Cathodoluminescence of phosphorus doped (111) homoepitaxial diamond thin films

A homoepitaxial diamond film with a thickness of approximately 1 μm was grown on the (111) surface of a type Ib diamond substrate with dopant concentration (PH₃/CH₄) of 500 ppm. Bound exciton recombination radiation due to the neutral donors has been observed in the peaks at 5.18 and 5.32 eV. The dominant peak at 5.18 eV is associated with one TO phonon. The peak at 5.32 eV is the radiation without phonon emission due to the localization of donors. This appearance of non-phonon emission is one of the important evidences to prove the two peaks at 5.18 and 5.32 eV to be the recombination radiation of bound exciton due to the donor. The binding energy of the free exciton to the donors, which is deduced from the energy difference between free and bound exciton, is 90 meV.     

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.     

Surface electronic properties on boron doped (111) CVD homoepitaxial diamond films after oxidation treatments

Surface electronic properties on oxidized boron (B) doped (111) homoepitaxial diamond films are investigated by Hall effect measurements and Schottky junction characterizations. Surface electronic properties on (111) diamond strongly depend on annealing treatments after wet-chemical oxidation, whereas for those on (001) diamond no change due to annealing can be detected. Hall effect results show that a p-type surface conductive layer (SCL) exists on (111) diamond surface in air after wet-chemical oxidation followed by annealing in Ar atmosphere (WO–AN) above 300 °C, but does not if only wet-chemical oxidation or air-oxidation is applied. This SCL disappears at annealing temperature above 350 °C in air. Schottky junction characteristics suggest that the Fermi level is unpinned at the (111) surface after WO–AN. Surface electronic characteristics on (111) diamond after WO–AN are similar to those generated by hydrogen termination.     

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.     

Thermionic electron emission from low work-function phosphorus doped diamond films

Thermionic electron emitters are a key component in applications ranging from travelling wave tubes for communications, space propulsion and direct energy conversion. As the conventional approach based on metallic emitters requires high operating temperatures the negative electron affinity (NEA) characteristic of diamond surfaces in conjunction with suitable donors would allow an electronic structure corresponding to a low effective work function. We have thus prepared phosphorus-doped polycrystalline diamond films on metallic substrates by plasma assisted chemical vapor deposition where an NEA surface characteristics was induced by exposure of the film surface to a hydrogen plasma. Thermionic electron emission measurements in an UHV environment were conducted with respect to the Richardson–Dushman relation observing an emission current at temperatures < 375 °C. Measurements were terminated at 765 °C without significant reduction in the electron emission current indicating a stable hydrogen passivation of the diamond surface. A fit of the emission data to the Richardson equation allowed for the extraction of emission parameters where the value of the materials work function was evaluated to 0.9 eV. This value could well be the lowest measured work function of any known material.     

Characteristics of homoepitaxial heavily boron-doped diamond films from their Raman spectra

As the boron incorporation level and the wavelength of the exciting laser were varied, we observed systematic modifications of the Raman spectra of homoepitaxial diamond films. A pronounced change in the lineshape of the zone-center optical Raman peak as well as a wide and structured signal at lower wavenumbers appeared simultaneously when the boron incorporation was increased above an abrupt threshold around 3×10²⁰ cm⁻¹. This threshold was found to depend on the excitation laser wavelength. Possible origins for the wide peaks at 500 and 1225 cm⁻¹ are also discussed.     

Epitaxial growth of phosphorus doped diamond on {111} substrate

A set of phosphorus doped CVD diamond films with various doping levels were grown epitaxially on {111} surfaces of synthetic type Ib diamond crystals and analysed by infrared absorption spectroscopy. Infrared absorption spectra clearly indicated the presence of neutral substitutional phosphorus in the samples. Two peaks located at 523 and 562 meV, corresponding to the electronic transitions from the phosphorus ground level 1S to the first and second excited states 2P₀ and 2P_+/− respectively, are present in the infrared spectra. Resistivity measurements were also performed to calculate the activation energy. All results (infrared spectra, activation energy, hopping conduction and photoconductivity) seem to indicate an n-type doping of the films. However, due to the high resistivity it was not possible to carry out Hall measurements.     

Hydrogen incorporation control in high quality homoepitaxial diamond (111) growth

The effect of growth conditions (temperature, microwave power, and pressure) on the growth rate and hydrogen concentration in the (111) homoepitaxial diamond grown by temperature-controlled microwave-assisted plasma chemical vapor deposition is systematically studied by secondary ion mass spectrometry. The most effective parameter in the hydrogen incorporation is the substrate temperature. The growth rate and hydrogen concentration are found to increase with elevated substrate temperature. The increase in microwave power enhances the growth rate and suppresses hydrogen incorporation. The effect of pressure rise below 50 Torr is similar to that of microwave power. The effect of microwave power and pressure on the decrease of hydrogen incorporation can be explained as a result of the hydrogen abstraction from the growing surface by atomic hydrogen in the gas phase.     

