Single-crystal Sapphire Substrates Research Articles

The study on the nanomachining property and cutting model of single-crystal sapphire by atomic force microscopy.

This study focused on the nanomachining property and cutting model of single-crystal sapphire during nanomachining. The coated diamond probe is used to as a tool, and the atomic force microscopy (AFM) is as an experimental platform for nanomachining. To understand the effect of normal force on single-crystal sapphire machining, this study tested nano-line machining and nano-rectangular pattern machining at different normal force. In nano-line machining test, the experimental results showed that the normal force increased, the groove depth from nano-line machining also increased. And the trend is logarithmic type. In nano-rectangular pattern machining test, it is found when the normal force increases, the groove depth also increased, but rather the accumulation of small chips. This paper combined the blew by air blower, the cleaning by ultrasonic cleaning machine and using contact mode probe to scan the surface topology after nanomaching, and proposed the "criterion of nanomachining cutting model," in order to determine the cutting model of single-crystal sapphire in the nanomachining is ductile regime cutting model or brittle regime cutting model. After analysis, the single-crystal sapphire substrate is processed in small normal force during nano-linear machining; its cutting modes are ductile regime cutting model. In the nano-rectangular pattern machining, due to the impact of machined zones overlap, the cutting mode is converted into a brittle regime cutting model.

Investigation of Removal Mechanism of Sapphire in Plasma Assisted Polishing

Single crystal sapphire is widely used as the material for precision equipments, due to its high hardness, chemical inertness and light transmission. However, it is difficult to obtain a scratch-free and damage-free sapphire surface with high-efficiency through traditional mechanical polishing or etching. We developed plasma assisted polishing (PAP) for the finishing of difficult-to-machine materials, such as silicon carbide, diamond, and sapphire. In this article, preliminary research results are showed about PAP applied to polishing of single crystal c-plane sapphire substrates. Combination of helium based atmospheric pressure water vapor plasma irradiation and silica abrasive polishing drastically increased removal rate of the sapphire c-plane. XPS measurements of the surfaces with and without irradiation of water vapor plasma revealed that alumina hydrate was formed by plasma irradiation at low temperature of less than 40°C. It is assumed that formation of alumina hydrate promoted the removal rate of sapphire.

Chemical vapor deposition of graphene on a "peeled-off" epitaxial Cu(111) foil: a simple approach to improved properties.

We present a simple approach to improving the quality of CVD grown graphene, exploiting a Cu(111) foil catalyst. The catalyst is epitaxially grown by evaporation on a single crystal sapphire substrate, thickened by electroplating, and peeled off. The exposed surface is atomically flat, easily reduced, and exclusively of (111) orientation. Graphene grown on this catalyst under atmospheric CVD conditions and without wet chemical prereduction produces single crystal domain sizes of several hundred micrometers in samples that are many centimeters in size. The graphene produced in this way can easily be transferred to other substrates using well-established techniques. We report mobilities extracted using field-effect (as high as 29 000 cm(2) V(-1) s(-1)) and Hall bar measurement (up to 10 100 cm(2) V(-1) s(-1)).

Growth and characterization of uranium–zirconium alloy thin films for nuclear industry applications

Polycrystalline and epitaxial U–Zr thin films have been grown on glass and single-crystal sapphire substrates using ultra-high vacuum magnetron sputtering at high temperatures (T = 800 °C). Mixed α- and γ-U phases were detected for polycrystalline U–Zr alloy thin films with the prevailing crystal structure controlled by composition. Epitaxial U–Zr thin film samples were determined to form bi-layered structures of single-crystal γ-U and α-U phases or γ-U, δ UZr2 and α-U phases depending on the concentration of the alloying element.

Properties of GaN-Related Epitaxial Thin Films Grown on Sapphire Substrates as Transparent Conducting Electrodes

Thin films of gallium nitride (GaN) and related nitride materials were prepared, and their properties as transparent conducting electrodes were investigated. GaN thin films were directly grown on sapphire single crystal substrates by the molecular beam epitaxy. Heavy doping of germanium was employed to reduce resistivity of the films, with sufficient reduction found to be possible while maintaining their epitaxial growth state. Optical transmission spectra of the films in the short wavelength region were slightly deteriorated by the heavy doping; however, this was successfully improved by growing GaN films under metal-rich conditions to increase the electron mobility and suppress unwanted increase of the carrier densities. In addition, the optical transmission spectra in the short wavelength region was improved also by alloying GaN with aluminum nitride, though the resistivities of these films were relatively higher than those of the unmodified GaN films. The prepared nitride thin films exhibited sufficiently suitable properties as transparent conducting electrodes for use in applications such as full-spectrum nitride-based solar cells.

