William R. Harrell

Observation of Poole–Frenkel effect saturation in SiO2 and other insulating films

Abstract An experimental method for observing the saturation of the Poole–Frenkel (PF) effect in insulating films is developed, allowing observation of this theoretically predicted effect for the first time. This method is also applied to measure the ionization potential of Coulombic attractive traps in SiO 2 films. New insights into the temperature variation of the PF effect shed light on the range of validity of the classical model of Frenkel. These methods and insights are general and can be applied to many insulating films, particularly those with leakage currents dominated by field-enhanced thermal emission.

Electronic properties of polyaniline/carbon nanotube composites

Composites of high molecular weight polyaniline and carbon nanotubes are investigated for electronic device applications. Physical characterization by Thermogravimetric analysis and atomic force microscopy indicates that polyaniline containing 1% Carbon Nanotubes is suitable for organic devices. Measured electrical characteristics of Schottky diodes fabricated using these materials exhibit current levels in the polyaniline/carbon nanotube composite devices nearly an order of magnitude higher than in the polyaniline devices.

Mechanical and Electrical Properties of Solution-Processed Polyaniline/Multiwalled Carbon Nanotube Composite Films

Mechanical and electrical properties of high molecular weight polyaniline/multiwalled carbon nanotube composite films were investigated. Addition of carbon nanotubes to polyaniline films was accomplished by solution processing. Physical characterization of these composites by thermogravimetric analysis, tensile testing, dynamic thermal mechanical analysis, and atomic force microscopy measurements indicate that polyaniline containing 1% carbon nanotubes is more mechanically and thermally stable than neat polyaniline. Rectifying aluminum contacts were fabricated using this composition of the composite material, along with neat polyaniline for comparison. The measured electrical characteristics indicate that the current levels of the polyaniline/carbon nanotube composite devices are nearly an order of magnitude higher than those of the polyaniline devices; thus, this composite material has the potential for applications in organic electronics.

Implications of advanced modeling on the observation of Poole–Frenkel effect saturation

Abstract The implications of a modern model of the Poole–Frenkel (PF) effect on an experimental method for observing PF saturation are investigated. It has previously been shown, using the classical PF model, that I – V curves measured at different temperatures should converge under the conditions of PF effect saturation, and such measurements were presented showing the first observation of saturation. However, saturation is not directly predicted from the classical model. The modern model used in this paper incorporates the Fermi–Dirac function to describe the population of Coulombic traps, and will predict saturation; therefore, the consistency between the modern model and this measurement technique are investigated. This is accomplished by studying the temperature variation of the modern model. We show that the modern model does predict that I – V ( T ) curves will converge at saturation, but this convergence is more complicated than in the classical model. Thus, the effective saturation condition predicted by the modern model is more complicated than the saturation condition usually considered. We discuss the interpretation of saturation measurements in light of the modern model.

Electrical Characterization of Tantalum Capacitors with Poly(3,4-ethylenedioxythiophene) Counter Electrodes

A combination of structural and electrical characterization techniques was applied to polymer Ta capacitors with in situ and prepolymerized poly(3,4-ethylenedioxythiophene) (PEDOT) cathodes with a broad range of Ta 2 O 5 dielectric thicknesses. The in situ PEDOT was produced by polymerizing the EDOT monomer using an oxidizer/dopant. The prepolymerized material was an aqueous suspension of doped polymer which was dried to produce the PEDOT film. Experimental data show that polymer Ta capacitors with prepolymerized PEDOT have lower dc leakage and higher breakdown voltage (BDV) compared to polymer Ta capacitors with in situ PEDOT. The difference in dc leakage and BDV between these two types of capacitors becomes greater with increasing thickness and improved structure of the Ta 2 O 5 dielectric film. These experimental results are inconsistent with current theories presented in the literature. An alternative model is presented based on classical metal/insulator/semiconductor (MIS) theory, where in this case M corresponds to the Ta metal, I corresponds to the Ta 2 O 5 insulator, and S corresponds to the semiconducting PEDOT. According to this model, a potential barrier for the current carriers at the insulator/semiconductor interface controls the current flow through the Ta 2 O 5 dielectric under normal operating conditions (positive polarity on the Ta anode and a temperature range of ―55°C ≤ T ≤ 105°C). In situ polymerization of the PEDOT adversely affects this barrier, while the prepolymerized PEDOT suspension leaves it essentially intact. The different migratory ability of the dopants in in situ and prepolymerized PEDOT also contributes to the differences in electrical performance of polymer Ta capacitors with in situ and prepolymerized PEDOT cathodes.

