Browsing by Author "Jon-Paul Maria, Committee Chair"

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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.
Integration of Ferroelectric Thin Films in Tunable Microwave Devices
(2010-04-30) Lam, Peter Gaifun; Jon-Paul Maria, Committee Chair; Michael Steer, Committee Member; Douglas Irving, Committee Member; Ramon Collazo, Committee Member; Jayesh Nath, Committee Member
Abstract LAM, PETER GAIFUN. Integration of Ferroelectric Thin Films in Tunable Microwave Devices. (Under the direction of Jon-Paul Maria). Recent growth in wireless communication industry has led to a growing search for alternative technologies that can provide higher flexibility and efficiency in the handling of radio frequency spectrum, and preferably at a similar or lower cost than the present technologies. Thin film barium strontium titanate has shown great promises for microwave applications. There are two goals in this thesis: 1) To show a methodology for integration of BST thin films in microwave devices operating at frequencies above 6 GHz, and 2) To explore the limits of material optimization for increased device performance. The integration of BST thin film with a 3rd order combline-based bandpass filter in the frequency range of 6 to 18 GHz was demonstrated. Alumina substrates with filled vias were used to support the devices. Optimized (Ba0.7Sr0.3)TiO3 film was deposited and patterned in alumina substrate. Planar gap capacitors were patterned with a Cr/Au metal stack while the rest of the device layout was patterned with silver metal and electroplated with copper. The measured microwave characteristics of the bandpass filters fall within the specifications of the design but more improvements were needed to lower the insertion loss and the frequency tunability for devices operating at frequency above 12 GHz. The metallization impact on the microwave devices was studied. Two variables were tested with electroplated copper: 1) Thickness, and 2) geometry with respect to the gap. Increasing the thickness of the copper decreases the insertion loss of the device. Metal thickness beyond three skin depths does not impart significant improvement. Plating distances to the edge of the gap in the capacitor has not effect in the insertion loss of the filter. Tunability limit of coplanar gap capacitors was investigated. A series of gap capacitors with different geometry were tested. In the limit of low capacitance values, a fringe capacitance is observed, which measured to be in the range of 50-100 fF. The non-tunable capacitance contributes to the low tunability of the 12-18 GHz range bandpass filter, where the required capacitance values for impedance matching, 200-400 fF, are close to the values of the fringe. A coplanar MIM capacitor structure is proposed to improve the tunability values. The configuration consists of a sapphire substrate, followed by the metal coplanar electrodes (Pt/Ti stack deposited by e-beam evaporation) and then capped with the BST layer. Tunability values of 50% were obtained with an electric field of 125 KV/cm for an interdigitated coplanar MIM capacitor.
Interfacing Epitaxial Oxides to Gallium Nitride
(2008-08-19) Losego, Mark Daniel; Jon-Paul Maria, Committee Chair; Mark Johnson, Committee Member; Zlatko Sitar, Committee Member; Gregory Parsons, Committee Member
Molecular beam epitaxy (MBE) is lauded for its ability to control thin film material structures at the atomic level. Controlling the chemistry and structure of epitaxial interfaces at the atomic level can improve performance of microelectronics and cultivate the development of novel device structures. This thesis explores the utility of MBE for designing interfaces between oxide epilayers and the wide band gap semiconductor gallium nitride (GaN). The allure of wide gap semiconductor microelectronics (like GaN, 3.4 eV) is their ability to operate at higher frequencies, higher powers, and higher temperatures than current semiconductor platforms. Heterostructures between ferroelectric oxides and GaN are also of interest for studying the interaction between GaN's fixed polarization and the ferroelectric's switchable polarization. Two major obstacles to successful integration of oxides with GaN are: (1) interfacial trap states; and (2) small electronic band offsets across the oxide / nitride interface due to the semiconductor's large band gap. For this thesis, epitaxial rocksalt oxide interfacial layers (˜8 eV band gap) are investigated as possible solutions to overcoming the challenges facing oxide integration with GaN. The cubic close-packed structure of rocksalt oxides forms a suitable epitaxial interface with the hexagonal close-packed wurtzite lattice of GaN. Three rocksalt oxide compounds are investigated in this thesis: MgO, CaO, and YbO. All are found to have a (111) MO || (0001) GaN; <1`10> MO || <11`20> GaN epitaxial relationship. Development of the epilayer microstructure is dominated by the high-energy polar growth