LaAlO3 Crystals Research Articles

Low-temperature NIR-VUV optical constants of (001) LaAlO3 crystal

The optical constants and dielectric function of (001) LaAlO3 crystal were investigated at low temperatures down to 10 K in the NIR-VUV spectral range (photon energies 0.8-8.8 eV). Reflection variable angle spectroscopic ellipsometry and transmission spectroscopy were applied. Interband transitions were examined using the Tauc plots and the critical-point analysis. At room temperature, the indirect bandgap of 5.6 ± 0.01 eV and the lowest-energy direct transition at 7.2 ± 0.03 eV were detected. On cooling to 10 K, a blueshift of ∼0.2 eV and ∼0.1 eV was observed for the indirect and direct transitions, respectively. In the transparency spectral range, the index of refraction was found to be nearly temperature-independent and vary with photon energy from 2.0 (1 eV) to 2.5 (5.5 eV). It was suggested that the excellent thermal stability of the index of refraction may be related to the revealed thermally stable interband transitions. The results are of importance for modeling and design of modern optical devices.

Structural damage response of lanthanum and yttrium aluminate crystals to nuclear collisions and electronic excitation: Threshold assessment of irradiation damage

A comparative analysis is performed on the structural damage response and associated mechanisms in lanthanum aluminate and yttrium aluminate crystals under various irradiation conditions by a combination of experimental and theoretical approaches. Under low-energy Au+ irradiation, the damage accumulation curve shows a higher damage rate for LaAlO3 crystals than YAlO3 crystals. The relatively low irradiation tolerance of LaAlO3 to the action of nuclear collisions is ascribed to the large amorphization cross-section and effective cross-section for defect-stimulated amorphization. Under swift Ar12+, Ni19+ and Kr17+ irradiation with different ion energies and velocities, the formed highly-disordered/amorphous latent tracks with different morphologies in pristine and predamaged crystals are discussed, and the corresponding electronic energy loss and lattice temperature thresholds are quantitatively determined. Compared to YAlO3, LaAlO3 exhibits lower sensitivity and higher damage tolerance to the electronic energy loss process, attributing to its relatively high recrystallization efficiency during the rapid quenching process. Furthermore, the introduction of lattice defects into LaAlO3 and YAlO3 crystals considerably enhances the sensitivity and intensity of thermal spike response to the electronic energy loss, and the induced effective modification of track morphologies demonstrates the synergistic effect between the electronic energy loss and pre-existing defects created by nuclear collisions. In this case, even under the action of electronic energy loss below the threshold, the lattice temperature in the nuclear-collision damaged crystalline system could still meet the criterion for track production. The irradiation energy deposited to atoms and induced lattice temperature evolution discussed in this work provide a deeper insight into the complex processes involved in irradiation-induced latent track behaviors.

Thermoluminescent properties of LaAlO3:C crystals synthesized by solid state reaction applied to UV Dosimetry

In this work we have investigated the thermoluminescent (TL) response of carbon doped lanthanum aluminate crystals grown by solid state reaction method. We conducted three syntheses by different combinations of Al2O3, La2O3 and 0.1 wt % of carbon atoms. The 1:1 mixture of aluminum and lanthanum oxide was mixed with 0.1wt.% carbon and annealed at 1700°C for two hours in a hydrogen atmosphere. The X-ray diffraction analysis revealed the formation of the rhombohedral LaAlO3 crystallographic phase. F and F+ centers were identified by ultraviolet-visible spectroscopy. Fourier transform infrared spectroscopic data show higher and lower frequencies assigned to AlO6 octahedra in LaAlO3. The UV irradiations were carried out using a commercial 8W UV lamp. Thermoluminescence measurements were performed at a Harshaw 4500 TL reader. The three syntheses were very efficient to obtain carbon doped LaAlO3 crystals with high thermoluminescent output, for low exposure rates of UVR fields.

