Fabrication of C12A7 Electride as a high Efficiency Electron Emitter by ALD Nanolaminate Technology

Principal Investigator: Helmut Baumgart, Professor, Department of Electrical and Computer Engineering

Institution: Old Dominion University

CO-PI: Qiliang Li, George Mason University

Fabrication of C12A7 Electride as a high-Efficiency Electron Emitter by ALD Nanolaminate Technology


The objective of this seed grant proposal is a feasibility study to develop a new high-efficiency electron emitter 12CaO·7Al2O3 [C12A7] by ALD nanolaminate technology to enable applications that require coating of non-planar substrates for example hollow cathode structures or nanostructures with high aspect ratios for field emission displays. The ALD technology proposed to be developed here, for high-efficiency electron emitter electride is of potential benefit to high-tech and optoelectronics companies in the Commonwealth as well as National Labs like Jefferson Labs, that require efficient photocathode electron guns for their accelerator technology.

Fig.1: Schematic of Cage structure of C12A7 Electride

State-of-the-Art: The electride 12CaO·7Al2O3 [C12A7] is a new electronic oxide compound with a work function as low as 2.4 eV, which is close to those of alkaline metals. Its crystal structure illustrated in Fig. 1 can be regarded as a stack of cage-like subunits that share their faces, and this arrangement differs from that of zeolite-based compounds. The inner diameter of the cage is approximately 0.4 nm. Owing to the mobility of these cage electrons, C12A7 exhibits a high electrical conductivity of up to 1500 S cm-1 , and the low work function of this electrode originates from the midgap states created by these electrons. The optical transparency, chemical stability in air and at room temperature, ionic and electronic conductivity, and natural abundance of this compound are highly desirable and predestine C12A7 as potential material for display applications among others. Field emission displays (FEDs) are widely considered in flat panel display applications due to several attractive qualities, such as wide viewing angle, slim panels, high contrast ratio, quick response, low weight, inertness to variations of temperature and power efficiency. Fig. 2 provides a schematic illustration of FED. The main challenges in the fabrication of FED devices are the choices of an efficient electron emitter and suitable phosphors with high brightness levels and long lifetimes. C12A7 has recently attracted considerable attention as both an electron emitter and phosphor for FED applications.

Fig.2. a) Schematic illustration of a field emission display. b) Schematic structure of an OLED based on An electrically conductive C12A7 acts as an electron emitter. C12A7:ethin film as cathode

Emitters with narrow elongated shapes and sharp tips can enhance the local electric field and reduce the turn-on field. Nanostructures with high aspect ratios, such as nanotubes, have been receiving special attention as field emitters. Such a complex surface topography necessitates ALD technology for conformal coating with C12A7. Electron incorporated C12A7 exhibits superior characteristics compared to previously known electron emitting materials. A work function of 3.7 eV has been determined for conductive C12A7, while the apparent work function based on field emission measurements is as low as 0.6 eV; much lower than that reported for carbon nanotubes. Polycrystalline C12A7:e- yields a rather small work function of ~0.6 eV for the electron emission, which has excellent potential for use in a cold-cathode electron-field emitter. The oxidization of the C12A7 surface and formation of a thin semiconductive surface layer is responsible for the dramatic decrease in the value of the apparent work function via the band bending effect, which is beneficial in order to improve the efficiency of cold emission. Additionally, the chemical stability of C12A7 in air or a humid environment at room temperature as well as considerably higher emission current makes it superior to organic electrodes.

Fig. 3: Configuration of FET test devices based on: (a) polycrystalline C12A7:ethin film and (b) single crystalline C12A7:e- bulk.

Conductive C12A7 thin films can be considered as electron emitters. The heat treatment of an insulating polycrystalline thin film with an amorphous C12A7 over-layer can produce a conductive thin film with excellent electrical conductivity of 800 S cm and a maximum Hall mobility of 2.5 cm V s. An increase in the film thickness can enhance the electrical conductivity due to an increase in the carrier mobility. For instance, a conductivity of 450 Scm is obtained from a film with 150nm thickness, while a conductivity of 800 Scm can be achieved for a film thickness of 400 nm or more. Therefore, it is possible to regulate the value of electronic properties by adjusting the film thickness. C12A7 has proven to be an excellent conductive transparent oxide with unique electrical and chemical properties and is amenable to FET test devices illustrated in Fig. 3.

III. Research Plan and Tasks
Currently there are no existing thin film technologies for the conformal deposition of C12A7 electride films on complex device surfaces with nanoscale precision. For today’s stringent nanotechnology device requirements, an R&D study to develop an Atomic Layer Deposition (ALD) fabrication process with hitherto novel ALD precursors is needed in order for the application potential of the C12A7 electride to materialize. This work is conceived as a collaborative research project between Dr. Baumgart’s group at Old Dominion University (ODU) and Dr. Li’s group at George Mason University (GMU). In broad outlines the proposed work can be subdivided into process development and nanomaterials synthesis by ALD, chemical surface termination and physical & electrical characterization of the resulting C12A7 films. ODU will focus on process development to develop from scratch a new ALD process technology for the electride 12CaO·7Al2O3. Conventional gas-phase ALD is characterized by its cyclic operational mode in which a binary film is being synthesized by the sequential self-limited reactions between volatilized precursor and co-reactant vapors that are introduced into the reactor chamber in an alternating manner and separated by intermediate inert gas purges thereby saturating every sample and reactor surface. For the C12A7 the challenge is to develop two ALD processes for two binary films in this case CaO and Al2O3 which have to be synthesized as alternating nanolaminates in the correct stoichiometric ratio and are subsequently reacted under high temperature annealing. For the CaO ALD process development we propose to utilize the new solid ALD precursor Bis(N,N-di-isopropylformamidinato)calcium(II) dimer [Ca(famd)2], which is a low vapor pressure Ca amidinate precursor and has to be reacted with ozone or DI H2O vapors. The chemical structure of the Calcium precursor is shown schematically in Fig. 4a. The Al2O3 binary compound will be synthesized with trimethylaluminum (TMA) and DI water vapor. The low vapor pressure of Ca amidinate precursor Ca(famd)2 in solid form is a major technical challenge and cannot be reacted by ALD in vapor draw mode but necessitates as new hardware a 150mL bubbler with dip tube for bubbling the molten precursor and use of an LVPD kit with bypass outlined in Fig 4b. ODU will take care of the physical characterization of the ALD electride films with FE-SEM surface morphology studies, SEM EDS elemental analysis of the final composition of the ALD synthesized electride film C12A7, X-ray diffraction XRD analysis and atomic force microscopy AFM. The GMU team will focus on a range of electrical measurements on test structures of the ALD electride films using their special expertise and facilities to photolithographically structure test devices and having access to NIST test facilities in Gaithersburg. GMU will emphasize understanding the characteristics of the material in a device, since it behaves quite differently in most cases due to its unique crystal structure.

Fig. 4 a) Schematic of chemical structure of the new solid Ca amidinate precursor Ca(famd)2  b) design for low pressure amidinate precursor bubbler with dip tube and bypass

Education Benefits: ODU & GMU will each delegate one PhD graduate student to work on the research related to the proposed electride film development and characterization part. The participation on this electride project will be an important part of the laboratory training for our graduate students and will also involve the undergraduate VMEC summer scholar interns, who are expected to come onboard next summer to join our respective laboratories.

Follow-up Proposals: This Seed Grant proposal and the experimental outcome is expected to enable the two participating university teams to apply for a new follow-up grant from other funding agencies, such as NSF, DOE and NIST and SBIRs.

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