surface (drives 3D nucleation) and the interfacial symmetry, which permits the formation of twin boundaries. Using STEM, strain relief for these ionicly bonded epilayers is observed to occur through disorder within the initial monolayer of growth. All rocksalt oxides demonstrate chemical stability with GaN to >1000°C. Concurrent MBE deposition of MgO and CaO is known to form complete solid solutions. By controlling the composition of these alloys, the oxide's lattice parameter can be engineered to match GaN and reduce interfacial state density. Compositional control is a universal challenge to oxide MBE, and the MgO-CaO system (MCO) is further complicated by magnesium's high volatility and the lack of a thermodynamically stable phase. Through a detailed investigation of MgO's deposition rate and subsequent impact on MCO composition, the process space for achieving lattice-matched compositions to GaN are fully mapped. Lattice-matched compositions are demonstrated to have the narrowest off-axis rocking curve widths ever reported for an epitaxial oxide deposited directly on GaN (0.7° in f-circle for 200 reflection). Epitaxial deposition of the ferroelectric (Ba,Sr)TiO3 by hot RF sputtering on GaN surfaces is also demonstrated. Simple MOS capacitors are fabricated from epitaxial rocksalt oxides and (Ba,Sr)TiO3 layers deposited on n-GaN substrates. Current-voltage measurements reveal that BST epilayers have 5 orders of magnitude higher current leakage than rocksalt epilayers. This higher leakage is attributed to the smaller band offset expected at this interface; modeling confirms that electronic transport occurs by Schottky emission. In contrast, current transport across the rocksalt oxide ⁄ GaN interface occurs by Frenkel-Poole emission and can be reduced with pre-deposition surface treatments. Finally, through this work, it is realized that the integration of oxides with III-nitrides requires an appreciation of many different fields of research including materials science, surface science, and electrical engineering. By recognizing the importance that each of these fields play in designing oxide ⁄ III-nitride interfaces, this thesis has the opportunity to explore other related phenomena including accessing metastable phases through MBE (ytterbium monoxide), spinodal decomposition in metastable alloys (MCO), how polar surfaces grown by MBE compensate their bound surface charge, room temperature epitaxy, and the use of surface modification to achieve selective epitaxial deposition (SeEDed growth).
Lead Zirconate Titanate (PZT) Based Thin Film Capacitors For Embedded Passive Applications
(2003-08-21) Kim, Taeyun; Robert Croswell, Committee Member; Paul Franzon, Committee Member; Jon-Paul Maria, Committee Chair; Gerd Duscher, Committee Member; Angus Kingon, Committee Chair
Investigations on the key processing parameters and properties relationship for lead zirconate titanate (PZT, 52/48) based thin film capacitors for embedded passive capacitor application were performed using electroless Ni coated Cu foils as substrates. Undoped and Ca-doped PZT (52/48) thin film capacitors were prepared on electroless Ni coated Cu foil by chemical solution deposition. The effects of processing parameters on the phase evolution, microstructures, dielectric properties, and reliability were investigated. Electroless Ni coated Cu foil was selected as substrate for its low cost, oxidation resistance and lamination capability. When annealed at 450 °C, electroless Ni coated Cu foil showed transformation from amorphous Ni to crystalline phase of Ni-P (mostly Ni₃P) and Ni metal. For PZT (52/48) thin film capacitors on electroless Ni coated Cu foil, voltage independent (zero tunability) capacitance behavior was observed. Dielectric constant reduced to more than half of the identical capacitor processed on Pt/SiO₂/Si. Dielectric properties of the capacitors were mostly dependent on the crystallization temperature. Capacitance densities of almost 350 nF/cm² and 0.02-0.03 of loss tangent were routinely measured for capacitors crystallized at 575-600 °C. Leakage current showed dependence on film thickness and crystallization temperature. It is speculated that space charge limited conduction (SCLC) seems to be consistent with conduction mechanism in PZT thin films on electroless Ni. From a two-capacitor model, the existence of a low permittivity interface layer (permittivity -30) was suggested. Also it is suggested a high concentration of traps exist inside the PZT capacitor. Interface reaction between PZT thin film and electroless Ni was suggested to be responsible for measured electrical properties. The interfacial layer might be composed of unreacted oxide, phosphate, and phosphides possibly from phosphorous diffused from electroless Ni into PZT bulk. For Ca-doped PZT (52/48) thin film capacitors prepared on Pt, typical ferroelectric and dielectric properties were measured up to 5 mol%Ca doping. Further addition up to 10 mol % changed the lattice parameter