Deformation Broadening and the Fine Structure of Spectral Lines in Optical Spectra of Dielectric Crystals Containing Rare-Earth Ions

The procedure of calculation of the spectral line shape in optical spectra of rare-earth ions in crystals with the inclusion of random deformations of an elastically anisotropic crystal lattice caused by point defects is developed. The distribution function of components of the random strain tensor in the case of a low defect concentration is obtained as the generalized six-dimensional Lorentz distribution. The distribution function parameters are represented by the integral functional of the strain tensor components on a sphere of unit radius containing an isotropic point defect in its center. The numerical calculations of the strain tensors induced by point defects and the parameters of the distribution functions of random strains in LiLuF4 and LaAlO3 crystals have been performed. The calculated envelope with the doublet structure corresponding to the Γ2(3H4) → Γ34(3H5) singlet–doublet transition in the absorption spectrum of Pr3+ ions in the LiLuF4 crystal agrees well with the data of the measurements.

Thermoluminescent properties of LaAlO3:C crystals synthesized by solid state reaction applied to UV Dosimetry

In this work, the thermoluminescent (TL) response of carbon doped lanthanum aluminate crystals grown by solid state reaction method was investigated. Three syntheses using different combinations of Al2O3, La2O3 and 0.1 wt % of carbon atoms were conducted. The 1:1 mixture of aluminum and lanthanum oxide was mixed with 0.1wt.% carbon and annealed at 1700°C for two hours in a hydrogen atmosphere. The X-ray diffraction analysis revealed the formation of the rhombohedral LaAlO3 crystallographic phase. Ultraviolet-visible spectroscopies were obtained and F and F+ centers were identified ascribed. Fourier transform infrared spectroscopic data show higher and lower frequencies assigned to AlO6 octahedra in LaAlO3. The UV irradiations were carried out using a commercial 8W UV lamp. Thermoluminescence measurements were performed at a Harshaw 4500 TL reader. The three syntheses were very efficient to obtain carbon doped LaAlO3 crystals with high thermoluminescent output, for low exposure rates of UVR fields.

Thermoluminescent properties of LaAlO3:C crystals synthesized by solid state reaction applied to UV Dosimetry

In this work we have investigated the thermoluminescent (TL) response of carbon doped lanthanum aluminate crystals grown by solid state reaction method. We conducted three syntheses by different combinations of Al2O3, La2O3 and 0.1 wt % of carbon atoms. The 1:1 mixture of aluminum and lanthanum oxide was mixed with 0.1wt.% carbon and annealed at 1700°C for two hours in a hydrogen atmosphere. The X-ray diffraction analysis revealed the formation of the rhombohedral LaAlO3 crystallographic phase. F and F + centers were identified by ultraviolet-visible spectroscopy. Fourier transform infrared spectroscopic data show higher and lower frequencies assigned to AlO6 octahedra in LaAlO3. The UV irradiations were carried out using a commercial 8W UV lamp. Thermoluminescence measurements were performed at a Harshaw 4500 TL reader. The three syntheses were very efficient to obtain carbon doped LaAlO3 crystals with high thermoluminescent output, for low exposure rates of UVR fields.

Ion beam damage assessment and waveguide formation induced by energetic Si-ion irradiation in lanthanum aluminate crystal

Lanthanum aluminate (LaAlO3) crystal has emerged as one of the most valuable functional-materials, and its physical, electronic and optical properties strongly depend on the crystal structure, which can be easily altered in an irradiation environment and therefore affect the performance of LaAlO3-based devices. On the other hand, the preparation of LaAlO3 waveguide is also a scientific challenge for its potential application prospects in optoelectronics field. In this work, the damage evolution behavior of LaAlO3 crystal under Si-ion irradiation has been discussed in detail utilizing complementary characterization techniques, and then, single-mode waveguide of LaAlO3 crystal in the visible band can be obtained based on ion-irradiation-induced lattice damage behavior. Waveguide optical-coupling techniques are used to show its competitive features. Thus, novel optical waveguides with optimized features in LaAlO3 crystals can be tailored by a proper selection of ion mass, energy and fluence using the modification of the target material during ion irradiation process.

