William Harrell - Academia.edu (original) (raw)

Papers by William Harrell

AIP Conference Proceedings, 2015

A new electron beam ion trap (EBIT) based ion source and beamline were recently commissioned at C... more 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.

Bulletin of the American Physical Society, 2017

Submitted for the MAR17 Meeting of The American Physical Society Ion beam induced hot electron ex... more Submitted for the MAR17 Meeting of The American Physical Society Ion beam induced hot electron excitations in thin metal films DHRUVA KULKARNI, DANIEL FIELD, DANIEL CUTSHALL, JAMES HAR-RISS, WILLIAM HARRELL, CHAD SOSOLIK, Clemson University-We present measurements on hot carrier excitations in a metal irradiated by hyperthermal energy ions. Specifically, alkali (Rb +) and noble gas(Ar +) ions were used to irradiate a Schottky diode consisting of a thin film of Ag (˜25nm) grown on an n-type Si (111) wafer. Measurements of the resultant current through the device were performed as a function of energy and angle of incidence of the incoming ions. Energy loss of the incident energetic ions inside the metal film leads to the generation of hot carriers which are detected as a kinetically-induced current or "kinecurrent" within the device, analogous to previous measurements of "chemicurrent" [H. Nienhaus, Surface Science, 45, 1-78 (2002)]. We observe the existence of a threshold with respect to the energy of the incoming ions for the generation and detection of hot electrons using Schottky diodes.

Bulletin of the American Physical Society, Nov 13, 2014

Submitted for the SES14 Meeting of The American Physical Society Measuring potential energy dissi... more Submitted for the SES14 Meeting of The American Physical Society Measuring potential energy dissipation effects via below-surface electronic excitations in solids using MOS devices DHRUVA KULKARNI, RADHEY SHYAM, DANIEL FIELD, JIM HARRISS, DANIEL CUTSHALL, WILLIAM HARRELL, CHAD SOSOLIK, Clemson Univ-We have conducted measurements on oxides irradiated by multicharged ions (Q > 1) to explore the potential energy/charge state effect on subsurface damage caused in the target. Our goal was to determine charge state dependence of stopping power, or energy lost per unit distance, relative to the singly charged (Q = 1) ions usually encountered in ionsolid interactions. Specifically, we have irradiated a 170 nm thick SiO 2-on-Si sample with Ar Q+ ions (Q = 1, 4, 8, 11) at a fixed K.E. of 1 keV and tracked the electronic excitations in the oxide by capacitance-voltage (C-V) characterization post irradiation. Energy dissipation within the oxide generates electron-hole pairs which leads to mobile holes that can transport across the oxide and have a finite probability of getting trapped within the oxide, which causes a flatband voltage shift in its C-V curve. To obtain C-V characteristics, we deposited top metal contacts on our oxides to create MOS devices (Al-SiO 2-Si). These measurements showed increased flatband voltage shifts dependent on impact position, ion dose and the ion charge state. the ion charge state. Preliminary analysis reveals a power law (Q 2.2) dependence on the charge state which is compared to theoretical predictions and other experimental studies.

Bulletin of the American Physical Society, Mar 1, 2012

have fabricated metal-oxide-semiconductor (MOS) devices for a study of implantation rates and dam... more have fabricated metal-oxide-semiconductor (MOS) devices for a study of implantation rates and damage resulting from low energy ion-solid impacts. Specifically, we seek to capture ion irradiation effects on oxides by exposing as-grown SiO 2 layers (50 nm to 200 nm) to incident beams of alkali ions with energies in the range of 100 eV to 10 keV. The oxide is analyzed post exposure by encapsulating the irradiated region under a top metallic contact or within a finished MOS device. Characterization of the resulting ion-modified MOS device involves the standard techniques of room temperature and bias-dependent capacitance-voltage (C-V) measurements. The C-V results reveal alkali ion-induced changes in the flatband voltage of irradiated devices which can be used to extract both the range and implantation probabilities of the ions. Biased C-V measurements are utilized to confirm the concentration or dosage of ions in the oxide. A triangular voltage sweep (TVS) measurement at elevated temperatures also reveals the total ionic space charge in the oxide and can be used to extract a mobility for the ions as they pass through the damaged oxide. Comparisons of these measurements to standard device models as well as to ion range calculations in the oxide are presented.

