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Spring 1999 logo1999 MRS Spring Meeting & Exhibit

April 5-9, 1999 | San Francisco
Meeting Chairs: Katayun Barmak, James S. Speck, Raymond T. Tung, Paul D. Calvert

Symposium C—Materials Issues in Vacuum Microelectronics II



Thomas Felter 
Lawrence Livermore National Lab 
Livermore, CA 94550 

Christopher Holland
SRI International
Menlo Park, CA 94025

Robert Nemanich 
Dept of Physics 
North Carolina State Univ 
Raleigh, NC 27695-8202 

A. Talin
Phoenix Corp Research Lab
Motorola Corp
Mail Drop EL508
Tempe, AZ 85284

* Invited paper

Monday Afternoon, April 5, 1999
Metropolitan I (A)
1:30 PM *C1.1/B3.1
STATUS ON THE DEVELOPMENT OF FIELD EMISSION DISPLAY TECHNOLOGY. Lawrence N. Dworsky , Motorola, Inc., Flat Panel Display Division, Tempe, AZ.

Field emission displays are currently moving from prototypes into products. An overview of the current state of the technology will be presented.

2:00 PM *C1.2/B3.2
PACKAGING OF FIELD EMISSION DISPLAYS . Jim Browning , Micron Display Technology, Boise, ID.

Alignment, assembly, and sealing of Field Emission Displays are important aspects of the display technology. The materials issues surrounding FEDs including spacers, seal temperatures, cathode compatability, and glass frits make component integration a challenge. These issues will be discussed.

3:00 PM *C1.3/B3.3
COATINGS, GASES, AND THEIR EFFECTS ON SILICON FIELD EMITTER ARRAYS. D. Temple and W. D. Palmer, MCNC, Electronic Technologies Division, Research Triangle Park, NC.

In the range of vacuum electronic devices, flat panel displays and microwave power amplifier tubes represent the extremes in cathode performance requirements. But both applications stand to benefit greatly from the incorporation of cold cathode technologies once the state of the art can offer the repeatability and reliability available from thermionic cathodes. The stability of the cathode emission over time and with exposure to various ambient gases is particularly important. In the flat panel display, variations in the emission current produce variations in the pixel brightness across the display. In the microwave amplifier tube, noise and drift in the emission current degrades the quality of the amplified signal. This talk will give an overview of fabrication and electrical performance of silicon field emitter arrays (FEAs) in the context of requirements for field emission flat panel display and microwave power amplifier applications. Both as-fabricated Si FEAs and FEAs incorporating thin coatings of various materials will be discussed, and experimental data illustrating the cathode performance will be presented. In particular, we will discuss stability of emission current over extended periods of time in high vacuum conditions, and changes in the emission current upon exposure to gaseous ambients at varying pressure. This work has been supported in part by DARPA via ONR.

3:30 PM *C1.4/B3.4

Whereas Thermionic Integrated Circuits (TICs) have been based on thermionic emission of electrons, most, but not all, Vacuum Microelectronics devices have been based on field emission from Field Emitter Arrays (FEAs). FEAs have been made from a variety of materials in many different structures. This talk will address a variety of different materials including silicon, refractory metals, resistive materials, diamond and diamond-like materials, compound semiconductors, carbon and carbon like materials including fullerene tubules, defected dielectric materials, etc. in structures including vertically oriented conical and pyramidal structures, multilayer thin-film-edges, surface conducting emitters, etc. Applications will include Field Emitter Displays (FEDs), RF power amplifiers, lasers, scientific instrumentation, multibeam electron lithography, etc.

4:00 PM *C1.5/B3.5
CATHODE LIFETIME ISSUES IN FIELD EMISSION DISPLAYS. Robert H. Reuss , Motorola, Inc., Flat Panel Display Division, Tempe, AZ.

Long device life is required for the successful commercialization of field emission display technology. In this paper, an overview of cathode lifetime issues in high voltage field emission displays will be presented.

4:30 PM C1.6/B3.6
FIELD EMISSION FROM POLYMER FILMS. I. Musa , W. Eccleston, Liverpool Univ, Dept of Electrical Engineering and Electronics, Liverpool, UNITED KINGDOM; G.A.J. Amaratunga, Cambridge Univ, Dept of Engineering, Cambridge, UNITED KINGDOM.

We have reported (Nature, Sept. 1998) low threshold field emission from conjuaged polymers: poly(3-alkylthiophene). The polymers are spun or cast on the surface of a polished n++ Si slice and are p type with an electron affinity of about 3 eV. Like most carbon materials the film requires conditiong before which the field dependece of the current is consistent with Fowler-Nordheim emission from the surface. The molecules are probably not aligned to give high carrier mobility in the direction of emission. After conditioning I $\propto$ V1.8 close to V2 expected for space charge limited current. There may be generation of a localised high density of traps caused by high current densities associated with these films during the conditioning process. There is evidence that some material is removed during the process. Two mechanisms then seem to be possible, one associated with the presence of field intensification at crater/void rim. The other is a process whereby electrons are emitted form the lower sidewalls of the voids where the electron concentration is, with SCLC, very high and the field is near to its maximum value. The changes in emission current with time will be discussed on the basis of trapping effects.

4:45 PM C1.7/B3.7
BAND-BENDING EFFECTS ON FIELD ELECTRON EMISSION FROM N- AND P-TYPE SILICON GATED EMITTER TIPS. Takashi Matsukawa , Seigo Kanemaru, Junji Itoh, Electrotechnical Laboratory, Tsukuba, Ibaraki, JAPAN; Kazuaki Tokunaga, Tokai University, Hiratsuka, Kanagawa, JAPAN.

Field emission characteristics from n- and p-type silicon gated emitter tips have been investigated in detail by means of experiments and theoretical calculation of band-bending induced by surface states. The single-tip emitters have been fabricated from n-type (4 $\Omega$-cm) and p-type (0.6 $\Omega$-cm) silicon, and their current-voltage (I-V) characteristics have been evaluated. The field emission from the p-type emitter has been found to occur at lower extraction voltage than that of the n-type emitter. As the theoretical approach to the origin of the lower extraction voltage for the p-type emitter, potential distribution in the emitter tip has been calculated by means of device simulation. The surface state density at the Si emitter tip has been assumed to be $3\times10^{13}$ cm-2eV-1, and doping concentration for n- and p-type Si has been decided as $1\times10^{16}$ cm-3. In the p-type Si emitter, the surface states are positively charged, and the potential for electrons falls down toward the top of the emitter tip. Electrons in the inversion layer under the extraction gate can be transferred to the tip top without a potential barrier. On the other hand, the surface state of the n-type emitter tip are negatively charged and forms a potential barrier against the electrons. This potential barrier prevents electrons from reaching the top of the emitter tip. These are the reason why the emission current of the n-type emitter is suppressed lower than that of the p-type emitter.

Tuesday Morning, April 6, 1999
Metropolitan I (A)
8:30 AM *C2.1/B4.1

I will describe the physical properties of both diamond and carbon nanotube field emitters. Both emit electrons efficiently at low electric fields and are capable of producing technologically meaningful emission current densities. I will discuss our current understanding of the emission mechanisms involved. I will also present data on the emission stability and uniformity, both of which are essential for successful vacuum microelectronic applications.

9:00 AM C2.2/B4.2
ELECTRON FIELD EMISSION PROPERTIES OF SiC/Si HETEROSTRUCTURES SYNTHESIZED BY HIGH DOSE CARBON IMPLANTATION INTO SILICON. Dihu Chen, S.P. Wong , W.Y. Cheung, Chinese Univ of Hong Kong, Dept of Electronic Engineering and Materials Science & Technology Research Centre, Shatin, N.T., HONG KONG, CHINA; R.W.M. Kwok, Chinese Univ of Hong Kong, Dept of Chemistry and Materials Science & Technology Research Centre, Shatin, N.T., HONG KONG, CHINA.

We have recently reported that electron field emission with a remarkably low turn-on field of 1V/$\mu$m can be achieved from planar SiC/Si heterostructures synthesized by high dose carbon implantation into silicon under appropriate conditions [D. Chen et al, Appl. Phys. Lett. 72 (1998) 1926.]. The good field emission property was attributed to the formation of a thin surface stoichiometric SiC layer and the formation of densely distributed small protrusions on the surface. This technique is attractive for its compatibility with Si technology and is promising for the fabrication of silicon-based flat emitter arrays. In this work, we shall report more of our recent results on the field emission properties from these structures prepared under various implantation and annealing conditions. The carbon implantation was performed using a metal vapor vacuum arc ion source. The chemical composition depth profiles were determined from x-ray photoelectron spectroscopy. The surface morphology was observed by atomic force microscopy. The work function of the surface SiC layer was determined by ultraviolet photoelectron spectroscopy. It was found that the good field emission property was mainly controlled by the surface morphology which is highly correlated with the field enhancement factor. Issues on the stability and uniformity of the emission will also be discussed. This work is supported in part by the Research Grants Council of Hong Kong (RGC reference number: CUHK 513/95E).

