Browsing by Author "Salah Bedair, Committee Member"

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Acceptor Conductivity In Bulk ZnO (0001) Crystals
(2005-05-04) Adekore, Bunmi Tolu; Robert F. Davis, Committee Chair; Salah Bedair, Committee Member; Douglas Barlage, Committee Member; Robert Kolbas, Committee Member; Klaus Bachmann, Committee Member
ZnO is a promising wide bandgap semiconductor. Its renowned and prominent properties as its bandgap of 3.37eV at 4.2K; its very high excitonic binding energy, 60meV; its high melting temperature, 2248K constitute the basis for the recently renewed and sustained scientific interests in the material. In addition to the foregoing, the availability of bulk substrates of industrially relevant sizes provides important opportunities such as homoepitaxial deposition of the material which is a technological asset in the production of efficient optoelectronic and electronic devices. The nemesis of wide bandgap materials cannot be more exemplified than in ZnO. The notorious limitation of asymmetric doping and the haunting plague of electrically active point defects dim the bright future of the material. In this case, the search for reliable and consistent acceptor conductivity in bulk substrates has been hitherto, unsuccessful. In the dissertation that now follows, our efforts have been concerted in the search for a reliable acceptor. We have carefully investigated the science of point defects in the material, especially those responsible for the high donor conductivity. We also investigated and herein report variety of techniques of introducing acceptors into the material. We employ the most relevant and informative characterization techniques in verifying both the intended conductivity and the response of intrinsic crystals to variation in temperature and strain. And finally we explain deviations, where they exist, from ideal acceptor characteristics. Our work on reliable acceptor has been articulated in four papers. The first establishing capacitance based methods of monitoring electrically active donor defects. The second investigates the nature of anion acceptors on the oxygen sublattice. A study similar to the preceding study was conducted for cation acceptors on the zinc sublattice and reported in the third paper. Finally, an analysis of the response of the crystal to hydrostatic strain and its recovery when such strain enforces a collapse of its crystallinity is reported in the fourth paper. For the sake of brevity and the need to be concise, our supplementary investigations on extrinsic donor conductivity is deferred to other journal publications.
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.
Nanoscale Investigation of the Piezoelectric Properties of Perovskite Ferroelectrics and III-Nitrides
(2004-02-09) Rodriguez, Brian Joseph; Angus Kingon, Committee Co-Chair; Robert Nemanich, Committee Chair; Salah Bedair, Committee Member; Jacqueline Krim, Committee Member; Alexei Gruverman, Committee Member
Nanoscale characterization of the piezoelectric and polarization related properties of III-Nitrides by piezoresponse force microscopy (PFM), electrostatic force microscopy (EFM) and scanning Kelvin probe microscopy (SKPM) resulted in the measurement of piezoelectric constants, surface charge and surface potential. Photo-electron emission microscopy (PEEM) was used to determine the local electronic band structure of a GaN-based lateral polarity heterostructure (GaN-LPH). Nanoscale characterization of the imprint and switching behavior of ferroelectric thin films by PFM resulted in the observation of domain pinning, while nanoscale characterization of the spatial variations in the imprint and switching behavior of integrated (111)-oriented PZT-based ferroelectric random access memory (FRAM) capacitors by PFM have revealed a significant difference in imprint and switching behavior between the inner and outer parts of capacitors. The inner regions of the capacitors are typically negatively imprinted and consequently tend to switch back after being poled by a positive bias, while regions at the edge of the capacitors tend to exhibit more symmetric hysteresis behavior. Evidence was obtained indicating that mechanical stress conditions in the central regions of the capacitors can lead to incomplete switching. A combination of vertical and lateral piezoresponse force microscopy (VPFM and LPFM, respectively) has been used to map the out-of-plane and in-plane polarization distribution, respectively, of integrated (111)-oriented PZT-based capacitors, which revealed poled capacitors are in a polydomain state.
Nitrogen Doping and Ion Beam Processing of Zinc Oxide Thin Films
(2006-01-05) Wellenius, Ian Patrick; John Muth, Committee Co-Chair; Robert Kolbas, Committee Member; Salah Bedair, Committee Member
The modification of single crystal epitaxial ZnO thin films grown by Pulsed Laser Deposition on c-axis oriented sapphire substrates by Ion Beam Processing was investigated. Nitrogen doping of the films was attempted using nuclear transmutation using the ¹⁶O (³He, ⁴He) ¹⁵O reaction at 6.6 MeV. The ¹⁵O product is unstable and decays to ¹⁵N after several minutes by positron emission. There are several potential advantages to using nuclear transmutation including producing nitrogen atoms on the correct lattice site for doping and reduced crystal damage as compared to conventional ion beam implantation. In the experiments in this thesis the doping levels achieved ~10¹⁴ cm⁻³ were too low to be expected to dope the films to p-type. However several beneficial effects due to the ion beam processing were observed, including large increases in resistivity, reduction of defect luminescence, and substantial increases in the response of photoconductive detectors. In addition to desired effects in some films it was also found that in some films bubble like structures approximately 10 μm in diameter were formed where the thin film delaminated from the surface. It was assumed that mechanism for the bubble formation was the build up of helium gas at the sapphire/ZnO interface.
Nonvolatile Spin Memory based on Diluted Magnetic Semiconductor and Hybrid Semiconductor Ferromagnetic Nanostructures
(2008-06-27) Enaya, Hani; Salah Bedair, Committee Member; Ki Wook Kim, Committee Chair; Jacqueline Krim, Committee Member; Venna Misra, Committee Member; John Zavada, Committee Member
Study of Phase Separation and Ordering in InGaN and AlInGaN: Experimental and Computer Modeling.
(2005-02-13) Behbehani, Mark Kian; Salah Bedair, Committee Member; Mark Johnson, Committee Member; Donald Brenner, Committee Co-Chair; Nadia El-Masry, Committee Chair
A comprehensive study examines the phase behaviour of InGaN and AlInGaN including growth characterization and computer modeling. InGaN alloys were grown with up to 50% InGaN and studied for phase separation and ordering. The AlInGaN system has been studied with discovery of the Self Assembled Super-Lattice (SASL) and the Strain Equilibrium Indium (In) Incorporation Effect. Computer modeling was performed using a strain based Valence Force Field (VFF) model combined with a Monte Carlo method to study both the composition pulling and the Strain Equilibrium In Incorporation Effect In the InGaN system, both phase separation and ordering behaviour in the InGaN system was studied extensively. Using Transmission Electron Microscopy, the presence of simultaneous phase separation and (0002) ordering was confirmed. The composition pulling effect was also studied in the InGaN system. VFF computer modeling was successfully used to predict the composition of highly strained InGaN films based on the bulk composition. In the AlInGaN system, the SASL effect was discovered using Transmission Electron Microscopy. In addition the Strain Equilibrium In Incorporation Effect was discovered using a combination of Transmission Electron Microscopy and X-ray diffraction. Using a VFF Metropolis Monte Carlo algorithm, the Strain Equilibrium In Incorporation Effect has been studied. The computer model has been able to predict the In incorporation behaviour of highly strained films in the AlInGaN system and has confirmed that the strain energy is the primary factor determining the In composition in strained AlInGaN films.