ACS Applied Materials & Interfaces, Jan 18, 2018

In this paper, aluminum-doped zinc oxide(AZO)/reduced graphene oxide nano-composite thin films ar... more In this paper, aluminum-doped zinc oxide(AZO)/reduced graphene oxide nano-composite thin films are synthesized by a one-pot, solution-processed method. The nanocomposite film has been extensively characterized using scanning electron microscopy (SEM), X-ray-diffraction (XRD), energy dispersive spectroscopy (EDS), Hall effect measurement and UV-Vis spectroscopy. It is found that the controlled addition of reduced graphene oxide into AZO can lower the film's resistivity without causing significant degradation of optical transparency. In addition, nanocomposite films post-annealed at process temperature at 500°C possesses the lowest resistivity and the highest optical transmittance and that further increases in the annealing temperature degrades the film's property due to nucleation of other phases of the AZO.

Vacuum, Feb 1, 2020

Using conductive and non-conductive source materials, an ion deflector plate was fabricated by fu... more Using conductive and non-conductive source materials, an ion deflector plate was fabricated by fused filament fabrication. The fully vacuum-compatible design was tested by measuring the voltage-dependent deflection of Rb þ ions, and the results, which are ion energy dependent, are consistent with predicted kinematics for the ions. The results also compare well with a metal plate of similar dimensions and with an ion trajectory simulation.

Silicon p-n junctions fabricated in a CMOS compatible process can be operated above breakdown in ... more Silicon p-n junctions fabricated in a CMOS compatible process can be operated above breakdown in Geiger mode. Geiger mode operation allows for single photon detection. These devices are known as Single Photon Avalanche Diodes (SPAD). SPAD quality can be assessed by the dark count present during Geiger mode operation. Higher dark counts have been attributed t o defects within the shallow p-n junction. While higher dark counts are detrimental t o SPAD operation t h e variance in dark count due t o defects makes t h e m suitable for monitoring the junction quality of a CMOS process. At present the dark count of a SPAD is a qualitative measure of the presence of defects within the junction of a diode. T h e SPAD geomet r y does not allow t h e extraction of the necessary terms t o fully characterize t h e SPAD. This prevents quantitative process characterization t o be made. Special test structures and test methodologies have been designed t o allow for t h e extraction of the parameters characterizing SPADs. T h e test characterization and methodology required t o extract the SPAD parameters will be shown. In addition it will be shown t h a t with suitable test structures SPADs can be characterized and used as process a n d device monitors in a CMOS process.

Photolithographic Fabrication of P3HT Based Organic Thin-Film Transistors with High Mobility

ECS Journal of Solid State Science and Technology, 2022

An original design and photolithographic fabrication process for Poly (3-hexylthiophene-2, 5-diyl... more An original design and photolithographic fabrication process for Poly (3-hexylthiophene-2, 5-diyl) (P3HT) based Organic thin-film transistors (OTFTs) is presented. The structure of the transistors was based on the bottom gate bottom contact OTFT. The fabrication process was efficient, cost-effective, and relatively straightforward to implement. Most of the fabrication steps were performed at room temperature and atmospheric pressure, with the only exceptions being the high temperatures used for annealing the films and the low pressures used for depositing the metal contacts. More than 226 transistors were fabricated on a single wafer. The electrical characteristics and the geometry of the transistors were consistent across the wafer. Current–voltage (I–V) and atomic force microscopy (AFM) measurements were performed to characterize the primary electronic properties of the transistors and morphology of the P3HT, respectively. Two key performance parameters were extracted from these m...

Electronic Device Fabricated From Polyaniline / Single walled Carbon Nanotubes Composite

MRS Proceedings, 2003

Composites of high molecular weight polyaniline and single walled carbon nanotubes are investigat... more Composites of high molecular weight polyaniline and single walled carbon nanotubes are investigated for applications to electronic devices. Electrical characteristics of devices fabricated using this composite show an order magnitude higher current level than those measured in the neat polyaniline devices. Composite materials and devices with various weight percentages of single walled carbon nanotubes were also fabricated. Current-voltage (I-V) characteristics of these devices indicate a significant increase in current with an increase in carbon nanotube concentration in the composite. Analysis of the forward I-V characteristics of these composite devices on a log-log scale shows two power law regions. In the lower voltage range, the exponent is 1, which is consistent with ohmic conduction, while in the higher voltage range, the exponent is between 1.1 and 1.5, which is consistent with space-charge-limited conduction.

