Single-crystalline Aluminum Research Articles

Understanding the spall behaviors of single crystalline aluminum under double decaying shock loadings: Atomistic simulations and theoretical models

The shock responses of single crystalline aluminum under double decaying shock loadings are systematically studied by atomistic simulations and theoretical models. Two successive spall events in the same region are observed, which were controlled by voids formation-collapse-formation mechanism. The spall strength for the second spallation was significantly reduced and gradually decreased with increasing time interval. The important dynamic properties, such as spall strength and void nucleation threshold, are significantly different and depend on microstructure and temperature. Furthermore, an analytical expression has been developed to quantify the temperature effect on the nucleation threshold for the second spallation. The damage evolution of two successive spall events in the same region is described by a combination of a nucleation threshold model and a nucleation-and-growth model complemented with the voids collapse process. The voids volume fractions from theoretical models and simulations show good agreement, demonstrating that the theoretical models are feasible for describing the nucleation threshold variation and damage evolution under double decaying shock loadings. The spall behaviors and theoretical models provide insights into further understanding the dynamic responses under complex shock loadings.

Novel nanoindentation strain rate sweep method for continuously investigating the strain rate sensitivity of materials at the nanoscale

We introduce a new nanoindentation method to continuously measure the hardness while sweeping through orders of magnitudes of strain rates within a single experiment. While nanoindentation already allows the determination of the strain rate sensitivity of materials by means of strain rate jump tests, these are typically limited to few discrete strain rates. With the new method, the strain rate sensitivity can be measured continuously as a function of the strain rate. Applications to fused silica, Zn-22 %Al superplastic alloy, single crystalline aluminum, various nanocrystalline metals and a palladium-based metallic glass are shown. Besides some discrepancy with the reference measurements, the new method seems only affected by the presence of a strong nanoindentation size effect. Provided this indentation size effect is not excessively large and can be corrected for accurately, the method proves robust, with no suggestion that the direction of the strain rate sweep affects the evaluation of the strain rate sensitivity.

Improved Singum Model Based on Finite Deformation of Crystals with the Thermodynamic Equation of State

The recently published simplified singum model has been improved by using the thermodynamics-based equation of state (EOS) of solids to derive a new interatomic potential based on elastic constants. The finite deformation formulation under hydrostatic load has been used to evaluate the pressure-volume (p-v) relationship for the EOS of a solid. Using the bulk modulus and its derivatives at the free-stress state, one can construct the EOS, from which a new form of interatomic potential is derived for the singum, which exhibits much higher accuracy than the previous one obtained from the Fermi energy and provides a general approach to construct the interatomic potential. The long-range atomic interactions are approximated to be proportional to the pressure. This improved singum model is demonstrated for the face-centered cubic (FCC) lattice of single-crystalline aluminum. The elastic properties at different pressures are subsequently predicted through the bond length change and compared with the available experimental data. The model can be straightforwardly extended to higher-order terms of EOS with better accuracy and other types of lattices.

Homoepitaxial growth of 3-inch single crystalline AlN boules by the physical vapor transport process

Single crystalline aluminum nitride (sc-AlN or AlN) boules with a diameter of 3-inch (Φ76 mm) were successfully prepared by the physical vapor transport (PVT) process. The initial homoepitaxial growth run was performed on an aluminum nitride seed sliced from a Φ51 mm aluminum nitride boule, and diameter enlargement was conducted iteratively via the lateral expansion technique until a Φ76 mm boule was achieved. During the diameter expansion growth runs, the crystal shape transitioned from a hexagonal pyramid to a cylindrical pyramid. After the standard slicing and wafering processes, the as-obtained substrates were characterized by high-resolution X-ray diffraction (HRXRD), preferential chemical etching, and optical spectroscopy. The characterization results revealed that the aluminum nitride substrates showed good crystallinity and excellent UV transparency, although a slight quality deterioration was observed when the crystal size was expanded from Φ51 to Φ76 mm, while the deep-UV (DUV) transparency remained very similar to that of the aluminum nitride seeds. The Φ76 mm aluminum nitride boules obtained in this study are an important milestone towards achieving Φ100 mm (4-inch) aluminum nitride, which are essential for the rapid commercialization of deep-UV optoelectronics and ultra-wide bandgap (UWBG) electronics.