Lattice damage assessment and optical waveguide properties in LaAlO3 single crystal irradiated with swift Si ions

As one of the representative ABO3 perovskite-structured oxides, lanthanum aluminate (LaAlO3) crystal has emerged as one of the most valuable functional-materials, and has attracted plenty of fundamental research and promising applications in recent years. Electronic, magnetic, optical and other properties of LaAlO3 strongly depend on its crystal structure, which could be strongly modified owing to the nuclear or electronic energy loss deposited in an ion irradiation environment and, therefore, significantly affecting the performance of LaAlO3-based devices. In this work, utilizing swift (tens of MeV) Si-ion irradiation, the damage behavior of LaAlO3 crystal induced by nuclear or electronic energy loss has been studied in detail utilizing complementary characterization techniques. Differing from other perovskite-structured crystals in which the electronic energy loss could lead to the formation of an amorphous region based on the thermal spike mechanism, in this case, intense electronic energy loss in LaAlO3 will not induce any obvious structural damage. The effects of ion irradiation on the mechanical properties, including hardness increase and elastic modulus decrease, have been confirmed. On the other hand, considering the potential applications of LaAlO3 in the field of integrated optoelectronics, the optical-waveguide properties of the irradiation region have been studied. The significant correspondence (symmetrical inversion) between the iWKB-reconstructed refractive-index profile and SRIM-simulated dpa profile further proves the effects (irradiation-damage production and refractive-index decrease) of nuclear energy loss during the swift-ion penetration process in LaAlO3 crystal. In the case of the rather-thick damage layer produced by swift-ion irradiation, obtaining a damage profile will be constrained owing to the analysis-depth limitation of the characterization techniques (RBS/channeling), and our analysis process (optical guided-mode measurement and subsequent refractive-index-profile reconstruction) also provides a new approach to study the damage behavior (damage profile) once the functional relationship between the refractive index and lattice disorder for the specific material could be determined.

Lattice damage and waveguide properties of medium- and high-energy C3+ ions-irradiated LaAlO3 crystals

To investigate irradiation effects of LaAlO3 crystals, planar waveguides were fabricated via the medium- and high-energy C3+ ions irradiation. The characterizations of waveguides showed that irradiation at different conditions induced diverse variations of the number of guiding modes, refractive index profiles and lattice damage. Rutherford backscattering/channeling spectra in combination with X-ray diffraction and micro-Raman spectra was used to probe the lattice damage distributions in the near surface of irradiated areas, where the electronic energy loss is predominant. The annealing process with the restoring of the lattice damage to some extent was investigated at temperatures ranging from 533 to 773 K. Meanwhile, as a crucial element of integrated optics and optoelectronics, the light propagation properties of optical waveguide were also investigated. These enable a feasible application of LaAlO3 in integrated optical system.

Deformation splittings in the spectra of LaAlO3: Ho3+, Pr3+, Tm3+single crystals

The work is devoted to a high-resolution spectroscopy study of the perovskite-type single crystals of LaAlO3 doped with non-Kramers rare-earth ions of praseodymium, holmium and thulium. Doublets of a specific shape were observed in the spectra, which we attribute to the presence of random deformations in the crystals.

Magnetic Modes in Rare Earth Perovskites: A Magnetic-Field-Dependent Inelastic Light Scattering study

Here, we report the presence of defect-related states with magnetic degrees of freedom in crystals of LaAlO3 and several other rare-earth based perovskite oxides using inelastic light scattering (Raman spectroscopy) at low temperatures in applied magnetic fields of up to 9 T. Some of these states are at about 140 meV above the valence band maximum while others are mid-gap states at about 2.3 eV. No magnetic impurity could be detected in LaAlO3 by Proton-Induced X-ray Emission Spectroscopy. We, therefore, attribute the angular momentum-like states in LaAlO3 to cationic/anionic vacancies or anti-site defects. Comparison with the other rare earth perovskites leads to the empirical rule that the magnetic-field-sensitive transitions require planes of heavy elements (e.g. lanthanum) and oxygen without any other light cations in the same plane. These magnetic degrees of freedom in rare earth perovskites with useful dielectric properties may be tunable by appropriate defect engineering for magneto-optic applications.

