Experimental study of positron motion in Kapton

The drift of positrons under the action of an electric field in Kapton has been observed by using a variable-energy positron beam and the Doppler broadening technique. Even if the maximum applied field (2 MV/cm) is so high that the electrostatic work for a displacement of less than 0.15 nm is larger than the thermal energy, the experimental data can be fit by the diffusion model with a field-independent mobility. The best-fit value of the mobility (ca. 1×10^–3 cm² V^–1 s^–1) is extremely low in comparison with other dielectrics. The results are discussed with regards to the interaction of positrons with the molecular field.     

Photoluminescence and electric conductivity in germanium at low temperatures

We have studied the exciton and electron-hole droplet (EHD) luminescence in optically irradiated germanium at temperatures between 1.8 and 4.2 K in the presence of an electric field. Simultaneously the electric conductivity was measured. The sample material was high-purity Ge (N _A –N _D =7·10¹⁰ cm^–3) andp-doped Ge withN _A =3·10¹⁴ cm^–3. In the high-purity Ge samples the exciton and EHD-luminescence intensity decreased nearly linearly as a function of the applied electric current, whereas the dependence upon the electric field was more complicated. Our results could be explained by a model in which carrier annihilation at the contacts following a rapid drifting process plays a dominant role (drift model). In thep-doped Ge samples the current-dependence of the luminescence intensity was qualitatively similar. However, here the drift model is not strictly valid any more because of the reduced carrier mobility and the generation of additional carriers by impurity impact ionization. During variation of the electric field, the luminescence intensity and the electric current show hysteresis. Here the capture of the moving carriers by the EHD appears to play an important role, in addition to the EHD-nucleation process.     

Anomalous plasma diffusion transverse to high magnetic fields in InSb

Investigations on the ambipolar diffusion of an electron-hole plasma transverse to a magnetic field have been carried out in InSb. A plasma layer, produced at the surface of the sample by a short laser pulse, was moved through the sample in crossed electric and magnetic fields by the Lorentz force. From the broadening of the plasma layer we found at 80K an enhanced diffusion coefficient which decreased proportional to 1/B for magnetic fields higher than 1T, constrary to the expected classical 1/B ² dependence. Furthermore, the diffusion coefficient was strongly dependent on the electric field. The ambipolar drift velocity, measured simultaneously showed a classical behaviour. Together with the enhanced diffusion we observed instabilites in the electric potential. The instability threshold decreased towards the cathode.     

Drift theory of charged-particle motion for a strong electric field in a weakly relativistic approximation

Drift equations of motion are derived for a charged particle in the case of a strong electric field with allowance for relativistic effects of order v²/c². The role of these effects is discussed along with the effects of a high-frequency field. The cases of weak and strong electric fields are distinguished [2] in the drift theory of the motion of charged particles in weakly inhomogeneous magnetic and electric fields. In the case of a weak electric field, the electric-drift velocity is v_E v, where v is the characteristic velocity of the particle. For a strong electric field,v _Ev.The drift theory has now been reasonably well developed for the case of weak electric fields in the classical and relativistic cases, for the absence of high-frequency fields and for the presence of these [1–3], Extension of the theory to strong electric fields involves considerable mathematical difficulties, and this has been done only in the classical approximation with and without hf fields [2–4], Here we consider the drift theory of charged-particle motion for the case of a strong electric field in the weakly relativistic approximation, incorporating terms of order v²/c², where c is the velocity of light. Also hf fields may be present.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 7–9, September, 1981.     

Threshold Frequency of Helical Electron–Hole Plasma Instability

Experimental evidence on the dependence of the threshold frequency of silicon oscillistors on the threshold electric field strength, magnetic induction, temperature, and injecting-contact separation is presented. In the temperature interval, where the weak magnetic field criterion is roughly satisfied, the experimental results are shown to be adequately explained by the classical theory of the bulk helical instability of an extrinsic plasma. The threshold frequency in this temperature interval is determined by the sum of two components. One component is due to the ambipolar drift of helical plasma perturbations, and the other results from the presence of the charge-carrier concentration gradient in a direction normal to the vectors of the electric field strength and magnetic induction. In short oscillistors (0.85·10^–3, 2.38·10^–3 m) at 77 K, a semiconductor plasma, wherein the helical instability is excited, approximates an intrinsic plasma, and the threshold frequency is determined by the rotation rate of helical perturbations.     

