Browsing by Author "Robert J. Nemanich, Committee Chair"

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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.
The Influence of a TiN Buffer Layer on the PECVD Growth and Field Emission Properties of Carbon Nanotubes
(2007-04-20) Bryan, Eugene Noboru; Robert J. Nemanich, Committee Chair
This study explores the growth and field emission properties of carbon nanotubes (CNTs) prepared using Fe catalyst layers on TiN coated Si substrates. TiN buffer layers were employed as a catalyst support in the plasma enhanced chemical vapor deposition (PECVD) growth of the CNT films. The TiN layers were characterized by x-ray photoelectron spectroscopy, atomic force microscopy, and photo electron emission spectroscopy to establish the interaction of the Fe catalyst film with the TiN buffer layer and underlying substrate. The TiN layers were shown to directly influence the growth process by enabling the catalyst to remain in an active state. The PECVD grown CNT films exhibited field emission current densities of 1 mA⁄cm2 at applied fields of ˜2.1 V⁄μm. The emission distribution from the films, observed using a phosphor coated screen, was found to be uniform over selectively deposited areas up to 3.24 cm2. Non-linearity in the Fowler-Nordheim emission plots of the CNT films was shown to originate from adsorbate effects which have in many cases led to misleading analysis of field emission data. The uniform distribution of emission sites and the excellent field emission properties of the CNT films indicate that TiN, unlike SiO2, effectively functions as a catalyst barrier for growth without significantly impeding electron transport from the substrate to the emitter structures.
Photoemission investigation of the electronic properties of Ga-face GaN (0001)-dielectric interfaces
(2003-05-12) Cook, Ted Edwin Jr.; Robert J. Nemanich, Committee Chair; Robert F. Davis, Committee Co-Chair; Zlatko S. Sitar, Committee Member; Gerry Lucovsky, Committee Member
The characteristics of clean n- and p-type GaN (0001) surfaces and the interface between this surface and SiO2, Si3N4, and HfO2 have been investigated. Both n- and p-type Ga-face GaN (0001) surfaces have been cleaned via an 860° C anneal in an ammonia atmosphere, and carbon and oxygen contaminants were reduced to below the detection limits. Layers of SiO2, Si3N4, or HfO2 were carefully deposited to limit the reaction between the plasma and the GaN surface. After stepwise deposition, the electronic states were measured with x-ray photoelectron spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS). A valence band offset (VBO) of 2.0 ° 0.2 eV with a conduction band offset (CBO) of 3.6 ± 0.2 eV was determined for the GaN/SiO2 interface. The large band offsets suggest SiO2 is an excellent candidate for passivation of GaN. For the GaN/Si3N4 interface, type II band alignment was observed with a VBO of 0.5 ± 0.2 eV with a CBO of 2.4 ± 0.2 eV. While Si3N4 should passivate n-type GaN surfaces, it may not be appropriate for p-type GaN surfaces. A VBO of 0.4 ± 0.2 eV with a CBO of 2.0 ± 0.2 eV was determined for the GaN/HfO2 interface. An instability was observed in the HfO2 film, with energy bands shifting ~0.5 eV during a 650° C densification anneal. The electron affinity measurements via UPS were 3.0, 1.1, 1.8, and 2.9 ° 0.1 eV for GaN, SiO2, Si3N4, and HfO2 surfaces, respectively. Electron affinity measurements, along with band alignment data, allow a deviation from the electron affinity model due to a change of the interface dipole to be observed. Interface dipoles of 1.7, 1.1 and 1.9 ° 0.2 eV were observed for the GaN/SiO2, GaN/Si3N4, and GaN/HfO2 interfaces, respectively. The existence of Ga-O bonding at the heterojunction significantly increases the interface dipole, which raises the dielectric bands in relation to the GaN.
Thermionic Emission from Doped and Nanocrystalline Diamond
(2003-04-21) Koeck, Franz Alexander; Robert J. Nemanich, Committee Chair; Hans D. Hallen, Committee Member; Zlatko Sitar, Committee Member
Microwave Plasma assisted Chemical Vapor Deposition (MPCVD) has been utilized to synthesize nitrogen doped and intrinsic nanocrystalline diamond films to investigate thermionic field emission behavior. Sulfur-doped nanocrystalline diamond films prepared by hot filament chemical vapor deposition (HFCVD) have been included in the thermionic field emission measurements. The samples were imaged in UHV by photo electron emission microscopy (PEEM) using a UV Hg lamp for photoemission excitation. The same instrument was used to obtain the thermionic-field emission electron microscopy images (T-FEEM) at temperatures up to 900°C. The Raman spectra of the films showed a strong diamond peak at 1332cm-1 and weaker signal from the graphitic regions in the sample. Field emission could not be measured at room temperature, but the PEEM images showed relatively uniform emission. The PEEM images showed little change as the temperature is increased. At temperatures as low as 640°C the T-FEEM images exhibited strongly enhanced electron emission with increasing temperature. Doped and undoped nanocrystalline diamond films showed localized emission from small emission sites with a significant temperature dependence of the electron emission for the sulfur doped films at around 600°C. This thesis focuses on developing a consistent model of thermionic emission from doped and nanocrystalline diamond films.