A. T. Sowers

Field emission properties of nitrogen-doped diamond films

This study explores the field emission properties of nitrogen-doped diamond grown by microwave plasma chemical vapor deposition. Over 70 nitrogen-doped diamond samples were grown on silicon and molybdenum under varying process conditions. Under certain conditions, films can be grown which exhibit photoluminescence bands at 1.945 and 2.154 eV that are attributed to single substitutional nitrogen. Photoelectron emission microscopy with UV free electron laser excitation indicated a 0 or negative electron affinity. Field emission characteristics were measured in an ultrahigh vacuum with a variable distance anode technique. For samples grown with gas phase [N]/[C] ratios less than 10, damage from microarcs occurred during the field emission measurements. Samples grown at higher [N]/[C] content could be measured prior to an arcing event. Contrary to other reports on nitrogen-doped diamond, these measurements indicate relatively high threshold fields (>100 V/μm) for electron emission. We suggest that the nitroge...

Thin films of aluminum nitride and aluminum gallium nitride for cold cathode applications

Cold cathode structures have been fabricated using AlN and graded AlGaN structures (deposited on n-type 6H-SiC) as the thin film emitting layer. The cathodes consist of an aluminum grid layer separated from the nitride layer by a SiO2 layer and etched to form arrays of either 1, 3, or 5 μm holes through which the emitting nitride surface is exposed. After fabrication, a hydrogen plasma exposure was employed to activate the cathodes. Cathode devices with 5 μm holes displayed emission for up to 30 min before failing. Maximum emission currents ranged from 10–100 nA and required grid voltages ranging from 20–110 V. The grid currents were typically 1 to 104 times the collector currents.

Phase transformations during microcutting tests on silicon

Controlled slow‐speed microcutting tests were made on single crystal silicon. Micro‐Raman spectroscopy confirmed the presence of amorphous silicon within the microcutting grooves as well as in the debris particles removed from the grooves. These results indicate that pressure‐induced transformation to metallic silicon can occur during microcutting and the ductile metallic phase will facilitate the cutting process. Raman spectroscopy further indicated the presence of large residual tensile strains in some areas of the microcutting grooves.

Electron emission properties of crystalline diamond and III-nitride surfaces

Abstract Wide bandgap semiconductors have the possibility of exhibiting a negative electron affinity (NEA) meaning that electrons in the conduction band are not bound by the surface. The surface conditions are shown to be of critical importance in obtaining a negative electron affinity. UV-photoelectron spectroscopy can be used to distinguish and explore the effect. Surface terminations of molecular adsorbates and metals are shown to induce an NEA on diamond. Furthermore, a NEA has been established for epitaxial AlN and AlGaN on 6H–SiC. Field emission measurements from flat surfaces of p-type diamond and AlN are similar, but it is shown that the mechanisms may be quite different. The measurements support the recent suggestions that field emission from p-type diamond originates from the valence band while for AlN on SiC, the field emission results indicate emission from the AlN conduction band. We also report PEEM (photo-electron emission microscopy) and FEEM (field electron emission microscopy) images of an array of nitride emitters.

Effect of nitrogen incorporation on electron emission from chemical vapor deposited diamond

Two different types of the nitrogen-doped chemical vapor deposited (CVD) diamond films were synthesized with N2 (nitrogen) and C3H6N6 (melamine) as doping sources. The samples were analyzed by scanning electron microscopy, Raman scattering, photoluminescence spectroscopy, and field-emission measurements. More effective substitutional nitrogen doping was achieved with C3H6N6 than with N2. The diamond film doped with N2 contained a significant amount of nondiamond carbon phases. The sample produced with N2 exhibited a lower field emission turn-on field than the sample produced with C3H6N6. It is believed that the presence of the graphitic phases (or amorphous sp2 carbon) at the grain boundaries of the diamond and/or the nanocrystallinity (or microcrystallinity) of the diamond play a significant role in lowering the turn-on field of the film produced using N2. It is speculated that substitutional nitrogen doping plays only a minor role in changing the field emission characteristics of CVD diamond films.