Structure and characteristics of Li-doped NiO thin films prepared by using pulsed-laser deposition process.

10 at% Li doped NiO thin films were prepared on glass and sapphire single crystal substrates by using pulsed-laser deposition (PLD) process. The effects of the substrate and the deposition temperature on the crystal structures, electrical, and optical properties of NiO thin films were studied by using XRD, 4 point probe, and UV-VIS spectrometer. The electrical conductivities and the optical properties of NiO thin films depend on the crystallinity of the thin films and substrate temperatures. The Li doped NiO thin film with electrical conductivity of 2.25 S/cm and optical transmittance of 80% was fabricated on the sapphire substrate when processing parameters were optimized.

Defect studies of thin ZnO films prepared by pulsed laser deposition

Thin ZnO films were grown by pulsed laser deposition on four different substrates: sapphire (0 0 0 1), MgO (1 0 0), fused silica and nanocrystalline synthetic diamond. Defect studies by slow positron implantation spectroscopy (SPIS) revealed significantly higher concentration of defects in the studied films when compared to a bulk ZnO single crystal. The concentration of defects in the films deposited on single crystal sapphire and MgO substrates is higher than in the films deposited on amorphous fused silica substrate and nanocrystalline synthetic diamond. Furthermore, the effect of deposition temperature on film quality was investigated in ZnO films deposited on synthetic diamond substrates. Defect studies performed by SPIS revealed that the concentration of defects firstly decreases with increasing deposition temperature, but at too high deposition temperatures it increases again. The lowest concentration of defects was found in the film deposited at 450° C.

In-Situ AlN Induced Valence State Variation of V in Vanadium Oxide Films Investigated by XPS

Vanadium oxide (VOx) thin films with and without in situ AlN layer were grown on single crystal sapphire substrates by pulsed laser deposition (PLD) with V2O5 target. The significant structure and morphology transforms resulted from the insertion of in-situ AlN layer were observed by X-ray diffraction (XRD) and field-emission scanning electron microscope (FESEM). To interpret the mechanism behind these phenomena, the valence state variation of vanadium (V) in VOx films induced by in-situ AlN layer was investigated by X-ray photoelectron spectroscopy (XPS) analyses. The results indicated that the valence state of V become more complicated and VOx films with the mixed V valence state of V5+,V4+and V3+ were obtained due to the introduction of AlN interfacial stress layer. Our achievements will be helpful for understanding the physical mechanism behind the exotic characteristics of VOx films induced by stress layer.

Heteroepitaxial growth of Cu2ZnSnS4 thin film on sapphire substrate by radio frequency magnetron sputtering

The heteroepitaxy of tetragonal Cu2ZnSnS4 (CZTS) thin films on hexagonal sapphire (0001) single crystal substrates is successfully obtained by radio frequency magnetron sputtering. The sputtered CZTS film has a mirror-like smooth surface with a root mean square roughness of about 5.44 nm. X-ray θ-2θ scans confirm that CZTS film is (112) oriented on sapphire with an out of plane arrangement of CZTS (112) ‖ sapphire (0001). X-ray Phi scan further illustrates an in plane ordering of CZTS [201¯] ‖ sapphire [21¯1¯0]. The high resolution transmission electron microscopy image of the interface region clearly shows that the CZTS thin film epitaxially grows on the sapphire (0001) substrate. The band gap of the film is found to be approximately 1.51 eV.

Optimization of the semiconductor-metal transition in VO2 epitaxial thin films as a function of oxygen growth pressure

High quality VO2 epitaxial thin films were deposited on sapphire single crystal substrates by pulsed laser deposition and their semiconductor-to-metal transitions (SMTs) were characterized as a function of film growth conditions. Varying the oxygen pressure during deposition affected the number of oxygen vacancies, which allowed tuning of the crystal structure and phase transition properties of the VO2 films. Films grown at optimized conditions exhibited a significant resistivity drop (>104 Ω-cm) across the SMT that is correlated with the strain due to oxygen vacancies. This resistivity drop is mainly accounted for by a large change in carrier density at the SMT.

Growth of Highly (112) Oriented Cu2ZnSnS4 Thin Film on Sapphire Substrate by Radio Frequency Magnetron Sputtering

ABSTRACTFor structural investigation, highly (112) oriented tetragonal Cu2ZnSnS4 (CZTS) thin films on hexagonal sapphire (0001) single crystal substrates were obtained by radio frequency (RF) magnetron sputtering. The influences of the deposition parameters, such as substrate temperature (Tsub) and working Ar pressure (PAr) on the chemical composition and structural properties of as deposited CZTS films were investigated. The film sputtered at 500°C has the only orientation of (112), also, it bears the best structural quality with pure CZTS phase and an estimated band gap of 1.51eV.