of the unit cell, and reduced dielectric properties were observed. The possibility of Ca acceptor doping is suggested. When Ca-doped PZT (52/48) thin film capacitors were prepared on electroless Ni coated Cu foil, phase stability was influenced by Ca doping and phosphorous content. Dielectric properties showed dependence on the crystallization temperature and phosphorous content. Capacitance density of -400 nF/cm² was achieved, which is an improvement by more than 30% compared to undoped composition. Ca doping also reduced the temperature coefficient of capacitance (TCC) less than 10%, all of them were consistent in satisfying the requirements of embedded passive capacitor. Leakage current density was not affected significantly by doping. Interface control by controlled pO² crystallization was found to be not effective in interface layer mitigation. Phase purity, dielectric properties, surface microstructure, and pO² were found to have a correlated dependence. To tailor the dielectric and reliability properties, ZrO² was selected as buffer layer between PZT and electroless Ni. Only RF magnetron sputtering process could yield stable ZrO² layers on electroless Ni coated Cu foil. Other processes resulted in secondary phase formation, which supports the reaction between PZT capacitor and electroless Ni might be dominated by phosphorous component. Incorporation of ZrO² layers reduced maximum capacitance density by 10 %(- 350 nF/cm²) due to lower permittivity of ZrO² layer. Significantly improved leakage current densities were measured for PZT thin film capacitors on ZrO₂. For PZT thin film capacitors incorporating 100 nm thick ZrO₂ layer, leakage current density of 10⁻⁸ A/cm² was measured at 25 VDC, which is more than three orders of magnitude lower than those directly deposited on electroless Ni coated Cu foil. The complete set of experimental data provides validation and process conditions for the use of PZT thin films on low cost electroless Ni coated Cu foil substrate as embedded capacitors in high density printed circuit boards.
Processing Science of Barium Titanate
(2009-04-22) Aygun, Seymen Murat; Gregory Parsons, Committee Member; Zlatko Sitar, Committee Member; Jon-Paul Maria, Committee Chair; Yuntian Zhu, Committee Member
Barium titanate and barium strontium titanate thin films were deposited on base metal foils via chemical solution deposition and radio frequency magnetron sputtering. The films were processed at elevated temperatures for densification and crystallization. Two unifying research goals underpin all experiments: 1) To improve our fundamental understanding of complex oxide processing science, and 2) to translate those improvements into materials with superior structural and electrical properties. The relationships linking dielectric response, grain size, and thermal budget for sputtered barium strontium titanate were illustrated. (Ba0.6Sr0.4)TiO3 films were sputtered on nickel foils at temperatures ranging between 100-400 Ã‚Â°C. After the top electrode deposition, the films were co-fired at 900 Ã‚Â°C for densification and crystallization. The dielectric properties were observed to improve with increasing sputter temperature reaching a permittivity of 1800, a tunability of 10:1, and a loss tangent of less than 0.015 for the sample sputtered at 400 Ã‚Â°C. The data can be understood using a brick wall model incorporating a high permittivity grain interior with low permittivity grain boundary. However, this high permittivity value was achieved at a grain size of 80 nm, which is typically associated with strong suppression of the dielectric response. These results clearly show that conventional models that parameterize permittivity with crystal diameter or film thickness alone are insufficiently sophisticated. Better models are needed that incorporate the influence of microstructure and crystal structure. This thesis next explores the ability to tune microstructure and properties of chemically solution deposited BaTiO3 thin films by modulation of heat treatment thermal profiles and firing atmosphere composition. Barium titanate films were deposited on copper foils using hybrid-chelate chemistries. An in-situ gas analysis process was developed to probe the organic removal and the barium titanate phase formation. The exhaust gases emitted during the firing of barium titanate films were monitored using a residual gas analyzer (RGA) to investigate the effects of ramp rate and oxygen partial pressure. The dielectric properties including capacitor yield were correlated to the RGA data and microstructure. This information was used to tailor a thermal profile to obtain the optimum dielectric response. A ramp rate of 20 Ã‚Â°C/min and a pO2 of 10-13 atm resulted in a permittivity of 1500, a loss tangent of 0.035 and a 90 % capacitor yield in 0.5 mm dot capacitors. Yield values above 90% represent a significant advantage over preexisting reports and can be attributed to an improved ability to control final porosity. Finally, the