Aluminum Vacancies Research Articles

Effects of Sc on the vacancy and solute behaviours in aluminium

Element Sc is a promising candidate for optimizing the high-temperature mechanical properties of Al alloys. In this study, the Sc-solute, Al3Sc-vacancy and Al3Sc-solute interactions in aluminium are investigated extensively by using first-principles calculations. The correlation between the various interaction energies and the solute atomic size, and the Sc-solute compound formation energy has been evaluated. A negative correlation between the first nearest neighbour Sc-solute binding energies and the lowest Sc-solute compound formation energies has been identified, while the second nearest neighbour Sc-solute binding energies increase monotonically with the solute atomic size. Al3Sc precipitates can bind vacancy strongly at the specific atomic site, but their relatively low number density limits their influence on vacancy behaviours during the ageing period shortly after quenching. Compared to the atomic size, the trend for solute segregating at the interface between Al3Sc precipitate and Al bulk is more strongly related to the Sc-solute binding energy. The calculated results can clarify the available experimental observations for Al-Sc, Al-Cu, Al-Mg-Si and Al-Zn-Mg-Cu alloys, and it is hoped to guide the design of high-performance Al alloys.

Direct Evidence on Effect of Oxygen Dissolution on Thermal and Electrical Conductivity of AlN Ceramics Using Al Solid-State NMR Analysis.

Aluminum nitride, with its high thermal conductivity and insulating properties, is a promising candidate as a thermal dissipation material in optoelectronics and high-power logic devices. In this work, we have shown that the thermal conductivity and electrical resistivity of AlN ceramics are primarily governed by ionic defects created by oxygen dissolved in AlN grains, which are directly probed using 27Al NMR spectroscopy. We find that a 4-coordinated AlN3O defect (ON) in the AlN lattice is changed to intermediate AlNO3, and further to 6-coordinated AlO6 with decreasing oxygen concentration. As the aluminum vacancy (VAl) defect, which is detrimental to thermal conductivity, is removed, the overall thermal conductivity is improved from 120 to 160 W/mK because of the relatively minor effect of the AlO6 defect on thermal conductivity. With the same total oxygen content, as the AlN3O defect concentration decreases, thermal conductivity increases. The electrical resistivity of our AlN ceramics also increases with the removal of oxygen because the major ionic carrier is VAl. Our results show that to enhance the thermal conductivity and electrical resistivity of AlN ceramics, the dissolved oxygen in AlN grains should be removed first. This understanding of the local structure of Al-related defects enables us to design new thermal dissipation materials.

Native and radiation induced point defects in AlN and Sc-doped AlN

We have performed first-principles calculations to investigate the electronic structure, configurations, formation, and binding energies of native and radiation induced point defects in pristine and Sc-doped wurtzite AlN. For the native defects, the nitrogen vacancy has the lowest formation energy in $p$-type material while the aluminum vacancy has the lowest formation energy in $n$-type material which is consistent with the previous studies. Several interstitial defect structures were modeled for Al, N, and Sc atoms. The effects of charge state on their relative stability were investigated. The binding energy of Sc with point defects was calculated and found to be dependent strongly on the defect type and charge state. The results obtained are discussed in light of the possible Sc effects on the radiation damage evolution in AlN. Thus the attraction of Sc atom to N vacancy and both Al and N interstitials reduces their mobility and increases Frenkel pair recombination distance.

Atomic and Electronic Structure of the Al2O3/Al Interface during Oxide Propagation Probed by Ab Initio Grand Canonical Monte Carlo.

