R.L. Boxman

Annealing and Sb-doping of Sn—O films produced by filtered vacuum arc deposition: structure and electro-optical properties

Abstract Tin oxide films were deposited using a filtered vacuum arc at rates up to 10 nm s−1. Optimal results were obtained with a 160 A arc when the deposition pressure was in the range 6–9 mTorr. The structure of the films was amorphous if the deposition temperature, or the post-deposition annealing temperature, was less than 350°C, while films containing orthorhombic, tetragonal, and amorphous phases were obtained at higher temperatures. Annealing or substrate heating with temperatures within the amorphous range improved the conductivity considerably, and was accompanied by increases in the optical gap, carrier density, and carrier mobility. The improvements are attributed to greater short-range ordering. Annealing or substrate heating into the crystalline range yielded higher resistivity films. Sb doping did not decrease the resistivity of the amorphous films. The lowest resistivities obtained, (5–7) × 10−4ωcm, were equal to or less than any reported in the literature for undoped Sn—O films, and were obtained at deposition rates considerably higher than with other techniques.

A Model of the Multicathode-Spot Vacuum Arc

A model is proposed for the multicathode-spot (MCS) vacuum arc. A zero-order model is filrst constructed, whereby the interelectrode plasma is produced by the multitude of cathode spots, and flows to the anode upon which it condenses. The electron density is calculated by assuming that the plasma is uniform within a cylinder bounded by the electrodes and using expenmental data for the ionic velocities and ion current fraction obtained in single cathode spot arcs. The electron density thus obtained is proportionate to the current density, and is equal to 5 × 1020 m-3 in the case of a 107-A/m2 Cu arc. The model predictions are a factor of 3-4 lower than measured values. First-order perturbations to the zero-order model are considered taking into account inelastic electron-ion collisions, plasma-macroparticle interactions, the interaction of the self-magnetic field with the plasma and electric current flows, and the interaction with the anode. Inelastic collisions tend to increase the ionicity of the plasma as a function of distance from the cathode, in agreement with spectroscopic observations. Macroparticles are heated by ion impact until they have significant evaporation rates. The vapor thus produced is ultimately ionized, and most probably accounts for the discrepancy between the zero-order prediction of electron densities and the measured values. Constrictions near the anode in both the plasma and electric current flows have been calculated. An overabundant electron current supply forces the anode to assume a negative potential with respect to the adjacent plasma.

Characteristics of Macroparticle Emission from a High-Current-Density Multi-Cathode Spot Vacuum Arc

Macroparticle mass transport, size distribution, and spatial distribution were studied in a 6.5-MA/M2 25-ms Cu multi-cathode spot (MCS) vacuum arc. The macroparticle erosion rate was determined to be 105 ¿g/C, and together with ionic emission, accounted for most of the cathodic erosion. The number of macroparticles emitted decreased exponentially with macroparticle diameter, with 20-80-¿m macroparticles carrying the bulk of the mass transport. Macroparticles are emitted preferentially at an angle of 20° with respect to the cathode surface. In comparison to previous investigations, higher macroparticle erosion rates, a larger proportion of large macroparticles, and a higher emission angle are observed, and the differences are attributed to the large current density used in the present experiment.

Anode Melting in a Multicathode-Spot Vacuum Arc

Melting of the anode surface in a multicathode-spot vacuum arc is expected when the incident energy flux is not balanced. The anodic energy influx is proportional to the arc-current collected by the anode and melting of the anode should be observed when peak arc-current exceeds a critical value. In this work, the critical peak arc-current Ipt was measured, and its dependence on anode and cathode materials was determined. The arc was sustained between two parallel cylindrical electrodes, 14 mm in diameter and spaced 4 mm apart. The almost critically damped current pulse lasted for 30 ms with a 6-ms rise time to peak value. Peak currents were in the range of 500-2300 A. In most of the experiments the anode material differed from that of the cathode. In the runs where the cathode-anode materials were Cu-Al or Mo-Cu, respectively, the time dependence of a spectral line intensity radiated by the anode atoms located in the plasma near the anode surface was recorded. We found that Ipt depended on both the anode and cathode materials. Thus for an Al anode and Al and Cu cathodes, Ipt equaled to 1100 and 900 A, respectively. In arcs with a peak current larger or equal to Ipt, a sudden jump of the spectral line intensity was observed. In all experiments, even when strong melting of the anode was observed, the arc-voltage stayed quiescent and in the range 15-35 V, suggesting that no anode spot was formed.

The numerical calculation of plasma beam propagation in a toroidal duct with magnetized electrons and unmagnetized ions

Electron-magnetized vacuum arc plasma transport in a magnetic toroidal duct is calculated numerically taking in account electron - ion collisions, electron and ion temperatures, and the high conductivity of the duct wall. The longitudinal magnetic field in the duct, the fully ionized plasma density and the electric potential distribution at the torus entrance are given, while the plasma density, electrical field and current, and macroscopic plasma velocity across the magnetic field inside the duct are calculated. Toroidal coordinates are used to describe plasma beam propagation. A Runge - Kutta routine is used for the calculations along the torus while a finite difference method is used across the torus cross section. It is found that plasma loss due to particle flux to the duct wall depends on the electron and ion temperatures and the plasma density distribution at the torus entrance cross section. With an electron temperature of , 30 000 K and 50 000 K, an ion temperature and a Gaussian distribution of plasma density at the torus entrance with a maximum value , we found that the duct efficiency was less than 10% for longitudinal magnetic field strengths of 10 mT and 20 mT. In the case where only the electrons are magnetized, filter efficiency depends only weakly on the magnetic field strength, on , and on .

