Nearest neighbor effect on capture cross sections measured in IR phototransients on Si:In

The extrinsic photoconductive decay at T=20–100 K is analyzed in FZ-grown Si: In material after pulsed irradiation by a PbSSe infrared laser (=4 m). Trapping time constants (=10 ns-100 s) are resolved for the prevalent In acceptor (N _In=10¹⁶–10¹⁷ cm^–3) and for additional shallow acceptors B, Al, and the X(In)-center present at low concentrations (N=10¹²–10¹⁴ cm^–3). Hole capture cross sections determined for the acceptor levels show a large scatter over up to 4 orders of magnitude. It is shown that the capture cross section is dependent on all the dopant concentrations present in the sample due to nearest neighbor interaction. Due to the formation of donor-acceptor dipoles, the capture cross section assumes low values. A model calculation of the interaction based on only fundamental parameters of Si is in accordance with the experimental data within the experimental error. The hole capture cross sections for isolated acceptors are _p=1×10^–12, 1×10^–14, 1×10^–13, 2.5×10^–13 cm² for indium, X-center, aluminum, and boron at the temperatures T=95 K, 100 K, 70 K, 45 K, respectively.     

Electron kinetics in dc CF4 + O2 mixture discharge

The electron distribution function, transport coefficients and rate coefficients for the dissociation of CF₄ and the excitation of a¹ _g and b¹ _g ⁺ states of oxygen have been calculated for a CF₄ + O₂ mixture discharge. The two-term approximation was used in solving the electron Boltzmann equation. The influence of the dissociation was ignored, so that the results are parametric in the density reduced electric fieldE/N and the ratio of the mixture components. Only slight dependence of the calculated quantities on the composition of the gas mixture has been observed. The critical electric field for the onset of the discharge was found to beE/N=1.1–1.25 × 10^–15 Vcm² and it is weakly decreasing with growth of the oxygen content. With a small amount of oxygen added to CF₄ the electron distribution function and the mean kinetic energy remain practically unchanged.     

Subthreshold K− production in pion-nucleus reactions

The inclusive subthresholdK ^– mesons production in charged pion-induced reactions is analyzed with respect to the one-step production processes on the basis of an appropriate first collision model, which allows one to take into account the various forms of an internal nucleon momentum distribution. Free elementary production processes are described by the phase space calculations normalized to the corresponding total experimental cross sections. Simple parametrizations for the total cross sections of theK ^– production in pion-nucleon collisions are given. Primary-pion energy dependence of the inclusive cross sections for K^– production in ⁺ C– collisions is calculated. The influence of the uncertainties in the elementaryK ^– production cross sections close to the production threshold on theK ^– yield is explored.The authors acknowledge stimulating discussions with V. Koptev on the initial stage of this study.     

Absolute cross-section measurements of inner-shell ionization

Cross section ratios for K- and L-shell ionization of thin silver and gold targets by positron and electron impact have been determined at projectile energies of 30–70 keV. The experimental results are confirmed by calculations in plane wave Born approximation (PWBA) which include an electron exchange term and account for the deceleration or acceleration of the incident projectile in the nuclear field of the target atom. We report first absolute cross sections for K- and L-shell ionization of silver and gold targets by lepton impact in the threshold region. We have measured the corresponding cross sections for electron (e^–) impact with an electron gun and the same experimental set-up.     

Electrical characteristics of the rf-excited oxygen plasma-cathodization-grown SiO2/Si interface

The electrical properties of the SiO₂/n-type Si(100) interface, where the silicon-oxide layer was grown by an electrodeless rf oxygen-plasma-cathodization technique, were investigated usingC-V and DLTS methods. Interface traps with high density in the range of 10¹² eV^–1 cm^–2 and a capture cross section as large as 10^–18 cm² were found in the upper region of the silicon forbidden gap. After a post-annealing process, typically at 400°C for 30 min in dry N₂ atmosphere, their densities and capture cross sections were reduced to the range of 1–2 × 10¹¹ eV^–1 cm^–2 and 10^–19 cm^–2, respectively. Apparant differences in DLTS curves before and after thermal annealing were also observed. Results are qualitatively explained by considering the specific oxidation and annealing mechanism of this low-temperature silicon-oxidation technique.     

