Single-crystalline Aluminum Research Articles

J212015 マイクロ波原子間力顕微鏡を用いた金属ナノワイヤの導電率の定量評価に関する研究([J212-01]マイクロ・ナノ材料創成とそのデバイス応用(1))

With the development of nanotechnology in recent years, many researchers have focused on the development of nanomaterials and nanostructures such as nanowires. To apply these nanomaterials and nanostructures into nanodevices, there are great needs of the quantitative measurement of electrical properties of materials in an infinitesimal area. Recently, we proposed a novel microwave atomic force microscopy (M-AFM). M-AFM probe was fabricated by forming a microwave transmission line on the probe cantilever. By combining the AFM technique with microwave-based measurement, M-AFM has the ability to sense the topography and microwave image simultaneously with a high spatial resolution. In this study, we tried to evaluate quantitatively electrical properties of single crystalline aluminum nanowire. Using M-AFM, both measurements of the surface topography and the microwave response of single crystalline aluminum nanowire were succeeded. As a result, the electrical property of microwave signals was detected as the difference of the voltage value.

Continuum modeling of dislocation starvation and subsequent nucleation in nano-pillar compressions

The mechanical behavior of single crystalline aluminum nano-pillars under uniaxial compression differs from bulk Al in that the former is characterized by a smoother transition from elasticity to plasticity. We propose an extension of the phenomenological model of dislocation starvation originally proposed in [Greer and Nix, Phys. Rev. B 73 (2006) 245410] additionally accounting for dislocation nucleation. The calibrated and validated continuum model successfully captures the intrinsic mechanisms leading to the transition from dislocation starvation to dislocation nucleation in fcc nano-pillars.

Dislocation microstructure evolution in cyclically twisted microsamples: a discrete dislocation dynamics simulation

Miniaturization in technical devices has increased interest in the investigation of the deformation and fatigue behaviour of metals in the micrometre regime. Due to the small dimensions of these devices, mechanical properties depend on the motion of a marginal number of dislocations. In this paper, the evolution of dislocation microstructure in torsion loaded single crystalline aluminium wires is analysed by three-dimensional discrete dislocation dynamics simulations. It is shown that the size of pile-ups and the number of the active slip systems is significantly influenced by cross-slip events independent of the crystallographic orientation. Dislocations are driven by the stress gradient from the applied loading to move into the centre of the sample. These dislocations cannot escape through the surface because of the reversal of the sign of the stress in the centre of the sample. If the micrometre-sized specimens are untwisted, the remaining dislocation microstructure in these samples depends on the maximum torsion angle reached before unloading. The larger the torsion angle, the higher is the remaining dislocation density in the unloaded specimens. These results are discussed with respect to cyclic deformation mechanisms at small scale.

Single-crystalline AlN growth on sapphire using physical vapor deposition

A novel technique for growing single crystalline aluminum nitride (AlN) films is presented. The novelty of the technique, specifically, comes from the use of an innovative physical vapor deposition magnetron sputtering tool, which embeds magnets into the target material. A relatively high deposition rates is achieved (∼0.2 μm/min), at temperatures between 860 and 940 °C. The AlN, grown onto sapphire, is single-crystalline as evidenced by observation using transmission electron microscopy. Tool configuration and growth conditions are discussed, as well as a first set of other analytical results, namely, x-ray diffraction and ultraviolet-visible transmission spectrophotometry.

Single-step, catalyst-free plasma-assisted synthesis and growth mechanism of single-crystalline aluminum nitride nanorods

Synthesis of one-dimensional AlN nanostructures commonly requires high process temperatures (>900 °C), metal catalyst, and hazardous gas/powder precursors. We report on a simple, single-step, catalyst-free, plasma-assisted growth of dense patterns of size-uniform single-crystalline AlN nanorods at a low substrate temperature (∼650 °C) without any catalyst or hazardous precursors. This unusual growth mechanism is based on highly effective plasma dissociation of N2 molecules, localized species precipitation on AlN islands, and reduced diffusion on the nitrogen-rich surface. This approach can also be used to produce other high-aspect-ratio oxide and nitride nanostructures for applications in energy conversion, sensing, and optoelectronics.

