Monocrystalline Sapphire Research Articles

The Optical Properties and Band Gap Energy of Nanocrystalline La0.4Sr0.6TiO3 Thin Films

The effects of microstructure on the optical properties of La 0.4 Sr 0.6 TiO 3 thin films were investigated. Dense films with the thickness of ∼200 nm and grain size 14-30 nm were produced on monocrystalline sapphire substrates by using a polymeric precursor spin coating technique at annealing temperatures under 800' C. X-ray data showed the formation of a single-phase cubic perovskite-type structure similar to undoped SrTiO 3 for annealing temperatures >500°C. The results of optical measurements showed that the optical spectra varied with the change of the grain size. From these data, the absorption coefficients were calculated and the band gap energy determined. In agreement with the quantum confinement model, it was shown that the band gap energy increased as the grain size decreased.

Magnetic anisotropy modification in ultrathin Co(0 0 0 1) film epitaxially grown on Mo(1 1 0)

Modification of anisotropy has been studied in Mo(110)/Co(0001)/Au structures grown by molecular beam epitaxy on a monocrystalline (112¯0) sapphire substrate. In order to fit the experimental data, the out-of-plane anisotropy energy contribution as well as that of the uniaxial in-plane were considered. Magnetic anisotropy constants as a dependence on Co thickness were determined in the range of spin-reorientation phase transition. Observed magnetic anisotropy can be explained by the presence of strains in Co film induced by the Mo(110) buffer.

Strength of silicon, sapphire and glass in the subthreshold flaw region

Strength properties are determined for monocrystalline silicon and sapphire and for soda-lime glass plates containing subthreshold flaws, i.e. flaws without detectable microcracks. Nanoindentations covering a wide range of loads are used to introduce flaws of predetermined sizes into highly polished or etched surfaces, to enable systematic study of the transition from the postthreshold to subthreshold regions. Strengths of the indented plates are measured using a simple bilayer test configuration in which the plates are bonded – indentations downward – onto a polycarbonate support base and loaded centrally at their top surfaces to failure. Failure occurs from flaw sites over the entire range of diminishing loads, up to a strength cutoff at >2 GPa associated with natural surface flaws. In none of the materials do the postthreshold strength/indentation-load data extrapolate unconditionally into the subthreshold region. However, whereas the glass data show an abrupt increase in strength at the threshold, the silicon and sapphire data show only a slight discontinuity in slope. A simple model for subthreshold flaws, based on the critical conditions to initiate radial cracking from embryonic shear faults within the contact damage sites, accounts for the main size trends in the data. The disparities between the glass and crystalline responses are attributed to differences in the precursor shear fault geometries. General applicability of the scaling concepts to a broad range of alternative flaw types, e.g. associated with microstructural defects, is discussed.

The dependence of the permittivity of sapphire on thermal deformation at cryogenictemperatures

The whispering gallery mode technique applied to single crystal sapphire, doped withtrivalent and tetravalent ions of titanium, has been used as a precise method of determiningthe dependence of the permittivity of pure sapphire on temperature-induceddimensional changes. According to the one-phonon model this is a measure of thetemperature dependence of the polarizability of the dielectric. The fractional change inthe square root of the permittivity near 17 GHz was found to be related to thefractional change in dimensions resulting from thermal expansion by a factor of16.0 ± 0.7,between 20 and 90 K. This was found to be the same both parallel and perpendicular to thecrystal axis in monocrystalline sapphire. Only the anisotropy in the thermal expansioncoefficient causes a difference between the temperature coefficients of permittivity paralleland perpendicular to the crystal axis.

Adhesion of oral streptococci to experimental bracket pellicles from glandular saliva

The aim of this study was to evaluate the functions of bracket pellicles as the binding receptors for Streptococcus mutans and Streptococcus gordonii. Four different types of orthodontic brackets were used: stainless steel, monocrystalline sapphire, polycrystalline alumina, and plastic. The bracket pellicles were formed by incubating orthodontic brackets with fresh submandibular–sublingual saliva or parotid saliva for 2 hours. The pellicles were extracted, and their components were confirmed by gel electrophoresis, immunodetection, and amino acid composition analysis. The roles of the bracket pellicles in the adhesion of oral streptococci were evaluated by incubating tritium-labeled streptococci with pellicle-transfer blots. The results showed that the salivary components adhered selectively according to type of bracket and glandular saliva. The selective adsorption was also proven by the amino acid composition profiles. Among the several salivary proteins, MG2, α-amylase, and the acidic proline-rich proteins provided the binding sites for S gordonii. However, none of these proteins in the bracket pellicles contributed to the adhesion of S mutans. These findings suggest that numerous salivary proteins can adhere selectively to the orthodontic brackets, and some of them contribute to the binding of S gordonii.