Spectral diagnostics of a unipolar high-frequency discharge excited in nitrogen and argon at pressures from 1 to 12 atm

By methods of spectral diagnostics, the temperature of neutral gas and the electron temperature and density have been determined in the channel of a unipolar high-frequency discharge excited at very high pressures. In nitrogen the h.f. discharge was excited at pressures of 1–5 atm, in argon at pressures of 1–12 atm. In the discharge excited in argon, the electron temperature does not change with increasing pressure and isT _e =(6–7)×10³ °K; the electron density increases with increasing pressure. It can be demonstrated that the electron velocity distribution is given by a Maxwellian distribution function although the plasma of a unipolar high-frequency discharge is non-isothermal (T _e –T _n 5×10³ °K).In conclusion, the author thanks Prof. Dr. V. Truneek for stimulating remarks and his kind interest in this work.     

Studies of divacancy in Si using positron lifetime measurement

The charge state dependence of positron lifetime and trapping at divacancy (V₂) in Si doped with phosphorus or boron has been studied after 15 McV electron irradiation up to a fluence of 8.0×10¹⁷ e/cm². The positron trapping cross sections for V ₂ ^2– , V ₂ ^– and V ₂ ⁰ at 300 K were about 6×10^–14, 3×10^–14 and 0.1–3×10^–14 cm², respectively. For V ₂ ⁺ , however, no positron trapping was observed. The marked difference in the cross sections comes from Coulomb interaction between the positron and the charged divacancy. The trapping rates for V ₂ ⁰ and V ₂ ^2– have been found to increase with decreasing temperature in the temperature range of 10–300 K. These results are well interpreted by a two-stage trapping model having shallow levels with energy of 9 meV (V ₂ ⁰ ) and 21 meV (V ₂ ^2– ). The appearance of a shallow level for V ₂ ⁰ can not be explained by a conventional Rydberg state model. The lifetime (290–300 ps) in V ₂ ⁰ is nearly constant in the temperature range from 10 to 300 K, while that in V ₂ ^2– increases from 260 ps at 10 K to 320 ps at 300 K. The lifetime (260 ps) in V ₂ ^2– is shorter than that in V ₂ ⁰ at low temperature, which is due to the excess electron density in V ₂ ^2– . At high temperature, however, the longer lifetime of V ₂ ^2– than that of V ₂ ⁰ is attributed to lattice relaxation around V ₂ ^2– .     

Results of the Bielefeld-Brookhaven e+-H experiment

Positronium formation (Ps) cross sections for positrons impinging on atomic hydrogen were measured in the impact energy range from 13 to 255 eV at the High Intensity Positron (HIP) beam at Brookhaven National Laboratory (BNL). The Ps-formation cross section was found to rise rapidly from the threshold at 6.8 eV to a maximum value of (2.98±0.18) ×10^–16 cm² for 15 eV positrons. By 75 eV it drops below the detection limit of 0.17×10^–16 cm² which is the present level of statistical uncertainty. A more elaborate data analysis was performed leading to somewhat smaller cross sections than originally published [1]. During the course of this experiment further relative data on impact-ionization cross sections were accumulated.     

Dissociative electron attachment to CF2Cl2

Using a recently constructed high resolution crossed electron/molecular beam apparatus consisting of a hemispherical electron monochromator and a quadrupole mass spectrometer we have measured the relative production cross sections for CI^– and F^– via electron attachment to CF₂Cl₂. The relative Cl^– cross section is placed on an absolute scale by reference to an absolute rate coefficient using a calibration method involving integration of the measured anion signal. The most efficient Cl^– production process is at about zero energy and its magnitude is resolution limited. The present high resolution value of 6 × 10^–16 cm² compares well with an earlier value reported by Chen and Chantry. A second peak is detected at around 0.8 eV in accordance with some of the earlier beam and swarm measurements. The observed production of F^– has an appearance energy of 1.9 eV and the energy of maximum cross section is 3.36 eV, the latter value comparing well with several previous studies.     