dramatic enhancement in film density was demonstrated by understanding the processing science relationships between organic removal, crystallization, and densification in chemical solution deposition. The in situ gas analysis was used to develop an each-layer-fired approach that provides for effective organic removal, thus pore elimination, larger grain sizes, and superior densification. The combination of large grain size and high density enabled reproducing bulk-like dielectric properties in a thin film. A room temperature permittivity of 3000, a 5 ÃŽÂ¼F/cm2 capacitance density, and a dielectric tunability of 15:1 were achieved. By combining the data sets generated in this thesis with those of comparable literature reports, we were able to broadly rationalize scaling effects in polycrystalline thin films. We show that the same models successfully applied to bulk ceramic systems are appropriate for thin films, and that models involving parasitic interfacial layers are not needed. Developing better models for scaling effects were made possible solely by advancing our ability to synthesize materials thus eliminating artifacts and extrinsic effects.
Reliability of Gap Capacitors
(2008-12-08) Kupferschmidt, James R.; Jon-Paul Maria, Committee Chair; Angus Kingon, Committee Member; J.C. Poindexter, Committee Member
The requirements for communication devices continue to become more challenging each year leading to rapid developments in varying technologies to meet the demands. Tunable filters, utilizing ferroelectric varactors, are merely one solution to the ever-demanding need for microwave communication systems. Barium Strontium Titanate (BST) can be employed as the dielectric material necessary to make tunable devices. The broad range of BSTâ€™s remarkable properties allow for easy integration into many devices, such as MEMS, oscillators, phase shifters, and tunable filters. BST deposition is done via RF magnetron sputtering for our purposes. Previous work completed here at NCSU has led to a sophisticated process to fabricate tunable filters chips that work over the 6-18 GHz range. Using polycrystalline alumina substrates with varactors made up of a pair of 4 Î¼m silver gap capacitors in series as well as utilizing the BST dielectric creates excellent tunable chips. There are however issues related to the reliability of the gap capacitors composing the varactors. Existing theory (Paschenâ€™s Law) describing capacitor failure reveals gaps with relatively close spacings (below 5 Î¼m) should maintain robust breakdown strengths. However, our research, as well as numerous additional literature, proves this theory to be inaccurate. Mooreâ€™s Law has led to a return to this research area, after almost a four decade hiatus, relatively recently as well as additional modifications to electrical systems that are presently occurring. Townsend discharge and electric field emission can be used to describe the mechanisms of gap capacitor breakdown. The Fowler-Nordheim equation and Weibull statistics, via the Bernard median rank equation for determining cumulative probability, are employed to analyze the breakdown data. Various mechanisms lead to breakdown occurring among these capacitors and further investigation into silver electromigration was undertaken to explore the possibility of migration as a significant factor towards failure. The details involved in fabricating tunable filters is detailed and shown to be relatively similar to the construction of samples with loads of gap capacitors. BST thin-film deposition is completed initially, followed by photolithography processes to define the outline for the filters. Silver metallization is accomplished using DC magnetron sputtering, and patterning is completed by lift-off in an acetone solution, due to the lack of harmful effects to the BST thin-film. Samples with roughly 250 gap capacitors on them were also fabricated on alumina substrates, with gaps ranging between 2 to 5 microns and widths between 100 to 750 microns. Large number of the gap capacitors were placed under ramping DC bias to observe if failure occurred using a suitable probe station. Proper analysis was undertaken using Weibull statistics, and several critical variables were examined, including roughness, patterning processes, adhesion of silver metal to the substrate, as well as varying the type of ground metal. SEM was utilized in examining the capacitor failure, including carrying out experiments looking into silver electromigration. In this thesis, we will show the inadequacies of Paschenâ€™s Law for describing capacitor failure with gap spacings of less than 5 Î¼m. Also, an explanation into the process improvements for utilizing Ag gap capacitors on alumina substrates, for our tunable filters purposes, to eventually using Cr-Au gap capacitors on sapphire substrates is provided. For silver gap capacitors, the existence of the electromigration process is established and verified confirmed through SEM and EDS analysis.