Optical and electrical properties of Pr0.8Sr0.2MnO3 thin films

The results of studies on the preparation, structure, optical, and electrical properties of Pr0.8Sr0.2MnO3 thin films were presented. Dense films with a thickness of 50–70 nm were produced on monocrystalline sapphire substrates using a polymeric precursor spin coating technique. The results of the optical measurements were correlated with the annealing temperatures of the films and showed that the shape of the optical spectra changed as the structure changed from an amorphous to crystalline structure. The optical spectra was also used to determine the energy dependence of the absorption coefficient. The electrical conductivity was observed to be about three orders of magnitude higher for specimens which were crystalline as compared to amorphous.

Optical properties of undoped and Gd-doped CeO2 nanocrystalline thin films

The results of studies of the preparation, structure, and optical properties of undoped and Gd-doped CeO2 thin films are presented. Dense films with 4–150 nm grain size have been obtained on monocrystalline sapphire substrates using a polymeric precursor spin coating method. The results of the optical measurements are presented and correlated with the microstructure of the films. The transmission spectra have been used to determine the energy dependence of the refractive index, n and the extinction coefficient, k. Both n and k of the thin films decreased as grain size decreased and these results showed that this change could be related to the transition from crystalline to amorphous CeO2. The effect of dopant has little influence on n, <5%, but doping resulted in about a 30%–40% reduction of k compared to undoped specimens, which could be related to the decrease of absorption centers due to the replacement of Ce by Gd.

Strains and stresses in an epitaxial Ni(111)/Mo(110) multilayer grown by direct ion beam sputtering

An epitaxial Ni(111)/Mo(110) multilayer was grown on a (1 1 2 ̄ 0) oriented monocrystalline sapphire substrate in a high vacuum sputtering deposition system. The strain and stress state of the layers has been measured with X-ray diffraction in symmetric and asymmetric geometries. Non-equal biaxial coherency stresses due to the Nishiyama–Wassermann epitaxial relation between both lattices are clearly evidenced. The values of the stress-free lattice parameters of molybdenum and nickel sublayers, deduced from the global stress analysis, supports the hypothesis of an interfacial alloying effect between layers with diffusion of nickel in the Mo layers and, to a lesser degree, of molybdenum in Ni layers. On the other hand, the Ni layers appear nearly fully relaxed. A detailed analysis of the stresses in the film shows a strong contribution of a compressive stress field due to possible post-growth atomic rearrangements occurring inside the multilayered film.

Low-loss sapphire waveguides for 75-110 GHz frequency range

Low-loss dielectric waveguides are promising for use instead of metal ones, but problems in transitions have to be overcome. A simple and effective structure made of a monocrystalline sapphire waveguide has been designed. Experimental results at 75-110 GHz indicate good matching with metal waveguides (VSWR /spl les/1.13) and low insertion loss (0.05-0.35 dB for 47 mm dielectric section).

Cryogenic measurement of the optical absorption coefficient in sapphire crystals at 1.064 μm for the large-scale cryogenic gravitational wave telescope

We have applied laser calorimetry to the measurement of optical absorption in mono-crystalline sapphire at cryogenic temperatures. Sapphire is a promising candidate for the mirror substrates of the large-scale cryogenic gravitational wave telescope. The optical absorption coefficients of different sapphire samples at a wavelength of 1.064 μm at 5 K were found to average 90 ppm/cm.

Electrical conductivity of nanocrystalline ceria and zirconia thin films

The results of studies of the preparation, structure and electrical conductivity of ZrO 2:16% Y and CeO 2 thin films are presented. Dense films with grain size controlled in the region of 1–400 nm have been obtained on monocrystalline sapphire and polycrystalline Al 2O 3 substrates using a polymeric precursor spin coating method. The electrical conductivity of nanocrystalline thin films has been studied as a function of oxygen activity and temperature and correlated with the microstructure. Nanocrystalline specimens are characterized by enhanced electrical conductivity and different stoichiometry compared with microcrystalline material.

An evaluation of plasma sprayed ceramic coatings by erosion, abrasion and bend testing

The mechanical properties of a series of plasma sprayed ceramic coatings made from alumina, chromia and alumina–titania precursor powders have been evaluated in particle erosion, two-body abrasion and bend testing. For the alumina coatings, both monocrystalline (sapphire) and polycrystalline alumina powders of varied grit sizes were used. Also, coatings of chromia and a mixture of alumina and titania were evaluated. The coatings were deposited with an axial injection spraying technique. Clear differences were found between the investigated coatings. Among the alumina coatings, those deposited from sapphire powders gave the best results in the wear tests. An addition of titania to alumina had a favourable effect in most tests. In abrasion, one chromia coating displayed a wear resistance comparable to the dense sintered alumina reference. The erosion rates were, however, several times higher for all sprayed coatings compared to the reference alumina.

Mechanical quality factor of a sapphire fiber at cryogenic temperatures

A mechanical quality factor of 1.1×10 7 was obtained for the 199 Hz bending vibrational mode in a monocrystalline sapphire fiber at 6 K. Consequently, we confirm that pendulum thermal noise of cryogenic mirrors used for gravitational wave detectors can be reduced by the sapphire fiber suspension.

