Field emission of polycrystalline diamond films grown by microwave plasma chemical vapor deposition. II. Effect of p-type doping in diamond

The effects of boron (B) doping on the field emission (FE) of diamond films grown by a microwave plasma chemical vapor deposition technique were studied. Raman scattering spectroscopic analysis revealed that B-doping significantly suppressed formation of non-diamond components in the diamond film. The B-doped p-type diamond films had low resistivity, ranging from 0.07 to 20 Ω cm, and various volume fractions of non-diamond components in the diamond films. The turn-on electric field, F_T, was independent of the resistivity, the film thickness, and the volume fraction of the non-diamond components. The lowest F_T value of 8 V μm⁻¹ and the highest emission current of 3×10⁻² A cm⁻² were obtained in the B-doped diamond films. The high efficiency of the electron emission in the B-doped diamond films was believed to be due to the increase in volume fraction of the conductive regions in the film and the high density of emission sites on the film surface.     

Electrical characterisation of silicon pn-junctions terminated with diamond

Properties of the silicon/diamond interface are investigated in terms of leakage current measurements of silicon pn-junction diodes. Two different types of pre-treatment and deposition methods of polycrystalline diamond were investigated, microwave assisted plasma CVD (MPCVD) with bias pre-treatment and hot filament CVD (HFCVD). It was found that when the depletion region was in contact with the silicon/diamond interface the leakage current was increased. Generation at surface states, with a surface recombination velocity of about 9 × 10⁶ cm s⁻¹, was found to be the responsible mechanism in the MPCVD samples, while surface conduction was also contributing to the leakage current in the HFCVD samples. However, near ideal diode characteristics with low leakage current were obtained by introducing a thin silicon dioxide layer between the silicon and diamond.     

Surface Pretreatments of Silicon Nitride for CVD Diamond Deposition

The efficiency of different surface pretreatments (four standard chemical etchings and four diamond powder abrasive procedures) on silicon nitride (Si₃N₄) substrates for chemical vapor deposition (CVD) of diamond has been systematically investigated. Blank Si₃N₄ samples were polished with colloidal silica (∼0.25 μm). Diamond nucleation and growth runs were conducted in a microwave plasma chemical vapor deposition apparatus for 10 min and 6 h, respectively. Superior results concerning nucleation density ( N _d∼ 10¹⁰ cm⁻² after 10 min), film uniformity, and grain size (below 2 μm after 6 h) were obtained for the mechanically microflawed samples, revealing that chemical etchings (hot and cold strong acids, molten base or CF₄ plasma) are not crucial for good CVD diamond quality on Si₃N₄.     

Diamond growth by carbon ion implantation of diamond

Medium energy (5–25 keV) ¹³C⁺ ion implantation into diamond (100) to a fluence ranging from 10¹⁶ cm⁻² to 10¹⁸ cm⁻² was performed for the study of diamond growth via the approach of ion beam implantation. The samples were characterized with Rutherford backscattering/channelling spectroscopy, Raman spectroscopy, X-ray photoemission spectroscopy and Auger electron spectroscopy. Extended defects are formed in the cascade collision volume during bombardment at high temperatures. Carbon incorporation indeed induces a volume growth but the diamond (100) samples receiving a fluence of 4 × 10¹⁷ to 2 × 10¹⁸ at. cm⁻² (with a dose rate of 5 × 10¹⁵ at. cm⁻² s⁻¹ at 5 to 25 keV and 800 °C) showed no He-ion channelling. Common to these samples is that the top surface layer of a few nanometers has a substantial amount of graphite which can be removed by chemical etching. The rest of the grown layer is polycrystalline diamond with a very high density of extended defects.     

Drift mobility determination using surface-charge decay technique in polyvinylidene fluoride (PVF2)

Drift mobility of charge carriers in pure and iodine doped PVF₂, has been determined by surface-potential decay method. The mobility 8 × 10⁻¹¹cm²V⁻¹s⁻¹ and 10⁻⁸cm²V⁻¹s⁻¹ have been encountered in pure and iodine doped PVF₂ respectively. The increase in mobility with doping is attributed to the charge transfer complex formation. The existence of charge transfer complex is found from the analysis of u.v. and visible spectra of doped samples. It is concluded that charge carrier conduction in PVF₂ may be due to a hopping process.     

