Sapphire Samples Research Articles

AFM adhesion force measurements on conversion-coated EN AW-6082-T6 aluminium

Atomic force microscopy (AFM) has been used to measure the adhesion force between functionalised AFM tips and smooth surfaces of an EN AW-6082-T6 aluminium alloy, both before and after application of different conversion coatings. In addition, the surface of a sapphire sample was studied as a model aluminium surface. The results obtained for the sapphire surface were highly reproducible, and were used as a mean to establish proper routines for the more complex industrial surfaces. The adhesion force between a chromate conversion-coated (CCC) EN AW-6082-T6 aluminium alloy and a COOH functionalised tip was significantly increased compared to the uncoated surface, probably as a result of strong hydrogen bonding. However, the adhesion force decreased with time during the first 24h after treatment due to aging of the CCC. Chromate-free Ti–Zr-based treatment also increased the adhesion, but the adhesion force varied significantly due to non-uniform deposition and composition of the conversion coating. The measured AFM adhesion forces correlated qualitatively with macroscopic adhesion test results obtained previously for these specific conversion coatings. The AFM technique may thus provide useful information on the adhesion behaviour of heterogeneous conversion-coated aluminium surfaces.

On the defect pattern evolution in sapphire irradiated by swift ions in a broad fluence range

Sapphire samples, irradiated with swift Kr (245 MeV) ions at room temperature in a broad fluence range, were investigated using a continuous and a pulsed positron beam to study the defect structure created by the passage of the ions in depths of a few micrometers. At small doses, monovacancies were identified as dominant defects and positron trapping centres. These monovacancies are assumed to be highly concentrated inside a cylindrical volume around the ion path with an estimated radius of ∼1.5 nm. For higher doses a second type of trapping centre emerges. This second class of structural imperfection was associated with the overlap of the individual ion tracks leading to the formation of larger vacancy clusters or voids.

Evolution of residual stress and crack morphologies during 3D FIB tomographic analysis of alumina

Three-dimensional focused ion beam (FIB) tomography is increasingly being used for 3D characterization of microstructures in the 50 nm-20 microm range. FIB tomography is a destructive, invasive process, and microstructural changes may potentially occur during the analysis process. Here residual stress and crack morphologies in single-crystal sapphire samples have been concurrently analyzed using Cr3+ fluorescence spectroscopy and FIB tomography. Specifically, maps of surface residual stress have been obtained from optically polished single-crystal alumina [surface orientation (1 ī 0 2)], from FIB milled surface trenches, from Vickers micro-indentation sites (loads 50 g-300 g), and from Vickers micro-indentation sites during FIB serial sectioning. The residual stress maps clearly show that FIB sputtering generates residual stress changes. For the case of the Vickers micro-indentations, FIB sputtering causes significant changes in residual stress during the FIB tomographic serial sectioning. 3D reconstruction of the crack distribution around micro-indentation sites shows that the cracks observed are influenced by the location of the FIB milled surface trenches due to localized stress changes.

Micro‐Raman spectroscopy applied to the study of inclusions within sapphire

AbstractThe present research concerns the characterization of samples of corundum (Al2O3), varieties of sapphire, by micro‐Raman spectroscopy. Ten samples of natural corundum from Australia, Madagascar, Sri Lanka, Thailand, Tanzania, and Vietnam were studied. After standard gemmological analysis, micro‐Raman analysis was also performed in order to identify mineral inclusions within the sapphire samples and to obtain information on the geological genres of the minerals. Several inclusions were evidenced within the sapphires and identified by Raman spectroscopy thanks to a confocal spectrometer, which allowed detection also below the external surface of the gem. Finally, the Raman spectra are discussed taking into account the provenance of the samples and their optical absorption. Copyright © 2008 John Wiley & Sons, Ltd.

