α-Al2O3 Single Crystals Research Articles

Determining residual impurities in sapphire by means of electron paramagnetic resonance and nuclear activation analysis

Sapphire (α-Al2O3) single crystals grown using the Verneuil and Kyropoulos methods have been analyzed using electron paramagnetic resonance and γ-ray spectroscopy with 12-MeV bremsstrahlung excitation. It is established that uncontrolled impurities in the final sapphire single crystals grown by the Kyropoulos method in molybdenum-tungsten crucibles are supplied both from the initial materials and from the furnace and crucible materials

Electrical conductivity in undoped α-Al2O3 crystals implanted with Mg ions

Undoped α-Al2O3 single crystals with the c axis either perpendicular or parallel to the broad face were implanted with Mg ions. The concentrations of defects induced by implantation were monitored by optical absorption measurements. In the Mg-implanted region, direct current electrical measurements were made between 296 and 445K. At room temperature, an increase in the electrical conductivity of about 15 orders of magnitude relative to the unimplanted region was observed only in crystals with the c axis perpendicular to the broad face of the samples. The I–V characteristics reveal an ohmic behavior of the electrical contact on the implanted area. Measurements at different temperatures suggest that the electrical conductivity is thermally activated with an activation energy of about 0.02eV. We relate the enhancement in conductivity observed in the implanted regions to intrinsic defects created by the implantation, rather than with the implanted Mg ions.

Amorphization of sapphire induced by swift heavy ions: A two step process

Single crystals of α-Al2O3 were irradiated with 0.7MeV/amu Xe ions at the GANIL accelerator with fluences extending from 5×1011 to 2×1014ions/cm2. The samples were characterized by Rutherford Backscattering Spectrometry in Channelling geometry (RBS-C) and surface profilometry for swelling measurements. RBS-C analysis evidence the presence of two incident ion effects on sapphire targets: the creation of partial disorder that saturates at ∼40% with a damage cross section of 7×10−14cm2, followed by a complete disorder starting from the surface and appearing at fluence larger than ∼1.2×1013ions/cm2. A correlation between relative disorder and swelling is observed. This study is in agreement with the appearance of amorphization of α-Al2O3 at high fluence as observed by Aruga et al. [T. Aruga, Y. Katano, T. Ohmichi, S. Okayasu, Y. Kazumata, Nucl. Instr. Meth. Phys. Res. B 166–167 (2000) 913–919].

Morphology and interface structure of α-Fe2O3 islands grown on sapphire by ion-implantation and annealing

The morphology and interface structure of α-Fe2O3 islands grown on α-Al2O3 single crystals (sapphire) by Fe-ion-implantation and annealing in an oxidizing atmosphere have been studied using transmission electron microscopy. The α-Fe2O3 islands have the orientation relationship of [0001]α-Fe2O3||[0001]sapphire and (112¯0)α-Fe2O3||(112¯0)sapphire with sapphire. The typical outline of α-Fe2O3 islands consists of two (0001) and six {101¯2} planes. The interfaces between α-Fe2O3 islands and sapphire are semicoherent, that is coherent regions separated by misfit dislocations at the interfaces. When imaged along the [1¯100]sapphire direction, the projected Burgers vector is determined to be 16[112¯0]sapphire. When imaged along the [112¯0]sapphire direction, the projected Burgers vector is determined to be 12[1¯100]sapphire. These misfit dislocations form a network structure at the interface to accommodate the mismatch between the lattices of the α-Fe2O3 and the α-Al2O3.

Low-Temperature Selective Growth of Heteroepitaxial α-Al2O3 Thin Films on a NiO Layer by the Electron-Beam Assisted PLD Process

AbstractSelective heteroepitaxial growth of α-Al2O3 thin films on a NiO layer was investigated using an electron-beam assisted pulsed laser deposition process. The epitaxial NiO layer was grown on an ultrasmooth sapphire (α-Al2O3 single crystal) (0001) substrate. The α-Al2O3 thin film could be grown epitaxially only in the electron-beam irradiated region of the epitaxial NiO layer at 300°C, while the amorphous Al2O3 film was grown in the non-irradiated region. The homoepitaxial growth of α-Al2O3 thin films could not be attained on the sapphire (0001) substrate at 300°C. This indicates that the electron-beam irradiation enhances heteroepitaxial growth of the α-Al2O3 thin films on the NiO layer at 300°C. When we annealed the epitaxial Al2O3/NiO bilayer film at 350°C in a hydrogen atmosphere, we could reduce only the NiO layer to an epitaxial Ni metal layer, allowing the fabrication of epitaxial Al2O3/Ni (insulator/metal structure) films. The fabricated Al2O3/Ni bilayer films exhibited a very smooth surface.