Bending and punching characteristics of aluminum sheets using the quasi-continuum method.

The nano-punching characteristics of single-crystalline aluminum are investigated using the quasi-continuum (QC) method. Four variables (i.e., crystal orientation, workpiece thickness, clearance between the punch and the substrate, and the taper angle of punch) are used to explore their effect during the nano-punching process. The shear stress distribution is used to express the punching effect on the punch and on both sides of the substrates. Besides, fracture strength, residual flash, and the atomic displacement vector are observed and discussed regarding the behaviors of the nano-punching process under various conditions. Based on the results, the Al workpiece with the X[111]Y[-110] orientation presents less lattice resistance during the punching process. Besides, the thickness of the workpiece has a significant effect on the punching quality. Workpieces with thickness values of 5 and 10 Å are more suitable for punching, due to stable loading and unloading stress-displacement curves and less residual flash on the cutting surfaces of these workpieces. In contrast, the effect of clearance has less impact on the punching behaviors of thinner workpieces. However, for thicker workpieces (i.e., 15 and 20 Å), a larger clearance will likely cause more residual flash. Furthermore, the taper angle of the punch should not be larger than 10°, otherwise, it might damage the workpiece and the substrate.

Investigation of the adhesive and abrasive wear mechanisms at the atomic scale using molecular dynamic simulations

In this study, the effect of adhesion on material removal is numerically investigated at the nanoscale level. In this regard, Molecular Dynamics (MD) simulations are conducted to distinguish the contribution of adhesive and abrasive wear mechanisms during a scratching process in terms of the degree of interfacial adhesion and scratching depth. The numerical model simulates the scratching of a flat workpiece made of a single crystalline aluminum using a rigid conical indenter with a blunted spherical tip at four different indentation depths (i.e. 0, 3, 7, and 10 Å). The classical Leonard-Jones interatomic potential is used to mimic the degree of adhesion by varying the adhesion parameter between 5–70% of the aluminum bonding energy. It is shown that, at shallow scratching depths, the contribution of adhesive work to the frictional work is much larger than the ploughing work. On the contrary, the ratio of adhesive to ploughing work reduces by increasing the scratching depth.

Phase relation in Ga-Al-N2 systems and nitrogen solubilities in Ga-Al melts equilibrated with aluminum nitride

The liquid phase epitaxial method has been applied using Ga-Al solutions to grow single-crystalline aluminum nitride (AlN). Understanding the solubility of nitrogen in such solutions is essential for improving the growth process; however, no solubility data have been reported. Therefore, we have developed two methods (a crucible method and a levitation method) that can be used to determine the nitrogen solubility in Ga-Al melts equilibrated with AlN. It was determined that the nitrogen solubilities in the Ga-Al melts were much lower than that in pure molten Al. Using the levitation method, the nitrogen solubility in pure molten Al was measured at 1282 and 1403 K. By combining the data obtained from both methods, the equilibrium constant and standard enthalpy of nitrogen dissolution reaction in molten Al were determined. Thermodynamic analysis of the nitrogen dissolution in Ga-Al melts was also performed to determine the activity coefficients of nitrogen in the melts.

Low loss single crystalline aluminum films obtained on Si (1 1 1) through interfacial modulation

Single-crystalline aluminum (Al) films were grown on Si (111) substrates by molecular beam epitaxy to study the growth mechanism and demonstrate the role of interface modulation. The effect of the growth temperature is demonstrated on a series of 10 nm thick Al films, showing improved surface wetting by lowering growth temperatures. Post-annealing is proved to be effective on the elimination of twins within a thin Al film. The best twin-free Al film in this study was achieved by a two-step method, with a roughness less than 0.2 nm. The transmission electron microscopy results reveal the atomically sharp Al/Si interface. The ellipsometry measurements show an overall reduction on ε2 compared to the Palik’s values, and especially a more than 40% reduction is achieved in the UV regions.

