Browsing by Author "Nadia El-Masry, Committee Member"

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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.
Characterization of Molecular and Atomic Species Adsorbed on Ferroelectric and Semiconductor Surfaces
(2009-07-27) Bharath, Satyaveda Chavi; Mark Luo, Committee Member; Thomas Pearl , Committee Chair; Gerd Duscher, Committee Co-Chair; Nadia El-Masry, Committee Member; C. Lewis Reynolds, Committee Member
BHARATH, SATYAVEDA CHAVI. Characterization of molecular and atomic species adsorbed on ferroelectric and semiconductor surfaces. (Under the direction of Thomas P. Pearl and Gerd Duscher). In order to clarify the mechanisms behind the adsorption of atomic and molecular species adsorbed on ferroelectric surfaces, single crystalline lithium niobate (LiNbO3, LN), Ã¢â‚¬ËœZ-cutÃ¢â‚¬â„¢ along the (0001) plane, has been prepared, characterized and subsequently exposed to molecular and atomic species. 4-n-octyl-4Ã¢â‚¬Â²-cyanobiphenyl (8CB) liquid crystal was chosen as a polar molecule for our model system for this study. Low-energy electron diffraction (LEED), atomic force microscopy (AFM), surface contact angles (CA), and X-ray photoelectron spectroscopy (XPS) were used to characterize the surface of LN as well as the nature of the liquid crystal films grown on the surface. Atomically flat LN surfaces were prepared as a support for monolayer thick, 8CB molecular domains. Also, for the purpose of gaining a fundamental understanding of low coverage interactions of metal atoms on ferroelectric surfaces, we choose to deposit gold onto the LN surface. These gold atomic layers were grown under UHV conditions and characterized. Understanding anchoring mechanisms and thin film organization for LC molecules and metal atoms on uniformly poled surfaces allows for a fuller appreciation of how molecular deposition of other polarizable molecules on patterned poled LN surfaces would occur as well as yielding greater insight on the atomic characteristics of metal on ferroelectric interfaces. Also, to reveal the mechanisms involved in the adsorption of organic aromatic molecules on high-index Si surfaces, thiophene (C4H4S) and pyrrole (C4H5N) molecules were dosed on prepared Si(5 5 12)-2x1 surfaces as our experimental system. The Si(5 5 12) surface was prepared to produce a 2x1 reconstruction after which molecules were dosed at low exposure to observe the preferred adsorption sites on the surface. All surface preparation and experiments were performed in UHV and measurements of the surface before and after deposition were performed using scanning tunneling microscopy (STM). Fundamental understanding of organic nanostructures on Si(5 5 12) surfaces will yield a broader appreciation of the mechanisms that drive the creation of these nanostructures for specific potential applications.
Characterizing the Ductile Response of Brittle Semiconductor Materials to Dynamic Contact Processes
(2004-06-22) Randall, Travis John; Ronald O. Scattergood, Committee Chair; Nadia El-Masry, Committee Member; John MacKenzie, Committee Member
It has been well documented that single crystal silicon and germanium exhibit plastic flow with the generation of high pressure on the surface by contact processes such as micro-indentation, scribing, and single point diamond turning. A high pressure phase transformation (HPPT) of the near surface region from the brittle, highly covalent diamond cubic (dc) structure to a metallic β-tin phase is thought be responsible for the anomalous plastic flow behavior during contact loading processes. The scope of this investigation is the study of the response of single crystal silicon and germanium to dynamic contact processes such as scratching and single point diamond turning. Plastic flow in silicon was noted for scribing experiments in SEM observation for various cutting directions on different crystal orientations. The both the ductile response and fracture behavior was shown to be greatly influence by the combination of cutting direction and tip geometry. Residual stress was measured by wafer deflection and quantified as a dipole force. The cutting direction and tip dependence of the fracture behavior was qualitatively explained using a stress model (modified from a model used for diamond turning of these materials) showing the directional propensity for fracture based on imparted tensile stresses on a certain set of fracture planes. Raman measurements to identify remnant phases indicative of HPPT in the scribe regions showing intense plastic behavior and the generated debris were inconclusive. Low RMS (1-9 nm,) optical quality surfaces were generated by diamond turning silicon and germanium at low feed rates (1-5 um/rev.) Symmetric damage patterns noted at high feed rates are partially explained using a damage orientation model based on elasticity theory developed from previous diamond turning work with these materials. Raman measurements of the machined areas showed signature of a near surface amorphous layer, perhaps remnant of the high pressure β-tin transformation, in both silicon and germanium samples. TEM observation of collected debris indicated dc structure perhaps to recrystallization of the amorphous material by heating. The implication of the ductile behavior is that using careful machining conditions, normally brittle materials such as single crystal silicon or germanium (and similar dc semiconductors) may be machined in the ductile regime to create high quality optics and substrates without intensive processing steps.
