Browsing by Author "Gerry Lucovsky, Committee Member"

Now showing 1 - 3 of 3

Approaches for High Permittivity in Barium Titanate
(2008-12-04) Daniels, Patrick Richard; Gerry Lucovsky, Committee Member; Jon-Paul Maria, Committee Chair; Nadia El-Masry, Committee Member
Abstract DANIELS, PATRICK R. High Permittivity Barium Titanate Thin Films. (Advisor: Jon-Paul Maria). As the demand for smaller, faster and more robust electronics increases, the sophistication of system design and the optimization of material properties must improve. In order to accommodate these goals, electrical components, such as capacitors, must be placed closer to the integrated circuit chip and consume less area on a printed wiring board. One approach to this end embeds these passive components into the printed wiring board directly beneath the integrated circuit chip. Previous work at NCSU pioneered a thin film ferroelectric capacitor technology that satisfies the principle demands. This thesis describes a set of advancements to this technology of the thin film dielectrics on flexible copper and platinum foils that target specifically enhanced permittivity through microstructure control. Barium titanate was deposited on copper and platinum foils by a chemical solution deposition process with the intent of investigating the effects of the A:B site ratio on the microstructure and electrical properties. The primary investigation involved preparing a range of dielectric compositions from 0 to 5% excess barium and annealing them to temperatures ranging from 900 to 1200Â°C for 20 hours. The annealing atmospheres and maximum temperature limits were chosen with respect to preserving the integrity of the metallic foil substrates. On Pt substrates, as annealing temperature and the amount of excess barium increased the average grain size increased dramatically. Average grain size grew from 70 nm for a stoichiometric film annealed at 900Â°C to 800 nm for films with 4% excess barium annealed at 1200Â°C. The grain size decreased in films with 5% excess barium annealed at 1200Â°C due to the development of a second phase identified by x-ray diffraction. This data is in sharp contrast to existing descriptions of the BaTiO3 binary phase diagram that suggest ppm levels of solid solubility associated with the BaTiO3 intermediate compound and present interesting new questions regarding stability of Ba excess crystals. Guided by this Pt substrate reference data, the microstruture â€“ dielectric property relationships were explored for BaTiO3 films on copper foil. Compositions of 1:1 barium titanate and 3% excess barium annealed at 900 and 1060Â°C were prepared and characterized respectively. The room temperature permittivity increased from 1800 to 4000 with the addition of excess barium and increased annealing temperature. The grain sizes ranged from x1 nm to x2 nm respectively. These results demonstrate a completely new method of controlling grain size in BaTiO3 with Ba excess, and the success of these methods to engineer extrinsic permittivity contributions consistent with well-prepared bulk ceramics. In addition, thin films of Ba0.7Sr0.3TiO3 were deposited on copper foils via RF magnetron sputtering with the intent of investigating the effects of process flow on the percent yield of working capacitors with respect to electrode size â€“ a second challenge in the development of a viable embedded high value capacitor technology. In the previously established conventional process, electrode metallization was performed after annealing at 900Â°C. In the newly developed co-firing process, electrode metallization is performed after sputtering deposition but before annealing at 900Â°C. By changing the process flow, of the fraction of working 5 mm diameter capacitors increased from 0% to 100%. These capacitors were prepared on copper foils with a dielectric thickness of less than 1 Âµm in the absence of clean room conditions. A model involving curvature-controlled de-wetting is proposed to explain the success of this method to obviate the short circuits that typically accompany geometric asperities associated with polycrystalline thin films and rough substrates.