Diffusion of deuterium in GaAs from a molecular gas source

It is shown for the first time that deuterium can diffuse into GaAs from a gaseous source. Experiments performed at 500° C show two-component diffusion profiles with diffusion coefficients in the range 10^–15–4×10^-1cm²s^–1 depending on the conductivity type of the samples. These diffusion coefficients are considerably lower than those determined after RF plasma hydrogenation. Such a slow diffusion process is related to the reaction of molecular deuterium with the sample surface leading to the dissociation of the deuterium molecules.     

Anomalous plasma phenomena of hollow cathode arc discharge

Magnetically confined argon plasma produced by hollow cathode arc discharge has been studied in different experimental conditions, with discharge current from 10–50 A, vessel argon pressure between 10^–3 and 10^–4 torr (1 torr=133·32 Pa) and axial magnetic field up to 0·12 T. The plasma density measured by a cylindrical Langmuir probe is found to be 10¹⁹ to 4 × 10¹⁹ m^–3 and the electron temperatureT _e varies between 2·5 and 4·8 eV. When an external axial magnetic field is applied the plasma temperature decreases with the increase in the magnetic field intensity until it reaches a minimum value at 0·075T and then increases with the same rate. This has been interpreted as high frequency waves excitation due to electron beam-plasma interaction, which explains the electron density jumps with the magnetic field intensity. Enhanced plasma transport across the magnetic field is studied and classified as anomalous diffusion.     

Hydrodynamic theory of electron transport in a strong magnetic field

The mode coupling contribution to the transverse transport coefficients of a three-dimensional one-component plasma in a strong external magnetic field is calculated. For very strong fields it is found that the tagged particle diffusion rate, the thermal diffusion rate, and the coefficient of viscosity in the plane orthogonal to the field have a Bohm-like B ^–1 behavior. The mode coupling mechanism responsible for such an effect is always one that involves the finite-frequency upper hybrid modes.     

Renormalized vertex modification of Hall conductivity for dilute magnetic alloys

By means of the renormalized vertex procedure for the motion of Green's function developed by the authors, the vertex function of magnetic alloys, based on thes-d exchange interaction, is solved exactly and the corresponding Hall conductivity tensors are obtained. It is found that the value of the renormalized Hall conductivity is (1+h ²)^–1 times less than that before the renormalization (hereh is a reduced magnetic field). It is shown that the renormalized modification of the conductivity is very important in the cases with not too weak external magnetic field and slow relaxation time.     

Scale transformations for renormalized field operators: Discussion of the soluble model

We discuss the operator formulation of the Zachariasen-Thirring model, describing the chain approximation to the propagator (the sum of three-particle massless bubbles) in massless λ⁴ theory. Such a model is formally scale-invariant and explicitly soluble. All intermediate steps of conventional renormalization procedure, regularization, introduction of appropriate counterterms, and cut-off free limit, are explicitly performed. In every step the scaling properties are discussed and respective dilatation currents are written down. After the proper choice of scale transformations for the renormalized field operator, we obtain the nonlocal dilatation current, defining the renormalized dilatation generator D_Λ^R(t). In the cut-off free limit Λ → ∞ the ET commutator of D_Λ^R(t) with renormalized field operators reproduces the Callan-Symanzik modification of “naive” canonical scale transformations. The renormalized scale transformations coincide in the cut-off free limit with renormalized dimensional transformations and define the exact symmetry of the renormalized theory.     

Beam coupling and decoupling in degenerate two-wave mixing in a reflection geometry with photorefractive Bi2GeO20 crystals

The dependence of the effective gain on the incident angle of the beams and on the initial beam intensity ratio is studied in the experiment of degenerate two-wave mixing (TWM) in a reflection geometry with photorefractive Bi₁₂GeO₂₀ (BGO) crystals. A saturation value of the effective coupling constant ¦g¦-0.4 cm^–1 is obtained. In contrast to TWM operated in the drift mode (i.e. with a nonzero electric field applied to the crystal), for TWM operated in the diffusion mode (zero external electric field) as is our case, beam coupling is reduced by moving the crystal or the interference fringes at a constant speed. At high moving speeds, complete beam decoupling can be reached. A comparison between the theoretical and measured dependence of the effective gain on the moving speed is also made. Using this technique, complete isolation of two intersecting coherent beams inside a nonlinear medium can be achieved.     