9:15 AM C2.3/B4.3
FIELD EMISSION ENERGY DISTRIBUTIONS AND CURRENT-VOLTAGE CHARACTERISTICS OF SINGLE TIP GATED DIODES. John B. Bernhard , Ambrosio A. Rouse, Edward D. Sosa, Bruce E. Gnade and David E. Golden, Univ N Texas, Depts Physics and Materials Science, Denton, TX; Babu R. Chalamala, Motorola, Flat Panel Display Division, Tempe, AZ; S. Aggarwal and R. Ramesh, Univ Maryland, Dept Materials Science and Nuclear Engineering, College Park, MD.

Field emission (FE) current-voltage characteristics and simultaneous electron energy spectra have been obtained using single tip gated diodes. The electron spectra are generated at each step of each current-voltage characteristic using a low-cost compact simulated hemispherical energy analyzer. A personal computer (PC) is used for data acquisition and control. The PC is programmed using graphics based data acquisition software and is connected to a CAMAC crate and a picoammeter through a GPIB interface. The picoammeter measures the current leaving the tip and the FE electrons are energy analyzed, detected and then processed in the CAMAC crate. The CAMAC crate also sends control voltages to the gate anode and the energy analyzer. This apparatus allows the simultaneously measurement of tip work functions and Fowler-Nordheim tip shape parameters. Work function measurements are compared to photoelectric work function measurements for molybdenum and iridium oxide.

9:30 AM C2.4/B4.4
METAL BORIDE CATHODE MATERIALS FABRICATED BY SELECTIVE AREA LASER INDUCED SOLUTION DEPOSITION. Zhenchen Zhong , Veronica Holmes, Peter A. Dowben and David J. Sellmyer, University of Nebraska-Lincoln, Center for Materials Research and Analysis and Department of Physics and Astronomy, Lincoln, NE.

We have developed a novel technique for the selective area deposition of rare earth hexaborides: laser-induced solution deposition (LISD). This technique is both simple and efficient and combines many advantages of both chemical vapor deposition and electrolytic deposition. The results of LISD deposition show that the polycrystalline thin films of rare earth hexaborides and sub-borides such as MB6, MB4, and MB2 (M = Gd, La) are formed through the light initiated chemical reaction of nido-decaborane (B10H14) and rare earth chloride in solution. These films grow with a strong texture and morphology that is dependent both on the selection of solvents and laser wavelengths and power used in LISD. The application as cathode materials in DC plasma display panels will be discussed.

10:15 AM *C2.5/B4.5
LIFETIME AND STABILITY OF CARBON COLD CATHODES. R.L. Fink , L.H. Thuesen, Z. Li Tolt and Zvi Yaniv, Field Emission Picture Element Technology, Inc. (FEPET, Inc.), Austin, TX.

We present the results of tests measuring the life of carbon cold cathodes and determining what conditions limit the life of the cathode. Cathode life and stability are important for a broad range of applications. Cathodes were tested in vacuum chambers using different gas environments as well as in sealed and gettered glass envelopes. We measured emission current half-lives of 10,000 - 20,000 hours or more in sealed display devices, depending on operating conditions. The presence of a significant oxygen or water partial pressure degrades the life of the cathode. After removing the gases the decay rate was restored to near the original value. A similar partial pressure of hydrogen gas has little or no effect on the life of the cathode. These results will be discussed with respect to the various applications available for carbon cold cathodes.

10:45 AM C2.6/B4.6

The effects of ultrasound treatment (UST) at the various temperatures on the electro-optical properties of amorphous carbon (a-C:H) active layer thin-film light-emitting diode (TFLED) have been investigated. The TFLED has a structure of glass/tin-oxide (SnO2)/p-$\mu$c-Si:H (2.3eV)/i-a-C:H (3.2eV)/n-$\mu$c-Si:H (2.4eV)/Al. For the USTs at the room temperature, the threshold voltages of the TFLED were not changed, while the series resistance decreased. For the USTs above the room temperature, both the threshold voltage and the series resistance were reduced. With increasing the UST temperature, electroluminescence (EL) intensity increases, the peak of the EL spectra moves to shorter wavelength region and the full-width half-maximum decreases. These improvement of electro-optical characteristics are due to the reduction of defect states in the a-C:H active layer and at the p/i interface.

11:00 AM C2.7/B4.7
PROPERTIES OF ELECTRON EMITTING DIODE FABRICATED WITH SINGLE-CRYSTALLINE DIAMOND. Toshimichi Ito , Masaki Nishimura, Osaka Univ, Dept of Electrical Engineering, Suita, Osaka, JAPAN.

Highly efficient electron emitting diodes with a flat emission surface have been fabricated using single-crystalline diamond thin-films homoepitaxially grown on high-pressure synthesized (100) diamond. The fabrication method employed includes suitable ion-implantation and diamond regrowth processes. The emitters examined in the present study contain a buried electrode and a hydrogenated diamond surface with negative electron affinity[1]. When a driving voltage ranging from 0.3 to 1.1 kV was applied between the buried electrode and the hydrogenated surface, highly efficient emission currents were measured. In the best case, the almost same amount of the emission current as the driving current (or the injection current) was observed although the emission stability was not very good, meaning that the emission efficiency (= emission current / injection current) reached 100 %[2]. The efficiency thus observed was always found to be almost independent of the amount of the current below 200 nA. Details of the performance will be addressed in relation to defects in the diamond films grown.
[1] M. Nishimura, A. Hatta and T. Ito, Jpn. J. Appl. Phys. Vol.37, L1011(1998).
[2] T. Ito, M. Nishimura and A. Hatta, Appl. Phys. Lett., (1998) in press.

11:15 AM C2.8/B4.8
THE POSSIBILITY OF FIELD EMITTER IMPROVEMENT BY MEANS OF FULLERENE COVERAGE. Gennadi G. Sominski, Tatjana A. Tumareva , St. Petersburg State Technical Univ, Dept of Physical Electronics, St. Petersburg, RUSSIA.

It is safely determined that carbon containing coverages can be used for the decrease of working voltages of field emitters and for the stabilization of their emission. The search of sufficiently simple methods of pure carbon coverage creation and of new containing carbon materials for the field emitter improvement represent now the special interest. The fullerene coverage influence on the emission ability of tungsten field emitter was studied in present work. The measurements were performed in the field projector. The coverages of C60 molecules were deposited upon the tungsten tip with top radius  1 micrometer. The deposited coverages were exposed to the thermal treatment. The coverage structure and field emitter characteristics were defined in the wide range of the fullerene amounts on the tip surface in temperature interval between 300 and 1800 K. The preliminary deposited fullerene coverage was found to transform to the well-known system W+C after the heat of fullerenes onto the tip at 850-950 K. The subsequent deposition of the large enough fullerene coverage on the prepared by this way system W+C made it possible to create the rough coverage with the typical microtip radius about 100 Angstrom or less. The emitter with this coverage secure stable emission current more than 100 mA at the static regime. These currents were registered at voltages that were less (up to 30%) than in the case of pure tungsten emitter. Thus the results of the present work show the possibility to create the pure carbon coverages and stable effective emitters with rough surface by means of fullerene deposition and thermal treatment. This work was supported by the Russian Foundation for Basic Research (grant N97-02-16080).

11:30 AM C2.9/B4.9
ORIENTED CARBON NANOTUBE GROWTH FOR FIELD EMISSION APPLICATIONS. Alexander N. Obraztsov , Igor Yu. Pavlovsky, Alexander P. Volkov, Dept. of Physics, Moscow State University, Moscow, RUSSIA; Vladimir L. Kuznetsov, Andrey L. Chuvilin, Boreskov Inst. of Catalysis, Novosibirsk, RUSSIA.

Oriented carbon nanotube films were grown using method of chemical vapor deposition in hydrogen/methane plasma activated by glow discharge. The film phase composition and structural features were studied by Raman, SEM, TEM, and HREM techniques. Field emission properties of the films were examined to obtain I-V characteristics and the field emission sites distribution. The I-V curves in Fowler-Nordheim coordinates were linear, that is typical for the field emission, with the threshold average field about 1.5 V/$\mu$m and the emission current density up to 50 mA/cm2 at the field of 5 V/$\mu$m. The field emission properties of the carbon nanotube films were studied at various temperatures ranging from 77K to 600K.

11:45 AM C2.10/B4.10
FIELD EMISSION PROPERTIES OF THIN MOLYBDENUM CARBIDE AND DIAMOND FILMS DEPOSITED BY DIELECTROPHORESIS. Ambrosio A. Rouse , John B. Bernhard, Edward D. Sosa and David E. Golden, Univ N Texas, Depts of Physics & Materials Science, Denton, TX.

Molybdenum carbide and diamond films were deposited at room temperature by dielectrophoresis on molybdenum foil and tips. The films have been characterized using UPS, XPS, SEM. In addition field emission current voltage characteristics and electron energy distribution measurements have been made using the tips as part of a single aperture gated diode. A VG ESCALAB system and Al x-rays (1481.8 eV) were used to determine the presence of molybdenum carbide, molybdenum trioxide, molybdenum dioxide, graphite and diamond as a function of annealing temperature and time at constant temperature. Molybdenum trioxide is present after molybdenum carbide deposition at room temperature on all samples and is found to be stable as the temperature is raised, until it undergoes a phase transition to molybdenum dioxide. Molybdenum dioxide also has a stable temperature range. The surfaces undergo a third transition to molybdenum carbide with a significant graphite content at about 1000 K. The molybdenum carbide films are stable when heated at 1073 K for 4 hours. UPS data for these samples was obtained at the different annealing temperatures and a number of photon energies to determine photoelectric work functions for molybdenum trioxide, molybdenum dioxide, molybdenum carbide and diamond. The VG ESCALAB system was used measure Fowler-Nordheim current-voltage characteristics, photoelectric work functions and field emission energy distributions for these materials.