Atomically flat diamond (111) surface formation by homoepitaxial lateral growth

A process of homoepitaxial growth of diamond (111) films by microwave plasma-enhanced chemical vapor deposition has been investigated characterizing areas by ex-situ atomic force microscopy. The evolution of surface morphology during a lateral growth of (111) diamond was visualized utilizing a mesa structure as a marker. Lateral growth forms atomically flat surfaces, which show atomically flat terraces over several hundred nm widths and single bilayer steps of (111) diamond.     

Effect of boron on the superconducting transition of heavily doped diamond

We have performed electronic structure calculations by means of density functional theory combining plane-wave method with cluster method for the heavily boron-doped diamond. The results for an isolated boron center with different doping levels indicate an acceptor energy level apart from the top of the valence band, and the results for the heavily boron-doped diamond, considering the interaction of boron centers, show the impurity levels mixed with valence band edge and the Fermi level located in the valence band. Consequently, the results support a mechanism of the type of metal–superconductor transition and indicate the critical temperature T_c should be related to the B concentration.     

High-pressure and high-temperature annealing effects on CVD homoepitaxial diamond films

High-pressure and high-temperature (HPHT) annealing effects on the chemical vapor-deposited (CVD) homoepitaxial diamond films were investigated. By the HPHT annealing, the intensity of free-exciton (FE)-related emission was increased by  2 times and the luminescence bands from 270 to 320 nm, which originate from 5RL and 2BD bands, were almost completely eliminated in the cathodoluminescence (CL) spectrum. The CL intensity of band-A emission, which is related to crystal defects in diamond, was also decreased. The hole mobility at room temperature was increased from 826 to 1030 cm²/Vs by HPHT annealing. These results suggest that HPHT annealing decreases the crystalline defects and improves the optical and electronic properties of homoepitaxial diamond films.     

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.     

Growth of homoepitaxial diamond doped with nitrogen for electron emitter

Although we had reported the remarkable low threshold emission from polycrystalline diamond heavily doped with nitrogen (N) [Nature 381 (1996) 140], the problems caused by polycrystallinity still remain for understanding the electron emission mechanism. This paper describes the growth of N-doped homoepitaxial diamond film {100}, {111} and {110}, and their electron emission properties. N-doped homoepitaxial diamond is grown on synthetic diamond by hot filament chemical vapor deposition. Urea [(NH₂)₂CO] is used as a dopant for N. Atomic force microscope (AFM) observations indicate that the relatively smooth surface morphologies are obtained for all the films. The epitaxial growth of all the film is confirmed using reflective high energy electron diffraction (RHEED) patterns. Reflective electron energy loss spectra (REELS) indicate that the very surfaces of {100} and {111} are diamond while {110} is graphite rather than diamond. Raman spectra suggest that the bulk of the obtained films are diamond. The resistivities of the films are found to be much higher than the detection limit of the system. The relatively low threshold emission was observed even from the smooth surface and the threshold voltage is confirmed to depend on the crystal orientation. It is speculated from the film characterizations and the electron emission properties that the low threshold emission is due to high resistance rather than rough surface and/or grain boundaries.     

Structural and electrical properties of nanocrystalline diamond (NCD) heavily doped by nitrogen

Gas mixtures containing up to 40% nitrogen by volume and 1% CH₄ with the balance being argon have been used for the deposition of nitrogen doped nanocrystalline diamond (NCD) films by means of microwave plasma enhanced chemical vapour deposition (MPECVD). The CVD plasma was monitored by optical emission spectroscopy to reveal the plasma species, e.g., CN molecules, as a function of the nitrogen additive. Structural properties of the deposited NCD films were studied by FESEM and Raman spectroscopy. Effects of nitrogen doping on the electrical resistivity and electron field emission characteristics of the NCD films were measured. In this work, correlation between the structural and electrical properties of NCD films and the nitrogen additive to the CVD plasma will be presented and discussed.     

Heavy phosphorus doping by epitaxial growth on the (111) diamond surface

Semiconducting n-type diamond can be fabricated using phosphorus as a substitutional donor dopant. The dopant activation energy level at 0.58 eV is deep. At high dopant concentrations of 10²⁰ cm^− 3 the activation energy reduces to less than 0.05 eV. Phosphorus doping at concentrations of 10²⁰ cm^− 3 or higher has been achieved with epitaxial growth on the (111) diamond crystallographic surface. In this work epitaxial growth of diamond with high phosphorus concentrations exceeding 10²⁰ cm^− 3 is performed using a microwave plasma-assisted chemical vapor deposition process with process conditions that include a pressure of 160 Torr. This pressure is higher than previous phosphorus doping reports of (111) surface diamond growth. The other growth conditions include a feedgas mixture of 0.25% methane and 500 ppm phosphine in hydrogen, and a substrate temperature of 950–1000 °C. The measured growth rate was 1.25 μm/h. The room temperature resistivity of the heavily phosphorus doped diamond was 120–150 Ω-cm and the activation energy was 0.027 eV.     