Fabrication and Characterization of Electrically Functional Lanthanum Hexaboride Thin Films on Ultrasmooth Sapphire Substrates

AbstractThe crystal growth of lanthanum hexaboride (LaB6) thin films was examined by applying the laser molecular beam epitaxy (laser MBE) process. C-axis (100) highly-oriented LaB6 thin films could be fabricated on ultrasmooth sapphire (α-Al2O3 single crystal) (0001) substrates with atomic steps of 0.2 nm in height and atomically flat terraces. The obtained film exhibited a smooth surface with root mean square roughness of 0.15 nm. The lattice parameter of the LaB6 thin film was close to the bulk value reported previously. In the case of deposition on commercial mirror-polished sapphire substrates, the grown film was amorphous. The resistivity of the prepared crystalline LaB6 thin films was as low as 2.2 × 10−4 Ω cm and almost constant in the temperature range of 10–300 K.

Hyperfine interaction of 25Al in α-Al2O3 and its quadrupole moment

The electric quadrupole (Q) moment of short-lived nucleus 25Al \((I^{\pi} = 5/2^{+}, T_{1/2} = 7.18~{\rm s})\) has been measured for the first time, by means of the β-NQR technique. The spin polarization of 25Al was produced in heavy ion collisions and was kept in a α-Al2O3 single crystal for as long as 2 s and the quadrupole coupling frequency was obtained as ∣ eqQ / h(25Al in Al2O3) ∣ = (4.05 ±0.30) MHz. From the result, the Q moment was determined as ∣ Q(25Al)∣ = (240 ±20) mb. The present Q moment is larger by 30% than the shell model value of 184 mb, calculated by OXBASH code, which may show additional deformation of the nucleus.

Electric quadrupole moments of neutron-rich nuclei 32Al and 31Al

The electric quadrupole moments for the ground states of 32Al and 31Al have been measured by the β ray-detected nuclear quadrupole resonance method. Spin-polarized 32Al and 31Al nuclei were obtained from the fragmentation of 40Ar projectiles at E/A = 95 MeV/nucleon, and were implanted in a single crystal α-Al2O3 stopper. The measured Q moment of 32Al, |Q(32Al)| = 24(2) mb, is in good agreement with a conventional shell-model calculation with a full sd model space and empirical effective charges, while that of 31Al is considerably smaller than the sd calculations.

Measurement of the electric quadrupole moment of 32Al

Abstract The electric quadrupole moment Q for the ground state of 32Al has been measured using the β-NMR technique. Spin-polarized 32Al nuclei were obtained from the fragmentation of 40Ar projectiles at E / A = 95 MeV / nucleon , and were implanted in a single crystal α-Al2O3 stopper. The quadrupole moment was deduced from the measured quadrupole coupling constant. The obtained value, | Q ( Al g.s. 32 ) | = 24 ( 2 ) mb , is remarkably small as compared with other Al isotopes, but is well explained by shell model calculations within the sd shell. The present result indicates that 32Al has a normal sd-shell structure, in contrast to the neighboring N = 19 isotone 31Mg for which a strongly deformed intruder ground state has been reported to occur.

Magnetotransport properties of Fe3O4 epitaxial thin films: Thickness effects driven by antiphase boundaries

We present an in-depth study of the magnetotransport properties of epitaxial Fe3O4 films as a function of film thickness. The films, grown on α-Al2O3(0001) single crystals by atomic-oxygen assisted molecular beam epitaxy, exhibit high structural order and abrupt interfaces. These films contain antiphase boundaries (APBs), the density of which is strongly dependent on film thickness. A series of resistivity and magnetoresistance measurements demonstrate a systematic evolution of these properties with decreasing film thickness, revealing the impact of APBs on the transport properties in the films. We present a model based on the spin-polarized transport across an antiferromagnetically coupled APB in order to successfully reproduce our experimental data over a large range of applied magnetic fields. The comparison of this model with experimental results further clarifies the mechanism of the anomalous magnetotransport behavior in Fe3O4.

