Browsing by Author "John F. Muth, Committee Chair"

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Electrostatic MEMs Fabry-Perot Modulators in the Visible Spectrum and Electrothermal Wrinkling of Bilayer Thin Films
(2008-12-02) Chintapatla, Shravan; Leda M. Lunardi, Committee Co-Chair; Robert M. Kolbas, Committee Member; John F. Muth, Committee Chair
Silicon micromachining was used to form silicon nitride microbridge mirrors and to make micromachined Fabry Perot cavities. The position of the mirror can be controlled either electrostatically, or electrothermally allowing resonance of the Fabry Perot cavity to be controlled resulting in the transmitted or reflected light to be modulated. Modulation rates of up to 150 KHz were observed when operated electrostatically. Chips with arrays of microbridges were fabricated and the uniformity in wavelength response across the chip was investigated. It was found that using indium bump bonding was superior to using SU-8 as the bonding layer since the uniformity was more controllable. Electrothermal operation revealed interesting wrinkling phenomena. When electric current was passed through the thin aluminum layer on the silicon nitride membrane joule heating caused the aluminum layer to thermally expand resulting in a compressive stress. The compressive stress and thin sheet geometry of the bridge then resulted in the formation of periodic series wrinkles. The number spacing between the wrinkles was found to be controllable with voltage. The formation of periodic wrinkles was found to be dependent on width to length ratio and the grain size of the evaporated aluminum film. The formation of wrinkles also allowed the finesse of the Fabry Perot cavity to be controlled rather than the distance between the mirror also allowing the light to be modulated or switched on/off in intensity.
Photoemission Spectroscopic Studies of Metal-Gated MOS structures based on ultra-thin High-k Dielectrics
(2009-03-05) Choung, Jiyoung; Robert M. Kolbas, Committee Member; Veena Misra, Committee Member; John F. Muth, Committee Chair; Jack E. Rowe, Committee Member; Robert J. Nemanich, Committee Co-Chair
The band structure and interface properties of high-Îº MOS gate stack structures have been studied using a combination of x-ray and ultraviolet photoemission spectroscopy to comprehend the effective work function (EWF) variation during thermal processing. The possibility of controlling the effective work function of a metal by inserting an intentional dipole layer or additional charges is investigated using high-k gate stacks. The band bending of a Si interface and the electric field across the buffer oxide is explored using the band structure analysis of MOS capacitors. Prior to the band structure analysis of MOS capacitors, a high-Îº/buffer oxide (SiO2) on p- and n-Si substrates is explored to observe the Si band bending and the electric field across the buffer oxide, which demonstrates the possibility for control of the effective work function of a metal by inserting an intentional dipole layer or additional charges. The Fermi level alignment of TiN/HfO2/p- and n-type Si is investigated, where three main conclusions are presented: 1) the Fermi level difference between TiN/high-Îº/p- and n-type Si substrates has been identified with photoemission spectroscopy (XPS, UPS) and the results used to determine the band structure; 2) a systematic study of the core level spectra shifts of the Si 2p and Hf 4f display the Fermi level alignment; 3) the results indicate that the interfacial dipole at high-Îº/buffer oxide contributes to the EWF of TiN in the structure. To observe the effects depending on different dielectrics on the EWF, TiN on SiO2, HfO2, and Al2O3 is investigated, and the results are found to be similar to the results obtained by Capacitance Voltage measurements. Finally, the process and temperature dependence of a PMOS capacitor (Ru/HfSiOx/n-Si) is studied. The process dependence showed that there is a large gap between the VWF and the EWF of Ru after annealing, and the interface at the high-Îº/buffer oxide needs to be considered to tune the EWF. The temperature dependence revealed the thermal instability of Ru with HfSiOx over 700Â°C. This study suggests that the EWF is affected by the dipole layer not only at a metal/high-Îº interface but also at a high-Îº / SiO2 interface.
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%.
Rare-Earth Doped Wide Bandgap Oxide Semiconductor Materials and Devices
(2009-09-16) Wellenius, Ian Patrick; Mark Johnson, Committee Member; Leda Lunardi, Committee Member; Robert Kolbas, Committee Member; Henry O. Everitt, Committee Member; John F. Muth, Committee Chair
The Simultaneous Determination of Thickness and Complex Refractive Index of a Thin Film on a Substrate
(2007-08-07) Kim, In Kyo; Gerald J. Iafrate, Committee Member; David E. Aspnes, Committee Co-Chair; John F. Muth, Committee Chair; Salah M. Bedair, Committee Member
The problem of the simultaneous determination on a wavelength-by-wavelength basis of the complex refractive index ñ = n + iκ and thickness d of an isotropic thin film deposited on an isotropic substrate has never been solved analytically over the last 35 years. Here I find the analytic solution of the three-phase model in the limit d/λ << 1 on a wavelength-by-wavelength (λ-by-λ) basis from polarimetric data Δρ/ρ and ΔR⁄R, where ρ and R are the complex reflectance ratio and either the p- or s-polarized power reflectance, respectively. My relatively simple quadratic equation eliminates several disadvantages of previous numerical-analysis approaches, including intrinsic instabilities and a need for accurate estimates of starting parameters for convergence, which become particularly acute as d⁄λ→0. I also investigate the source of the numerical difficulties, the correlations in both parameters and data, with an approach based on the Jacobian of the exact equations. These correlations prevent a simple quadrature addition of uncertainties in Δρ⁄ρ and ΔR⁄R in the calculation of uncertainties in the parameters. I also show that a simple mathematical transformation of the data can improve the accuracy of the first-order equations. The results will be useful directly for optically analyzing films with d ≤ 1 nm, and for providing starting values for more accurate numerical calculations for thicker films. Results are demonstrated for the cyclic physisorption and desorption of a layer of H₂O on oxidized GaAs, and as numerical estimates of performance specifications for application to several materials combinations important to semiconductor technology.
Wide Band Gap SemiconductorOptical Waveguide
(2006-10-10) Cai, Ailing; John F. Muth, Committee Chair
This dissertation involves the design, fabrication and characterization of wide band gap semiconductor optical waveguide devices. Prism coupling was used as the primary characterization method to obtain design parameters of the refractive index and propagation loss of the wide band gap semiconductor materials such as c-plane, a-plane GaN and ZnO grown on c-plane, r-plane sapphire. AlxGa1-xN alloys and ZnO co-doped with nitrogen, tellurium and chromium diffused sapphire waveguides were also investigated. The simulation and layout of optical waveguide devices, including 1x2 GaN splitters, Mach-Zehnder interferometers and asymmetric twin waveguide devices, were performed using a commercial software package using the beam propagation method. As-grown GaN materials were characterized by using cathodoluminescence, optical transmission measurement, atomic force microscopy and prism coupling. GaN waveguides, 1x2 splitters and Mach-Zehnder interferometers were fabricated and tested at visible wavelengths using HeNe and a tunable Argon laser. The end faces of the waveguide chips were polished by using the sample preparation techniques used in transmission electron microscopy. The morphology of the rib waveguides were characterized by using atomic force microscopy and scanning electron microscopy. A novel light coupling system suitable for high numerical aperture and small dimensional waveguides was designed using the lensed-fiber and a ball lens combination and used to couple light into the waveguide structures.