Surface preparation and homoepitaxial deposition of AlN on (0001)-oriented AlN substrates by metalorganic chemical vapor deposition

Chemical surface treatments were conducted on mechanically polished (MP) and chemomechanically polished (CMP) (0001)-oriented single crystalline aluminum nitride (AlN) substrates to determine a surface preparation procedure for the homoepitaxial deposition of AlN epitaxial layers by metalorganic chemical vapor deposition. MP AlN substrates characterized by atomic force microscopy exhibited 0.5 nm rms roughness and polishing scratches, while CMP AlN substrates exhibited 0.1 nm rms roughness and were scratch-free. X-ray photoelectron spectroscopy analysis of MP and CMP AlN substrates indicated the presence of a surface hydroxide layer composed of mixed aluminum oxide hydroxide and aluminum trihydroxide. Wet etching with sulfuric and phosphoric acid mixtures reduced the amount of surface hydroxide. Ammonia annealing at 1250 °C converted the substrate hydroxide layer to AlN and increased the rms roughness of MP and CMP AlN substrates to 2.2 nm and 0.2 nm, respectively. AlN epitaxial layers were deposited at 1100–1250 °C under 20 Torr total pressure with a V/III ratio of 180–300 in either N2 or H2 diluent. High-resolution x-ray diffraction measurements revealed that AlN epitaxial layers deposited on MP substrates were strained due to nucleation and coalescence of AlN grains on the mechanically damaged surfaces. AlN deposited on CMP substrates was epitaxial and strain-free. Thermodynamic models for nitridation and AlN deposition were also proposed and evaluated.

Single crystalline aluminum nanowires with ideal resistivity

We present the stress-induced synthesis of aluminum nanowires having almost perfect crystallinity. Their resistivity is comparable to the lowest bulk value of aluminum, and thus unprecedentedly smaller than the ones observed in other metal nanostructures. We analyze the measured resistivity using the standard theory of scattering in metal wires, and the observed resistivity values are consistent with the infinite average grain size.

Synthesis of h-AlN Nanowires via Carbothermal Reduction and Nitridation Method Using Acetylene Black

Using acetylene black as an assistant reagent in the ammonia atmosphere, one dimensional (1D) hexagonal single crystalline aluminum nitride (AIN) nanowires were successfully synthesized via carbothermal reduction and nitridation method. With smooth surface and uniform diameter, the wool-like product of AIN nanowires grow along [001] direction and are 80-120 nm in diameter and several tens micrometers in length. According to thermodynamic and kinetic analysis, AI2O3 initially was reduced to a volatile suboxide gas Al2O at 1800 degrees C. Then gaseous Al2O was transported by N2/NH3 mixtures to about 1200 degrees C, where 1D AIN nanowires formed by the reaction of Al2O gas, graphite and NH3. Due to high surface area, small particle size and amorphous structure, acetylene black maybe possess more active sites and more effective contact area, which benefits the first step reaction. The electrostatic polar charge model and crystallographic characteristics are employed to explain the growth mechanism of AIN nanowires. To our knowledge, acetylene black is the first time to be used for synthesizing 1D AIN nanostructures via carbothermal reduction and nitridation method, which would be an efficient, economical assistant reagent for fabricating nitride nanostructures.

Stability of Phosphonic Acid Self-Assembled Monolayers on Amorphous and Single-Crystalline Aluminum Oxide Surfaces in Aqueous Solution

The formation of octadecylphosphonic acid (ODPA) self-assembled monolayers (SAMs) and their stability in water has been studied on four distinctly different aluminum oxide surfaces. The aim was to improve the understanding of the state of binding between the phosphonic acid to the oxide surface and how this interaction depends on the structure and termination of the oxide surface. Single crystalline Al(2)O(3)(0001) and Al(2)O(3)(1102) surfaces were compared to amorphous oxide passive films on aluminum and physical vapor deposited (PVD) amorphous aluminum oxide films on gold. The monolayers were adsorbed from ethanol solution, characterized by means of high-resolution in situ atomic force microscopy (AFM), contact angle measurements, polarization modulated infrared reflection absorption spectroscopy (PM-IRRAS), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and proved to be self-assembled. On Al(2)O(3)(1102) surfaces and amorphous Al(2)O(3) surfaces, the ODPA self-assembled monolayers showed high stability in aqueous environments. However, the adsorbed ODPA monolayers were substituted by the adsorption of interfacial water on the Al(2)O(3)(0001) surface via the intermediate formation of micelles. The different stability of the monolayers in aqueous environments is explained by the variation of interfacial binding states ranging from ionic interactions between phosphonate groups and the positively charged hydrolytated oxide surface to directed coordination bonds between the phosphonate group and Al ions.

Single-crystal growth of aluminum nitride on 6H-SiC substrates by an open-system sublimation method

Single-crystalline aluminum nitride (AlN) has successfully been grown on 6H-SiC (0 0 0 1) substrates by sublimation using an open-system crucible at 2273 K within 30 h. The thickness of the AlN single-crystal layer is about 1 mm. The dislocation density in the vicinity of the crystal surface has been calculated to be less than 10 7 cm −2 from transmission electron microscopy observation and etch pit density measurement of the crystal. Single-crystal growth of AlN has been carried out by varying supersaturation of Al vapor and employing on- and off-axis SiC substrates. Supersaturation of Al vapor has critically influenced the crystalline quality and morphology, while it has not affected the growth rate so much. Thus, precise control of supersaturation is a key to ensuring the quality of AlN single crystals. The quality of the crystals grown on off-axis SiC substrates is superior to that grown on on-axis SiC substrates. Moreover, the quality has been improved as the thickness of the crystals has increased.

