Single Crystal Sapphire Research Articles

Doping and compensation in Al-rich AlGaN grown on single crystal AlN and sapphire by MOCVD

In order to understand the influence of dislocations on doping and compensation in Al-rich AlGaN, thin films were grown by metal organic chemical vapor deposition (MOCVD) on different templates on sapphire and low dislocation density single crystalline AlN. AlGaN grown on AlN exhibited the highest conductivity, carrier concentration, and mobility for any doping concentration due to low threading dislocation related compensation and reduced self-compensation. The onset of self-compensation, i.e., the “knee behavior” in conductivity, was found to depend only on the chemical potential of silicon, strongly indicating the cation vacancy complex with Si as the source of self-compensation. However, the magnitude of self-compensation was found to increase with an increase in dislocation density, and consequently, AlGaN grown on AlN substrates demonstrated higher conductivity over the entire doping range.

Bondless Abrasive Pellet の開発及びサファイアウエハの仕上げ加工への応用

Single crystal sapphire has excellent mechanical, optical and chemical properties which widen its applications but also make it a difficult-to-cut material. Currently, CMP (Chemical-Mechanical-Polishing) is used for sapphire substrate in its final finishing, but counts problems due to the nature of loose abrasive machining. CMG (Chemical-MechanicalGrinding) is a fixed abrasive process by integrating chemical reaction and mechanical grinding into one-stop process and shows advantages against CMP in finishing efficiency, geometric controllability, and waste disposal. So far, we have been succeeded in developing BAP (Bondless Abrasive Pellet) made of Cr2O3 for CMG of sapphire but due to lack of strength, it had a problem the amount of wear is higher than including a bond material. In order to enhance the wear resistance of BAP, in this report, sintering is added into the procedure of BAP manufacturing. Pellet wear, material removal rate and finished surface roughness are investigated when varying the sintering temperature.

Numerical study on the effect of additional resistive heating and crystal rotation on sapphire single crystals (Al2O3) grown by the Kyropoulos method

Reduction of melt–crystal interface convexity during Kyropoulos growth of sapphire single crystals through rotation and crucible bottom heating.

Numerical investigation of thermal and residual stress of sapphire during c-axis vertical Bridgman growth process considering the solidification history effect

Sapphire single crystals have been highlighted for epitaxial of gallium nitride films in high-power laser and light emitting diode industries. In this study, the evolution of thermally induced stress in sapphire during the vertical Bridgman crystal growth process was investigated using a finite element model that simplified the real Bridgman process. A vertical Bridgman process of cylindrical sapphire crystal with a diameter of 50 mm was considered for the model. The solidification history effect during the growth was modeled by the quite element technique. The effects of temperature gradient, seeding interface shape and seeding position on the thermal stress during the process were discussed based on the finite element analysis results.

Начальные стадии роста пленок цирконата-титаната бария и станната-титаната бария на монокристаллических подложках сапфира и карбида кремния

AbstractThe initial stages of growth of barium zirconate titanate and barium stannate titanate ferroelectric films on single-crystal sapphire and silicon carbide are studied for the first time. The choice of substrates is dictated by the possibility of using such structures in ultra-high frequency devices. The growth of discontinuous BaZr_ x Ti_1– x O_3 films is found to be mediated by the gas phase mass transport mechanism in the studied temperature range. For deposition of BaSn_ x Ti_1– x O_3 films, the mechanism of mass transport switches at ~800°C from surface diffusion to gas phase diffusion; also, the films deposited on sapphire and silicon carbide have considerably different elemental composition. The formation of an intermediate SiO_2 layer is noted on silicon carbide during the growth of oxide films on this substrate, its thickness depending on the deposition temperature.

Study of stress intensity factor on the anisotropic machining behavior of single crystal sapphire

The ductile–brittle transition was investigated based on material crystallography and stress intensity factor on the four distinctive planes of sapphire. Cutting forces were analyzed to explain the anisotropic transition characteristics. The projected stress on the objective plane at the transition point was estimated from the critical stress intensity factor of the corresponding crystal plane. Per cutting direction, the effect of crystal planes on the machinability was discussed, and the estimated stress was compared with measured values. Experimental results showed that the anisotropic machinability can be predicted in terms of crystal orientation and the critical stress intensity factor.

Новые оксидные покрытия на монокристаллическом сапфире и спеченной керамике

AbstractThe formation of the protective and hardening oxide coatings on the surface of the single-crystal sapphire and sintered corundum (ceramics) is studied. Corundum systems with oxide coatings are as yet unknown and our studies are original. The compositions of the coating based on the mixture of Al_2O_3 and B_2O_3 oxides and a technique of their deposition on the samples are worked out. The strength of the samples during the bending test is studied. It is shown that the hardening of several samples achieves ~30% and microhardness (HV) reaches 30 GPa. Basing on the results of an investigation of the significant number of the samples obtained under different conditions, it is concluded that the developed method of the protection and hardening of the corundum products is promising.

