Electrical properties of GaAs layers irradiated by H4 ions

The quantities(D) and(T) are studied in n- and p-GaAs, irradiated at T = 300°K by H⁺ ions (5 MeV). It is shown that the resistance of lightly doped GaAs specimens increases from original values of ₀ to 10⁹ ·cm upon irradiation by H⁺ ions (5 MeV) to integral fluxes up to D^* – 10¹⁵ H⁺/cm². For D > D^* the layer resistance decreases from 10⁹ ·cm to 1 ·cm at 300°K. It was found that all the GaAs specimens intensely irradiated by H⁺ ions had p-type conductivity near 300°K. Isochronic annealing of radiation defects was studied in the temperature interval 20–700°C.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 39–43, January, 1982.     

Magnetic properties of amorphous Bi2DyFe5O12

Amorphous Bi₂DyFe₅O₁₂ films have been prepared by rf sputtering technique. The magnetic properties, structure and effect of annealing have been investigated. The magnetization increases with increasing magnetic field and is not saturated at a high magnetic field of 65 kOe. In the temperature range 20 K<T<200 K the susceptibility can be described by a Curie-Weiss law with a Weiss constant of =–15 K. The effective paramagnetic moment for the iron ion (4.1 _B ) is much smaller than the theoretical moment (5.9 _B ). Above 200 K the slope in ^–1 vsT curve changes. The Weiss constant and effective paramagnetic moment for the iron ion increase. After annealing above 600°C the amorphous samples crystallize to a single garnet phase with the magnetization of 21 emu/g at 1.5 K and 7.2 emu/g at 300 K, which are the same as those of the bulk Bi₂DyFe₅O₁₂ garnet ferrites.     

Self-gravitating radiation in anti-de sitter space

Equilibrium configurations of self-gravitating massless thermal radiation inside spherical boxes of radiusR in asymptotically anti-de Sitter space (A = -3/b ₂) are constructed numerically for a range of central densities. For each box radius considered (R/b = 0, 1/2, 1, 2, 4, ), there is a unique configuration with maximal total mass and entropy, and another (at a lower central density) with maximum asymptotic red-shifted temperature. With the box removed toR=, the maximum total mass and entropy of self-gravitating thermal radiation areM _max 0.4598b0.7964(–A)^–1/2 andS _max1.3560a ^1/4 b ^3/2 3.0910a ^1/4(–A)^–3/4, and the maximum red-shifted temperature is     

Shallow acceptor population and free hole concentration in Si: In and Si:Ga with IR-photoexcitation

Hall measurements at low temperaturesT<50 K have been performed on Si:In (N _In10¹⁷ cm^–3) and Si:Ga (N _Ga10¹⁸ cm^–3) with infrared photoexcitation of holes into the valence band. It is shown in quantitative agreement with a theoretical model that the population of shallow acceptors, e.g. B and Al, which are present as impurities in concentrations ofN _B,Al10¹²-10¹⁴ cm^–3 strongly affects the photoexcited hole concentration. Photo-Hall measurements can, therefore, serve as a tool for the determination of low impurity acceptor concentrations in the case of high In- or Ga-doping. Hole capture coefficientsB _In=6×10^–4 (T/K)^–1,8 cm³ s^–1 andB _Ga=2×10^–4 (T/K)^–1 cm³ s^–1 have been determined.     

Variation of electronic properties and evidence for a reversible defect induced metal to semiconductor transition in PbMo6S8 observed by ion irradiation and subsequent annealing of thin films

The influence of lattice disorder varied by low temperature irradiation with 20 MeV³²S-ions and subsequent isochronal thermal annealing on Hall-effectR _H (T), resistivityp(T) and superconducting critical temperatureT _c of thin films of the Chevrel-phase PbMo₆S₈ is reported. It is found that the well known, unusual sensitivity ofT _c is correlated with drastic changes of normal state transport properties e.g.R _H (T) andp(T). In the low fluence regime (6·10¹³ cm^–2,T _c 2 K) annealing leads to a monotonous restoration of the initial properties with the main recovery occurring at temperatures as low as 500 K. Contrary to this, annealing of highly disordered samples (10¹⁵ cm^–2) creates semiconductor-like conduction behavior. This manifests itself by a strong increase of the electrical resistivity with decreasing temperature which becomes more pronounced at higher annealing temperaturesT _A . AfterT _A =800 K the resistivityp(15K) is enhanced by more than a factor of 1000 withp(15K)/p(280 K)=210 compared to as irradiated. Further annealing at 900 K and 1000K results in the reappearance of metallicp(T)-behavior and superconductivity (T _c >10K). The observed effects can be understood by systematic changes of the electronic density of states consistent with an earlier proposed defect model.     

