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Materials Science at Oregon State University

Graduate Student Seminars - Nasir Alimardani and Richard Oleksak

Date: Thursday, Apr 5th
Presenter: Nasir Alimardani and Richard Oleksak, OSU EECS and OSU CBEE


Nasir Alimardani abstract:
Metal-Insulator-Insulator-Metal (MIIM) Tunnel Diodes with Enhanced Asymmetric Properties Using Dual Dielectric Stacks Deposited by Atomic Layer Deposition (ALD)

Metal-insulator-metal (MIM) based tunneling devices, with very thin insulator layers, have been proposed for a variety of applications, such as liquid-crystal display (LCD) backplanes, hot electron transistors, infrared (IR) detectors, macroelectronics, and optical rectennas for IR energy harvesting. The desired properties for the majority of these applications are high nonlinearity and asymmetric current density versus electric field (J-E) behavior at small voltage and in most of the applications at ultra-high frequencies. To achieve mentioned properties, the dominating conduction mechanism through the insulator layer of the MIM tunnel device at desired operating bias should be Fowler-Nordheim (FN) tunneling. Although asymmetry can be achieved using different metal electrodes, the amount of asymmetry is limited by the work-function difference that can be obtained using practical metal electrodes. Another way to introduce low voltage asymmetry and nonlinearity into an MIM tunnel diode is through the use of a dual insulator tunnel barrier or an MIIM structure in which the insulators used have different bandgaps and band offsets thus making asymmetric energy barrier heights with electrodes. An electron tunneling from one metal electrode to another electrode will see a different shape barrier depending on the direction of tunneling. This can improve diode performance via enhancement of FN tunneling dependence to polarity and resonant tunneling due to formation of a quantum well.

In this talk, the impact of dual dielectric tunnel barriers on MIM tunnel diode performance is discussed. It is shown that the current density versus electric field (J-E) characteristics of MIIM diodes can show enhanced asymmetric response and asymmetric behavior at small electric field; however all insulator stacks investigated in this study do not show such an asymmetric enhancement. It is found that high asymmetry can be seen in devices when FN tunneling is the dominant conduction mechanism.

Richard Oleksak abstract:
Microwave-enhanced Synthesis of Copper Indium Diselenide Nanoparticles in Non-absorbing Solvents

Printing-based methods for the fabrication of solar cells have gained considerable interest. This is due to the potential to reduce manufacturing cost while maintaining fairly high efficiencies in thin film solar cells. Inorganic nanoparticle ink approaches have recently been shown to produce copper indium gallium diselenide (CIGS) and copper zinc tin sulfide (CZTS) solar cells with good photon to electron conversion efficiencies. Herein we have investigated the solution-based synthesis of CuInSe2 (CIS), which is a promising material for thin film photovoltaic applications. CIS nanoparticles were synthesized via a microwave batch reactor from dissolved precursors in microwave transparent solvents tri-n-octylphosphine and oleic acid. These chemistries were used to investigate the effect of directed heating of dissolved precursors at concentrations much higher than typical solution-based nanomaterial syntheses. Reaction temperature, reaction time, and precursor concentrations were varied to determine optimum synthesis conditions for the CIS nanoparticles. Results indicate the formation of relatively monodisperse and phase-pure chalcopyrite CIS nanoparticles (less than 10 nm) which were synthesized from stoichiometric precursor concentrations, for short reaction times (5 min) and moderate reaction temperatures (215 C). It is hypothesized that direct heating of the precursor species leads to uniform nucleation events and homogeneous nanoparticle dispersions. A benefit of this approach is that the direct heating of the precursors may reduce the high temperatures required for some nanoparticle syntheses. CIS nanoparticles were characterized for phase, composition, structure, size and morphology using x-ray diffraction, Raman spectroscopy, transmission electron microscopy, scanning electron microscopy and energy dispersive x-ray spectroscopy.