Magnetic anisotropy in MBE-grown epitaxial gadolinium ultra-thin films

The structure and magnetic properties of gadolinium thin films in two systems: W/Gd/W and Y/Gd/Y were studied. Gd films were grown epitaxially on tungsten and yttrium buffer layers in UHV MBE facility using e-guns as the evaporation source. Monocrystalline sapphire substrates with (112̄0) crystallographic orientation were used. Critical thickness of dc=3 Å for gadolinium deposited epitaxially on tungsten was determined - in the first monolayer lattice expansion of 3.5% was observed comparing to the bulk gadolinium. Significant decrease of Curie temperature with decreasing of Gd thickness was found. In Y/Gd/Y system for all studied range of Gd thickness (160>X>9 Å) in-plane anisotropy was observed. On the contrary, in W/Gd/W system for Gd thickness below 40 Å the transition from in-plane to perpendicular anisotropy took place.

Properties of Nanocrystalline Gallium Nitride Thin Films Grown on Monocrystalline Substrates and Buffer Layers

Properties of GaN thin films deposited by rf-magnetron sputtering on monocrystalline sapphire, Si, and MgO substrates as well as on ZnO, MgAl2O4, and AlN buffer layers were investigated. The X-ray structural and crystallochemical analysis of phase composition, structure parameters, texture parameters and crystallite size of GaN films was carried out. Luminescent and electrical properties of GaN films on various substrates and buffer layers are described.

Growth of oxide buffer layers and YBCO films on various substrates by pulsed injection CVD

Bilayers composed from a YSZ or CeO 2 buffer layer and a YBCO film were in situ grown on monocrystalline sapphire, MgO. silicon, vicinally polished sapphire and MgO substrates by single source pulsed injection CVD. Zr. Ce. Y. Ba and Cu 2,2,6,6-tetramethyl-3,5-heptandionates dissolved in organic solvent were used for film deposition. CeO 2 /YBCO bilayers on sapphire were grown epitaxially and exhibited critical current densities up to 10 6 A/cm 2 at 77 K. YSZ and CeO 2 buffer layers on MgO substrates did not improved the properties of YBCO layers compared with YBCO films deposited directly on MgO surface; however, interesting microstructural properties of these heterostructures were found. Homogeneous YSZ films were also deposited on large Si substrates (3 inches in diameter).

Frequency-temperature compensation in Ti/sup 3+/ and Ti/sup 4+/ doped sapphire whispering gallery mode resonators

A new method of compensating the frequency-temperature dependence of high-and monolithic sapphire dielectric resonators near liquid nitrogen temperature is presented. This is achieved by doping monocrystalline sapphire with Ti(3+) ions. This technique offers significant advantages over other methods.

Electrical conductivity of glass and sapphire cells exposed to dry cesium vapor

We demonstrate that the walls of glass cells become electrically conductive when exposed to a dry cesium vapor, glasses of different compositions leading to similar effects. We find dramatically different results for monocrystalline sapphire cells, which exhibit typical resistances of a few GΩ, nearly five orders of magnitude larger than glass cells in similar conditions. In spectroscopy experiments requiring the application of an electric field, low cell resistances imply leakage currents generating stray magnetic fields. The latter, at the origin of harmful effects in precise tests of the fundamental laws of physics performed in cesium cells, will thus be suppressed in sapphire cells. Moreover, with such cells it becomes possible to place the electrodes outside. A tentative interpretation of the surface electrical conductivity of glass cells is propounded by establishing a connection with the surface coverage by cesium atoms physically adsorbed on the glass surface. This results from the observed dependences of the cell conductance versus Cs vapor density in the mtorr range and versus the wall temperature up to 200 °C, which indicates an activation energy of 0.66±0.05 eV. In the sapphire cell, where there is no hint of surface effects, the conductivity looks instead attributable to collisional processes occuring inside the vapor for Cs number densities ≳1014 at/ cm3.

Temperature dependence of Ti 3+ doped sapphirewhispering gallery mode resonator

A new method of compensating the frequency-temperature dependence of high-Q sapphire dielectric resonators near liquid nitrogen temperature is presented. This is achieved by doping monocrystalline sapphire with Ti/sup 3+/ ions. This technique offers significant advantages over other methods.

Measurements of Radiation Pressure Effect in Cryogenic Sapphire Dielectric Resonators

We report observations of the radiation pressure induced expansion of a solid dielectric resonator. This effect causes a fundamental limit to the frequency stability of the resonator. The measurements were made on four high $Q$-factor quasi-TE ``whispering gallery'' modes from 9.9 to 12.6 GHz in a monocrystalline sapphire resonator at liquid helium temperatures. The fractional frequency shift is $\ensuremath{-}1.0\ifmmode\pm\else\textpm\fi{}0.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}$ per \ensuremath{\mu}J of energy stored in the resonator. This result is consistent in sign, magnitude, and linearity with the radiation pressure induced lattice expansion term predicted by Braginsky.