Thermionic electron emission from nitrogen-doped homoepitaxial diamond

Nitrogen-doped homoepitaxial diamond films were synthesized for application as low-temperature thermionic electron emitters. Thermionic electron emission measurements were conducted where the emission current was recorded as a function of emitter temperature. At a temperature < 600 °C an emission current was detected which increased with temperature, and the emission current density was about 1.2 mA/cm² at 740 °C. The electron emission was imaged with photoelectron emission microscopy (PEEM) and thermionic field-emission electron microscopy (T-FEEM). The image displayed uniform electron emission over the whole surface area. Thermionic emission and ultraviolet photoemission spectroscopy were employed to determine the temperature dependent electron emission energy distribution from the nitrogen-doped homoepitaxial diamond films. The photoemission spectra indicated an effective work function of 2.4 eV at 550 °C. These values indicate reduced band bending and establish the potential for efficient electron emission devices based on nitrogen-doped homoepitaxial diamond.     

Light-assisted adsorption processes in nanocrystalline diamond membranes studied by femtosecond laser spectroscopy

We report on femtosecond photoluminescence spectroscopy of nanocrystalline diamond membranes (thickness ～ 1000 nm) prepared by microwave plasma enhanced chemical vapour deposition (CVD) technique. The decay of photoluminescence excited by the blue femtosecond light pulses (405 nm) reflects the photoexcited charge carrier dynamics in the sub-band gap energy states. The photoluminescence is strongly influenced by ambient conditions and by the laser irradiation (405 nm, 70 fs pulses). Under lower ambient air pressure (5–300 Pa) the photoluminescence intensity increases and the photoluminescence decay gets faster. For higher air pressures (> 600 Pa) the photoluminescence intensity decreases and the photoluminescence decay rates do not evolve. We interpret the observed different behaviour of the photoluminescence in the two air pressure intervals in terms of a thin water layer condensed on the surface at higher air pressures. Due to a low coverage of the sample surface by water molecules under low pressure the air species can be adsorbed to NCD and influence the sub-band gap energy states.     

Mechanism of diamond nucleation enhancement by electron emission via hot filament chemical vapor deposition

The mechanism of diamond nucleation enhancement by electron emission in the hot filament chemical vapor deposition process has been investigated by scanning electron microscopy, Raman spectroscopy and infrared (IR) absorption spectroscopy. The maximum value of the nucleation density was found to be 10¹¹ cm⁻² with a −300 V and 250 mA bias. The electron emission from the diamond coating on the electrode excites a plasma, and greatly increases the chemical species, as we have seen by in situ IR absorption. The experimental studies showed that the diamond and chemical species were transported and scattered from the diamond coating on the electrode and through the plasma towards the substrate surface, where they caused enhanced nucleation.     

Highly and heavily boron doped diamond films

From the high ionization energy E_i = 0.368 eV and high solid solubility ≥ 1.4 × 10²² cm^− 3 of boron in diamond, metallic conductivity is expected on the boron impurity band within the band gap (Mott model). On the contrary, the numerical models used to describe the superconductivity of metallic diamond mainly use the Bardeen model with a Fermi level within the valence band. Taking into account the decrease to zero of E_i through the high [B] range and the band gap narrowing through the high and heavy [B] ranges, both specific of the Bardeen model, we discuss the validity of the Mott and Bardeen models from the literature and cathodoluminescence and Raman experiments. They agree with the Mott rather than the Bardeen model. Several experiments independently show the coupling of boron related levels with the zone centre optical phonons which soften for heavy [B]. The Mott model might explain the similar range of the superconductivity temperature of homoepitaxial and polycrystalline films from the similarity of their boron impurity band.     

Interface and interlayer barrier effects on photo-induced electron emission from low work function diamond films

Nitrogen-doped diamond has been under investigation for its low effective work function, which is due to the negative electron affinity (NEA) produced after surface hydrogen termination. Diamond films grown by chemical vapor deposition (CVD) have been reported to exhibit visible light induced electron emission and low temperature thermionic emission. The physical mechanism and material-related properties that enable this combination of electron emission are the focus of this research. In this work the electron emission spectra of nitrogen-doped, hydrogen-terminated diamond films are measured, at elevated temperatures, with wavelength selected illumination from 340 nm to 450 nm. Through analysis of the spectroscopy results, we argue that for nitrogen-doped diamond films on metallic substrates, photo-induced electron generation at visible wavelengths involves both the ultra-nanocrystalline diamond and the interface between the diamond film and metal substrate. Moreover, the results suggest that the quality of the metal–diamond interface can substantially impact the threshold of the sub-bandgap photo-induced emission.     

