B. Díaz

Strain profile of (001) silicon implanted with nitrogen by plasma immersion

In this work, we investigate the strain and defect state of silicon implanted with nitrogen by plasma immersion ion implantation, with doses between 4.5×1016 and 8.7×1016 cm−2. For this purpose, we have used Auger electron spectroscopy, x-ray reflectivity, and high-resolution x-ray diffraction. Auger spectra showed that nitrogen concentration profiles broaden and shift deeper into the substrate as the dose increases. High oxygen concentration in the first 20 nm suggested the presence of an amorphous oxide layer at the sample surface, which was confirmed by x-ray reflectivity measurements. Reciprocal space maps revealed a tensile strain perpendicular to the surface, while no in-plane strain was detected. Since no significant diffuse scattering was found, randomly distributed point defects must be predominant in the strained region compared to large displacement field defects such as clusters and dislocations. ω∕2θ scans around (004) Bragg reflection were fitted using dynamical theory of x-ray diffraction. ...

Electrical properties of PbTe doped with BaF2

We study here the p-type doping of PbTe with BaF2. For the investigation, PbTe layers were grown on (111) BaF2 substrates by molecular beam epitaxy. The beam flux ratio between BaF2 and PbTe, defined as the nominal doping level, was varied from 0.02% to 1%. The hole density increases from 5×1017 to 1×1019 cm−3 as the doping level rises from 0.02% to 0.4% and saturates at p∼1019 cm−3 for higher levels. The saturation effect was attributed to self-compensation. The carrier concentration of all samples remained almost constant as the temperature was varied from 10 to 350 K, indicating that no thermal activation is present in the whole doping range. It suggests that the impurity level in PbTe doped with BaF2 remains resonant with the valence band, similar to the native defects behavior. The low-temperature mobility showed a pronounced reduction from 50 000 to 2 000 cm2/V s as the doping level rises from 0.02% to 1%, mainly due to the substantial increase in the hole concentration. For temperatures higher than...

Plasma Immersion Ion Implantation With Lithium Atoms

A new method was developed to produce lithium plasma for plasma immersion ion implantation. Initially, an argon glow discharge with operation pressure ranging from 2 times 10-1 to 1 mbar is generated by negatively polarizing an electrode from -400 to -1500 V. Small pieces of metallic lithium that are 99.9% pure fill the top of a conic crucible, with a depth of 2 cm, in electric contact with the electrode. Argon ions from the plasma are used to bombard this target, where heat is created by the momentum transfer from the impacting ions to the crucible. By controlling the operation pressure and the electrode voltage polarization, it is possible to easily heat the crucible to temperatures above the lithium melting point (180degC), causing its evaporation. Lithium atoms are then ionized, mainly due to collisions, with argon ions moving toward the crucible. Double Langmuir probe measurements indicated variation in the density of the discharge from 4 times 109 cm-3 to 1010 cm-3 after lithium evaporation. Silicon wafer pieces immersed in this mixed plasma were submitted to repetitive negative high-voltage pulses (3 kV/6 mus/2.5 kHz) to accelerate plasma ions. High strain in the treated layers was measured by high-resolution X-ray diffraction. Photoluminescence intensity increased after annealing. X-ray photoelectron spectrometry measurement revealed lithium implantation in silicon with an atomic concentration of 78% on the top surface and a penetration depth of about 75 nm.

Magnetic ordering of EuTe∕PbTe multilayers determined by x-ray resonant diffraction

In this work we use resonant x-ray diffraction combined with polarization analysis of the diffracted beam to study the magnetic ordering in EuTe∕PbTe multilayers. The presence of satellites at the (12 12 12) magnetic reflection of a 50 repetition EuTe∕PbTe superlattice demonstrated the existence of magnetic correlations among the alternated EuTe layers. The behavior of the satellites intensity as T increases toward the Neel temperature TN indicates that these correlations persist nearly up to TN and suggests the preferential decrease of the magnetic order parameter of external monolayers of each EuTe layer within the superlattice.

Zero-phonon emission and magnetic polaron parameters in EuTe

A phonon structure in the photoluminescence of EuTe was discovered, with a well-defined zero-phonon emission line (ZPL). The ZPL redshifts linearly with the intensity of applied magnetic field, indicating spin relaxation of the photoexcited electron, and saturates at a lower magnetic field than the optical absorption bandgap, which is attributed to formation of magnetic polarons. From the difference in these saturation fields, the zero-field polaron binding energy and radius are estimated to be 43 meV and 3.2 (in units of the EuTe lattice parameter), respectively.

Structural and Magnetic Characterization of EuTe/SnTe Superlattices Grown by Molecular Beam Epitaxy

Here we investigate the structural and magnetic properties of 24 repetitions EuTe/SnTe superlattices (SLs), with 3 monolayers (ML) EuTe films and SnTe thicknesses between 13 and 36 ML. The SLs were grown by molecular beam epitaxy on 3 μm SnTe buffer layers, grown on top of (111)BaF2 substrates. High resolution x‐ray diffraction measurements indicated that the SLs with thicker SnTe layers have higher structural quality. This is due to the SnTe growth mode on EuTe, which starts in islands and evolves to layer‐by‐layer. The magnetic diffraction peak observed for the higher quality SLs proved the existence of antiferromagnetic order within the individual EuTe layers. Decreasing the width of the non‐magnetic SnTe layers resulted in rougher interfaces, and the fading of the magnetic peak signal. The magnetization versus applied field curves indicated that the magnetic moments of SLs with thinner SnTe layers were also harder to align along the field direction. We interpret our results considering the loss of Eu ...

Exchange Interaction Effects on the Optical Properties of EuxPb1−xTe

We measured the absorption band‐edge in EuxPb1−xTe in the ferromagnetic order induced by an external magnetic field (Faraday configuration). A narrow dichroic line, superimposed on a rising monotonous background, describes the spectrum. Here we examine if this line could be associated to an excitonic character of the electronic transition. Measurements of the absorption line width dependence on x (the molar fraction of Eu) allowed us to obtain an estimate for the characteristic radius of the excited wave function, and hence of the hypothetical exciton binding energy, which is found to be ∼120 meV. When the magnetic field is removed, anti‐ferromagnetic order is restored, and the absorption intensity decreases by more than one order of magnitude. Such a large change in the absorption cannot be explained in the framework of the excitonic absorption model.