Browsing by Author "Robert J. Nemanich, Committee Member"

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Electromagnetic and Experimental Modeling of Waveguide-Based Spatial Power Combining Systems
(2003-02-23) Hicks, Chris Wayne; Robert J. Nemanich, Committee Member; James W. Mink, Committee Co-Chair; Michael B. Steer, Committee Chair; Lazzi Gianluca, Committee Member; Alexander B. Yakovlev, Committee Member
Recent technological advancements and demands for high power sources at microwave and millimeter-wave frequencies have initiated extensive theoretical and experimental research in the area of quasi-optical and spatial power combining. The work described here was motivated by the necessity to develop a modeling environment for the electromagnetic analysis of planar quasi-optical and spatial power combining systems, in order to understand physical fundamentals and provide a basis for the design process. Two types of planar quasi-optical (QO) and spatial power combining systems are investigated. Propagation in a QO parallel plate system is investigated with the aim of establishing the mode structure and characteristics of the modes. Theoretical electromagnetic properties of a Gauss-Hermite beammode expansion was developed, and verified experimentally, for the prediction of the resonant frequencies of the structure and beammodes dispersion behavior. The system was designed, fabricated, tested, and showed good agreement between the experimental and theoretical results. In addition, a QO parallel-plate stripline-slot amplifier system was designed, tested and compared to a QO open HDSBW amplifier system with Vivaldi-type antennas. Experimental results verify that a QO parallel-plate stripline-slot amplifier proposed in this dissertation can be modeled using Gauss-Hermite beammodes. A full-wave electromagnetic model is developed and verified for a spatial power combining system consisting of slotted rectangular waveguides coupled to a strip line. The waveguide-based structure represents a portion of the planar QO power combiner discussed above. The electromagnetic simulator is developed to analyze the stripline-to-slot transitions in a waveguide-based environment. The simulator is based on the method of moments (MoM) technique to model a power combining array of slotted waveguide modules coupled to a strip line. The simulator uses Galerkin projection technique with piecewise sinusodial testing and basis functions in the electric and magnetic surface current density expansions. Electric and magnetic dyadic Green's functions are developed for an infinite rectangular waveguide in the form of partial expansions over the complete system of eigenfunctions of a transverse Laplacian operator. Numerical results are obtained and compared with a commercial microwave simulator for a few representative slot-strip-slot spatial power combining transitions and arrays.
Investigation of Polymer Phase Behavior at Heterogeneous Polymer-Polymer Interfaces using Secondary Ion Mass Spectrometry.
(2006-03-07) Harton, Shane Edward; Dieter P. Griffis, Committee Member; Bruce M. Novak, Committee Member; Robert J. Nemanich, Committee Member; Phillip E. Russell, Committee Co-Chair; Harald Ade, Committee Chair
Changes in the thermodynamic behavior of polymer blends from bulk to heterogeneous interfaces is investigated using secondary ion mass spectrometry (SIMS). The use of a magnetic sector spectrometer (CAMECA IMS-6f) is fully explored in order to determine the optimal conditions in which to probe polymer surfaces and heterogeneous interfaces using three bilayer film systems, namely polystyrene (PS) with poly(methyl methacrylate) (PMMA), poly(cyclohexyl methacrylate) (PCHMA) with PMMA, and PS with poly(2-vinylpyridine) (P2VP). Two primary ion beams have been employed, O2+ with detection of positive secondary ions, and Cs+ with detection of negative secondary ions. It was found that each polymer thin film system must be closely investigated in order to determine the optimal conditions for depth profiling using SIMS. Three types of systems were further investigated using SIMS.
