High-current electron emission of thin diamond films deposited on molybdenum cathodes

The results of investigation of emission characteristics of cold cathodes employing diamond and related films are presented. The films were deposited in a new millimeter wave plasma-assisted CVD reactor using Ar–H₂–CH₄ and Ar–H₂–CH₄–N₂ gas mixtures. To study the emission properties of the high-current cathodes they were subjected to ~ 50 ns high-voltage pulses with amplitudes up to 100 kV. Experiments show that the emission properties strongly depend on methane and nitrogen concentration in gas mixture. The homogeneous emission with current density of 220 A/cm² has been obtained. The prepared cathodes were successfully tested in high-power rf pulse compressor employing electron beam triggering.     

Surface engineering of diamond tips for improved field electron emission

Results are reported on the study of surface engineering of diamond microtips for improved field electron emission. Two-dimensional (2D) arrays of diamond pyramids were prepared using a molding technique. As-grown diamond pyramids of 3 and 9 μm in size with sharp apexes were insulating and showed a relatively poor field electron emission. Several ways are examined to provide an electrical conductivity of diamond in order to supply electrons for the emission: diamond boron doping, partial graphitization/amorphization by nitrogen ion implantation and/or high temperature annealing, formation of built-in conductive metal layers. A perceptible improvement of surface electrical conductivity and reduction of field electron emission thresholds down to 10 V/μm was observed. For explanation of the results it was enough to take into account only the geometric field enhancement on pyramids apexes.     

Enhanced field electron emission properties of hierarchically structured MWCNT-based cold cathodes

Hierarchically structured MWCNT (h-MWCNT)-based cold cathodes were successfully achieved by means of a relatively simple and highly effective approach consisting of the appropriate combination of KOH-based pyramidal texturing of Si (100) substrates and PECVD growth of vertically aligned MWCNTs. By controlling the aspect ratio (AR) of the Si pyramids, we were able to tune the field electron emission (FEE) properties of the h-MWCNT cathodes. Indeed, when the AR is increased from 0 (flat Si) to 0.6, not only the emitted current density was found to increase exponentially, but more importantly its associated threshold field (TF) was reduced from 3.52 V/μm to reach a value as low as 1.95 V/μm. The analysis of the J-E emission curves in the light of the conventional Fowler-Nordheim model revealed the existence of two distinct low-field (LF) and high-field (HF) FEE regimes. In both regimes, the hierarchical structuring was found to increase significantly the associated β_LF and β_HF field enhancement factors of the h-MWCNT cathodes (by a factor of 1.7 and 2.2, respectively). Pyramidal texturing of the cathodes is believed to favor vacuum space charge effects, which could be invoked to account for the significant enhancement of the FEE, particularly in the HF regime where a β_HF as high as 6,980 was obtained for the highest AR value of 0.6.     

Observation of electron field emission patterns from B-doped diamond films

To develop electron beam sources of carbon materials, field emission patterns were observed in three different setups. The first was a diode-type, in which a carbon specimen was facing to a positively biased fluorescence plate. The second was a triode-type, in which a positively biased grid was placed between them. In the third setup, a commercial electron gun was modified so that it could accommodate a carbon specimen and a grid. A fluorescence plate was placed in a vacuum chamber outside the gun. As the carbon specimen for electron emission source, B-doped diamond films, a single crystal diamond with a B-doped layer, an undoped diamond film and a glass-like carbon both with a fibrous structure at the surface, and a sponge carbon were used. It was found that electron emission from edges was dominant for 1×1 cm diamond films and carbon specimens in the diode-type setup. In the triode-type setup, the edges of the specimens were masked with a Kapton® tape. The electron emission occurred only from some spots on the specimen. In the electron gun setup, it was confirmed that an electron beam was generated, and a fairly uniform circle was seen on the fluorescence plate under defocused situation, while the circle became smaller by adjusting the current of the focusing lens. Although more uniform emission from the electron source materials seemed to be necessary for practical applications, it was demonstrated that an electron beam could be generated even in such a simple setup.     

