Diamond Electron Emitters Research Articles

Thermionic Emission from Diamond Films in Molecular Hydrogen Environments

Diamond-based low work function thermionic electron emitters are in high demand for applications ranging from electron guns and space thrusters to electrical energy converters. A key requirement of such diamond-based electron sources is hydrogen termination of the surfaces which can significantly reduce the emission barrier. However, at high temperatures (> 600°C), terminated hydrogen begins to desorb causing degradation in thermionic emission performance. The purpose of this study is to examine low-pressure hydrogen operating environments as a means to overcome this high temperature performance limitation by enabling increased thermionic emission currents with improved stability at temperatures > 600°C. A series of isothermal and isobaric experiments were performed in both nitrogen and hydrogen gas environments to determine the performance enhancement. Diamond electron emitters in both the as-grown and hydrogenated states were characterized at temperatures of 600°C, 625°C, and 650°C. An increase in thermionic emission current over vacuum operation was observed following the introduction of hydrogen. Upon evacuation of hydrogen to vacuum, the emission current decreased back to baseline levels. Further experiments in gas environments at a constant pressure (~5.5 x 10-6 Torr) were conducted at temperatures ranging from 700-900°C. It was observed that the hydrogen environment promoted increased emission current while also enabling the diamond electron emitters to stably emit at increased temperatures compared to vacuum operation. Analogous experiments using nitrogen environments did not show any measurable performance enhancements, thus verifying that hydrogen is responsible for the observed effect. These results suggest diamond-based electron emitters can have improved thermionic emission performance at temperatures > 600°C when operating in hydrogen gas environments.

Electron emission characteristics of needle type semiconductor diamond electron emitters by pulsed bias operation and X-ray generation

Electron emission characteristics of needle-type semiconductor diamond electron emitters with pulsed bias operation were evaluated. An X-ray generation experiment was performed. Fowler–Nordheim plotting confirmed that field emission completely governed the electron emission. Maximum emission current of 4.2 mA was achieved using an n-type diamond needle. The needle tip, with area smaller than 1 μm 2, had estimated electron emission density greater than 4.2 × 10 5 A/cm 2. The effective emission area obtained from the Fowler–Nordheim plot was several 10 − 13 cm 2. For adopting and emission area of 1 × 10 − 12 cm 2, the estimated electron emission density was higher than 4.2 × 10 9 A/cm 2. Furthermore, the average emission current was 0.5–0.6 mA. This large electron emission was continued for several seconds and repeatable. A threshold electric field existed for electron emission higher than 50 kV/mm; pulsed electron emissions of less than 30 ms were created by slow triangular waveform shaped bias voltage supplied at frequencies of 5–10 Hz. An improved vacuum level and pulsed bias operation prevented damage to diamond electron emitters and steady electron emission better than with thermoelectronic emission and high bias voltage supply in DC mode; continuous X-ray generation of 1 h was achieved.

Development of an evaluation system for electron emission characteristics and generation of X-rays using a needle type semiconductor diamond electron emitter

Emission currents and other characteristics of needle type diamond electron emitters were evaluated for application to diamond X-ray sources. The evaluation system consisted of a Pyrex glass vacuum chamber, a turbo molecular pump, and a − 50 kV high-voltage supplier. A needle type phosphorus-doped semiconductor diamond electron emitter was kept at 600 °C by ohmic heating in a vacuum environment; emission current of ca. 1.1 μA was observed with bias voltage of − 50 kV. In addition, the electron orbit and intensity were estimated using an electron optics and electron gun design program ‘EGUN’. Great differences were apparent between experimental and calculated results: 64 μA electron emission current. This result was probably the result of a change in the shape of the tip of the diamond electron emitter. X-rays were generated using electrons from the diamond electron emitter with a tungsten target; energy spectra of the generated X-rays were measured. Then fluoroscopy was carried out.

Ion-beam processing of single crystal diamond using SOG mask

Various ion-beam etching characteristics of diamond and selectivity between diamond and spin-on-glass (SOG) were examined. The maximum selectivity of diamond and SOG was 12.7 in oxygen reactive ion-beam etching process at 100 V acceleration voltage. Using this etching condition and dot-shaped SOG mask, conical diamond field electron emitter arrays with 30 nm curvature radius, 2.58 μm base radius and 5.86 μm height were fabricated.

Effect of sp2/sp3 Ratio on Electron Emission Properties of Nitrogen-Doped Diamond Electron Emitter

sp 3 and sp 2 -rich nitrogen (N)-doped diamond electron emitters are prepared by modifying the substrate temperature. Their electron emission properties have been measured in order to examine the effect of the sp 2 /sp 3 ratio on the electron emission from N-doped diamond. The obtained electron emitters have been characterized by reflective high energy electron diffraction, X-ray diffraction and Raman spectroscopy. The electrical resistivities of the sp 3 -rich diamond is 100 G Ω cm or higher while that of the sp 2 -rich diamond is 100 Ω cm. In electron emission characteristics, the threshold voltage of the sp 3 -rich emitter is found to be lower than that of the sp 2 -rich emitter. From the threshold voltage versus the anode-diamond spacing characteristics, it has been found that most of the anode voltage drops in the bulk of the sp 3 -rich emitter and almost no voltage drop takes place in the sp 2 -rich emitter. The difference in electron emission properties should be explained by the resistivity of the N-doped diamond bulk.

Uniform electron emission from a nitrogen-doped diamond-based electron emitter fabricated by the sintering technique

A nitrogen (N)-doped diamond-based electron emitter has been fabricated by the sintering technique prior to the chemical vapor deposition process in order to improve the uniformity. There are no spatial differences in reflective electron energy loss spectra (REELS) from the diamond-based electron emitter, suggesting that uniform surface conditions are obtained. The uniform electron emission from the obtained electron emitter is confirmed through emission current vs anode voltage characteristics measurements. It seems that the uniformity of the emitter surface results in uniform electron emission from the diamond electron emitter.

Potential profile between boron-doped diamond electron emitter and anode electrode

The voltage drop of vacuum and boron (B)-doped diamond electron emitter bulk has been discussed in order to understand the electron emission mechanism of B-doped diamond. It is confirmed that the electron emission from B-doped diamond depends on its film thickness and the localized electron pass/channel is formed in the film. From the results of the threshold voltage versus anode-diamond spacing characteristics, it is found that most of the anode voltage is applied in vacuum, and the high electric field near the B-doped diamond surface is required for the electron emission from B-doped diamond regardless of the film thickness.

Formation of backcontacts on diamond electron emitters

Electron emission from boron (B)- and nitrogen (N)-doped diamond electron emitter with silver (Ag)-loaded conductive epoxy, titanium (Ti)/gold (Au), aluminum (Al) and amorphous carbon contacts are studied. Both Ti/Au and amorphous carbon contacts on B-doped diamond electron emitters lead to lower turn-on voltage compared to other contacts, while the turn-on voltage of N-doped diamond electron emitter is independent on the backcontacts.

Fabrication and Characterization of Planar Diamond Electron Emitters

A planar electron emitter was fabricated employing chemical vapor deposited (CVD) diamond particles. This device is composed of isolated particles of CVD diamond selectively deposited on a pair of patterned Pt films separated 4 micrometers from each other. Field emission current from the diamond particles was observed when the voltage was applied between the Pt films. The mechanism of electron emission from the diamond particles used in this device was discussed by comparing with the electron emission from a continuous diamond film. It was found that the effective work functions differ between the isolated particles and the continuous films. This result suggests a difference in the emission site of electrons.