Characterization of low pressure chemically vapour- deposited thin silicon nitride films

Silicon nitride films prepared on silicon by low pressure chemical vapour deposition (LPCVD) were characterized by electrical measurements (current-voltage and capacitance-voltage), Auger electron spectroscopy (AES) and reflection high energy electron diffraction (RHEED). The contact current versus contact field characteristics were interpreted in terms of the Fowler-Nordheim tunnelling of holes from silicon into silicon nitride, with the field of charged traps and the effect of changing the triangular shape of the barrier under steady state conditions taken into account and on the assumption of a Poole-Frenkel detrapping mechanism. AES data show that the LPCVD process yields stoichiometric Si₃N₄ films. RHEED data shows that films of thickness more than 10 nm are amorphous. Some crystalline structures of Si₃N₄ and SiO₂ are observed for thicknesses of less than 10 nm.     

Preparation and d.c. conductivity studies of amorphous layers with the composition Ge35Se53Te10Hg2

The preparation of thin amorphous layers with the integral composition Ge₃₅Se₅₃Te₁₀Hg₂ by free evaporation of the glass Ge_33.3Se_50.0Te_8.35Hg_8.35 (mol.%) in vacuo is reported. The origin of concentration gradients perpendicular to the film plane is discussed with the help of data obtained from chemical analyses of the layers and from mass spectrometric investigations. The influence of the concentration gradients on a slight asymmetry of the current-voltage characteristics controlled by the Poole-Frenkel effect of single centres is shown. The analysis of the high field conductivity also demonstrates different effective permittivities for the two field directions.     

Sur les propriétés diélectriques en courant alternatif de structures or-monoxyde de silicium-or

Thin gold-silicon monoxide-gold films have been prepared by thermal evaporation in vacuo; the oxide thickness was about 5000 Å, with composition SiO_x (x slightly greater than one). A study of capacity and loss tangent as functions of frequency and temperature was carried out. The experimental results agree approximately with the Simmons model (established for alternating current). They show the existence of Schottky barriers of approximately 70 Å thickness. The conduction in the interior of the insulator is ascribed to a Poole-Frenkel effect with activation energy 0.45 eV; the concentration of donor centres has been found to be about 2×10¹⁹ cm^-3.     

Unipolar resistive switching behavior of BiFeO3 thin films prepared by chemical solution deposition

Bismuth ferrite (BiFeO₃, BFO) thin films were spin-coated on Pt/Ti/SiO₂/Si substrates by a chemical solution deposition method. The ferroelectric BFO films annealed at 500 °C and 550 °C were found to possess unipolar resistive switching behaviors. The resistance ratio of the high resistance state (HRS) to the low resistance state (LRS) of the unipolar resistance switching is about 10³ for the ferroelectric BFO films. The conduction mechanisms are concluded to be space charge-limited conduction for the initial state and Ohmic conduction for the LRS. As for the HRS, the Poole-Frenkel emission fits well in the whole voltage region. Traps composed of oxygen vacancies are considered to play a key role in forming conducting paths. The relaxation time of electronic carriers is much shorter than that of ionic oxygen vacancies; therefore, the resistance switching is considered more probably due to carrier injection and emission through the Poole-Frenkel model after forming.     

Electrical properties of plasma-deposited polysiloxane and metal/plasma polysiloxane/GaAs structures

Thin films of polymer were grown on metallized glass or GaAs substrates using plasma polymerization of hexamethyldisiloxane ((CH₃)₃SiOSi(CH₃)₃). Results concerning the electrical properties of the polymer sandwiched between two metal electrodes are first reported and then those of metal/(plasma polymer)/GaAs structures are presented. On the basis of their electrical behaviour, the possibility of using plasma polymer films as a passivating layer for III–V compounds is discussed.Step response experiments were carried out on metal/(plasma polymer)/metal structures. Both transient and steady state currents were recorded as a function of the electric field, the temperature, the nature of the electrodes and the applied pressure. At low fields the transient current is produced by dipolar orientation. At higher fields the dielectric response becomes non-linear and a space charge builds up in the films.The steady state behaviour involves electronic transport. It is a bulk-controlled process limited by a Poole-Frenkel mechanism.In the case of metal/(plasma polymer)/GaAs structures the density of states at the polymer-GaAs interface features a U-shaped distribution with a minimum of 10¹² states eV^-1 cm^-2 located 1.1 eV below the conduction band of the semiconductor.     

Electrical conduction through co-evaporated SiOx-SnO thin films

D.c. electrical properties of SiO _x -SnO (300 nm) thin films forming part of a metal-insulatormetal (MIM) structure are reported. The conduction process is related to the field-assisted thermal excitation of electrons from the trapping centres in the forbidden band gap to the conduction band (Poole-Frenkel effect). The energy gap between the trapping level and the bottom of the conduction band in 80% SiO _x -20% SnO in thin films (300 nm) is found to be equal to 0.50 eV. The level of conduction in the composite films is observed to increase with an increase in the content of SnO.     