Secondary electron emission from CVD diamond films

Secondary electron emission from boron doped diamond polycrystalline membranes (hole concentration 5×10¹⁸ cm⁻³), prepared by microwave plasma assisted CVD, was investigated in both the reflection and transmission configurations. The model of secondary electrons behavior taking into account the distribution and diffusion mechanism of secondary electrons is proposed to explain the yield dependencies on primary electron energy in both configurations. The model predicts the SEE yield K=19 at the primary electron energy E₀ close to 1 keV for reflection configuration and K=3–7 at E₀=15–30 keV for transmission configuration for polycrystalline films used in the study. Experimental measurements of the SEE yield vs. primary electron energy (18 at E₀=950 eV for the reflection scheme and 3.5–4 at E₀=25 keV for the transmission one) are found to accord well with the theoretical results. Estimations, which were made using the model, show that SEE yield in transmission configuration can be increased up to 60 for the primary electron energy of about 10 keV. Since such high yields in transmission scheme may be obtained in monocrystalline membrane, another approach using porous polycrystalline diamond membranes is considered. Porous diamond membranes having SEE yield in transmission scheme of more than 10 at the primary electron energy E₀=1 keV were fabricated.     

Electron emission suppression from hydrogen-terminated n-type diamond

By total photoelectron yield spectroscopy (TPYS), we have found a thermal instability of a negative electron affinity in hydrogen-terminated n-type diamond films, which has never been observed for intrinsic and p-type diamond films. Experimentally, we have succeeded in detecting surface defect states and surface valence band extended states on the same n-type sample after soft annealing from 100 to 300 °C, which allowed us to evaluate the surface Fermi level and surface band bending using phosphorus doping parameters. Our results show that “quasi-positive” electron affinity prevents electron emission from an n-type diamond film even though its surface has a negative electron affinity.     

Enhanced thermionic energy conversion and thermionic emission from doped diamond films through methane exposure

Thermionic electron emitters are a crucial component in applications ranging from high power telecommunication, electron guns, space thrusters and direct thermal to electrical energy converters. One key characteristic of diamond based electron sources is the negative electron affinity (NEA) properties of hydrogen terminated surfaces which can significantly reduce the emission barrier. Nitrogen and phosphorus doped diamond films have been prepared by plasma assisted chemical vapor deposition on metallic substrates for thermionic emitter application. Electron emission current versus temperature was measured and analyzed with respect to the Richardson-Dushman relation, with work function and Richardson constant deduced from the results. Initial emission measurements up to 500 °C in vacuum were followed by emitter characterization while the sample was exposed to methane. Vacuum measurements indicated a work function of 1.18 eV and 1.44 eV for phosphorus and nitrogen doped diamond films, respectively. Introduction of methane resulted in a significant increase of the emission current which was ascribed to contribution from ionization processes which increase charge transfer from the emitter surface. This phenomenon was utilized in a thermionic energy conversion structure by introduction of methane in the inter electrode gap where a two-fold increase in output power was observed upon introduction of the gaseous species.     

Field emission of polycrystalline diamond films grown by microwave-plasma chemical vapor deposition. I. Effects of surface morphology of diamond

The effects of surface morphology on the field emission of non-doped polycrystalline diamond films with thicknesses ranging from 5 to 55 μm were studied. Diamond films grown by a microwave-plasma chemical vapor deposition technique had both the diamond and non-diamond components with pyramidal and angular crystalline structures. Although the average crystallite size increased with increasing the film thickness (d), the volume fraction of the non-diamond components in the films was insensitive to the film thickness. However, the turn-on electric field, F_T, (defined as the low-end electric field to emit electrons) showed a U-shape dependence on the film thickness. This U-shape dependence was explained by a model in which the emission current was controlled by Fowler–Norheim tunneling of electrons at surface of the pyramids when d was thinner than 20 μm and by carrier transport in the polycrystalline diamond film when d was thicker than 20 μm. The lowest field of 4 V/μm was obtained in the film with 20 μm thick.     