Hydrothermally Grown ZnO Crystals of Low and Intermediate Resistivity

Single crystals of zinc oxide in the resistivity range 10¹ to 10¹⁰ ohm-cm were prepared by hydrothermal crystallization using controlled lithium doping. Lithium concentration in the crystals ranged from 0.4 to 6 × 10¹⁸ atoms/cm³ when the LiOH concentration in the hydrothermal growth solutions ranged from 0 to 2.0 molal. The logarithm of the resistivity is nearly linear with the lithium concentration of the grown crystal. Calculations of apparent mobility show that either the donor concentration is a function of the lithium concentration during growth or not all the lithium in the grown crystals acts as acceptors. Crystals with resistivities as low as 10⁻² ohm-cm were prepared by doping with indium which acts as a donor. Crystals were grown in solutions containing NH₄⁺+ LiOH. The NH₄⁺ had a negligible effect on resistivity but it markedly increased the rate of crystallization on the (1010) (prism) face. A study of the occasionally produced anomalous crystals grown in silver-lined autoclaves showed that resistivity anomalies were not due to silver-doping of the grown crystals.     

Defective structure, oxygen mobility, oxygen storage capacity, and redox properties of RE-based (RE = Ce, Pr) solid solutions

Defective structures, surface textures, oxygen mobility, oxygen storage capacity (OSC), and redox properties of RE_0.6Zr_0.4O₂ and of RE_0.6Zr_0.4−xY_xO₂ (RE=Ce, Pr; x=0, 0.05) solid solutions have been investigated using X-ray diffraction (XRD), temperature-programmed desorption (TPD), temperature-programmed reduction (TPR), O₂−H₂ and O₂−CO titration, ¹⁸O/¹⁶O isotope exchange, CO pulsing reaction, and X-ray photoelectron spectroscopy (XPS) techniques. The effects of doping noble metal onto RE_0.6Zr_0.4−xY_xO₂ on oxygen mobility and surface oxygen activities have also been studied. Based on the experimental outcomes, we conclude that: (i) a Pr-based solid solution has better redox behavior than a Ce-based one; (ii) incorporation of yttrium ions in the lattices of CZ and PZ solid solutions could result in an enhancement in oxygen vacancy concentration, Ce⁴⁺/Ce³⁺ and Pr⁴⁺/Pr³⁺ redox properties, lattice oxygen mobility, and oxygen storage capacity; and (iii) doping the noble metal (Rh, Pt, and Pd) onto RE-based solid solution has positive effect on the properties concerned in this work.     

Diamond tip fabrication by air-plasma etching of diamond with an oxide mask

We have fabricated an array of cone-shaped diamond tips for use as a field electron emitter by air-plasma etching a polycrystalline diamond film with a silicon oxide mask. The difference in etching speed between the mask and the diamond resulted in the formation of cone-shaped diamond tips. Post-treatment with hydrogen plasma was effective in cleaning the diamond tips and increasing the surface conductivity. The emission from the diamond tips was measured with a diode configuration. The threshold field was 3 V μm⁻¹, and the emission current was 0.8 nA tip⁻¹ when the field was raised to 10 V μm⁻¹.     

Homoepitaxial growth and characterization of phosphorus-doped diamond using tertiarybutylphosphine as a doping source

Phosphorus-doped diamond was grown by plasma-enhanced chemical vapor deposition (CVD) using tertiarybutylphosphine (TBP) as doping gas. It was confirmed that phosphorus was successfully doped into homoepitaxial diamond films. The films exhibited a negative Hall coefficient throughout the temperature range, from room temperature to 673 K, indicating n-type conductivity. The best sample showed a mobility of 250 cm²/V s at room temperature and the activation energy of the carrier conduction was 0.6 eV. From observations of the bound-exciton emission in cathodoluminescence measurements, it was also shown that phosphorus atoms are located in substitutional sites of the diamond lattice as donor atoms.     

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.     