Electronic conduction in thin amorphous MoO3/SiO films deposited by co-evaporation

A study of the effects of changes in composition and temperature on the electrical properties of MoO₃/SiO thin amorphous films is presented. The high-field conduction is probably due to the Poole-Frenkel effect as it is in simple SiO. At low temperature and low field the electron hopping process is dominant but conduction at higher temperatures is a contact-limited process. The decrease in conductivity with increasing concentration of SiO in MoO₃ may be attributed to the increasing number of trapping centres introduced in MoO₃/SiO films during the evaporation process. The increase in conductivity in MoO₃/SiO films with increasing temperature is attributed to the increasing concentration and higher mobility of charge carriers.     

Photo-injection of electrons from metal films into single-crystal Al2O3 layers

Photoemission from metal films into insulators has been confined in the past to experiments where the thickness of the insulating films was less than one micron. In this paper we report results of photo-injection of electrons from thin metallic films into single-crystal sapphire films of 10–20 mil thickness. Results are given for the interface barrier height between the metal electrode and the sapphire layer for several metals. The results of the study of the transport of these injected carriers in Al₂O₃ show a possible Poole-Frenkel type of transport through the insulator. Both transient and steady state transport data have been obtained as functions of the electric field in the insulator and the temperature. It is also shown that this structure with metal films on both sides of the insulator layer is a convenient tool for the study of transport properties of photo-injected electrons in both metal and insulating layers.     

D.c. and a.c. conduction in amorphous titanium dioxide thin films

The d.c. and a.c. measurements were performed with two groups of vacuum- deposited amorphous TiO₂ thin films which had metal-TiO₂-metal sandwich structures. The d.c. conduction process and the frequency dependence of the a.c. conductivity σ_ac of “group 1” samples,which contained numerous oxygen vacancies, showed the three-dimensional Poole-Frenkel effect and σ_ac was proportional to the frequency ? over the range 50 Hz–50 kHz. “Group 2” samples, in which the vacancies were subsequently removed by heat treatment in air, showed the three-dimensional anomalous Poole-Frenkel effect and σ_ac was proportional to ?ⁿ where n was 0.7–0.8.     

Decay of Counterflow Quantum Turbulence in Superfluid 4He

We have simulated the decay of thermal counterflow quantum turbulence from a statistically steady state at T=1.9 K, with the assumption that the normal fluid is at rest during the decay. The results are consistent with the predictions of the Vinen equation (in essence the vortex line density decays as t ^?1). For the statistically steady state, we determine the parameter c ₂, which connects the curvature of the vortex lines and the mean separation of vortices. A formula connecting the parameter χ ₂ of the Vinen equation with c ₂ is shown to agree with the results of the simulations. Disagreement with experiment is discussed briefly.     

Influence of carrier injection on resistive switching of CaCu3Ti4O12 thin films with Ni electrode

The influence of electron injection on the electric-pulse-induced resistive switching of perovskite CaCu₃Ti₄O₁₂ (CCTO) films was studied by current-voltage (I-V) measurements. The electron injection was reduced by annealing the sample in an O₂ atmosphere. The switching from the high-resistance state HRS to the low-resistance state LRS by a filamentary mechanism was suppressed when the carrier injection occurs by Poole-Frenkel emission. The interfacial potential barrier plays a crucial role in determining the carrier injection.     

Transient and steady state photoelectronic analysis in TlInSe2 crystals

The temperature and illumination effects on the transient and steady state photoconductivities of TlInSe₂ crystals have been studied. Namely, two recombination centres located at 234 and at 94 meV and one trap center located at 173 meV were determined from the temperature-dependent steady state and transient photoconductivities, respectively. The illumination dependence of photoconductivity indicated the domination of sublinear and supralinear recombination mechanisms above and below 160 K, respectively. The change in the recombination mechanism is attributed to the exchange of roles between the linear recombination at the surface and trapping centres in the crystal, which become dominant as temperature decreases. The transient photoconductivity measurement allowed the determination of the capture coefficient of traps for holes as 3.11 × 10⁻²² cm⁻².     

Nanograins dependent dielectric constant, tunability, phase transition, impedance spectroscopy and leakage current of (Pb1 − xSrx)TiO3 thin films

Nanostructured (Pb_1 − xSr_x)TiO₃ (PST) (x = 0.1, 0.2 and 0.3) thin films have been prepared by chemical solution deposition process using spin coating technique. The solution as such was deposited on Pt/Ti/SiO₂/Si substrates and annealed at 650 °C/3h. Nanograins dependent dielectric properties of PST films show dielectric constant up to the higher frequency region, low losses, large tunability and phase transition at small temperature. The impedance data has been fitted by Cole-Cole model to study the effect of grain boundaries on the dielectric properties. The current-voltage characteristics have been measured to study leakage current in PST films and described by Poole-Frenkel emission model. It is suggested that the key carrier transport process in PST films is emission of electrons from a trap state near the metal-film interface into a continuum of states associated with each conductive dislocation. The activation energy value for carrier transport in PST films is obtained from temperature-dependent current-voltage characteristics.     

