Cabrera-Mott Model Research Articles

Transport in magnetically doped magnetic tunnel junctions

We report striking transport behavior of magnetic tunnel junctions which are engineered to incorporate localized magnetic impurities inside the tunnel barrier. Such junctions exhibited zero-bias anomalies (giant conductance dip or equivalently, giant resistance peak) in dynamic conductance curves, and striking bias and temperature dependencies of tunneling magnetoresistance (TMR), i.e., suppression of TMR at zero-bias and the decrease of TMR with decreasing temperature. Logarithmic dependencies of the conductance on bias and temperature agree well with Applebaum's theory describing Kondo-type spin flip scattering between tunneling electrons and impurity spins. The systematic variation of the transport anomalies with oxidation time confirmed that an over-oxidation of the tunnel barrier leads to diffusion of Co (Fe) ions from the bottom electrode into the barrier, which is consistent with the Cabrera-Mott oxidation model.

Electrochemical Properties Of Copper Oxide Surfaces, Buried Interfaces, And Subsurface Zones And Their Use To Characterize These Entities

ABSTRACTElectrochemistry of oxides is an expanding area of oxide characterization. Although, interfacial characterization techniques including surface science methods have contributes substantially to our current understanding of the processes involved in the oxidation of metals and alloys. The characterization of subsurface zones and buried interfaces still remain a major challenge. Copper reactions with oxygen have been studied by high vacuum based techniques of AES, ELS, ISS, XPS. SIMS, LEED. STM, SEXAFS, HEIS and PFDMS and with optical methods, like UV-Vis-NIR, diffuse reflectance spectroscopy, FTIR and photoluminescence spectroscopes. However it has become evident that the processes that produce thermally and plasma grown oxide films on metals and alloys are electrochemical in nature and can be modeled by electrochemical concepts. Therefore, it is important that the oxide over layers, thin films and thick films be characterized by electrochemical means -with electrochemical methods, such as linear potential sweep voltammetry, cyclis voltammetry, galvanostatic reduction and coulometry which allow the identification of copper (I), copper (II) and copper (III) oxides. Interest in copper as a technologically important material needs to be met with greater understanding of the fundamental nature of copper oxide structures. In this study, the authors demonstrate the use of Linear Sweep Voltammetry (LSV) to study buried structures in the thermally grown oxide layers on copper. In particular, LSV can be used to detact reactions at buried interfaces. It also recognizes Cu3O2 and the decomposition of copper oxides at the metal-oxide layers on copper. In particular, LSV can be used to detect reactions at buried interface. The two key parameters that drive oxide growth and decomposition are demonstrated to be oxygen activity and the free energies of formation of the oxides. The complex nature of the oxidation of copper, as well as other metals and alloys, will be described qualitatively using the Modified Cabrera-Mott (C-M) Model. Surface studies of oxidation of metals and alloys need to be supported and complemented by other techniques such as electrochemical methods.

Nanooxidation of n-In0.53Ga0.47As with an atomic force microscope

It is shown that the features formed when an InGaAs layer lattice-matched with the InP substrate is modified by an atomic force microscope are of the oxide nature. Various models of InGaAs nanooxidation by an atomic force microscope are considered. The conventional Cabrera-Mott model refined by introducing the space charge into the oxide is taken as the basic model of nanooxidation by an atomic force microscope.

Oxidation of GaAs/AlGaAs heterostructures studied by atomic force microscopy in air

Oxide growth on cleaved (011̄) cross sections of GaAs/AlGaAs heterostructures exposed to air was investigated using atomic force microscopy. Measurements of the height profile at constant force provide information on the oxide growth rate as a function of time and Al-mole fraction. We observe an initial field-aided oxidation mode that follows the Cabrera–Mott model for metal oxidation. After a critical time, oxidation proceeds more slowly, probably assisted by a thermal diffusion process. We used the dependence of oxide thickness on alloy composition for imaging the Al content in GaAs/AlGaAs heterostructures deposited on nonplanar substrates, with a lateral resolution of ≊1 nm.

Prediction of alloy component oxidation order from a kinetic model using free energies of oxide formation

Described is a method based on the modified Cabrera–Mott model to predict the order of oxidation of alloy components under conditions of varying temperature and oxygen pressure.

The oxidation behavior of amorphous and polycrystalline ZrNi alloys

The surface reactivity of amorphous and crystalline ZrNi alloys with oxygen have been examined with X-ray photoelectron spectroscopy. Oxidation at ambient conditions (1 atm air and room temperature (RT)) and over the temperature range RT to 300°C and at pressures between atmospheric and 10 −8 Torr have been studied. The preferential oxidation of zirconium and the structure of the resulting oxide films are discussed in terms of the Cabrera-Mott model. The pressure and temperature dependence appears consistent with the model. The instability of thin ZrO 2 films with respect to oxygen absorption into the bulk or subsurface region and electrochemical charge exchange has been observed. A comparative discussion of surface reactivity to oxygen of the crystalline and amorphous alloys is given.