Dopant Segregation at Silicon-oxide Interfaces
(2007-05-26) Pei, Lirong; Nadia El-Masry, Committee Member; Michael Rigsbee, Committee Member; Salah Bedair, Committee Member; Gerd Duscher, Committee Chair
With the fast scaling of MOSFET devices, interfaces between silicon and dielectric layers are becoming increasingly important. However, a physical understanding of dopant segregation at such interfaces using atomic resolution remains elusive in spite of intensive study. In this dissertation, As and Sb are selected as dopants to achieve different levels of segregation in equilibrium conditions. This study utilizes a combination of theoretical and experimental concepts. Due to the fact that each experimental method has its own artifacts, we use a combination of three different methods (SIMS, GI-XRF and Z-contrast imaging⁄EELS) to allow accurate determination of position and concentration of dopants. Additionally, ab initio calculations provide appropriate structure model by calculating the energy of different preferred segregation sites. After implanting As (10¹⁵ and 10¹⁶ cm⁻²) into Czochralski Si (100) wafer at 32keV, a SiO₂ layer is thermally grown. Then Si⁄SiO₂ samples are annealed at 900°C for 360min in N₂, with a final SiO₂ thin film less than 15nm measured by ellipsometry. Combining the above three experimental methods, the segregation of As to the Si⁄SiO₂ interface is observed. The As concentration profiles of both samples are analyzed close to the interface region by EELS, and compared with those measured by GI-XRF and SIMS. A maximum of 4˜5x10²¹ cm⁻³ arsenic (10¹⁶ cm⁻²) and 1.2x10²¹ cm⁻³ arsenic (10¹⁵ cm⁻²) are observed at the last monolayer of Si. The total dose loss at the interface of the 10¹l⁶ cm⁻² As doped sample is 8˜9%. With the incorporation of ab initio calculations, a physical explanation of the segregation mechanism is given based on both theoretical and experimental results. Using Z-contrast imaging, Sb segregation at Si-SiO₂ interface is also observed on Sb doped Si⁄SiO₂ samples. Unlike the As doped samples, pentagon-shaped Sb precipitates are detected 8nm from interface on the Si side. For the As doped Si⁄Hf[subscript x]Si[subscript 1-x]O samples, an unexpected silicate interfacial layer is observed between hafnium oxide thin film and silicon substrate. Therefore, As segregation at the novel interface turns out to be exactly same as As at Si⁄SiO₂ interfaces.
Integration of Functional Oxide Thin Film Heterostructures with Silicon (100) Substrates.
(2010-04-30) Aggarwal, Ravi; Roger Narayan, Committee Chair; Jagdish Narayan, Committee Chair; Nadia El-Masry, Committee Member; John Prater, Committee Member
On the Interactions of Point Defects, Dopants and Light Element Impurities in Silicon as Stimulated by 200 kV Electron Irradiation.
(2005-07-21) Stoddard, Nathan Gregory; Robert Nemanich, Committee Member; George Rozgonyi, Committee Co-Chair; Gerd Duscher, Committee Co-Chair; Nadia El-Masry, Committee Member; Phil Russell, Committee Member
The purpose of this research has been the investigation of atomic manipulation in silicon. It has been demonstrated that bulk vacancies and interstitials are created and spatially separated one Frenkel pair at a time during 200 kV electron irradiation of nitrogen-doped silicon. The mechanism by which the nitrogen pair allows Frenkel pair separation is shown to be a combination of the lowering of the energy barrier to a knock-on event combined with a more stable end-state. Anomalous nitrogen diffusion has been observed as a result of low energy ion milling, and the diffusion of nitrogen is studied theoretically, revealing a new, low energy model for N2 pair diffusion. For the first time, 200 kV irradiation has been demonstrated not only to create Frenkel pairs during broad-beam irradiation, but also to allow the formation of extended defects like voids, oxygen precipitates and interstitial complexes. Using electron energy loss spectroscopy combined with first principles simulations, dark and bright areas induced in Z contrast images by 200 kV irradiation are demonstrated to be due to vacancy and self-interstitial complexes, respectively, with N>2. Finally, the manipulation of dopants in silicon is induced by using the difference in energy transferable from a 200 kV electron to light versus heavy elements (e.g. B vs. Sb). Atomic Force Microscopy is used to demonstrate that n-type regions with a size corresponding to the beam diameter are created in p-type material by short periods of 200 kV e-beam exposure. In this way, a method can be developed to create p-n-p type devices of arbitrary size in codoped silicon using a room temperature process.