Electric-field and temperature effects on electric currents in polycrystalline ZnGa<Subscript>2</Subscript>Se<Subscript>4</Subscript>

The current (electrical transport) through In/ZnGa₂Se₄/In structures has been measured as a function of temperature and applied electric field at temperatures from 77 to 400 K in fields from 10 to 3 × 10⁴ V/cm. The results are analyzed in terms of the Poole-Frenkel effect and space-charge-limited currents. The activation energy of traps and trap concentration in ZnGa₂Se₄ and its refractive index are determined to be E _t= 0.8 eV, N _t = 4 × 10¹³ cm^?3, and n = 2.4, respectively.     

dc conduction properties of TlSbSe2 crystals

TlSbSe₂ samples used for electrical measurements were cleaved from larger crystals grown by using the Bridgman-Stockbarger method. Electrical properties of TlSbSe₂ have been carried out as a function of temperature (203-258 K) at different dc electric fields. The dc studies revealed the non-ohmic type of conduction (log I versus log V plot). The field lowering coefficientβ is evaluated from log I versus E^1/2 plot. It is found that the dominant conduction mechanism in these samples is Poole-Frenkel type. The activation energies are also calculated at different voltages. It is seen that the activation energy decreases with increase of applied voltage.     

Unsteady free convective boundary-layer flow near a stagnation point in a heat-generating porous medium

The free convection boundary-layer flow near a stagnation point in a porous medium is considered when there is local heat generation at a rate proportional to (T ? T _∞) ^p , (p ≥ 1), where T is the fluid temperature and T _∞ the ambient temperature. Two cases are treated, when the surface is thermally insulated and when heat is supplied at a constant (dimensionless) rate h _s from the boundary. If h _s = 0 the solution approaches a nontrivial steady state for time t large in which the local heating has a significant effect when p ≤ 2. For p > 2 the effects of the local heating become increasingly less important and the solution dies away, with the surface temperature being of O(t ^?1) for t large. When h _s > 0 and there is heat input from the surface, the solution for p ≤ 2 again approaches a nontrivial steady state for t large and all h _s . For p > 2 there is a critical value h _s,crit (dependent on the exponent p) of h _s such that the solution still approaches a nontrivial steady state if h _s < h _s,crit. For h _s > h _s,crit a singularity develops in the solution at a finite time, the nature of which is analysed.     

Transient and steady state creep behaviour of polycrystalline MgO

Stress change experiments during compressive creep tests at high stresses on polycrystalline MgO at 1596 K have shown that the creep rate at any instant during transient and steady state creep is predicted by the ratio,r/h, wherer is the rate of recovery (=??σ/t6t) andh is the coefficient of strain hardening (=?σ/?ε). Over most of transient and steady state creep, whenh is constant and the decrease in creep rate, \(\dot \in\) , is a direct result of a decrease inr, the variation of the creep strain,ε, with time,t, is accurately described as $$ \in = \in _0 + \in _T (1 - e^{ - mt} ) + \dot \in _s t$$ whereε ₀ is the instantaneous strain on loading,ε _T the transient creep strain,m relates to the rate of exhaustion of transient creep and \(\dot \in _s\) is the steady creep rate. Deviations from this equation occur during the initial 10 to 15% of the transient creep life, whenh increases rapidly. The variations in \(\dot \in\) ,r andh during transient and steady state creep are explained in terms of a model for creep in which the rate-determining process is the diffusion controlled growth of the three-dimensional dislocation network within subgrains to form dislocation sources allowing slip to occur.     

Numerical Studies of Counterflow Turbulence

We performed the numerical simulation of quantum turbulence produced by thermal counterflow in superfluid ⁴He by using the vortex filament model. The pioneering work was made by Schwarz, which has two defects. One is neglecting non-local terms of the Biot-Savart integral (localized induction approximation, LIA), and the other is the unphysical mixing procedure in order to sustain the statistically steady state of turbulence. We succeeded in making the statistically steady state without the LIA and the mixing. This state shows the characteristic relation L=γ ² v _ns ² between the line-length-density L and the counterflow relative velocity v _ns with the quantitative agreement of the coefficient γ with some typical observations. We compare our numerical results with the observation of experiment by Paoletti et al., where thermal counterflow was visualized by solid hydrogen particles.     

Mechanisms of current conduction in Pt/BaTiO3/Pt resistive switching cell

The 80-nm-thickness BaTiO₃ (BT) thin film was prepared on the Pt/Ti/SiO₂/Si substrate by the RF magnetron sputtering technique. The Pt/BT/Pt/Ti/SiO₂/Si structure was investigated using X-ray diffraction and scanning electron microscopy. The current-voltage characteristic measurements were performed. The bipolar resistive switching behavior was found in the Pt/BT/Pt cell. The current-voltage curves were well fitted in different voltage regions at the high resistance state (HRS) and the low resistance state (LRS), respectively. The conduction mechanisms are concluded to be Ohmic conduction and Schottky emission at the LRS, while space-charge-limited conduction and Poole-Frenkel emission at the HRS. The electroforming and switching processes were explained in terms of the valence change mechanism, in which oxygen vacancies play a key role in forming conducting paths.