Identifying the structure of the Al2O3/Al interface is important for advancing its performance in a wide range of applications, including microelectronics, corrosion barriers, and superconducting qubits. However, beyond the study of a few select terminations of the interface using computational methods, and top-down, laterally averaged spectroscopic and microscopic analyses, the explicit structure of the interface and the initial stages of propagation of the interface into the metal are largely unresolved. In this study, we utilize ab initio grand canonical Monte Carlo to perform a physically motivated, unbiased exploration of the interfacial composition and configuration space. We find that at equilibrium, the interface is atomically sharp with aluminum vacancies and propagates in a layer-by-layer fashion, with aluminum excess in the oxide layer at the interfacial plane. Oxygen incorporation, aluminum vacancy formation, and aluminum vacancy annihilation are the building blocks of Al2O3 formation at the interface. The localized interfacial mid-gap states from under-coordinated aluminum atoms from the oxide and the immediate depletion of aluminum states near the Fermi level upon oxygen incorporation prevent oxygen dissolution ahead of the interface front and result in the layer-by-layer propagation of the interface. This is in sharp contrast to the ZrO2/Zr system, which forms interfacial sub-oxides, and also explains the favorable self-healing nature of the Al2O3/Al system. The occupied interfacial mid-gap states also increase the calculated n-type Schottky barrier heights. Additionally, we identify that interfacial aluminum core-level shifts linearly depend on the aluminum coordination number, whereas interfacial oxygen core-level shifts depend on long-range ordering at the interface. The detailed geometric and electronic insights into the interface structure and evolution expand our understanding of this fundamental interface and have important implications for the engineering and design of Al2O3/Al-based corrosion coatings with enhanced barrier properties, controllable transistor technologies, and noise-free superconducting qubits.

Investigation of thermal conductivity and dielectric properties of AlN-based ceramics with ZnO addition by impedance spectroscopy analysis

ABSTRACT The effect of ZnO on phase composition, microstructure, thermal and dielectric properties of AlN ceramics was studied. The X-ray diffraction indicated that ZnAl2O4 and ZnO phases were detected in samples. With 0.5 wt% ZnO added, the thermal conductivity of AlN sample increased to 193.7 W/m·K. Whereas, the thermal conductivity dropped sharply due to excess ZnO. Moreover, the addition of ZnO increased the permittivity and altered the relaxation mechanism of AlN ceramics. The fitting data and activation energy from impedance spectroscopy revealed the variation of defect mechanisms in samples. The addition of 0.5 wt% ZnO reduced the concentration of aluminum vacancy in AlN grains to improve thermal conductivity. Nevertheless, excess ZnO dissolved into the AlN lattice and created additional defects, causing the decrease in thermal conductivity and electrical resistivity for AlN ceramics. Overall, suitable addition of ZnO can effectively increase the thermal conductivity and alter the dielectric properties of AlN ceramics.

Defect enhanced CoMnNiOx catalysts derived from spent ternary lithium-ion batteries for low-temperature propane oxidation

Herein, a feasible strategy was reported to reuse the waste ternary lithium-ion batteries (TLIBs) as precursors to develop efficient CoMnNiOx catalysts for propane oxidation. Interestingly, the T90 value of the defect enhanced-CoMnNiOx catalyst for propane oxidation was only 200 °C. The de-aluminum and de-lithium process generated by the alkaline etching promoted the increase of the oxygen vacancy defects, which was confirmed by ICP-OES, XPS, EPR and DFT calculations. At the atomic level, the oxygen vacancies near the aluminum and lithium vacancies (NiO-Alv-Ov and NiO-Liv-Ov) were more likely to promote the activation of oxygen molecules. Furthermore, it was observed that the reducibility, acidity and lattice oxygen mobility of the defective CoMnNiOx catalyst were boosted by the presence of abundant oxygen vacancy defects, and thus enhancing the catalytic activity. Remarkably, the TLIBs recycling and defect enhancement engineering are conducive for the development of the green and efficient catalysts for VOCs oxidation.

Iron‐Doped Ni−Al Layered Double Hydroxide as an Efficient Oxygen Evolution Reaction Electrocatalyst

AbstractTwo‐dimensional (2D) layered double hydroxide (LDH) is a good alternative to precious metals in terms of oxygen evolution electrocatalysis. The doping of various elements into LDH has been widely investigated in order to further minimize the oxygen evolution overpotential. Herein, by using the cation exchange method, we design and construct a Fe‐doped Ni−Al LDH nanoplatelets (NAF LDH‐25). Fe3+ enter the nickel and aluminum vacancies smoothly as a result of the dual action of corrosion and cation exchange, and iron ion activity is optimized. Meanwhile, strong interaction of Ni and Fe, effect of Al on electronic structure and the structural advantages of two‐dimensional nanoplatelets, as a result, the as‐prepared NAF LDH‐25 exhibits extremely low overpotential (288 mV) at high current density of 100 mA cm−2. The NAF LDH‐25 also showed excellent stability in the 72 h chronopotentiometry test. Therefore, cation exchange strategy provides a new option for the preparation of practical OER catalysts.

