Diffusion Of Fluorine Ions Research Articles

Improved package reliability of AlGaN/GaN HFETs on 150 mm Si substrates by SiNx/polyimide dual passivation layers

Abstract In order to prevent reduction of output power of AlGaN/GaN heterostructure field effect transistors(HFETs) with inorganic passivation layer such as SiO 2 and SiN x after TO-220 plastic package, SiN x /photosensitive polyimide(PSPI) dual layers were proposed as passivation layers for high power, enhanced mode AlGaN/GaN HFETs on 6 in. Si substrates. The PSPI layer can endure the filler-induced mechanical stress during the plastic package, followed by prevention of the passivation layer cracking and metal deformation. Furthermore the SiN x layer can suppress the diffusion of fluorine ions during curing of the PSPI layer, which causes the reduction of 2DEG concentration due to the strong electronegativity of fluorine ions. The results showed that output power of the AlGaN/GaN HFETs with the SiN x /PSPI dual passivation layers was not deteriorated even after plastic package process, which can be attributed to prevention of the filler-induced stress and fluorine ion diffusion by the dual passivation layers.

Ionic mobility and phase transition in heptafluorodiantimonate(III) Cs0.4Rb0.6Sb2F7

The ionic mobility and conductivity in the crystalline phase of Cs0.4Rb0.6Sb2F7 (I) is studied using 19F NMR, DSC, and impedance spectroscopy methods. The analysis of the 19F NMR spectra makes it possible to follow the character of ion motions in the fluoride sublattice in the temperature range of 150-470 K, and to determine the types and temperature ranges of these motions. It is found that diffusion of fluorine ions becomes the dominant type of ion motions in the modification formed as a result of the reversible phase transition (T = 415 K, the DSC data) in compound I. According to the preliminary results of electrophysical studies, the ionic conductivity in this compound is ~10−3 S/cm at temperatures above 450 K.

Ion mobility and conductivity in solid solutions in the KBiF4–ZrF4 system

The ion mobility and conductivity in solid solutions with the fluorite structure (100–x)KBiF4 + xZrF4, where x = 2.5–15 mol %, is studied by the methods of differential scanning calorimetry (DSC), X-ray diffraction (XRD), and 19F NMR. The character and type of ion transport in the fluorite sublattice of solid solutions are studied as well as the temperature intervals in which it is observed (150–570 K). For solid solutions containing 5 and 10 mol % ZrF4, the fluorine ion diffusion coefficient is assessed. It is found that the conductivity in the solid solution systematically decreases as the content of zirconium tetrafluoride in the sample decreases, which is probably due to the formation of strongly bound complexes formed by interstitial fluoride ions with zirconium cations. The presence of the high ionic conductivity in the tested solid solutions (from ~10–3 to 10–2 S/cm) makes this material a good candidate for preparation of materials with the high ion-conducting properties.

NMR spectroscopy of bismuth fluoride glasses with alkali cations

Results of the 7Li, 19F, and 23Na NMR studies of ionic mobility in bismuth fluoride glasses in the systems BiF3-LiF and BiF3-MF-ZrF4 (M = Li, Na, K, Cs) are summarized. Analysis of the 7Li, 19F, and 23Na NMR spectra made it possible to reveal changes in the nature of ion motions in the fluoride, lithium and sodium sublattices of glasses upon temperature variation and to determine their types. The temperature ranges were found where main types of ion motions in the tested glasses are represented by diffusion of lithium ions, reorientations of fluorine-containing groups constituting the glass network, and diffusion of fluorine ions. The role of alkali cations in the formation of ionic mobility in bismuth fluorozirconate glasses is considered.