Air annealing effects on the optical properties of ZnO-SnO2thin films deposited by a filtered vacuum arc deposition system

ZnO–SnO2 transparent and conducting thin films were deposited on microscope glass substrates by a filtered vacuum arc deposition (FVAD) system. The cathode was prepared with 50%:50% atomic concentration of Zn:Sn. The films were annealed in air at 500 °C for 1 h. Structural and compositional analyses were obtained using XRD and XPS diagnostics. X-ray diffraction analysis indicated that as-deposited and air-annealed thin ZnO–SnO2 films were amorphous. The atomic ratio of Zn to Sn in the film obtained using the 50%:50% cathode as determined by XPS analysis was ~2.7:1 in the bulk film. The optical properties were determined from normal incidence transmission measurements. Film transmission in the visible was 70% to 90%, affected by interference effects. Annealed films did not show higher transmission in the VIS compared to as-deposited films. Assuming that the interband electron transition is direct, the optical band gap was found to be in the range 3.34–3.61 eV for both as-deposited and annealed films. However, the average Eg for annealed films was 3.6 eV, larger by 0.2 eV than that of as-deposited. The refractive index n increased while the extinction index k decreased significantly with annealing.

The Current Distribution and the Magnetic Pressure Profile in a Vacuum Arc Subject to an Axial Magnetic Field

The steady-state electric-current distribution and the magnetic pressure in a uniform conducting medium, flowing in a cylindrical configuration between two circular electrodes, was determined by solving the magnetic field transport equation with a superimposed axial magnetic field. This medium models the interelectrode plasma of the diffuse mode metal vapor vacuum arc. The results show the following. a) The electric current and the flux of the poloidal magnetic field are constricted at the anode side of the flowing plasma. Most of the constriction takes place within a boundary layer, with a characteristic length of 1/Rme, where Rme is the magnetic-Reynolds number for axial electron flow. b) The electric-current constriction inversely depends on K¿, where K¿ is the azimuthal surface current density which produces the axial magnetic field. c) The magnetic-pressure profile shows a radial pinch force in most of the interelectrode region, but in the anode boundary layer it is axially directed, thus retarding the plasma flow. d) The peak of the magnetic pressure is at the anode, and its amplitude directly depends on K¿. As K¿ increases, the peak location moves toward the anode center.

Optical properties of transparent ZnO?SnO2 thin films deposited by filtered vacuum arc

ZnO–SnO2 films were deposited onto glass substrates by filtered vacuum arc deposition. The source was equipped with a 70 at% Zn and 30 at% Sn cathode that was the source of Zn and Sn ion beam. Arc current was 200–300 A. The oxygen background pressure was 4–8 mTorr. The deposition time was 60 or 120 s, resulting in film thickness in the range 100–900 nm. The maximum deposition rate was 7.6 nm s−1. All films were found to be amorphous. The transmission of the film in the VIS was 80%–90%, affected by interference. The refractive index and the extinction coefficient were determined from the measured optical transmission in the range 300–1100 nm by fitting a theoretically calculated film transmission to the measured one, using a single oscillator model. The values of n and k were determined from spectroscopic ellipsometry data and were in the ranges 2.38–1.97 and 0.24–0.013, respectively, depending on wavelengths and deposition parameters. The optical band gap (Eg) was determined by the dependence of the absorption coefficient on the photon energy at short wavelengths. Its values were in the range 3.5–3.62 eV, depending on the deposition conditions.

Optical characterization of filtered vacuum arc deposited zinc oxide thin films

ZnO thin films, 100–250 nm thick, were deposited on microscope glass slides and UV fused silica (UVFS) substrates using a filtered vacuum arc deposition (FVAD) system, operating at room temperature (RT) and 200 A for 60 s. The cathode was prepared from 99.9% pure Zn metal and the oxygen background pressure during deposition was in the range 0.67–0.93 Pa. The films were characterized by x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), optical transmission and spectroscopic ellipsometry. As-deposited ZnO films were found to be polycrystalline with c-axis orientation. The atomic concentration ratio of Zn to O in the film as determined by XPS analysis was stoichiometric. Film transmission in the visible was 70–90%. The maximum and minimum values of the refractive index n and the extinction coefficient k in the visible, for all samples, were 2.23 to 1.90 and 0.6 to approximately 0, respectively. The type of inter-band electron transition was found to be direct transition with optical band gap in the range of 3.25–3.42 eV.

Role of the magnetic field in the cathode region during vacuum arc operation

Arc operation was studied in a vacuum arc deposition apparatus consisting of a Ti cathode, spacer, annular anode, straight duct, quarter torus macroparticle magnetic filter, and a deposition chamber. Superposition of fields from different magnetic coils allowed the formation of different field configurations in the vicinity of the cathode and in the cathode-anode gap. The analytical study of these fields together with the observation of cathode spot motion, made it possible to demonstrate the action of two mechanisms causing unstable arcing: (1) cathode spot movement off of the cathode surface to the side in the direction of the opening of the acute angle formed by the intersection of the field lines with the cathode surface, and (2) cutoff from the anode of the magnetized electron flow by the field lines. With an optimal field configuration both of the above mechanisms were avoided. The field configuration included (1) an arched field on the cathode surface, which rotated the cathode spots on the cathode surface, and (2) connection of a sufficient portion of the cathode surface to the anode with magnetic field lines, so that the random electron current in the vicinity of the anode can supply the required arc current.