Schottky Field Research Articles

Facility Implementation and Comparative Performance Evaluation of Probe-Corrected TEM/STEM with Schottky and Cold Field Emission Illumination

We report the installation and performance evaluation of a probe aberration-corrected high-resolution JEOL JEM-ARM200F transmission electron microscope (TEM). We provide details on construction of the room that enables us to obtain scanning transmission electron microscope (STEM) data without any evident distortions/noise from the external environment. The microscope routinely delivers expected performance. We show that the highest STEM spatial resolution and energy resolution achieved with this microscope are 0.078 nm and 0.34 eV, respectively. We report a direct comparative evaluation of the performance of this microscope with a Schottky thermal field-emission gun versus a cold field-emission gun. Cold field-emission illumination improves spatial resolution of the high current probe for analytical spectroscopy, the TEM information limit, and the electron energy resolution compared to the Schottky thermal field-emission source.

State of the art in atomic resolution off-axis electron holography

As proposed by Hannes Lichte, to resolve structure–property relations not only the question “Which atom is where?” but also the question “Which fields are around?” has to be answered. High-resolution off-axis electron holography opens up an access to these key questions in that it allows accessing the complete exit-wave of the object provided within the information limit of the microscope, i.e. amplitude and phase including atomic details such as position and species, and moreover, information about large area electric potentials and magnetic fields, which a conventional transmission electron microscope is blind for—also when using a Cs-corrector. For an excellent object exit-wave reconstruction, special care has to be taken on the hologram quality, i.e. interference fringe contrast and electron dose. Severe restrictions are given to signal resolution by the limited brightness of the electron source. Utilizing a new high-brightness Schottky field electron emitter in a state-of-the-art transmission electron microscope operated at 300 kV, the phase signal resolution at atomic resolution can significantly be enhanced. An improvement by at least a factor of 2.88 compared to the most recently reported single hologram at atomic resolution is found. To proof the applicability of this setup to real materials science problems, a grain boundary of gold has been investigated holographically. ► Impact of the brightness on the reconstructed signal in electron holography. ► Factor 2.8 gain in signal quality by setup with a high brightness electron gun. ► Investigation of a grain boundary in gold with a state-of-the-art holography setup. ► A-posteriori aberration fine-tuning for true one Angstrom resolution in the object wave. ► Mistilt analysis on the atomic scale by numerical wave optics.

Electron Gun Technologies for High Resolution Electron Microscopes

High-brightness electron gun technologies for high resolution electron microscopes are reviewed. High performance electron beam apparatuses today are equipped with either Schottky emission or field emission type cathodes, both of which have sharply etched tips for electric field enhancement that promotes electron emission. One of the key elements in these pointed cathodes is a proper control of the tip geometry. It substantially affects the emitter optics and performance. It is shown that the geometry is dictated by the faceting of the tip, which is in turn determined by the Equilibrium Crystal Shape (ECS). The ECS is the tip geometry that minimizes the surface free energy and dependent on the emitter operation environment. By proper choice of the tip field and temperature, one can control the degree of faceting and achieve optically desirable tip geometries.

Strong field electron emission and the Fowler–Nordheim–Schottky theory

We found that the well-established Fowler–Nordheim–Schottky field emission theory needs to be revisited for strong electric fields F. The classical derivation of the electron tunneling probability through the triangular potential barrier is re-examined and specified. This probability is studied in the fields of arbitrary strength and found as a function with a maximum at some value of the field and decaying when F goes to zero and to ∞. The location and height of this maximum depend only on the ratio of the electron kinetic energy to the work function, but the maximum cannot be realized for real materials. A simple interpolation formula for all possible electric fields is given. The domain of validity of the standard Fowler–Nordheim approximation is shown to be very wide and evaluated in detail. By solving the Schrödinger equation with the help of a power series expansion of the electron wavefunction, the standard Schottky potential is shown to make the tunneling impossible. This can be fixed tentatively by replacing the image potential with a more realistic modification which eliminates its non-physical singularity. The power series method promises to find a wider application in the field emission theory.

Neutron radiation effect on 4H-SiC MESFETs and SBDs

4H-SiC metal Schottky field effect transistors (MESFETs) and Schottky barrier diodes (SBDs) were irradiated at room temperature with 1 MeV neutrons. The highest neutron flux and gamma-ray total dose were 1 × 1015 n/cm2 and 3.3 Mrad(Si), respectively. After a neutron flux of 1 × 1013 n/cm2, the current characteristics of the MESFET had only slightly changed, and the Schottky contacts of the gate contacts and the Ni, Ti/4H-SiC SBDs showed no obvious degradation. To further increase the neutron flux, the drain current of the SiC MESFET decreased and the threshold voltage increased. φB of the Schottky gate contact decreased when the neutron flux was more than or equal to 2.5 × 1014 n/cm2. SiC Schottky interface damage and radiation defects in the bulk material are mainly mechanisms for performance degradation of the experiment devices, and a high doping concentration of the active region will improve the neutron radiation tolerance.

