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– .     

Quantitative assessment of solid oxide electrochemical doping

Quantitative analysis of metal cation doping by solid oxide electrochemical doping (SOED) has been performed under galvanostatic doping conditions. A M–β″-Al₂O₃ (M=Ag, Na) microelectrode (contact radius: about 10 μm) was used as cation source to attain a homogeneous solid–solid contact between the β″-Al₂O₃ and doping target. In Ag doping into alkali borate glass, the measured dopant amount closely matched the theoretical value. High Faraday efficiencies of above 90% were obtained. This suggests that the dopant amount can be precisely controlled on a micromole scale by the electric charge during electrolysis. On the other hand, current efficiencies of Na doping into Bi₂Sr₂CaCu₂O_y (BSCCO) ceramics depended on the applied constant current. Efficiencies of above 80% were achieved at a constant current of 10 μA (1.6 A cm⁻²). The relatively low efficiencies were explained by the saturation of BSCCO grain boundaries with Na. By contrast, excess Na was detected on the anodic surface of ceramics at a constant current of 100 μA (16 A cm⁻²). In the present study, we demonstrate that SOED enables micromole-scale control over dopant amount.     

The sub-micrometer thickness n-InSb/i-GaAs epilayers for magnetoresistor applications at room temperatures of operation

Magnetotransport at fields up to 500 mT and LF-noise characteristics are reported for miniature magnetoresistors with ferrite concentrators based on Sn-doped n-InSb/i-GaAs heterostructures grown by MBE. The thickness of the InSb epilayers lie in the range 0.55–1.5 μm giving room temperature mobilities of 2.5–5.5 m² V⁻¹ s⁻¹ with carrier densities of (0.5–1.5)×10¹⁷ cm⁻³. The room temperature magnetoresistance (MR) for our two terminal devices could be as high as 115% at 50 mT which is comparable to the extraordinary MR (ExMR) recently reported in microscopic composite van der Pauw disks four terminal devices [Science 289 (2000) 1530]. In addition, a high signal-to-noise ratio and a good temperature stability of R(B)/R₀=0.5–0.83% K⁻¹ was observed for B<60 mT (below the saturation field B_sat for ferrite). Device resistance stability R₀(T) was equal to 0.27–0.66% K⁻¹ in zero field with a nominal device resistance R₀=197–224 Ω for DC currents in the range I=0.01–1.0 mA. The minimum detectable magnetic field is estimated from the reduced differential MR (∂R/∂B)/R=2000% T⁻¹ at B=31 mT and normalised 1/f current noise power spectral density measured at the same field. The resolution limit B_min=2.6 nT at 10² Hz and B_min=0.82 nT at 10³ Hz. These resolution limits are seven times better than those recently reported for the same material n-InSb/i-GaAs and ferrite fabricated Hall sensors [Magnetotransport and Raman characterization of n-InSb/i-GaAs epilayers, for Hall sensors applications over extremely wide ranges of temperature and magnetic field, Proceedings NGS 10, IPAP Conference Series 2, IPAP, Tokyo, 2001, pp. 151–154].     

Dielectric relaxation effect in MIM systems

On applying voltage to a thin film Al-Al₂O₃-M system, one observes a decrease of the leakage current with time. After a long period (10³–10⁴ s) the current settles at a new value, lower by several orders of magnitude. After shortcircuiting the sample, there appears a discharge current of reversed polarity, with decay time again of the order of 10³ s. This effect has been reproducibly observed in the temperature range 80–500 K. The present paper is devoted to the experimental study of this relaxation phenomenon in dependence on the parameters of the system (thickness of the dielectric layer, temperature, and material of the upper electrode).The paper is based on RNDr Thesis of the first author.     

Infrared and Raman spectra of M2Cu2O5 (M=Y,Ho)

We report both Raman and infrared reflectivity spectra of M₂Cu₂O₅ (M=Y, Ho) at room temperature in the spectral range of 30–1000 cm^–1.37 (31) ir and 18 (15) Raman active modes of Y₂Cu₂O₅ (Ho₂Cu₂O₅) are observed. A factor group analysis has been performed to identify the symmetries of the observed modes. Comparing the vibrational spectra of these compounds we conclude that the phonons above 300 cm^–1 originate from the Cu–O vibrations and those under 300 cm^–1 from M–O vibrations.Alexander von Humboldt Foundation fellow     

Temperature anomalies in the dispersion and absorption of sound near phase transition points at low temperatures

We examined the attenuation and dispersion of sound during phase transitions with an overdamped soft mode at low temperatures. The obtained temperature and frequency dependences differ from results known at the high-temperature asymptote. The temperature anomalies are sharper. At low frequencies, the attenuation behaves like (T – T_c)^–5/2 for a critical phonon spectrum isotropic in k, and like (T–T_c)^–2 for uniaxial ferroelectrics. The presence of a temperature anomaly is characteristic also for high-frequency attenuation, behaving like (T–T_c)^–1/2 and ln(T–T_c)., respectively. We discuss briefly the results obtained from an experimental test.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 70–74, January, 1989.     