Spectroscopy of Oxide-GaN Interfaces
(2009-03-05) Craft, Hughes Spalding; Zlatko Sitar, Committee Member; Donald Brenner, Committee Member; Thomas Pearl, Committee Member; Jon-Paul Maria, Committee Chair
GaN-based devices are of interest for applications requiring high-frequency, high-power operation at elevated temperatures. As in traditional, silicon-based devices, integration of semiconducting phases with insulators is critical. Additionally, applications involving the integration of GaN with polar oxides such as perovskite ferroelectrics have been proposed, due to the coupling that may be achieved between the respective polar vector. Devices utilizing such a coupling behavior would make possible two-dimensional electron gases of high charge densities that could be modulated by the oxideâ€™s polarization. The current status of oxide-GaN research is far behind that of oxide-Si research, and large-scale realization of GaN devices will require detailed understanding of oxide-GaN interfaces. This thesis focuses on the characterization of several oxide-GaN interfaces using x-ray photoelectron spectroscopy (XPS), as well as the identification of issues relating to the GaN surface. The rocksalt oxides MgO and CaO have been proposed as candidates for GaN MOSFET gate oxides, passivating layers, and buffer layers in GaN-ferroelectric structures. Thus, knowledge of film growth modes and band alignments is critical. Utilizing in-vacuo molecular beam epitaxy (MBE) and XPS, the growth of MgO on GaN was found to occur by the Volmer-Weber mode, with coalescence occurring at ~12 nm. This coalescence behavior was not found to affect the band alignment. As measured by XPS, the valence band offset at the MgO-GaN interface is 1.2 Â± 0.2 eV, leading to a conduction band offset of 3.5 eV. A similar study was undertaken for the CaO-GaN system, in which more rapid coalescence was observed, leading to the conclusion of a Stranski-Krastanov growth mode. The difference in coalescence behavior is attributed to the increased reactivity of the CaO surface. The band offsets at the CaO-GaN interface were found to be 1.0 Â± 0.2 eV at the valence band, and 2.5 eV at the conduction band. The band structures measured in this thesis are considered to be sufficient for limiting leakage current by Schottky emission for high-temperature devices. Surface chemical stability of rocksalt oxides is a known issue with respect to hydroxylation through water adsorption. XPS characterization of water uptake was performed using the O 1s photoelectron line after several in-vacuo exposures, culminating in a one-hour exposure to a water/oxygen mixture at 1 x 10-6 Torr. Characterization of polycrystalline MgO showed a saturating coverage of â€“OH groups at approximately 1 monolayer, regardless of exposure. CaO films exhibited increased reactivity, with hydroxyl coverage increasing to 3 monolayers, in addition to a similar amount of physisorbed water, suggesting the possibility for further reaction. Complete recovery of both oxide surfaces is shown to be achievable using mild vacuum anneals. Finally, the surface of GaN has been characterized with respect to several issues encountered during these investigations. GaN surfaces are found to be significantly Ga-rich, with surface stoichiometries routinely in excess of Ga2N. Several wet chemistries for GaN preparation were evaluated for their ability to modify the electrical behavior of subsequently grown oxide films. XPS could not unambiguously identify any change in surface chemistry that promotes these effects. Finally, p-type GaN films were noted to consistently possess greater oxide contamination in the as-grown state. Typical n-type or undoped GaN were marked by submonolayer quantities of oxide surface coverage, while p-type GaN typically exhibited coverages in the 1-2 nm scale. This difference has been found to be due to the p-type dopant activation anneal, during which GaN oxidation cannot be suppressed
Sputtered (Bax, Sr1-x)TiO3, BST, Thin Films on Flexible Copper Foils for Use as a Non-Linear Dielectric
(2006-11-03) Laughlin, Brian James; Jon-Paul Maria, Committee Chair; Angus I. Kingon, Committee Member; Michael B. Steer, Committee Member; Raoul Schlesser, Committee Member; Alexei Gruverman, Committee Member
