Single-crystal Sapphire Substrates Research Articles

Characterization of Polish-Induced Gouges on Single Crystal Sapphire Substrates

Abstract Single-crystal sapphire is known to be among the hardest insulators. Its mechanical properties and chemical inertness make it a challenging material to polish for the atomic-level surface smoothness required for its applications. Mechanical polish with diamond abrasives renders high removal rates but creates unacceptable levels of polish-induced gouges. Chemical mechanical polish on the other hand results in atomic smoothness but is a slow process. Hence, a combination of the two is used in the industry. In this work, we have attempted to characterize gouging and subsurface damage using atomic force microscopy, X-Ray diffraction, and cross-section transmission electron microscopy on C-plane and A-plane sapphire induced by diamond abrasive mechanical polish and chemical mechanical polish with colloidal silica.

NaCl flux growth of non-polar m-plane ZnO epitaxial thin film on c-plane sapphire substrate

We have developed a technique to fabricate non-polar m-plane ZnO thin films: the post NaCl flux treatment of Zn-containing precursors, such as amorphous zinc carbonate hydroxide, Zn5(CO3)2(OH)6. Conventionally, single-crystal m-plane sapphire or MgO (001) substrates have been utilized to fabricate m-plane ZnO thin films. In contrast, this method facilitates the growth of m-plane ZnO thin films on various substrates, such as sapphire and quartz glass, despite its c-plane growth habit. Furthermore, the m-plane ZnO thin film was demonstrated to epitaxially grow even on a c-plane sapphire substrate. Owing to the good lattice matching with the substrate, the m-plane ZnO epitaxial film had high crystallinity comparable to a c-plane ZnO epitaxial film that was created using a conventional pulsed laser deposition method, and exhibited similar photocatalytic activity to the c-plane ZnO film. This research offers valuable insights into the fundamental mechanisms of flux growth orientation control, which can be beneficial in designing ZnO-based electronic devices and photocatalysts.

Low-loss interconnects for modular superconducting quantum processors

Scaling is now a key challenge in superconducting quantum computing. One solution is to build modular systems in which smaller-scale quantum modules are individually constructed and calibrated, and then assembled into a larger architecture. This, however, requires the development of suitable interconnects. Here, we report low-loss interconnects based on pure aluminium coaxial cables and on-chip impedance transformers featuring quality factors up to $8.1 \times 10^5$, which is comparable to the performance of our transmon qubits fabricated on single-crystal sapphire substrate. We use these interconnects to link five quantum modules with inter-module quantum state transfer and Bell state fidelities up to 99\%. To benchmark the overall performance of the processor, we create maximally-entangled, multi-qubit Greenberger-Horne-Zeilinger (GHZ) states. The generated inter-module four-qubit GHZ state exhibits 92.0\% fidelity. We also entangle up to 12 qubits in a GHZ state with $55.8 \pm 1.8\%$ fidelity, which is above the genuine multipartite entanglement threshold of 1/2. These results represent a viable modular approach for large-scale superconducting quantum processors.

From transparent to black amorphous zinc oxide thin films through oxygen deficiency control

Despite the fact that zinc oxide is a well-known transparent oxide, several recent studies on “black” ZnO have renewed its potential for photocatalytic applications. We report on the control of oxygen deficiency in ZnO thin films grown at 300 °C on c-cut sapphire single-crystal substrates by pulsed electron beam deposition (PED) through a slight variation of argon pressure in PED. At a pressure of 2 × 10−2 mbar transparent, stoichiometric (ZnO) and crystalline films are obtained, while at 9 × 10−3 mbar black, oxygen-deficient (ZnO0.85) and amorphous films result. Stoichiometry, structural, and optoelectronic properties of transparent and black ZnO thin films were comparatively analyzed as a function of oxygen deficiency. Black ZnO thin films exhibit enhanced absorption in the visible and near-infrared due to oxygen deficiency, thus extending the range of applications of zinc oxide thin films from transparent electronics to solar absorbers and photocatalysis.