Tuesday Afternoon, April 6, 1999
Franciscan III (A)
1:30 PM *C3.1
SOME EXOTIC ASPECTS OF PHYSICS WITH MICROTIPS. R. Baptist , CEA-LETI, Departement de Microtechnologies, Grenoble, FRANCE.

Vacuum microelectronics is presently dominated by the Display application ; other applications such as microwaves tubes, light sources, pressure sensors and vacuum gauges are still in infancy and may never emerge due to commercial reasons or due to the competition with other technologies (solid state devices with wide band gap semiconductors, electro-luminescence from organic material, other types of sensors).
Other applications of integrated field emission, somewhat more exotic, have also been studied in various laboratories ; some ones with success and other without. In this paper we will present some of the tentative which were studied by our group in collaboration with other laboratories to produce electron sources with particular properties. We will also try to explain why experiments were or were not successful.
An (unsuccessful) tentative to produce spin polarised electron beams with NiFe microtips was the first experiment. Although the degree of polarisation was carefully checked with a Mott polarimeter, no evidence of a spin polarisation was found [1].
The production of interferences with low energy electrons was the second, mitigated experiment. To our sense, the result, although positive, is incomplete because no entire interference field could be recorded as is done currently in laboratories working on electron-holography with macroscopic W tips.
The third, was the demonstrated feasibility of an optical command for microtips. This feasibility was, however, not pursued to prove the possibility of pulsing the electronic emission at very high frequency (microwave regime).
Finally, we will report on the injection of low energy electrons emitted by microtips immersed in liquid xenon (an environment very different from vacuum). This type of experiments should enable a better understanding of the electronic structure of liquids apart of having already shown a strong generation of UV light [2].
[1] Collaboration with Ecole Polytechnique, Palaiseau.
[2] Collaboration of the Grenoble High Magnetic Field Laboratory with CEA-LETI.

2:00 PM *C3.2
SYNTHESIS AND CHARACTERIZATION OF Si/Cs/O NANOCLUSTER THIN FILMS WITH NEGATIVE ELECTRON AFFINITY. L.N. Dinh , W. McLean II, M.A. Schildbach, M. Balooch, Lawrence Livermore National Laboratory, Livermore, CA.

Patterned and unpatterned thin films of Si/Cs/O nanoclusters have been synthesized by the technique of supersaturated thermal vaporization of Si and Cs in an oxygen background gas. These films, which were deposited onto conducting or semiconducting substrates, exhibit negative electron affinity (NEA) as evidenced by ultraviolet photoemission spectroscopy (UPS). Photo, secondary, and field electron emission properties of these nanocluster films were investigated with photo-electron emission microscopy (PEEM), field electron emission microscopy (FEEM), secondary electron microscopy (SEM), and current-voltage measurements. Flat cathodes covered with thin films of Si/Cs/O nanoclusters exhibited high current outputs and lower turn-on fields (<8.7 V/mm) than most NEA diamond surfaces and gated Si or Mo tip arrays. The films' NEA is unaffected by air exposure and is stable to high temperature annealing (550 C). The electron emission property of the NEA nanocluster films can be best explained by a sub-band gap abundant surface state nanocluster model. A field emission display unit with a simple diode structure containing a flat cathode coated with a thin film of Si/Cs/O nanoclusters has also been built to demonstrate the potential application of this material in cold cathode electron emitting devices, particularly field emission flat panel displays.

2:30 PM C3.3
DESIGN AND FABRICATION OF A InP/CdS/LaS COLD CATHODE. M. Cahay , A. Malhotra, Y. Modukuru, H. Tang, W. Bresser, P. Boolchand, University of Cincinnati, Dept of Electrical Engineering, Cincinnati, OH; P. Mumford, Air Force Research Laboratory, Sensors Directorate, WPAFB, Dayton, OH; W. Friz, Multi Area Research in Science (MARS) Consultants, Fairborn, OH.

We will describe our recent efforts to develop a InP/CdS/LaS cold cathode. The main elements of the cathode are: (1) a wide bandgap semiconductor slab (CdS, Cadmium Sulfide) sandwiched between a heavily doped semiconductor (n++ - InP) that supplied electrons at a sufficient rate into the conduction band of CdS and a thin semimetallic film (LaS). The latter provides Negative Electron Affinity to the CdS surface. We analyze various physical phenomena affecting the operation of the proposed cathode including: dynamical work function shift, current crowding and self-heating effects, and the importance of space-charge effects as a function of the anode to cathode spacing. We show that the cold cathode has the following advantages: (1) low voltage ( < 20 V) operation, (2) emission current densities of several 100 A/cm2, and (3) large power efficiencies. We will report on some experimental results including the growth of bulk LaS samples and the characterization of CdS thin films grown by Molecular Beam Epitaxy on InP substrates.

3:15 PM *C3.4
SECONDARY ELECTRON EMISSION STUDIES OF DIAMOND AND GaN MATERIALS. J.E. Yater , A. Shih and D.S. Katzer, Naval Research Laboratory, Washington, DC.

Wide bandgap materials such as diamond and group III-nitrides have attracted recent interest as potential cold electron emitters due to the low or negative electron affinity reported at specific surfaces. While electron emission has been demonstrated from several of these materials, it is important to understand the emission process and the factors that determine the emitted electron intensity, energy and angular spread, and emitter robustness. In this study, we use secondary electron emission spectroscopy to examine the transport and emission of low-energy electrons in several wide bandgap materials. In particular, we compare the secondary emission properties of C(100) and C(111) samples in order to examine the effect of crystallographic orientation on the emission characteristics. Very high yields are obtained from negative-electron-affinity surfaces of both samples, indicating that low-energy electrons are transported and emitted very efficiently at both surfaces. Although the angular spread of the emission is not determined in our study, the energy distribution of the emitted electrons is found to be sharply peaked at low energy for both samples. However, the energy distributions measured from the C(111) surfaces are broader and reveal structure in the energy gap of the diamond. The different emission processes at the C(100) and C(111) surfaces, as indicated by the energy distribution data, may account for the broad energy distributions observed previously from polycrystalline CVD diamond samples. Finally, we examine secondary emission measurements from GaN and AlGaN films grown by MBE. The secondary emission is not as strong as from the diamond samples, and the measurements reveal the impact of interface and surface barriers on the emission process.

3:45 PM C3.5
ANODIC ALUMINA - MATERIAL FOR HIGH ASPECT RATIO TECHNOLOGY. Alexander Govyadinov and Ivan Grigorishin, Vacuum Microelectronics Lab of Institute of Electronics, Belarus National Academy of Sciences, Minsk, BELARUS.

Anodic aluminium oxide films formed by electrochemical oxidation of aluminium are a promising material for vacuum microelectronics and microtechnology. The anodic alumina (AA) films have a unique self-ordered regular cellular-porous structure. The parallel pores extend through all thickness of the AA. It gives opportunity to perform deep etch process on AA analogous anisotropic wet etching. A simple wet etchants were used for that purpose. It was shown that deep AA etching had rectangle profile with features size less 2 um and aspect ratio up to 30. The rectangle profiles based on aluminium supported and free (separated from aluminium sheet) AA films with thickness in a range from 1 um up to 150 um have been realised. The AA technology based on combination of AA growing and etching, lithography, thin solid films deposition and high temperature treatment have been elaborated. It has permited to form freestanding 3-D microstructures. A number of applications of this AA technology for vacuum integrated circuits, microchannel plates, soft X-ray windows, gas sensors, precision apertures and high transparency grids for electron microscopy, etc. has been demonstrated. Wide prospects of anodic alumina deep etch technology for vacuum microelectronics and microtechnology has been shown. The elaborated AA technology can become a cheap alternation of LIGA process.

4:00 PM C3.6
OPTIMUM THICKNESS OF ALUMINUM NITRIDE COATINGS ON ELECTRON EMITTERS. Donghun Kang , Gregory J. Wojak, Dan E. Jonsen, Ed Preble, Jerry J. Cuomo and John J. Hren, Department of Material Science and Engineering, NC State University, Raleigh, NC; Victor V. Zhirnov, Semiconductor Research Corporation, Research Triangle Park, NC.