Features of current carrier scattering in epitaxial films

The properties of epitaxial cadmium selenide films obtained by condensation in a vacuum on mica substrates under almost equilibrium conditions are investigated. The temperature dependences of the conductivity and current carrier mobility and concentration are studied. The electron concentration in the films depended on the gas phase composition (coevaporation of CdSe + Se or CdSe + In) and varied between 5·10¹⁰cm^–3 and 3.5·10¹⁸. It is shown that the current carrier scattering mechanism depends on their concentration and production conditions. For n₁ 10¹⁶ cm^–2 (T_S520C),n₂ < 10¹⁵ (T_S=630C), scattering on intercrystallite barriers predominated. For n₁ and n₂ greater than the quantities mentioned, scattering by ionized defects becomes dominant. It is established that the magnitude of the intercrystallite barrier in films with 10¹⁵ < n < 10¹⁶ cm^–3 is comparatively small and does not exceed 4·10^–3 eV, whereupon scattering at the barriers is not explicitly manifest. Concentrations of the ionized centers, magnitudes of the intercrystallite barriers, and ionization energies of the donor levels are determined for films obtained under different conditions.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 98–103, September, 1977.     

Photoconductivity of highly excited A IV B VI thin films

We report the results of picosecond photoconductivity measurements in photosensitive electrolytically deposited PbS and vacuum evaporated PbTe polycrystalline films. We determine Auger recombination to be the prevailing carrier recombination mechanism in highly excited PbTe and PbS films and found Auger coefficients _A5×10^–28 cm⁶ s^–1 for PbTe and _A5.3×10^–29 cm⁶ s^–1 for PbS for carrier concentration changes N>10¹⁸ cm^–3. The results indicate that the low mobility values are controlled by intergrain carrier scattering. We have studied the thermal annealing influence on picosecond photoconductivity of the films.     

Pulsed laser crystallization of hydrogen-free a-Si thin films for high-mobility poly-Si TFT fabrication

The possibility to fabricate high-mobility polysilicon TFTs by nanosecond pulsed laser crystallization of unhydrogenated amorphous Si thin films has been investigated. Two types of lasers have been used: a large area ( 1 cm²) single ArF excimer laser pulse and a small diameter ( 100 m) frequency-doubled Nd:YAG laser beam, working in the scanning regime. Processed films have been characterized in detail by different optical and microscopic techniques. Device performances indicate that the best results are achieved with the excimer laser leading to high mobility values (up to 140 cm²/Vs) which are much larger than in polysilicon TFTs fabricated onto the same quartz substrates by low-temperature thermal (630° C) crystallization of amorphous Si films (_fe55 cm²/Vs).     

Spectral response of CdS with Ag,Cu, and in impurities

High-resistance (dark 10⁸ · cm) photosensitive CdS samples (light 10²–10³ · cm at saturation) were obtained from low-resistance ( 0.1 · cm) nonphotosensitive CdS single crystals by thermal diffusion of Cu, Ag, and In in vacuum at 2 · 10^–5 torr. The spectral response of impurity and pure high-resistance photosensitive single-crystal CdS samples was studied in the wavelength range from 0.35 to 3. The short-wave and long-wave limits of the spectral curves, the wavelength range of maximum photoconductivity and the forbidden band width, the depth of impurity levels, the upper edge of the impurity band, and the depth of maximum-density activation centers have been determined from the spectral curves. The results are discussed and compared with published data.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 117–121, June, 1972.     

Negative mass cyclotron resonance maser

Physical principles, design and operation characteristics of a negative mass cyclotron resonance maser inp-type germanium are considered in this paper. The formation of anisotropic inverted distributions of negative effective mass heavy holes in strong electric and magnetic fieldsE H [001], resulting in negative conductivity in the millimetre and submillimetre wavelength ranges, is discussed. The generation is observed at 0.9 to 8mm in low compensated germanium samples with the hole concentration rangeN ₀ 2×10¹² to 2×10¹⁴ cm^–3 at low temperaturesT 25 K in electric fieldsE 40 to 350Vcm^–1. The maser frequency is tuned by the magnetic field corresponding to a cyclotron resonance frequency of carriers with an effective mass ofm _c 0.4m ₀. The spectral width of the emission in single mode operation does not exceed several megahertz. A pulse duration of 1 to 200 s and a repetition rate off _rep 1 to 200 Hz has been obtained limited by sample heating. Possibilities of improving the maser characteristics inp-Ge as well as in other A_III B_v semiconductors and the perspectives of new frequency tuning methods due to the application of uniaxial stress and magnetic field are demonstrated.     