Volume and grain boundary diffusion of implanted 113Sn in aluminium

Volume and grain boundary diffusion of ¹¹³Sn in aluminium was investigated with the radiotracer method. The implantation technique was used for tracer deposition to avoid problems of tracer hold-up caused by the oxide layer always present on aluminium. The diffusion penetration was chosen large enough to permit serial sectioning of samples with the aid of a microtome.The temperature dependence of the volume diffusivity was determined as D(T)=4.54×10^–5×exp[–(114.5±1.2)kJmol^–1/RT] m ² s ^–1. This confirms previous measurements from our group which already showed that Sn is the fastest foreign metal diffusor so far investigated in aluminium.Grain boundary diffusion of ¹¹³Sn in Al polycrystals was measured in the type-B kinetic regime. The grain boundary diffusion product P=sD _gb (s=segregation factor, =grain boundary width, D _gb=grain boundary diffusivity) was found to be strongly affected by the impurity content of aluminium. For Al polycrystals of 99.9992% nominal purity we obtained P _5N(T)=1.08×10^–8exp [–(96.9±7.5) kJ mol^–1/RT] m³ s^–1 and for less pure Al polycrystals of 99.99% nominal purity P _4N(T)=3.0×10^–10 exp [–(90.1±4.2) kJ mol^–1/RT] m³ s^–1 was determined. The grain boundary diffusion product in the purer material is more than one order of magnitude higher than in the less pure material. Very likely this is an effect of co-segregation of non-diffusant impurities into the grain boundaries.     

The observed relationship between the south pole 225-GHz atmospheric opacity and the water vapor column density

We compare our previously reported measurements of South Pole 225 GHz atmospheric opacity,, to the column of precipitable water vapor (PWV) which was derived from concurrent upper air soundings. From this comparison we found that=(2.8±0.1)×10^–2+(6.9±0.2)×10^–2×PWV with in units of nepers/airmass andPWV in units of mm of precipitable H₂O. We compared our results to predictions from Grossman's AT atmospheric transparency model which is widely used in the radio astronomy community. The coefficient of the second term of the above relation, 0.069, was consistent with the predictions from the model; however, the first term, 0.028, which represents the dry air opacity, was about five to ten times larger than expected. Most of this discrepancy between the observed and the predicted dry air opacity can be accounted for by including contributions from continuum emission from N₂ and O₂ as is done in Liebe's MPM atmospheric model.     

Electrical properties and defect model of tin-doped indium oxide layers

Tin-doped In₂O₃ layers were prepared by the spray technique with doping concentrationsc _Sn between 1 and 20 at. % and annealed at 500 °C in gas atmospheres of varying oxygen partial pressures. The room-temperature electrical properties were measured. Maximum carrier concentrationsN=1.5×10²¹cm^–3 and minimum resistivities =1.3×10^–4 cm are obtained if the layers are doped withc _Sn9 at. % and annealed in an atmosphere of oxygen partial pressurep _O2 10^–20 bar. At fixed doping concentration, the carrier mobility increases with decreasing oxygen pressure. The maximum obtainable mobility can be described in terms of electron scattering by ionized impurities. From an analysis of the carrier concentration and additional precision measurements of the lattice constants and film thicknesses, a defect model for In₂O₃:Sn is developed. This comprises two kinds of interstitial oxygen, one of which is loosely bound to tin, the other forming a strongly bound Sn₂O₄ complex. At low doping concentrationc _Sn4 at. % the carrier concentration is governed by the loosely bound tin-oxygen defects which decompose if the oxygen partial pressure is low. The carrier concentration follows from a relationN=K ₁ ·p _O2 ^–1/8 ·(3 ×10¹⁰ × c_Sn –N)^1/4 with an equilibrium constantK ₁=1.4×10¹⁵ cm^–9/4bar^1/8, determined from our measurements.     