Optical spectroscopy study of type 1 natural and synthetic sapphires

Low iron, pale blue natural and synthetic sapphire samples were studied bylow temperature absorption and luminescence spectroscopy. For comparison,a bicolour pink and blue corundum from Vietnam was also considered.From radioluminescence and photoluminescence excitation spectra of bothCr3+ and Ti3+, experimental evidence was obtained for attributing an absorption band at17 500 cm−1, currently interpreted as intervalence charge transfer, to overlapping crystal field transitions ofCr3+ andTi3+. An important rolewas also proposed for Cr2+; indeed, it is possible to propose that the colour of pale blue sapphiresis mainly determined by chromium in its two valence states whileTi3+ and Fe3+ have a minor role.

InGaN/GaN multi-quantum well and LED growth on wafer-bonded sapphire-on-polycrystalline AlN substrates by metalorganic chemical vapor deposition

We report growth of InGaN/GaN multi-quantum well (MQW) and LED structures on a novel composite substrate designed to eliminate the coefficient of thermal expansion (CTE) mismatch problems which impact GaN growth on bulk sapphire. To form the composite substrate, a thin sapphire layer is wafer-bonded to a polycrystalline aluminum nitride (P-AlN) support substrate. The sapphire layer provides the epitaxial template for the growth; however, the thermo-mechanical properties of the composite substrate are determined by the P-AlN. Using these substrates, thermal stresses associated with temperature changes during growth should be reduced an order of magnitude compared to films grown on bulk sapphire, based on published CTE data. In order to test the suitability of the substrates for GaN LED growth, test structures were grown by metalorganic chemical vapor deposition (MOCVD) using standard process conditions for GaN growth on sapphire. Bulk sapphire substrates were included as control samples in all growth runs. In situ reflectance monitoring was used to compare the growth dynamics for the different substrates. The material quality of the films as judged by X-ray diffraction (XRD), photoluminescence and transmission electron microscopy (TEM) was similar for the composite substrate and the sapphire control samples. Electroluminescence was obtained from the LED structure grown on a P-AlN composite substrate, with a similar peak wavelength and peak width to the control samples. XRD and Raman spectroscopy results confirm that the residual strain in GaN films grown on the composite substrates is dramatically reduced compared to growth on bulk sapphire substrates.

Improving the Properties of Sapphire by Coatings

SiO2 coatings and double layer films of SiO2/Si3N4 have been designed and prepared on sapphire (α-Al2O3) by radio frequency magnetron reactive sputtering in order to improve the optical and mechanical properties of infrared windows of sapphire. The transmission of the coated and uncoated sapphire in the wavelength range from 2.5 to 6.7um was determined. Surface morphology and roughness of coated and uncoated sapphire have been measured using a talysurf. The flexure strength of coated and uncoated sapphire samples has been studied by 3-point bending tests at room temperature and high temperatures. The results show that the coatings can improve the surface morphology and reduce the surface roughness of sapphire substrate. What’s more, the designed single layer of SiO2 film and two-layer of SiO2/Si3N4 films can both increase the transmission of sapphire in mid-wave IR and strengthen sapphire at high temperatures. Coated sapphire transmits over 95% at wavelength from 3 to 5um for the two designs. Flexure tests reveal that SiO2 coatings and SiO2/Si3N4 films increase the strength of c-axis sapphire by a factor of about 1.5 and 1.4, respectively, at 800°C.

Growth and fluorescence measurements of neodymium: Yttrium Aluminum Garnet, neodymium fluoride, Ti:Sapphire, ytterbium fluoride and mixture of neodymium fluoride with ytterbium fluoride thin films

The main objective of the present work was to fabricate and find evidence of fluorescence in the optical films of some well established laser materials and as well as some other new materials have also been probed. Materials such as Nd:Yttrium Aluminum Garnet (Nd:YAG), Ti:Sapphire, NdF 3 and YbF 3 were deposited on BK7 type optical glass, fused silica (Suprasil), Sapphire and YAG substrates by conventional electron beam evaporation processes. In case of Nd:YAG, the films were prepared by using pure single crystals of Nd:YAG as evaporation material. Samples with higher Nd content were also prepared in order to enhance the absorption in the film at the pump wavelength. A theoretical analysis for the lasing action in the Nd:YAG film has also been made and parameters like gain, threshold population inversion, threshold power etc. have been calculated for the realisation of laser action in thin films. In case of Ti:Sapphire, the films were prepared by an Ion Beam Sputtering process. It has been demonstrated that fluorescence can be observed in Electron-beam deposited films of Nd:YAG and NdF 3 when pumped with a diode laser at 802 nm. The maximum peak emission was observed at 1066 nm in case of Nd:YAG and 1046 nm in case of NdF 3. A Ti:Sapphire sample has also been excited using an Nd:YAG laser at 532 nm. Here a broad band fluorescence spectrum was observed. Films of YbF 3 and YbF 3 + NdF 3 (co-deposition) prepared by Electron Beam Evaporation were also studied.