It was shown recently that the emission properties of wide band gap coatings depend on the coating thickness. Until now, however, this effect was studied only in relation to emission threshold voltage, i.e. emission in the low-current region, and not for coatings thicker than 100 nanometers. We present here results about the thickness effect of magnetron sputtered AlN coatings on Mo tips ranging from a few nanometers to 1 micron in thickness. All changes in the emissivity were referenced to uncoated tips. Three parameters were chosen for characterization: threshold voltage Vth, maximum current Imax, and high-current transconductance Imax/Vmax. For very thin coatings (2 nanometers), an increase in emission threshold voltage was observed. This result was explained as being due to the formation of an AlN dipole on the Mo surface with an orientation which increases the local work function (nitrogen terminated surface). A further increase in the coating thickness results in a decrease of Vth below that of the original Mo emitter, with Vth reaching a minimum at a thickness of 20 nanometers. This result is consistent with the model of internal field emission and the decrease in emission current is associated with the appearance of bulk properties of AlN (e.g. band gap, electron affinity, dielectric constant, etc.) as a certain minimum thickness is achieved. A further increase in coating thickness beyond 20 nanometers results in an increase in Vth. This effect has been observed earlier for diamond, AlN, and c-BN coatings, and it is most likely due to electron transport limitations in the poorly conductive dielectric coatings. Maximum current limits of AlN based emitters were studied for the first time. The value of high current transconductance Imax/Vmax is proposed as a figure of merit for characterization of the practical efficiency of AlN cathodes.

4:15 PM C3.7 INCREASING THE EMISSION EFFICIENCY OF A FORMED MIM STRUCTURE. Sergei V. Kalistratov , Pavel E. Troyan and Alexander A. Zhigalski, Tomsk State University of Control Systems and Radioelectronics, Physical Electronics Dept, Tomsk, RUSSIA.

The classic formed MIM-structure works at voltages in the range from 0 to 15 V and has the N-type circulating current versus volage characteristic. The values of the emission efficiency coefficient (EEC) lie within the limits of 1-5*10-3 in a pumping system and about x*10-4 in a sealed lamp [1]. The MIM-cathode with these characteristics also have a short lifetime, therefore its practical usage is limited.
We conducted an investigation of possibility of the EEC increasing in formed MIM-structures. The authors of [1] pointed to the absence of influence of lower electrode material on properties and characteristics of MIM-structures. It was observed the fact of influence of electrode properties of the lower electrode possessing surface resistance and representing insular metal film of Mo. At the same time with the replacement of the lower metal electrode by a resistive layer resulted in a change in working conditions of MIM-cathode. The work voltage needed to operate such structures were about 120-140 V. We observed several stages of work of this new investigation object.
1. Electrical forming under voltages about 25-30 V.
2. Combined process: repeated breakdown of the structure and electrical forming continuation at Up=70-100 V.
3. Termination of the combined process. The value of the circulating current (Ic) had a minimum level. The volume of the emission current (Ie) was out of record device sensitivity.
4. As the applied voltage reached about 120-140 V the emission current has been recorded. After some time (from several seconds to several minutes) has elapsed the value of Ie increased in ten and more times without any external changes in electric circuit parameters. The range of EEC values for the obtained samples was from 0.5 to 5%. The output of samples with increased EEC was from 10% to 20%. The EEC value of these samples variate from 0.1-0.5% to 5% standing at the last from few to dozens of minutes in succession. The total working time of samples with EEC > 1% laid in the range from 0.5 to 3 hours.
We made a supposition that in this investigation object, which can be called new in comparison to formed MIM-structures, the emission centers can be nano-clearances [2] between upper and lower electrodes making as a result of sparring breakdown thin film structure. One fact in favor of this point of view, in our opinion, is that the samples worked after the breakdown and the gradual increase in the emission current when required voltage was reached, which we connect with adsorption of a conducting phase from surrounding atmosphere in the mentioned clearance.
1. Dearnaley G., Stoneham A., Morgan D., UFN-1974.-T.112-N1- p.83-128.
2. V.M. Mordvincev, B.L. Levin. The model of nano-MIM-diod with carbon active condition with percolation in insulating slot. Microelectronics 1998 tom 27, N4, c.265-274.


Tuesday Evening, April 6, 1999
8:00 P.M.
Metropolitan Ballroom (A)
PREPARATION OF ULTRASHARP DIAMOND TIP EMITTERS BY ION BEAM ETCHING. A.N. Stepanova, E.I. Givargizov and L.V. Bormatova, Institute of Crystallography RAS, Moscow, RUSSIA; E.S. Mashkova and A.V. Molchanov, Institute of Nuclear Physics, Moscow, RUSSIA.

Ion-beam milling was used for sharpening of diamond coatings on ends of silicon tips. The sharpened diamond tips were used as field-emission electron cathodes. I-V characteristics of the electron emitters were measured. An effect of conditioning of the emitters was observed: after the emitters worked during at least several hours, their currents increased for several orders of magnitude and became stabilized.

PREPARATION OF STM/AFM PROBES OF SPECIAL SHAPE WITH DIAMOND TIPS. E.I. Givargizov , L.N. Obolenskaya, A.N. Stepanova, and M.E. Givargizov, Institute of Crystallography, Russian Academy of Sciences, Moscow, RUSSIA; and I.W. Rangelow, University of Kassel, Kassel, GERMANY.

For investigations in submicron grooves with vertical walls, typical of microelectronic technologies, and of objects with coarse surfaces such as biological macromolecules special STM/AFM probes with cylindrical or prismatic upper parts of probes are necessary. Such probes have been prepared by growing/sharpening of single-crystalline whiskers. To improve their robustness, the probes were coated with sharpened crystaline diamond tips.

A GATE CURRENT SUPPRESSED LATERAL FEAS INTEGRATED WITH TFTS. Moo-Sup Lim , Cheol-Min Park and Min-Koo Han, Seoul Nat'l Univ., School of Electrical Engineering, Seoul, KOREA.

It is well known that the stability of field emission current and the efficiency of control gate are key problems to real applications. There are many researches to improve the stability and uniformity of field emission current using the active device such as Field-Effect- Transistors (FETs). However, most of them have four terminals and the fabrication process becomes complicated due to the additional process of the integration of FETs and FEAs. We have reported the three terminal lateral FEAs integrated with TFTs without additional process steps[1]. But the gate current of the previous device is relatively large due to the fact that there is a current path to the metal on the active layer. In this paper, we propose new method to suppress the gate current. The details of fabrication steps are in Ref. [1]. Lower insulator layer including 500nm thick nitride and 50nm thick oxide was deposited on Si wafer. 100nm thick amorphous Si was deposited and selective n+ doping is performed. Upper insulator layer including 50nm thick oxide, 100nm thick nitride was deposited. Tip patterning was performed and upper insulator layer, poly-Si layer and oxide were etched with anisotropic dry etching and then the poly-Si layer was over-etched intentionally in order to make micro-cavity. It should be noted that the region of cathode is n+ doping poly-Si, and that of tip is undoped poly-Si. The oxidation was performed to sharpen the poly-Si tip, and isolates an anode and a cathode. The thermal oxide was removed by BOE etchant. Then, we deposited oxide using e-beam evaporation instead of deposition of molybdenum. Al was deposited sequentially. Finally an electrical interconnection was then fabricated in proper locations by employing mask steps. The proposed device has three terminals and stable anode current. And, the gate current of the device is negligible because insulator isolates gate electrode from active layer. Reference [1] M.S. Lim, C.M. Park, M.H. Han, and Y.I. Choi, A New Lateral Field Emitter Arrays inherently integrated with thin film transistor, MRS spring meeting, 1998

THE INFLUENCE OF THE BASE ELECTRODE MATERIAL ON THE OPERATION OF A FORMED MIM STRUCTURE. Sergei V. Kalistratov and Pavel E. Troyan, Tomsk State University of Control Systems and Radioelectronics, Physical Electronics Dept, Tomsk, RUSSIA.

An investigation of resistance layer In2O3 influence on the work of metal-insulator-metal structure was made. This structure was exposed to electric forming, after which the circulating current versus voltage characteristics exhibited a section with the N-type negative resistance, electron emission and electroluminiscense [1]. The thickness of insulating layer and upper electrode are 30-40 nm and 10-20 nm.
All investigators note the absence of influence of the base electrode material on electric properties of a formed MIM-structure with different materials of insulating layer (SiOx, SiN4, Al2O3, Ta2O5, etc.) and the top electrode (Al, Ni, Mo, etc.) [2].
Our research showed that properties of the structure In2O3-SiOx-Me (Al, Ni) strongly depend on material, thickness and the resistance of the base electrode. The result of replacement of metal base electrode by a In2O3 layer having surface resistance value about 50 Om/Ø and thickness value about 100 nm is a significant change in the I-V characteristic which consists in gradual smoothing of the N-type one and its complete transformation into a characteristic which obeys the Ohm law.
We found out this effect to depend on the polarity of the applied voltage. This effect became five times more rapid with the reversed polarity. The change in the formed MIM-structure I-V is schematically presented on picture 1 (reverse polarity: - - on top, + - on base electrode). In some cases an effect of momentary restoring the N-type characteristic is observed, which consists in conversion of the Ohm characteristic to the N-type one (duration of this effect is about several minutes) and reverse conversion. This gives us the possibility to talk about the origin of the conducting crosspiece being responsible for the effects shown in figure 1.
We think the building material for these crosspieces can be In atoms formed as a result of interaction positive charging In ions receiving because of In2O3 film decomposition when a formed channel and conductivity electrons are created.
1. Dearnaley G., Stoneham A., Morgan D., UFN-1974.-T.112-N1- p.83-128.
2. Cold cathodes. Under edition M.I. Elinson. M., Sov. radio, 1974, 336p.