Effects of Crystal Orientation on the Crack Propagation of Sapphire by Sequential Indentation Testing

Single-crystal sapphire (α-Al2O3) is a hard and brittle material. Due to its highly crystalline nature, the fracture behavior of sapphire is strongly related to its crystal structure, and understanding the effects of crystal structure on the crack propagation of sapphire is essential for the successful application of this important material (e.g., as wafers in the electronics industry). In the present work, crack propagation that is induced by sequential indentation was investigated on the A-plane and C-plane of sapphire using a Vickers indenter on a micrometer scale. It was found that increasing indentation depth obviously increases the rate of crack propagation on the A-plane, but this effect is not so obvious on the C-plane because of the different slip systems induced by indentation on the different crystal planes of sapphire. Moreover, some parallel linear traces along the A-plane, which fracture with increasing indentation depth, are observed from the residual indentation on the A-plane. The fracture toughness of both A-plane and C-plane sapphire is smaller after indentation testing than that obtained using conventional testing methods. The subsurface damage was detected by transmission electron microscopy (TEM).

A sapphire single-crystal cell for in situ neutron powder diffraction of solid-gas reactions

Solid-gas reactions play an important role in many technologically important processes from ore smelting to hydrogen storage and the synthesis of functional materials. In situ investigations are very useful for unraveling basic steps of such reactions, rationalizing them and gaining control. For investigating time-resolved solid-gas reactions, we have constructed a gas pressure cell for elastic neutron diffraction. By proper orientation of a single-crystal sapphire tube as sample holder, Bragg peaks from the container material can be completely avoided, thus yielding high-quality powder diffraction data with very clean diffraction background. This enables the extraction of high precision crystal structure data as a function of gas pressure and temperature (laser heating) in time-resolved studies. The potential of the gas pressure cell is demonstrated by in situ studies of the reaction of solids with hydrogen, which yielded detailed models of the reaction pathways including high quality crystal structures of reaction intermediates and products. These were used to predict successfully the existence of further metal hydrides, to explain unusual bonding properties, and to optimize the synthesis of metastable compounds.

Raman scattering in single-crystal sapphire at elevated temperatures.

Sapphire is a widely used high-temperature material, and this work presents thorough characterization of all the measurable Raman scattering modes in sapphire and their temperature dependencies. Here, Raman scattering in bulk sapphire rods is measured from room temperature to 1081°C and is illustrated as a method of noncontact temperature measurement. A single-line argon ion laser at 488nm was used to excite the sapphire rods inside a cylindrical furnace. All the anti-Stokes peaks (or lines) were observable through the entire temperature range of interest, while Stokes peaks were observable until they were obscured by background thermal emission. Temperature measurements were found to be most reliable for A1g and Eg modes using the peaks at ±418, ±379, +578, +645, and +750 cm-1 (+ and - are designated for Stokes and anti-Stokes peaks, respectively). The 418 cm-1 peak was found to be the most intense peak. The temperature dependence of peak position, peak width, and peak area of the ±418 and ±379 peaks is presented. For +578, +645, and +750, the temperature dependence of peak position is presented. The peaks' spectral positions provide the most precise temperature information within the experimental temperature range. The resultant temperature calibration curves are given, which indicate that sapphire can be used in high-temperature Raman thermometry with an accuracy of about 1.38°C average standard deviation over the entire >1000°C temperature range.

Point-by-point fabrication and characterization of sapphire fiber Bragg gratings.

The Letter reports the inscription of fiber Bragg gratings (FBGs) in a single-crystal sapphire optical fiber via a point-by-point method by 780nm infrared-femtosecond laser pulses. Compared to phase mask exposure, the use of the point-by-point method for the inscription provides a flexible way to fabricate sapphire FBGs and to make wavelength division multiplexing in sapphire fiber more practicable. The multiplexing of three cascade gratings is demonstrated, and their performance up to 1400°C is tested. The permanent enhancement of reflectivity by a factor of about 5 after heat treatment and the nearly linear temperature response with a slope of 25.8pm/°C are demonstrated.

Laser-Induced-Plasma-Assisted Ablation and Metallization on C-Plane Single Crystal Sapphire (c-Al₂O₃).

