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Spin-dependent recombination (SDR) electron paramagnetic resonance (EPR) spectroscopy is applied for investigation of paramagnetic recombination centers in irradiated silicon p−n junction detectors (diodes) formed on float-zone (FZ) silicon wafers. The main radiation defects, associated with SRD-EPR spectra arising from excited triplet states, are assigned to complexes of two substitutional carbon atoms and one interstitial silicon atom (CS+SiI+CS) and to oxygen + vacancy (O+V) complexes (A-centers). In spite of the low concentration of oxygen in FZ silicon the A-centers are found to play an important role in the recombination process in the diodes. At temperatures T

Kelvin probe force microscopy is used to observe the bulk potential redistribution across the high power InGaAs/AlGaAs/GaAs separate confinement heterostructure quantum-well laser diodes for a wide range of injection currents, including the lasing regime. By increasing the injection current, the development of a parasitic voltage drop is detected at initial calibration layers and the buffer layer of the laser structure. Catastrophic degradation of the laser mirror was observed at the level of injection current ∼19 times the threshold value. Atomic force microscopy images of the mirror revealed a 100 nm deep crater of maximum width ∼2.5 μm in the vicinity of the buffer/emitter interface. By combining the surface morphology results of the destructed mirror with those of Kelvin probe force microscopy in operating devices, it is concluded that the parasitic voltage drop is responsible for a substantial energy dissipation and the nonoptical degradation of the laser mirror.

The plume generated by a pulsed XeCl laser from a novel nanostructured YBaCuO target (n target) is investigated by methods of optical emission spectroscopy and atomic force microscopy. While the spectral positions of the emission lines are the same, stronger line intensities, pertinent to higher kinetic energy of the particles, are observed in the plumes generated from the n target than from a target having micron size grains (m target). The size of small clusters captured on Si plates assembled inside the plume grows in directions perpendicular to the axis of the plume. As shown by x-ray photoelectron spectroscopy investigations, in the particles deposited on a SrTiO3 substrate at Ts=700 °C in oxygen the correct 1-2-3 composition is achieved. The average ratio of the heights of the particles deposited from the n target and from the m target is hn/hm=0.6, both in the plume and on SrTiO3. This can explain the smoothness of YBaCuO films prepared by laser deposition from the n targets.

Electron paramagnetic resonance (EPR) of centers produced in porous silicon (PS) by heat treatment in air is investigated. The properties of these centers are compared with the defects present in as‐prepared PS or activated by treatment of the specimens with ethanol. The native defects of PS and those produced by thermal annealing show different saturation behavior of the EPR spectra when the microwave power is increased. Also, their spin‐dependent recombination processes are different. The dependence of the EPR line intensity on the thermal annealing time at temperatures of 150–500 °C obeys a law characteristic to a diffusion process with an activation energy of 0.9 eV. This value is close to the activation energy of oxidation of crystalline silicon surfaces with atomic oxygen.

The results of the Shubnikov-de Haas (SdH) effect and magnetization of single crystals of (Cd1−x−yZnxMny)3As2 (CZMA) are presented. Specimens with x+y=0.3 and y=0.04, 0.06 and 0.08, subjected to long-time thermal annealing, were studied. SdH effect was investigated in fields B up to 11 T in the temperature range T=4.2–20 K. Magnetization was measured at B⩽5. 5 T and 5 K⩽T⩽20 K. SdH oscillations were analyzed taking into account (i) the anomalous magnetic field dependence of the cyclotron mass observed recently in CZMA, and (ii) influence of the s−d interaction to the effective g-factor (geff) of conduction electrons. The dependence of geff on B and T was introduced in the analysis of the SdH oscillations by fitting the magnetization data with the effective Brillouin function. Values of the exchange-independent part of geff, the exchange constant in the conduction band and the spin-dependent part of the Dingle temperature were determined.

Electron paramagnetic resonance (EPR) of centers produced in porous silicon (PS) by heat treatment in vacuum is investigated. Annealing in the temperature range of 400–600 °C and vacuum up to 5×10−5 Torr increases the EPR line intensity of Pb centers in a way depending on the time of annealing. This can be explained by hydrogen depassivation of the Pb centers. An isotropic line with g=2.0055, usually attributed to disordered Si dangling bonds, appears after vacuum annealing of some minutes but at a longer heat treatment its intensity decreases. At room temperature this decrease is exponential with a time constant of a few minutes, depending on the level of vacuum. Heat treatment of PS in vacuum leads to amplification of the Pb EPR spectrum after exposition of samples to air at room temperature.

The effect of BaZrO3 (BZO) dopant concentration on superconducting properties in YBa2Cu3O6+x thin films is investigated in a wide magnetic field range up to 30 T. Based on the magnetization and resistivity measurements, the optimal BZO concentration for flux pinning is found to be 4% up to 20 T and increasing up to around 8% in higher fields. This result is qualitatively explained by a model where more BZO is needed in order to organize optimal columnar defect structure at high fields because the amount of single BZO particles acting as isotropic point pinning centers is increased, especially in high BZO concentrations. This means that for applications of thin film or coated conductor structures carried out at high magnetic fields, even higher dopant concentration than used in a low-field range should be produced in order to attain the optimal flux pinning conditions.

