Tyson C. Back

Approach to multifunctional device platform with epitaxial graphene on transition metal oxide

Heterostructures consisting of two-dimensional materials have shown new physical phenomena, novel electronic and optical properties, and new device concepts not observed in bulk material systems or purely three dimensional heterostructures. These new effects originated mostly from the van der Waals interaction between the different layers. Here we report that a new optical and electronic device platform can be provided by heterostructures of 2D graphene with a metal oxide (TiO2). Our novel direct synthesis of graphene/TiO2 heterostructure is achieved by C60 deposition on transition Ti metal surface using a molecular beam epitaxy approach and O2 intercalation method, which is compatible with wafer scale growth of heterostructures. As-grown heterostructures exhibit inherent photosensitivity in the visible light spectrum with high photo responsivity. The photo sensitivity is 25 times higher than that of reported graphene photo detectors. The improved responsivity is attributed to optical transitions between O 2p orbitals in the valence band of TiO2 and C 2p orbitals in the conduction band of graphene enabled by Coulomb interactions at the interface. In addition, this heterostructure provides a platform for realization of bottom gated graphene field effect devices with graphene and TiO2 playing the roles of channel and gate dielectric layers, respectively.

Pulsed-Laser Deposited Transition-Metal Carbides for Field-Emission Cathode Coatings

Thin films of transition-metal carbides ZrC, HfC, and TiC were deposited by pulsed-laser deposition under vacuum. The surface chemistry of the films was characterized with ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy, and Auger electron spectroscopy in situ. X-ray diffraction was used to characterize the film structure. TiC was shown to be nearly stoichiometric and polycrystalline. The TiC was applied to a vertically aligned carbon nanotube sample and characterized by field emission. Field-emission results showed enhanced current and current density at a film thickness, 5 nm, not previously reported in the literature. Emission from TiC films was also shown to be less affected by adsorbates during field emission. Pulsed-laser deposition of TiC offers a distinct advantage over other techniques in that high-quality films can be obtained under ultrahigh vacuum conditions without the use of a reactive background gas or excessively high annealing temperatures. The application of TiC by pulsed-laser deposition as a cathode coating shows potential for integration into a fabrication process.

Review Article: Rare-earth monosulfides as durable and efficient cold cathodesa)

In their rocksalt structure, rare-earth monosulfides offer a more stable alternative to alkali metals to attain low or negative electron affinity when deposited on various III-V and II-VI semiconductor surfaces. In this article, the authors first describe the successful deposition of lanthanum monosulfide via pulsed laser deposition on Si and MgO substrates. These thin films have been characterized by x-ray diffraction, atomic force microscopy, high resolution transmission electron microscopy, ellipsometry, Raman spectroscopy, ultraviolet photoelectron spectroscopy, and Kelvin probe measurements. For both LaS/Si and LaS/MgO thin films, the effective work function of the submicron thick thin films was determined to be about 1 eV from field emission measurements using the scanning anode field emission microscopy technique. The physical reasons for these highly desirable low work function properties were explained using a patchwork field emission model of the emitting surface. In this model, nanocrystals of ...

Field emission from carbon nanotube fibers in varying anode-cathode gap with the consideration of contact resistance

This paper studies field emission (FE) from a single carbon nanotube (CNT) fiber with different anode-cathode (AK) gap distances. It is found that the field enhancement factor depends strongly on the finite AK gap distance, due to the combination of geometrical effects and possible fiber morphology change. The geometrical effects of AK gap distance on the field enhancement factor are confirmed using COMSOL simulations. The slope drop in the Fowler-Northeim (FN) plot of the FE data in the high voltage is related to the electrical contact resistance between the CNT fiber and the substrate. It is found that even a small series resistance to the field emitter (<30% of the emission gap impedance) can strongly modify the FE characteristics in the high voltage regime, inducing a strong deviation from the linear FN plot.

Tunable stoichiometry of BCxNy thin films through multitarget pulsed laser deposition monitored via in situ ellipsometry

Abstract. Pulsed laser deposition is an energetic deposition technique in which thin films are deposited when a laser pulse at 248-nm wavelength strikes a target and material is subsequently deposited onto a substrate with ideally the same stoichiometry. By synchronizing a high-speed mirror system with the pulsing of the laser, and using two separate targets, thin films having tunable stoichiometry have been deposited. Depositions were performed in a high vacuum environment to obtain as much kinetic energy as possible during growth. Typically, some 150 pulses at 300  mJ/pulse were required to deposit 1 nm. Island growth must occur on a per pulse basis since over 100 pulses are required to deposit a 1 nm film thickness. Films were deposited to ∼100-nm thickness, and in situ ellipsometry data were modeled to calculate thickness, n and k. X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and atomic force microscopy (AFM) were all performed on each of the films. XPS demonstrated change in film composition with change in laser pulse ratio; ellipsometry displayed thickness from the model generated as well as the optical properties from 370 to 1690 nm. AFM thickness measurements were in agreement with independently modeled ellipsometry thickness values.

Effect of in-situ oxygen on the electronic properties of graphene grown by carbon molecular beam epitaxy

We report that graphene grown by molecular beam epitaxy from solid carbon (CMBE) on (0001) SiC in the presence of unintentional oxygen exhibits a small bandgap on the order of tens of meV. The presence of bandgaps is confirmed by temperature dependent Hall effect and resistivity measurements. X-ray photoelectron spectroscopy (XPS) measurements suggest that oxygen incorporates into the SiC substrate in the form of O-Si-C and not into the graphene as graphene oxide or some other species. The effect is independent of the carrier type of the graphene. Temperature dependent transport measurements show the presence of hopping conduction in the resistivity and a concurrent disappearance of the Hall voltage. Interactions between the graphene layers and the oxidized substrate are believed to be responsible for the bandgap.

