R. González

Characterization of MgO thin films grown by rf-sputtering

Abstract Thin films of MgO were grown on an Si(1xa00xa00) substrate at 1000xa0K by rf-sputtering using two different targets: Mg and MgO. The total pressure during the growing process was maintained at 7.0×10 −3 xa0mbar and the O 2 to Ar percentage was varied from 5% to 50%. X-ray diffraction measurements indicate that two different crystalline structures are formed depending on whether the target was Mg or MgO. In MgO films grown using an Mg target, the diffraction peak corresponds to MgO(2xa00xa00) of the rock salt structure, and in MgO films grown with an MgO target the diffraction peak corresponds to MgO(4xa00xa00) of the cubic spinel structure. In both structures, the maximum intensity of the diffraction peak occurs when the oxygen to argon percentage is 20%. The surface stoichiometry of both types of films was determined by Auger electron and X-ray photoelectron spectroscopy. X-ray photoelectron spectroscopy corroborates the presence of OH radicals.

Microstructural characterization of MgO thin films grown by radio-frequency sputtering. Target and substrate-temperature effect

The microstructure of thin films deposited by radio-frequency (rf)-sputtering on a silicon substrate at several temperatures and with two different targets was investigated by x-ray diffractometry (XRD) and scanning electron microscopy. XRD spectra reveal that films deposited at room temperature from either an MgO or an Mg target contain small (∼5 nm) periclase MgO crystallites. Thermal treatments in air followed by a fast cooling improved the degree of crystallinity and increased the grain size. The films grown from an Mg target at high temperatures are polycrystalline with a rock-salt structure. However, in thin films deposited from a sintered MgO target at T⩾873 K, the cubic spinel structure due to a mix of periclase (MgO) and brucite [Mg(OH)2] is observed; hydrogen comes from the target contamination. Thermal treatments in air at high temperatures improved the degree of crystallinity and texturing. The film structure depends on the cooling rate from elevated temperatures. Nanoindentation measurements ...

Optical properties of vacancies in thermochemically reduced Mg-doped sapphire single crystals

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 irradiat...

p -type semiconducting properties in lithium-doped MgO single crystals

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. Theconcentration of [Li]° 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]° 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.

High temperature semiconducting characteristics of magnesium-doped α-Al2O3 single crystals

In the temperature interval 300–773 K, ac and dc electrical measurements show a phenomenally large enhancement of the electrical conductivity of otherwise excellent insulator Al2O3 crystals when doped with magnesium impurities. The conductivity increases linearly with the concentration of hole-trapped [Mg]0 centers and is four times higher in the direction parallel to the crystallographic c axis than in the perpendicular direction. The conductivity activation energy is 0.68 eV, independent of both [Mg]0 content and crystallographic orientation. Electroluminescence experiments indicate that holes are the majority carriers. These results favor the small-polaron-motion mechanism.

Enhancement of electrical conductivity in α-Al2O3 crystals doped with magnesium

Direct current and alternating current electrical measurements were performed to investigate the electrical conductivity of α-Al2O3:Mg samples with different concentrations of [Mg]0 centers (Mg ions each with a trapped hole) in the temperature interval 250–800 K. The concentration of [Mg]0 centers was monitored by the optical absorption peak at 2.56 eV. These centers were produced by oxidation at temperatures above 1050 K. The formation rate of [Mg]0 centers depends on the previous thermal history of the sample in either reducing or oxidizing atmosphere. At low electrical fields, dc measurements reveal blocking contacts. At high fields, the I–V characteristic is similar to that of a diode (corresponding to a blocking contact at one side of the sample and an ohmic contact at the other side) connected in series with the bulk resistance of the sample. Steady electroluminescence is emitted at the negative electrode when a current in excess of ≈10 μA passes through the sample, indicating that the majority of c...

Hardness and elastic modulus from nanoindentations in nominally pure and doped MgO single crystals

Abstract Using a nanoindentation technique, the hardness and Young′s modulus were determined for nominally pure MgO single crystals and for MgO crystals doped with H, Li, Ni or Co impurities, subjected to different thermal treatments. The resulting defects were monitored by optical absorption spectroscopy. Undoped crystals have a hardness of 9.2 ± 0.2 GPa. After thermochemical reduction up to about 2300 K in Mg vapour at 7 atm, resulting in a deficiency of anions, a hardness value of 10.4±0.1 GPa was obtained. The enhancement is attributed to O vacancies. For doped crystals, hardening was observed and was attributed to impurities, point defects, cavities and metallic precipitates. A constant value of 300 GPa for Young′s modulus was obtained in all cases, indicating that the elastic properties are not influenced by either impurities or intrinsic defects.

Optical and mechanical properties of MgO crystals implanted with lithium ions

Defect profile induced by implantation of Li+ ions with an energy of 175 keV and a fluence of 1×1017u2009ions/cm2 in MgO single crystals was characterized by Rutherford backscattering and optical absorption measurements. Several absorption bands at 5.0, 3.49, 2.16, and 1.27 eV, identical to those found in neutron irradiated crystals, were observed and have been previously associated with oxygen vacancies and higher-order point defects involving oxygen vacancies. Despite the high fluence of Li+ ions, no evidence was found for the formation of Li nanocolloids during implantation. Nanoindentation experiments demonstrated that both the hardness and Young’s modulus were higher in the implanted layer than in the sample before implantation. The maximum values were H=(17.4±0.4) and E=(358±9)u2009GPa, respectively, at a contact depth of ≈165 nm. Thermal annealings in flowing argon at increasing temperatures improved the crystalline quality of the implanted layer. After annealing at 500 K, two extinction bands at ≈2.75 and...

Photochromism of vacancy-related defects in thermochemically reduced α‐Al2O3:Mg single crystals

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.

Radiation-damage recovery in undoped and oxidized Li doped MgO crystals implanted with lithium ions

Abstract Undoped MgO and oxidized Li-doped MgO single crystals were implanted with 1xa0×xa01017 Li+/cm2 at 175 keV. The Rutherford backscattering spectrometry (RBS)/channeling data obtained after implantation shows that damage was produced throughout the entire range of the implanted ions. Optical absorption measurements indicate that after implantation the most intense band occurs at ≈5.0 eV, which has been associated with anion vacancies. After annealing at 450 K the intensity of the oxygen-vacancy band decreases monotonically with temperature and completely disappears at 950 K. A broad extinction band centered at ≈2.14 eV associated with lithium precipitates emerges gradually and anneals out at 1250 K. RBS/channeling shows that recovery of the implantation damage is completed after annealing the oxidized samples at 1250 K.