First performance measurements and application results of a new high brightness Schottky field emitter for HR-S/TEM at 80-300kV acceleration voltage

Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008

Optically induced voltage pulses by resonant excitation of a passive GaAs photoconductive switch

We have investigated the all-optical generation of ultrashort voltage pulses that are launched by resonant excitation of a passive GaAs photoconductive switch with no electric bias field applied. Due to a specially designed geometry, we can distinguish between electrical transients generated in the Schottky field region of the metal/GaAs contact and shift currents resulting from nonlinear optical effects in the GaAs material. The shift current contribution is smaller than the Schottky field contribution but possesses a higher bandwidth with frequency components exceeding 1 THz. For small bias fields, the contribution of all-optically-generated currents to the overall voltage signal may be significant and needs to be accounted for when modeling photoconductive switches.

Probe current, probe size, and the practical brightness for probe forming systems

Probe size, shape, and current are important parameters for the performance of all probe forming systems such as the scanning (transmission) electron microscope, the focused ion beam microscope, and the Gaussian electron beam lithography system. Currently, however, the relation between probe current and probe size is ill defined. The key lies in a lacking definition of “size.” This problem is solved with the introduction of the “practical brightness.” In literature, many different definitions of “brightness” can be found, but for systems in which the whole of the virtual source is imaged onto the target, it is the practical brightness of a source that determines how much current is in the probe. This means that only with the practical brightness the performance of a probe forming system can be calculated quantitatively. The beauty of the practical brightness is that this source property is unaffected by the quality of the column: without interactions between electrons in the beam, the practical brightness is conserved down to the target. This makes it the only relevant brightness for probe forming systems to be used to compare different sources. The practical brightness can be measured, but can also be calculated when the source intensity profile is known. The Gaussian source intensity profile of thermionic, Schottky, and cold field emitters yields a practical brightness of 1.44ej∕π⟨ε⟩, where j is the current density on the emitting surface and ⟨ε⟩ is the average tangential electron energy.

A Simple Method for Imaging DNA using SEM

Abstract The intricate relationship between molecular structure and function is a common theme in molecular biology. Visualizing the structure of biological macromolecules through imaging is therefore useful in understanding their varied biological roles. The process is often complex; imaging in a high voltage Transmission Electron Microscope (TEM) involves extensive staining and freezing. The aim of this experiment was to image DNA easily in a close to natural environment in a simple microscope. Samples were imaged using a Leo (Zeiss) 1550 Scanning Electron Microscope (SEM) with a Schottky field emitter in the 15-30kV range to reduce radiation damage. After imaging off-the-shelf DNA, two more DNA samples were dialyzed with RbCl and NaCl and imaged to elucidate what made the DNA visible.

Investigation of an electron string ion source with field emission cathode

The string mode of operation for an electron beam ion source uses axially oscillating electrons in order to reduce power consumption, also simplifying the construction by omitting the collector with cooling requirements and has been called electron string ion source (ESIS). We have started a project (supported by INTAS and GSI) to use Schottky field emitting cathode tips for generating the electron string. The emission from these specially conditioned tips is higher by orders of magnitude than the focused Brillouin current density at magnetic fields of some Tesla and electron energies of some keV. This may avoid the observed instabilities in the transition from axially oscillating electrons to the string state of the electron plasma, opening a much wider field of possible operating parameters for an ESIS. Besides the presentation of the basic features, we emphasize in this paper a method to avoid damaging of the field emission tip by backstreaming ions.

Effect of gate interface on performance degradation of irradiated SiC-MESFET

The radiation damages and their degradation mechanism of device performance on 4H-SiC Metal Schottky Field Effect Transistors (MESFETs), which have been irradiated at room temperature with 2 MeV electrons and 20 MeV protons, is studied together with recovery behavior. No performance degradation is observed by 1×10 12 e/cm 2 and 5×10 11 p/cm 2, while a slight increase of the linear drain current together with a decrease of the threshold voltage are noticed above higher fluence. The damage coefficient for protons is about three orders of magnitude larger than that for electrons. The radiation source dependence of the device performance degradation is attributed to the difference of mass and the possibility of nuclear collision for the formation of lattice defects. Based on thermal annealing results of electron-irradiated MESFETs, it is found that the recovery of the drain current characteristics principally takes place from 100 °C, and that the drain current recovers to the pre-rad value after 200 °C annealing, while capacitance and induced deep levels do not recover. From the results annealing behavior with 0.27 eV, it is concluded that the degradation of the drain current is mainly sensitive to the decrease of the Schottky barrier height at the gate contact.

