The electrical conductivity in α-Al2O3 single crystals implanted with Mg ions in two different crystalline orientations, parallel and perpendicular to c axis, was investigated. The samples were implanted at room temperature with energies of 50 and 100keV and fluences of 1×1015, 5×1015 and 5×1016ions/cm2. Optical characterization reveals slight differences in the absorption bands at 6.0 and 4.2eV, attributed to F type centers and Mie scattering from Mg precipitates, respectively. DC electrical measurements using the four and two-point probe methods, between 295 and 490K, were used to characterize the electrical conductivity of the implanted area (Meshakim and Tanabe, 2001). Measurements in this temperature range indicate that: (1) the electrical conductivity is thermally activated independently of crystallographic orientation, (2) resistance values in the implanted region decrease with fluence levels, and (3) the I–V characteristic of electrical contacts in samples with perpendicular c axis orientation is clearly ohmic, whereas contacts are blocking in samples with parallel c axis. When thin layers are sequentially removed from the implanted region by immersing the sample in a hot solution of nitric and fluorhydric acids the electrical resistance increases until reaching the values of non-implanted crystal (Jheeta et al., 2006). We conclude that the enhancement in conductivity observed in the implanted regions is related to the intrinsic defects created by the implantation rather than to the implanted Mg ions (da Silva et al., 2002; Tardío et al., 2001; Tardío et al., 2008).

With the aim to study the effects of heavy ion implantation on the morphology of α-Al2O3 single crystals with c, m and r orientations, ion implantation at room temperature, with Au or Ag ions, energy of 160keV and fluences ranging from 5×1015 to 1×1017ionscm−2 was performed. The microstructure and morphology of irradiated surfaces were characterized by atomic force microscopy (AFM) and scanning electron microscopy (SEM). The presence of metal precipitates was explored by AFM and monitored by the variation of the intensity of the extinction bands associated with Mie scattering from Au and Ag colloids located at about 2.2eV (563nm) and 2.8eV (443nm), respectively. The results show a dependence on the surface roughness with the ion fluence and the implantation orientation. The dependence of thermal conductivity with the implanted conditions was studied by scanning thermal microscopy. Thermal images indicate that thermal conductivity of Al2O3 is higher in samples implanted with Ag ions at a fluence of 6×1016ionscm−2 than in samples implanted with Au at a higher fluence. Successive etchings of an Ag implanted sample reveal a dependence of the thermal conductivity with the distance to the surface due to the layered damage structure produced by the implantation. The DC electrical measurements, between 273 and 445K, using the four and two-point probe methods were used to characterize the electrical conductivity of the implanted area. The I–V characteristic of the electrical contacts reveals an ohmic behavior, independent of the ions implanted and the crystallographic orientation. Measurements of the electrical conductivity at different temperatures in the investigated range predominantly suggest a band conduction mechanism thermally activated, with energies of about 0.09 and 0.07eV, in samples implanted with Au or Ag ions, respectively. We conclude that the enhancement in conductivity observed in the implanted regions is related to the intrinsic defects created by the implantation, rather than to the implanted Au and Ag ions.

Undoped and Mg-doped α-Al2O3 single crystals were implanted with Mg ions, with an energy of 90 keV and a fluence of 1017 ions/cm2. DC electrical measurements using the four-point probe method, between 295 and 428 K, were used to characterize the electrical conductivity of the implanted area. Measurements in this temperature range indicate that the electrical conductivity after implantation is thermally activated with an activation energy of about 0.03 eV both in undoped and in reduced Mg-doped α-Al2O3 crystals, whereas the activation energy in oxidized Mg-doped α-Al2O3 crystals remains close to that before implantation. The I–V characteristics of the latter samples reveal a blocking behavior of the electrical contacts on the implanted area in contrast to the ohmic contacts observed in α-Al2O3 single crystals with the c-axis perpendicular to the broad face, where the Mg ions were implanted. We conclude that the enhancement in conductivity observed in the implanted regions is related to the intrinsic defects created by the implantation, rather than to the implanted Mg ions. The relationship between the oxygen vacancy concentrations at different stages of etching and the changes in the electronic structure, the chemical bonding, and the Al3+(2p)/O2−(1s) and Mg2+(1s)/O2−(1s) relative intensities was studied by X-ray Photoemission Spectroscopy.