Searching for light Higgs scalar bosons in the next generation of electron-positron collider at LEP

Thee ⁺ e ^–-collider facilities at LEP II, with the CM energy S in the range 100–170 GeV, may be able to detect light Higgs bosons, assuming a high luminosity. The production cross sections of a light Higgs bosonH ⁰ in association with the neutral gauge bosonZ ⁰ are calculated for varying ranges of the CM energy expected to be available to LEP II and VLEEP (Novosibirsk) and for various values of the light Higgs mass. It is found that production cross sections are sizable in comparison with those for the very massive Higgs bosons in proton-anti(proton) supercolliders, Tevatron, Sp¯pS, and SSC, respectively. The implication of this feature is pointed out. Further, prospects for light Higgs production in association with the charged gauge bosonW ^– in ultraenergetic neutrino beams are examined.     

Hadronic vacuum polarization contribution to the Lamb shift in muonic hydrogen

We report on the accuracy of the measurement of the total cross section of the process e⁺e^– ZH and of the Higgs boson mass that would be achieved in a linear collider operating at a centre-of-mass energy of 350 GeV, assuming an integrated luminosity of 500 fb^–1. For that we have exploited the recoil mass off the Z using its leptonic decays into electron and muon pairs. The Higgs mass is determined with 150 MeV accuracy, the recoil mass resolution is about 1.5 GeV and the cross section is obtained with a statistical error of 3%.     

Intensity distributions of spectral lines and elementary processes in a negative glow discharge column

The connection is established between the intensity distributions of the spectral lines along a negative glow discharge column and the individual elementary processes which take place in a low-temperature nonequilibrium plasma (direct electron excitation, step processes, recombination, collisions of the second kind, and charge exchange). The experiments were made in pure inert gases and mixtures of them at pressures 1–30 mm Hg and discharge current densities of 10^–2–10⁺¹ mA/cm². Knowledge of the intensity distributions together with some additional data (such as the electron density, the energy distribution function of the electrons, and the population of the levels) makes it possible to estimate more accurately the rates and cross sections of the reactions which lead to excitation of the glow discharge.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 41–49, July, 1980.     

Pulsed laser deposition of hard coatings in atmospheric air

A new laser plasma technique for non-vacuum deposition of thin films has been proposed and experimentally realized. It is based on the fact that the plasma plume, which occurs under ablation of a target in air by high-intensity short laser pulses, can penetrate through a dense gas environment without significant cooling at the distance of about 1 mm. The technique has been applied to deposit diamond-like carbon (DLC) coatings on stainless steel substrates using four different values of pulse duration: 10 ns, 300 ps, 5 ps and 130 fs. Optimization of different experimental parameters including distance between the target and the substrate, laser intensity and gases (He, Ar, N₂, compressed air) blown in the deposition zone, has been performed. The deposition rate in the experiments was estimated as 2–5×10^-4 nm/(cm²pulse) for the pulse energy of 1–4 mJ. The deposited amorphous carbon films with thickness of several hundred nanometers have shown high average nanohardness (10–25 GPa depending on the irradiation conditions) and good adhesion to substrates (60 MPa). According to X-ray electron spectroscopy analysis the films consist of both sp²- and sp³-bonded carbon and contain 3–7% of free oxygen in bulk. The mechanisms of DLC non-vacuum laser deposition are discussed. To demonstrate the large potential of this technique, the first results on deposition of titanium nitride using ablation of titanium in air with nitrogen jet assistance are also presented. PACS 52.38.Mf; 81.15.Fg; 81.05.Uw     