NANOSCALE FIELD EMISSION STRUCTURES OF ULTRA-HIGH PACKING DENSITY. Nikolai I. Tatarenko , Scientific Research Institute of Precision Devices, Microelectronics Department, Moscow, RUSSIA.

Novel nanoscale field emission structures (NFESs) have been fabricated and tested in vacuum and in air at atmospheric pressure. The process of fabricating these structures is described. The nanoscale-tip field emission system was formed on a glass substrate and consisted of a titanium layer with regular nanoscale cylinder-like titanium pillars of about 70 nm height, (38$\pm$10) nm diameter and packing density $\lambda$ = 3.7x 1010 tips/cm2, which were incorporated into cylinder-shaped through nanochannels of a dielectric layer of a porous anodic alumina. Very small overall dimensions of tip emitters and opening diameters (18$\pm$8) nm in a dielectric layer with the gained ultra-high packing density contributed to reducing the potential required for field emission to units of volts. While studying the I-V characteristics of such NFESs in air at atmospheric pressure in a thin-film two-electrode device with a 150 nm in-between electrode distance at the operation voltage of 2.7 V, average densities of field emission currents of about 50 mA/cm2 have been observed. The regular titanium tip array without a dielectric layer of a porous anodic alumina (after its removing by a chemical way) in the conventional vacuum system with a simple diode configuration with an in-between electrode distance of 300 $\mu$m at the pressure of residual gasses of 2x10-3 Pa has been also tested. These tests showed that the threshold field emission of such structures took place when the electric-field intensity in a vacuum in-between electrode gap gained about 8 V/$\mu$m.
The Fowler-Nordheim plots associated with the observed voltage-current traces for NEFSs tested both in vacuum and in air at atmospheric pressure in a thin-film two-electrode device confirmed the field emission character of such structures.

FIELD ELECTRON EMISSION CHARACTERISTCS OF DIAMOND AFTER SURFACE TREATMENT. Wang Weibiao , Jin Changchun, Yuan Guang, Yin Xiuhua, Zhao Haifeng, Fan Xiwu, Changchun Institute of Physics, Chinese Academy of Sciences, Changchun, CHINA; Ji Hong, Department of Physics, Jilin University, Changchun, CHINA.

Diamond cold cathodes are made using diamond grit powder which was synthesized from graphite by high pressure method. Diamond grit is treated with Cs-salt and annealed in H2 at 1000$^{\circ}$C and then mixed with AgO-base inorgranic conductive paste. The mixture of diamond and inorgranic conductive paste is coated onto a Mo substrate. The micture becomes a diamond coductive ceramic after heat treatment at temperatures above 300$^{\circ}$C. Field emission characteristics are measured in a high vacuum chamber at a pressure of 10-5 Pa. ITO glass plate is used as an anode. The distance between the anode and the cathode is 120 $\mu$m. The experimental results show that the turn-on voltage of diamond conductive ceramic is about 400. Results also show that diamond conductive ceramic has better emission ability and stable till about 3x10-2 Pa.

DIODE AND TRIODE CATHODOLUMINESCENT DEVICE PROTOTYPES FOR BACK LIGHT APPLICATIONS. Alexander N. Obraztsov , Igor Yu. Pavlovsky, Alexander P. Volkov, Department of Physics, Moscow State University, Moscow, RUSSIA.

Diode and triode cathodoluminescent device prototypes were fabricated with use of carbon thin film field emission cathodes. The film cathodes were obtained by using techniques of chemical vapor deposition in hydrogen/methane plasma activated by glow discharge. I-V characteristics, brightness, power efficiency, and time dependence of the lamp parameters were studied. We found that, for a diode type of device, the I-V curves in Fowler-Nordheim coordinates were linear, that is typical for the field emission, with the threshold average field about 1.5 V/um. The lamp brightness and power efficiency were found to depend on a kind of used phosphors and reached the values of 700 Cd/sq.m and 15%, correspondingly. Thikness of the different types of sealed lamp prototypes was 3 to 10 mm that allows their use as a back light source.

SUBSTRATE SENSITIVITY OF THE DEPOSITION RATE AND MATERIAL PROPERTIES OF RF-PECVD AMORPHOUS CARBON. Shashi Paul , F.J. Clough, Emerging Technologies Research Centre, Department of Electrical and Electronic Engineering, De Montfort University, Leicester, UNITED KINGDOM.

Amorphous hydrogenated carbon (a-C:H), deposited by the rf-plasma enhanced chemical vapour deposition (PECVD) technique, is a promising material for flat panel display metal-semiconductor-metal (MSM) switches and interlayer dielectric applications [1]. The properties of PECVD a-C:H have been shown to be sensitive to the substrate on which the thin film is deposited. For MSM and interlayer dielectric applications this effect may result in significant variations in the operating performance of the material. This paper presents a detailed investigation of this effect and examines the resulting issues for optimised a-C:H manufacture. Test structures have been prepared by thermally evaporating thin films of Al, Cr and Cu, through a shadow mask, on to c-Si and C7059 substrates. The shadow mask defines thin metal strips and pads (with a range of dimensions) on the substrate surface. Control substrates, with a continuous metal coating, were also prepared without the shadow mask. Thin films of a-C:H were then deposited at room temperature by rf-PECVD from CH4/He gas mixtures over a range of growth conditions [2]. A top contact was then formed by the thermal evaporation of Al through a second shadow mask. Our investigation shows that the choice of substrate (c-Si or C7059), and the existence and geometrical dimensions of any metallic pattern on the substrate surface, can result in significant spatial variations in the deposition rate and material properties of rf-PECVD a-C:H thin films. The observed effects can be attributed to potential variations across the metal patterned substrates which influence the `local' dc self-bias. This leads to spatial variations in the growth conditions and hence material properties. The nature of the substrate and any overlying metallisation pattern are therefore important considerations which can greatly influence the behaviour of a-C:H films incorporated as an interlayer dielectric or as a semiconductor in MSM switches. [1]S. Egret, J. Robertson, W.I. Milne and F.J. Clough, Diamond and Related Materials, 6 (1997) 879. [2]K.J. Clay, S.P. Speakman, N.A. Morrison, N. Tomozieu, W.I. Milne, and A. Kapoor, Diamond and Related Materials, 7 (1998) 1100.

FABRICATION OF PLANAR DIAMOND ELECTRON EMITTERS FOR FLAT PANEL DISPLAYS. Hideki Kawamura , Shinichiro Kato, Tetsuro Maki, Takeshi Kobayashi, Osaka Univ, Graduate School of Engineering Science, Osaka, JAPAN.

This work is on planar diamond electron emitters, which are expected to realize large screen flat panel displays and low voltage operation owing to its structural and material merit. This device is composed of chemical vapor deposited (CVD) diamond particles selectively deposited on a pair of patterned Pt film separated by several micrometers each other. By applying voltage to a pair of Pt film, electrons are emitted from diamond particles into vacuum and flow into the other side. In this study, planar diamond electron emitters were fabricated for the first time and electron-emission properties were closely investigated. Emitted current was observed above 150 V. If part of this current is drawn to the anode placed above the device, whole the device will work as a cathode equipped with a controlling electrode. The electron-emission properties of continuous polycrystalline diamond film were also measured for comparison, and the mechanisms of the electron-emission were discussed. The effective work function of continuous film was lower than that of isolated particles. In case of continuous films, electrons are emitted through electronic states induced by some kinds of defects, which results in lower work function. On the other hand, in case of planar electron-emitters using isolated particles, it was suggested that electrons were emitted from near valence-band maximum. As the electron emission from isolated particles is not affected by uncertain factor in inter-grain boundaries, this device is also interesting from a scientific point of view.

CHANGES OF CONTACT POTENTIAL DIFFERENCE INDUCED BY FRICTIONAL DAMAGE IN ULTRA-HIGH VACUUM. Lulu Zhang , Keiji Nakayama, Mechanical Engineering Laboratory, Plasticity and Forming Division, Tsukuba, JAPAN.

Frictional electrification has been found to be important in many areas. In the early 1950s, Harper proposed an electron transfer mechanism, which is proportional to the contact potential difference of the metals, to explain the contact charging of metals to metals. If it were possible to identify the frictional electrification phenomena with near atomic resolution, then a deeper understanding of this process might obtain. In this study, we carried out the scratch test by using Ultra-High Vacuum Atomic Force Microscope (UHV-AFM) and measured in-situ the contact potential differences before and after scratch by means of the Kelvin probe Force Microscope (KFM). By using KFM and Non-Contact AFM, we measured both the contact potential difference and the topographic image simultaneously without contacting the sample surface. In the AFM scratch test, we used Si and metal deposited Si cantilevers as scratch tool and used the same one to obtain the images. Si(111) wafers, which were deposited by metal, were used as samples. The scratch loads were in the order of $\mu$N and speed was about 0.45 $\mu$m/s. The in-situ measurements show the changes in both topographic image and contact potential difference after the scratch. In the topographic images, scratch track and metal transfer from the cantilever to the surface were observed. The contact potential difference images showed clear change between the area with and without the scratch.

TOWARDS SUPRAMOLECULAR HYBRID MATERIALS FROM NANODIAMOND. Peter I. Belobrov , Inst of Biophysics SB RAS, Krasnoyarsk, RUSSIA; Sergey K. Gordeev, Central Research Inst of Materials, St. Petersburg, RUSSIA; Olivier M. Kuettel, Louis Schlapbach, Univ of Fribourg, Fribourg, SWITZERLAND.

