Papers by Keyword: Sapphire

Abstract: The nucleation of dislocations in sapphire of various structural perfection has been investigated by nanoindentation. The studies were carried out on crystallographic planes С(0001), a(1 1 2 0), R (0 112). In the curve of indentation of a Berkovich indenter into the single crystals, an abrupt transition from elastic to plastic deformation has been observed at a depth of about 75 nm due to the nucleation of dislocations in the initially dislocation free region under the contact. Deterioration of structural perfection results in a decrease in shear stresses under which dislocations nucleated. It is shown that the anisotropy of sapphire nanohardness is less pronounced than the static one.

Abstract: This research aims to study the properties of polishing pads made from polyurethane mixed with rice husk fiber for use in chemical mechanical polishing (CMP) of sapphire. After cleaning and sizing, the rice husk fiber was modified using hydrochloric acid (HCl). Then, both unmodified and modified rice husk fibers were mixed with polyurethane at ratios of 7.5, 10, and 12.5 phr to form polishing pads. The hardness and polishing performance of these pads in sapphire CMP were then tested. The experimental results showed that polishing pads from rice husk fiber could be successfully formed and remained stable. The natural fibers were evenly distributed across the contact surface of the pads. The hardness of polishing pads from rice husk fiber was smaller than conventional polishing pad (SUBA800) in range of 40.5–47.5%. Polishing results revealed that pads made from polyurethane mixed with unmodified and modified rice husk fiber achieved the highest material removal rates (MRR) of 94.2% and 64.7%, respectively, compared to the conventional pad. These results indicate that both types of fibers able to be used as a material for manufacturing polishing pads for adding value and reducing the waste from the agricultural.

Abstract: This paper introduced an accurate empirical model for the thermal resistance of a single-finger AlGaN-GaN high electron mobility transistor (HEMT) on three different substrates including Sapphire, SiC and Si. The model reckons the constant thermal conductivity of GaN and substrate, thickness of host substrate layers, gate length (Lg) and width (Wg). The model plausibility is verified by comparing it with DC channel temperature measurement method and charge controlled based device modeling which agrees very favorable observation of the model data. Having nimble expression for the channel temperature is of inordinate importance in the field of designers of power device and monolithic microwave integrated circuits. Proposed model gives a variety of inquiries that would be impossible or impractical to do using time-consuming numerical simulations.

Abstract: Surface structuring on the nano/micro level is important for a huge range of areas. A critical bottle neck for many industrial applications is upscaling; that is to say it must be possible to do the structuring in a comparatively simple and inexpensive manner. However, this is a challenge for many industrially important materials, including sapphire. A few years ago, solid state conversion was introduced as a relatively simply nanoscale structuring method for sapphire. Here we show that the method, by careful choice of parameters, can be extended to work also on the micron scale. We show that a microstructured aluminium film deposited on a Al₂O₃ (0001) surface is converted into a crystalline Al₂O₃ surface, when thermally annealed in air, using optimized parameters.

Abstract: The depth of surface/subsurface damage layer is the key index of surface quality of sapphire. In this paper, that depth model of the surface/subsurface damage lay characterized by the crack length was established according to the mechanical theory of indentation fracture. The cutting relation between abrasive and workpiece and the difference of the depth of subsurface damage crack are analyzed. It is preliminarily estimated that the length of sub-surface damage crack of free abrasive sapphire is about 2.46 times that of fixed abrasive when considering only the contact hardness of abrasive grain under static load. Diamond abrasives with size of W20 were adopted to carry out experiments in free and fixed lapping methods. The results show that the surface/subsurface damage depth is 9.87μm and 3.63μm respectively. It is easier to obtain good sub-surface quality by using the fixed abrasive method than free abrasive at the same particle size.

