Browsing by Author "Gerd Duscher, Committee Chair"

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Atomic and Electronic Structure of Interfaces in Materials Systems for Future Semiconductor Devices
(2004-02-12) Lopatin, Sergei; Gerd Duscher, Committee Chair; Mark Johnson, Committee Co-Chair; George Rozgonyi, Committee Member; Michael Rigsbee, Committee Member; Christopher Roland, Committee Member
Because of the intrinsic limits of the Si/SiO₂ based industry, there is a great trend towards the monolithic integration of new materials into already well developed silicon technology. Having lasted for several decades now, downscaling reaches the limit, in which a critical device dimension approaches the size of one atom. At this level of the miniaturization, it is not the bulk material, but the interface between the two materials that what controls the properties of the resulting optoelectronic device. Thus, the characterization of precise atomic arrangements at different interfaces and the influence of these arrangements on the optoelectronic properties of interfaces is required. Therefore, in this study, a combination of scanning transmission electron microscopy (STEM) techniques and density functional theory calculations was used as a research tool for the characterization of interfaces. The STEM instruments used for the study were equipped with prototypes of spherical aberration correctors, enabling to achieve the highest resolution currently available both in space and energy. The combination of experimental and theoretical methods was applied to study interfaces between Si/GaAs, Si/Ge, Ge/SiO₂, Si/HfO₂ and Si/Al₂O₃. As the result of the present research, a new dislocation configuration at the Si/GaAs interface was reported for the first time. The influence of this dislocation structure on the electrical properties of the Si/GaAs interface was analyzed. Also, the transition from Si to GaAs and from Si to Ge at corresponding interfaces was described with atomic precision. For the first time, the interface between Ge and SiO₂ was shown to have 'ideal' characteristics (chemical abruptness and sharpness). This indicates the potential, both for a more successful use of Ge in high-speed devices and for advances in interface engineering to enhance performance in electronic devices. The features of Si/HfO₂ and Si/Al₂O₃ interfaces, namely the distribution and bonding of Si and Hf across the interface, and the formation of charged SiO₂ islands at the Si/Al₂O₃ interface were also studied. These results for materials systems show the significance of a basic understanding of the atomic structures of interfaces for a rapid development of new electronic devices.
Bio-Related Noble Metal Nanoparticle Structure Property Realtionships
(2007-07-29) Leonard, Donovan Nicholas; Stefan Franzen, Committee Co-Chair; Phillip Russell, Committee Member; Don Brenner, Committee Member; John Mackenzie, Committee Member; Gerd Duscher, Committee Chair
Structure property relationships of noble metal nanoparticles (NPs) can be drastically different than bulk properties of the same metals. This research study used state-of-the-art analytical electron microscopy and scanned probe microscopy to determine material properties on the nanoscale of bio-related Au and Pd NPs. Recently, it has been demonstrated the self-assembly of Au NPs on functionalized silica surfaces creates a conductive surface. Determination of the aggregate morphology responsible for electron conduction was studied by atomic force microscopy (AFM) and scanning electron microscopy (SEM). In addition, changes in the electrical properties of the substrates after low temperature (<350°C) annealing was also studied. It was found that coalescence and densification of the Au NP aggregates disrupted the interconnected network which subsequently created a loss of conductivity. Investigation of bio-related Au⁄SiO2 core-shell NPs determined why published experimental results showed the sol-gel silica shell improved, by almost an order of magnitude, the detection efficiency of a DNA detection assay. Novel 360° rotation scanning TEM (STEM) imaging allowed study of individual NP surface morphology and internal structure. Electron energy loss spectroscopy (EELS) spectrum imaging determined optoelectronic properties and chemical composition of the silica shell used to encapsulate Au NPs. Results indicated the sol-gel deposited SiO2 had a band gap energy of ˜8.9eV, bulk plasmon-peak energy of ˜25.5eV and chemical composition of stoichiometric SiO2. Lastly, an attempt to elicit structure property relationships of novel RNA mediated Pd hexagon NPs was performed. Selected area electron diffraction (SAD), low voltage scanning transmission electron microscopy (LV-STEM), electron energy loss spectroscopy (EELS) and energy dispersive spectroscopy (EDS) were chosen for characterization of atomic ordering, chemical composition and optoelectronic properties of the novel nanostructures. Data from control experiments found the hexagons could be made without RNA and confirmed the presence of nanocrystalline Pd metal NPs in unpurified Pd2(DBA)3 reagent powder. Furthermore, the study determined the hexagon platelets to have a chemical composition of ˜90at% carbon and ˜10at% Pd and a lattice parameter corresponding to molecular crystals of Pd2(DBA)3 precursor, not Pd metal.
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.
INTERFACES IN NOVEL ELECTRONIC MATERIALS
(2008-01-10) Liu, Fude; Nadia A. El-Masry, Committee Member; Carl Osburn, Committee Member; Robert Nemanich, Committee Co-Chair; Gerd Duscher, Committee Chair
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.