UV Raman characteristics of nanocrystalline diamond films with different grain size

Nanocrystalline diamond films with different size were characterized by ultraviolet (UV) (244 nm) Raman spectroscopy. It was found that a diamond peak at 1333 cm⁻¹ was enhanced, while the D and G peak of graphite as well as photoluminescence was suppressed, compared with that measured by visible (514.5 nm) Raman. With decreasing the particle size from 120 to 28 nm, the diamond peak shifts from 1332.8 to 1329.6 cm⁻¹, the line width of the peak becomes broader, the intensity ratio of diamond and G peak decreases. The down shift and broadening of the diamond peak depending on the particle size by UV Raman measurements are consistent with the phonon confinement model.     

Application of diamond film to cold cathode fluorescent lamps for LCD backlighting

A cold cathode fluorescent lamp (CCFL) is a gas discharge light source widely used for liquid crystal display (LCD) backlighting. We proposed applying diamond as a new cathode material to reduce the power consumption of the CCFL. In this work, we show stable and low (less than 50% of metal) cathode-fall voltage for a glass discharge tube.     

The effects of nitrogenation on the electrochemical properties of nanocrystalline diamond films

The effect of the nitrogenation on the electrochemical properties of nanocrystalline diamond films produced by microwave plasma CVD in CH₄–Ar–H₂–N₂ gas mixtures was studied systematically, using cyclic voltammetry and electrochemical impedance spectroscopy measurements, for the first time. Differential capacitance, kinetic parameters of reactions in [Fe(CN)₆]^3-/4-redox system and potential window were found to be sensitive to the nitrogen concentration in the process gas. With its increase (from 0 to 25%), a transition of the NCD film behavior from “poor conductor” to metal-like character takes place. The heavily N-doped nanocrystalline diamond films have satisfactory electrochemical properties to be used as electrodes.     

Secondary electron emission measurements on synthetic diamond films

During the electron irradiation of synthetic diamond films, three successive regimes are encountered as a function of the electron dose: (1) a reduction of the downward band bending of energy levels at the sample surface because an excess of secondary electrons leaves the sample; (2) the creation of an internal electric field in which secondary electrons drift to the surface, leading to an appreciable increase in the secondary emission and to a linear relation between the primary electron energy and the secondary electron yield; and (3) the desorption of hydrogen terminating the carbon surface bonds. The secondary emission thus decreases to very low values. The rate of decrease of secondary emission is similar for C:H- and C:H:Ba-terminated diamond surfaces.     

Field penetration and its contribution to field enhanced thermionic electron emission from nanocrystalline diamond films

Field emission from sulfur doped nanocrystalline diamond films is characterized by intense emission sites with nm scale diameters. Field emission measurements were obtained at room temperature and analyzed in terms of the Fowler–Nordheim expression where electron emission is due to tunneling through a diminished barrier. The electron emission versus temperature was also recorded at a series of applied fields from 0.5 to 0.8 V/μm. These results were analyzed in terms of a modified Richardson–Dushman relation which describes field dependent thermionic emission. It was found that both sets of data could be fit with a work function of 2.0 eV and a field enhancement factor of  1750. The large field enhancement could not be correlated with specific structures on the relatively flat surfaces. The field and thermionic-field emission from the sulfur doped nanocrystalline diamond films is evaluated by a model which includes barrier lowering as a result of field penetration effects.     

Thermionic emission of amorphous diamond and field emission of carbon nanotubes

Thermal-field emission characteristics from nano-tips of amorphous diamond and carbon nanotubes at various temperatures are reported in this study. Amorphous diamond emitted more than 13 times more electrons at a temperature of 300 °C than at room temperature. In contrast, CNTs exhibited no increase of emitted current upon heating to 300 °C. The thermally agitated emission of amorphous diamond is attributed to the presence of defect bands. The formation of these defect bands raises the Fermi level into the upper part of the band gap, and thus reduces the energy barrier that the electrons must tunnel through. From defect bands within the band gap, the conduction band electrons were significantly increased due to electron tunnels from defect bands. The enhanced thermal-field emission originating from defect bands was observed in this study. This thermally agitated behavior of field emission for amorphous diamond was highly reproducible as observed in this research.     