C26 電界砥粒制御技術を用いた単結晶サファイア基板の高効率研磨加工 : 第1報(OS10 研磨技術(1))

It takes a lot of time to process the single crystal sapphire substrates for LED, because it is hard process material and its diameter trend becomes larger for productivity improvement. Therefore we have attempted to develop the novel processing technologies that enhance the process ability of sapphire substrates. In this paper, we described the results pertaining to the evaluation of the novel lapping process for sapphire substrates. It was confirmed applied AC electric field lead to enhanced removal rate, and the surface roughness was improvement.

Optical properties of rare earth-doped TiO2 anatase and rutile thin films grown by pulsed-laser deposition

Ln3+ (Ln=Tm, Eu and Yb) doped titanium dioxide anatase and rutile films have been grown by pulsed-laser deposition at 700°C under 0.1mbar O2. By using c-cut (0001) Al2O3 sapphire or (100) LaAlO3 single crystal substrates, TiO2 films doped with Ln3+ are constituted with either highly oriented (200) rutile or (004) anatase, respectively. Energy transfer from TiO2 to Ln3+ is studied by photoluminescence spectroscopy with UV excitation (364nm) under band gap excitation of the oxide matrix. It is demonstrated that Tm3+ dopant is not efficient as sensitizers. On the contrary, energy transfer from TiO2 to Eu3+ and Yb3+ occurs in both matrixes, which make this material suitable for down-shifting purpose. Results obtained for Yb3+ compared with our previous study on Nd3+ show that Nd3+ doped-rutile and Yb3+ doped-anatase are the more efficient combinations to convert UV to NIR photon. Finally, a cooperative conversion mechanism is suggested to explain the higher integrated photoluminescence intensity found in anatase Yb3+ rather than in rutile.

High-throughput synthesis of thermoelectric Ca3Co4O9 films

Properties of complex oxide thin films can be tuned over a range of values as a function of mismatch, composition, orientation, and structure. Here, we report a strategy for growing structured epitaxial thermoelectric thin films leading to improved Seebeck coefficient. Instead of using single-crystal sapphire substrates to support epitaxial growth, Ca3Co4O9 films are deposited, using the Pulsed Laser Deposition technique, onto Al2O3 polycrystalline substrates textured by spark plasma sintering. The structural quality of the 2000 Å thin film was investigated by transmission electron microscopy, while the crystallographic orientation of the grains and the epitaxial relationships were determined by electron backscatter diffraction. The use of a polycrystalline ceramic template leads to structured films that are in good local epitaxial registry. The Seebeck coefficient is about 170 μV/K at 300 K, a typical value of misfit material with low carrier density. This high-throughput process, called combinatorial substrate epitaxy, appears to facilitate the rational tuning of functional oxide films, opening a route to the epitaxial synthesis of high quality complex oxides.

Antiphase inversion domains in lithium cobaltite thin films deposited on single-crystal sapphire substrates

Antiphase inversion domains in LiCoO2 thin films prepared by pulsed laser deposition on sapphire single-crystal substrates are analyzed using a combination of (scanning) transmission electron microscopy and first-principles calculations. Domains form epitaxially on the substrates with orientation relationships of [112¯0]LiCoO2(0001)LiCoO2//[11¯00]α-Al2O3(0001)α-Al2O3 and [1¯1¯20]LiCoO2(0001)LiCoO2//[11¯00]α-Al2O3(0001)α-Al2O3. In addition, substrate/film interfaces with the above orientation relationships always have the same stacking sequence of Al–O–Co–O–Li–O. This is confirmed to be the most energetically stable stacking arrangement according to first-principles calculations. Individual domains form as a result of steps one (0001) O–Al–O spacing in height on the otherwise flat substrate surface. Because the orientation of adjacent (0001) AlO6 octahedra in Al2O3 are rotated by 180°, while LiO6 and CoO6 octahedra in LiCoO2 are all aligned in the same direction, substrate steps produce LiCoO2 domains rotated 180° relative to their neighbors. The similar size of oxygen octahedra in the two materials also means that the step height is close to the layer spacing in LiCoO2, so that (0001) Li and Co layers of adjacent domains are shifted by one layer relative to each other at each domain boundary, aligning Li layers with Co layers across the boundary. The combination of these two effects generates antiphase inversion domains. The domain boundaries effectively sever Li-ion diffusion pathways in the (0001) planes between domains and thus are expected to have a detrimental effect on Li-ion conductivity.