Imaging electron emission from diamond and III–V nitride surfaces with photo-electron emission microscopy

Abstract Wide bandgap semiconductors such as diamond and the III–V nitrides (GaN, AlN, and AlGaN alloys) exhibit small or even negative electron affinities. Results have shown that different surface treatments will modify the electron affinity of diamond to cause a positive or negative electron affinity (NEA). This study describes the characterization of these surfaces with photo-electron emission microscopy (PEEM). The PEEM technique is unique in that it combines aspects of UV photoemission and field emission. In this study, PEEM images are obtained with either a traditional Hg lamp or with tunable UV excitation from a free electron laser. The UV-free electron laser at Duke University provides tunable emission from 3.5 to greater than 7 eV. PEEM images of boron or nitrogen (N)-doped diamond are similar to SEM of the same surface indicating relatively uniform emission. For the N-doped samples, PEEM images were obtained for different photon energies ranging from 5.0 to 6.0 eV. In these experiments, the hydrogen terminated surface showed more intense PEEM images at lower photon energy indicating a lower photothreshold than annealed surfaces which are presumed to be adsorbate free. For the nitrides, the emission properties of an array of GaN emitter structures is imaged. Emission is observed from the peaks, and relatively uniform emission is observed from the array. The field at the sample surface is approximately 10 V/μm which is sufficient to obtain an image without UV light. This process is termed field emission electron microscopy (FEEM).

Measurement of field emission from nitrogen-doped diamond films

Abstract This study explores issues related to the measurement of the field emission properties of nitrogen-doped diamond grown by microwave plasma chemical vapor deposition (CVD). Growth conditions have been optimized to produce films with a low concentration of sp 2 -bonded carbon which results in high electrical resistance. Field emission characteristics were measured in an ultrahigh vacuum with a variable distance anode technique. For samples grown with gas phase [N]/[C] ratios less than 10, damage from micro-arcs occurred during the field emission measurements. Samples grown at higher [N]/[C] content could be measured prior to an arcing event. The occurrence of a micro-arc is related to the film properties. The measurements indicate relatively high threshold fields (>100 V μm −1 ) for electron emission.

Highly oriented diamond films on Si: growth, characterization, and devices

Highly oriented, (100) textured diamond films have been grown on single-crystal Si substrates via microwave plasma enhanced chemical vapor deposition. A multistep deposition process including bias-enhanced nucleation and textured growth was used to obtain smooth films consisting of epitaxial grains with only low-angle grain boundaries. Boron-doped layers were selectively deposited onto the surface of these oriented films and temperature-dependent Hall effect measurements indicated a 3 to 5 times improvement in hole mobility over polycrystalline films grown under similar conditions. Room temperature hole mobilities between 135 and 278 cm2/V-s were measured for the highly oriented samples as compared to 2 to 50 cm2/V-s for typical polycrystalline films. Grain size effects and a comparison between the transport properties of polycrystalline, highly oriented and homoepitaxial films will be discussed. Metal-oxide- semiconductor field-effect transistors were then fabricated on the highly oriented films and exhibited saturation and pinch-off of the channel current.

Field Emission from Nitrogen-Doped Diamond Film

Nitrogen-doped diamond films were prepared for the first time using melamine (C 3 H 6 N 6 ) as the nitrogen source. To explore the differences in the films produced with different precursors, nitrogen-doped films were also produced using pure nitrogen gas as the source. Since melamine has a ring structure with pre-existing C-N bonds, the incorporation of nitrogen on substitutional sites of diamond lattice are expected. Relatively large amounts of substitutional nitrogen were successfully doped into diamond without degrading its quality. However, when pure nitrogen gas was used as a doping source, the quality of the diamond was not as high as the sample doped with nitrogen by melamine. Raman spectroscopy, photoluminescence spectroscopy (PL), and field emission measurements were carried out to characterize the samples. Nitrogen-doped diamond samples did not exhibit any significant reduction in turn-on fields. It is suggested that nitrogen doping has only a minor effect on the field emission properties of the diamond films.

Field emission induced damage from nitrogen doped diamond films grown by microwave plasma CVD

This study explores the field emission properties of nitrogen doped diamond grown on n-type silicon by microwave plasma CVD. Nitrogen gas was used as the nitrogen doping source and was added directly to the process gas. With this configuration, nitrogen could be added as an impurity to the process gas with gas phase [N]/[C] ratios spanning from 0 to 80. Following deposition, the micro-Raman and photoluminescence (PL) spectra were recorded using the 514.5 nm line of an argon ion laser as the excitation source. The sample surfaces were also examined using SEM and optical microscopy. Field emission measurements were obtained in UHV with a position-variable anode system. With this arrangement current voltage measurements are possible at numerous cathode to anode spacings.