Synthesis-by-design of advanced hybrid materials from nanodiamond (Nd) and its supramolecular aggregates in a solutions based on the approach to molecular associates of coordinately saturated compounds offered on a supermolecule (Ubermolekule) introduced by Wolf and developed by Lehn. After transformation of carbon of explosive substances into Nd we use chosen from detonation soot and purified Nd. Nanodiamond contains carbon (main part of which contains in a diamond phase), hydrogen, nitrogen, oxygen, the doping additives, metal impurity (iron, calcium etc.) and different functional groups on a surface. Therefore an adequate definition of Nd is supermolecule. As up to 5 nm Nd is more stable then graphite, i.e. the phase of supramolecular hydride of diamond is most stable, as Nd is good precursor for synthetic routes of organic-inorganic hybrids. Except for hydrides at explosion and purification are always formed as well oxides. The functionalization of this carbon supermolecule with diamond-like core was made. All surface functional groups were modified by Cl, CH3 etc. The properties of Nd in microemulsions and in nanoporous carbon composites synthesized in confined volumes have been describe. The stripped Nd was prepared too by hydrogen plasma etching and electrophoretic deposition of the one on silicon substrate. Field emission, XPS, UPS, Raman and X-ray measured the physical characteristics. We plan to analyze sp3-sp2-sp relation of Nd, to compare with other carbon scaffolding and to discuss prospect of Nd supermolecule for carbon electronics and hydrogen storage. We will compare the field electron emission and semiconductor properties of porous nanocomposites as new organic-inorganic hybrid materials from nanodiamond.

Wednesday Morning, April 7, 1999
Franciscan III (A)
8:30 AM *C5.1

A number of mechanisms have been put forward to clarify the characteristics of field emission from both natural single crystal and CVD diamond films. Fundamental understanding of the field emission properties of diamond is incomplete. In this paper, we examine electronic state origin and related characteristics of field emission from synthetically produced thin film diamond materials using the technique of simultaneous field emission and photoemission (FEPES). FEPES is achieved by placement of a grounded grid about 100$\mu$m above the diamond surface. The electrostatic potential of the diamond cathode is set to several kV (negative) simultaneous with a routine He I photo-emission measurements. The utility of this technique includes the ability to directly reference the field emission initial state energy to electron binding energies at the surface as determined by He I photoemission. In the case of a natural semiconducting (p-type) single crystal polished diamond flat we find that the valence band is the origin of electron field emission at an applied (macroscopic) field of 16 V/ $\mu$m. FEPES linewidth analysis finds a local field of ca. 2kV/ $\mu$m. FEPES measurements from synthetic N-doped diamond (from K. Okano) have found a decrease in the required applied field together with a change of the field emission initial state energy from the valence band edge to the Fermi level, indicating that metallic states at the surface dominate field emission. The spatial distribution of emission can be examined using an imaging electron spectrometer. In a variety of materials studied, field emission is found to be localized. Spectroscopic examination finds that the field emission current instabilities are characterized by time varying resistive voltage drops through the bulk of the WBS. Measurement of UPS spectra under field emission conditions are shown to be useful to determine cathode surface potential, and the spatial variation of the cathode surface potential.

9:00 AM C5.2
ELECTRON FIELD EMISSION FROM UNDOPED AND DOPED DLC FILMS. Volodimir G. Litovchenko , Anatoli A. Evtukh, Nikolai I. Klyui, Institute of Semiconductor Physics, Kiev, UKRAINE; Andrei G. Chakhovskoi*, University of California, Electrical and Computer Engineering Dept., Davis, CA; Thomas E. Felter**, Lawrence Livermore National Laboratory, Livermore, CA.

In this presentation the electron field emission and electrical conductivity of the undoped and nitroged doped DLC films have been investigated. Undoped and nitrogen doped DLC films were grown by PE CVD method from CH4:H2 and CH4:H2:N2 mixtures, correspondingly. During nitrogen doped DLC films deposition the nitrogen content in gas mixture was varied within the range from 0 to 45$\%$. In-situ the gas-phase doping allowed us to deposit DLC films with different content nitrogen in them. DLC films were deposited under three different levels of gas pressure in chamber: 0.2, 0.6 and 0.8 Torr. The measurement of emission current from samples was perforrned in the vacuum system which could be pumped to a stable pressure of 10-6 Torr. The emission current was measured in the diode structure. The emitter-anode spacing L was constant and equal to 20 $\mu$m. The current - voltage characteristics of the Si field electron emission arrays covered with undoped and nitrogen doped DLC films show that at the beginning the threshold voltage (Vth) remarkably increases with nitrogen content growth, then the decreasing of Vth is observed and finally Vth increases. Corresponding Fowler-Nordheim (F-N) plots follow F-N tunneling over a wide field range. The F-N plots were used for determination of the work functions, threshold voltage, field enhancement factors, effective emission areas. The effective work function on refractive index dependences of nitrogen doped and undoped DLC films are nonmonotonous with minimum for both undoped and nitrogen doped DLC films. For the qualitative explanation of experimental results we base on the model of DLC film as a diamond-like (sp3 - bonds) matrix with graphite-like inclusions in it.

9:15 AM C5.3
TETRAHEDRAL AMORPHOUS CARBON FILM FOR ELECTRON FIELD EMISSION APPLICATION. L.K. Cheah , X. Shi, Ion Beam Processing Lab, School of Electrical and Electronic Engineering, Nanyang Technological University, SINGAPORE.

The fact that carbon field emission cathodes can be produced with varying degrees of sp3, sp2 and sp1 hybridizations means that these films can be tuned for particular applications. It also means that these films can be produced with a whole gamut of different properties resulting in the difficulties to propose a single model to explain the observed electron emission at low electric fields. The tetrahedral amorphous carbon (ta-C) thin films deposited using a filtered cathodic vacuum arc (FCVA) system has been shown to have different degrees of sp3 and sp2 and surface morphology by varying the carbon ion energy, deposition rate and substrate temperature. The field emission properties of the ta-C films were measured, i.e. field emission current density versus applied electric field and field emission spots projected from a tin oxide coated glass. The field emission properties of the ta-C films are influenced by the microstructure. In addition, the electron field emission properties of the ta-C films have been shown to improve as a result of surface treatment with H, O and Ar ions. The surface treatment transforms the mirror-smooth samples into the films with visible nano-scale regions or protrusions formed on the surface within the slightly receded network. Thus, we concluded that the front surface and the microstructure of the ta-C films play an important role in the utilization of carbon films for field emission application.

9:30 AM C5.4
MICROSTRUCTURES, OPTICAL AND ELECTRON EMISSION PROPERTIES OF TETRAHEDRAL AMORPHOUS CARBON SYNTHESIZED BY FILTERED ARC DEPOSITION. Y.H. Yu , Z.Y. Chen, J.P. Zhao, X. Wang, Ion Beam Laboratory, Shanghai Institute of Metallurgy, Chinese Academy of Science, Shanghai, CHINA; N.Z. Lou, S.P. Wong, I.H. Wilson, Department of Electronic Engineering, The Chinese University of Hong Kong, Hong Kong, CHINA.

A new kind of carbon materials-tetrahedral amorphous carbon (ta-C) films has received much attention in recent years. ta-C films have been found to be amorphous and contain up tp 90% sp3-bonded C. This kind of film is semiconducting, exhibits photoconductivity and has an optical band gap 2.0-2.5eV, and is potentially useful as a stable wide-band-gap semiconductor. In this work, tetrahedral amorphous carbon (ta-C) films have been prepared by filtered arc depostion (FAD) technique. The surface morphology, defect, microstructure of ta-C films are investigated by atomic force microscopy (AFM), high-resolution transmittance electron microscopy (HRTEM), and slow positron annihilation (SPA). Results show that the surface ta-C films is smooth and compactive, electron emission properties are affacted by deposition processes. Optical properties of ta-C films are studied and a new optical method has been developed to determined the ratio of sp3 and sp2 in the films. The method is based on the simulation of the infrared reflection spectrum by applying the Bruggeman effective medium approximation (EMA). By the simulation of infrared reflection spectra with the established model, the volume fraction of sp3 bonds is obtained, which is in good agreement with the value obtained by the electron energy loss spectroscopy (EELS).

10:15 AM C5.5
ANALYSIS OF HIGH-CURRENT YIELD OF DIAMOND-BASED FIELD EMITTERS FOR MICROWAVE VACUUM MICROELECTRONICS. Victor V. Zhirnov , Semiconductor Research Corporation, Research Triangle Park, NC; C. Lizzul Rinne, Gregory J. Wojak, J.J. Cuomo, John J. Hren, North Carolina State University, Dept of Materials Science and Engineering, Raleigh, NC.