ECS Journal of Solid State Science and Technology, 2017

Polymer Tantalum capacitors with PEDOT cathodes fabricated with an aqueous pre-polymerized PEDOT ... more Polymer Tantalum capacitors with PEDOT cathodes fabricated with an aqueous pre-polymerized PEDOT dispersion (slurry PEDOT) were investigated for stability with temperature. In particular, capacitance dependence on temperature was investigated in humidified and dry capacitors fabricated with both coarse and fine tantalum powders, variable thicknesses of the Ta 2 O 5 dielectric, and insitu vs. slurry PEDOT cathodes. Electrical measurements along with scanning electron microscopy (SEM) and secondary ion mass spectrometry (SIMS) were used to characterize the samples. The capacitance of all samples was observed to increase with temperature; however, the extent and nature of this increase depends significantly on the nature of the polymer cathode and its interface with the dielectric. Humidified devices showed less capacitance variation above room temperature while dry devices showed less capacitance variation below room temperature for both coarse and fine powders. These results cannot be explained by the inherent variation of the dielectric constant with temperature. A model was developed for the capacitance dependence on relative humidity and temperature based on surface area changes due to heating or cooling, complimented by changes in the dielectric constant, which is in good agreement with the experimental data. The results of this work demonstrate the critical impact of the dielectric-polymer interface on capacitance stability in these capacitors.

Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2018

We discuss how a buried interface device, specifically a metal-oxide-semiconductor (MOS) capacito... more We discuss how a buried interface device, specifically a metal-oxide-semiconductor (MOS) capacitor, can be utilized to track effects of ion irradiation on insulators. We show that the exposure of oxides within unfinished capacitor devices to ions can lead to significant changes in the capacitance of the finished devices. For multicharged ions, these capacitive effects can be traced to defect production within the oxide and ultimately point to a role for charge-dependent energy loss. In particular, we attribute the stretchout of the capacitance-voltage curves of MOS devices that include an irradiated oxide to the ion irradiation. The stretchout shows a power law dependence on the multicharged ion charge state (Q) that is similar to that observed for multicharged ion energy loss in other systems.

IEEE Transactions on Nuclear Science, 2015

The Effect of a Copolymer Interfacial Layer on the Performance of Organic Thin-Film Transistors

ECS Transactions

The effects of adding a copolymer interfacial layer on the performance of Poly (3-hexylthiophene-... more The effects of adding a copolymer interfacial layer on the performance of Poly (3-hexylthiophene-2, 5-diyl) (P3HT) based organic thin-film transistors (OTFT) were investigated. Poly (oligo (ethylene glycol) methyl ether methacrylate- glycidyl methacrylate- lauryl methacrylate), which is referred to as POGL, was used as an interfacial layer between P3HT and the dielectric. OTFTs with and without a POGL interfacial layer were fabricated. The field-effect mobility and the threshold voltage were extracted for all the devices. The OTFTs with a POGL interfacial layer were observed to have a smaller threshold voltage than the OTFTs without an interfacial layer, which makes the POGL devices attractive for low power applications. The POGL OTFTs were also observed to have much more ideal drain current saturation characteristics with very small I-V curve slope. This is explained by the deep trap states on the POGL surface and the reduction of the contact resistance at the electrode/organic sem...

Polymers

We fabricated and characterized poly(3-hexylthiophene-2, 5-diyl) (P3HT)-based Organic thin-film t... more We fabricated and characterized poly(3-hexylthiophene-2, 5-diyl) (P3HT)-based Organic thin-film transistors (OTFTs) containing an interfacial layer made from virgin Graphene Oxide (GO). Previously chemically modified GO and reduced GO (RGO) were used to modify OTFT interfaces. However, to our knowledge, there are no published reports where virgin GO was employed for this purpose. For the sake of comparison, OTFTs without modification were also manufactured. The structure of the devices was based on the Bottom Gate Bottom Contact (BGBC) OTFT. We show that the presence of the GO monolayer on the surface of the OTFT’s SiO2 dielectric and Au electrode surface noticeably improves their performance. Namely, the drain current and the field-effect mobility of OTFTs are considerably increased by modifying the interfaces with the virgin GO deposition. It is suggested that the observed enhancement is connected to a decrease in the contact resistance of GO-covered Au electrodes and the particul...