Abstract: In this work, sol-gel spin coated of magnesium (Mg) doped gallium nitride (GaN) thin films grown on AlN sapphire substrate was reported. The structural, lattice vibrational, and electrical properties of the deposited films were investigated and compared. X-ray diffraction results show that the deposited films composed of wurtzite structure with preferred orientation of GaN(002). The Raman active phonon modes correspond to the E₂(high) and A₁(LO) at 568 cm^-1 and 733 cm^-1 phonon modes of the hexagonal GaN were observed, while a broad peak attributed to the Mg-related lattice vibrational mode was detected at 669 cm^-1. The Raman phonon modes were detected by using Raman spectroscopy. Hall effects results show that the resistivity, carrier concentration, and hall mobility of the Mg-doped GaN film was 0.1397 Ω cm, 1.77 × 10¹⁸ cm³, and 6.04 cm²/Vs, respectively. Besides, the characteristics of the ultraviolet (UV) photoresponse of the fabricated detector were investigated. The current-voltage characteristics of the Mg-doped GaN UV photodetector exhibits Schottky behaviour. The current-voltage measurements were carried out at room temperature with a computer-controlled integrated Source Meter (Keithley 2400). Lastly, the ideality factor and Schottky barrier heights were calculated using thermionic emission theory.

Abstract: In this paper, a preliminary study in the wetting behavior and interface reaction between active Sn-Ag-4Ti solder alloy and C-plane sapphire was given. An in-situ observation of Sn-Ag-4Ti alloy on C-plane sapphire revealed a decrease in contact angles at temperature close to 550°C. Moreover, sapphire/sapphire and sapphire/copper sandwich joints were brazed using Sn-3.5Ag-4Ti alloy at 500 ^oC, 550°C and 600 °C to investigate the microstructure evolution and interface reaction. Microstructure characterization and element analysis indicated that the temperature affected the diffusion of active Ti element by modifying the formation of Sn-Ti intermetallics compounds in Sn-Ag-Ti solder alloy. The absorption of Ti together with the release of Al from sapphire suggested the interface reaction between Sn-Ag-Ti alloy and sapphire was triggered at 550°C.

Abstract: Sapphire, an inorganic gem-material in a variety of corundum, mainly consists of alpha-alumina (α-Al₂O₃) structure. The geological origins of sapphire are related to either basaltic or metamorphic rocks. The causes of the color on sapphire are some trace elements such as Cr, Fe, and Ti. It could be mentioned that Ti atoms have cooperated with Fe atoms for creating the blue color. In this study, X-ray absorption spectroscopy (XAS) technique focused on the x-ray absorption near edge structure (XANES) and the extended x-ray absorption fine structure (EXAFS) is employed to identify the oxidation state of Ti atoms and Ti-O bond length on sapphire samples. The Ti K-edge XANES and EXAFS spectra of natural sapphires were carried out using the 13-channel array germanium detector in fluorescence mode. The XANES spectra showed that the oxidation state of Ti was Ti⁴⁺ regardless of Fe content. Moreover, the Ti-O bond length on a-Al₂O₃ was equal to the Ti-O bond length on rutile (TiO₂) analyzed from the EXAFS spectra. From these results, it could be concluded that the oxidation state of Ti atoms on natural sapphires was Ti⁴⁺ which substitutes Al³⁺ on the sapphire structure.

Abstract: Sapphire is one of difficult-to-machine materials because of its high hardness and brittleness. It can be used for an optical window or cover named as sapphire glass and a substrate for semiconductor circuits. Before preparing the required surface roughness of sapphire substrate, the geometrical shape should be retained through mechanical machining processes. The lapping and diamond mechanical polishing (DMP) are essentially used for achieving the required thickness and surface roughness of sapphire substrate prior to chemical mechanical polishing (CMP). In this study, we introduce a lap grinding process using fixed abrasives to substitute lapping and DMP. The material removal rates (MRRs) were measured under various machining conditions. The semi-empirical model on MRR was introduced based on the information of grinding pallet. This paper may provide a preliminary experimental study on the lap grinding of sapphire substrate.

Abstract: Growth of gallium oxide thin film was realized with MOCVD on (0001) sapphire substrate. Structural and compositional properties of thin film were studied employing trimethylgallium and water as precursors, carrier gases were H₂ and N₂. Obtained film is polycrystalline and predominantly consisted of (201) oriented β-Ga₂O₃. Sample exhibited blue luminescence which is attributed to oxygen vacancies. H₂ gas proved to have beneficial effect on film quality and overall growth process.