E. Abramof

Room temperature ferromagnetism in cubic GaN epilayers implanted with Mn + ions

Mn ions were implanted in p-type cubic GaN at doses from 0.6 to 2.4×1016cm−2 at 200 keV energy. A 200-nm-thick epitaxial layer, grown by molecular beam epitaxy on GaAs(001) substrate, is used for the Mn implantation. The Mn implanted samples were subjected to an annealing at 950 °C for 1–5 min. The structural quality of the samples was investigated by high resolution x-ray diffraction and Raman spectroscopy. The annealing procedure leads to a significant increasing of the crystalline quality of the samples. Hysteresis loops were observed for all cubic GaMnN annealed samples and ferromagnetism was detected up to room temperature.

Electrical and optical properties of PbTe p-n junction infrared sensors

Lead telluride mesa diodes were fabricated from a series of p-n junctions grown on (111) BaF2 substrates, in which the hole concentration p was kept constant at 1017cm−3 and the electron concentration n varied between 1017 and 1019cm−3. Capacitance-voltage analysis showed that for n>1018cm−3 the PbTe p-n junction is one sided and abrupt. The parameters (incremental resistance, series and parallel resistances, and ideality factor) obtained from the current-voltage (I-V) characteristics and the detectivity D* exhibited a large fluctuation among the photodiodes. In spite of these fluctuations, it was possible to correlate the noise and D* values to the parameters obtained from the I-V analysis. These results allow predicting the PbTe detector’s figures of merit from the data obtained from the I-V curves. The best PbTe photodiodes fabricated here showed D* values close to 1011cmHz1∕2W−1, comparable to InSb and HgCdTe commercial detectors and to PbTe sensors fabricated on Si substrates.

Electrical properties of Pb1-xSnxTe layers with 0⩽x⩽1 grown by molecular beam epitaxy

In this work, the electrical properties of Pb1−xSnxTe epitaxial layers with Sn composition covering the whole range were investigated. The samples were grown on (111)BaF2 substrates in a molecular beam epitaxy system using PbTe, SnTe, and Te solid sources. As the alloy composition varies from PbTe to SnTe, the hole concentration increases exponentially from 1017 to 1020 cm−3 for Te-rich sources and from 1017 to 1019 cm−3 for stoichiometric ones. The resistivity of the samples, which depends mainly on their hole concentrations, shows an exponential dependence on the temperature with a slope which decreases as x goes from 0 to 1. For all Pb1−xSnxTe samples with x in the range of 0.35–0.7, the resistivity curve shows a very well defined minimum at low temperatures. This anomalous behavior is supposed to be related to the band crossing, where the energy gap temperature coefficient changes sign. The temperatures where the minimum in the resistivity occurs only agree with the ones predicted by the band inversio...

Nanostructure of sol–gel films by x-ray specular reflectivity

Recently, several studies have been carried out on sol–gel films for optical applications, mostly motivated by the quickness and low cost of the film preparation process. In order to preserve the coherence properties of the light, improvements in the current quality of such films are necessary as well as appropriated techniques for structural characterization and quality control. X-ray specular reflectivity could be one of such techniques, but it is limited by the complexity of the internal nanostructure of the films. In this work, we have developed a procedure to extract the exact density profile of sol–gel films, and applied it to analyze a sol–gel derived Er2O3 film.

Strain determination in PbEuTe/PbTe multi-quantum wells

A series of Pb1−xEuxTe/PbTe multi-quantum well (MQW) samples were grown on (111) cleaved BaF2 substrates by molecular beam epitaxy. The Eu content was maintained at x∼0.05–0.06 and the PbTe well width was varied from 23 to 206 A. The samples were characterized structurally by high resolution x-ray diffraction in the triple axis configuration. The ω/2Θ scans of the (222) Bragg reflection showed very well resolved satellite peaks up to the tenth-order for all samples indicating that sharp interfaces were obtained. Reciprocal space mapping around the (224) lattice point indicated that the MQW structure tended to the free-standing condition. The (222) ω/2Θ scans were calculated by dynamical theory of x-ray diffraction and compared to the measured ones. Using the in-plane lattice constant as the main fitting parameter, the strain in the PbTe well inside the MQW structure was obtained as a function of its width. It decreased monotonically from an almost fully strained layer to 26% of strain relaxation as the Pb...