Study of the thermoluminescence mechanisms in LaAlO3: Ce, Dy crystals

In the present paper a complementary investigation about the mechanisms behind the thermoluminescence of LaAlO3:Ce,Dy crystals have been performed. The aim of this study is to present a model that, by using as guess values the parameters obtained by employing the GOK model, explain the role of Ce3+ and Dy3+ in the huge increase of the thermoluminescence signal of undoped LaAlO3 crystals. In the first section of this paper, the previous recorded TL glow peaks were further analyzed and their corresponding kinetic parameters such as order of kinetics (b), trap depth (E) and frequency factor (s) were determined; in this approach, the GCD analysis is mainly used to find the individual glow-peaks of a composite TL glow-curve and further to evaluate the sets of trapping parameters of each glow-peak.

Substrate-related structural, electrical, magnetic and optical properties of La0.7Sr0.3MnO3 films

La0.7Sr0.3MnO3 (LSMO) film exhibits certain unique properties which make it an ideal material for heterostructure-based multifunctional devices. In this paper, La0.7Sr0.3MnO3 thin films were grown epitaxially on SrTiO3, LaAlO3(LAO) and (LaAlO3)0.3-(SrAl0.5Ta0.5O3)0.7(LSAT) substrates, and their structural, electrical, magnetic and optical properties were examined and compared, aiming at revealing and explaining the effects of substrate on fundamental physical properties of LSMO films. Results show that (i) the effects of substrate materials on the electrical conductivity and paramagnetic–ferromagnetic transition temperature of the LSMO films could be ignored, and (ii) the LSMO film on LAO substrate has much higher coercive field and slower photoinduced relaxation process than other samples. This unusual behavior observed in the LSMO film on the LAO substrate could be related to the nature of the LAO substrate itself, on which the LSMO film showed different easy magnetization direction, and the film also exhibited relatively poor crystalline quality due to the twin domain structure of LAO crystal. Our results may help to understand the substrate-related electrical, magnetic and optical properties for perovskite manganite films.

Research on the thermal shifts and electron–phonon coupling parameters of Ri lines for Cr3+ ions in LaGaO3 and LaAlO3 crystals

The thermal shifts of Ri lines for Cr3+ ions in LaGaO3 and LaAlO3 crystals are investigated by an expression including both the static contribution due to thermal expansion of lattice and the vibrational contribution due to electron–phonon interaction. The results show that the relative importance of static contributions (characterized by the ratio between the static contribution and vibrational one) is larger than 20% for these systems. So the assumption in the previous papers that the static contributions are very small and can be neglected is doubtful. The true electron–phonon coupling coefficients α' obtained through the above expression are increased by about 30% and 40%, respectively, for LaGaO3:Cr3+ and LaAlO3:Cr3+ in comparison with the corresponding parameters α acquired previously by taking account of only the vibrational contribution. So, in the reasonable and precise studies of the thermal shifts and the electron–phonon parameters for a spectral line in crystals, both the static and vibrational contributions should be contained.

Comparison of waveguide properties and Raman spectroscopic visualization of C and O ion implantation on LaAlO(3) crystals.

LaAlO3 crystals were implanted by C ions and O ions at an energy of 6.0MeV with a fluence of 1.5×1015 ions/cm2. The profiles of the guided modes were measured through prism coupling and end-face coupling methods with a 633nm laser source. A nonleaky waveguide structure in the TM mode was fabricated by O ion implantation after a proper annealing treatment. Characteristics of the implanted C and O ions were compared. Some changes of the full width at half of the maximum and intensity of the Raman spectra were observed between the waveguide and substrate regions in LaAlO3 crystals. Thus, the Raman spectra can be used to visualize any damage or defects in the LaAlO3 crystals during the implantation process.