New method for selective growth of diamonds by microwave plasma chemical vapour deposition

Selective nucleation and deposition of diamonds were achieved on an SiO₂-patterned Si substrate. The substrate was pre-treated with an electric field in plasma to introduce diamond nuclei. This treatment did not affect the SiO₂ area. Consequently, diamonds grew only on the area where Si was exposed under the conventional conditions of diamond growth. The maximum nucleation density on the area of SiO₂was about 5 × 10⁷ cm⁻². The ratio of the selectivity was 2 × 10² or higher. This process will be useful and very promising for manufacturing diamond electronic devices.     

Influence of nitrogen doping on photoconductivity properties of a: DLC films

We present photoconductivity, photosensitivity and decay time of photocurrent measured as a function of temperature for both nitrogen-doped and undoped a:DLC films. The a:DLC films were deposited using radio-frequency (RF) glow discharge of methane gas (CH₄) as a source of carbon. Several films were doped employing nitrogen (N₂) as the doping gas. The doped and undoped a:DLC films have shown photoconductivity effects in a wide range of temperatures. All photoconductivity parameters, i.e. spectral response, photosensitivity, decay time and photocurrent, were measured for both undoped and doped films. The maximum spectral photosensitivity of doped films shifts to a higher energy, similar to the optical energy-gap measurements. The photocurrent of the doped film is larger by two orders of magnitude than that of undoped film, while the photosensitivity shows an opposite effect. The mobility of doped films (2.43 × 10⁻⁵) is larger by two orders of magnitude than that of undoped films (5.64×10⁻⁷) at room temperature. In order to provide nanoscale information about the morphological properties of the undoped a:DLC films surface, we have used atomic force microscopoy (AFM). It was found that the roughness of our films increased with increasing thickness of the films, from 0.3 to 1.5 μm.     

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.     

Positron lifetime investigations of diamond films

Positron lifetime investigations on B-doped and undoped chemical vapor deposition diamond films have shown that B-doping at 500 ppm almost completely removes vacancies from the film. The 1.68 eV photo luminescence (PL) line is also removed. For undoped films, large concentrations (≈10¹⁸ cm⁻³) of vacancy clusters (approximately 6 vacancies) and mono- or divacancies are observed. Small observable effects arising from annealing up to 1100°C could be found in both B-doped or undoped films. There is no correlation between the width of the 1.68 eV PL line and the vacancy concentration.     

Polypyrrole materials doped with weakly coordinating anions: influence of substituents and the fate of the doping anion during the overoxidation process

The influence of weakly coordinating anions with different shapes and substituents has been studied to get the overoxidation resistance limit of the material, ORL. The anions utilized are derivatives of [Co(C₂B₉H₁₁)₂]⁻, [B₁₂H₁₂]²⁻ and [B₁₂H₁₁NH₃]⁻. The following tendencies have been established (1) boron cluster monoanions are to date the anions that offer the highest stability to overoxidation of PPy doped materials (2) the ORL stability of the material can not be attributed only to the shape of the cluster (3) monoanionic clusters are far superior than dianionic to get an ORL rise (4) cluster charge density reduction results in ORL rise as has been observed in [Co(C₂B₉H₁₁)₂]⁻ after incorporation of electron-withdrawing substituents with no electron back-donation (5) globular, rigid and large monoanions are less suitable for enhanced ORL values than elongated and non-rigid species (6) adequate anion's substitution produce a rise in the ORL of the material, thus polyether side-arms are beneficial with [Co(C₂B₉H₁₁)₂]⁻, whereas, T-shaped methylaryl groups are appealing in [B₁₂H₁₁NH₃]⁻ based materials, respectively, (7) substituents on the anions usually imply higher difficulty in the materials' growth. The high boron contents in these materials has permitted to learn on the fate of the doping anions during the overoxidation process. There is a built-up of the concentration of the doping anion in the electrolyte near surface area, whereas, a depletion is observed in the nearest inner layers.     

Effect of crystal structure on the electrochemical behaviour of synthetic semiconductor diamond: Comparison of growth and a nucleation surfaces of a coarse-grained polycrystalline film

Comparative studies of the electrochemical behaviour of the growth and nucleation surfaces of a free-standing boron-doped polycrystalline diamond film grown in a microwave plasma CVD reactor are performed. The uncompensated acceptor concentration in diamond is determined from the electrochemical impedance (Mott–Schottky plots), uncompensated boron acceptor concentration from infrared absorption measurements, and the total boron concentration, by the SIMS method. In the diamond bulk adjacent to the nucleation surface, constituted from submicrometre-sized crystallites, both the boron concentration and the total acceptor concentration are found to be significantly higher than near the growth surface, where the film crystallinity is more perfect. This difference is tentatively attributed to the increased concentration of crystal lattice defects near the nucleation surface. These defects, in addition to boron atoms, play the role of acceptors in diamond.     