ABSTRACT The search of new organic molecules with improved properties is of fundamental relevance for hybrid organic-inorganic based devices (OLED, FET, PV, injection layers, flexible large area devices, lasers, etc.). Triindole based materials present extended aromatic cores with disk-like geometry that allow tailoring their electronic properties through chemical functionalization. In this work we present an optical and electronic study of new triindole based single crystals. Pi-stacking gives rise to highly ordered columnar structures yielding to high mobilities, around 0.4 cm 2 V -1 s -1 . External oxidation is found to increase orders of magnitude the conductivity. Slight modifications of this platform modify substantially the crystallization dynamics and therefore the quality of the thin films obtained by spin coating from solutions with different solvents. The morphology, stability and properties of the optimized films are found to be promising for device fabrication. A comparative study of the absorption and emission efficiency of solutions and thin films of the different derivatives is presented. The first tests for OLED and OFET devices are under way. Keywords: organic electronics, carbazole, triindo le, electronic structure, mobility, FET, thin film, single crystal.

Optical absorption and emission experiments were used to characterize defects and defect aggregates in Mg-doped Al2O3 crystals due to thermochemical reduction at high temperatures. Oxygen vacancies and higher-order defects are produced much more readily in Mg-doped than in undoped Al2O3 crystals. F+ and F centers (oxygen vacancies with one or two electrons, respectively) were monitored by their optical absorption bands at about 4.8 and 6.0eV, respectively. In contrast with undoped crystals, where the reduction produces primarily F centers and a small amount of F+ centers, in Mg-doped crystals both F and F+ centers are created in comparable concentrations. These thermally generated F and F+ centers are much more stable than those produced in undoped crystals irradiated with neutrons. Clustering of individual oxygen vacancies forming higher-order defects, such as anion divacancy F22+ and F2+ centers, was investigated by low temperature absorption and luminescence experiments, in conjunction with UV irradiation and thermal treatments. The strong absorption bands at 2.87 and 3.69eV were shown to be due to Mg-perturbed F22+ and F2+ centers, respectively. In addition, photoconversion of F22+ and F2+ centers was observed. In crystals containing large concentrations of F22+ centers, electrons excited by 5.0eV light are trapped by F22+ and F2+ centers resulting in the conversion of F22+ centers into F2+. A model of F-type centers was extended to F22+ and F2+ centers. The calculated optical parameters are in very good agreement with those determined experimentally. A simple analysis of the lattice energy suggests that the environments of the F2-type centers are different in TCR Al2O3 crystals and in n-irradiated undoped crystals.

The effects of atmosphere and cooling rate from elevated temperatures on the distribution of Li impurities in MgO, and Mg impurities in Al2O3 crystals are reported. At sufficiently high temperatures, regardless of the atmosphere, Li+ ions from Li2O precipitates in MgO, and Mg2+ ions from MgAl2O4 precipitates in Al2O3 are dispersed around the precipitates, forming complex-oxide regions rich in these impurities, referred to as microgalaxies. Fast cooling of these crystals freezes the nonequilibrium distribution of these impurities surrounding the precipitates. However, if the crystals are cooled sufficiently slowly, the system has ample time to establish an equilibrium distribution, with most of the Li and Mg ions returning to the Li2O and MgAl2O4 precipitates respectively. Based on the microgalaxy model, it is not unrealistic to expect that oxygen vacancies would be formed within the microgalaxy, regardless of the atmosphere.

Oxygen vacancies and their aggregates are produced much more readily in Mg-doped α-Al2O3 than in undoped α-Al2O3 single crystals during thermochemical reduction at high temperatures. A reversible photochromic effect was discovered in Mg-doped Al2O3 crystals containing large concentrations of oxygen divacancies. Alternate excitation with 5.0 and 3.69 eV light results in brown or yellowish–green coloration, respectively. The yellowish–green coloration can also be restored by thermal anneals at temperatures of about 750 K.