Better Impedance Measurement Using Kelvin-Type Test Fixturing

Journal of Materials Research, Jan 5, 2015

We describe measurements aimed at tracking the subsurface energy deposition of ionic radiation by... more We describe measurements aimed at tracking the subsurface energy deposition of ionic radiation by encapsulating an irradiated oxide target within multiple, spatially-separated metal-oxide semiconductor (MOS) capacitors. In particular we look at incident kinetic energy and potential energy effects in the low keV regime for alkali ions (Na +) and multicharged ions (MCIs) of Ar Q+ (Q = 1,4,8,11) incident on as-grown layers of SiO 2 on Si. With the irradiated oxide encapsulated under Al top contacts, we record an electronic signature of the incident ionic radiation through capacitance-voltage (C-V) measurements. Both kinetic and potential energy deposition give rise to shifted C-V signatures that can be directly related to internal electron-hole pair excitations. The MCI data reveal an apparent power law dependence on charge state which is at odds with some prior thin foil studies obtained at higher incident energies.

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

Journal of Vacuum Science & Technology B Microelectronics Processing and Phenomena, 2003

... The authors would like to thank Dr. Rajendra Singh for his suggestion of plotting the data on... more ... The authors would like to thank Dr. Rajendra Singh for his suggestion of plotting the data on a log-log scale, and for many helpful discussions on the space-charge ... TP Chow, V. Khemka, J. Fedison, N. Ramungul, K. Matocha, Y. Tang, and RJ Gutmann, Solid-State Electron. ...

ACS applied materials & interfaces, Jan 29, 2017

Despite noteworthy progress in the fabrication of large-area graphene sheet-like nanomaterials, t... more Despite noteworthy progress in the fabrication of large-area graphene sheet-like nanomaterials, the vapor-based processing still requires sophisticated equipment and a multistage handling of the material. An alternative approach to manufacturing functional graphene-based films includes the employment of graphene oxide (GO) micron-scale sheets as precursors. However, search for a scalable manufacturing technique for the production of high-quality GO nanoscale films with high uniformity and high electrical conductivity is still continuing. Here we show that conventional dip-coating technique can offer fabrication of high quality mono- and bi-layered films made of GO sheets. The method is based on our recent discovery that encapsulating individual GO sheets in a nanometer-thick molecular brush copolymer layer allows for the nearly perfect formation of the GO layers via dip coating from water. By thermal reduction the bi-layers (cemented by a carbon forming polymer linker) are converted...

Journal of Vacuum Science & Technology B, Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena

Hot electron generation was measured under the impact of energetic Ar and Rb ions on Ag thin film... more Hot electron generation was measured under the impact of energetic Ar and Rb ions on Ag thin film Schottky diodes. The energy-and angular-dependence of the current measured at the backside of the device due to ion bombardment at the frontside is reported. A sharp upturn in the energy dependent yield is consistent with a kinetic emission model for electronic excitations utilizing the device Schottky barrier as determined from current-voltage characteristics. Backside currents measured for ion incident angles of 630 are strongly peaked about 0 (normal incidence) and resemble results seen in other contexts, e.g., ballistic electron emission microscopy. Accounting for the increased transport distance for excited charges at non-normal incidence, the angular results are consistent with the accepted mean free path for electrons in Ag films. V

Journal of Vacuum Science & Technology B

Measurements were performed to characterize and better understand the effects of slow highly char... more Measurements were performed to characterize and better understand the effects of slow highly charged ion (HCI) irradiation, a relatively unexplored form of radiation, on metal oxide semiconductor (MOS) devices. Si samples with 50 nm SiO 2 layers were irradiated with ion beams of Ar Q+ (Q = 4, 8, and 11) at normal incidence. The effects of the irradiation were encapsulated with an array of Al contacts forming the MOS structure. High frequency capacitance-voltage (CV) measurements reveal that the HCI irradiation results in stretchout and shifting of the CV curve. These changes in the CV curve are attributed to dangling Si bond defects at the Si/SiO 2 interface and trapped positive charge in the oxide, respectively. Charge state dependencies have been observed for these effects with the CV curve stretchout having a dependence of Q ∼1.7 and the CV curve shifting with a dependence of Q ∼1.8. These dependencies are similar to the results of previous studies focused on the Q-dependence of the stopping power of HCIs.