Laser-induced-plasma-assisted ablation (LIPAA) is a promising micro-machining method that can fabricate microstructure on hard and transparent double-polished single crystal sapphire (SCS). While ablating, a nanosecond pulse 1064 nm wavelength laser beam travels through the SCS substrate and bombards the copper target lined up behind the substrate, which excites the ablating plasma. When laser fluence rises and is above the machining threshold of copper but below that of SCS, the kinetic energy of the copper plasma generated from the bombardment is mainly determined by the laser fluence, the repetition rate, and the substrate-to-target distance. With a lower repetition rate, SCS becomes metallized and gains conductivity. When micro-machining SCS with a pulsed laser are controlled by properly controlling laser machining parameters, such as laser fluence, repetition rate, and substrate-to-target distance, LIPAA can ablate certain line widths and depths of the microstructure as well as the resistance of SCS. On the contrary, conductivity resistance of metalized sapphire depends on laser parameters and distance in addition to lower repetition rate.

The effect of ventricular assist device-associated biomaterials on human blood leukocytes.

Ventricular assist devices (VADs) are an effective bridging or destination therapy for patients with advanced stage heart failure. These devices remain susceptible to adverse events including infection, bleeding, and thrombus; events linked to the foreign body response. Therefore, the biocompatibility of all biomaterials used is crucial to the success of medical devices. Biomaterials common in VADs-DLC: diamond-like carbon coated stainless steel; Sap: single-crystal sapphire; SiN: silicon nitride; Ti: titanium alloy; and ZTA: zirconia-toughened alumina-were tested for their biocompatibility through incubation with whole human blood for 2 h with mild agitation. Blood was then removed and used for: complete cell counts; leukocyte activation and death, and the production of key inflammatory cytokines. All were compared to time 0 and an un-exposed 2 h sample. Monocyte numbers were lower after exposure to DLC, SiN, and ZTA and monocytes showed evidence of activation with DLC, Sap, and SiN. Neutrophils and lymphocytes were unaffected. This approach allows comprehensive analysis of the potential blood damaging effects of biomaterials. Monocyte activation by DLC, Sap, ZTA, and SiN warrants further investigation linking effects on this cell type to unfavorable inflammatory/thrombogenic responses to VADs and other blood handling devices. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 1730-1738, 2018.

The physical and mechanical properties and local deformation micromechanisms in materials with different dependence of hardness on the depth of print

The size hardness effects are studied via the micro- and nanoindentation methods over the wide range of the depth of print h (from dozens of nanometers to several dozen micrometers) for several classes of materials, such as ionic and covalent single crystals (sapphire, silicon, lithium fluoride); metals (single-crystal Al, polycrystalline Cu, Ni, and Nb); ceramics (high-strength nanostructured TZP-ceramic based on the natural zirconium dioxide–baddeleyite mineral); amorphous materials (fused quartz); and polymers (polycarbonate and polytetrafluoroethylene). As is shown, some of them possess severe size hardness effects, whereas the others reveal the weak ones or even a lack of these effects. The thermoactivation analysis is implemented, as well, and the activating and energy characteristics of local deformation processes induced by an indenter are compared with the dominant plasticity micromechanisms of the studied materials at different stages of the print formation and with the size peculiarities. The materials with low hardness coefficients and meeting the requirements of ISО 14577 and GOST R 8.748-2011 standards in the nanohardness measurements are highlighted, as well. In the established load ranges, these materials are the promising candidates for their use as reference samples, which are designed to ensure the uniformity of the hardness measurements at the nano- and microscales, as well as for calibrating and testing the nanoindentometers.

The influence of thermal processing on microstructure of sol–gel-derived SrSnO3 thin films

Polycrystalline SrSnO3 thin films were fabricated using the aqueous sol–gel process and deposited on the single crystal sapphire (R-Al2O3 and C-Al2O3) and Si substrates by spin coating technique. Two different processing of thermal treatment was explored to produce SrSnO3 thin films of different porosity. The XRD analysis showed that polycrystalline films with preferential growth of SrSnO3 (200) plane were obtained on C-Al2O3 substrates, while films deposited on R-Al2O3 demonstrated a random polycrystalline growth. FE-SEM analysis revealed that the higher porosity of SrSnO3 films can be achieved by introducing additional thermal treatment step during the deposition procedure. The UV–Vis reflectance spectroscopy was used to study the effect of porosity on optical properties of the films.

Effect of interface on mid-infrared photothermal response of MoS2 thin film grown by pulsed laser deposition

This study reports on the mid-infrared (mid-IR) photothermal response of multilayer MoS2 thin films grown on crystalline (p-type silicon and c-axis-oriented single crystal sapphire) and amorphous (Si/SiO2 and Si/SiN) substrates by pulsed laser deposition (PLD). The photothermal response of the MoS2 films is measured as the changes in the resistance of the MoS2 films when irradiated with a mid-IR (7 to 8.2 μm) source. We show that enhancing the temperature coefficient of resistance (TCR) of the MoS2 thin films is possible by controlling the film-substrate interface through a proper choice of substrate and growth conditions. The thin films grown by PLD are characterized using X-ray diffraction, Raman, atomic force microscopy, X-ray photoelectron microscopy, and transmission electron microscopy. The high-resolution transmission electron microscopy (HRTEM) images show that the MoS2 films grow on sapphire substrates in a layer-by-layer manner with misfit dislocations. The layer growth morphology is disrupted when the films are grown on substrates with a diamond cubic structure (e.g., silicon) because of twin growth formation. The growth morphology on amorphous substrates, such as Si/SiO2 or Si/SiN, is very different. The PLD-grown MoS2 films on silicon show higher TCR (−2.9% K−1 at 296 K), higher mid-IR sensitivity (ΔR/R = 5.2%), and higher responsivity (8.7 V·W–1) compared to both the PLD-grown films on other substrates and the mechanically exfoliated MoS2 flakes transferred to different substrates.