Electron emission mechanism of hydrogenated natural type IIb diamond (111)

Clarification of the electron emission mechanism of diamond has been one challenging topic in the field of vacuum nanoelectronics. Electric field of less than 5 V/µm is enough to extract electrons from diamond, which is orders of magnitude lower than the value required for electron emission from metal emitter in general. There have been number of studies on the clarification of electron emission mechanism, however, unified model has not been proposed. The difficulty largely lies in determining the origin of emitted electrons. In this study, we succeeded in clarifying the electron emission mechanism of hydrogenated natural type IIb diamond (111) surface by using combined x-ray photoemission spectroscopy/ultraviolet photoemission spectroscopy/field emission spectroscopy (XPS/UPS/FES) system. Obtained spectra clearly defined the origin of emitted electrons at the valence band maximum (VBM). As applied voltage was increased, the number of emitted electrons increased, however, the origin remained at VBM independent of the applied voltage.     

Electrical properties of B-doped homoepitaxial diamond (001) film

Relationship between growth condition and quality of homoepitaxially grown B-doped diamond (001) film has been studied using physical measurements of defect density as a function of doping concentration. In particular, electrical properties of the homoepitaxial diamond film were characterized using measurements of conductivity, carrier concentration and mobility. The highest mobility is found to be about 1000 cm²V⁻¹s⁻¹ at 293 K, indicating that the quality of the CVD diamond film is further improved through optimizing the growth condition. The density of the compensation donor was determined from the temperature-dependent hole concentration. The lowest donor density is found to be 8.4 × 10¹⁵ cm⁻³ in the present work. This is an order of magnitude greater than the lowest value measured in natural IIb diamond. Furthermore, it is also found that the donor density increases with increasing doping concentration during the growth. On the other hand, the mobility decreases rapidly with increasing doping concentration. From these results, we speculate that the compensation donor is an origin of an additional scattering center in diamond, and excessive B-doping makes the quality of the CVD diamond worse.     

Substrate temperature effects on the electron field emission properties of nitrogen doped ultra-nanocrystalline diamond

For the purpose of improving the electron field emission properties of ultra-nanocrystalline diamond (UNCD) films, nitrogen species were doped into UNCD films by microwave plasma chemical vapor deposition (MPCVD) process at high substrate temperature ranging from 600° to 830 °C, using 10% N₂ in Ar/CH₄ plasma. Secondary ion mass spectrometer (SIMS) analysis indicates that the specimens contain almost the same amount of nitrogen, regardless of the substrate temperature. But the electrical conductivity increased nearly 2 orders of magnitude, from 1 to 90 cm^− 1 Ω^− 1, when the substrate temperature increased from 600° to 830 °C. The electron field emission properties of the films were also pronouncedly improved, that is, the turn-on field decreased from 20 V/μm to 10 V/μm and the electron field emission current density increased from less than 0.05 mA/cm² to 15 mA/cm². The possible mechanism is presumed to be that the nitrogen incorporated in UNCD films are residing at grain boundary regions, converting sp³-bonded carbons into sp²-bonded ones. The nitrogen ions inject electrons into the grain boundary carbons, increasing the electrical conductivity of the grain boundary regions, which improves the efficiency for electron transport from the substrate to the emission sites, the diamond grains.     

Electrochemical preconditioning of moderately boron doped diamond electrodes: Effect of annealing

This paper is concerned with the study of electrochemical preconditioning on moderately boron doped diamond electrodes. Samples were submitted to an isothermal annealing at 1100 °C in order to outgas the hydrogen introduced into the layer during the deposition process. Consequences of anodic and cathodic galvanostatic steps (1 C cm^− 2), in H₂SO₄ 0.5 M, have been studied on both as-deposited samples and annealed ones, by capacitance measurements, cyclic voltammetry in presence of Ce^4+/3+ redox system and XPS measurements. The results of Mott–Schottky plots and current voltage curves show that the electrochemical responses of BDD electrodes are strongly influence by annealing. After preconditioning, an enhancement of charge transfer is observed for as-deposited samples, while a more and more passivated behavior is recorded for annealed electrodes. On as-deposited samples a “new” superficial conductive layer linked to the creation of surface defects high above the valence band, is suggested after a specific electrochemical treatment which is not possible on annealed ones.     

Measurement of field emission from nitrogen-doped diamond films

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 sp²-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.