Browsing by Author "David Aspnes, Committee Member"

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EXAFS Studies of Ge-Sb-Te Alloys for Phase-Change Applications
(2007-12-07) Baker, David Andrew; David Aspnes, Committee Member; Michael A. Paesler, Committee Co-Chair; Gerald Lucovsky, Committee Co-Chair; Gerald Iafrate, Committee Member
Studies of amorphous (a-) semiconductors have been driven by technological advances as well as fundamental theories. Observation of electrical switching, for example, fueled early interest in a-chalcogenides. More recently a-chalcogenide switching has been applied successfully to programmable memory devices as well as DVD technology where the quest for the discovery of better-suited materials continues. Thus, switching grants researchers today with an active arena of technological as well as fundamental study. Bond constraint theory (BCT) and rigidity theory provide a powerful framework for understanding the structure and properties of a-materials. Application of these theories to switching in a-chalcogenides holds the promise of finding the best composition suited for switching applications. Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy is an ideally suited technique to investigate the switching properties of these materials. Films of amorphous Ge2Sb2Te4, Ge2Sb2Te5, and Ge2Sb2Te7 exhibit differing bonding structures and bond statistics, which result in different electronic and optical properties. Results of new EXAFS experiments on these three critical compositions in the Ge-Sb-Te system are presented in light of BCT and rigidity theory.
Formation of Metal Silicide and Metal Germanosilicide Contacts to Si[subscript 1-x]subscript Ge[subscript x] Alloys
(2004-04-20) Burnette, James E. Jr.; Dale Sayers, Committee Member; Robert Nemanich, Committee Member; Gregory Parsons, Committee Member; David Aspnes, Committee Member
The goals of this research were to study the phase stability and formation of Ti-Si[subscript 1-x]Ge[subscript x] and Co-Si[subscript 1-x]Ge[subscript x] thin film reactions. The Ti-Si[subscript 1-x]Ge[subscript x] and Co-Si[subscript 1-x]Ge[subscript x] solid phase reactions result in the formation of precipitates within the grain boundaries of the films thus formed. The precipitates are either Ge or a Si-Ge compound, depending on the type of metal used in the reaction. The formation of Ti(Si[subscript 1-y]Ge[subscript y])₂ thin films on Si[subscript 1-x]Ge[subscript x] has been examined. It has been found that the generation of Ge-rich Si-Ge precipitates which form in the Ti-Si[subscript 1-x]Ge[subscript x] solid phase reaction could be reduced or eliminated by the insertion of an amorphous Si layer before the metallization step. A Gibbs free energy model, which was parameterized in terms of Ge concentration by atomic percentage was used to determine stability between the Ti(Si[subscript 1-y]Ge[subscript y])₂ layer and the Si[subscript 1-x] Ge[subscript x] substrate. The films in this study were characterized using x-ray diffraction (XRD) to investigate phase formation, stability, and the composition of the Ti(Si[subscript 1-y]Ge[subscript y])₂ layer. Scanning electron microscopy (SEM) was used to determine the surface morphology and phase stability. It was found that amorphous Si layers of a certain thickness could prevent precipitate formation, depending on the composition of the underlying Si[subscript 1-x] Ge[subscript x] layer. The formation of CoSi₂ on Si[subscript 1-x]Ge[subscript x] was also examined. The solid phase reaction of Co and Si[subscript 1-x]Ge[subscript x] results in the formation of a poly-crystalline CoSi₂ layer, and the occurrence of a Ge precipitate. The TIME (Titanium Interlayer Mediated Epitaxy) process has been used in the formation of epitaxial CoSi₂ on Si (100). A Ti layer of varying thicknesses, which serves as a barrier to retard the diffusion of Co atoms was deposited on a c-Si/Si[subscript 1-x]Ge[subscript x] substrate pseudomorphically strained to Si (100), before the final Co metallization step. The films in this study were characterized using x-ray absorption fine structure (XAFS) to determine the short-range crystalline order, XRD to determine phase formation and long-range crystalline order, Auger electron spectroscopy (AES) to determine surface chemistry, and SEM to determine the surface morphology. This work shows that the formation of epitaxial CoSi₂ on Si[subscript1-x]Ge[subscript x] can be achieved, depending on the thickness of the diffusion barrier. In addition, the optimal diffusion barrier thickness has been determined for the Co layer thickness used in these studies.
Resonant Soft X-ray Reflectivity: A Tool for the Study of Polymer Thin Films
(2008-01-22) Wang, Cheng; Harald Ade, Committee Chair; Jan Genzer, Committee Member; David Aspnes, Committee Member; Jack Rowe, Committee Member
Spectroscopic and Electrical Studies of Hafnium-Based High-k Thin Film Dielectrics on Germanium Surfaces
(2008-11-10) Long, Joseph Preston; Gerald Lucovsky, Committee Chair; Jack Rowe, Committee Member; David Aspnes, Committee Member; Robert Kolbas, Committee Member
The research discussed here has been carried out in order to advance the basic understanding of the compatibility between germanium surfaces and hafnium-based high Îº dielectric materials with a particular emphasis on their potential for microelectronic applications. To this end, spectroscopic studies were carried out to determine the physical and electronic properties of Ge/high-Îº structures, and MOS capacitors were fabricated to study their electrical characteristics. Crystallinity, thermal stability, electronic defect levels, and Hf d state degeneracy removal in this material system were studied via x-ray absorption spectroscopy (XAS). The presence of conduction band edge defect states was confirmed by spectroscopic ellipsometry (SE), and medium energy ion spectroscopy (MEIS) was employed to investigate chemical distributions within these structures. Capacitance-voltage (C-V) measurements made on related MOS structures provided valuable insight into the nature of carrier trapping and charged traps in these devices, and lastly, current-voltage (I-V) measurements revealed information about the integrity of these gate materials while in contact with germanium surfaces. While the nitridation of germanium surfaces was shown to mitigate atomic migration and diffusion to a degree during device processing, both hafnia and hafnium nitro-silicate films demonstrated a large degree of physical and chemical instability when in contact with germanium. Moreover, these difficulties were found to be correlated with electrically active defects which make this technology unsuitable for CMOS applications at present.
Thermionic Energy Conversion and Particle Detection Using Diamond and Diamond-Like Carbon Surfaces
(2007-10-02) Smith, Joshua Ryan; Robert J. Nemanich, Committee Co-Chair; Griff L. Bilbro, Committee Co-Chair; David Aspnes, Committee Member; Thomas Pearl, Committee Member