Browsing by Author "Thomas P. Pearl, Committee Member"

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Electrical Characterization of Transition Metal Silicide Nanostructures Using Variable Temperature Scanning Probe Microscopy
(2007-12-07) Tedesco, Joseph Leo; Carlton M. Osburn, Committee Member; Thomas P. Pearl, Committee Member; Robert J. Nemanich, Committee Chair; J.E. (Jack) Rowe, Committee Member
Cobalt disilicide (CoSi2) islands have been formed on Si(111) and Si(100) through UHV deposition and annealing. Current-voltage (I-V) and temperature-dependent current-voltage (I-V-T) curves have been measured on the islands using conducting atomic force microscopy (c-AFM) with a doped diamond like carbon cantilever. Thermionic emission theory has been applied to the curves and the Schottky barrier heights, ΦB, and ideality factors, n, for each island have been calculated. Barrier heights and ideality factors are evaluated as functions of temperature, island area, and each other. While all islands were prepared in UHV conditions, one set was removed from UHV and measurements were performed in ambient conditions while the other set remained in UHV. The islands measured in ambient conditions were known as "air-exposed samples" due to the fact that the surface was assumed to be passivated upon exposure to atmospheric conditions. The islands measured in UHV were known as "clean samples" because the surface was not passivated. Air-exposed samples were CoSi2 islands on Si(111) and exhibited a negative linear correlation between the barrier height and the ideality factor. Measured values of ΦB on the air-exposed samples approached reported bulk values. Measurements from CoSi2 islands on clean Si(111) and Si(100) revealed no correlation between ΦB and n. Furthermore, it was observed that the measured barrier heights of CoSi2 islands on clean Si surfaces are ˜0.2 — 0.3 eV less than the barrier heights measured from CoSi2 islands on air-exposed surfaces. This negative shift in the clean surface barrier heights was attributed to Fermi level pinning by the non-passivated silicon surface states. Additionally, a slight trend toward lower barrier height as a function of decreasing island area was detected in all samples. This trend is attributed to increased hole injection and generation-recombination in the smaller islands, but it may also be due to effects caused by increased spreading resistance as the island size decreases. Non-linearity in activation energy plots, as well as correlations between decreasing barrier height and decreasing island area-to-island periphery ratio, are attributed to generation-recombination. These measurements indicate that the Schottky barrier height decreases and ideality factor increases with decreasing temperature, even if there is no direct linear correlation between ΦB and n. These temperature-dependent relationships are attributed primarily to hole injection and generation-recombination, with barrier height inhomogeneity as a minor effect. Titanium silicide (TiSi2) islands have been formed by UHV deposition of titanium on atomically flat Si(100) and Si(111). Scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and a variant of current imaging tunneling spectroscopy (CITS) have been used to characterize single electron tunneling (SET) through the islands. SET is observed to occur in the islands and is evaluated based on the predictions of the orthodox model. The observation of SET suggests that the Schottky barrier could be effective in future SET-based electronic devices. SET was not observed as often as expected, however, suggesting that there is a mechanism limiting SET. Possible mechanisms for SET limiting are evaluated and it is concluded that SET is limited due to a combination of Schottky barrier lowering, a low resistance substrate, and Fermi level pinning by the non-passivated surface states of the silicon. These factors make SET in TiSi2 islands on silicon potentially too variable to be used in future devices unless the SET-limiting mechanism is resolved.
First principles theory of metal/insulator interfaces and nanostructures, from crystalline oxides to ferroelectrics.
(2008-07-28) Nunez, Matias; Donald Brenner, Committee Member; Lubos Mitas, Committee Chair; Thomas P. Pearl, Committee Member; Marco Buongiorno Nardelli, Committee Chair; Jerry Bernholc, Committee Co-Chair
Low Resistivity Contact Methodologies for Silicon, Silicon Germanium and Silicon Carbon Source/Drain Junctions of Nanoscale CMOS Integrated Circuits.
