Crystalline Sapphire Research Articles

Time resolved detection of particle removal from dielectrics on femtosecond laser ablation

The dynamics of the ablation process in different oxides (amorphous and crystalline quartz, sapphire and MgO) after intensive laser excitation was studied by a combination of the femtosecond pump–probe technique with optical surface scattering. A 120-fs laser pulse ( λ=800 nm) was focused onto the sample with a fluence at least twice the single-shot surface damage threshold. A much weaker probe laser pulse optically aligned collinear to the pump beam illuminated the excited area at variable delay times up to 100 ps. We observe a material dependent increase in the scattering signal after a certain delay time which we interpret as the onset of the ablation process. This occurs much earlier for amorphous and crystalline silica than for sapphire or MgO, as a consequence of differences in the electron–phonon coupling strength. The morphology of the irradiated spot on sapphire, amorphous and crystalline quartz is indicative of melting and vaporisation in contrast to that on MgO which clearly shows fracturing.

Highly Textured Films of Layered Metal Disulfide 2H‐ WS 2: Preparation and Optoelectronic Properties

Highly textured films of 2H‐ can be obtained by sulfurization of up to 4 μm thick layers in the presence of hydrogen using amorphous (quartz glass, glassy carbon) or crystalline (sapphire, muscovite, highly oriented pyrolytic graphite) substrates. Best conditions have been found employing (00.1) oriented sapphire substrates with a 5 nm thick nickel layer interposed between substrate and oxide film (0.5 to 4 μm thickness) and a reaction temperature ranging from 973 to 1173 K. Depending on time the crystallites, oriented with their hexagonal basal planes parallel to the substrate, exhibited a lateral extension of up to 20 μm and a thickness of ⩽300 nm. Conductivity measurements of the films showed a p‐type conductivity in the range from 0.1 to 3 Ω−1 cm−1 and a lateral mobility as high as 105 cm2 V−1 s−1 at room temperature. The conductivity type has been confirmed by ultraviolet photoelectron and x‐ray photoelectron spectroscopy which were compared with n‐type single crystals. A freestanding film pealed off from a Pt coated quartz substrate and mounted on a brass holder was investigated photoelectrochemically. Using a 0.2 M redox electrolyte in 0.5 M an open‐circuit voltage of ≈100 mV and a short‐circuit current of 5 mA/cm2 has been detected for the first time.

Pulsed laser deposition of Y3Al5O12:Tb photoluminescent thin films

Transparent-photoluminescent Y3Al5O12:Tb (YAG:Tb) thin films were deposited on single- crystalline sapphire at two different substrate temperatures by means of the pulsed laser deposition technique. The ablation targets were fabricated from YAG:Tb powders obtained with a new ceramic processing technique (combustion synthesis). A KrF excimer laser with an ultraviolet wavelength of 248 nm was used to evaporate the target and to grow the films. Analysis and characterization of the films were performed by scanning electron microscopy, x-ray diffraction, Auger electron spectroscopy, and photoluminescence measurements. As-deposited films were amorphous and deficient in oxygen as compared to the target. Additionally, these films were not luminescent. Postannealing treatments in air for at least 1 hr in the range 800–1200 °C were found to be necessary in order to induce crystallization and oxygenation in the films thus recovering the green photoemission of YAG:Tb with these treatments.

Epitaxial growth of LiNbO 3 thin films on (001) sapphire by Pulsed Laser Deposition

LiNbO 3 thin films have been deposited on c-cut single crystalline sapphire substrates (α-Al 2O 3) by the pulsed laser deposition technique. Crystalline quality and structural properties have been determined using Rutherford backscattering spectroscopy (RBS) and x-ray diffraction experiments. Under optimised conditions, the (001) epitaxial growth has been obtained with some grain misorientations in the plane and an average surface roughness of about 2 nm as measured by atomic force microscopy (AFM).