SELF-INDUCED SURFACE TEXTURING OF Al2O3 BY MEANS OF INDUCTIVELY COUPLED PLASMA REACTIVE ION ETCHING IN CL2 CHEMISTRY

In this work we investigate a pseudo-random surface texturing technique of sapphire by means of inductively coupled plasma reacting ion etching in chlorine chemistry, for which no sophisticated lithographic process is required. Such a surface texturing technique, which we believe offers indicative promise for enhanced light extraction in deep ultraviolet light-emitting diodes has allowed us to texture sapphire samples having a surface larger than 1 cm2 with controlled structures. Fabrication parameters have been characterized, and textured Al 2 O 3 surfaces having submicron features, and nano-scale periodicity have been obtained. Performance, and characterization of our textured Al 2 O 3 surfaces is the hinge of addition work in progress.

Room temperature annealing of low-temperature ion implanted sapphire

Sapphire samples of different orientation were irradiated with different ion species at 15K (LT) and subsequently left to warm up to room temperature (RT) at which the defect analysis by Rutherford backscattering spectrometry was repeated several times over the course of more than one year. For samples where the aligned yield had reached the random level up to the surface, no change of the defect concentration is observed even after storage for several months. For buried damaged layers with a maximum defect concentration of ndef=0.5…0.9 after LT measurement the warming-up to RT already leads to a reduction of the defect concentration followed by an even stronger annealing when keeping the samples at RT and air for some weeks. Depth profiles and time dependence of the defect concentration are discussed for the various samples investigated.

Strengthening sapphire at elevated temperatures by SiO 2 films

SiO 2 films have been prepared on sapphire by radio frequency magnetron reactive sputtering in order to increase the optical and mechanical properties of infrared windows and domes of sapphire at elevated temperatures. Infrared transmission and flexural strength of uncoated and coated sapphires have been investigated at different temperatures. SiO 2 films were shown to have apparent antireflective effect on sapphire substrate at room temperature. With increasing temperature, the coated sapphires have larger average transmission than the uncoated ones. The temperature was proven to only weakly affect the absorption coefficient and antireflection capability of the deposited films. It is also indicated that the flexural strengths of the c-axis sapphire samples coated with SiO 2 films are increased by 1.2 and 1.5 times than those of uncoated at 600 and 800 °C, respectively.

Effects of annealing on the photoluminescence of He ion implanted sapphire after 230 MeV Pb ion irradiation

Single crystal sapphire (Al2O3) samples implanted with 110 keV He and irradiated at 320 K by 208Pb27+ ions with energy of 1.1 MeV/u to the fluences ranging from 1×1012 to 5×1014 ion/cm2 and subsequently annealed at 600, 900 and 1100 K. The obtained PL spectra showed that emission peaks centred at 375, 390, 413, and 450 nm appeared in irradiated samples. The peak of 390 nm became very intense after 600 K annealing. The peak of 390 nm weakened and 510 nm peak started to build up at 900 K annealing, the peak of 390 nm vanished and 510 nm peak increased with the annealing temperature rising to 1100 K. Infrared spectra showed a broadening of the absorption band between 460 cm-1, and 510 cm-1 indicating strongly damaged regions being formed in the Al2O3 samples and position shift of the absorption band at 1000—1300 cm-1 towards higher wavenumber after Pb irradiation.