Ionic mobility and phase transitions in heptafluorodiantimonates MSb2F7 and Cs(1−x)M x ′Sb2F7 (M′ = K, NH4) according to NMR and DSC data

Ionic mobility and phase transitions in heptafluorodiantimonates with homo- and heteroatomic cations in the systems MSb2F7 (M = K, Cs, NH4) and CsM’Sb2F7 (M’ = K, NH4) have been studied using the 19F, 1H NMR and DSC methods. Analysis of the 19F and 1H NMR spectra made it possible to reveal the nature of ion motions in the fluoride and proton sublattices upon temperature variation and to determine the types and temperature ranges of such motions. It was found that diffusion of fluorine ions becomes the dominant form of ion motions in the high-temperature modifications, the amount of diffusing ammonium ions depending on the composition of a sample. The observed phase transitions in cesium-potassium and cesium-ammonium fluoroantimonates(III) leading to the formation of high-temperature modifications are the transitions to superionic state. According to preliminary results of electrophysical studies, hightemperature phases Cs1−x(NH4)xSb2F7 (0.05 ≤ x ≤ 0.6) are superionic, their conductivity reaching −10−3-10−4 S/cm at 463–483 K.

Effect of glass tempering on microstructure and functional properties of SnO2:F thin film prepared by atmosphere pressure chemical vapor deposition

The low-emission glass was prepared via depositing fluorine-doped tin oxide thin film on glass substrate by atmospheric pressure chemical vapor deposition method. The as-deposited low-emission glass was found to present a SnO2:F/SiCxOy/glass sandwich structure via focused ion beam technique and transmission microscopic measurement. After tempering process at ~650°C with varied periods, the electrical and optical properties of the SnO2:F thin film remained stable for less than 10min, but decreased dramatically when the tempering period exceeded 10min, which was mainly due to the oxygen chemisorptions and fluorine ion diffusion. It was observed that the SnO2:F thin films presented uniform polycrystalline nature of cassiterite structure throughout the tempering process. The study has therefore suggested the appropriate tempering conditions for the SnO2:F low-emission glass, and provided a critical guidance for further energy-saving glass applications.

Ion mobility and thermal properties of glasses in the BiF3–LiF and BiF3–LiF–ZrF4 systems

Ion mobility and the characteristic temperatures of bismuth fluoride glasses in the BiF3–LiF and BiF3–LiF–ZrF4 systems were studied by Nuclear Magnetic Resonance and Differential Scanning Calorimetry methods. Analysis of 7Li and 19F Nuclear Magnetic Resonance spectra made it possible to reveal the character of ion motion in the fluoride and lithium sublattices of the glasses under temperature variation, and to determine the types and temperature ranges in which they took place. It was found that the main types of ion motion in the glasses were the diffusion of lithium ions and the reorientation of fluorine-containing groups. The diffusion of fluorine ions was observed only in crystallized glasses.

Ionic mobility and structure of glasses in Zrf4—Bif3—Mf2 (M = Sr, Ba, Pb) systems according to Nmr, Ir, and Raman spectroscopic data

The NMR (19F and MAS NMR 19F), IR, and Raman spectroscopic methods are used to study the ionic mobility and structure of a series of new glasses in ZrF4—BiF3—MF2 (M = Sr, Ba, Pb) systems in a temperature range of 180 K to 500 K. The temperature range, in which diffusion of fluorine ions becomes the dominant form of ionic motion, is determined by the nature of the M2+ cation. The factors determining the basic model of the structure of glasses in ZrF4—BiF3—MF2 (M = Sr, Ba, Pb) systems and conditions under which bismuth polyhedra can participate in the construction of the glass network are considered. According to the data of impedance spectroscopy, the studied glasses have relatively high ionic conductivity (δ ≥ 10–4 S/cm above 480 K).