Structural properties of LiNbO 3 thin films grown by the pulsed laser deposition technique

LiNbO 3 thin films have been deposited onto R-cut and C-cut single crystalline sapphire substrates ( α-Al 2O 3, (1102) and (0001)) by the pulsed laser deposition technique at different oxygen pressures and substrate temperatures. Thin film composition and structure have been determined using Rutherford backscattering spectroscopy (RBS) and X-ray diffraction experiments. The atomic composition is dependent on the oxygen pressure in the range 0.5–1.3 mbar. At pressures lower than 1.3 mbar, we have observed a deviation from the stoichiometry. Nearly stoichiometric thin films have been obtained for a pressure equal to 1.3 mbar. For that pressure, the atomic composition does not depend on the substrate temperature in the range 650–800°C. Under optimised conditions the 〈1102〉 and 〈0001〉 preferential orientations of growth have been obtained on (1102) and (0001) sapphire substrates, respectively.

Preparation of lanthanum aluminate thin films by a sol-gel procedure using alkoxide precursors

The preparation by a chemical method of thin lanthanum aluminate (LaAlO 3) films on substrates of single crystalline silicon and sapphire as well as on optical silica glass substrates has been investigated. A sol-gel procedure is used during which an isopropanol sol of La-isopropoxide and Al-isopropoxide along with acetylacetone as a chelating agent is spin-coated on the substrate. Well crystallized LaAlO 3 films are formed on the substrates after heat-treatment of the amorphous gel coatings. These films, having a thickness of about 300 Å, are characterized by a very good adhesion to the substrate, the lack of cracks and a very smooth surface.

Orientation and mobility in ultrathin oligothiophene films: UV-Vis, IR and fluorescence studies

Thin films of oligothiophenes αnTs are prepared by vapour deposition on fused silica and crystalline sapphire substrates, and characterized with respect to molecular orientation by polarized fluorescence, UV-Vis and IR absorption spectroscopy. Isolated αnTs are adsorbed in the submonolayer region with their long molecular axes parallel to the surface plane. In the monolayer region, the αnTs organize with their long axes perpendicular to the surface plane forming aggregates of the pin cushion type with a mean tilt angle of θ = 15 ± 3° relative to the normal of the macroscopic surface. Terminal alkylation of αnT or multilayer coverage results in a loss of orientation with respect to the surface. Heating the substrate during film deposition either reduces or enhances the molecular order depending on temperature and molecular chain length.

Preparation of yttria-stabilized zirconia thin films by a sol-gel procedure using alkoxide precursors

The preparation by a chemical method of thin yttria-stabilized zirconia (YSZ) films on substrates of single crystalline silicon and sapphire, optical silica glass and alumina ceramics has been investigated. A sol-gel procedure is used during which an isopropanol sol of Zr-propoxide and Y-isopropoxide along with ethyl acetoacetate as a chelating agent and an equivalent amount of water is spin coated on the substrate. Well crystallized YSZ films are formed on the substrates after heat-treatment of the amorphous gel coatings. These films, having a thickness of 1200–1500 Å, are characterized by a very good adhesion to the substrate, the lack of cracks and, especially on silicon and silica glass substrates, a very smooth surface.

Preparation of crystalline SiC thin films by plasma-enhanced chemical vapour deposition and by ion beam modification of silicon

Thin crystalline films of SiC have been produced by ion beam modification of heteroepitaxial silicon thin films on sapphire and by plasma-enhanced chemical vapour deposition on crystalline silicon and sapphire substrates. Both methods yield highly transparent crystalline material. The preparation procedures and the optical properties of these materials are described and applications to optical lithography are pointed out.