Exchange-coupling across SiO

Strongly temperature-dependent ferromagnetic exchange coupling is observed between Fe films separated by an amorphous SiO barrier. Heating reversibly enhances the coupling strength which monotonically decreases with increasing SiO thickness. AtT=40K the coupling disappears for d_SiO20Å whereas at room temperature it persists up to a barrier thickness of 60 Å. The coupling strengthJ is determined by externally compensating the exchange field of samples grown on an antiferromagnetically biased Fe/Cr/Fe structure. It amounts toJ2·10^–6 J/m² for a sample with d_SiO=25 Å atT=300K. As a tentative explanation we propose that impurity or defect states within the large mobility gap of SiO carry the magnetic interaction across the insulating barrier.     

Trapping by vacancies and mobility of positive muons in neutron-irradiated aluminium

By means of the muon spin rotation (µSR) technique the trapping at vacancies and the mobility of positive muons are investigated in the temperature range 2.5 K to 297 K in polycrystalline aluminium irradiated with fast neutrons at 4.6 K. The observed damping of the µSR signal at temperatures below 50 K is in good agreement with saturation trapping at monovacancies. The temperature dependence of the damping above 50 K indicates that the muon diffusivity exhibits a minimumD _min ^µ+ 110^–13 m² s^–1 near 125 K, which is explained by a transition from coherent to incoherent tunneling. With the help of an estimated trapping cross section a monovacancy concentration of ~3 × 10^–4 after 125 K annealing is deduced, in good agreement with measurements of the residual electrical resistivity. As expected, the vacancy recovery shows up in an irreversible decrease of the damping above ~ 175 K.Work supported by the Bundesminister für Forschung und Technologie and by Schweizerisches Institut für Nuklearforschung (SIN). Requests for reprints and correspondence should be addressed to A. Seeger, MPI für Metallforschung, Heisenbergstraße 1, D 7000 Stuttgart-80, Germany     

Electrical resistivity and thermoelectric power of amorphous niobium-nickel alloys synthetized by vapour quenching

Thin film samples (10–20 thick) of niobium-nickel alloys in the composition range Nb-5 to 95 at % Ni were vapour quenched by R. F. sputtering onto fused quartz substrates held at a temperature of 450 K. It was found that fully glassy alloys were synthetized in the composition range Nb-30 to 85 at % Ni, 2·5 times larger than reported for splat-quenched alloys. Crystallization temperatures exhibited maxima near the eutectic composition and are comparable to those of splat-quenched materials. At room temperature, the electrical resistivity of these alloys lies between 176–210 cm, and the absolute thermoelectric power S between 2·20–2·52 V/K. On increasing the temperature from 4·2 to 775 K, up to which the amorphous alloys are stable, the resistivity of the alloy with=0·50 decreases by about 1·5%; the value of d/dT progressively increases with increasing Ni content, becoming positive at 0·50dS/dT of all alloys lies between 6–8·5×10^–3V deg^–2. The electrical behaviour of these alloys may be treated in terms of electron scattering in disordered structures assuming the nearly free-electron model, in a manner analogous to Ziman's theory of electronic transport in liquid metals.     

Hall effect in heterogeneous Fe-Ni vacuum condensates

The temperature dependence of the electric conductivity and the Hall and Nernst-Ettinghausen effects of amorphous and microcrystalline Fe-Ni films obtained by ion-plasma sputtering, with a content of technological impurities of about 3 at. %, was studied for the first time. The relationship R_a(T) a²(T), was found between the anomalous Hall constant R_a and . R_a was found to be more sensitive to peculiarities of the complex transformation amorphous, microheterogeneous-crystalline state.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 29–33, September, 1990.     

Superplasticity in single-phase Zn-0·25 wt. % Cd alloy

A single-phase Zn-0·25 wt. % Cd alloy was prepared with the mean grain sized¯ 1m. Superplastic characteristics (A _max=320% andm _max>0·4) were established at room temperature at the strain rates.10^–4s^–1. Because of the absence of any second phase particles the fine-grained structure was not stable and an intense grain coarsening was observed both during the long-term ageing and during straining at room temperature. The increasing temperature accelerated this grain coarsening and suppressed the superplastic behaviour. Nevertheless, the stress relaxation tests suggested that the superplastic behaviour might be expected at higher temperatures in coarser grained specimens, too, but at substantially lower strain rates.     