(2009-12-03) Alptekin, Emre; Mehmet C. Ozturk, Committee Chair; Thomas P. Pearl, Committee Member; Michael Escuti, Committee Member; Veena Misra, Committee Member
State-of-the-art p-channel metal oxide semiconductor field effect transistors (MOSFETs) employ Si(1-x)Ge(x) source/drain junctions to induce uniaxial compressive strain in the channel region in order to achieve hole mobility enhancement. It is also know that the elec- tron mobility can be enhanced if the MOSFET channel is under uniaxial tension, which can be realized by replacing Si(1-x)Ge(x) with Si(1-y)C(y) epitaxial layers in recessed source/drain regions of n-channel MOSFETs. This dissertation focuses on epitaxy of Si(1-y)C(y) layers and low resistivity contacts on Si, Si(1-x)Ge(x), and Si(1-y)C(y) alloys. While these contacts are of particular importance for future MOSFETs, other devices based on these semiconductors can also benefit from the results presented in this dissertation. The experimental work on Si(1-y)C(y) epitaxiy focused on understanding the impact of various process parameters on carbon incorporation, substitutionality, growth rate, phosphorus incorporation and activation in order to achieve low resistivity Si(1-y)C(y) films with high substitutional carbon levels. It was shown, for the first time, that phosphorus lev- els above 1.3x10^(21) cm^(-3) can be achieved with 1.2% fully substitutional carbon in epitaxial layers. Specific contact resistivity (C) on strained Si(1-x)Ge(x) layers was evaluated using the existent results from the band structure calculations. Previous work on this topic mainly focused on barrier height and the doping density at the interface. In this work, the impact of the tunneling effective mass on specific contact resistivity was calculated for the first time for strained Si(1-x)Ge(x) alloys. It was shown that due to the exponential dependence of contact resistivity on this parameter tunneling effective mass may have a strong impact on contact resistivity. This is especially important for strained alloys in which the tunneling effective mass is dependent on the strain. The contact resistivity was found to decrease with Ge concentration due to the smaller tunneling effective mass in strained Si(1-x)Ge(x). These calculations can also be extended to Si(1-y)C(y) junctions when better models for the Si(1-y)C(y) band structure are available. Two different metallization schemes have been considered. In the first approach, two band edge silicides are used to achieve low-resistivity contacts on complimentary MOSFETs. For this purpose, experiments on band edge silicides including PtSiGe, NiSiC and ErSiC were conducted. The impact of Ge and C on silicide formation and the barrier height at the interface was investigated. Barrier height values around 0.3 eV were achieved with PtSiGe and ErSiC contacts formed on p-Si(1-x)Ge(x)and n-Si(1-y)C(y), respectively. Due to the exponential dependence of contact resistivity on barrier height, this barrier height is low enough to yield contact resistivity figures below 10^(-8) Ohm-cm^(2) even with modest doping levels. On the other hand, smaller barrier height values will be needed for Schottky barrier MOSFETs. It is more desirable to use a single metal contact metal on both p- and n-channel MOSFETs, which requires tuning of the barrier height. Impurity implantation was considered as a means to achieve barrier height tuning. Extremely small barrier height values below 0.1 eV were obtained by sulfur segregation for the silicides of Pt, Ni and NiPt on n-type Si and Si(1-y)C(y). Indium segregation was used for the first time to lower the hole barrier height to obtain barrier height values below 0.2 eV on p-Si. The results provide several approaches that can be used to form low resistivity contacts. We believe that the knowledge gained from this work is expected to have a significant impact on choosing the most effective and economical approach to form low-resistivity contacts in CMOS manufacturing.
Tribo-Induced Temperature Rise and Melting at a Single Asperity Sliding Contact
(2010-04-29) Dawson, Benjamin D; Thomas P. Pearl, Committee Member; Laura I. Clarke, Committee Member; Mohammed A. Zikry, Committee Member; Jacqueline Krim, Committee Chair
The fundamental issue of temperature rise at a rubbing interface due to frictional energy dissipation is of great practical and theoretical importance. Many tribological phenomena, such as tribo-chemistry and wear rates, have been attributed at least in part to the temperature rise felt at the contact between rubbing asperities. However, the difficulty of investigating this interface has lead to a lack of in situ experimental results. In this study, the interfacial mechanics of a sliding interface are probed using a combined quartz crystal microbalance and scanning tunneling microscope in ultra high vacuum conditions. The unique capability of a quartz crystal microbalance to monitor sensitive changes in the contact region, along with the wide range of surface velocities that are readily attainable, allow for a novel investigation of interfacial phenomena at the asperity contact formed by the tip contacting the oscillating crystal surface. The imaging ability of the scanning tunneling microscope permits the detailed characterization of the surface before and after indentation, as well as a method for calibrating the velocity of the QCM surface by directly measuring the crystal amplitudes. I have employed Indium as the substrate due to its low melting point, and a velocity and temperature dependent transition in the response of the quartz crystal microbalance is observed. This transition is attributed to the contact region melting or becoming liquid-like due to frictional energy dissipation. This is the first time such a velocity dependent transition has been observed at the scales studied here.