Cavity electro-optic circuit for microwave-to-optical conversion in the quantum ground state

In the development of microwave-to-optical (MO) quantum transducers, suppressing added noise induced by the optical excitation remains a major challenge. Here we report an integrated superconducting cavity electro-optic circuit based on single crystalline thin-film aluminum nitride of ultralow microwave and optical losses. We demonstrate efficient bi-directional MO conversion at milli-Kelvin temperatures, with near-ground state microwave thermal excitation $({\overline{n}}_{\mathrm{e}}=0.09\ifmmode\pm\else\textpm\fi{}0.06)$, despite the peak power of the optical drive exceeding the cooling power of the dilution refrigerator mixing chamber. Our dynamical study further reveals different light-induced noise generation mechanisms and provides crucial guidelines for optimizing electro-optic circuits in future hybrid microwave-optical quantum links.

Luminescence and up-conversion of single crystalline Lu3Al5O12:Pr3+

This work deals with the spectroscopic properties of praseodymium doped single crystalline lutetium aluminum garnet (LuAG:Pr3+). A special focus was set on temperature- and time-dependent spectroscopy. Beyond the well-known down-conversion luminescence of LuAG:Pr3+, also UV-A/B up-conversion luminescence under excitation with a 488 nm laser was thoroughly investigated. Furthermore, the results of the spectroscopic investigations on the single crystalline material were supplemented and compared with measurements on a microscale powder sample.In addition, to the spectroscopic investigations, mechanistic considerations are presented to obtain a closer look at the up-conversion process in LuAG:Pr3+. We promote the thesis of a temperature-dependent energy transfer up-conversion mechanism.

Hydrogen enhanced cracking via dynamic formation of grain boundary inside aluminium crystal

By in situ bending tests inside an environmental transmission electron microscope, we found that from the pre-crack tip of single crystalline aluminium cantilevers, dislocations are emitted and self-organized to build a low angle grain boundary (LAGB), along which the crack propagates more easily in hydrogen atmosphere than in vacuum. Atomistic modelling suggests that the LAGB formed can strongly trap hydrogen atoms, and its cohesive strength can be substantially reduced. These results imply that the hydrogen-induced transgranular crack in a polycrystalline metal can proceed by repeating a process of dynamically forming a hydrogenated LAGB and its subsequent separation in grain.

Aluminum Plasmonics Enriched Ultraviolet GaN Photodetector with Ultrahigh Responsivity, Detectivity, and Broad Bandwidth.

Plasmonics have been well investigated on photodetectors, particularly in IR and visible regimes. However, for a wide range of ultraviolet (UV) applications, plasmonics remain unavailable mainly because of the constrained optical properties of applicable plasmonic materials in the UV regime. Therefore, an epitaxial single‐crystalline aluminum (Al) film, an abundant metal with high plasma frequency and low intrinsic loss is fabricated, on a wide bandgap semiconductive gallium nitride (GaN) to form a UV photodetector. By deliberately designing a periodic nanohole array in this Al film, localized surface plasmon resonance and extraordinary transmission are enabled; hence, the maximum responsivity (670 A W−1) and highest detectivity (1.48 × 1015 cm Hz1/2 W−1) is obtained at the resonance wavelength of 355 nm. In addition, owing to coupling among nanoholes, the bandwidth expands substantially, encompassing the entire UV range. Finally, a Schottky contact is formed between the single‐crystalline Al nanohole array and the GaN substrate, resulting in a fast temporal response with a rise time of 51 ms and a fall time of 197 ms. To the best knowledge, the presented detectivity is the highest compared with those of other reported GaN photodetectors.