Surface modification of silicate glasses by nanoimprint using nanostriped NiO thin film molds

Nanoscale surface modification of silicate glasses was examined by applying nanoimprint technique using a nanostriped NiO thin film mold. The mold had the pattern composed of regularly arranged straight nanogrooves, which was formed by high-temperature annealing of the Li-doped NiO epitaxial thin film deposited on the atomically stepped sapphire (α-Al2O3 single crystal) substrate. The glass imprint was proceeded through the simple steps of heating (∼600°C), pressing (∼1kPa) and then cooling in air. The nanoimprinted glass surface transferred reversely from the mold exhibited the multi nanowire array having an interval of ∼80nm, wire width of ∼70nm, and wire height of ∼20nm.

Radiative color centers in doped sapphire crystals

Experimental and theoretical results of investigation of the spectral properties of sapphire (α-Al2O3) single crystals are analyzed. Crystals grown under different conditions, both nonactivated and activated with impurity ions of the iron group (Cr, V, Mn, Fe, Co, Ni), primarily with chromium ions, were investigated before and after irradiation with photons and high-energy electrons. It is shown that the impurity composition of crystals significantly affects the radiative and optical properties and the character of changes in the electronic state of impurities.

Optical absorption of metallic Zn nanoparticles in Zn ion implanted sapphire

Zn+ ion implantation (48keV) was performed at room temperature up to a fluence of 5×1017cm−2 in α-Al2O3 single crystals. X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and optical absorption spectroscopy were utilized to characterize the optical properties, chemical charge states and the microstructure of embedded metallic Zn nanoparticles, respectively. XPS analysis indicated that implanted Zn ions are in the charge state of metallic Zn0. TEM analysis revealed the metallic Zn nanoparticles of 3–10nm in the as-implanted sample at a fluence of 1×1017cm−2. A selected area electron diffraction (SAD) pattern indicates the random orientation of the Zn nanoparticles. A clear absorption peak appeared gradually in the optical absorption spectra of the as-implanted crystals, due to surface plasma resonance (SPR) of Zn nanoparticles. The wavelength of the absorption peak shifted from 260nm to 285nm with the increasing ion fluence, ascribed to the growth of Zn nanoparticles.

Bleaching and micro-cracking phenomena induced in various types of sapphires by keV-electron beam irradiations

Electron-beam-induced phenomena on α-Al2O3 single-crystals have been investigated using a scanning electron microscope with a cathodoluminescence (CL) apparatus. Various types of sapphires were irradiated at room temperature by keV electrons of the total fluences up to ≈6×1020electronscm−2. In the case of colored specimens, increasing amounts of electron irradiations induced a reversible “bleaching” phenomenon and subsequently an irreversible “cracking” phenomenon on nanometer scales in the surface and subsurface layers. The details of the fluence dependences of these beam-induced changes differed among the various natural and synthetic sapphires. These changes were dramatically reduced by the presence of thin metal layers on the insulating sapphire surfaces, indicating that these phenomena were induced by the presence of charges accumulated in the specimens. Such electron irradiations also varied CL intensities of the F+ center peaked at 330nm while the Cr3+ center CL peak observed at 697nm was almost unchanged in intensity with increasing the electron fluence. Furthermore, information on these CL centers along the depth direction from the specimen surface was obtained using variable incident electron energies ranging from 1 to 25keV. The above phenomena are discussed in relation to the crystalline quality of the specimens examined.

Morphological Evolution of Ba(NO3)2 Supported on α-Al2O3(0001): An In Situ TEM Study

A key question for the BaO-based NOx storage/reduction catalyst system is the morphological evolution of the catalyst particles during the uptake and release of NOx. Notably, because the formed product during NOx uptake, Ba(NO3)2, requires a lattice expansion from BaO, one can anticipate that significant structural rearrangements are possible during the storage/reduction processes. Associated with the small crystallite size of high-surface area gamma-Al2O3, it is difficult to extract structural and morphological features of Ba(NO3)2 supported on gamma-Al2O3 by any direct imaging method, including transmission electron microscopy. In this work, by choosing a model system of Ba(NO3)2 particles supported on single-crystal alpha-Al2O3, we have investigated the structural and morphological features of Ba(NO3)2 as well as the formation of BaO from Ba(NO3)2 during the thermal release of NOx, using ex-situ and in-situ TEM imaging, electron diffraction, energy dispersive spectroscopy (EDS), and Wulff shape construction. We find that Ba(NO3)2 supported on alpha-Al2O3 possesses a platelet morphology, with the interface and facets being invariably the eight [111] planes. Formation of the platelet structure leads to an enlarged interface area between Ba(NO3)2 and alpha-Al2O3, indicating that the interfacial energy is lower than the Ba(NO3)2 surface free energy. In fact, Wulff shape constructions indicate that the interfacial energy is approximately 1/4 of the [111] surface free energy of Ba(NO3)2. The orientation relationship between Ba(NO3)2 and the alpha-Al2O3 is alpha-Al2O3[0001]//Ba(NO3)2[111] and alpha-Al2O3(1-210)//Ba(NO3)2(110). Thus, the results clearly demonstrate dramatic morphology changes in these materials during NOx release processes. Such changes are expected to have significant consequences for the operation of the practical NOx storage/reduction catalyst technology.