Micrographs of the micro-particle in sapphire by ion implantation

Sapphire crystals with (0001), (101¯0), (011¯2) and (112¯0) orientations were implanted by Cu +, Nb + or CuCl + ions of 300–380 keV, (5–50) × 10 16 ions/cm 2 at room temperature or 100 K. These samples were then annealed at the temperature of 500 to 1100 °C in reducing atmosphere. In this paper, the morphologies are presented for the surfaces and for the precipitates within the near-surfaces of the ion implanted sapphire samples after annealing. The AFM shows that the island like distribution morphologies with jagged and craggy characteristics can be seen for the ion implanted sapphire before annealing, while the morphologies become smooth with the boundaries, and the number of the islands decreased after annealing. Micro-particles of precipitates can be found for all the sapphire samples implanted with different ion species after annealing. Different sizes of precipitates can be found in the same sample. The growth process of these precipitates can be estimated based on the particles observation. The factors which affect shape, size as well as distribution of the precipitates are discussed in this paper.

Size-Related Plasticity Effects in AFM Silicon Cantilever Tips

ABSTRACTWe are developing dynamic atomic force microscopy (AFM) techniques to determine nanoscale elastic properties. Atomic force acoustic microscopy (AFAM) makes use of the resonant frequencies of an AFM cantilever while its tip contacts the sample surface at a given static load. Our methods involve nanosized silicon probes with tip radius R ranging from approximately 10 nm to 150 nm. The resulting radius of contact between the tip and the sample is less than 20 nm. However, the contact stress can be greater than a few tens of GPa, exceeding the theoretical yield strength of silicon by a factor of two to four. Our AFAM experiments indicate that, contrary to expectation, tips can sometimes withstand such stresses without fracture. We subjected ten tips to the same sequence of AFAM experiments. Each tip was brought into contact with a fused quartz sample at different static loads. The load was systematically increased from about 0.4 μN to 6 μN. Changes in tip geometry were observed in images acquired in a scanning electron microscope (SEM) between the individual AFAM experiments. All of the tips with R < 10 nm broke during the first AFAM experiments at static loads less than 1.6 μN. Tips with R > 40 nm plastically deformed under such loads. However, a group of tips with R from 25 nm to 30 nm neither broke nor deformed during the tests. In order to reach higher contact stresses, two additional tips with similar values of R were used in identical experiments on nickel and sapphire samples. Although the estimated stresses exceeded 40 GPa, we did not observe any tip fracture events. Our qualitative observations agree with more systematic studies performed by other groups on various nanostructures. The results emphasize the necessity of understanding the mechanics of nanometer-scaled bodies and the impact of size effects on measurements of mechanical properties on such scales.

APPLICATION OF SAPPHIRE BONDING FOR INTERFEROMETRIC GRAVITATIONAL WAVE DETECTOR WITH CRYOGENIC MIRRORS

Thermal conductance and light scattering were measured for two bonded sapphire samples. A sample with adhesion free bonding (AFB) showed the same thermal conductivity as that of the bulk at cryogenic temperatures. In contrast, a finite thermal resistance was observed in the sample with hydroxide-catalysis bonding (HCB). Light scattering at λ = 488 nm from bonded region was about 10% larger than the bulk part and was about 800 ppm of an incident laser power for AFB sample.

Optical properties tailoring by high fluence implantation of Ag ions on sapphire

Optical and structural properties of single crystalline α-Al2O3 were changed by the implantation of high fluences of Ag ions. Colourless transparent (101¯0) sapphire samples were implanted at room temperature with 160keV silver ions and fluences up to 1×1017Agcm−2. Surface amorphization is observed at the fluence of 6×1016Agcm−2. Except for the lower fluences (below 6×1016Agcm−2) the optical absorption spectra reveal the presence of a band peaking in the region 450–500nm, depending on the retained fluence. This band has been attributed to the presence of silver colloids, being thus 1×1016Agcm−2 below the threshold for colloid formation during the implantation. Annealing in oxidizing atmosphere promotes the recrystallization along with segregation of Ag followed by loss through evaporation. Recrystallization is retarded for annealing in reducing atmosphere and the Ag profile displays now a double peak structure after evaporation. Playing with the implantation fluence, temperature and annealing atmosphere controllable shifts of the position and intensity of the optical bands in the visible were achieved.