Highly sensitive strained AlN on Si(1 1 1) resonators

In this study, we report two methods to fabricate freestanding single-crystalline aluminium nitride micromechanical device structures on Si(1 1 1) substrates. A method based only on Si etching techniques is demonstrated, and the differences with the conventional process are detailed. The mechanical properties of the released AlN structures are characterized by resonance measurements and micro-Raman spectroscopy. An enhancement of the resonant frequency and quality factor of resonant structures, and thus the sensitivity, is proposed using epitaxial AlN. This high quality material, highly strained due to thermal mismatch with Si substrate, allows the fabrication of improved high sensitivity AlN-based micro/nano electro-mechanical systems and sensors.

Epitaxial growth of aluminum nitride on AlGaN by reactive sputtering at low temperature

We report the synthesis of 1μm thick single crystalline aluminum nitride films by dc magnetron sputtering on AlGaN∕GaN layer grown on sapphire substrate at low temperature (substrate temperature <250°C). The microstructure of c-axis oriented AlN films deposited on Si (100) and AlGaN ⟨0001⟩ substrates was studied by x-ray diffraction, selected area electron diffraction, and transmission electron microscopy. The optimization of process parameters, involving low energetic ion bombardment on film surface (20–30eV) during the growth, leads to an increase in the surface mobility and thus promotes AlN epitaxial growth on AlGaN substrate at 250°C.

Molecular-dynamics simulation of edge-dislocation dynamics in aluminum

The ability of a crystal to exhibit plastic deformation is related to the presence of dislocations in the crystal lattice. The motion of dislocations provides for the formation of a real atomic structure in crystalline solids and the kinetics of deformation of crystals under load; it underlies the control of many important physical properties of solids [1‐3]. It is known that the dependence of the yield stress on deformation rate in many metals sharply increases when the deformation rate exceeds ~10 3 –10 4 s ‐1 . This phenomenon can be interpreted as the consequence of a change in the mechanism of dislocation motion. Moving at low velocities, dislocations overcome obstacles as a result of the joint action of the applied stress and thermal fluctuations. Due to this, an increase in the temperature is accompanied by a decrease in the yield stress of the material. For a high-rate deformation, it is necessary to apply higher stresses. At a deformation rate exceeding a certain threshold ( ~10 4 s ‐1 for pure metals), the acting stresses prove to be sufficient for the dynamic overcoming of obstacles without an additional contribution from thermal fluctuations. In this case, the pumping of the dislocation energy to the crystal lattice vibrations or, depending on temperature, to the electron subsystem becomes the dominating mechanism of the retardation of dislocations. In contrast to the region of thermofluctuational mobility, the dislocation velocity in the dynamic region decreases with temperature in accordance with an increase in the density of the gas of elementary excitations. For this reason, an anomalous increase in the yield stress with increasing temperature is observed for some materials at very high rates of deformation [1]. In this study, we have compared the results of simulation with the values of the dynamic yield stress for single-crystalline aluminum in the shock-wave experiments, which offer a powerful method of studying the properties of materials dynamically loaded under well controllable conditions. The behavior of materials under high-rate deformation in shock-wave experiments is very diverse, which is manifested both in the temperature dependence of the yield stress and in the character of deformation in the samples upon storage [1, 4].

Dislocation Nucleation and Interaction under Nanoindentation in Single Crystalline Al and Cu: Molecular Dynamics Simulations

Recent advances in miniaturization and highly-accurate measurement techniques have allowed mechanical properties to be measured at the nanometer scale. Nanoindentation has been widely used because of its applicability in ambient conditions. Unstable displacement burst or the abrupt growth of indent displacement after homogeneous elastic deformation observed in crystalline materials is a unique plastic deformation characteristic (nanoplasticity). In the present paper, a series of atomistic simulations of nanoindentation in single crystalline aluminum and copper are performed in analyzing the critical state for dislocation nucleation and interaction between dislocations beneath the indenter. With reference to the Hertzian solution based on isotropic linear elastic theory, both the anisotropic effect and nonlinear behavior of nanoindentation are discussed in detail. The discovery was made that the incipient yield process is strongly related to the triaxial stress state created beneath the indenter, and that energetically unfavorable interactions accompanied with cross slip induce the formation of prismatic dislocations.

