We report a significant decrease in the photoelectric threshold of chemical vapor deposition grown diamond films as the fraction of sp3 carbon to sp2 plus sp3 carbon in the films decreases. Raman spectroscopy and x-ray photoelectron spectroscopy are used to characterize the different forms of carbon in the films and the sp3/(sp2 + sp3) carbon fraction at the surface. We observe a decrease in the photoelectric threshold from 4.5 eV to 3.9 eV as the sp3/(sp2 + sp3) carbon fraction at the surface decreases from 71% to 55%. Ultraviolet photoelectron spectroscopy of the films shows that they have a negative electron affinity surface. Therefore, the work function of the films decreases from 4.5 eV to 3.9 eV. We propose that the decrease in photoelectric threshold is due to a decrease in the band gap of sp2-sp3 carbon networks at the grain boundaries. The observed decrease in photoelectric threshold can be used to tailor the electronic properties of diamond films for specific applications.

Boron doped polycrystalline diamond films grown on p-type single-crystal Si substrates using chemical vapor deposition with a gas mixture of hydrogen, methane and diborane were characterized with scanning electron microscopy, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, Raman spectroscopy and photoelectric current measurements. The energy distributions are not sensitive to boron doping for diborane concentrations from 0 to 4.75 ppm, although the boron doping modifies the surface morphology and the photoemission intensity. The photoemission intensity is high where the microcrystalline content is highest (at diborane concentrations of 2.91 and 4.75 ppm. The photoelectric threshold is found to be at 4.38 eV, in agreement with earlier measurements. The present results are characteristic of valence band emission at 4.38, 4.63, 4.92, 5.12 and 5.30 eV for incident photons between 4.87 and 5.63 eV.

The effect of deposition parameters and substrate on the morphology of carbon films prepared in VHF plasma at low temperature has been studied by Atomic Force Microscopy. Carbon films demonstrating superior emission characteristics were the smoothest, but not all of the smoothest films demonstrated good emission. Significant influence of pre-growth treatment of the substrate surface on the emission characteristics of the films was found.

We have deposited carbon films from mixtures of methane and N2 using Plasma Enhanced Chemical Vapor Deposition. By changing the percentage of N2 in the feed gas, we were able to produce films that have various aspect ratios and sp2/sp3 contents. The film with the highest field emission contains spears of aspect ratio of 10:1. We also found that in our sp3-rich films, higher sp2 content enhanced field emission. This is ascribed to improved charge transport to the field emission sites.

The effects of Cs on oxygenated epitaxial diamond (100) films are studied using ultrahigh vacuum (UHV) scanning tunneling microscopy (STM). The epitaxial diamond (100) films are grown on synthetic diamond substrates using chemical vapor deposition and are boron doped. Before Cs deposition and oxygenation, UHV STM imaging of the epitaxial (100) films reveals a (2×l) dimer reconstruction. After Cs deposition and oxygenation, steps with relatively smooth surfaces are observed using positive tip voltages. Using negative tip voltages, many round bright structures approximately 20 Å in diameter are observed on the surface. We propose that these bright structures are Cs atoms or clusters of Cs atoms. Since these structures are only observed for negative tip voltages, they have a large number of empty states. Our observations are compared with recent theoretical predictions for Cs adsorbed on oxygenated diamond (100).

We report ultrahigh vacuum (UHV) scanning tunneling microscopy (STM) studies of the structural and electronic properties of epitaxial diamond (110) films. We observe that epitaxial diamond (110) films grow very rough due to striations. The striations are found to be due to the appearance of(111) faces and contain (100) steps. UHV STM atomic resolution images of the diamond (110) films show a (lxi) zigzag structure that measures 1.5 ± 0.1 Å × 1.5 ± 0.1 Å, in agreement with theoretical predictions for the hydrogen terminated diamond (110) surface. Ultraviolet spectroscopy shows that the epitaxial films have a photoelectric threshold of 5.3 ± 0.1 eV, providing evidence that the films have a negative electron affinity surface.