In order to prepare Ni clusters embedded in amorphous Si3N4, Ni and Si3N4 layers have been alternatively deposited using sputtering techniques. The nominal Ni layer thickness ranged from 2 to 60 Å and the number of layers was varied accordingly so as to keep the total amount of Ni constant. Extended x-ray-absorption fine-structure (EXAFS) spectroscopy has been used to study the Ni clustering as well as the isolated Ni ions in the silicon nitride matrix. For small Ni layer thickness, the Ni layer becomes discontinuous and the average size of Ni clusters can be determined. Aiming to modify the Ni surrounding, samples with the thinnest Ni layers have been irradiated with He and P. The EXAFS results show that the main effect is the removal of the isolated Ni in the Si3N4 matrix. The size evolution of the Ni clusters depends on the type of the irradiating ion. A complete magnetic characterization is presented in order to correlate Ni surrounding with the magnetic properties of Ni clusters.

It is presented the effect of the reactive and non-reactive sputtering preparation, as well as, the atomic composition on the electrical properties and conduction mechanisms. Depending on the preparation conditions, samples present ohmic, Poole–Frenkel and space charge limited current conduction behaviours, as well as several orders of magnitude in resistivity values. As it is reported, impurities and non-stoichiometry play a very important role in the conduction mechanism and therefore on the thin film resistivity.

Infrared-absorption measurements were used to characterize OH- and OD- stretching frequencies in Al2O3 crystals both nominally pure and doped with either Ti, V, or Mg impurities. Impurities, cooling rates, and ultraviolet irradiation affect the distribution of various OH- (OD-) band intensities. Polarization experiments determined the precise angle of OH- (OD-) ions protruding from the basal plane for several OH- (OD-) bands. Most were <15degrees, with one at 21degrees. Diffusion of isotopic species was performed with and without an electric field. Without an electric field, indiffusion is possible only by exchanging with an existing species. With an electric field, indiffusion occurs by exchange as well as occupying new sites. Incorporation of hydrogen (deuterium) was investigated by subjecting the crystals to a moderate electric field both parallel and perpendicular to the crystallographic c axis, in the temperature range 973-1300 K in H2O (or D2O) vapor. An initial linear dependence of the percent of exchange with annealing time and applied voltage was observed, indicating that ionic conduction is the dominant mechanism. The activation energy for the H(+)double left right arrowD(+) exchange was determined to be approximate to2.4 eV with an electric field of 3000 V/cm applied either parallel or perpendicular to the c axis. The estimated proton (deuteron) mobility is mu=(6+/-1)x10(-8) cm(2)/(V s).

Alternating and direct current electrical measurements between 293 and 450 K were used to characterize the electrical conductivity of the implanted region in as-grown and oxidized MgO:Li single crystals. Both types of crystals were implanted with Li+ ions with an energy of 175 keV and a fluence of 1×1017 ions/cm2. The electrical conductivity of the implanted regions was ≈14 and 7 orders of magnitude higher than that of the unimplanted areas, respectively. Electrical measurements at different temperatures of the implanted regions suggest thermally activated processes with activation energies of about 0.14 and 0.06 eV in as-grown and oxidized samples, respectively. In both type of crystals, the I–V characteristics reveal that the contacts are ohmic, in contrast to blocking contacts in unimplanted crystals. The enhancement in conductivity observed in the implanted region is associated with the intrinsic defects created by the implantation, rather than with the implanted Li ions. The differences in both conductivity and activation energy relative to undoped crystals are likely due to free carriers already present in different concentrations in as-grown and oxidized MgO:Li crystals before implantation.

A reversible photochromic effect was investigated in Mg-doped α-Al2O3 single crystals oxidized at elevated temperatures. Alternate illumination with blue and UV light at T⩽210 K results in reversible disappearance and reappearance of a gray-purple coloration, respectively. The coloration can also be fully rejuvenated by warming the crystal to T⩾215 K.

We present a study of the magnetotransport behavior around Tc of epitaxial La0.7Ca0.3MnO3 thin films with controlled chemical defects introduced in order to change the electron-lattice coupling (λ). We found unexpected enhancement of the very low field (below 250 Oe) magnetoresistance around Tc and relate it to a reduction of the magnetic domain size. This process is strongly favored by the presence of chemical defects that also promote weak localization of the carriers into polaron clusters that can be overcome by relatively low magnetic fields. This behavior is characteristic of temperatures near Tc where the bandwidth is reduced and localization, due to intrinsic disorder and chemical defects, is more probable. In addition, the increase of λ raises the polaron binding energy and reduces the temperature at which percolation occurs.