Terahertz filters based on frequency selective surfaces for high-speed terahertz switch

We design and fabricate two terahertz (THz) filters based on frequency selective surfaces' structures (FSSs) on single crystal sapphire substrate, which show high and low transmissivities to THz electromagnetic waves, respectively. The difference of THz transmissivity between two filters reaches up to 72% at the resonance frequency of 1.35 THz. The THz filtering effect is due to the resonant coupling between THz wave and periodic metallic elements and the filter operating frequency can be tuned systematically by the geometry of FSSs. The good agreement between experiments and finite difference time domain simulation indicates that a high-speed THz switch could be realized for THz active devices based on the FSSs.

Characterization of critically cleaned sapphire single-crystal substrates by atomic force microscopy, XPS and contact angle measurements

A contaminant-free surface of single-crystal α-Al2O3 (or sapphire) substrates is key to the experimental studies of its surface and interfacial properties at ambient conditions. Here we critically evaluated methods reported in the literature using comprehensive surface analysis techniques including atomic force microscopy, XPS and contact angle measurements. We found that reported methods did not perform well in terms of removing both organic and particulate contaminants from the (0001) basal surface. After thoroughly examining the cleaning effect of various chemical solutions and UV light and plasma irradiation, and based on modified RCA cleaning protocols, we proposed a new wet-cleaning method showing outstanding cleaning performance. This new reliable method will be very useful for the next-step surface chemistry study of single-crystal α-Al2O3. It was also demonstrated that AFM, due to its high spatial resolution and sensitivity as a local probe technique, was an indispensable tool for surface contamination control studies.

Enhanced Jc of MOD-YBCO Films by Modifying Surface States of CeO2 Buffer Layers on Sapphire Substrates

Abstract Simple and easy treatments for the CeO2 buffer layers on sapphire single crystal substrates were demonstrated to enhance the superconducting properties of the metalorganic YBa2Cu3O7-y films using (i) acid solution dipping, (ii) UV light irradiation, and (iii) thermal annealing. When the CeO2 buffer layers were exposed to UV light, water contact angles diminished drastically, however, an apparent effect on the superconducting properties was not observed. On the other hand after the thermal annealing at 750°C under the mixture of dry-air and N2 gas, superconducting current density of 1.4 MA/cm 2 at 77.3K was achieved independent of treatment time.

Pulsed laser deposition and characterization of epitaxial CuInS2 thin films on c-plane sapphire substrates

The epitaxial growth of CuInS2 thin films on single-crystal sapphire (0001) substrates was first attempted by using pulsed laser deposition. A highly dense CuInS2 ceramic target with chalcopyrite structure was prepared by sintering high-purity (5N) powders in vacuum at 960°C for an hour. CuInS2 thin films deposited at various temperatures all grew in a (112) preferred orientation, suggesting a three-dimensional island-like growth mechanism. When deposited at 700°C, epitaxial growth of CuInS2 films on c-plane sapphire was achieved with an out-of-plane orientation of CuInS2 (112)|| sapphire (0001) and in-plane orientation of CuInS2[1¯10]‖sapphire(101¯0). A typical rocking curve half width of 0.17° demonstrates a high degree of crystallinity as well as excellent alignment of the CuInS2 films along [221], whereas in-plane two domains rotated by 180° with respect to each other are found coexisting in the epilayers. High-quality epitaxial thin films will allow further research into the fundamental properties of CuInS2 absorber layers, which will eventually help to improve conversion efficiencies of CuInS2-based solar cells.

Large Crystallographic Orientation Tilting Induced by Postoxidation Annealing in Layered Cobaltite CaxCoO2 Thin Films

A drastic change in crystallographic orientation has been observed in incline-oriented thin films of layered cobaltite CaxCoO2 by postoxidation annealing. The as-grown CaxCoO2 films fabricated on n-plane sapphire single crystal substrates exhibited an inclined CoO2 plane alignment with respect to the substrate surface by an angle of 58°. Upon postoxidation annealing, we found that the tilt angle of the aligned CoO2 planes changes dramatically up to 80°. The large crystallographic orientation change was observed uniquely in the incline-oriented films and could not be observed in films with low-index plane orientations. We discuss that the large crystallographic orientation tilting results from a strain relief process so as to compensate the internal lattice misfit due to CaxCoO2 lattice modification, which was also induced by the oxidation process.