Electrical properties of doped 3-tetradecylpolypyrrole/metal devices

The electrical properties of devices made of doped 3-tetradecylpolypyrrole (PPy-C₁₄) thin films sandwiched between indium-tin-oxyde (ITO) and gold metal electrodes are reported. The current density–voltage (J–V) curves are asymmetric and nonlinear implying a non Ohmic rectifying contact. Using standard thermionic emission theory (Schottky) J–V characteristics were satisfactorily fitted with a saturation current of J₀=1.5×10^-5 A cm^-2, a barrier height of ϕ_b=0.7 eV, and an ideality factor of n=5.3. Characteristics from the plot of J–V versus 1/T show that the activation energy of the thermionic emission process is higher below the glass transition temperature of PPy-C₁₄ (T_g=45 °C) than above, which seems to indicate that the hopping conduction process is enhanced at T>T_g. The carrier concentration has been calculated from capacitance–voltage (C-V) measurements (N=1.9×10¹⁷ cm^-3) allowing estimation of the carrier mobility μ=2.6×10^-2 cm² V^-1 s^-1. PACS 73.61.Ph; 73.40.Sx; 73.30.+y     

Tunnett

Conclusion The p⁺–n⁺–i(v)–n⁺ GaAs Tunnett diodes have been fabricated with control of the growing diffusion technique in TDM CVP liquid phase epitaxial growth. The threshold current density (J_th) of 2.8×10⁴ A/cm², the threshold voltage (V_th) of 5.3V for the oscillation and the 1% duty cycle operation have been succeeded. The more reduction of J_th and V_th will be able to realize the CW operation.     

Tritium isotope separation by CO2-laser irradiation at low temperatures

Tritium isotope separation by CO₂-laser induced multiphoton dissociation of CTF₃ is investigated. For the optimization of the performance of this working substance, trifluoromethane, the conditions to yield high-selectivity at high-operating pressure and low-critical fluence for complete dissociation are studied using our deconvolution procedure. The irradiation conditions are varied over the following ranges; wavenumber: 1052–1087 cm^–1, gas temperature: 25°C to –78°C, CHF₃ pressure: 5–205 Torr. The selectivities exceeding 10⁴ are observed for 85–205 Torr CHF₃ at –78°C by the irradiation at 1057 cm^–1.     

Dynamic properties of P4O6S and P4O7: P spin–echo and P MAS–NMR investigations

The rotational dynamics of P₄O₆S and P₄O₇ in the solid state were studied by means of ³¹P NMR spectra of spinning and static powder samples in the temperature range of 153–295 K and 295–388 K, respectively. All spectra were simulated to confirm the type of the motion and to extract the time scales as a function of the temperature. Good agreement between experimental and theoretical data was obtained on the basis of a three-site jump model. For P₄O₆S, the activation energy and the pre-exponential factor derived from the lineshape simulations amount to 51(2) kJ/mol and 6(3)·10¹⁵ s⁻¹. For P₄O₇, the spectral analysis yields an activation energy of 67(1) kJ/mol and a pre-exponential factor of 6(2)·10¹⁴ s⁻¹. The dynamic behavior was checked independently by lineshape analyses under both MAS and static conditions. Activation energies are consistent within the errors for the lineshape analyses. Additionally, we have analyzed spin–lattice relaxation measurements, which show the correct trends for the activation energies.     

Time and temperature dependent breakdown characteristics of ZnS:Mn films obtained by rf-magnetron sputtering

Breakdown delay times (t_del) for films of managanese-doped zinc sulfide (ZnS:Mn) were measured in the range 10^–6–10^–1 s. The maximum value was t_del=10^–3–10^–2 s. The electrical strength (E_br) was found to increase as the voltage pulse duration was reduced, the more so the thinner the ZnS:Mn film. The temperature dependence of E_br exhibited a weak reduction in E_br as the temperature was raised to roughly 80°C and a sharp reduction in E_br for T>130°C. A maximum in E_br was observed at T130°C which is presumably explained by a structural modification of the ZnS:Mn film. The experimental results obtained are explained in terms of a combined electronic and thermal breakdown mechanism.State Academy of Control Systems and Radioelectronics, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 3–6, April, 1994.     