Electron emission from CVD diamond p-i-n junctions with negative electron affinity during room temperature operation

We successfully observed electron emission from hydrogenated diamond p-i-n junction diodes with negative electron affinity during room temperature operation. The emissions started when the applied bias voltage produced flat-band conditions, where the capacitance-voltage characteristics showed carrier injection in the i-layer. In this low current injection region, the electron emission efficiency (η) of the p-i-n junction diodes (p is top layer) was about 5 × 10^− 5, while that of the n-i-p diodes (n is top layer) was about 10^− 8. With increasing diode current, both diodes showed an increase in η and a nonlinear increase in emission current. In the high current injection region with high diode current of 5-50 mA, both diodes had an emission current of almost 10 μA, where η of a p-i-n junction diode was 0.18%, while that of a n-i-p junction diode was 0.02%.Note that η, which corresponds to the electron emission mechanism, depended on the diode current level.     

Microstructure of mayenite 12CaO·7Al2O3 and electron emission characteristics

Electron emission characteristic, electrical conductivity of polycrystalline mayenite (12CaO·7Al₂O₃) electride, formation of [Ca₂₄Al₂₈O₆₄]⁴⁺(e⁻)₄ framework as a function of phase content, and microstructure have been investigated. The mayenite microstructure was investigated using high-resolution transmission microscopy which revealed the type cage structure of 12CaO·7Al₂O₃ partially filled by extra-framework oxygen ions. Incorporation of electrons by means of carbon ion template 12CaO·7Al₂O₃ produces complex structure, and an incomplete ion template 12CaO·7Al₂O₃ structure consisting of mixture of a [Ca₂₄Al₂₈O₆₄]⁴⁺(e⁻)₄ and [Ca₂₄Al₂₈O₆₄]⁴⁺(O²⁻)₂ framework had a direct effect on the electron emission. Surface chemistry and stability of the 12CaO·7Al₂O₃ electride have been studied using x-ray photoelectron spectroscopy. The work function of phase pure 12CaO·7Al₂O₃ electride was determined from direct thermionic emission data and compared to the measurement from ultraviolet photoelectron spectroscopy (UPS). Depending on the extent of ion template of 12CaO·7Al₂O₃ structure, a work function of 0.9–1.2 eV and 2.1–2.4 eV has been measured and thermionic emission initiating at 600°C.     

Surface roughness of AlN films deposited on negatively biased silicon and diamond substrates

Our previous studies on AlN microstructures have shown that smooth amorphous films (a-AlN) can be grown on negatively biased Si substrates by the versatile physical vapour deposition technique under reactive magnetron sputtering. These a-AlN films are produced by energetic Ar ion bombardment under negative bias whereas those grown without bias were columnar crystallized ones (c-AlN). Here, we show first that depositing an a-AlN layer on c-AlN/Si structures by switching a suitable bias to the Si substrate can efficiently reduce their surface roughness. We then extend this smoothening method to a c-AlN/Poly-crystallized diamond (PCD) structure to reduce its high surface roughness that hampers using such structures in SAW device design. In fact, the piezoelectric c-AlN surfaces grown on rough diamond surfaces are equally rough. Effectively, the a-AlN layer deposited on the c-AlN/PCD structure brings down the latter's RMS surface roughness to one tenth of its initial RMS roughness, as confirmed here by TEM and AFM observations. The insulating property of the diamond as biased substrate doesn't impede the growth of this a-AlN layer. This smoothening method is without process interruption, where simply a negative bias is switched on to the diamond substrate once the desired piezoelectric c-AlN film thickness as monitored here by in-situ reflectometry, is attained. This as-grown smoothening method can be therefore easily and rapidly implemented and can thus replace time-consuming and costly PCD ionic and/or mechanical polishing. Hopefully, the method can be advantageously applied to c-AlN/nano-crystallized diamond structures (NCD) where the NCD films are not prepared under rigorous conditions meant to minimize their surface roughness.     

