A.T. Rakhimov

Electron field emission for ultrananocrystalline diamond films

Ultrananocrystalline diamond (UNCD) films 0.1–2.4 μm thick were conformally deposited on sharp single Si microtip emitters, using microwave CH4–Ar plasma-enhanced chemical vapor deposition in combination with a dielectrophoretic seeding process. Field-emission studies exhibited stable, extremely high (60–100 μA/tip) emission current, with little variation in threshold fields as a function of film thickness or Si tip radius. The electron emission properties of high aspect ratio Si microtips, coated with diamond using the hot filament chemical vapor deposition (HFCVD) process were found to be very different from those of the UNCD-coated tips. For the HFCVD process, there is a strong dependence of the emission threshold on both the diamond coating thickness and Si tip radius. Quantum photoyield measurements of the UNCD films revealed that these films have an enhanced density of states within the bulk diamond band gap that is correlated with a reduction in the threshold field for electron emission. In additio...

The mechanism of low-k SiOCH film modification by oxygen atoms

The interaction of oxygen atoms with three types of plasma enhanced chemical vapor deposition low-k SiOCH films is studied. The samples were treated by O atoms in the far plasma afterglow conditions in a special experimental system designed for this study. The experimental system allowed avoiding the effect of ions and vacuum ultraviolet (VUV) photons on surface reactions and controlling the oxygen atom concentration over the samples. Fourier-transform infrared spectroscopy, x-ray fluorescence, and atomic force microscopy techniques were used to analyze the changes occurring in low-k films. Monte Carlo model for O atom interaction with low-k material that includes penetration, recombination, and reactions with methyl groups was developed. It is shown that the surface recombination on the pore wall surface determines the profile and penetration depth of O atoms into the films. The reaction of O atoms with methyl groups has lower probability and therefore proceeds in the background mode.

Two-dimensional simulation of a hot-filament chemical vapor deposition reactor

Abstract A two-dimensional model of a hot-filament (HF) CVD reactor has been developed to study the gas-phase and surface processes of diamond growth. Full transport equations were solved numerically to calculate the gas temperature, fluid flow, and species concentration fields. Catalytic chemistry at the filament surface was considered. The distribution of the hydrogen atom concentration and the gas temperature in an HFCVD reactor were obtained analytically. The expressions for diamond growth rate and the hydrogen atom destruction coefficient at the substrate were derived from the surface kinetics. The gas-phase reaction mechanism was obtained using a brute force sensitivity analysis. The calculated results were compared with existing experimental data.

Nanocrystalline graphite: Promising material for high current field emission cathodes

Electron field emission properties of nanocrystalline graphite (NCG) films, grown by plasma enhanced chemical vapor deposition method on conductive Si substrates without using of any catalyst, were investigated. Current-voltage characteristics were measured in pulse-periodic regime. It was shown that grown NCG films can operate at field emission current density up to 10 A/cm2. It was found that NCG films contain, along with the normally oriented to the substrate nanoflakes, carbon whiskers consisted of graphene nanoribbons and nanowires with length considerably higher than of the nanoflakes.Electron field emission properties of nanocrystalline graphite (NCG) films, grown by plasma enhanced chemical vapor deposition method on conductive Si substrates without using of any catalyst, were investigated. Current-voltage characteristics were measured in pulse-periodic regime. It was shown that grown NCG films can operate at field emission current density up to 10 A/cm2. It was found that NCG films contain, along with the normally oriented to the substrate nanoflakes, carbon whiskers consisted of graphene nanoribbons and nanowires with length considerably higher than of the nanoflakes.

Performance of nanocrystalline graphite field emitters

Nanocrystalline graphite field emitters fabricated on silicon substrates have been characterized in terms of current–voltage, pressure dependency, long term stability, work function and lateral momentum. The work function is 4.0–4.2 eV and the cone angle of emission due to the lateral momentum of the emitting electrons in less than 1°. It is shown that with proper pre-testing treatment and copper anodes, these emitters can operate for over 5000 h without significant changes in emission current. They also operate at pressures of 5×10−5 Torr with current fluctuations ΔI/I of less than 1%. A quite complex emission pattern develops in conjunction with CRT phosphors. The transmission coefficient of electrons exiting a grid can vary from 1–50% depending on which phosphor is being used.

Carbon nanowalls: the next step for physical manifestation of the black body coating

The optical properties of carbon nanowall (CNW) films in the visible range have been studied and reported for the first time. Depending on the film structure, ultra-low total reflectance up to 0.13% can be reached, which makes the CNW films a promising candidate for the black body-like coating, and thus for a wide range of applications as a light absorber. We have estimated important trends in the optical property variation from sample to sample, and identified the presence of edge states and domain boundaries in carbon nanowalls as well as the film mass density variation as the key factors. Also we demonstrated that at much lower film thickness and density than for a carbon nanotube forest the CNWs yield one order higher specific light absorption.