Effect of H2/Ar plasma on growth behavior of ultra-nanocrystalline diamond films: The TEM study

Incorporation of H₂ species into Ar plasma was observed to markedly alter the microstructure of diamond films. TEM examinations indicate that, while the Ar/CH₄ plasma produced the ultrananocrystalline diamond films with equi-axed grains (~ 5 nm), the addition of 20% H₂ in Ar resulted in grains with dendrite geometry and the incorporation of 80% H₂ in Ar led to micro-crystalline diamond with faceted grains (~ 800 nm). Optical emission spectroscopy suggests that small percentage of H₂-species (< 20%) in the plasma leads to partially etching of hydrocarbons adhered onto the diamond clusters, such that the C₂-species attach to diamond surface anisotropically, forming diamond flakes, which evolve into dendrite geometry. In contrast, high percentage of H₂-species in the plasma (80%) can efficiently etch away the hydrocarbons adhered onto the diamond clusters, such that the C₂-species can attach to diamond surface isotropically, resulting in large diamond grains with faceted geometry. The field needed to turn on the electron field emission for diamond films increases from E₀ = 22.1 V/μm (J_e = 0.48 mA/cm² at 50 V/μm applied field) for 0% H₂ samples to E₀ = 78.2 V/μm (J_e < 0.01 mA/cm² at 210 V/μm applied field) for 80% H₂ samples, as the grains grow, decreasing the proportion of grain boundaries.     

High resolution electron energy loss spectroscopy of hydrogenated polycrystalline diamond: Assignment of peaks through modifications induced by isotopic exchange

In this work we unambiguously determine the origin of the different peaks which appear in the High Resolution Electron Energy Loss Spectrum (HREELS) of hydrogenated polycrystalline diamond films for an incident electron energy of 5 eV and loss energies extending to 700 meV. High quality diamond films deposited by hot filament chemical vapor deposition from various isotopic gas mixtures: ¹²CH₄ + H₂, ¹²CD₄ + D₂, ¹²CH₄ + D₂, ¹²CD₄ + D₂, ¹³CH₄ + H₂ were characterized. The different vibrational modes, fundamentals and overtones, were directly identified through the modifications of the HREEL spectra induced by the isotopic exchange of H by D and ¹²C by ¹³C Three types of peaks were identified: (1) pure C–C related peaks (a diamond optical phonon at ∼ 155 meV and its overtones at 300, 450 and 600 meV), (2) pure C–H related peaks (C–H bend at ∼ 150 meV and C–H stretch of sp³ carbon at 360 meV), (3) coupling of C–H and C–C peaks (510 meV peak due to coupling of the C–H stretch at 360 meV with either the C–C stretch or the C–H bend at ∼ 155 meV). The overtones at 300, 450 and 600 meV (associated with electron scattering at diamond optical phonons) indicate a well defined hydrogenated diamond surface since they are absent in the HREEL spectrum of low energy ion beam damaged diamond surface.     

Nucleation and growth surface conductivity of H-terminated diamond films prepared by DC arc jet CVD

High-quality polycrystalline diamond film has been extremely attractive to many researchers, since the maximum transition frequency (f_T) and the maximum frequency of oscillation (f_max) of polycrystalline diamond electronic devices are comparable to those of single crystalline diamond devices. Besides large deposition area, DC arc jet CVD diamond films with high deposition rate and high quality are one choice for electronic device industrialization. Four inch free-standing diamond films were obtained by DC arc jet CVD using gas recycling mode with deposition rate of 14 μm/h. After treatment in hydrogen plasma under the same conditions for both the nucleation and growth sides, the conductivity difference between them was analyzed and clarified by characterizing the grain size, surface profile, crystalline quality and impurity content. The roughness of growth surface with the grain size about 400 nm increased from 0.869 nm to 8.406 nm after hydrogen plasma etching. As for the nucleation surface, the grain size was about 100 nm and the roughness increased from 0.31 nm to 3.739 nm. The XPS results showed that H-termination had been formed and energy band bent upwards. The nucleation and growth surfaces displayed the same magnitude of square resistance (R_s). The mobility and the sheet carrier concentration of the nucleation surface were 0.898 cm/V s and 10¹³/cm² order of magnitude, respectively; while for growth surface, they were 20.2 cm/V s and 9.97 × 10¹¹/cm², respectively. The small grain size and much non-diamond carbon at grain boundary resulted in lower carrier mobility on the nucleation surface. The high concentration of impurity nitrogen may explain the low sheet carrier concentration on the growth surface. The maximum drain current density and the maximum transconductance (g_m) for MESFET with gate length L_G of 2 μm on H-terminated diamond growth surface was 22.5 mA/mm and 4 mS/mm, respectively. The device performance can be further improved by using diamond films with larger grains and optimizing device fabrication techniques.     