Experimental Test of a Time-Temperature Formulation of the Uncertainty Principle Via Nanoparticle Fluorescence

No Heading The uncertainty in the measured fluorescence decay lifetimes of 30 nm particles of YAG:Cc was used to evaluate the predictions of a novel form of the Heisenberg uncertainty principle suggested by de Sabbata and Sivaram, T t h/k. The worst-case uncertainty in temperature of 4.5 °K (as derived from the relationship between temperature and lifetime) and the measured uncertainty in decay lifetime, 0.45 ns, yielded an internal estimate of T t = 2.0 × 10^–9 °K s, which is 263 times larger than /k = 7.6 × 10^–12 °K s. An external estimate of T t = 4.5 × 10¹¹ °K s (which is = 6 times /k) is derived from the independently measured uncertainty in the temperature of the sample and the experimentally determined uncertainty in lifetime. These results could be low by a factor of 5.6 if signal averaging must be taken into account. If valid, the findings are consistent with the predictions of this version of the uncertainty principle and they imply the existence of a type of thermal quantum limit.     

Direct observation of laser-induced crystallization of a-C:H films

The post-growth modification of diamond-like amorphous hydrogenated carbon a-C:H films by laser treatment has been studied by transmission electron microscopy and Raman spectroscopy. a-C:H films grown on Si substrates by benzene decomposition in a rf glow discharge were irradiated with 15 ns pulses of a KrF-excimer laser with fluences in the range of E=50–700 mJ/cm². At fluences below 100 mJ/cm² an increase in the number of graphitic clusters and in their ordering was evidenced from Raman spectra, while the film structure remained amorphous according to electron microscopy and electron diffraction observations. At higher fluences the appearance of diamond particles of 2–7 nm size, embedded into the lower crystallized graphitic matrix, was observed and simultaneously a progressive growth of graphite nanocrystals with dimensions from 2 nm to 4 nm was deduced from Raman measurements. The maximum thickness of the crystallized surface layer (400 nm) and the degree of laser annealing are limited by the film ablation which starts at E>250 mJ/cm². The laser-treated areas lose their chemical inertness. In particular, chemical etching in chromium acid becomes possible, which may be used for patterning the highly inert carbon films.     

Redox reaction of μ−O in water determined by the chemical shift

The chemical shift of ^–O in water, ice and in aqueous hydrogen peroxide (H₂O₂) was examined using ^–O resonance. Two distinct signals are obtained in water and ice. Using ⁺ resonance signal as a standard, the observed shiftsg are –7.0×10^–4 (signal A) and –12.1×10^–4 (signal B) respectively. The intensity of the two fractions are almost equivalent in water; however, in ice, the intensity of signal B decreases with increase of signal A. After the addition of hydrogen peroxide to the water signal B disappears, A increases, and the total amplitude is increased.     

Low-temperature properties of amorphous (Mo1−x Ru x )0.8P0.2 alloys

Amorphous (Mo_1–x Ru _x )_0.8P_0.2 alloys (0.3x0.7) have been investigated with measurements of the specific heatC and thermal conductivity . Also the superconducting properties (critical temperatureT _c and upper critical field) have been determined. Well belowT _c , all alloys show the familiar behavior known for glasses, i.e.CT ⁿ and T _m withn1 andm2 which is attributed to tunneling states (TLS). The largeT _c allows an unambiguous determination of the coefficients ofC and . Compairing our data with literature data, we find no correlation between the TLS density of states and the glass temperature or crystallization temperature, as opposed to insulating glasses where such a correlation appears to exist. The unusual annealing behavior found previously in amorphous Zr–Ni and Zr–Cu, which was attributed to a change in the TLS relaxation-time spectrum, is confirmed in the present work.     

Creep of cadmium single crystals

The creep of cadmium single crystals has been investigated in the temperature range of 78 to 297 K. It has been shown that the transient creep of Cd single crystals is described by a time dependence of the creep strain rateå t ^–m wherem is a function of the resolved shear stress and temperature. The activation areaA* depends on the resolved shear stress,, A ^{* –r} wherer=1 for room temperature andr 1·4 for 204 and 78 K. The activation enthalpy does not practically depend on the resolved shear stress in the whole region of the resolved shear stress measured and with increasing temperature it increases roughly withT ². For temperatures above 200 K the nonconservative jog motion is proposed to be the dominant thermally activated process.The authors are very grateful to Dr. P.Kratochvíl for valuable discussions.