ishikawa, H. Tsuji, Y. Gotoh, T. Sasaki, T. Kaneko, M. Nagao, K. Inoue, J. Vac. Sci Technol. B, vol. 11 (1993), p. 403. ²B.R. Johnson, A.I. Akinwande, D. Murphy, J. Vac. Sci. Technol. B, vol. 15, p. 535 (1997).