Large-Scale Multilayer MoS2 Nanosheets Grown by Atomic Layer Deposition for Sensitive Photodetectors

Two-dimensional (2D) transition-metal dichalcogenides have promising optoelectronic applications. However, most reported MoS2-based photodetectors are based on MoS2 flakes, dozens of microns in size, prepared by mechanical exfoliation or chemical vapor deposition, which limits their practical applications. In this study, a wafer-scale, continuous, and uniform multilayer MoS2 nanosheet was grown on an insulating single-crystal sapphire substrate by atomic layer deposition (ALD), with MoCl5 and hexamethyldisilathiane as precursors, because of the smaller lattice mismatch between crystalline sapphire and hexagonally arranged MoS2 compared with polycrystalline substrates. Then, large-area MoS2 nanosheets with high uniformity and homogeneity were obtained on Al2O3/Si and SiO2/Si substrates via large-scale peeling and vacuum stacking transfer. Subsequently, MoS2 photodetectors were fabricated on both substrates with ALD-MoS2 as a photosensitive channel material and high-k Al2O3 or SiO2 as a dielectric layer, which resulted in gate control with relatively low and high voltages, respectively. A high photoresponsivity (R) of 577.4 A W–1 was achieved for the MoS2 photodetector fabricated on the Al2O3/Si substrate at an illumination power density (Pin) of 5.01 μW cm–2 (λ = 500 nm). In addition, the MoS2 photodetectors exhibited a light response over a wide wavelength range (400–700 nm). Moreover, a MoS2 photodetector array containing 4 × 3 photodetectors with a uniform photoelectric response was demonstrated. These results will facilitate future applications of 2D materials in large-scale, high-performance photoelectronic devices and circuits.

Tailorable properties of Nd-doped ZnO epitaxial thin films for optoelectronic and plasmonic devices

Transparent conductive oxides with both low resistivity and high transparency in the visible and near-infrared wavelengths are required for solar cells and plasmonic applications. In this work we demonstrate that highly conducting and transparent Nd:ZnO thin films have been grown on c-cut sapphire single crystal substrates by pulsed electron beam deposition under argon (10−2 mbar). At 2% at. Nd doping, these films crystallize in the wurtzite structure at T > 300 °C, with well-defined epitaxial relationships with the c-cut sapphire as a function of the substrate temperature. The improvement of the film structural quality with increasing growth temperature led to the lowest resistivity value of 5.12 × 10−4 Ωcm and highest transparency that were maintained more than 36 months under ambient conditions. X-ray photoelectron valence band measurements revealed that the dominant Zn-polar face of Nd:ZnO thin films explains the improved electrical properties. The dielectric functions of films were determined and show that these films are suitable as transparent contact electrodes for solar cell devices or tunable epsilon-near-zero materials for plasmonics.

Evaluation of Preferential Orientation in the Film Depth Direction by Two-Dimensional Grazing Incident Wide-Angle X-ray Diffraction of CeO2 Film on Sapphire Single Crystal Substrate with Different Film Formation Conditions

Evaluation of Preferential Orientation in the Film Depth Direction by Two-Dimensional Grazing Incident Wide-Angle X-ray Diffraction of CeO₂ Film on Sapphire Single Crystal Substrate with Different Film Formation Conditions

Formation of In-=SUB=-2-=/SUB=-O-=SUB=-3-=/SUB=- films by magnetron sputtering on Al-=SUB=-2-=/SUB=-O-=SUB=-3-=/SUB=- (012) substrates

The results of studies of the microstructure and optical characteristics of In2O3 films deposited by the dc-magnetron sputtering of a polycrystalline target onto single-crystal sapphire substrates are summarized. The technological regimes of films preparation differed in the deposition time, substrate temperature, and the presence of additional heat treatment of film structures in air. It has been established that the optical refractive index of films deposited on a "cold" substrate increases in the direction from the substrate to the external interface. Heat treatment of the films eliminates the inhomogeneity of the refractive index and leads to a decrease in the band gap. The observed optical properties are explained by the thickness inhomogeneous microstructure of the films, which is formed during sputtering of a target with a relatively low mechanical strength. Keywords: In2O3 films, magnetron sputtering, Al2O3 substrates, refraction index, bandgap width.

Формирование плёнок In-=SUB=-2-=/SUB=-O-=SUB=-3-=/SUB=- методом магнетронного напыления на подложках Al-=SUB=-2-=/SUB=-O-=SUB=-3-=/SUB=- (012)

The results of studies of the microstructure and optical characteristics of In2O3 films deposited by the dc-magnetron sputtering of a polycrystalline target onto single-crystal sapphire substrates are summarized. The technological regimes of films preparation differed in the deposition time, substrate temperature, and the presence of additional heat treatment of film structures in air. It has been established that the optical refractive index of films deposited on a "cold" substrate increases in the direction from the substrate to the external interface. Heat treatment of the films eliminates the inhomogeneity of the refractive index and leads to a decrease in the band gap. The observed optical properties are explained by the thickness inhomogeneous microstructure of the films, which is formed during sputtering of a target with a relatively low mechanical strength.

A high temperature cell for investigating interfacial structure on the molecular scale in molten salt/alloy systems.