Modeling of the Point Defect Migration across the AlN/GaN Interfaces-Ab Initio Study.

The formation and diffusion of point defects have a detrimental impact on the functionality of devices in which a high quality AlN/GaN heterointerface is required. The present paper demonstrated the heights of the migration energy barriers of native point defects throughout the AlN/GaN heterointerface, as well as the corresponding profiles of energy bands calculated by means of density functional theory. Both neutral and charged nitrogen, gallium, and aluminium vacancies were studied, as well as their complexes with a substitutional III-group element. Three diffusion mechanisms, that is, the vacancy mediated, direct interstitial, and indirect ones, in bulk AlN and GaN crystals, as well at the AlN/GaN heterointerface, were taken into account. We showed that metal vacancies migrated across the AlN/GaN interface, overcoming a lower potential barrier than that of the nitrogen vacancy. Additionally, we demonstrated the effect of the inversion of the electric field in the presence of charged point defects and at the AlN/GaN heterointerface, not reported so far. Our findings contributed to the issues of structure design, quality control, and improvement of the interfacial abruptness of the AlN/GaN heterostructures.

First-principles study of Ti-doped sapphire. II. Formation and reduction of complex defects

Eliminating the effect of residual infrared absorption in titanium sapphire remains a crucial task to fulfill, despite that this kind of laser crystal has been developed for decades. The ${\mathrm{Ti}}^{3+}\text{\ensuremath{-}}{\mathrm{Ti}}^{4+}$ (${\mathrm{Ti}}_{\mathrm{Al}}^{0}\text{\ensuremath{-}}{\mathrm{Ti}}_{\mathrm{Al}}^{1+}$) pair model is the most widely accepted explanation to this residual absorption, but theoretical analyses based on first-principles calculation have not yet depicted a clear picture for the variation of the ${\mathrm{Ti}}_{\mathrm{Al}}^{0}\text{\ensuremath{-}}{\mathrm{Ti}}_{\mathrm{Al}}^{1+}$ pair and a variety of other potentially important defects in titanium sapphire when fabricating conditions change. Here, we extend the work in [W. Jing, M. Liu, J. Wen, L. Ning, M. Yin, and C.-K. Duan, Phys. Rev. B 104, 165103 (2021)] about binding tendencies of ${\mathrm{Ti}}_{\mathrm{Al}}\text{\ensuremath{-}}{\mathrm{Ti}}_{\mathrm{Al}}$ pairs and optical transition properties to providing a comprehensive understanding of the formation and processing condition dependence of various potentially significant defects and complexes besides the (${\mathrm{Ti}}_{\mathrm{Al}}\text{\ensuremath{-}}{\mathrm{Ti}}_{\mathrm{Al}}{)}^{0,1+}$ pairs. Apart from the complexes composed of ${V}_{\mathrm{Al}}^{3\ensuremath{-}}$ and ${\mathrm{Ti}}_{\mathrm{Al}}^{0,1+}$ defects, two new significant negatively charged defects, ${({V}_{\mathrm{Al}}\text{\ensuremath{-}}{\mathrm{Al}}_{i}\text{\ensuremath{-}}{V}_{\mathrm{Al}})}^{3\ensuremath{-}}$ and ${({V}_{\mathrm{Al}}\text{\ensuremath{-}}{\mathrm{Ti}}_{i}\text{\ensuremath{-}}{V}_{\mathrm{Al}})}^{2\ensuremath{-}}$ are revealed. These two complexes are correspondingly an interstitial ${\mathrm{Al}}^{3+}$ and ${\mathrm{Ti}}^{4+}$ surrounded by two neighboring aluminium vacancies ${V}_{\mathrm{Al}}^{3\ensuremath{-}}$. We show that ${V}_{\mathrm{Al}}^{3\ensuremath{-}}$ plays the key role to stabilize ${\mathrm{Ti}}_{\mathrm{Al}}^{0}\text{\ensuremath{-}}{\mathrm{Ti}}_{\mathrm{Al}}^{1+}$ and form various stable complexes with ${\mathrm{Ti}}_{\mathrm{Al}}^{1+}$ and ${\mathrm{Ti}}_{\mathrm{Al}}^{0}\text{\ensuremath{-}}{\mathrm{Ti}}_{\mathrm{Al}}^{1+}$ in weak and moderate reductive atmospheres. Thus, besides annealing at strong reductive atmosphere at elevated temperatures, Al ion injection or annealing in Al vapor is a potential method to eliminate the harmful residual infrared absorption, which is pinpointed at the reduction of the concentration of ${V}_{\mathrm{Al}}^{3\ensuremath{-}}$ and its variant ${({V}_{\mathrm{Al}}\text{\ensuremath{-}}{\mathrm{Al}}_{i}\text{\ensuremath{-}}{V}_{\mathrm{Al}})}^{3\ensuremath{-}}$. While in extremely strong reductive atmosphere, isolated Ti substitution dominates over those complexes containing ${V}_{\mathrm{Al}}^{3\ensuremath{-}}$, but there still remains a tiny amount of ${\mathrm{Ti}}_{\mathrm{Al}}^{0}\text{\ensuremath{-}}{\mathrm{Ti}}_{\mathrm{Al}}^{1+}$ and ${\mathrm{Ti}}_{\mathrm{Al}}^{1+}$, which are charge compensators to balance the charge of trace amount ${\mathrm{Ti}}_{\mathrm{Al}}^{1\ensuremath{-}}$ further reduced from ${\mathrm{Ti}}_{\mathrm{Al}}^{0}$. And this effect can be further eliminated by lower temperature reductive-atmosphere annealing. In addition, we obtain a simple numerical expression to predict the achievable figure of merit when the concentration of ${\mathrm{Ti}}_{\mathrm{Al}}^{1+}$ is given. Formation energies for simple defects and binding energies for complexes obtained in this work may serve as the basis for simulations and design various quenching and annealing processes to further reduce harmful defect species in titanium sapphire.