For microwave vacuum electronic devices, the most important parameters for cathodes are to provide high maximum currents and integral current densities. This work is the first attempt to summarize results of high-current experiments with diamond-coated cathodes, to provide analysis and recommendations for material properties, and to design an electron source. Experimental results cover several cathode designs (single tips, array of tips, and planar cathodes), and various diamond/carbon coatings (CVD diamond, HPHT diamond, nanodiamond, and laser-ablated carbon). It was found that the maximum current, Imax, varied significantly as different coating materials were used. Imax was also found to be a function of the coating thickness, d. The experimental dependencies of Imax(d) and Imax/Vmax(d) for different diamond and carbon materials were studied for the first time. It was found that for a single emission site (or single-tip emitter), diamond coatings enhance the maximum current by about two orders of magnitude. For large area (1 cm2) arrays, the maximum current was about 2-7 times the maximum current of an un-coated cathode, depending on the parameters of the coating. The possible physics controlling the current limitation will be explored. There are two main mechanisms controlling the current limitation: overheating of the emission site due to heat dissipation at high local current densities, and environmental effects (ion bombardment, arcing, etc.). Differences in maximum emission currents can be partly explained by the various thermal conductivities of the diamond and carbon coatings. This assumption was used to obtain relative estimates of the thermal conductivity for the different coating materials. The integral thermal conductivity depends on structural material properties such as porosity, crystal size, and phase composition. The results of a model developed for simulating maximum current operation of a cathode will be presented. The model is based on cathode parameters such as emission site density, thermal conductivity, thickness of the coating, and random variations in the local field enhancement. Various methods to increase the total current yield from diamond-based cathodes will also be discussed.

10:30 AM C5.6

We characterize the spatial distribution of unoccupied defects in chemical vapor deposition grown polycrystalline diamond films using two-photon spectroscopy and Raman spectroscopy. Photons of energy 2.4 eV, 2.5 eV, 2.7 eV and other energies from an argon ion laser are focussed on a 10 micron location of the sample and used to excite electrons from unoccupied defect states to vacuum. The emitted electrons are collected with high efficiency using a microchannel plate detector. The number of emitted electrons per incident photon is used as a measure of the defect density at the location. Raman spectroscopy is simultaneously performed at the location to determine the morphology and diamond versus graphite content. For a given location, the defect density is compared with the morphology and diamond versus graphite content. The results are compared with the field emission properties.

10:45 AM C5.7
FIELD EMISSION FROM CARBON CARBON FILMS DEPOSITED BY VHF CVD ON DIFFERENT SUBSTRATES. A.I. Kosarev , A.S. Abramov, A.J. Vinogradov, M.V. Shutov, A.F. Ioffe Phys-Techn. Inst., St. Petersburg, RUSSIA; T.E. Felter, Lawrence Livermore National Laboratory, CA; A.N. Andronov, S.V. Robozerov, Technical University, St. Petersburg, RUSSIA.

As previously demonstrated, non diamond carbon (NDC) films deposited at low temperatures (200-300 C)on silicon tips reduced threshold field value of field emission. In this paper we will present the results of study of field emission from flat NDC films prepared by VHF CVD. Effect of different metals of back contact and of film thickness on electron emission were observed. Emission measurements were performed in diode configuration in vacuum at about 10-10 Torr. NDC films were deposited on ceramic and on c-Si substrates coated by metall layers of Ti, Cu, Ni and Pt. The metals were deposited by sputtering. Correlation of work function of back contact metal and emission characteristics has been observed. Model of field emission from metall-NDC film structure will be discussed.

11:00 AM C5.8
VERY LOW THRESHOLD FIELD EMISSION FROM NANO-CRYSTALLINE DIAMOND FILMS GROWN BY HOT FILAMENT CVD PROCESS. B.S. Satyanarayana , X.L. Peng*, J. Robertson, W.I. Milne & T.W. Clyne*, Department of Engineering, Cambridge University, Cambridge, UNITED KINGDOM; *Department of Material Science, Cambridge University, Cambridge, UNITED KINGDOM.

There is a great interest in field assisted electron emission from diamond. The main attraction being its electro -negativity in addition to its properties like the thermal stability, mechanical hardness and chemical inertness. All forms of Diamond including crystalline, poly - crystalline, doped diamond, nano diamond etc have been studied. Generally low threshold field emission has been reported for doped diamond films grown by various process. Here we report very low threshold field emission from undoped discontinuous diamond films grown by the hot filament CVD process. The diamond films were grown by the hot filament chemical vapour deposition [HFCVD] process using methane and hydrogen as source gases. First the effect of the crystal size were studied. It was observed that the threshold field decreases from 19 V/$\mu$m for 1 $\mu$m sized crystal diamond film to 0.5 V/$\mu$m for about 0.1 $\sim$ 0.2 $\mu$m sized crystal diamond film. The threshold field is defined as that field at which an emission current density of 1 $\mu$A/cm2 is obtained. Next the effects of temperature [775$^{\circ}$C - 975 $^{\circ}$C] and methane concentration [0.5 $\%$ to 3 $\%$ CH4 / H2] were studied, trying to grow similar thin discontinuous/defective or nano-crystalline films under all conditions. With change in temperature it was observed that a low threshold field of nearly 1 V/$\mu$m was obtained for samples grown at 975$^{\circ}$ and 825$^{\circ}$C. Hence the effect of methane concentration variation was studied at 825 $^{\circ}$C. It was observed that with increase in methane concentration from 1 $\%$ to 3 $\%$ the threshold field increased from 0.5 V/$\mu$m to 1.25 V/$\mu$m. However there was also a corresponding increase in current density and emission site density. The varying morphological properties are compared using the SEM.

11:15 AM C5.9
BAND-GAP STRUCTURE AND ELECTRON EMISSION PROPERTY OF CHEMICAL VAPOR DEPOSITED DIAMOND FILMS. J. Liu , D.Y.T. Chiu, D.C. Morton and W.H. Chang, Army Research Laboratory, Sensors and Electron Devices Directorate, AMSRL-SE-EO, Adelphi, MD.

Diamond has been regarded as an ideal material of high-current- density cold cathode for vacuum microelectronics devices over the years. Diamond films synthesized using chemical vapor deposition (CVD) are able to yield large current density at low electric field. Theoretical investigations concluded that the emission could come from the surface and defects states near the conduction band. Quasiballistic electron transport through band-gap states was proposed to be the mechanism for sustaining the emission current. It is widely accepted that the distribution of impurity bands and band-gap states caused by defects and impurities in the CVD diamond. However, no direct experimental measurement was performed or reported for such bands and band-gap states and the resultant electron emission behavior. Using visible to vacuum ultra-violet (VUV) photoluminescence spectroscopy, we investigated the structure of the band gap and possible impurity states of intrinsic, doped, and amorphous CVD diamond films. Natural diamond crystals were used as reference for the measurement. Experiments revealed that in doped and high-defect diamond films band-gap states are distributed very closely to the conduction band and extended deep into the band gap. Such band-gap structure resulted in a low-field electron emission. Amorphous diamond, with a narrower band gap, showed no significant behavior in electron emission.

11:30 AM C5.10
ELECTRON EMISSION FROM NEA DIAMOND: EXCITON TRANSPORT STUDIES. W. Chang, B.B. Pate , Washington State University, Physics Department, Pullman, WA.

Photoelectron emission is a powerful spectroscopy for the study of the electronic structure of solids and surfaces. UV photoelectron emission studies of negative electron affinity (NEA) diamond recently discovered electron emission from the breakup of bound electron-hole pairs (Mott-Wannier excitons) at the diamond surface. In that work Spicer's three-step model (absorption, transport, escape) identified the key role of exciton transport. In this paper, we demonstrate that the observed exciton diffusion length is intrinsic; that is, governed by phonon scattering events. Measurements of photoelectron quantum yield (emitted electrons per incident photon) versus excitation photon energy from natural type IIb diamond (as­polished NEA (111) surface) were made at temperatures from 86 to 295 K. Note that the electron yield decreases significantly as the temperature is increased for a fixed photon energy. Moreover, the characteristic oscillatory structures of the yield as a function of photon energy gradually vanishes as the temperature is lowered. Assuming that intrinsic phonon-mediated exciton dissociation (ionization) and recombination dominate the exciton lifetime and diffusion coefficient we are able to reproduce the observed temperature dependence of the quantum photoelectron yield. Consistent with numerical simulations, we find that the disappearance of the photoyield oscillations at low temperature result from the decrease of available acoustic phonons of sufficient wavelength to dissociate (ionize) the excitons. A detailed physical model will be presented.


Wednesday Afternoon, April 7, 1999
Franciscan III (A)
1:30 PM *C6.1

In thermionic vacuum devices the use of space charge limited flow of the electron current has the following desirable properties.It enables the anode current to be virtually independent of the current the emitter is capable of delivering, it smooths out spatial variations in the emission from the cathode surface, and it substantially reduces (by a factor of 10 to 1000) the current fluctuation noise (both flicker and shot).The field emission sources used in vacuum microelectronics require by their very nature that there is a very high electric field at the emitting surface and this introduces problems for producing space charge limited flow along subsequent portions of the electron path between the electron source and the anode if similar desirable effects are to be obtained. In this paper I explore on a theoretical basis how space charge limited flow may be obtained from both a single gated micro-field emission source and from planar arrays of large numbers of such sources.