In this work, we describe the designand development of an in situ neutron reflectometry cell for high temperature investigations of structural changes occurring at the interface between inorganic salts, in their molten state up to 800°C, and corrosion resistant alloys or other surfaces. In the cell, a molten salt is confined by an annular ring of single crystal sapphire constrained between the sample substrate and a sapphire plate using two gold O-rings, enclosing a liquid salt volume of 20 ml, along with a dynamic cell volume to accommodate expansion of the liquid with heating. As a test case for the cell, we report on an in situ neutron reflectometry measurement of the interface between a eutectic salt mixture of MgCl2-KCl (32:68 molar ratio) and a single crystal sapphire substrate at 450°C, resulting in the formation of a 60 Å layer having a scattering length density of 1.72 × 10-6 Å-2. While the origin of this layer is uncertain, it is likely to have resulted from the salt reacting with an existing impurity layer on the sapphire substrate.

Behavior of Polyaniline Particles in Vacuum on Silicon and Sapphire Single-Crystal Substrates

Behavior of Polyaniline Particles in Vacuum on Silicon and Sapphire Single-Crystal Substrates

Rapid hydrogenation of VO2 thin films via metal-acid contact method using mild electric fields at room temperature

Five thin films of VO2 were grown on single-crystal sapphire substrates in identical deposition conditions using pulsed excimer laser with an objective to study hydrogenation under different electric fields at room temperature. For hydrogenation, a self-made steup was used with the provision of the negative (film) and positive electrodes in acidic solution. The microstructural, crystallographic and electrical properties were studied in detail. By applying only a trivial voltage of 0.001 mV at 3 cm distance for hydrogenation just for 30 s, the resistivity of the film decreased by one order of magnitude at room temperature, indicating incorporation of a good amount of hydrogen in VO2. We find that the hydrogenation is not only sustainable and reversible but a lot more rapid as compared to other techniques.

Structural transformation and tuning of electronic transitions by W-doping in VO2 thin films

V1-xWxO2 (x = 0, 0.005, 0.01, 0.015, 0.02, 0.03, 0.04) thin films have been deposited on single-crystal sapphire substrates using pulsed laser deposition (PLD) technique. The PLD-grown films are pure and crystallographically oriented. The films are found to sustain their crystallinity even at higher percentages of doping. Even a minor inclusion of tungsten (W) starts the process of driving the lattice towards rutile phase, which is confirmed by structural analysis. The doping of W at V-site was confirmed by beamline experiments of X-ray photoemission spectroscopy. As observed from the resistivity curves, initially the transition temperature reduced up to 10 K per 0.5 at % of W doping at V-site, and this decrement is further amplified with an increase in doping. Thus, W doping was successfully employed to alter the structure quite systematically with a significant reduction in the phase transition temperature.

Understanding the effect of thickness on the thermoelectric properties of Ca3Co4O9 thin films

We are reporting the effect of thickness on the Seebeck coefficient, electrical conductivity and power factor of Ca3Co4O9 thin films grown on single-crystal Sapphire (0001) substrate. Pulsed laser deposition (PLD) technique was employed to deposit Ca3Co4O9 films with precisely controlled thickness values ranging from 15 to 75 nm. Structural characterization performed by scanning electron microscopy (SEM) and atomic force microscopy (AFM) showed that the growth of Ca3Co4O9 on Sapphire (0001) follows the island growth-mode. It was observed that in-plane grain sizes decrease from 126 to 31 nm as the thickness of the films decreases from 75 to 15 nm. The thermoelectric power measurements showed an overall increase in the value of the Seebeck coefficient as the films’ thickness decreased. The above increase in the Seebeck coefficient was accompanied with a simultaneous decrease in the electrical conductivity of the thinner films due to enhanced scattering of the charge carriers at the grain boundaries. Because of the competing mechanisms of the thickness dependence of Seebeck coefficient and electrical conductivity, the power factor of the films showed a non-monotonous functional dependence on thickness. The films with the intermediate thickness (60 nm) showed the highest power factor (~ 0.27 mW/m-K2 at 720 K).

Thickness effect on structural properties of post annealed barium hexaferrite films deposited by ion beam sputtering

Barium hexaferrite thin films were prepared by ion beam deposition on sapphire (102) single crystal substrates. The structural properties of the films were analyzed by means of X-ray diffraction, atomic force microscopy, magnetic force microscopy and scanning electron microscopy. The obtained results were interpreted as a presence of layered structure with different types of texture and its formation mechanism is proposed. The significant influence of BaFe12O19 thickness on its microstructure was observed. Films obtained under certain conditions are potentially fit for further growing of oriented hexaferrite with out-of-plane orientation of c-axis.