Ablation replacement of iron with Co, Mn, Ni, and Cu during growth of iron-based superconductor films in the Fe0.9 M 0.1Se0.92 system

Thin films of iron-based chalcogenide superconductors FeSe0.92, with iron partially replaced (at least up to 10 at %) by elements such as cobalt, nickel, manganese, or copper, have been grown on the surface of LaAlO3 crystals. Growth is performed by the laser ablation of a target prepared in the form of a ceramic pellet by high-temperature synthesis and the sintering of preliminarily pressed stoichiometric mixture of powders. Iron in these ceramics is replaced with an alloying metal by no more than 3 at %. The rest (7 at %) of the metal is in the form of precipitates of other phases. X-ray diffraction analysis of the grown films has shown that they are single-crystal and free of any precipitates of other crystallographic orientations and phases. This is evidence of the complete (10 at %) replacement of iron with a doping metal in the film structure. This circumstance indicates that the synthesis of components occurs more actively and completely during laser ablation (than in solid-phase chemical reactions) as a result of the transformation of multicomponent target material into plasma. Thus, one can fabricate film materials in a wider range of chemical compositions than in the form of solid-phase synthesized ceramics.

Synthesis and investigation of the luminescent properties of carbon doped lanthanum aluminate (LaAlO3) for application in radiation dosimetry

LaAlO3 single crystals grown under hydrothermal conditions and co-doped with Ce and Dy atoms have been recently reported to show high thermoluminescent (TL) outputs for ultraviolet (UV) radiation fields (Oliveira et al., 2011). Due to this property, they have been considered for further investigation for applications in UV dosimetry. Encouraged by these results, we start an investigation about the TL properties of polycrystalline LaAlO3 grown by an alternative method. In this method, equimolar amounts of Al2O3 and La2O3 are sintered, producing polycrystalline LaAlO3 powder. Polycrystals doped with amounts of carbon ranging from 0.0 to 5.0 at.% were synthesized by sintering under hydrogen reducing atmosphere. After irradiation with a UV commercial lamp, the best TL outputs were observed for the undoped sample. The recorded TL glow curves show a main TL peak centered at 175 °C. The TL emission spectrum show a broad emission peak centered at 634 nm and another three narrow peaks centered at 724 nm, 738 nm and 754 nm, respectively. The undoped material show a huge TL output response for UV spectral irradiances ranging from 0.04 to 1.68 mJ cm−2 that can be fitted by a 2nd order polynomial regression. The investigation demonstrates that undoped polycrystalline LaAlO3 crystals sintered under reducing atmosphere are very attractive to be investigated as high sensitivity ultraviolet TL dosimeters.

Investigation into the growth and structure of thin-film solid solutions of iron-based superconductors in the FeSe0.92-FeSe0.5Te0.5 system

Thin films of FeSe0.92 and FeSe0.5Te0.5 iron chalcogenide superconductors and solid solutions containing these components in different ratios have been grown on the surface of LaAlO3 (102) crystals by pulsed laser deposition. Films of solid solutions have been deposited by simultaneous laser ablation from two targets of the FeSe0.92 and FeSe0.5Te0.5 stoichiometric compositions onto one substrate. An X-ray diffraction study of the film structure shows that the films grown are epitaxial and their lattice parameters regularly vary with the ratio of the deposited components, which was controllably varied by changing the ablation intensities from the targets.

Defect dynamics and spectral observation of twinning in single crystalline LaAlO3 under subbandgap excitation

We have investigated the photoluminescence and ultrafast dynamics of LaAlO3 crystal. The photoluminescence consists of a broad spectrum and two sharp peaks, which arise from various defect levels within the bandgap. A doublet splitting of roughly 6 nm is seen in these two sharp peaks. An Al displacement of 0.09 Å in a sublattice, which is possible because of twinning, is adequate to explain the spectral splitting. Femtosecond pump probe experiments reveal further that many of these defect levels have a few picosecond decay times while the lowest defect states have decay times longer than nanosecond to the valence band.

Magnetism in wide band gap semiconductors implanted with non-magnetic ions

Single crystals of ZnO, TiO2 and LaAlO3 have been implanted with Ar with 100 keV and different fluencies. The Ar implanted crystals showed a week ferromagnetic-like signal between 10 K and 400 K. Hysteresis curves obtained at room temperature allowed confirming the ferromagnetic behaviour of the implanted samples. Spin polarised first principles density functional calculations were performed in the case of ZnO considering Zn interstitials and O vacancies. No net magnetic polarisation was found for O vacancies, but in the case of Zn vacancies a magnetic moment of 1μB was obtained.