Ferroelectric thin film dielectrics have a non-linear DC bias dependent permittivity. Ferroelectric thin film can be used as the dielectric between metal electrodes to make tunable Metal-Insulator-Metal (MIM) capacitors. Varactors can be used to change the resonance frequency of a circuit allowing high speed frequency switching intra- and inter-band. 2-D geometric arrays of circuitry, where resonant frequency is independently controlled by tunable elements in each section of the array, allow electromagnet radiation to be focused and the wave front spatial trajectory controlled. BST thin films varactor allow large DC bias fields to be applied with modest voltages allowing a large tunability to be realized. If ferroelectric thin film based devices are to supplant semiconductor varactors as tunable elements then devices must be synthesized using a low cost processing methodology. The Film on Foil process methodology for depositing BST thin films on copper foil substrates was used to create BST/Cu specimens. Sputtering conditions were determined via BST deposition on platinized silicon. Sputtered BST thin films were synthesized on Cu foil substrates and densified using high T, controlled pO2 anneals. XRD showed the absence of Cu2O in as-deposited, post crystallization annealing, and post "re-ox" annealing. Data showed a polycrystalline BST microstructure with a 55 - 80 nm grain size and no copper oxidation. HRTEM imaging qualitatively showed evidence of an abrupt BST⁄Cu interface free from oxide formation. Dielectric properties of Cu/BST⁄Pt MIM devices were measured as a function of DC bias, frequency, and temperature. A permittivity of 725 was observed with tunability >3:1 was observed with zero bias tan δ of 0.02 saturating to tan δ < 0.003 at high DC bias fields. No significant frequency dispersion was observed over five decades of frequency. Temperature dependent measurements revealed a broad ferroelectric transition with a maximum at -32°C which sustains a large tenability over -150°C to 150°C. Sputtered BST thin films on copper foils show comparable dielectric properties to CVD deposited films on platinized silicon substrates proving sputtered BST⁄Cu specimens can reproduce excellent properties using a more cost-effective processing approach. A concept for reducing the temperature dependence was explored. Stack of multiple compositions of BST thin films were considered as extension of core-shell structures to a thin film format. Temperature profiles of BST°Cu films were modeled and mathematically combined in simulations of multi-composition film stacks. Simulations showed singular composition BST thin films could meet X7R specifications if film has a 292 K < TC < 330 K. Simulations of series connected film stacks show only modest temperature profile broadening. Parallel connected dual composition film stack showed a 75°C temperature range with essentially flat capacitance if a ΔTC = 283°C between the compositions were simulated. Assumptions of the simulations were maximum permittivity and temperature profile shape independent of film thickness or composition. BST⁄Cu thickness and compositions series were fabricated and dielectric properties characterized. These studies showed films could be grown from 300 nm and approaching 1 μm without changing the dielectric temperature response. In studying BST composition, a increasing TC shift was observed when increasing Ba mole fraction in BST thin films yet tunability was maintained > 3:1. These results provide a route to creating temperature stable capacitors using a BST⁄Cu embodiment. In an effort to reduce surface roughness of copper foil substrates adversely impacted BST film integrity by impairing adhesion. XPS analysis of high surface roughness commercially obtained Cu foils revealed a surface treatment of Zn-Cu-O that was not present on smooth Cu, thus an investigation of surface chemistry was conducted. Sessile drop experiments were performed to characterize Cu-BST adhesion and the affects of metallic Zn and ZnO in this system. The study revealed the work of adhesion of Cu-BST, WaCu-BST ≈ 0.60 J⁄m2, is intermediary to noble metals commonly used as electrodes and substrates for electroceramic materials. Examination of metallic Zn-BST adhesion revealed a dramatic decrease of WaZn-BST ≈ 0.13 J⁄m2, while increasing the content of Zn in metallic (Cux, Zn1-x) alloys monotonically reduced Wa(Cux, Zn1-x). Conversely, the Cu-ZnO interface showed a large work of adhesion, WaCu-ZnO = 2.0 J⁄m2. These results indicate that a ZnO interlayer between the substrate Cu and the BST thin film provides adequate adhesion for robust films on flexible copper foil substrates. Additionally, this study provided characterization of adhesion for Zn-Al2O3 and Zn-BST; data that does not exist in the open literature. A process has been developed for preparing ultra-smooth copper foils by evaporation and subsequent peel-off of copper metal layers on glass slides. These 15 ⁄m thick substrates exhibited roughness values between 1 and 2 nm RMS and 9 nm RMS over 25 μm2 and 100 μm2 analysis areas respectively. The deposition and crystallization of barium strontium titanate layers on these ultra-smooth foils is demonstrated. The fully processed dielectric layers exhibited field tunability greater than 5:1, and could withstand fields in excess of 750 kV⁄cm. High field loss tangents below 0.007 were observed, making these materials excellent candidates for microwave devices. Finally, a process of lamination and contact lithography was used to demonstrate patterning of micron-scale features suitable for microwave circuit element designs.