Optimal parameters to produce high quality diamond films on 3D Porous Titanium substrates

Diamond films grown on three-dimensional (3D) porous titanium substrate were obtained by hot filament chemical vapor deposition (HFCVD) technique. The growth parameters strongly influenced the film properties during this complex film formation process. The pressure inside the reactor as well as the methane concentration showed their influence on the film morphology, quality, and growth rate. The substrates were totally covered by a diamond coating including deeper planes leading to a 3D porous diamond/Ti composite material formation. The sp²/sp³ ratio as “purity index” (PI) and the “growth tendency index” (GTI), evaluated from Raman and X-ray spectra respectively, were obtained for these composite materials as a function of their growth parameters.     

Array geometry, size and spacing effects on field emission characteristics of aligned carbon nanotubes

Microwave plasma-enhanced chemical vapor deposition (MPECVD) has been shown capable of producing vertically aligned mutli-walled CNTs as a result of self-bias of the microwave plasma. These CNTs are relevant to field emission applications. However, it is also known that closely packed or mat-like CNTs are not effective field emitters due to field screening effects among neighboring tubes. In this study, an approach whereby “micro-” patterning of CNT arrays, adjusting their geometry, size and array spacing by conventional photolithography, rather than “nano-” patterning a single CNT by electron-beam lithography, is employed to fabricate efficient emitters with enhanced field emission characteristics. MPECVD with catalysts are used on Si substrate to fabricate micropatterned vertically aligned CNT arrays with various geometries, sizes and spacing. The field emission results show that a circular array with 20 μm spacing has the lowest turn-on field of 2 V/μm at 1 μA/cm² and achieves the highest current density of 100 μA/cm² at 3 V/μm. Investigation on the array spacing effect shows that 10 × 10 μm CNT square array with an array spacing of 20 μm displays the lowest turn-on field of 9 V/μm and achieved a very high current density of 100 mA/cm² at 20 V/μm. Furthermore, the results suggest that the array spacing of the 10 × 10 μm CNT square array can be reduced to at least 20 μm without affecting the field enhancement factor of the emitter. The results clearly indicate further optimization of spacing in the arrays of CNT emitters could result in lower turn-on field and higher current density.     

Tribological investigation of nanocrystalline diamond films at low load under different tribosystems

The mechanical and frictional properties of hydrogen- and oxygen-terminated nanocrystalline diamond films (NCD) grown by hot-filament chemical vapor deposition (HFCVD) have been investigated in the present work.The structure and morphology of the NCD films have been characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman-effect spectroscopy. In addition, X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS) have been used to investigate the surface chemical groups on the NCD surface. Mechanical and frictional properties are determined using atomic force microscopy (AFM), nano-indentation, nano-scratching and micro-tribometer. The friction behavior of these films in the load range of 25 to 200 mN under reciprocating sliding conditions, using steel counter-body material has been thoroughly studied.It is noted that these films are highly crystalline with nanometer size grains and contain a very high fraction of sp³ carbon bonds. They exhibit high hardness and high elastic modulus. The friction coefficient of the film is lower under unidirectional scratch with diamond indenter than the friction coefficient under low load reciprocating sliding against steel ball. Transfer of the film from the counter-body, oxidation of transfer film and mixing of transfer film with carbonaceous layer on the worn surfaces are responsible for such behavior. Although, the friction responses of H-terminated and O-terminated films are similar under unidirectional scratch with diamond indenter, the friction coefficient of O-terminated film is always higher than the friction coefficient of H-terminated film under reciprocating sliding condition against steel counter-body material.