Three-dimensional simulation of a HFCVD reactor

Abstract A three-dimensional (3D) model of a hot-filament CVD reactor is developed to study the gas-phase and surface processes of diamond growth. The gas-phase and surface reaction mechanisms, the molecular diffusion and thermodiffusion, catalytic hydrogen dissociation at the filament and surface kinetics at the substrate are taken into account in transport equations that are numerically solved. The model has been checked using experimental measurements of absolute methyl and atomic hydrogen concentration. An importance of 3D effects for the species concentration and gas temperature distribution is revealed. The filament and substrate temperature dependence of species concentrations are calculated and compared with experimental data.

Comparison of a one-dimensional particle-in-cell–Monte Carlo model and a one-dimensional fluid model for a CH4/H2 capacitively coupled radio frequency discharge

A one-dimensional particle-in-cell–Monte Carlo (PIC–MC) model was developed for a capacitively coupled rf discharge in a mixture of CH4 and H2. The electron behavior is kinetically simulated by solving Newton’s equations and treating the electron collisions with the Monte Carlo algorithm, whereas the behavior of the ions and radicals is treated by a set of continuity equations. The distinctive feature of this model is its self-consistency, i.e., the motion of the electrons is considered in the real electric field calculated from the Poisson equation, and not in the time-averaged electric field. The PIC–MC results were compared with the data calculated by means of a pure fluid model. In both models, exactly the same type of species, reactions, and cross sections are used. The results of both models, such as the electron energy distribution function, the average electron energy, and the densities of the various plasma species, are compared at a gas pressure of 0.14 Torr and a discharge frequency of 13.56 MH...

Pressure scaling of an electro-discharge singlet oxygen generator (ED SOG)

This work is devoted to the study of the possibility of obtaining the highest O2(a 1 � g) yield in ED SOG at the high absolute O2(a 1 � g) concentration needed for developing a powerful oxygen–iodine laser pumped by electric discharge. A singlet oxygen was produced in a transversal rf discharge in the pressure range 10–30 Torr of pure oxygen in the small-diameter (7 mm) quartz tube with HgO coating of the inner walls for removing atomic oxygen to eliminate fast O2(a 1 � g) quenching. It is shown that pd scaling (p—pressure, d—tube diameter) of the rf discharge actually allows an increase of the absolute O2(a 1 � g) density. The increase in the rf frequency from 13.56 to 81 MHz results in the essential increase of the O2(a 1 � g) yield (beyond 15% at such a high oxygen pressure as 15 Torr), but the subsequent transfer to the higher rf frequency of 160 MHz only slightly influences the maximally obtained O2(a 1 � g) yield. The effect of the NO admixture on the O2(a 1 � g) production has been also studied. The rate constant of O2(a 1 � g) quenching by NO k NO = (8.5 ± 1.5) × 10 −17 cm 3 s −1 was directly measured. The NO admixture (up to 20%) resulted in the noticeable increase in the O2(a 1 � g) yield mainly at low energy inputs. But this gain in the O2(a 1 � g) concentration drops with increasing energy input. Nevertheless it is shown that by combining the O2 + NO mixture with the HgO coating of the discharge tube walls one can provide the O2(a 1 � g) yield on the level of ∼21% at 10 Torr, ∼17% at 20 Torr and ∼13% at 30 Torr of O2 with the efficiency of ∼4–6%. The analysis of the NO admixture influence on the discharge structure and O2(a 1 � g) production has been carried out by using the 2D model. It was found that at the low energy input the NO admixture acts as an easily ionized species that enlarges the region occupied by plasma. Thus, in the O2 + NO discharge the normal current density is lower than in the pure oxygen discharge. As a result a higher energetic efficiency of O2(a 1 � g) production is also observed in the case of the O2 + NO mixture and the low energy input. In order to provide the optimal conditions for O2(a 1 � g) production (with regard to the yield and efficiency) in the continuous wave transversal VHF discharge at such high oxygen pressures as of 10–30 Torr it is necessary to find out the range of energy inputs where the VHF discharge operates in the regime of normal current density on the boundary with the abnormal regime and to remove atomic oxygen produced in the discharge by some volume or surface processes.

Experimental and Theoretical Study of Ion Energy Distribution Function in Single and Dual Frequency RF Discharges

Ion energy distribution functions (IEDFs) at the electrodes in single frequency (SF) and dual frequency (DF) radio-frequency discharges in Ar at pressures of 20 and 45 mtorr are measured and calculated. A numerical simulation of the IEDF on the base of a self-consistent particle-in-cell model with Monte Carlo collisions was performed. In addition, a semianalytical model was developed to calculate the IEDF in collisionless and collisional SF and DF plasmas. The IEDF width for the intermediate frequency case was determined from both experimental and theoretical results. The possibility of frequency decoupling is discussed.