Saturation of Electrostatic Potential: Exactly Solvable 2D Coulomb Models

We test the concepts of renormalized charge and potential saturation, introduced within the framework of highly asymmetric Coulomb mixtures, on exactly solvable Coulomb models. The object of study is the average electrostatic potential induced by a unique “guest” charge immersed in a classical electrolyte, the whole system being in thermal equilibrium at some inverse temperature β. The guest charge is considered to be either an infinite hard wall carrying a uniform surface charge or a charged colloidal particle. The systems are treated as two-dimensional; the electrolyte is modelled by a symmetric two-component plasma (TCP) of point-like ±e charges with logarithmic Coulomb interactions. Two cases are solved exactly: the Debye–Hückel limit β e²→ 0 and the Thirring free-fermion point β e²=2. The results at the free-fermion point can be summarized as follows: (i) The induced electrostatic potential exhibits the asymptotic behavior, at large distances from the guest charge, whose form is different from that obtained in the Debye–Hückel (linear Poisson–Boltzmann) theory. This means that the concept of renormalized charge, developed within the nonlinear Poisson–Boltzmann (PB) theory to describe the screening effect of the electrolyte cloud, fails at the free-fermion point. (ii) In the limit of an infinite bare charge, the induced electrostatic potential saturates at a finite value in every point of the electrolyte region. This fact confirms the previously proposed hypothesis of potential saturation.     

Diffusion of Au in amorphous Zr x Ni100−x alloys studied by Rutherford Backscattering Spectrometry

The diffusion coefficients (D) of Au in three binary amorphous Zr _x Ni_100–x (x=61, 65, and 67) alloys were measured in the temperature range 549–623 K using the technique of the Rutherford Backscattering Spectrometry (RBS). The D values were found to lie in the range 1.0×10^–21–9.0×10^–20 m²s^–1 for different alloys. The activation energy (Q) was calculated in each case on the basis of an observed Arrhenius temperature dependence of D. The activation energy was found to scale with the crystallization temperature (T _x) of the alloy. Other published measurements for Au diffusion in amorphous Zr-Ni alloys also appear to follow the scaling relation between Q and T _x.     

Investigation of Diffusion of Magnesium in Lithium Fluoride Crystals by the SIMS Method

Using secondary ion mass spectroscopy, diffusion of magnesium impurity in lithium fluoride is investigated. A temperature dependence of the magnesium diffusion coefficients within the temperature interval 870–1073 K is established, which is described by an expression of the following type: D = 2.8·10^–3·exp(–1.5/kT). Combining the data on self-diffusion of cations with the results of the ion conductivity measurements, estimation is made of the thermodynamic parameters characterizing the jump of a diffusant. The calculated frequencies of the impurity ion jump to the cation vacancy are reported.     

Dynamics of electron-hole plasma in semiconductor opening switches for ultradense currents

A physicomathematical model for calculating the dynamics of the electron-hole plasma in semiconductor opening switches for ultradense currents is developed. The model takes account of the real doping profile of a semiconductor p ⁺-p-n-n ⁺ structure and the following elementary processes in the electron-hole plasma: current-carrier diffusion and drift in high electric fields, recombination on deep impurities and Auger recombination, and collisional ionization in a dense plasma. The electrical pumping circuit of the opening switch is calculated by solving the Kirchhoff equations. The motion of the plasma in the semiconductor structure is analyzed on the basis of the model. It is shown that for ultrahigh pumping levels the interruption of the current in the opening switch occurs in the heavily doped regions of the p ⁺-p-n-n ⁺ structure and is due to saturation of the particle drift velocity in high electric fields. Zh. Tekh. Fiz. 67, 64–70 (October 1997)     

Drift mobility and kinetics of transport of excess charge carriers in glassy CdGexAs2

Conclusions We tried to measure transient conductivity response to pulse strongly absorbed excitation (light, accelerated electrons) in sandwich type samples of glassy CdGe _x As₂ compounds. We observed the signal due to transport of free excess carriers. From analysis of experimental results we conclude that in our materials strong trapping effects are present, so the range of excited carriers is very short (10^–4-10^–3 cm) even in the highest electrical fields used (to 10⁴ V. cm^–1). Estimates of upper limit of drift mobility give the values 10^–1- 1 cm² V^–1 sec^–1. We did not succeed in determining the type of carriers which are responsible for the observed effects.     