Interfacial modulation on single-crystalline aluminum films grown on GaAs by ErAs insertion

Single-crystalline aluminum (Al) films are grown on GaAs (100) substrates by molecular beam epitaxy. The Al/GaAs interface is modified by ErAs insertion, and the Al quality is further improved. The full-width at half-maximum for Al (111) diffraction peak is 0.06°, and the root-mean-square surface roughness is 0.69 nm. In addition, the Al growth orientation can be tuned by ErAs insertion, which is attributed to the smaller lattice mismatch and higher surface symmetry provided by ErAs. The interfacial interdiffusion between Al and GaAs can be suppressed by ErAs as well. The sharp interfaces as well as the high quality of Al are confirmed by cross-sectional transmission electron microscopy studies. The lattice arrangements of the two stable Al/GaAs and Al/ErAs structures are proposed to show that the lattice matching and the interfacial bonding are two main driving forces.

ZnO Nanowires on Single-Crystalline Aluminum Film Coupled with an Insulating WO3 Interlayer Manifesting Low Threshold SPP Laser Operation.

ZnO nanowire-based surface plasmon polariton (SPP) nanolasers with metal–insulator–semiconductor hierarchical nanostructures have emerged as potential candidates for integrated photonic applications. In the present study, we demonstrated an SPP nanolaser consisting of ZnO nanowires coupled with a single-crystalline aluminum (Al) film and a WO3 dielectric interlayer. High-quality ZnO nanowires were prepared using a vapor phase transport and condensation deposition process via catalyzed growth. Subsequently, prepared ZnO nanowires were transferred onto a single-crystalline Al film grown by molecular beam epitaxy (MBE). Meanwhile, a WO3 dielectric interlayer was deposited between the ZnO nanowires and Al film, via e-beam technique, to prevent the optical loss from dominating the metallic region. The metal–oxide–semiconductor (MOS) structured SPP laser, with an optimal WO3 insulating layer thickness of 3.6 nm, demonstrated an ultra-low threshold laser operation (lasing threshold of 0.79 MW cm−2). This threshold value was nearly eight times lower than that previously reported in similar ZnO/Al2O3/Al plasmonic lasers, which were ≈2.4 and ≈3 times suppressed compared to the SPP laser, with WO3 insulating layer thicknesses of 5 nm and 8 nm, respectively. Such suppression of the lasing threshold is attributed to the WO3 insulating layer, which mediated the strong confinement of the optical field in the subwavelength regime.

Ultraviolet to mid-infrared supercontinuum generation in single-crystalline aluminum nitride waveguides.

We demonstrate ultrabroadband supercontinuum generation from ultraviolet to mid-infrared wavelengths in single-crystalline aluminum nitride waveguides. Tunable dispersive waves are observed at the mid-infrared regime by precisely controlling the waveguide widths. In addition, ultraviolet light is generated through cascaded second-harmonic generation in the modal phase-matched waveguides. Numerical simulation indicates a high degree of coherence of the generated spectrum at around the telecom pump and two dispersive waves. Our results establish a reliable path for multiple octave supercontinuum comb generation in single-crystalline aluminum nitride to enable applications including precision frequency metrology and spectroscopy.

Vacancy diffusion coupled discrete dislocation dynamic modeling of compression creep of micro-pillars at elevated temperature

A two-dimensional discrete-continuous scheme (DCS), which couples the motions of dislocations and diffusion of vacancies, is developed. There are three modules in this scheme. The glide and climb of dislocations are performed by the discrete dislocation dynamics (DDD) module, by which the evolution of dislocations is captured and the plastic strain due to dislocation motions is obtained. The diffusion of vacancies is performed by the finite difference method (FDM) module, by which the vacancy concentration field and the inelastic strain due to vacancy diffusion are obtained. The plastic strain by the DDD and the inelastic strain by the FDM, as the eigen-strains, are transferred to the finite element method (FEM) module, by which the stress field is obtained and then transferred back to the DDD module and FDM module to drive dislocation motions and vacancy diffusion. By this vacancy diffusion coupled discrete dislocation dynamics scheme, the compression creep of single crystalline and bi-crystalline aluminum micro-pillars at elevated temperature is modeled. Some interesting creep behaviors are computationally captured. The compression creep of single crystalline and bi-crystalline micro-pillars is size dependent: the creep strain of micro-pillars at higher compressive stress increases with increasing micro-pillar diameter, which is in good agreement with experimental results reported in the literature. This is believed to result from higher dislocation density in those micro-pillars with larger diameter.