Growth kinetics of passive films

A new model of growth of passive films is presented which includes both the transport through the film in the presence of a strong and varying electric field and the transport through the film—electrolyte interface by a reaction with a varying potential drop at the interface. By neglecting the second mechanism the Cabrera-Mott and Vetter model is obtained as a special case of the general model. Galvanostatic and potentiostatic results for passive iron in acid and neutral solutions are explained qualitatively and quantitatively. Only this new model is in overall agreement with experimental findings for different pH values. The thickness of the passive layer is calculated to be 2.7 ± 0.5 nm in 1 N H 2SO 4 at 1.08 V H which is in excellent agreement with experimental values.

Oxidation at low temperatures

Abstract The growth of oxide films on metals and semiconductors at low temperatures was not adequately explained until Mott proposed electron tunnelling as the mechanism of charge neutralization. This led to the Cabrera-Mott model of oxidation in which a self-induced field reduces the activation energy for ion movement. The resulting equation was shown by Fehlner to fit the oxidation kinetics when network-forming oxides can grow. The present work examines kinetic data for network modifiers and attempts to fit them to a direct logarithmic expression of Uhlig. Trapped charge is assumed to limit electron transfer. Charge densities in the 1018−1020cm−3 range are found for Cu, Na, Be, Zn, and Fe.

Passivation kinetics of zircalloy-4 electrode in mineral acids under potentiodynamic conditions

The E/ I response of a zircalloy-4 electrode in mineral acids is examined at room temperature under potentiodynamic conditions. On electrodes that have been mechanically polished and cathodically polarized, growth of the oxide phase, ZrO 2, follows the formalism of the Cabrera-Mott model of high field assisted formation of thin oxide films. It is possible to propose a reaction model based on the potentiodynamic experimental parameters, in which the rate determining step is attributed to ionic transfer at the metal-oxide interface.

Plasma Enhancement in Direct Nitridation of Silicon and Silicon-Dioxide

AbstractEffects of plasma on direct nitridation of Si and SiO2 have been investigated. The reactor consisted of a quartz tube, SiC-coated carbon-susceptors and an RF-induction coil. The electrical probe technique was used to detect floating potential of the susceptor having Si substrates. The voltage distribution between the susceptor and plasma is influenced by nitridation ambient, pressure, reactor geometry and RF power. The predominant species in NH3 plasma is NH radicals which are thought to be negatively charged in an ion sheath formed around the susceptor. The Cabrera-Mott model has been successfully applied to analyze the nitridation mechanism. Plasma enhancement is mainly caused by activation of characteristic velocity of nitridation species rather than by an increase in the electric field appearing in a film. Enhancement due to fluorine incorporation is also explained with the model. Bombardment of ionic species causes sputtering and deposition resulting in damage, contamination and thickness nonuniformity. Optimizing the plasma conditions, decrease in nitridation temperature, increase in growth rate, and improvements in film composition and interfacial structure have been obtained. It has been found that the further nitridation proceeds, the lower Nss becomes. This technique offers various advantages to the VLSI dielectric process.

Anodic oxide films at nickel electrodes in alkaline solutions—I. Kinetics of growth of the β-Ni(OH) 2 phase

The growth kinetics of thin anodic oxide films at nickel electrodes are examined in 0.1 N KOH solutions at room temperature under potentiodynamic and galvanostatic conditions. At electrodes that have been mechanically polished and cathodically reduced, growth of the lower oxidation state phase, β-Ni(OH) 2, follows, under both experimental conditions, the formalism of the Cabrera—Mott model of high field assisted formation of thin oxide films. At 10 −3 Acm −2, the field within the oxide phase is 8.5 × 10 6 V/cm. The field decreases 1.3 × 10 6 V/cm for each ten-fold decrease of current density. The potential at which β-Ni(OH) 2 begins to grow, V 0 = −0.39 V vs she, is identified as the reversible potential of the phase. It is, however, 270 mV positive than the reversible potential listed in literature for Ni(OH) 2 formation in solutions of the same pH. It is suggested that the observed reversible potential refers to the β-Ni(OH) 2 phase. The exchange current density for the oxide film growth is 2.3 × 10 −10 Acm −2. Comparison of the kinetics at Pt and Ni electrodes indicates that the activation energy for the growth of the β-Ni(OH) 2 phase is 21 kcal/mole.

Anodic behavior of platinum electrodes in oxygen-saturated acid solutions

The anodic behavior of Pt electrodes in O 2-saturated sulfuric acid solution is shown to be critically dependent on the treatment of the electrode. The open circuit potentials, 1.48, 1.13 and 0.98 V, observed in purified solution, are related to the thermodynamic platinum-oxide potentials. The erratic behavior of Pt electrodes in non-purified solutions is also explained. The thermodynamic value of 1.23 V for the H 2O-O 2 reaction was obtained when certain conditions were fulfilled. The Tafel (b) parameter for the anodic oxygen evolution reaction was o.113, and i o = 2.5 × 10 −10 A/cm 2. Chemisorption of oxygen leads to a dipole potential of 0.98 V. An oxide film is produced by anodic oxidation. This was confirmed by an optical method (thin film ellipsometer) which showed that the growth of a film commences only above 1 V. Cathodic transient technique was used to measure the oxygen coverage ( Q) of the Pt electrode as a function of the applied potentiostatic voltage ( V). A linear relationship between Q and V was found. This relationship, Q = f( V), is explained on the basis of a modified Cabrera-Mott model. The evidence suggests that growth takes place on certain sites of the first layer only.