Effect of structure doping profile on the current switching-off process in power semiconductor opening switches

Using a physicomathematical model, the process of current breaking in power semiconductor opening switches was investigated in p ⁺-p-n-n ⁺ structures with different doping profiles. The model takes account of the actual doping profile of a structure, diffusion and drift of current carriers in a strong electric field, recombination via deep impurities and Auger recombination, and impact ionization in a dense plasma. The calculation of the electrical circuit of an opening switch is based on solution of Kirchhoff’s equations. It has been shown that in the nanosecond regime of breaking superhigh current densities with densities of the interrupted currents from a few to tens of kA/cm², the dominant factor in the current breaking process is the width of the p-region in the initial doping profile of a structure. An increase in the p-region width from 100 to 200 μm makes the velocity of the excess plasma front propagating in the p-region in the reverse pumping stage higher by a factor of 5–7. Higher propagation velocity of the plasma front makes the current breaking process more intensive, which is manifested in the shorter current breaking time and higher overvoltage across the opening switch.     

Anomalous polarization dependence of the photon drag and optical rectification fields inn-type gallium phosphide

The photon drag and optical rectification response fromn-type gallium phosphide containing 2.4×10¹⁶ electrons cm^–3 has been studied using laser sources at 10.61 and 2.83m. By using different contact configurations, the existence of an additional electric field term in the propagation direction has been deduced. This field is shown to arise from the spatial derivatives of the photon drag and optical rectification field components caused by absorption of the radiation. The importance of this effect in detector design is considered.     

Numerical Simulation of Grain-Boundary Grooving by Level Set Method

A numerical investigation of grain-boundary grooving by means of a level set method is carried out. An idealized polycrystalline interconnect which consists of grains separated by parallel grain boundaries aligned normal to the average orientation of the surface is considered. Initially, the surface diffusion is the only physical mechanism assumed. The surface diffusion is driven by surface-curvature gradients, while a fixed surface slope and zero atomic flux are assumed at the groove root. The corresponding mathematical system is an initial boundary value problem for a two-dimensional equation of Hamilton–Jacobi type. The results obtained are in good agreement with both Mullins analytical “small-slope” solution of the linearized problem (W. W. Mullins, 1957, j. Appl. Phys. 28, 333) (for the case of an isolated grain boundary) and with the solution for a periodic array of grain boundaries (S. A. Hackney, 1988, Scripta Metall. 22, 1731). Incorporation of an electric field changes the problem to one of electromigration. Preliminary results of electromigration drift velocity simulations in copper lines are presented and discussed.     

Diffusion of gold in dislocation-free or highly dislocated silicon measured by the spreading-resistance technique

The diffusion of Au in dislocation-free or plastically deformed Si (10¹¹ to 10¹³ dislocations/m²) was measured with the aid of the spreading-resistance technique. The Au profiles produced indislocation-free Si slices by in-diffusion from both surfaces possess nonerfc-type U shapes as predicted by the so-called kick-out diffusion model. This model is used to calculate the contribution of self-interstitials to the (uncorrelated) Si self-diffusion coefficient,D _I ^SD =0.064×exp(–4.80 eV/kT)m² s^–1, from the present and previous data on the diffusivity and solubility of Au in Si in the temperature range 1073–1473 K. Inhighly dislocated Si the diffusion of Au is considerably faster than in dislocation-free Si. From the erfc-type penetration profiles found in this case, effective Au diffusion coefficients were deduced and combined with data on the solubility of Au in Si. ThusC _i ^eq D _i=0.0064 ×exp(–3.93 eV/kT)m² s^–1 was obtained in the temperature range 1180–1427 K, whereC _i ^eq andD _i are the solubility and diffusivity of interstitial Au in Si.