Differences of Short Straight-Chain Monoalcohols in the Value-Added Conversion of Furfural Catalyzed by Zr3Al1-MMO: Effect of Hydroxyl Position and Carbochain Length

We synthesized a catalyst of zirconium–aluminum bimetallic oxides (Zr3Al1-MMO) by microwave and investigated the effect of alcohol solvents on furfural valorization. Zr3Al1-MMO had high catalytic activity under mild conditions (80–140 °C). The hydroxyl position of the alcohol sources had a decisive influence on the reaction type. Furfural would condense with the primary alcohols to form high-carbon compounds, and the secondary alcohols prefer to act as a hydrogen donor to promote the transfer hydrogenation process to produce furfuryl alcohol. As the length of the carbon chain of alcohol solvent increased, both types of reactions showed a decrease in yield, which can be attributed to solvent polarity and steric hindrance. Reaction kinetics results verified the above conclusions. Combined with characterization results, we believed that the acid–base sites provided by the zirconium, aluminum, and oxygen vacancies in the catalyst were the main active sites. Through stability and recyclable experiments, Zr3Al1-MMO further confirmed its potential for industrial applications.

Bionic hierarchical porous aluminum nitride ceramic composite phase change material with excellent heat transfer and storage performance

In this paper, we synthesized bionic bone hierarchical porous AlN ceramics which encapsulate polyethylene glycol2000 (PEG2000) to form a new composite phase change material. Argon adsorption and SEM demonstrate that the AlN ceramic has hierarchical structure with most probable diameters of 2.4 nm and 27.3 nm. Raman spectroscopy show that no aluminum vacancies are formed inside the material, which effectively reduce the probability of phonon scattering and improve the thermal performance. EDS and XRD demonstrate that the main crystalline phase of the ceramics is AlN with a small amount of impurity peaks. The thermal conductivity of AlN ceramics with a porosity of 75.2% and 67.5% are 5.10 and 13.86 W/m·K, and that of the composite material is 17.16 W/m·K. The latent heat of melting of the composite material is 88.73 J/g and the melting point is 54.75 °C. The material breaks through the defects of slow dynamic response time and low thermal conductivity of traditional energy storage materials.