Aging of theHF−H2SO4electrolyte used for the electropolishing of niobium superconducting radio frequency cavities: Origins and cure

Electropolishing (EP) in the $\mathrm{HF}\mathrm{\text{\ensuremath{-}}}{\mathrm{H}}_{2}{\mathrm{SO}}_{4}$ electrolyte is the most desirable surface treatment for niobium superconducting radio frequency cavities yet demonstrated, in terms of performance and surface finish. However, the efficiency of the electrolyte declines quickly with time (decrease in removal rate, deterioration of the niobium surface, increased sulfur generation). Previous studies at CEA Saclay have highlighted the impact of the water content in EP mixtures rather than the content of dissolved niobium. Knowledge of the electrochemical system was improved thanks to studies using a rotating disk electrode (RDE). Measurements with a RDE give precious information concerning mass transport of the different ionic groups present in the solution. The performed measurements prove that EP is controlled by the diffusion of fluorine ions and the value of the related diffusion coefficient ${\mathrm{D}}_{\mathrm{F}\ensuremath{-}}$ was estimated for different mixtures. Electrochemical impedance spectroscopy (EIS) measurements were also performed with different EP mixtures. Both volt ampere metric and EIS measurements prove the central role of fluorine during EP and show that EP mechanisms evolve with the aging of the bath. Another major problem related to electrolytes is the formation of impurities such as sulfur. We have proved that working at a reduced voltage of 5 V does not alter cavity performance and makes it possible to reduce the undesirable particulate contamination in electrolytes and to increase their lifetime.

On the stability of fluorine ions in AlGaN/GaN heterostructures: a molecular dynamics simulation study

AbstractFluorine ions can be effectively incorporated into AlGaN/GaN high electron mobility transistor (HEMT) structures, enabling the modulation of local potential and carrier density. The physical mechanism of fluorine incorporation in AlGaN/GaN heterojunctions is of fundamental importance to the stability of fluorine ions in AlGaN/GaN HEMTs. In this work, the molecular dynamic (MD) simulation method is used to calculate the potential energies of interstitial and substitutional fluorine atoms in AlGaN/GaN material system. Ziegler‐Biersack‐Littmark (ZBL), Lindhard‐Sorensen (L‐S) and Coulomb potential functions are applied in the MD simulation. The geometric lattice structures, spontaneous and piezoelectric polarizations, and temperature dependence are also included in the simulation. The activation energies associated with interstitial‐substitutional and interstitial‐interstitial diffusions are obtained. It is revealed that the fluorine ions are most likely located at the substitutional group‐III cation sites S(III) and the diffusion of fluorine ions should be dominated by S(III)‐interstitial process which exhibits an activation energy of 1.1 eV in Al0.25Ga0.75N and 1.4 eV in GaN in the presence of group‐III vacancies. It is expected that the removal of group‐III vacancies can significantly suppress the fluorine diffusion, which in turn, leads to excellent fluorine stability in III‐nitride materials. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Diffusion mechanism and the thermal stability of fluorine ions in GaN after ion implantation

The diffusion mechanisms of fluorine ions in GaN are investigated by means of time-of-flight secondary ion mass spectrometry. Instead of incorporating fluorine ions close to the sample surface by fluorine plasma treatment, fluorine ion implantation with an energy of 180 keV is utilized to implant fluorine ions deep into the GaN bulk, preventing the surface effects from affecting the data analysis. It is found that the diffusion of fluorine ions in GaN is a dynamic process featuring an initial out-diffusion followed by in- diffusion and the final stabilization. A vacancy-assisted diffusion model is proposed to account for the experimental observations, which is also consistent with results on molecular dynamic simulation. Fluorine ions tend to occupy Ga vacancies induced by ion implantation and diffuse to vacancy rich regions. The number of continuous vacancy chains can be significantly reduced by a dynamic thermal annealing process. As a result, strong local confinement and stabilization of fluorine ions can be obtained in GaN crystal, suggesting excellent thermal stability of fluorine ions for device applications.

Molecular dynamics calculation of the fluorine ions’ potential energies in AlGaN/GaN heterostructures

Fluorine ions can be effectively incorporated into AlGaN/GaN high electron mobility transistors (HEMTs), enabling the modulation of local potential and carrier density. The physical mechanism of fluorine incorporation in AlGaN/GaN heterojunctions is of fundamental importance to the stability of fluorine ions in AlGaN/GaN HEMTs. In this work, the molecular dynamics simulation method is used to calculate the potential energies of interstitial and substitutional fluorine ions in AlGaN/GaN material system. It is revealed that the fluorine ions are most likely located at the substitutional group-III cation sites S(III), and the diffusion of fluorine ions in implanted III-nitride semiconductors should be dominated by S(III)-interstitial process.