Nanoscale Investigation of Degradation Effects in Lead Zirconate Titanate by Piezoresponse Force Microscopy
(2005-08-22) Blair, Thomas; Angus I. Kingon, Committee Chair; Alexei Gruverman, Committee Co-Chair; Robert J. Nemanich, Committee Member
This thesis reports results obtained by piezoresponse force microscopy (PFM) on the nanoscale mechanisms of imprint and fatigue, two of the most important degradation effects in ferroelectric materials. Imprint is the preference of a ferroelectric material for one polarization state over another. Fatigue is the loss of polarization due to repeated switching. Imprint was investigated in three PZT films with thicknesses of 100, 300, and 500 nm. It was found that the imprint was most severe in the thinnest sample. This was attributed to the increasing dominance of interface effects in the thin sample. Furthermore, it was found that the presence of top electrodes moderated the imprint. This was explained by top electrode leading to a symmetric distribution of trapped space charge, as opposed to the asymmetric distribution when only the bottom electrode is present. Fatigue was investigated in bulk ceramic PZT samples cycled to three different fatigue states (virgin, semifatigued, and fatigued) and then polished at an angle such that a range of thicknesses in the sample were available to be studied. It was found that the fatigued samples exhibited different domain structures. Backswitching occurred in the thick regions of the fatigued sample, but did not occur at any thickness in the virgin sample. This was explained by the cascaded backswitching model, positing that charged defects agglomerating at grain boundaries and provoking backswitching in all grains above it.
Nanoscale studies of switching behavior of ferroelectric thin films by using Piezoresponse Force Microscopy
(2009-12-07) Wu, Dong; Alexei Gruverman, Committee Co-Chair; Tom Pearl, Committee Member; Jack Rowe, Committee Chair; Robert J. Nemanich, Committee Member
Ferroelectrics as the special group of materials not only have piezoelectricity and pyroelectricity but also have a spontaneous polarization which can be switched by an applied external electric field. Basing on these properties there are a lot applications since the war years when BaTiO3 was discovered and broadly used as the capacitors in radio and radar equipment. At present ferroelectrics are widely used in different fields, such as the ferroelectric capacitors, transducers, actuators, and thermistors. One of the most promising applications is nonvolatile ferroelectric random access memories or FeRAMs. [14, 15] From a fundamental aspect, we need to directly verify several different theoretical models used for studies of polarization switching in ferroelectric thin film capacitors to determine which model works for a specific class of FeRAM capacitor structure. There are several main problems addressed in the present study: (1) Direct observations of the domain dynamics in the thin film capacitors during polarization reversal need study and analysis. (2) The dependence of microstructure on the switching mechanism of ferroelectric thin film capacitors is not clear and needs to be investigated. (3) The capacitor scaling and time dependence of switching also needs to be investigated. There are only a few techniques which can measure the piezoelectric response and the surface polarization directly, especially with nanoscale spatial resolution. Traditional electrical measurements such as the bias-dependent P-E hysteresis loop measurements or transient switching current measurements are very difficult to use for study of the switching behavior of ferroelectric capacitors at micron length scale. However a new local probing technique called Piezoresponse Force Microscopy or PFM can be applied in a straightforward way to these measurements. This is a nondestructive characterization technique with high lateral resolution and it can directly observe spatially resolved vertical displacement due to local switching behavior. By using PFM, it is also possible to contact individual capacitors of micro- and submicron-dimension and study the scaling effect of polarization switching.
A Planar Violet Electroabsorption Modulator and Modeling of Electric Field Effects on Zinc Oxide Excitons
(2007-12-12) Zhang, Xiyao; John F. Muth, Committee Chair; David E. Aspnes, Committee Co-Chair; Robert J. Nemanich, Committee Member; Leda Lunardi, Committee Member