ZnO nanoparticles embedded in sapphire fabricated by ion implantation andannealing

ZnO nanoparticles were fabricated in sapphire(α-Al2O3 single crystal) by Zn ion implantation (48 keV) at an ion fluence of1 × 1017 cm−2 and subsequent thermal annealing in a flowing oxygen atmosphere. Transmissionelectron microscopy (TEM) analysis revealed that metallic Zn nanoparticlesof 3–10 nm in dimensions formed in the as-implanted sample and thatZnO nanoparticles of 10–12 nm in dimensions formed after annealing at600 °C. A broad absorption band, peaked at 280 nm, appeared in the as-implanted crystal, due tosurface plasma resonance (SPR) absorption of metallic Zn nanoparticles. After annealing at600 °C, ZnO nanoparticles resulted in an exciton absorption peak at 360 nm. The photoluminescence(PL) of the as-implanted sample was very weak when using a He–Cd 325 nm line as theexcitation source. However, two emission peaks appeared in the PL spectrum of ZnOnanopraticles, i.e., one ultraviolet (UV) peak at 370 nm and the other a green peak at500 nm. The emission at 500 nm is stronger and has potential applications in green/bluelight-emitting devices.

Mechanism of formation of nonlinearity in the dose dependence of the thermoluminescence output for anion-defect α-Al2O3 crystals

The mechanism of formation of nonlinearity in the dose dependence of the thermoluminescence output was investigated for anion-defect α-Al2O3 single crystals. The experiments revealed a new specific feature: the nonlinearity of the dose dependence of the thermoluminescence output depends on the heating rate of crystals during recording of thermoluminescence. It was established that this effect and other thermoluminescence features studied earlier for anion-defect α-Al2O3 crystals are adequately described in the framework of a unified mechanism associated with the interactive interaction of dosimetric and deep traps. Recommendations are given for a decrease in the range of the superlinear portion in the dose dependence of the thermoluminescence output for the crystals under investigation.

Anisotropy of wetting of molten Au on differently oriented α-Al2O3 single crystals

Abstract Wetting of molten Au ( r Max = 33–226 μm) on three differently oriented α-Al 2 O 3 single crystals, R ( 0 0 1 ¯ 2 ) , A ( 1 1 2 ¯ 0 ) and C (0 0 0 1), is studied by the microscopic sessile drop method under an Ar atmosphere at 1373 K. The contact angle differences for the three differently oriented α-Al 2 O 3 single crystals are less than 4°, corresponding to changes in the work of adhesion of less than 55 mJ/m 2 .

Thermal quenching of 3.0-eV photoluminescence in α-Al2O3 single crystals

The thermal quenching of 3.0-eV photoluminescence (PL) in oxygen-deficient α-Al2O3 single crystals upon preliminary UV irradiation has been studied. It is established that the parameters of the PL quenching kinetics vary within broad limits depending on the duration of UV pretreatment. The compensation effect related to a change in the energy state of the thermodynamic system “F-centers-electron traps” has been observed.

Development of a Novel Metal Epitaxy Method towards Ni Based Electro-magnetic Hybrid Systems

ABSTRACTThe epitaxial Ni (111) thin film on the oxide substrate could be obtained by a novel epitaxy method, employing pulsed laser deposition (PLD) of NiO (111) epitaxial film on the sapphire (α-Al2O3single crystal) substrate and successive hydrogen reduction of NiO. The NiO (111) epitaxial film was deposited on the sapphire (0001) substrate at room-temperature by PLD, and then reduced into the Ni epitaxial film by annealing (300 °C to 500 °C) in the hydrogen-atmosphere. On the other hand, the polycrystalline Ni metal thin film was obtained by reduction of the polycrystalline NiO film, indicating necessity of epitaxial growth for the precursor oxide thin film in the metal epitaxy. The present epitaxy method suggests the possible formation of [Ni/α-Al2O3] epitaxial multilayer via selective reduction of oxide multilayer.