Browsing by Author "Harald Ade, Committee Member"

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    Ab Initio Electronic Structure Calculations For High-K Dielectrics
    (2005-01-04) Zhang, Yu; Harald Ade, Committee Member; Jerry L. Whitten, Committee Member; Dave Aspnes, Committee Member; Gerald Lucovsky, Committee Chair
    In current semiconductor industry, continuing improvement in the performance of MOSFET requires aggressive scaling down of the dimensions of CMOS devices. A better capacitance/unit area can be gained as gate oxide thickness decreases. An equivalent oxide thickness (EOT) less than 1.0nm is required according to the 2002 International Technology Roadmap for Semiconductor (ITRS). However, as gate oxide thickness scaling down, tunneling current will increase, which will lower the device performance. SiO2, as the widely used gate oxide material, has reached its scaling limit due to the high current leakage at this thickness. Non-crystalline alloys of i) group IIIB, IVB and VB TM oxides and ii) first row RE oxides with SiO2 and Al2O3 have been proposed as alternative high-k gate dielectrics for advanced Si devices. This dissertation addresses differences between the electronic structure of alternative high-k transition metal dielectrics and SiO2. Ab inito calculations, based on small clusters identify unique aspects of electronic structure that are associated with the TM atoms. The lowest conduction band states are derived from atomic d-states of the TM atoms, and are localized on these atoms. Excitations into these states i) from TM core states, ii) from oxygen K1, iii) from oxygen atom derived valence band states, are simulated by using ab inito calculations at self-consistant-field (SCF) Hartree-Fock and Configuration Interaction (CI) level. And these electronic structure calculations are used to interpret optical, ultra-violet (UV), X-ray and electron spectroscopies, including UV and X-ray photoemission (UPS and XPS, respectively), and Auger electron spectroscopy (AES), and also provide a basis for interpretation of electrical results and narrowing the field of possible replacement dielectrics for advanced semiconductor devices.
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    Electronic Transition Imaging of Carbon Based Materials: The Photothreshold of Melanin and Thermionic Field Emission from Diamond
    (2006-12-06) Garguilo, Jacob Marshall; Glenn Edwards, Committee Member; Harald Ade, Committee Member; Robert Nemanich, Committee Chair; Keith Weninger, Committee Member; Ron Scattergood, Committee Member
    This study explores electronic transitions in carbon based materials through the use of a custom built, non rastering electron emission microscope. The specifics and history of electron emission are described as well as the equipment used in this study. The materials examined fall into two groups, melanosome films isolated from the human body and polycrystalline diamond tip arrays. A novel technique for determining the photothreshold of a heterogeneous material on a microscopic or smaller scale is developed and applied to melanosome films isolated from the hair, eyes, and brain of human donors. The conversion of the measured photothreshold on the vacuum scale to an electrochemical oxidation potential is discussed and the obtained data is considered based on this conversion. Pheomelanosomes isolated from human hair are shown to have significantly lower photoionization energy than eumelanosomes, indicating their likelihood as sources of oxidative stress. The ionization energies of the hair melanosomes are checked with complimentary procedures. Ocular melanosomes from the retinal pigment epithelium are measured as a function of patient age and melanosome shape. Lipofuscin, also found in the eye, is examined with the same microscopy technique and shown to have a significantly lower ionization threshold than RPE melanosomes. Neuromelanin from the substantia nigra is also examined and shown to have an ionization threshold close to that of eumelanin. A neuromelanin formation model is proposed based on these results. Polycrystalline diamond tip arrays are examined for their use as thermionic energy converter emitters. Thermionic energy conversion is accomplished through the combination of a hot electron emitter in conjunction with a somewhat cooler electron collector. The generated electron current can be used to do work in an external load. It is shown that the tipped structures of these samples result in enhanced emission over the surrounding flat areas, which may prove valuable in limiting the negative space charge effect in vacuum energy converting devices. Additionally, the effects of exceeding a threshold temperature for the films are shown, establishing a maximum operating regime for any device which incorporates hydrogen terminated diamond.