Analysis of the I–V characteristics of Al/4H-SiC Schottky diodes

The capability of silicon carbide in general to operate at high temperatures, and the fact that the electron mobility of the 4H-SiC polytype is twice that of the 6H polytype, has led to significant interest in 4H-SiC devices. Experimental data for Al/4H-SiC Schottky diodes is limited, and in this article we present forward and reverse I–V characteristics measured over a temperature range of 298–378 K. An analysis of the results shows that the charge transport mechanism is dominated by thermionic emission at forward biases less than 0.3 V, and series resistance at forward biases greater than 0.9 V. However, for a forward bias in the range of 0.3–0.9 V, we found the data fitted the space-charge-limited emission model. The competing mechanisms of thermionic emission and space-charge-limited emission lead to nonideal I–V characteristics, resulting in an increase in measured ideality factor. Space-charge-limited currents are dependent on the trap distribution in the material, and the estimated trap density is ...

Integration and Distribution of Carbon Nanotubes in Solution-Processed Polyaniline/Carbon Nanotube Composites

Composites of high-molecular-weight polyaniline (PANI) and single-walled carbon nanotubes as well as composites of PANI and multiwalled carbon nanotubes of various weight percentages were prepared using solution processing. The integration and distribution of the carbon nanotubes (CNTs) in the PANI matrix were investigated by various characterization methods. CNT distribution was evaluated using scanning electron microscopy, while CNT dispersion and alignment were characterized by transmission electron microscopy. The specific gravity of the composite material was measured using a density gradient column, which indicates how well the nanotubes are integrated into the polymer matrix. Results indicate that effective integration and distribution of CNTs into a PANI matrix can be achieved with solution processing for composites containing between 1 and 5% CNTs by weight.

First multicharged ion irradiation results from the CUEBIT facility at Clemson University

A new electron beam ion trap (EBIT) based ion source and beamline were recently commissioned at Clemson University to produce decelerated beams of multi- to highly-charged ions for surface and materials physics research. This user facility is the first installation of a DREEBIT-designed superconducting trap and ion source (EBIS-SC) in the U.S. and includes custom-designed target preparation and irradiation setups. An overview of the source, beamline, and other facilities as well as results from first measurements on irradiated targets are discussed here. Results include extracted charge state distributions and first data on a series of irradiated metal-oxide-semiconductor (MOS) device targets. For the MOS devices, we show that voltage-dependent capacitance can serve as a record of the electronic component of ion stopping power for an irradiated, encapsulated oxide target.

Mechanical Properties Of Polyaniline / Multi-walled Carbon Nanotube Composite Films

High molecular weight polyaniline / multi-walled carbon nanotube composite films were fabricated using solution processing. Composite films with various weight percentages of multiwalled carbon nanotubes were fabricated. Physical properties of these composites were analyzed by thermogravimetric analysis, tensile testing, and scanning electron microscopy. These results indicate that the addition of multiwalled nanotubes to polyaniline significantly enhances the mechanical properties of the films. In addition, metal–semiconductor (composite) (MS) contact devices were fabricated, and it was observed that the current level in the films increased with increasing multiwalled nanotube content. Furthermore, it was observed that polyaniline containing one weight percent of carbon nanotubes appears to be the most promising composition for applications in organic electronic devices.

Encapsulating Ion-Solid Interactions in Metal-Oxide-Semiconductor (MOS) Devices

We report on a measurement of low energy ion irradiation effects on as-grown films of SiO2 on a Si substrate. Beams of normally incident Na+ ions with kinetic energies of 2 keV to 5 keV were focused onto ~ 1900 Å SiO2 films. Aluminum top metal contacts were subsequently deposited onto these targets such that irradiated regions and unexposed (pristine) regions of the target could be compared using capacitance-voltage (C-V) measurements of individual metal-oxide-semiconductor (MOS) devices. The C-V data reveal an energy-dependent shift in the flatband voltage ( VFB) that can be returned to its near-pristine value by a low temperature anneal. An increase in the density of interface states ( Dit) inferred from the C-V curves is found to have a superlinear dependence on the incident kinetic energy. These data are consistent with previously observed UV radiation effects on MOS oxides, where transferred energy leads to electron-hole pair production and the diffusion and trapping of holes throughout the oxide. Our measured trapped hole densities are compared with calculated densities, which are based on the incident ion dose and the predicted ion implantation range, to arrive at a fractional yield for hole survival and measurement within an encapsulated MOS device.