The Chemical Solution Deposition of Lead Zirconate Titanate (PZT) Thin Films Directly on Copper Surfaces
(2005-07-13) Losego, Mark Daniel; Mark Johnson, Committee Member; Gerry Lucovsky, Committee Member; Jon-Paul Maria, Committee Chair
Traditionally, multifunctional complex oxide thin films, like the common ferroelectric materials lead zirconate titanate (PZT) and barium titanate (BaTiO₃) have been limited to substrates with noble metal or conductive oxide bottom electrodes. This constraint originates from the vulnerability of base metals to oxidation when traditional ceramic processing parameters—high temperatures and oxygen rich atmospheres—are used to synthesize ferroelectric films. With current technology, ferroelectric thin films have demonstrated vast applicability as tunable capacitors, sensors, piezoelectric actuators, and non-volatile memories. By integrating ferroelectrics thin films with base metals, the barrier to mass production is lowered through reduced expense and simplified electrode patternability. Moreover, base metals have higher conductivities and offer the possibility for increased functionality by incorporation of ferromagnetic or shape memory alloys. Recent research efforts have adapted 1970s thick film multilayer capacitor technology to process thin films of the (Ba,Sr)TiO₃ family directly on nickel and copper substrates. This methodology relies on processing these materials within a window of temperature and oxygen partial pressure (pO₂) that affords thermodynamic equilibrium between the oxidized perovskite film and unoxidized base metal substrate. Although the family of (Ba,Sr)TiO₃ materials offers excellent dielectric properties, the material PZT could provide a complementary set of functionality to satisfy applications that require an enhanced ferroelectric or piezoelectric response. Unfortunately, fundamental materials differences—particularly PbO volatility and a narrow thermodynamic stability window—make equilibrium processing impractical for PZT/base metal systems. In this thesis, integration of PZT directly on copper surfaces via a chemical solution deposition (CSD) route is investigated. Using this platform a new methodology is developed for achieving perovskite / base metal compatibility. Unlike the traditional equilibrium approach, this new method focuses on using a knowledge of sol-gel science to design a process window that is compatible with the copper substrate while maintaining the integrity of the PZT film. Using this approach, the chelating ligands (organic molecules that impart stability to the metal cations in solution) have been identified as a critical process parameter. If these chelating species cannot provide sufficient gel consolidation and volatilization prior to crystallization within a processing window compatible with the copper substrate, then various complications can result such as substrate oxidation, non-perovskite phase development, or film cracking. By proper chelating agent selection and a unique composite gel architecture, this thesis demonstrates that PZT films can be processed directly on copper substrates with dielectric and ferroelectric properties comparable to films deposited on conventional platinized silicon. Dielectric constants in excess of 800 with tanδ values below 0.02 have been achieved as well as remanent polarization of 33 μC/cm². C-V and P-E loops exhibit classical ferroelectric shapes with well-saturated intrinsic regimes. Electrical fatigue experiments show a classic response with loss of P-E loop squareness and a recoverable remanent polarization upon annealing above the Curie point. Hence, this work demonstrates a methodology for obtaining PZT thin films on copper substrates with remarkable dielectric and ferroelectric properties that are competitive with current noble metal / conductive oxide bottom metal electrode technologies.
Photoemission investigation of the electronic properties of Ga-face GaN (0001)-dielectric interfaces
(2003-05-12) Cook, Ted Edwin Jr.; Robert J. Nemanich, Committee Chair; Robert F. Davis, Committee Co-Chair; Zlatko S. Sitar, Committee Member; Gerry Lucovsky, Committee Member
The characteristics of clean n- and p-type GaN (0001) surfaces and the interface between this surface and SiO2, Si3N4, and HfO2 have been investigated. Both n- and p-type Ga-face GaN (0001) surfaces have been cleaned via an 860° C anneal in an ammonia atmosphere, and carbon and oxygen contaminants were reduced to below the detection limits. Layers of SiO2, Si3N4, or HfO2 were carefully deposited to limit the reaction between the plasma and the GaN surface. After stepwise deposition, the electronic states were measured with x-ray photoelectron spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS). A valence band offset (VBO) of 2.0 ° 0.2 eV with a conduction band offset (CBO) of 3.6 ± 0.2 eV was determined for the GaN/SiO2 interface. The large band offsets suggest SiO2 is an excellent candidate for passivation of GaN. For the GaN/Si3N4 interface, type II band alignment was observed with a VBO of 0.5 ± 0.2 eV with a CBO of 2.4 ± 0.2 eV. While Si3N4 should passivate n-type GaN surfaces, it may not be appropriate for p-type GaN surfaces. A VBO of 0.4 ± 0.2 eV with a CBO of 2.0 ± 0.2 eV was determined for the GaN/HfO2 interface. An instability was observed in the HfO2 film, with energy bands shifting ~0.5 eV during a 650° C densification anneal. The electron affinity measurements via UPS were 3.0, 1.1, 1.8, and 2.9 ° 0.1 eV for GaN, SiO2, Si3N4, and HfO2 surfaces, respectively. Electron affinity measurements, along with band alignment data, allow a deviation from the electron affinity model due to a change of the interface dipole to be observed. Interface dipoles of 1.7, 1.1 and 1.9 ° 0.2 eV were observed for the GaN/SiO2, GaN/Si3N4, and GaN/HfO2 interfaces, respectively. The existence of Ga-O bonding at the heterojunction significantly increases the interface dipole, which raises the dielectric bands in relation to the GaN.