Synthesis and Properties of Barium Titanate Solid Solution Thin Films on Copper Substrates
(2006-09-26) Ihlefeld, Jon F.; Jon-Paul Maria, Committee Chair; Angus Kingon, Committee Member; Robert Nemanich, Committee Member; Zlatko Sitar, Committee Member; William Borland, Committee Member
Barium titanate thin films were deposited via chemical solution deposition using a hybrid-chelate chemistry directly on copper foil substrates. A process was developed to crystallize and densify the ferroelectric films at 900C by using a reductive atmosphere containing nitrogen, hydrogen, water vapor, and oxygen impurities such that film constituents were oxidized to form barium titanate and the foil substrate remained metallic. The crystallized films are polycrystalline with equiaxed morphology and average grain diameters in excess of 100 nm. The dielectric properties exhibit permittivities in excess of 1800 at room temperature and zero bias with tunabilites of greater than 90% and high field loss tangents of less than 1%. A series of samples was prepared with varying grain and crystallite sizes by dividing and processing a single film over a range of temperature from 700 to 900C. This ensures that the chemical composition and film thickness is invariant for each sample. It is shown that the grain size increases with higher process temperatures and results in a concomitant increase in permittivity and tunability. These enhancements, combined with the constant paraelectric⁄ferroelectric phase transition temperature, indicated that a combination of film crystallinity and grain size is responsible for diminished performance. The phase transition temperature and temperature coefficient of capacitance modified by partially substituting zirconium, hafnium, and tin for titanium. The resulting films were single phase and the phase transition shifts were consistent with bulk materials. A reduction in permittivity was observed for increasing substituent level and was attributed to a reduction in grain size for both barium titanate zirconate and barium titanate hafnate. Processing conditions were chosen to stabilize Sn2+ during the firing process in an attempt to flux the system and increase grain size. The barium titanate stannate films had less reduction in grain size per substituent level than either zirconium or hafnium, however a similar reduction in permittivity was observed. The diminished dielectric response was explained by a defect reaction involving divalent tin and oxygen vacancies that quenched the extrinsic domain response to the dielectric constant. Defect equilibria were investigated with respect to processing atmosphere, stoichiometry, and dopant concentration. The solubility of excess barium and titanium was found to be greater in the films than is expected in the bulk, however it is unclear that equilibrium is achieved in the process. It was demonstrated that dopants could successfully eliminate the necessity of a reoxidation anneal to compensate for oxygen point defects resulting from the low pO2 atmospheres. The dopant levels necessary and insulation resistance of pure BaTiO3 were greater than expected from thermodynamic calculations. It was suggested that this is the result of a reduction in the enthalpy of reduction, stemming from an increase in grain boundary volume. Barium borate fluxes were used to improve densification and crystallinity. Barium borate additions between 0 and 3% uniformly increased grain size and density, while levels greater than 3% resulted in anomalous grain growth. Films with exaggerated grains show tetragonal peak splitting in the X-ray diffraction patterns, consistent with bulk barium titanate. In materials without exaggerated grain growth, dielectric measurements revealed permittivities in excess of 3000 at room temperature (for average grain sizes of approximately 160 nm). This value is equivalent to the finest-prepared bulk ceramics and substantially greater than any polycrystalline film ever reported. This has been attributed to in improvement in film crystallinity. These two accomplishments — tetragonal crystal symmetry and permittivities in excess of 3000 — represent dramatic breakthroughs in ferroelectric thin film technology.