Crystallization of fibres inside a matrix: a new way of fabrication of composites

Fibrous composites are normally fabricated by inserting premade fibres into a matrix and trying to tailor mechanical or physical properties of the material by a proper choice of fibre arrangement, fibre volume fraction, structure and properties of interface, etc. As a rule, this method satisfies all the needs fairly well. But in many cases, particularly when heat-resistant composites are involved, it leads to complications which cause composite experts to refrain from being involved in technically very attractive projects. So the need for alternative methods of composite fabrication obviously exists. The process described here is an example of such an alternative. It is based on the fibres growing from the melt within the volume of the matrix. The matrix should have prefabricated continuous cylindrical channels to be filled with the melt of the fibre material. The process is described using as a model a composite with a molybdenum matrix and single crystalline sapphire fibres. It is shown that the productivity of oxide fibre fabrication based on the process described can be some orders of magnitude higher than that based on the well known Czochralsky's and Stepanov's methods. The strength of the single-crystalline sapphire fibres obtained has been studied, as well as the high-temperature creep strength of composites containing such fibres. Some of the results of these experiments are reported here.

Far-infrared time-domain spectroscopy with terahertz beams of dielectrics and semiconductors

Using the method of time-domain spectroscopy, we measure the far-infrared absorption and dispersion from 0.2 to 2 THz of the crystalline dielectrics sapphire and quartz, fused silica, and the semiconductors silicon, gallium arsenide, and germanium. For sapphire and quartz, the measured absorptions are consistent with the earlier work below 0.5 THz. Above 1 THz we measure significantly more absorption for sapphire, while for quartz our values are in reasonable agreement with those of the previous work. Our results on high-purity fused silica are consistent with those on the most transparent fused silica measured to date. For the semiconductors, we show that many of the previous measurements on silicon were dominated by the effects of carriers due to impurities. For high-resistivity, 10-kΩ cm silicon, we measure a remarkable transparency together with an exceptionally nondispersive index of refraction. For GaAs our measurements extend the precision of the previous work, and we resolve two weak absorption features at 0.4 and 0.7 THz. Our measurements on germanium demonstrate the dominant role of intrinsic carriers; the measured absorption and dispersion are well fitted by the simple Drude theory.

Picosecond laser sputtering of sapphire at 266 nm

We observe that 266 nm laser pulses of 30 ps duration and fluence near 10 J/cm2 causes reproducible surface sputtering and etching of crystalline sapphire in air. The etching rate for the initial ten pulses is relatively slow, 0.04 μm/pulse, producing a smooth surface of the etched area and a broad-angle plume emission. After some 20 pulses, the etching rate is dramatically faster, 0.5 μm/pulse, producing a rough etched surface and a plume composed of broad-angle emission as well as a narrow perpendicular jet emission. Micron-sized depressions can be made on the sapphire with no visible damage or cracking to the surroundings.

Ion implantation in ceramics-residual stress and properties

The average and integrated stresses have been measured by the indentation technique for a series of sapphire crystals implanted with chromium or iron using a wide range of ion beam energy and fluences. The stress is compressive in nature and the average residual stress is in the range of 1100 MPa, i.e. about the value of the bulk rupture strength of defect-free single crystalline sapphire. Data for speciments implanted with iron indicate that a saturation value in average stress is reached at 10 16 Fe cm −2 (160 keV) but a saturation for chromium implantation is not apparent. The residual compressive stress has a marked influence on the apparent fracture toughness as measured by the indentation technique and the fracture strength measured in bend tests.

In-situ crystal growth and neutron four-circle diffractometry under high pressure

Pressure cells designed for work on four-circle diffractometers equipped with an Eulerian cradle and pressures up to 2.5 GPa are described. Single crystalline sapphire anvils allow the optical inspection of the sample crystals, e.g., during in-situ crystal growth and the pressure measurement with the ruby fluorescence technique. Experiments on normal and deuterated ice VI are reported.

Effects of tensile stress on the R lines of Cr^3+ in a sapphire fiber

We have investigated the spectroscopic properties of a crystalline sapphire fiber unintentionally doped with Cr(3+). We find that tensile stress produces blue shifts of the R lines and changes in their radiative lifetimes and integrated intensities that can be correlated to stress-induced changes of the crystal-field parameters.