Investigation of SiO2/Si3N4 films prepared on sapphire by r.f. magnetron reactive sputtering

Sapphire is a desired material for infrared-transmitting windows and domes because of its excellent optical and mechanical properties. However, its thermal shock resistance is limited by loss of compressive strength along the c -axis of the crystal with increasing temperature. In this paper, double layer films of SiO 2 /Si 3 N 4 were prepared on sapphire (α-Al 2 O 3 ) by radio frequency magnetron reactive sputtering in order to increase both transmission and high temperature mechanical performance of infrared windows of sapphire. Composition and structure of each layer of the films were analyzed by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), respectively. Surface morphology and roughness of coated and uncoated sapphire have been measured using a talysurf. Flexural strengths of sapphire sample uncoated and coated with SiO 2 /Si 3 N 4 have been studied by 3-point bending tests at different temperatures. The results show that SiO 2 /Si 3 N 4 films can improve the surface morphology and reduce the surface roughness of sapphire substrate. In addition, the designed SiO 2 /Si 3 N 4 films can increase the transmission of sapphire in mid-wave infrared and strengthen sapphire at high temperatures. Results for 3-point bending tests indicated that the SiO 2 /Si 3 N 4 films increased the flexural strength of c -axis sapphire by a factor of about 1.4 at 800 °C.

Surface microroughness of ion-beam etched optical surfaces

Ion-beam etching (IBE) and ion-beam figuring techniques using low-energy ion-beam sources have been applied for more than ten years in the fabrication and finishing of extremely smooth high-performance optics. We used optical interferometric techniques and atomic force microscopy to study the evolution of the surface root-mean-square (rms) microroughness, Rq, as a function of depth of a material removed (0–3000nm) by a broad ion-beam source (Ar+ ions of energy 600eV and ion current density of 1mAcm−2). Highly polished samples of fused silica and Zerodur (Rq∼3.5Å) showed a small decrease in microroughness (to 2.5Å) after 3000-nm IBE removal while an ultrapolished single-crystal sapphire sample (Rq∼1Årms) retained its very low microroughness during IBE. Power spectral density functions over the spatial frequency interval of measurement (f=5×10−3−25μm−1) indicate that the IBE surfaces have minimal subsurface damage and low optical scatter.

PHONON-INDUCED 1/F NOISE IN MOS TRANSISTORS

Existing experimental data for the temperature dependence of 1/f noise in both n- and p-channel MOS transistors are heuristically compared with either bulk or surface phonon spectra or with both of them. It is found that the noise structure mirrors different van Hove singularities in both bulk and surface phonon spectra. This is thought to be the signature of surface and bulk phonons in the 1/f noise of MOS transistor. For a Debye phonon spectrum, an intriguing 1/τ distribution is obtained. The famous connection between oxide states and 1/f noise can be understood in terms of phonon scattering if the tunneling is inelastic. Striking similarities were found in the temperature dependence of the frequency exponent in different MOS transistors and all of them feature similarities with those of the frequency exponent in silicon on sapphire. This indicates that a common structural factor is controlling the temperature variation in both systems. Starting from the observation that the noise intensity vs. temperature is the image of the phonon density of states, the temperature dependence of the frequency exponent was calculated. It is in a reasonable agreement with the experiments for both n-channel transistors and silicon on sapphire sample.

Large-scale inhomogeneity in sapphire test masses revealed by Rayleigh scattering imaging

Rayleigh scattering in test masses can introduce noise and reduce the sensitivity of laser interferometric gravitational wave detectors. In this paper, we present laser Rayleigh scattering imaging as a technique to investigate sapphire test masses. The system provides three-dimensional Rayleigh scattering mapping of entire test masses and quantitative evaluation of the Rayleigh scattering coefficient. Rayleigh scattering mapping of two sapphire samples reveals point defects as well as inhomogeneous structures in the samples. We present results showing significant non-uniform scattering within two 4.5 kg sapphire test masses manufactured by the heat exchanger method.