Deposition and characterization of nanocrystalline diamond films on Co-cemented tungsten carbide inserts

Nanocrystalline diamond films were deposited on Co-cemented tungsten carbides using bias-enhanced hot filament CVD system with a mixture of acetone, H₂ and Ar as the reactant gas. The effect of Ar concentration on the grain size of diamond films and diamond orientation was investigated. Nanocrystalline diamond films were characterized with field emission scan electron microscopy (FE-SEM), Atomic force microscopy (AFM), Raman spectroscopy and X-ray diffraction spectroscopy (XRD). Rockwell C indentation tests were conducted to evaluate the adhesion between diamond films and the substrates. The results demonstrated that when the Ar concentration was 90%, the diamond films exhibited rounded fine grains with an average grain size of approximately 60–80 nm. The Raman spectra showed broadened carbon peaks at 1350 cm^− 1 and 1580 cm^− 1 assigned to D and G bands and an intense broad Raman band near 1140 cm^− 1 attributed to trans-polyacetylene, which confirmed the presence of the nanocrystalline diamond phase. The full width at half maximum of the <111> diamond peak (0.8°) was far broader than that of conventional diamond film (0.28°–0.3°). The Ra and RMS surface roughness of the nanocrystalline diamond film were measured to be approximately 202 nm and 280 nm with 4 mm scanning length, respectively. The Ar concentration in the reactant gases played an important role in the control of grain size and surface roughness of the diamond films. Nanocrystalline diamond-coated cemented tungsten carbides with very smooth surface have excellent characteristics, which made them a promising material for the development of high performance cutting tools and wear resistance components.     

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 μm², had estimated electron emission density greater than 4.2 × 10⁵ A/cm². The effective emission area obtained from the Fowler–Nordheim plot was several 10^? 13 cm². For adopting and emission area of 1 × 10^? 12 cm², the estimated electron emission density was higher than 4.2 × 10⁹ A/cm². 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.     

Thermionic emission from phosphorus (P) doped diamond nanocrystals supported by conical carbon nanotubes and ultraviolet photoelectron spectroscopy study of P-doped diamond films

Materials with low work function values (< 2 eV) are highly in demand for low temperature thermionic electron emission, which is a key phenomenon for waste heat recovery applications. Here we present the work function reduction of phosphorus (P) doped (i) diamond nanocrystals grown on conical carbon nanotubes (CCNTs) and (ii) diamond films grown on silicon substrates. Thermionic emission measurements from phosphorus doped diamond crystals on CCNTs resulted in a work function value of 2.23 eV. The CCNTs provide the conducting backbone for the P-doped diamond nanocrystals and the reduced work-function is interpreted as due to the presence of midband-gap state and no evidence for negative electron affinity was seen. However, ultraviolet photoelectron spectroscopy studies on phosphorus doped diamond films yielded a work function value of ~ 1.8 eV with a negative electron affinity (NEA) value of 1.2 eV. Detailed band diagrams are presented to support the observed values for both cases.     

Photoelectron emission from heavily B-doped homoepitaxial diamond films

We discuss the energy band structure near the valence band maximum based on photoemission yield spectroscopy experiments using a hydrogen-terminated heavily boron-doped homoepitaxial diamond film with concentration of 3 × 10²⁰ cm^− 3. The experimental results showed a metallic photoemission behavior with a negative electron affinity surface. Based on the fitting as metallic photoemission behavior with a Fowler plot, the Fermi level should be at 5.35 eV below the conduction band minimum, which means that the Fermi level lies at 0.12 eV (5.47–5.35 eV) above the valence band maximum. Thus the film shows metallic conduction by the Mott transition, but not as degenerate semiconductor.     