2:00 PM C6.2

Due to the high performance sensitivity of field emission devices on system parameters, realistic models are highly desirable. In the present study we consider the effects of tunneling, quickly changing potentials, and ballistic electron motion on the anode current and on the width of the electron beam for a two-dimensional system. Despite the quantitative limitations imposed by the lower dimensionality of the model, our qualitative results offer useful insights. To accurately calculate the electric field in typical field emission devices containing elements ranging from the nanometer to the millimeter scale, we have chosen the boundary element method which avoids solving the problem on a grid, thus allowing greater flexibility. Our emission model evaluates the current density at the cathode surface from the tunneling transmission coefficient, which is calculated from the solution of the one-dimensional Schrödinger equation using a potential barrier that includes the effect of image charges. Electrons are emitted stochastically from the cathode with a velocity and angle (measured from the normal to the surface) following the tunneling distribution function. For very sharp tips the electric field changes from its surface value over a very short distance away from the surface, which may be comparable to the tunneling distance. We have compared the effect of a position-dependent versus a constant field approach on the current density. Once emitted at a rate following a Poisson distribution, we propagate electrons ballistically through the device. We have compared the effect of the electrons lateral kinetic energy and emission angle distribution on the electron beam width at the anode with a pure electrostatic approach, which assumes that the electrons exactly follow the electric field lines, and in particular are emitted perpendicular to the surface.

2:15 PM C6.3
ANALYSIS OF MEASURED I(V) RELATIONS FOR ELECTRON EMISSION FROM INSULATING DIAMOND FILMS ON VARIOUS SI SUBSTRATES. K.L. Jensen , Naval Research Laboratory, Washington, DC; A. Goehl, G. Mueller, Fachbereich Physik, Universitaet Wuppertal, Wuppertal, GERMANY.

Field emission from diamond microfabricated structures hold promise for high power applications, in which the operating environment is known to be deleterious to other cold-cathode candidates. In addition, the surface/vacuum interface may have a negligible energy barrier, so electron emission into vacuum should be more easily accomplished. Emission measurements from insulating diamond on p- and n-doped Si substrates suggest that the substrate-diamond interface plays a dominant role in the electron emission of a diamond film [1]. In particular, Fowler Nordheim plots of the current-voltage relationship for diamond films on lightly n- and p- and on highly p-doped Si substrates display convexity at low currents, analogous to the findings of a heuristic model of internal field emission based on ad hoc modifications to a WKB analysis of the potential at the substrate-diamond interface [2]. In this work, we shall present a substantially revised analysis of the internal field emission problem, directed towards analyzing and understanding the low voltage regime of the aforementioned data. Experimentally, the data was taken from diamond samples approximately 8 µm thick and grown in a microwave plasma-assisted CVD setup. Repeatable and characteristic I(V) behavior was obtained from the samples. Theoretically, we shall develop a more rigorous WKB analysis of the interface barrier and a current vs. applied field model based on parameters ascertained from the data. Theoretical current-applied field simulations and predictions shall be described and related to experiment. We endeavor to show that the deviation from Fowler Nordheim behaviour is explicable in terms of the nature of the interface barrier. [1] A. Gö hl, et al.,, Local field emission features of oriented diamond films on various silicon substrates. Tech. Digest of the 11th IVMC (July 12-24, 1998, Asheville, NC) p200. [2] K. L. Jensen, et al., Advanced emitters for next generation rf amplifiers, JVSTB16, 2038 (1998).

2:30 PM C6.4

The analytical Markov models are suggested for the description of the stochastic dynamics of some physical parameters (e.g. current, number of emission sites, work function) of field emission sources. These models give relationships between emission statistical theoretical and applied characteristics (autocorrelation functions, spectra, microparameters of models and reliability parameters). We introduce the range of working states of FEA and the probabilistic characteristics of lifetime, ``ageing'', failures and other reliability parameters of FEA's reliability in the context of various models of the ``drift''(decay) of emission from FEA. Some linear and nonlinear (with random parameters) are presented.

3:15 PM *C6.5
MATERIALS ISSUES IN INVERSE NOTTINGHAM EFFECT COOLING. R.F. Greene , R. Tsu, U. of N. Carolina at Charlotte, NC; J.J. Cuomo, N. Carolina State University, Raleigh, NC.

Field emission from metal emitters produces excess ``Nottingham heat'', with emission coming from states slightly below the Fermi-level EF. We proposed [1] that field emission from p-type semiconductors with low work function WF would be strongly cooling, with cooling power and efficiency significantly greater for this inverse Nottingham Effect (INE) than, e.g., for thermoelectrics. Strong INE cooling power from strongly doped p-type semiconductors occurs if emission from the conduction band CB dominates, while EF remains near the valence band edge. Then, heat of transport w $\approx$ energy gap, i.e. much higher than for Peltier. Cooling efficiency figure of merit (FOM) in solid state coolers is explicitly restricted by the thermal heat leak between cold and warm junctions, hence by the Wiedemann-Franz ratio. In INE cooling, the vacuum electron path has zero heat leak, immediately doubling the FOM. Since heat flux density is w times the electric current density, the latter must be optimized, posing three materials issues: The p-doping must be heavy so that the conduction band edge remains well above EF despite field penetration. The semiconductor surface recombination velocity must be high to prevent exhaustion of the CB states, indicating direct-gap emitters. Third, lowering WF, e.g., by cesiation, favors CB emission. Ridge emitters should be used to maximize emission area. Retarding-potential collection can minimize electron bombardment heating of the collector. An INE cooler for spot-cooling Si or III-V chips using integral (etched) porous -Si [2] micro-heat pipes for produce ``spot-cooling'' of chips is described.
1. R.F. Greene, 1997 DARPA Thermal Management Workshops Dec.11, 1997.
2. R. Tsu, 1994 Porous Silicon Workshop, UNC-Charlotte, April 5-9,1994.

3:45 PM C6.6
DIRECT IMAGING OF BIAS DEPENDENT p-n JUNCTION BY SCANNING CAPACITANCE MICROSCOPY. Chaiin Im , G. H. Buh, C. J. Kang, S. Lee, C. K. Kim, Y. Kuk, National Creative Research Initatives, Center for Science in Nanometer Scale, Seoul National Unversity, Dep. of Phyics, Seoul, KOREA.

With downsizing of feature sizes in very large scale integration (VLSI), nanometer scale characterization of electrical devices become important. Especially the characterization under the operating condition has drawn much attention to understand the mechanism of their degradation and breakdown. We directly observed the movement of a p-n junction with the applied bias for the first time. Since the local C-V spectroscopy by scanning capacitance microscopy (SCM) is determined by the carrier density at the location the location of the p-n junction can be estimated. In order to control the width of the depletion layer a DC bias voltage was applied to the n-type region while a small modulation AC voltage was added to the grounded p-type region in order to increase the sensitivity. At zero DC bias condition, SCM image clearly showed static built-in depletion region. The depletion region extends with the applied voltage in reverse bias condition and shrinks in forward bias case. Therefore the bias dependent change of depletion width could be imaged directly. An 1-D depletion approximation model was given to explain the bias dependent depletion width quantitatively. By comparing the experimental results with the 2D device simulation it was suggested that the SCM result can be used to aid the device simulation.

4:00 PM C6.7

To have a knowledge on charging mechanism at newly formed insulating solid surfaces, potential distribution on cleaved insulating solid surface has been investigated in ultrahigh vacuum. Insulating solids such as MgO single crystals were cleaved in ultrahigh vacuum of 10-8 Pa. Then the surface potential and topography at the fresh cleaved surface were measured simultaneously in the UHV using Kelvin force microscope (KFM) and non-contact atomic force microscope (NC-AFM), respectively. A Silicon cantilever tip coated with metal was used. For measuring surface potential, alternating voltage with the frequency of 2 kHz was applied. For MgO sigle crystal, (100) surface was cleaved. The measured KFM images on the MgO showed some distribution patterns of surface potential on the cleaved surface, while AFM images showed step lines on them. The distribution of the potential was investigated in connection to those of the steps. The results showed that the potential distribution corresponded to those of the step distributions where charging was low. On the other hand, where charging was high, the correspondence was not seen because too intense surface charging was generated over the surfaces including the terraces and ledges. The cleaved surface morphologies were investigated also using surface analytical tools such as micro-X-ray diffraction method.

4:15 PM C6.8
NUMERICAL SIMULATION OF ELECTRON FIELD EMISSION FROM SILICON WEDGE MICROCATHODE. V.A. Fedirko and S.V. Polyakov, Joint Center for Mathematical Modelling of Moscow State Univ of Technology ``Stankin'' and Institute for Mathematical Modelling RAS, Moscow, RUSSIA.

We report on the results of the numerical simulation of electron field emission from a silicon wedge microcathode. Hot electron effects in semiconductor are accurately taken into account in the framework of 2D quasi-hydrodynamic approach. Electric field and tunneling current density at the emitting surface are calculated and used as boundary conditions for the quasi-hydrodynamic set of equations in semiconductor. An original finite difference scheme with alternating mesh is used which guarantee the conservatism and weak monotony of the solution.
Two dimensional distributions of the electron density and the electron temperature near the emitting surface, as well as electric field and current density distributions are calculated and analyzed. The results of 1D and 2D modelling are compared.
Electron heating drastically change electron dynamics in the semiconductor and strongly affects the emitting characteristics. It may also dramatically influence the stability of the tip. It is shown that 2D modelling is essential for adequate simulation of field emission from a wedge microcathode.


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