Oxidation Mapping by Postpeak Technique

Department of Physics, University of Alberta, Edmonton T6G 2G7, CanadaSpatially resolved energy -loss near-edge structure (ELNES) reveals geometric and electronic structure of materials at high spatial resolution [1]. In scanning transmission ele ctron microscope (STEM), ELNES allows to study the valence state of atoms at interfaces of a nanocomposite system. The obtainable signal-to-noise ratio (SNR), set by specimen drift and radiation damage, is an obstacle in obtaining interpretable ELNES data at high spatial resolution. The white line ratio (WLR) is often used as a “fingerprinting” technique in identifying the oxidation of transition metals (TMs) [2], but hard to extract when the SNR is low. We recently found that a broad postpeak following the L-edge, observed in thin specimens where plural scattering is negligible, is characteristic of TMs in an oxidized state [3]. The postpeak was applied to identify surface oxidation of nanoparticles (NP) by examining average oxidation state of many NPs [4]. Here we show that the postpeak can be a convenient and sensitive indicator of oxidation of single iron NP by forming point analysis using a STEM with a small (0.5 nm) probe. The measurements were performed on isolated Fe NPs embedded in silica, shown in the high-angle annular dark-field (HAADF) image (Fig. 1a). The data were obtained using a JEOL 2200 FS (a 200 kV Schottky field emission instrument equipped with an W filter). Fig. 1b shows the ELNES of Fe L-edges recorded at different sites of a selected particle, as labelled in HAADF image in Fig. 1a. The signal comes mostly from the surface of the NP when the beam is focused at its edge, and mainly from the interior when the beam is focused at the center. A broad postpeak, about 40 eV above the onset of Fe L

Effect of substrate temperature and roughness on the solidification of copper plasma sprayed droplets

Flattening of an individual thermal sprayed particle on the substrate is a fundamental process for the coating formation. Coating properties such as porosity and adhesion depend strongly from the splats morphology. Also, the microstructure of the coating depends on the nucleation and the grain growth during splats solidification. The influence of the substrate temperature and roughness on the microstructure of copper splats was examined in the present study. Copper (−90 + 45 μm) was plasma sprayed under atmospheric conditions on mirror polished ( R a ∼ 0.02 μm) and on grit-blasted ( R a ∼ 1.33 μm) stainless steel substrates (AISI 304L). The substrates were preheated at three different temperatures: 200 °C, 250 °C and 300 °C. A Schottky field emission scanning electron microscope (SFE-SEM) was used in order to study the splats microstructure. In order to measure the grain size of the splats, image analysis software was used, while the chemical composition of the splats was studied by an electron probe microanalysis (EPMA). It was found that the splats cooling velocity is decreased with the increase of substrate temperature above the transition temperature. Also a further decrease of splats cooling velocity was provoked by increasing the mean substrate roughness.

Degradation of SiC-MESFETs by irradiation

The impact of radiation source dependence on the device performance degradation of 4H-SiC MESFETs (Metal Schottky Field Effect Transistors), which have been irradiated at room temperature with 2-MeV electrons and 20-MeV protons, is studied. The damage coefficient for protons is about three orders of magnitude larger than that for electrons. The radiation source dependence of the device performance degradation is attributed to the difference of mass and the possibility of nuclear collision for the formation of lattice defects. From thermal annealing of electron-irradiated MESFETs, it is found that the recovery of the drain current characteristics principally takes place from 100 °C, and that the drain current recovers to the pre-rad value after 200 °C annealing. It is concluded that the degradation of the drain current is mainly sensitive to the radiation-induced decrease of the Schottky barrier height at the gate contact.

Effects of fast secondary electrons to low-voltage electron beam lithography

In 1981 A. N. Broers suggested that the spatial limit of direct writing electron beam lithography (DWEBL) would be limited to ~10 nm by the laterally scattered fast secondary electrons (FSE) even in atomically thin resist. One possible solution to this restriction would be to use low- or ultralow-energy electrons. Experiments and simulations have been carried out to quantify the contribution of FSE to the energy deposition that results in exposure of the resist over high-beam energies. To examine the effects of FSE on low-voltage operations, studies of electron-beam lithography (EBL) in the low- to ultralow-energy range, employing commonly used resists such as PMMA, were performed, and the results were compared to those from conventional high-voltage processing. DWEBL was performed in a Schottky field emission gun scanning electron microscope (SEM), used in cathode-lens mode for ultralow-voltage operation. The exposure characteristics and sensitivity of the system at these energies have been investigated using Monte Carlo simulation methods. Saturation doses were calculated at low energies, which would give a useful condition to target for routine exposure because it ensures the critical dimensions will not be affected by any random changes in beam intensity.