Bending Nanoindentation and Plasticity Noise in FCC Single and Polycrystals

We present a high-throughput nanoindentation study of in situ bending effects on incipient plastic deformation behavior of polycrystalline and single-crystalline pure aluminum and pure copper at ultranano depths (< 200 nm). We find that hardness displays a statistically inverse dependence on in-plane stress for indentation depths smaller than 10 nm, and the dependence disappears for larger indentation depths. In contrast, plastic noise in the nanoindentation force and displacement displays statistically robust noise features, independently of applied stresses. Our experimental results suggest the existence of a regime in Face Centered Cubic (FCC) crystals where ultranano hardness is sensitive to residual applied stresses, but plasticity pop-in noise is insensitive to it.

Electromechanical losses in carbon- and oxygen-containing bulk AlN single crystals

Bulk single-crystalline aluminum nitride (AlN) is potentially a key component for low-loss high-temperature piezoelectric devices. However, the incorporation of electrically active impurities and defects during growth of AlN may adversely affect the performance of piezoelectric resonators especially at high temperatures. The electrical conductivity and electromechanical losses in bulk AlN single crystals are analyzed in the temperature range of 300–1200 K with respect to various contents of growth-related impurities in them. For AlN with [O]/[C] ≤ 1, an increase of electrical conductivity due to thermal activation of charge carriers in the temperature range of 850–1200 K has been observed and was determined to be a major contribution to electromechanical losses Q−1 rising up to maximum values of about 10−3 at 1200 K. As the oxygen content in AlN increased, the magnitude and the activation energy of high-temperature electrical conductivity increased. In oxygen-dominated AlN, two major thermally activated contributions to electromechanical losses were observed, namely, the anelastic relaxations of point defects at temperatures of 400–800 K and electrical conductivity at T > 800 K.

Controlled growth of single-crystalline metal nanowires via thermomigration across a nanoscale junction

Mass transport driven by temperature gradient is commonly seen in fluids. However, here we demonstrate that when drawing a cold nano-tip off a hot solid substrate, thermomigration can be so rampant that it can be exploited for producing single-crystalline aluminum, copper, silver and tin nanowires. This demonstrates that in nanoscale objects, solids can mimic liquids in rapid morphological changes, by virtue of fast surface diffusion across short distances. During uniform growth, a thin neck-shaped ligament containing a grain boundary (GB) usually forms between the hot and the cold ends, sustaining an extremely high temperature gradient that should have driven even larger mass flux, if not counteracted by the relative sluggishness of plating into the GB and the resulting back stress. This GB-containing ligament is quite robust and can adapt to varying drawing directions and velocities, imparting good controllability to the nanowire growth in a manner akin to Czochralski crystal growth.

Neutralization of slow helium ions scattered from single crystalline aluminum and tantalum surfaces and their oxides

We investigated the impact of surface oxygen on the ion yield for He+ ions scattered from different single crystalline surfaces in low-energy ion scattering. Initially clean Al(111) and Ta(111) were exposed to molecular oxygen and ion spectra for different oxidation stages and different primary energies were recorded. A comparison of ion yields normalized to the differential scattering cross section as well as experimental factors allows obtaining information about the influence of oxygen on charge exchange processes. The decrease in the ion yield of both metals with exposure cannot be explained by different surface coverages exclusively, but requires the neutralization efficiency to be dependent on the chemical structure of the surface. For Ta, additionally, a different energy dependency of the ion yield obtained in the metal and oxide occurs. The ion yield for O shows in both surfaces a significantly weaker energy dependency than the investigated metals.