Layer-by-Layer assembly and redox properties of undoped HPHT diamond particles

The surface properties of undoped diamond particles are investigated by a combination of zeta potential measurements in solution and electrochemical studies in thin layer assemblies. High-Pressure High-Temperature (HPHT) 500 nm diamond particles exhibit positive and negative zeta potentials depending on pH. The estimated point of zero zeta potential (pzzp) was 6.6, while mobility measurements provided an average charge per particle of ?(843 ± 31)e at high pH. The charge indicates that approximately 50 ppm of surface atoms involves ionisable impurities. The positive charge measured at low pH is of similar magnitude and could be related to nitrogen impurities. The surface charge in basic solutions allows the electrostatic adsorption of diamond particles on poly(diallyldimethylammonium chloride) (PDADMAC) modified In-doped SnO₂ electrodes (ITO). The particle number density shows a strong dependence on pH, with a maximum value of (1.7 ± 0.3) × 10⁸ cm^?2. Electrochemical studies carried out in the absence of redox species in solution revealed signals associated with sp² type surface states. Analysis of electrochemical responses concluded that 1 × 10⁴ redox centres per particle are involved in a single electron transfer process. We demonstrate that this simple yet versatile approach is rather sensitive to the extent of sp² hybridisation at the surface of diamond powders.     

Cathodoluminescence of boron-doped heteroepitaxial diamond films on platinum

Cathodoluminescence (CL) spectra of diamond films epitaxially grown on single crystal platinum (111) have been investigated at room temperature and 89 K. It was found that the CL spectra of the heavily boron-doped (>3×10²⁰ cm⁻³) diamond films of more than 16 μm thickness consist only of a near-edge emission at 248±1 nm (5.00±0.02 eV), while any other emissions are absent. It was also found that the temperature dependence of the 248 nm band is very unusual, since its intensity increases as temperature increases. This result is in strong contrast to CL intensities of both the free exciton and the bound exciton, which decrease significantly with temperature. It is concluded that a new electronic band due to heavily-doped boron is the origin of the 248 nm emission.     

Estimating band gap of amorphous diamond and nanocrystalline diamond powder by electron energy loss spectroscopy

Electronic properties such as band gap and density of states were estimated by electron energy loss spectroscopy for amorphous diamond synthesized from C₆₀ fullerene by shock compression. The imaginary part of the dielectric function, ε₂, obtained showed that the magnitude of the gap was 3.5 to 4.5 eV, a little smaller than that of crystalline diamond (5.5 eV), and that excitation of interband transition was not observed at X and L points but only at Γ points. The density of states around the gap was rather broad. These characteristic electronic properties observed can be explained by the unique atomic configuration of this material examined by radial distribution function (RDF) analysis.     

Iron-mediated growth of epitaxial graphene on SiC and diamond

Ordered graphene films have been fabricated on Fe-treated SiC and diamond surfaces using the catalytic conversion of sp³ to sp² carbon. In comparison with the bare SiC (0 0 0 1) surface, the graphitization temperature is reduced from over 1000 °C to 600 °C and for diamond (1 1 1), this new approach enables epitaxial graphene to be grown on this surface for the first time. For both substrates, a key development is the in situ monitoring of the entire fabrication process using real-time electron spectroscopy that provides the necessary precision for the production of films of controlled thickness. The quality of the graphene/graphite layers has been verified using angle-resolved photoelectron spectroscopy, scanning tunneling microscopy and low energy electron diffraction. Graphene is only formed on treated regions of the surface and so this offers a method for fabricating and patterning graphene structures on SiC and diamond in the solid-state at industrially realistic temperatures.     