19F NMR Study of the Anion Mobility in Tin Fluorozirconate Glasses of SnF2–ZrF4–LiF and SnF2–ZrF4(HfF4)–SbF3Systems

The influence of the concentration of lithium and antimony(III) fluorides on the ion mobility in the fluoride Subscriptsystem of tin fluorozirconate glasses is investigated by 19F NMR spectroscopy in the temperature range 180–450 K. The types of ion motions are determined, and their activation energies are estimated. It is revealed that the introduction of antimony trifluoride into the composition of tin fluorozirconate glass leads to a decrease in the temperature of defreezing the diffusion of fluorine ions. By contrast, the presence of lithium fluoride hinders diffusion processes in the fluoride Subscriptsystem.

Molecular dynamics simulation of ionic conductivity in the fluoride-perovskite KCaF 3

Constant-temperature constant-volume molecular dynamics simulations were performed for the fluoride-perovskite KCaF 3. Ionic conductivity values were predicted using systems designed so as to simulate crystals containing extrinsic defects; these values were compared with experimental measurements made between 575 and 690 K. At these temperatures, extrinsic defects were found to be prerequisite for diffusion of fluorine ions. During defect-free simulations, intrinsic disorder was not generated spontaneously. Fluorine migration procedes via a discrete vacancy “hopping” mechanism, in which the anion migration time is of at least an order of magnitude less than the average residence time at a given lattice site. This is in contrast to the partially correlated mechanism observed at temperatures approaching the simulated melting point for KCaF 3, in which vacancies are generated by the formation of fluorine ion pairs occupying interstitial sites.

Molecular dynamics study of ion transport in CaF2(10 mol % LaF3)

Abstract We present the results of a molecular dynamics study of CaF2(10 mol % LaF3). No diffusion of fluorine ions is observed in pure CaF2 below the transition temperature (TC ≈ 1460K) to the fast-ion phase. However, doping with La3+ ions promotes fluorine ion diffusion in this temperature range, and tends to suppress it above TC. Various ion migration mechanisms are discussed.

NMR Study of Internal Motions in a New Family of Ionic Conductors M2SbF5

Abstract19F NMR spectra and relaxation times T,1, T2, T1e and electric conductivity in isostructural polycrystalline pentafluoroantimonates (III) M2SbF5 (M = Rb, K, Cs, Tl, NH4) are measured. It is found that the observed changes in the NMR spectra and relaxation times with temperature are determined by three types of motion: anisotropic and isotropic reorientations of [SbF5E]2− octahedra and diffusion motion of the fluorine ions. Parameters of these motions are found. All the compounds investigated show high conductivity at high temperature, amounting in the case of thallium and ammonium salts the values of σ = 101 to 100Ω−1 m−1 typical of superionic crystals. It is shown that the observed high conductivity is determined by fast diffusion motion of fluorine ions. On the basis of the data obtained a mechanism of fluorine ion diffusion is proposed. In (NH4)2SbF5 at 398 K a phase transition is found.

Transport Properties of LaF3

We report measurements of the lattice and bulk thermal expansion, fluorine nuclear magnetic resonance, and electrical conductivity of single-crystal La${\mathrm{F}}_{3}$ in the temperature range 300-1000\ifmmode^\circ\else\textdegree\fi{}K. In the lower portion of this temperature range, the measurements yield activation energies for the formation of Schottky defects of \ensuremath{\sim}0.07 eV and for fluorine ion diffusion of \ensuremath{\sim}0.45 eV. The activation energy for diffusion appears to decrease at higher temperatures. We propose a model for the crystal with low activation energy for the formation of neutral defects, and a higher energy for defect dissociation and diffusion. The apparent changes of activation energy are ascribed to the excitation of lattice vibrations near the Debye temperature.