Reciprocal space maps of PbTe/SnTe superlattices

PbTe/SnTe superlattices were grown on (111) BaF2 substrates by molecular beam epitaxy using PbTe as buffer layers. The individual layer thickness and number of repetitions were chosen in order to change the strain profile in the superlattices from completely pseudomorphic to partially relaxed. The superlattices structural properties were investigated by making reciprocal space maps around the asymmetric (224) Bragg diffraction points and ω/2Θ scans for the (222) diffraction with a high resolution diffractometer in the triple axis configuration. With the strain information obtained from the maps, the (222) ω/2Θ scan was simulated by dynamical diffraction theory. The simulated spectra of the pseudomorphic superlattices, in which the in-plane lattice constant is assumed to be the same as the PbTe buffer throughout the superlattice, fitted in a remarkably good agreement with the measured data, indicating that almost structurally perfect samples were obtained. For the thicker superlattices, the (224) reciproca...

Characterization of SnTe films grown by molecular beam epitaxy

A series of SnTe layers with thicknesses varying from 0.42 to 9.1 µm were grown by molecular beam epitaxy on (111) BaF2 substrates. The SnTe lattice parameter was found to be 6.331 A as determined from x-ray diffraction spectra measured in the triple-axis configuration. The FWHM of the (222) SnTe x-ray rocking curves indicated a good crystalline quality and an unusual dependence on layer thickness. Atomic force microscopy (AFM) of the SnTe surface revealed spirals with monolayer steps formed around threading dislocations, similar to the PbTe on BaF2 epitaxy. The dislocation density was estimated from the AFM picture to be 9x108 cm-2. Small black pits corresponding to holes that were left during growth were also observed on the AFM images. Sn diffusion can be a possible reason for these pits and the relatively high dislocation density. Electrical measurements showed that the SnTe epilayers present a typical p-type carrier concentration around 1020 cm - 3 almost temperature independent and a Hall mobility which decreases from 104 to 103 cm2/V.s as the temperature increases from 10 to 350K.

Optical second harmonic generation in the centrosymmetric magnetic semiconductors EuTe and EuSe

Spectroscopy of the centrosymmetric magnetic semiconductors EuTe and EuSe reveals spin-induced optical second harmonic generation (SHG) in the band gap vicinity at 2.1-2.4 eV. The magnetic field and temperature dependence demonstrates that the SHG arises from the bulk of the materials due to a novel type of nonlinear optical susceptibility caused by the magnetic dipole contribution combined with spontaneous or induced magnetization. This spin-induced susceptibility opens access to a wide class of centrosymmetric systems by harmonics generation spectroscopy.

Sponge-like and columnar porous silicon implanted with nitrogen by plasma immersion ion implantation (PIII)

Sponge-like and columnar porous silicon (PS) were prepared from p- and n-type (100) monocrystalline silicon wafers using different anodization conditions (hydrofluoric acid concentration, current density and anodization time) and then implanted with nitrogen by plasma immersion ion implantation (PIII). The effect of the implantation and of the compounds formed was analyzed by measuring the reflectance of the implanted samples for wavelengths between 220 and 800 nm. A reduction in reflectance in the ultraviolet (UV) region of the spectrum was observed for polished Si samples and for all kinds of PS samples. Increased UV-induced photoluminescence in these samples caused by the increase in absorption in the UV region is expected.

Structural properties of Bi2Te3 topological insulator thin films grown by molecular beam epitaxy on (111) BaF2 substrates

Structural properties of topological insulator bismuth telluride films grown epitaxially on (111) BaF2 with a fixed Bi2Te3 beam flux were systematically investigated as a function of substrate temperature and additional Te flux. A layer-by-layer growth mode is observed since the early stages of epitaxy and remains throughout the whole deposition. Composition of the epitaxial films produced here stays between Bi2Te3 and Bi4Te5, as determined from the comparison of the measured x-ray diffraction curves with calculations. The substrate temperature region, where the growth rate remains constant, is found to be the most appropriate to obtain ordered Bi2Te3 films. Line width of the L = 18 Bi2Te3 diffraction peaks as low as 140 arcsec was obtained, indicating high crystalline quality. Twinning domains density rises with increasing growth temperature and reducing Te extra flux. X-ray reflectivity curves of pure Bi2Te3 films with thickness from 165 to 8 nm exhibited well defined interference fringes, evidencing ho...