C17 微細塑性加工による単結晶金属の結晶構造の変化(OS-12 ナノ加工と表面機能(4))

In this paper, the deformation mechanism is examined by nano forming of a single crystalline copper and aluminum specimen. And knife edge tool was indented into crystalline copper or aluminum specimen of various crystal orientations. And change of crystal orientation was measured by EBSD analysis. Spin vectors of crystal rotation were calculated by tensor analysis based on the EBSD data. Results revealed that most of the spin vectors indicates [100] crystal orientation around an indent groove. Next, single crystalline specimen was annealed after nano forming. FIB observation revealed an indent groove. This result indicates controllability of crystalline structure by combination of nano plastic forming and annealing. And the changes of crystal structure were examined.

Nanoindentation-Induced Collective Dislocation Behavior and Nanoplasticity

The present paper summarizes the crystallographic dependence of the displacement burst behavior observed in nanoindentation using two single crystalline aluminum (Al) materials and copper (Cu) with three kinds of surface indices, namely (001), (110) and (111). From the critical indent load at the first burst, the critical resolved shear stresses (CRSSs) of the collective dislocation nucleation were estimated in reference to molecular dynamics (MD) simulations. These are almost one-tenth of the shear modulus, which are close to the ideal values. We explain the nanoplastic mechanics by a comprehensive energy balance model to describe the linear relation between the indent load and the burst width of the first displacement burst and by the nucleation model consisting of three-dimensional discrete dislocations to evaluate the number of dislocations nucleating. The distance between the emitted dislocation loops of Al is found to be fairly large. Thus, Al is expected to exhibit a less tangled network of dislocations just below the indentation than Cu, which has a lower stacking fault energy.

Initial yield process around a spherical inclusion in single-crystalline aluminium

The initial yield process and the subsequent formation of prismatic dislocation loops around a spherical inclusion embedded in a single-crystalline Al matrix are studied by atomistic simulations. In conjunction with linear elastic theory, it is confirmed that the maximum shear stress is created at the inclusion–matrix interface on the {1 1 1} plane intersecting the spherical inclusion at a height of (RP: radius of spherical inclusion). The critical pressure, shear stress and strain for dislocation nucleation are then quantitatively determined. Afterwards, prismatic dislocation loops, which are constructed by four pure edge dislocations with the same Burgers vector but not on the same slip planes, are formed by energetically unstable interactions around the inclusion. Consequently, analytical considerations and atomistic simulation provide a clear explanation of experimental observations and an instructive insight into the precipitation problem.

Nanoplastic deformation of nanoindentation: Crystallographic dependence of displacement bursts

The present paper summarizes the crystallographic dependence of the displacement bursts in nanoindentation using single crystalline aluminum and copper with three kinds of surface indices, namely (0 0 1), (1 1 0) and (1 1 1). It is found that the experimental linear relation between the critical indent load and the burst width of the indent depth at the first displacement burst has significant crystallographic dependence. From the critical indent loads, the critical resolved shear stresses of the dislocation nucleation were estimated to be 3.3 GPa for Al and 3.6 GPa for Cu, using Hertz contact mechanics, which are both close to the ideal values. We explain the nanoplastic mechanics by a comprehensive energy balance model to describe the linear relation between the indent load and the burst width, and by the collective dislocation nucleation model consisting of three-dimensional dislocation loops to evaluate the number of dislocations nucleating. The former model can derive the linear relation qualitatively and link the burst width to the collective dislocation slipping. The latter relates the theoretical critical load and burst width with the function of the punch radius and the distance between dislocations nucleating d D, in which setting d D as about 10 b ( b is the Burgers vector) can fairly represent the Al experimental data.

Low-temperature performance of Al 4B 2O 9 nanowires

In this paper we report on the synthetic investigation of single-crystalline aluminum borate (Al 4B 2O 9) nanowires in large scale by a direct calcination of a precursor powder made of Na 2B 4O 7·10H 2O and Al (NO 3) 3·9H 2O at a low temperature of 850 °C. The nanowires, with the diameter of 20–40 nm and the length up to several micrometers, possess smooth surfaces and uniform sizes along the entire wire. The growth mechanism of the nanowires is attributed to a solid–liquid–solid process, which controls the nanowire morphology.

Single crystalline aluminum nitride films fabricated by nitriding α-Al2O3

Single crystalline aluminum nitride (AlN) is a key material for deep-ultraviolet light emitting devices. In the present study, high-quality single crystalline AlN films have been fabricated by nitriding α-Al2O3 with a precise control of driving force of nitridation reaction. This process provides the ⟨0001⟩-axis oriented AlN film as large as 50.8mm in diameter formed on (0001) and (112¯0) planes of α-Al2O3 substrates. The crystalline qualities of the films have the values of full width at half maximum of rocking curves using (0002) and (101¯0) planes of AlN for tilt and twist components as follows; Tilt=165arcsec and twist=540arcsec in the case of nitriding (0001) plane of α-Al2O3 and tilt=127arcsec and twist=237arcsec in the case of nitriding (112¯0) plane of α-Al2O3.