Structure influenced rapid hydrogenation using metal-acid contacts on crystallographically oriented VO2 thin films

We report on the effects of hydrogenation using metal-acid contacts with a simple method using the detachable electrode-like structure on crystallographically oriented VO2 thin films, deposited on single-crystal sapphire substrates. As the temperature increases, these films show insulator to metal transition accompanied by a structural phase transition from monoclinic to rutile phase. The said method of hydrogenation is tested around the room temperature as well as at elevated temperatures for the rutile structure. A rapid diffusion rate of hydrogen is indicated by a drastically reduced resistance of the films hydrogenated at the elevated temperatures. Moreover, when cooled down to the room temperature, the hydrogenated VO2 thin films incline to partially retain a stable rutile phase. In comparison to the hydrogenation in the monoclinic phase of VO2 film, the process of hydrogenation becomes much faster and more effective when VO2 acquires the rutile phase.

Effect of the Morphology of an Ensemble of ZnO Microrods on the Optical and Luminescence Properties

The effect of morphology on the optical and X-ray luminescence properties of an ensemble of zinc oxide microrods on single-crystal sapphire substrates has been studied. It has been shown that the intensity of X-ray luminescence depends on the volume of the ZnO single-crystal phase and the corresponding effective density of the absorbing medium. It has been found that the total transmission spectra of the samples have a complex structure. The transmittance primarily depends on the conditions of synthesis and nucleation, on the morphology of ZnO microrods, and on the degree of their misorientation in an ensemble. An array of ZnO microrods characterized by the rapid component of the X-ray luminescence kinetics in the range of 0–5 ns with a descending time of about 0.7 ns (taking into account the pump pulse) has been obtained.

Fabrication of vertically aligned ferromagnetic ZnO nanopillar arrays on sapphire substrates by polymer-assisted deposition

Vertically aligned ferromagnetic ZnO nanopillar arrays have been fabricated on single-crystal sapphire substrates through polymer-assisted deposition. X-ray diffraction and transmission electron microscopy analysis show that the arrays are ZnO hexagonal crystal structure along the c-axial direction. The existence of room temperature ferromagnetism of the arrays was confirmed by the superconducting quantum interference device measurements. The enhancement of ferromagnetism in ZnO nanopillar arrays is found to be correlated with the increase in Zn1− vacancies and the decrease in oxygen interstitials, which was shown by photoluminescence measurements of the samples. Our results provide a viable way to fabricate ferromagnetic vertical arrays of ZnO nanopillars.

Effect of thickness on the performance of solar blind photodetectors fabricated using PLD grown β-Ga2O3 thin films

Herein, we are reporting the fabrication of high-performance solar blind photodetectors using β-Ga2O3 thin films of varying thicknesses grown on single crystal (0001) Sapphire substrate by pulsed laser deposition (PLD) technique. Thickness of the β-Ga2O3 films was varied by varying the number of ablating laser shots from 1000 to 15,000 (1 k to 15 k). The effect of thickness on the structure, surface morphology and optical properties of the films was investigated using several state-of-the-art techniques such as X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM) and ultraviolet–visible (UV–Vis) spectroscopy. All the films were found to be made of phase-pure β-Ga2O3 with high crystal quality. The orientation of the films was found to gradually change from (−201) to (−201)/(400) as the number of laser shots increased from 1 k to 15 k. The bandgap of the films made with 1 k laser shots was found to be 5.26 eV which decreased monotonously with increase in number of laser shots and became 4.79 eV for the films made with 15 k laser shots. The detailed time-resolved photoresponse measurements showed that the films made with 2 k shots exhibit the best photoresponse characteristic among all the films with a high Iphoto/Idark ratio of ∼6.7 × 104 and short rise (0.38 μs) and decay times (142 μs). Our results indicate that the PLD-grown β-Ga2O3 thin films, with optimized thickness, can play a major role in next-generation solar-blind ultraviolet photodetector technology.

Polysaccharide-bonded abrasive tool for green machining of single crystal sapphire

ABSTRACT A novel polysaccharide-bonded soft abrasive tool is proposed for the green machining of single-crystal sapphire. The composition and manufacturing process of the new abrasive tool are described which are completely environmentally friendly. The machining of the single-crystal sapphire substrate was carried out using a novel abrasive tool; the experimental results showed that the new abrasive tool could effectively improve the surface quality of sapphire substrate with a wear ratio of 0.41. Polysaccharide binder was encapsulated on the abrasive’s surface, and solid-state reactions occurred between the abrasives and the sapphire substrate. After machining, a large number of cracks were observed in the blocking area of the abrasive tool, which is beneficial for self-sharpening. Finally, a process to recycle the abandoned polysaccharide-bonded abrasive was established; this process can extract dextrin and SiO2 powder as well.