Luminescence of CsPbBr3-like quantum dots in CsBr single crystals

Luminescence and decay kinetics of the Pb²⁺ aggregates in CsBr host crystals were measured in the 4–300 K temperature interval and in 10⁻¹⁰–10⁻³ time scale. Their emission properties are similar to those of CsPbBr₃ bulk crystal showing a subnanosecond free exciton emission in the 520–540 nm spectral region and slower trapped exciton emission in the 530–580 nm spectral region. An efficient energy exchange between the free and trapped exciton states is shown by the temperature dependencies of emission spectra. The quantum size effect is demonstrated in the high energy shift and broadening of the absorption and emission spectra and an estimate of the size of the CsPbBr₃-like aggregates is provided. Independent evidence of the presence of the CsPbBr₃ and Cs₄PbBr₆ aggregated phases in the CsBr host was obtained by X-ray structural studies.     

Superconductivity induced phonon anomalies in the Raman spectra of Zn and Ni doped YBa2Cu3O7

Here we present Raman spectra of YBa₂(Cu_1–x Zn _x )₃O₇ and YBa₂(Cu_1–x Ni _x )₃O₇ as a function of temperature and Zn or Ni content. The temperature dependence of two modes at 340 and 440 cm^–1 is analyzed. Similarly to the infrared measurements it is found that Zn substantially suppresses the superconductivity induced phonon softening whereas, Ni does not affect much that effect. Moreover, the superconductivity induced phonon stiffening of the 440 cm^–1 mode completely disappeared with the Zn doping. We find this behaviour might support the model where Zn acts effectively as a magnetic pair breaker.     

Visible laser spectroscopy of cobalt monofluoride

The laser-induced fluorescence excitation spectrum of jet-cooled CoF molecules has been studied in the range of 18 800–22 000 cm⁻¹. Ten observed vibronic bands have been classified into three transitions with the 0–0 band at 18 909, 19 236, and 20 654 cm⁻¹, assigned as the [18.8]³Φ₄–X³Φ₄, [19.2]³Φ₄–X³Φ₄, and [20.6]³Γ₅–X³Φ₄ transition, respectively, the two ³Φ states, [18.8]³Φ and [19.2]³Φ, are consistent with Adam’s results (10). The previously unanalyzed [20.6] state is identified in the current work. A rotational analysis of [20.6]³Γ₅–X³Φ₄ transition has been performed and effective equilibrium molecular constants have been determined for the first time. In addition, lifetime measurements of the three electronic transitions were carried out under the collision-free condition. From the lifetime analysis, we consider that the V=1, 2, and 3 vibrational levels of [18.8]³Φ state are perturbed by another state.     

Charge carriers and mobility of Cu-Ti ferrite

The concentration and drift mobility of charge carriers in Cu_1–x Ti _x Fe₂O₄ ferrite are calculated, over a wide range of temperatures (300–773 K), employing d.c. conductivity and thermoelectric power data. With increasing temperature the concentration of charge carriers decreases whilst the drift mobility exhibits an exponential increase. Over the above-mentioned temperature range, the obtained density of charge carriers varies between 10²¹ and 10²² cm^–3 whereas the drift mobility has values between 10^–8 and 10^–4 cm²/V s. The results are discussed on the basis of a small-polaron hopping conduction. The activation of the d.c. conductivity has been attributed to the thermal activation of the mobility.     

Superconductivity and processing of single Bi−O layered cuprates in the (Bi,Pb)−Sr−(Ca,Y)−Cu−O system

The effects of improved materials processing on single Bi–O layered cuprates in the (Bi, Pb)–Sr–(Ca, Y)–Cu–O system have been investigatged. For Bi-1212 we have improvedT _c to 102 K. The bulk nature of superconductivity is confirmed by the presence of superconducting volume fractions ( ZFC) around 30–40%. The critical current density is 2×10⁶ Acm^–2 at 5 K and 0 T. Moreover, indications for the presence of a second phase probably Bi-1223 with a transition to superconductivity in the range of 115–150 K have been found.     

Far-infrared spectra of Pb1−xMnxTe alloys

Far-infrared reflectivity spectra of Pb_1−xMn_xTe (0.0001x0.1) single crystals were measured in the 10–250 cm⁻¹ range at room temperature. The analysis of the far-infrared spectra was made by a fitting procedure based on the model of coupled oscillators. In spite of the strong plasmon–LO phonon interaction, we found that the long wavelength optical phonon modes of these mixed crystals showed an intermediate one–two mode behavior.     

Temperature dependence of the far infrared reflectivity spectra of a KD2PO4

Complete data on far infrared reflectivity spectra of a KD₂PO₄ single crystal in the temperature range 7–300 K, from the millimeter wave up to 400 cm^–1, for all crystallographic orientations of the crystal, in the ferro- and paraelectric phase, are given for the first time. The temperature dependence of the phonon spectra is similar to that we have observed and published for KH₂PO₄.     

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.