First principle electroabsorption calculations based on WTK spectral density theorem and Dow and Redfield theory were performed and used as a basis of a model to fit experimental electroabsorption data. Absorption measurements were taken from 4.5 K to 300 K on a c-plane ZnO thin film sample with a high-resolution spectrometer to obtain the temperature broadening linewidth. The free A and B exciton peaks and two major neutral donor bound excitons were observed. The calculated zero temperature electroabsorption spectrum from Dow and Redfield theory was convolved with Lorentzian and Gaussian temperature dependent linewidth to model these absorption spectra at various temperatures. It is found that Gaussian lineshape works better due to the strong electron-LO-phonon interaction of ZnO, especially at higher temperatures. Gaussian broadening parameters are then extracted and expressed as a function of temperature. Two material related coefficients in the broadening linewidth expression are the exciton-acoustic-phonon interaction strength = 79.6 ± 3 μeV⁄K and exciton-LO-phonon interaction strength = 242 ± 10 meV. This expression is independent of sample qualities, or is 'generalized' to accommodate the exciton band edge. A concept called "effective microfield intensity" was introduced to represent the crystalline quality. The microfield intensities of two ZnO samples with rocking curve FWHMs ˜ 0.25° and ˜ 0.42° are ˜ 1.3 105 and ˜ 2.26 105 kV⁄cm respectively. Buffer-assisted growth technique was applied to improve the quality of PLD grown ZnO on sapphire, with the optimal buffer condition of 8 nm thickness, 500 °C temperature and 35 mTorr pressure. The same technique can also be applied to MgxZn1-xO. ZnO thin films were deposited on ATO⁄ITO⁄glass substrate at 500 to 700 K, with higher temperature producing better films. XRD of these samples show highly c-oriented ZnO growth on the ATO substrate. TLM measurements shows that the contact resistivity of Ti⁄Au (50⁄150 nm) on Al-doped MgxZn1-xO is reduced by rapid thermal annealing (RTA) from ˜ 1.2 10-1 ΩΩcm2 to 4 10-2 ΩΩcm2. ZnO electroabsorption (EA) optical modulators were fabricated based on ZnO⁄ATO⁄ITO⁄glass structure. Two types of top electrodes, Ni semi-transparent electrode (TE) and conducting indium gallium zinc oxide (IGZO) were experimented with the IGZO-coated device reducing the insertion loss by ˜50%. In both device, I-V characteristics shows leakage less than 20 nA within the device operation voltage range. The DC percentage modulation of these devices have two peaks, over 40% near ˜ 370 nm and around 20% near ˜ 385 nm at 140 V bias. AC testing confirms purely field modulation and shows no evident of frequency cut-off up to 100 kHz. A simple device model attributes the threshold voltage of ZnO EA modulators to the charge screening effects caused by the free electrons in the ZnO active layer. Applied voltage was obtained both by converting the electric field based on device model and fitting the calculated EA spectra with the first principle calculation and Gaussian broadening. The field strengths obtained from the two approaches is consistent, with only a small proportional discrepancy of ˜ 30%.
Reliable Local Strain Characterization in Si/SiGe Based Electronic Materials System
(2007-12-21) Zhao, Wenjun; Gerd Duscher, Committee Chair; George Rozgonyi, Committee Co-Chair; Robert J. Nemanich, Committee Member; Nadia A. El-Masry, Committee Member
In this research we first developed a procedure to determine the strain in a TEM sample. This procedure includes HOLZ line detection from a Convergent beam electron diffraction (CBED) pattern, kinematic calculation of high order Laue zone (HOLZ) line position and searching lattice parameters by χ2 minimization. With only CBED technique, strain measurement on the strained Si layer is not possible in a blanket strained Si⁄SiGe structure due to HOLZ line splitting (deformation). For sub-100nm short channel SiGe CMOS device structures strain could be determined in the center of the channel. We demonstrated the CBED strain measurement can be implemented in new generation short channel technology node with a nano meter spatial resolution and high accurate. For the first time, we developed a new approach combined with CBED and finite element (FE) modeling and quantitatively investigated the correlation of the strain in a thin TEM sample with that in the bulk. The new method successfully determined the strain in the strained Si layer on a blanket strained Si/SiGe wafer, in a good agreement with other measurements. The new results also gave some insight in strain relaxation in a TEM sample. We found the [-1,-1,0] strain component which is perpendicular to the TEM sample thinning direction stays the same in the TEM sample and in the bulk, while the [001]) strain component is relaxed because it is along the same direction as the TEM sample thinning direction. This relaxation causes the deformation of the TEM foil and HOLZ line splitting. Therefore a clear CBED pattern can not be obtained from a TEM sample with a biaxial stain state. Our findings from a recessed SiGe PMOS test structure with a uniaxial compressive strain showed a different strain redistribution behavior. The data showed that the εx [-1,1,0] strain is actually more than 20% higher in a TEM sample than in the bulk. The εy [-1,-1,0] strain which is parallel to the TEM sample thinning direction turns to tensile in the TEM sample due to the loss of constraints, while it is zero in the bulk. The new results can explain our experimental data and others' (which could not be explained before) and are consistent with UV Raman measurements.