Partially stabilized zirconia substrate for chemical vapor deposition of free-standing diamond films

In this work we investigated the use of partially stabilized zirconia (PSZ) as the substrate for deposition of CVD diamond films. The polycrystalline PSZ substrates were sintered at high temperatures and the results showed that this material has unique properties which are very appropriated for the growth of free-standing diamond films. The diamond nucleation density on PSZ is high, even without seeding, and the CVD diamond film was totally released from the substrate after the deposition process, without cracking. Micro-Raman analysis revealed that the free-standing diamond film had a good crystallinity on both surfaces with practically no stress in the structure. The same PSZ substrate can be reutilized for the deposition of a large number of diamond films. The average growth rate is about 5–6 μm/h in a microwave plasma reactor at 2.5 kW. The deposition process causes the reduction of ZrO₂, producing ZrC. The high mobility of oxygen in the zirconia matrix at high temperature would probably help to etch the interface region between the substrate surface and the diamond film, decreasing the adhesion strength and eliminating some defects in the film structure related to non-diamond carbon phases.     

High-quality homoepitaxial diamond (100) films grown under high-rate growth condition

We present advantages of high-power microwave plasma chemical vapor deposition (MPCVD) in homoepitaxial diamond film deposition. Diamond films grown at comparatively high growth rate of 3.5 μm/h showed intense free-exciton recombination emission at room temperature. The free-exciton decay time of the diamond film at room temperature, 22 ns, was much longer than that of type-IIa single crystal, indicating electronically high quality of the homoepitaxial films. Dislocation-related emissions were locally observed, a part of which created by mechanical polishing process was successfully removed by surface etching process using oxygen plasma. Another advantage of the high-power MPCVD is effective impurity doping; boron-doped diamond films with high carrier mobility and high carrier concentration were reproducibly deposited. An ultraviolet photodetector fabricated using the high-quality undoped diamond film showed lower noise equivalent power as well as higher photoresponsivity for ultraviolet light with better visible-blind property, compared to those of standard Si-based photodetectors. The high-power MPCVD is, thus, indispensable technique for depositing high quality diamond films for electronic devices.     

Growth of faceted, ballas-like and nanocrystalline diamond films deposited in CH4/H2/Ar MPCVD

The influence of Ar addition to CH₄/H₂ plasma on the crystallinity, morphology and growth rate of the diamond films deposited in MPCVD was investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy. X-Ray diffraction patterns indicate that diamond films of strong (111) and weak (400) texture are produced in these samples. Faceted diamond gradually turns into ballas-like diamond with graphitic inclusions when the Ar concentration increases to above 30 vol.%, as indicated by Raman spectra. As the Ar concentration goes above 90 vol.%, nanocrystalline diamond films are formed, characterized by a 1150-cm⁻¹ peak in the Raman spectra and morphology observation. Diamond growth by CH₃ or by C₂ mechanism is proposed to interpret the change in the growth rate of diamond films with the variation of Ar content in the plasma.     

The anodic dissolution of copper in hydrochloric acid solutions

The electrodissolution of copper in hydrochloric acid solutions at the rotating ring-disk electrode was found to be controlled by both mass transfer and reaction in the apparent-Tafel region in HCl concentrations of between 0.1 and 1.0 M. The proposed mechanism describes the adsorption of CuCl_ads on the corroding copper surface and the diffusion of CuCl⁻₂ from the copper surface. The reaction in the limiting-current region was found to be controlled by the diffusion of Cl⁻ to the copper surface through a porous CuCl layer that forms on the surface. The thickness of this porous layer is dependent on the stirring conditions, and independent of the Cl⁻ concentration. Cu²⁺ is also produced at the Cu surface during electrodissolution. A mechanism describing the formation of a porous film of CuCl on the surface, the diffusion of Cl⁻ through this film and the formation of Cu²⁺ has been proposed.