Low-energy Ion Fluxes Research Articles

Hydrogen Incorporation in Amorphous Silicon Prepared at High Deposition Rates by the VHF-GD Technique

ABSTRACTThe influence of deposition temperature on hydrogen incorporation in a-Si:H prepared by VHF glow discharge at 70 MHz is investigated using hydrogen evolution, elastic recoil detection analysis and infrared spectroscopy.The films were further characterized by dark- and photoconductivity and by photothermal deflection spectroscopy. While the electronic film properties deteriorate in the usual manner with decreasing substrate temperature it is found that the total hydrogen content CH and the degree of microstructure that can be directly correlated to CH increase only moderately. It is concluded that a higher flux of low energy ions in the VHF plasma plays a key role in this context, possibly by increasing the surface mobility of the H atoms and thereby preventing the build-in of a large amount of hydrogen at low substrate temperatures.

Anisotropy of low-energy ion etching via electron cyclotron resonance plasma

Low-energy reactive ion etching is desirable as a method to fabricate devices, because it does not damage the sample surface. However, anisotropical properties are difficult to obtain with low energy ions. Thus, anisotropical properties with an electron cyclotron resonance (ECR) plasma etcher, with which it is easy to obtain a directional low energy (≤20 eV) ion flux, were examined. The conditions under which a clear enough anisotropical cross section figure was produced, were found and applied to sub-half-micron patterns. An ECR plasma etcher is used here with a standing wave cavity TE113 storing 2.45 GHz microwaves and with a sample chamber separate from the cavity. This kind of etcher does not permit control of an ion’s energy apart from the radicals. However, it is possible to control ionic performance by reducing the gas pressure to ∼1.0 mTorr and obtaining a satisfactory anisotropical etching mode. However, ion and molecule collision appear in the ion flux at that gas pressure. The collision makes disanisotropy; the collisional angle changed from about 15–25 deg for gas pressures from 0.2 mTorr to 1.0 mTorr. According to a simple theoretical calculation, the angle is about 30 deg on the assumption of a one time collision, about twice the value of the experimental result. At last, we obtained the high aspect ratio up to 10 at a gas pressure of 0.1–0.2 mTorr. It follows that it was possible to etch the sample well anisotropically with a low-energy ion flux without causing damage by using an Ion-Flux Type ECR etcher and controlling the gas pressure, and further without spoiling its undamaged nature.

Voids in Jovian magnetosphere revisited: Evidence of spacecraft charging

As Voyager 2 moved inward toward Jupiter in the vicinity of Ganymede's orbit, the low‐energy ion and electron fluxes recorded by the Plasma Science Experiment (PLS) intermittently fell to very low levels. The low fluxes, which lasted for times of the order of 10 min and recurred over an 8‐hour interval, have been called “voids.” Early interpretations associated the voids with Ganymede wake effects. Further analysis has demonstrated that the location and extent of the voids are inconsistent with such an interpretation. In this study, we have reexamined the PLS data in conjunction with data from the magnetic field experiment and the low‐energy charged particle experiment (LECP) in the relevant interval. The LECP data showed that the PLS voids were accompanied by large enhancements of the flux of energetic electrons and ions. No systematic signatures were found in the magnetic data. We suggest that increased energetic electron fluxes in the void regions intermittently charged the spacecraft negatively to values between a few kV and tens of kV. For a realistic phase space distribution of cold and hot particles in the Jovian magnetosphere, spacecraft charging can produce dropouts in the measured cold ion and electron fluxes and enhancements in the measured fluxes of hot particles consistent with the observations. Spacecraft charging would not produce a signature detectable by the flux gate magnetometer. We propose that the plasma density in the nominal voids is not exceptionally low and that the near‐Ganymede anomalies arose from an unusual combination of enhanced fluxes of ∼10 to ∼100 keV electrons in a low‐density plasma environment. Further work is needed to explain why this phenomenon occurred intermittently and only over a limited portion of the Voyager trajectory.

Enhanced low energy (1 MeV) ion fluxes in the outer heliosphere

The Low Energy Charged Particle (LECP) experiment on the Voyager 2 spacecraft in the outer heliosphere ( > 10 a.u.) has observed several occasions when there was a peak in the interplanetary ion spectra for ions of energies ∼ 0.5–1.0 MeV. Such enhancements can last for several days, suggesting that at these times particles of these energies dominate the low energy cosmic population in this region of the heliosphere. Two specific cases are discussed. The enhancements seem to be associated with the passage of transient interplanetary shock events, with the ion anisotropies generally showing outflow. The most straight-forward explanation for the observations seems to involve only a propagation effect of ions from the inner to the outer solar system. This conclusion is supported by simple modeling of the propagation of an event observed at 1 a.u. to the spacecraft at ∼ 12 a.u.