Direct measurements of space-charge-regions in individual nanometer-scale crystals of Al2O3-rich magnesium-aluminate-spinel using off-axis electron holography and electron energy-loss spectroscopy

Space charge regions (SCR) in ionic solids, a result of an uneven distribution of point defects, alter their thermodynamic, transport, mechanical, and optical properties.We examine the formation of SCR in nanometer-scale sized crystals of magnesium-aluminate-spinel with excess Al2O3, MgO·1.27Al2O3. For this composition, the expected dominant point defects are anti-sites and cation vacancies.We determine the types, distribution and formation energies of charged point defects that form the SCR. Off-axis electron holography and electron energy-loss spectroscopy enabled to measure the electrostatic potential and distribution of Al·Mg anti-site charged point defects, respectively in individual crystals.We extract the width of the SCR, LSCR, between 1.7 and 2.1 nm. We measured the formation energy of charged vacancies following two extreme states, only aluminum or magnesium vacancies resulting in EfV″′Al=1.09±0.16eV and EfV″Mg=1.04±0.19eV, respectively.For crystals considerably larger than LSCR, V'''Al and VʺMg cation vacancies are the majority species at the particle core resulting in a negative charge density. When approaching the surface of the crystal, the space charge region is richer in Al·Mg defects.Finally, we examine the SCR as the crystal size is reduced, approaching LSCR. Measurements at particle cores of these crystals results in a positive charge due to the overlap of the SCR.

Residual oxygen driven defect mediated room temperature magnetism in dilute nitrogen incorporated amorphous Al-N-O alloy thin film

In the present work, the effect of residual oxygen during reactive sputtering of Al target in a dilute nitrogen environment has been investigated. We have deposited a thin film by reactive sputtering of high purity Al target (99.999%) using a mixture of ultra-high pure Ar & N2 gas in a percentage ratio of 95% and 5%, respectively. The characterization of as-deposited film using Grazing Incidence X-ray Diffraction (GIXRD), X-ray Photoelectron Spectroscopy (XPS), and Energy-Dispersive X-ray (EDX) spectroscopy reveals that in dilute nitrogen environment Al preferred to react with residual oxygen to form amorphous Al2O3 (a-Al2O3) along with aluminum vacancies (AlV). The dilute nitrogen incorporated in the film occupies the AlV sites in Al2O3 and bonded with nearby oxygen to form Al(NOy)x phase. The bulk of the film mainly consists of Al(NOy)x phase, which decreases as we move towards the film surface. At close to the film surface the oxygen vacancies are significantly present. Magnetic and electrical transport properties measurement of the film shows room temperature (RT) magnetism along with n-type semiconducting behaviour. The origin of RT magnetism and observed transport properties have been investigated using a first-principle calculation based on density functional theory (DFT). The residual oxygen driven defects present in the film are found to play a critical role and discussed in detail. The current study is significant in the sense that it provides a new insight to explore the potentials of Al-N-O alloy-based materials as an n-type magnetic semiconductor at room temperature by manipulating defects and optimizing nitrogen concentration.

Physico-chemical model for dopant charge state conversion during Ti:sapphire crystal growth

An original physico-chemical model is proposed that accounts for the conversion mechanism between Ti3+ and Ti4+ during Ti:sapphire crystal growth process. This model involves release of O2 from the crystal surface, which consumes aluminum vacancies. Consequently, these vacancies diffuse from the bulk crystal toward the surface, leading to the conversion of Ti4+ into Ti3+. A set of mass balance differential equations is established for these chemical species, taking into account diffusion and chemical reaction. The boundary condition for concentration of aluminum vacancies on crystal surface is given as the function of PO2 and temperature. A preliminary numerical simulation is performed to study the proposed model, using COMSOL Multiphysics® software. The obtained radial concentration distribution profiles of Ti3+ and Ti4+ are in qualitative agreement with the experimental results, showing that the proposed model has potential for further studying point defect reactions during crystal growth of ionic crystals.