Crystal Orientation and Electrical Properties of Al and Ga Co-Doped Zinc Oxide Transparent Conducting Films Deposited on Sapphire Surface

Transparent conducting films of zinc oxide co-doped with aluminum and gallium (AGZO) were deposited on sapphire (single crystal of aluminum oxide) substrates by spray chemical vapor deposition. The AGZO films were epitaxial or with strongly preferred-orientation. The highest mobility was 48 cm2 V-1 s-1 when deposited on r-plane. Deposition on other surfaces doubled the mobility to approximately 30 cm2 V-1 s-1 than that on glass substrate. Resistivity of all films deposited on sapphire substrate were lower than 1.0×10-3 ·cm and lower than that (2.4×10-3 ·cm) deposited on glass substrate. The lowest resistivity was 5.8×10-4 ·cm when deposited on c-plane.

Single Mode Air-Clad Single Crystal Sapphire Optical Fiber

The observation of single mode propagation in an air-clad single crystal sapphire optical fiber at wavelengths at and above 783 nm is presented for the first time. A high-temperature wet acid etching method was used to reduce the diameter of a 10 cm length of commercially-sourced sapphire fiber from 125 micrometers to 6.5 micrometers, and far-field imaging provided modal information at intervals as the fiber diameter decreased. Modal volume was shown to decrease with decreasing diameter, and single mode behavior was observed at the minimum diameter achieved. While weakly-guiding approximations are generally inaccurate for low modal volume optical fiber with high core-cladding refractive index disparity, consistency between these approximations and experimental results was observed when the effective numerical aperture was measured and substituted for the theoretical numerical aperture in weakly-guiding approximation calculations. With the demonstration of very low modal volume in sapphire at fiber diameters much larger than anticipated by legacy calculations, the resolution of sapphire fiber distributed sensors may be increased and other sensing schemes requiring very low modal volume, such as fiber Bragg gratings, may be realized in extreme environment applications.

Growing of bulk sapphire single crystals using laser material deposition

A device to grow oxide single crystals via powder based laser additive manufacturing has been built and successfully tested. Single crystals of sapphire as demonstrator materials have been grown to prove the ability of this technique. A sapphire seed disk with a diameter of 25.4 mm is heated in a high temperature furnace up to 1550 °C. Through a hole of the furnace-lid, the seed disk is irradiated by CO2 laser radiation. Simultaneously, Al2O3 powder is fed into the furnace via ceramic tubes. The molten material solidifies on the seed material with exactly the same crystallographic orientation, thus retaining the single crystal structure. A crack free single crystal of a few millimeters in height is produced.

Cathodo-luminescence of color centers induced in sapphire and yttria-stabilized zirconia by high-energy electrons

We have studied the color-center production in sapphire and yttria-stabilized zirconia (YSZ) single crystals by cathodo-luminescence (CL) spectroscopy for electron energies ranging between 400 and 1250 keV in a high-voltage electron microscope. Emission spectra were collected for in-beam conditions near room temperature and at 200 K. Comparison was made with CL spectra recorded for 3-keV–20-keV electrons in a scanning electron microscope. For high-energy electrons, CL spectra for sapphire revealed broad emission bands centered at photon energies about 3.0 eV and 3.8 eV that were, respectively, assigned to oxygen vacancies (F0 and F+ centers) induced by elastic collisions, on the basis of photoluminescence (PL) data. No such bands were recorded for 3-keV and 5-keV electrons. Two similar bands were also recorded for YSZ (with 9.5 and 18 mol. % yttria) at about 2.8 eV and 4.1 eV that can be, respectively, attributed to the native oxygen vacancies (F2+ centers) and F+ centers. The 4.1-eV band was not seen for 20-keV electrons: it was only produced for high electron energies by elastic collision processes. Instead, the small side band was also found at 2.85 eV for 20-keV electrons. PL excitation contour plots of virgin and irradiated YSZ were also recorded to support our discussion on point-defect identification. CL band intensities show a maximum versus electron energy, whereas point-defect concentrations should increase due to the increase of oxygen atom displacement cross section. The effect of electron energy on the different steps of the CL process is discussed to account for such a behavior.