The effect of persistent photoconductivity is observed in zinc oxide (ZnO) ceramics together with persistence of the electron paramagnetic resonance (EPR) spectra of defects and impurity centers in the samples. The spectral and time dependences of the EPR signals and microwave photoconductivity are investigated under excitation by light with different quantum energies below the ZnO band gap. The mechanisms of the persistent photoconductivity and coexistence of pnotoconductivity and optically induced EPR spectra after switching off the light are discussed. Based on the experimental results it is concluded that surface and intergranular conductivities dominate in ZnO ceramics.

Photoluminescence (PL) spectra are investigated in n-ZnSe single crystals at different temperatures from 4.4 to 300 K immediately after doping with Au from melt of Se+Au or Zn+Au and after storage of the doped samples for 4 years in the dark under normal room conditions. Due to the formation of Aui interstitial donors in the n-ZnSe:Se:Au crystals with time, the origin of the near band edge PL changes from acceptor-bound to donor-bound excitons. Taking into account the results of PL characterization, we proposed that the Aui donors are generated by displacement of Au ions from regular lattice sites to interstitial sites with the help of lattice deformation forces. Transport measurements show dramatic increase in the electrical conductivity and the free electron concentration after storage of the n-ZnSe:Zn:Au crystals, thus confirming the proposed model.

Optical detection of magnetic resonance (ODMR) and electron paramagnetic resonance (EPR) spectra are investigated in ZnO single crystals. The strong negative ODMR line with axial symmetry of the g-tensor around the c axis with g∥=2.0133±0.0001 and g⊥=2.0135±0.0001 is ascribed to Pb3+ ions. The strongest Pb3+ ODMR spectrum is observed under excitation with visible light from an Ar+ ion laser and below 40Hz of on/off microwave power modulation. The negative Pb3+ line, accompanied with effective mass (EM) donors over the spectral range of the photoluminescence (PL), shows spin dependent transfer of electrons from EM donors to Pb3+, competing with PL. The intensity of the Pb3+ EPR line as well as the EPR line of oxygen vacancy (VO+) investigated in samples irradiated with 2.5MeV electrons show a fast increase when the light is switched on and gradual decrease during illumination. The dependence of the growth and decay kinetics of the Pb3+ and VO+ EPR spectra on wavelength of light is analyzed by taking into a...

We report the effect of La0.7Sr0.3MnO3 (LSMO) electrodes on the temperature dependence of the magnetoresistance (MR) of LSMO/polymer/cobalt spin valves (SVs). LSMO films have been prepared by pulsed laser deposition on three different single crystal substrates using different deposition parameters. The films were characterized for their surface morphologies, structural, magnetic, and magnetotransport properties. Low deposition rate is found to be detrimental for growth of good quality films and polycrystalline films with grain boundary effects are observed in thicker films. The films on MGO (100) substrate show a broad paramagnetic to ferromagnetic transition, accompanied with a metal-insulator transition below room temperature. This indicates growth of some strained structures due to large lattice mismatch (9%) between the substrate and the film and presence of polycrystalline grain boundaries. The deposited films on STO (100) and NGO (001) show much sharper magnetic transition and metallic behavior indicating higher spin polarization (SP) of LSMO on these substrates at room temperature. SVs made on STO (100) show improvement in switching behavior and better MR response compared to the devices made on MGO (100) at low temperatures. No difference in MR response was found at room temperature in either case. We conclude that the bulk spin polarization of LSMO films is not very important in the SV operation. The loss of most of the SP carriers at the LSMO/organic semiconductor interface at room temperature is a more dominant effect and drastically reduces the MR signal.

A method for direct observation of carrier leakage from active regions of working semiconductor light-emitting diodes and lasers is suggested. In this method, Kelvin probe force and surface photovoltage microscopies are used to measure local changes in the surface potential of the device mirror on which a high concentration of the leaked carriers is expected. The applicability of the method is demonstrated by studying in detail the leakage current on the mirrors of high-power InGaAs∕AlGaAs∕GaAs laser diodes in action. It is shown that minority holes arrive at the mirror surface from the active zone of the laser and spread over to regions of the n emitter and n substrate. This observation is confirmed by exposing the mirror to external light with photon energy exceeding the band gap of the laser structure and measuring the generated surface photovoltage. Owing to surface channels formed by the surface band bending, the holes can move tens of micrometers from the place of their generation. The leakage curre...

Resistivity of La0.7−δ Ca0.3Mn1−yFeyO3 (LCMFO) with y=0–0.05, δ=0 and 0.017 is investigated between 4.2–350 K, showing that La vacancies increase the effect of Fe doping by influencing electronic states through additional microscope disorder. The Shklovskii–Efros variable-range hopping conductivity, governed by generation of a soft parabolic gap in the spectrum of localized states due to Coulomb interaction between charge carriers, is observed both above and well below the ferromagnetic-to-paramagnetic transition temperature. Our results give evidence for absence of a true metal–insulator transition both in undoped material and in LCMFO.

Nonuniversal critical behavior of the dc magnetic susceptibility χ−1(T)−χ−1(TC)∼(T/TC−1)γ≡τγ is observed near the paramagnetic (PM) to ferromagnetic (FM) transition in La0.7Ca0.3MnO3 film samples. The value of γ≈1.4 obtained for 0

Surface structures of the high‐Tc superconductor YBa2Cu3Ox have been investigated with a scanning tunneling microscope. Four typical features are revealed, e.g., (i) flat area of the size up to 0.2×0.2 μm2, (ii) step structures with the height of the step from tens to hundreds of angstroms, (iii) regular arrays of subgrains having the width down to 100 A, and (iv) a complex formed by these features. The sharp surface corrugations suggest strong local fluctuations in the elastic and electronic properties of this material.