The phenomenally large enhancement in conductivity observed when Li-doped MgO crystals are oxidized at elevated temperatures was investigated by dc and ac electrical measurements in the temperature interval 250-673 K. The concentration of [Li](0) centers (substitutional Li+ ions each with a trapped hole) resulting from oxidation was monitored by optical absorption measurements. At low electric fields, dc measurements reveal blocking contacts. At high fields, the I-V characteristic is similar to that of a diode connected in series with the bulk resistance of the sample. Low-voltage ac measurements show that the equivalent circuit for the sample consists of the bulk resistance in series with the junction capacitance connected in parallel with a capacitance, which represents the dielectric constant of the sample. Both dc and ac experiments provide consistent values for the bulk resistance. The electrical conductivity of oxidized MgO:Li crystals increases linearly with the concentration of [Li](0) centers. The conductivity is thermally activated with an activation energy of (0.70+/-0.02) eV, which is independent of the [Li](0) content. The standard semiconducting mechanism satisfactorily explains these results. Free holes are the main contribution to band conduction as they are released from the [Li](0)-acceptor centers. In as-grown MgO:Li crystals (without [Li](0) centers) the electrical current increases with time as [Li](0) centers are being formed. When ample [Li](0) centers are formed, an activation energy of 0.7 eV was observed. At sufficiently high current, Joule heating thermally destroys the [Li](0) centers.

Silicon nitride has excellent properties as a refractory material. Crystalline Si3N4 ceramics exhibit stability at high temperatures, high electrical resistivity and extreme hardness. On the other hand, in electronic device fabrication, Si3N4 thin layers are used for encapsulation and packaging in integrated circuits and they may be used as the gate dielectric layer in thin film transistors and in metal-nitride-oxide–silicon devices. In the present work, we studied the composition and the effect on the electrical behavior of thin films prepared by r.f. magnetron sputtering with some different gases (reactive and non-reactive).

MgO single crystals were implanted with a fluence of 1×1017 Li+/cm2 with 175 keV. Using ac and dc techniques, the electrical conductivity of these crystals was investigated in the temperature range 296–440 K. The electrical conductivity of the implanted region was 14 orders of magnitude higher than the unimplanted area. Measurements at different temperatures suggest a thermally activated process with an activation energy of about 0.33 eV. In the implanted area, electrical contacts are found to be ohmic whereas contacts are blocking in unimplanted crystals. Removal of thin layers of the implanted region by immersing the crystal in hot phosphoric acid suggests that the enhancement in conductivity in the implanted region is associated with the intrinsic defects created by the implantation, rather than with the Li ions.

Electroluminescence (EL) was investigated in Al2O3:Mg single crystals containing thermally generated [Mg](0) centers when a current in excess of 10 muA was passed through the samples in the temperature interval 323-673 K. The EL is emitted from a narrow region at the negative electrode suggesting an electron-hole recombination process due to the injection of electrons which recombine with holes. EL bands at 3.8, 2.5, 1.8, and 1.5 eV were observed. The bands at 3.8, 1.8, and 1.5 eV are related to photoluminescence (PL) bands previously attributed to F+ centers, Cr3+, and Ti3+, respectively. The PL band at 2.5eV has been tentatively associated with excess Al3+ in the form of interstitials. The dependence of the EL spectra on the electrical current and temperature was also studied.

AC and DC electrical measurements between 273 and 800 K were used to characterize the electrical conductivity of Al_2 O_3:Mg single crystals containing [Mg]^{0} center. At low fields contacts are blocking. At high fields, electrical current flows steadily through the sample and the I-V characteristic corresponds to a directly biased barrier whit a series resistance (bulk resistance). AC measurements yield values for the junction capacitance as well as for the sample resistance, and provide perfectly reproducible conductivity values. The conductivity varies linearly whit the [Mg]^{0} concentration and a thermal activation energy of 0.68 eV was obtained, which agrees very well with the activation energy previously reported for motion of free holes.