Precision millimeter-wave dielectric measurements of birefringent crystalline sapphire and ceramic alumina

A quasi-optical technique, namely, dispersive Fourier transform spectrometry, has now been improved to provide high-precision continuous data of complex refractive index, complex dielectric permittivity, and loss tangent of materials at millimeter wavelengths. The use of a polarizing two-beam interferameter is ideally suited for the measurement of birefringent materials. A massive biréfringent effect (δ∊′ 2.193) is now observed for crystalline sapphire (Al <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> ). The bifringent effect for crystalline quartz is much smaller (δ∊′ ⋍ 0.2). New millimeter-wave results of commercially available ceramic alumina (Al <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> ) are compared with those of crystalline sapphire.

A Model of Lattice Defects in Sapphire

A model is proposed to account for the changes under ultraviolet illumination of several of the prominent optical absorption bands in neutron irradiated crystalline sapphire. This model, based on both new and previously observed photobleaching and predicted optical anisotropic properties of single and paired anion vacancies, assigns absorption and luminescent bands to specific charge states of anion divacancies.

Room temperature deposition of superconducting NbN for superconductor–insulator–superconductor junctions

Reactive dc magnetron sputtering is used to deposit hard, refractory, superconducting thin films of niobium nitride on amorphous (glass) as well as crystalline (sapphire) substrates held at room temperature. Stoichiometric NbN films so obtained possess the desired B1 crystal structure with a distinct (111) texture, and exhibit high (∼16 K) superconducting transition temperature. Room temperature deposition of the high Tc films is achieved primarily by optimizing the reactant fluxes of niobium and nitrogen reaching the substrate, through a systematic study of the nitrogen consumption versus injection characteristics which are found to be an absolute indicator of the quality of NbN formed. Superconductor–insulator–superconductor (SIS) junctions are fabricated by using high Tc NbN as the base electrode, a thin layer of native oxide grown in oxygen plasma or in room air as the insulator, and vacuum deposited Pb as the counterelectrode. The current versus voltage (I–V) characteristics of the junctions exhibit high (∼110) nonlinearity parameter and the superconducting gap parameter for NbN, Δ∼2.8 meV. The transition temperature, superconducting gap parameter, and nonlinearity parameter of the junctions are identical for the films deposited at room temperature with predominant (111) texture as well as the films deposited at elevated temperatures possessing (200) texture.

FRACTURE OF CERAMIC-TO-METAL INTERFACES

The methods used for studying fracture at metal-ceramic interfaces are based on fracture mechanical principles. Sandwich-like specimens consisting of two rectangular pieces of ceramic solid-state bonded by a intermediate metal layer and notched or precracked at one interface between the two materials are fractured in a threeor four-point bend test device. The interface fracture energy Gc is obtained from the measured loaddeflection curve, the fracture load, the dimensions of the specimen and by use of a correction function YG which pays regard to the elastic properties of the materials involved and the geometry of the specimen. The fracture behaviour of ceramic-to-metal joints with and without intermediate reaction layers is studied. Internal stresses due to differences in the thermal expansion of the constituents are reduced by microcrack formation. For polycrystalline material transitions a mixed failure mode is observed including both adhesive and cohesive fracture contributions. The fracture energy is influenced by impurity phases and by the microstructure of the interface region. Experiments with combinations of single crystalline niobium and sapphire indicate that the interface fracture energy depends on the crystallographic orientationship of the two crystal surfaces bonded together. It is shown that the work of adhesion is only a very small fraction of the interface fracture energy. The contributions of energy dissipation processes to the fracture energy are discussed in more detail.

The Defect Structure of Unintentionally Doped α‐Al2O3 Crystals

Electrical conductivity and the ionic and electronic transference numbers were determined for two types of unintentionally doped single crystalline sapphire for current directions perpendicular to the r plane (1102). One was acceptor dominated and the other was initially donor (Hix) dominated, changing to acceptor domination after a prolonged anneal at 1600°C. The positions of the electronic energy levels of dominant impurities and the constants regulating the oxidation‐reduction of these impurities and of pure Al2O3 are determined. The latter shows a discrepancy with an expression reported previously.