In situ reactivation of low-temperature thermionic electron emission from nitrogen doped diamond films by hydrogen exposure

Nitrogen doped, hydrogen terminated diamond films have shown a work function of less than 1.5 eV and thermionic electron emission (TE) has been detected at temperatures less than 500 °C. However, ambient exposure or extended operation leads to a deterioration of the emission properties. In this study thermionic electron emission has been evaluated for as-received surfaces and for surfaces after 18 months of ambient exposure. The initial TE current density of the freshly deposited diamond film was ~ 5 × 10^− 5 A/cm² at 500 °C. In contrast, the initial TE current density of a film aged for 18 months was ~ 1.8 × 10^− 9 A/cm² at 500 °C. The decreased emission current density is presumed to be a consequence of oxidation, surface adsorption of contaminants and hydrogen depletion from the surface layer. In situ reactivation of the aged film surface was achieved by introducing hydrogen at a pressure of 1.3 × 10^− 4 mbar and using a hot filament of a nearby ionization gauge to generate atomic and/or excited molecular hydrogen. After 2 h of exposure with the sample at 500 °C, the surface exhibited a stable emission current density of ~ 2.3 × 10^− 6 A/cm² (an increase by a factor of ~ 1300). To elucidate the reactivation process thermionic electron energy distribution (TEED) and XPS core level spectra were measured during in situ hydrogen exposure at 5 × 10^− 8 mbar. During the isothermal exposure it was determined that atomic or excited hydrogen resulted in a much greater increase of the TE in comparison to exposure to molecular hydrogen. During exposure at 400 °C the surface oxygen was substantially reduced, the TEED cut-off energy, which indicates the effective work function, decreased by ~ 200 meV, and the TE intensity increased by a factor of ~ 100. The increase in thermionic emission with hydrogen was ascribed to the reactivation of the surface through the formation of a uniform surface dipole layer and a reduction of the surface work function.     

Band-gap tuning of monolayer graphene by oxygen adsorption

We have performed high-resolution angle-resolved photoemission spectroscopy of oxygen-adsorbed monolayer graphene grown on 6H–SiC(0 0 0 1). We found that the energy gap between the π and π¹ bands gradually increases with oxygen adsorption to as high as 0.45 eV at the 2000 L oxygen exposure. A systematic shrinkage of the π¹ electron Fermi surface was also observed. The present result strongly suggests that the oxidization is a useful technique to create and control the band gap in monolayer graphene.     

Nanodiamod lateral device field emission diode fabricated by electron beam lithography

Nitrogen incorporated nanodiamond film is known to aid in promoting enhanced electron emission via the induced graphitic behavior both in the bulk material and also the surface of the film. Since electron emission current is inversely proportional to the cathode to anode inter-electrode distance; it is necessary to implement electron beam lithography (EBL) to obtain a small emission gap. To achieve high resolution from EBL, a thinner nanodiamond film is required. In this work, we fabricated lateral field emitters on a 0.65 µm nanodiamond film. The nanodiamond film was deposited onto a silicon-on-insulator (SOI) substrate in CH₄/H₂/N₂ plasma ambient by microwave chemical vapor deposition. The SOI was prepared for diamond nucleation using mechanical abrasion and ultrasonication in nanodiamond powder. Electron beam lithography (EBL) was used to delineate a 10 emitter tipped diode with a 2 µm anode-to- anode emission gap.     

Growth of diamond by DC Arcjet Plasma CVD: From nano-sized poly-crystal films to millimeter-sized single crystal grain

A 30 kW-powered DC Arcjet Plasma enhanced chemical-vapor deposition (CVD) system was applied to grow diamonds which included the nano-crystal free-standing film, the nano-/micro-crystal layered free-standing film, the gradient micro-crystal free-standing film and the millimeter-sized grain. The free-standing film quality, such as the roughness, the sp² content, the residual stress and the grain morphology, was studied by an atomic force microscope (AFM), Raman spectra, a scanning electron microscope (SEM) and a high resolution electron microscope (HREM). In large-sized grain deposition, as-grown deposit was obtained about 1 × 1 × 1 mm³ in size under the condition of 10 μm/h of the substrate moving speed without Nitrogen enhancement. Characterized by Raman spectra and Laue back reflection X-ray diffraction, the deposit was proven to be single crystal diamond with small grains coving its surfaces. The growth rate was about 30 μm/h. Optical emission spectrum (OES) was utilized to characterize gas phases in the plasma for diamond deposition. The mean electron temperature (Te) in the plasma was calculated based on the value of the emission intensity ratio of I_Hγ/I_Hβ. Te varied from 0.33 eV to 0.5 eV depending on the concentration of CH₄ in H₂ from 1.0% to 25%. C₂ radical was found to be the dominant carbon source compared with CH radical. The influence of the radical on the morphology of diamond was discussed. It was found that the nano-crystal could be grown when the ratio of the emission intensity, I_C2/I_CH, was larger than 8.