Radiation source dependence of device performance degradation for 4H-SiC MESFETs

The impact of radiation source dependence on the device performance degradation of 4H-SiC MESFETs (Metal Schottky Field Effect Transistors), which have been irradiated at room temperature with 2 MeV electrons and 20 MeV protons, is studied. No performance degradation is observed by 1×10 15 e/cm 2 and 5×10 11 p/cm 2 , while a slight increase of the linear drain current together with a decrease of the threshold voltage are noticed above higher fluence . The damage coefficient for protons is about three orders of magnitude larger than that for electrons. The radiation source dependence of the device performance degradation is attributed to the difference of mass and the possibility of nuclear collision for the formation of lattice defects. Based on thermal annealing results of electron-irradiated MESFETs, it is found that the recovery of the drain current characteristics principally takes place from 100 ∘ C, and that the drain current recovers to the pre-rad value after 200 ∘ C annealing. On the other hand, capacitance and induced deep levels do not recover by 200 ∘ C annealing. It is concluded that the degradation of the drain current is mainly sensitive to the radiation-induced decrease of the Schottky barrier height at the gate contact.

Corrections to “On the Modeling and Design of Schottky Field-Effect Transistors” and “Comparison Study of Tunneling Models for Schottky Field Effect Transistors and the Effect of Schottky Barrier Lowering”

Corrections to “On the Modeling and Design of Schottky Field-Effect Transistors” and “Comparison Study of Tunneling Models for Schottky Field Effect Transistors and the Effect of Schottky Barrier Lowering”

Comparison study of tunneling models for Schottky field effect transistors and the effect of Schottky barrier lowering

An Airy function transfer matrix tunneling model and the Wentzel-Kramers-Brillouin (WKB) tunneling model have been compared in the application to Schottky field effect transistors (SFETs) with and without the incorporation of Schottky barrier lowering (SBL). Model calculations have shown that the WKB model can predict tunneling current through a Schottky barrier with reasonable accuracy when SBL is excluded. It is also shown that the WKB model can mimic the total current behavior in an SFET when SBL is included and when thermal current is excluded. In both cases, though, the actual physical behavior of the SFET is misrepresented, and so it is finally shown that in the design cases of interest (low Schottky barrier heights), thermal current dominates the total on-state current in SFETs

Layer-by-Layer assembly and humidity sensitive behavior of poly(ethyleneimine)/multiwall carbon nanotube composite films

Raw CVD grown multiwall carbon nanotubes (MWCNTs) were oxidized in a mixture of concentrated sulfuric acid and nitric acid to obtain water soluble MWCNTs. Composite multilayer films of the oxidized MWCNTs and poly(ethyleneimine) (PEI) were fabricated through the layer-by-layer (LbL) assembly approach. The oxidized MWCNTs were characterized with Fourier transform infrared spectroscopy. The LbL assembly process of the multilayer films was monitored with a UV–vis spectrophotometer. The surface and sectional morphology of the films were observed under a Schottky field emission scanning electron microscope. On the assembled thin films, two parallel metal electrodes were vapor deposited. The resistance between the two electrodes was measured at different relative humidity levels at room temperature. The data show that the resistance increases with increasing the relative humidity over the range of 5–97% RH and that, in the range of 5–85% RH, the resistance increases almost linearly. The experimental data also show that the humidity sensitive multilayer films have response and recovery times of about 2 and 30 s, respectively.

Radiation-induced defects in SiC-MESFETs after 2-MeV electron irradiation

The impact of radiation damage on the device performance of 4H-SiC metal Schottky field effect transistors, which are irradiated at room temperature with 2-MeV electrons, is studied. No performance degradation is observed by 1 × 10 15 e / cm 2 , while a slight increase of the linear drain current together with a decrease of the threshold voltage are noticed for 1 × 10 16 e / cm 2 . The degradation for low electron fluence is mainly attributed to the radiation-induced decrease of the Schottky barrier height at the gate contact. For exposures over 1 × 10 16 e / cm 2 , the drain current and transconductance decrease. The maximum transconductance for 1 × 10 17 e / cm 2 is only 18% of the value before irradiation. Although no electron capture levels are observed before irradiation, three electron capture levels ( E 1 – E 3 ) are induced after irradiation. The observed increase of the channel resistance is due to the induced lattice defects creating electron traps.