We have been working for many years on electron emission and surface properties of transition metal carbides. These studies have covered field, photo, and thermionic emission from bulk carbides as well as field and photo emission from carbide coatings and films on various substrates. Thin film carbide overcoatings have also been used on field emitter arrays of both Mo and Si. These unique carbide materials have electron emission properties making them attractive candidates for several applications. Uses for these carbides include FEA video displays, microwave applications, high current and small spot electron sources (e.g. accelerators and FELs), and cold cathodes for operation at tube pressures and in poor vacuums (e.g. FEAs, vacuum detectors, and neutralizing beams for ion thrusters).

We report here on one aspect of our current research which deals with electron emission of ZrC films coating single Mo field emitters. The relevant application is Spindt-type field emitter arrays. Processing steps are evaluated showing the need for a clean substrate. After deposition treatments are also investigated. Emission degradation has been noted after exposure to air and we report on successful use of a hydrogen atmosphere to reverse those adverse effects.

We have fabricated poly-Si, Si, and Ti-silicide field emitter arrays employing in-situ vacuum encapsulated lateral field emitter structures and investigated the field emission characteristics such as turn-on voltage, emission current density, and the stability of emission current. Although poly-Si and Si emitter have almost identical turn-on voltages, Si emitter has a sharper turn-on than poly-Si emitter due to its uniform surface. The current densities of poly-Si, and Si emtter are 0.47, 0.43 μA/tip respectively at anode to cathode voltage of 90 V. The turn-on voltage and current density of Ti-silicide emitter are about 31 V, and 1.81 μA/tip at VAK of 90 V. The data of the normalized current fluctuations indicate that Ti-silicide emitter has the most stable current.

Our experiment shows that Ti-silicide is most promising among these three materials due to its low work function, uniform surface, and the stable characteristics.

It is demonstrated that porous polysilicon (PPS) diode fabricated on the silicon substrate operates as efficient and stable surface-emitting cold cathode. A 1.5 pm of non-doped polysilicon layer is formed on n-type (100) silicon wafer and anodised in a solution of HF(50%): ethanol = 1:1 at a current density of 10 mA/cm2 for 30 seconds under illumination by a 500W tungsten lamp from a distance of 20 cm. Subsequently, PPS layer is oxidized in a rapid thermal oxidation(RTO) furnace for one hour at a temperature of 700°C. A semi-transparent thin Au film (about 10 nm thick) is deposited onto the PPS layer as a positive electrode and an ohmic contact is formed at the back of the silicon wafer as a negative electrode. When a positive bias is applied to the Au electrode in vacuum, the diode uniformly emits electrons. No electron emission is observed in the negatively biased region. Emission current is about 10−4 A/cm2 at 20V bias, and no fluctuation of emission current is observed as a function of time. Emission current is not affected by a surrounding pressure up to around 10 Pa. It is envisaged that mechanism of this emission is attributed to hot electron tunnelling.

Based on the previously-reported porosity multilayer technique, cold electron emission properties of porous silicon (PS) electroluminescent diodes with a structure of Au/PS/n-type Si are further improved by introducing a graded-band multilayer structure. It is shown that electrons are quasiballisticly emitted from PS diodes owing to a significantly reduced electron scattering in PS layer. As a result, the emission current shows a fluctuation-free behavior. These observations are very important for both understanding the electron transport in PS and developing high performance electron emitters in application to vacuum microelectronic technology.

Planar SiC/Si heterostructures were formed by high dose carbon implantation using a metal vapor vacuum arc ion source. The variations of the field emission properties with the implant dose and annealing conditions were studied. A remarkably low turn-on field of IV/μm was observed from a sample implanted at 35 keV to a dose of 1.0×1018 cm−2 with subsequent annealing in nitrogen at 1200°C for 2h. The chemical composition depth profiles were determined from x-ray photoelectron spectroscopy and the surface morphology was observed by atomic force microscopy. The formation of a thin surface stoichiometric SiC layer and the formation of densely distributed small protrusions on the surface are believed to be the two factors responsible for the efficient electron field emission.