Nickel bolometers

A modified method of the production of nickel bolometers and their properties are described. A typical example is a bolometer of dimensions 005×2 mm and resistance 14.9 , the NEP (noise equivalent power) of which is 6.23×10^–11 Wc^–1/2 in air and 1.9×10^–11 Wc^–1/2 in vacuo. The paper also describes the influence of cooling the surroundings with liquid nitrogen on the properties of the bolometer. The NEP of a bolometer that works in vacuo with cooled surroundings is 5 times lower. A method is given for correcting the characteristics of two similar bolometers for use in a d.c. bridge which makes the equilibrium of the bridge independent of bridge-current fluctuations in a large range of current.     

Subthreshold antiproton andK − production in heavy ion collisions

Subthreshold ¯p andK ^– and energetic ^– production was studied in Ne + NaF, Cu, Sn and Bi, and in Ni + Ni collisions with incident energies between 1.6 and 2 GeV/u. The measured cross sections indicate a dominant contribution of baryonic resonances. This is also consistent with a generalized scaling behaviour of the cross sections with the energy available in the collision and the energy necessary to produce particles as observed with Ne induced reactions. Deviations from scaling especially pronounced in the Ni-Ni system will be discussed in terms of absorption effects. The flat slope of the excitation function for ¯p production can be related to a reduced production threshold caused by a reduction of the antiproton mass in the dense and heated medium by about 100—150 MeV/c². A similar in-medium mass reduction is also indicated forK ^– mesons. An increased ¯p reabsorption probability for the heavier systems is concluded from the comparison of the ¯p yields in Ne + NaF, Ne + Sn and Ni + Ni collisions.Dedicated to Professor Klaus Dietrich on the occasion of his 60th birthday.     

Characterization of vacancy-related defects introduced into silicon during heat treatment by deep-level transient spectroscopy and gamma-ray diffraction techniques

Vacancy-related defects introduced into n-Si during annealing or aluminium diffusion at high temperature (1000–1250°C) have been studied. Different ambients (argon, nitrogen, vacuum and chlorine-containing atmosphere) were used to create a vacancy supersaturation during heat treatments. Three deep-level centers whose formation is governed by the presence of vacancies have been identified. They were characterized by the following temperature dependences of the thermal emission rate:e3 = 7.92 × 10⁷ T ² × exp(– 0.455/kT),e ₅ = 2.64 × 10⁶ T ² × exp( – 0.266/kT),e ₇ = 7.26 × 10⁶ T ² × exp (– 0.192/kT). The influence of different factors, such as heat-treatment conditions, concentration of oxygen and doping level in initial crystals, on center formation was studied. An asymmetric diffuse-ray scattering was observed near the surface of a crystal irradiated by thermal neutrons and annealed in a chlorine-containing atmosphere. This scattering is related to the formation of structural defects of the vacancy type. In the same region of the crystal, the concentration of the E7 center was one order of magnitude higher than that of other deep-level centers. Comparison of the-ray diffraction and deeplevel transient spectroscopy (DLTS) data suggests that the formation of the center occurs under the conditions of Si supersaturation with vacancies.     

Heavy doping with Sn of GaAs layers grown by molecular beam epitaxy for non-alloyed ohmic contacts

High Sn-doping of molecular beam epitaxy (MBE) grown GaAs layers for non-alloyed ohmic contacts is presented. The highly doped layers were studied by van der Pauw-Hall measurements, Raman spectroscopy, low-temperature photoluminescence and scanning electron microscope. The highest free electron concentration of 8×10¹⁸ cm^–3 and the lowest resistivity of 8× ×10^–4 cm were achieved. Doping limit of Sn in GaAs is discussed. Using the heavily Sn-doped GaAs layers and the in situ Sn deposition the non-alloyed ohmic contacts with specific contact resistance of 8×10^–5 cm² and highly linearI–V characteristics were obtained.This results were presented at the symposium Molecular Beam Epitaxy, Frankfurt/Oder, GDR, March 23–27, 1987.