Epitaxial titanium nitride on sapphire: Effects of substrate temperature on microstructure and optical properties

Titanium nitride (TiN) is a mechanically robust, high-temperature stable, metallic material receiving considerable attention for resilient plasmonics. In this work, the authors fabricated six heteroepitaxial TiN films on sapphire using controllably unbalanced reactive magnetron sputtering. They examined the effect of substrate growth temperature on the plasmonic and crystalline quality of the film. Optical properties of all films were obtained from spectroscopic ellipsometry; plasmonic quality factors were determined from the real and imaginary parts of the dielectric function. The authors determined crystallinity using x-ray diffraction and surface morphology using atomic force microscopy. X-ray diffraction showed (111) TiN peaks with Pendellosung fringes indicating consistent heteroepitaxy. Atomic force microscopy showed smooth surfaces with root mean square surface roughness ranging from 0.2 to 2.6 nm. Based on this characterization, the authors determined that the substrate deposition temperature of 550 °C yielded (111)-oriented heteroepitaxial TiN with minimal surface roughness. The authors found that 550 °C also gave highest plasmonic quality factors for all wavelengths, approaching the values of todays best plasmonic materials (such as Au and Ag). Further, the Q-factors at wavelength 1550 nm inversely correlated with calculated lattice constants. Their results indicate that the plasmonic response of TiN is directly linked with structural quality of the film.Titanium nitride (TiN) is a mechanically robust, high-temperature stable, metallic material receiving considerable attention for resilient plasmonics. In this work, the authors fabricated six heteroepitaxial TiN films on sapphire using controllably unbalanced reactive magnetron sputtering. They examined the effect of substrate growth temperature on the plasmonic and crystalline quality of the film. Optical properties of all films were obtained from spectroscopic ellipsometry; plasmonic quality factors were determined from the real and imaginary parts of the dielectric function. The authors determined crystallinity using x-ray diffraction and surface morphology using atomic force microscopy. X-ray diffraction showed (111) TiN peaks with Pendellosung fringes indicating consistent heteroepitaxy. Atomic force microscopy showed smooth surfaces with root mean square surface roughness ranging from 0.2 to 2.6 nm. Based on this characterization, the authors determined that the substrate deposition temperature of 5...

A new formula for secondary emission yield in the low voltage region: An improvement of Vaughan's expression

Reducing the emission of secondary electrons from anode materials is critical to improved efficiency and increased performance in high power vacuum electronics for defense systems. The focus of this proposed effort is to leverage advances in materials technology, specifically thin films, to reduce secondary electron generation and outgassing from anode surfaces. By using advanced thin film deposition techniques, hybrid materials can be developed that provide the thermal and electrical conductivity required for operation, while reducing secondary electrons and desorption of gas species from the anode surface. Proposed solutions to these issues need to be robust, yet cost effective and applicable through available manufacturing processes. In this paper, we will introduce an improved mathematical expression for the secondary emission yield as a function of the impact voltage which is an extension of the formula first introduced by Vaughan [1]. Our expression gives a better fit to some of our experimental data of secondary emission yield versus impact voltage for polycrystalline Copper for which the maximum secondary emission yield is only slightly larger than unity.

Electron-Beam Induced Activation of Catalyst Supports for CNT Growth

The widespread use of carbon nanotubes (CNTs) in numerous practical applications has motivated multi-parameter studies using many support systems, growth conditions and synthesis methods. Among the most widely used support systems in electronics, sapphire is known to be catalytically inactive for CNT growth. Previously, our group demonstrated that the use of ionic bombardment can lead to an increase in CNT growth yield at the irradiated surface of catalytically inactive sapphire.[1] The engineering process was observed to reduce catalyst particle size and extend catalytic activity via the tuning of support stoichiometry and roughness. Therein, activation selectivity could be achieved by means of physically placing a micron-scale mask to impede growth on masked regions. Other studies have also demonstrated the use of ionic bombardment for selective growth of CNTs on slots and pits in other support systems.[2-3] We expand on the use of ionic species for patterned growth by conducting local and selective activation of c-cut sapphire using electron beam (e-beam) irradiation. We demonstrate that we can modify the surface stoichiometry and roughness thus activating the support surface. Exposing the activated sapphire to typical CNT growth conditions resulted in selective growth of vertically aligned CNTs in the activated regions. By varying the irradiation conditions, the process can be tuned to enhance catalytic activity and achieve patterned growth with high-precision.

Influence of surface roughness on secondary electron emission from graphite

In this study, the authors address how surface roughness alters secondary electron emission. By using specific grades of metallographic polishing pads, controlled levels of roughness and surface features were imparted. As expected, the smoothest surface (root mean square roughness 0.110 ± 0.022 μm) produced the highest secondary electron yield; however, a moderate rough surface (0.990 ± 0.019 μm) produced a slightly lower yield as compared to a rougher surface (7.10 ± 1.23 μm) at lower primary electron energies. This inversion, that a macroscopic rougher surface yields a higher emission, has been explained by differences between large and small scale variations in the surface roughness and the frequency that these features appeared on the surface. The surface roughness was quantified using optical profilometry and a fast Fourier transform of the surface topology.