Inelastic electron-tunneling studies of the effect of hydrogen on the growth of MgO.

Mg-MgO-metal tunnel junctions were studied by inelastic electron-tunneling spectroscopy. Junctions with the Mg film exposed to a flux of low-energy hydrogen ions exhibit a large peak in the tunneling spectrum at 124 meV. This peak is attributed to chemisorbed hydrogen absorbed at subsurface sites on the Mg film. MgO junctions prepared in low partial pressures of water were found to have poor tunneling characteristics. These properties are ascribed to a crystalline barrier with nonuniform coverage. In junctions with chemisorbed hydrogen the oxide growth was more uniform. The hydrogen provides a nucleation site for initial oxidation which results in a more amorphous barrier, with better tunneling characteristics.

Collisional aspects of preferential sputtering using the Monte Carlo method

In fusion reactors, surface coatings such as TiC and TiB2 are susceptible to the influence of preferential sputtering caused by plasma ion bombardment. The preferential sputtering is the result of chemical, diffusional, and kinetic processes. The kinetic processes are dominant at low energy and moderate ion flux. A Monte Carlo code, TRIPOS, is developed for analysis of the preferential sputtering phenomenon. Argon, helium, deuterium, and tritium ion bombardment of LuFe, CuAu, TiC, and laminated AuPt alloys are simulated. The results from TRIPOS are compared with the results from other theoretical and experimental works. For helium incident on CuAu, TRIPOS gives good agreement with experiment. While TRIPOS shows general agreement with the corresponding program TRIDYN, results for argon on laminated AuPt are not in agreement with EVOLVE. Finally the case of TiC is investigated, where preferential sputtering of carbon atoms under fusion ion bombardment is observed.

Low-energy high flux reactive ion etching by rf magnetron plasma

The magnetron plasma etching of SiO2 and Si in fluorocarbon gas has been investigated. The plasma has high ionization degree, collisionless high density, and low energy ion flux (I∼1.0 mA/cm2, 30≲Ei ≲250 eV at 1 W/cm2) and a controllable etch uniformity. The bulk of the plasma supports an electric field which can be shaped to achieve normal ion bombardment of the etching surface. Good etching anisotropy, high SiO2/Si selectivity, and rates six times greater than conventional reactive ion etching were demonstrated.

GaAs compensation by intense fluxes of low-energy particles

A compensation of n-GaAs, when bombarded by intense fluxes ( q= 10 18 − 10 2 o cm −2s −1) of low-energy ions (≈10 2 ÷ 10 3eV), is described. At bombardment temperatures of 400–500°C the carrier mobility in the region of compensation (≈ 1 ÷ 3 10 −5cm) remains high and conductivity tends to be intrinsic one. A behaviour model, assuming diffusion of elementary defects deep into a semiconductor and a formation neutral complexes with a contribution of donor atoms, is proposed. The diffusion length of vacancies is found to be about 600 Å at 500°C.

Plasma diffusion into the wake of Venus

A model of the diffusion of ionosheath plasma into the wake region of Venus is presented. It is shown that particle diffusion, which is assumed to be a consequence of the fluctuating magnetic field observed in the wake of Venus by Pioneer Venus and Veneras 9 and 10, can explain the plasma observations made in the wake by Veneras 9 and 10. The pressure due to these diffusing particles when balanced against the ionospheric pressure yield ionopause heights less than 1000 km for zenith angles less than about 135°. The model also predicts significant fluxes of low energy electrons and ions for zenith angles less than 135°, which are capable of producing the observed nightside ionosphere.

Energy fluxes of low-energy protons and positive ions in the Earth's inner radiation zone

Differential energy spectra of low energy protons and positive ions in earth inner radiation zone, using electrostatic analyzers aboard OGO-3 satellite

Detection of an intense flux of low-energy protons or ions trapped in the inner radiation zone

Employing a set of CdS crystal energy-flux detectors flown in satellite Injun I, we have observed an energy flux in excess of 50 ergs/cm2 sec sterad at approximately 1000 km altitude in the inner Van Allen zone. Through discrimination by magnetic rigidity and with the help of data from other on-board detectors, it was found that the flux is comprised of protons of energy 0.5 kev ≲E≲1 Mev and/or other heavy ions, apparently trapped in the geomagnetic field. So large a flux of low-energy heavy particles seems to be markedly inconsistent with prevailing ideas of the source function and/or the trapped lifetime of such particles.