Modeling the spatial control over point defect spin states via processing variables

Contemporary models that are used to search for solid-state point defects for quantum-information applications tend to focus on the defect’s intrinsic properties rather than the range of conditions in which they will form. In this work, a first-principles based multi-scale device model is used to explore how the conditions (i.e., growth temperature, doping concentration, unintentional impurity concentration) influence the formation of a neutral aluminum vacancy complexed with an oxygen impurity at a neighboring nitrogen site vAl-1ON in an Si/Mg:AlN homojunction. Varying the donor (Si) concentration is predicted to lead to the greatest change in both the maximum height and shape of the (vAl-1ON)0 profile. The shape is found to depend on the acceptor (Mg) concentration as well, and a critical ratio between the acceptor and unintentional impurities below which the (vAl-1ON)0 center would not form was identified. A detailed analysis of the electrostatic potential, electric field, and defect chemistry obtained with the model was used to reveal the underlying causes of these changes. These results show the potential of varying processing parameters to manipulate the local electronic structure as a means to control the properties of point defects for quantum-information applications.

Mapping the Structure of Oxygen-Doped Wurtzite Aluminum Nitride Coatings from Ab Initio Random Structure Search and Experiments.

Machine learning is changing how we design and interpret experiments in materials science. In this work, we show how unsupervised learning, combined with ab initio random structure searching, improves our understanding of structural metastability in multicomponent alloys. We focus on the case of Al-O-N alloys where the formation of aluminum vacancies in wurtzite AlN upon the incorporation of substitutional oxygen can be seen as a general mechanism of solids where crystal symmetry is reduced to stabilize defects. The ideal AlN wurtzite crystal structure occupation cannot be matched due to the presence of an aliovalent hetero-element into the structure. The traditional interpretation of the c-lattice shrinkage in sputter-deposited Al-O-N films from X-ray diffraction (XRD) experiments suggests the existence of a solubility limit at 8 at % oxygen content. Here, we show that such naive interpretation is misleading. We support XRD data with accurate ab initio modeling and dimensionality reduction on advanced structural descriptors to map structure-property relationships. No signs of a possible solubility limit are found. Instead, the presence of a wide range of non-equilibrium oxygen-rich defective structures emerging at increasing oxygen contents suggests that the formation of grain boundaries is the most plausible mechanism responsible for the lattice shrinkage measured in Al-O-N sputtered films. We further confirm our hypothesis using positron annihilation lifetime spectroscopy.

Effect of stress on vacancy formation and diffusion in fcc systems: Comparison between DFT calculations and elasticity theory

This paper discusses the effect of stress on the solubility and diffusivity of vacancies using the elasticity theory of point defects. To support the discussion, results are compared with DFT calculations to verify model accuracy. The particular case of vacancies in aluminum is discussed in detail (DFT-elasticity), while three other metallic fcc systems – Ni, Cu and Pd – are discussed through the elasticity approach only. Different types of loading were considered: hydrostatic, multi-axial and shear stresses. In the case of a uni-axial loading, two different directions were investigated: the first along a main crystallographic direction, i.e. [001], and the second perpendicular to the dense plane (111). In order to quantify the effect of stress on diffusivity, the diffusion coefficient of each configuration was expressed for further calculations. By analyzing the symmetry break during the loading process, non-equivalent atomic jumps were identified and diffusion equations obtained. A multi-physic approach was carried out by combining first-principles calculations, to study atomic-scale processes, and a multi-state formalism, to obtain exact diffusion equations. Results show that elasticity accurately captures the effects of stress on vacancy diffusion and solubility and an application method is presented.

Below bandgap photoluminescence of an AlN crystal: Co-existence of two different charging states of a defect center

The below bandgap optical transitions of an aluminum nitride (AlN) crystal grown on a tungsten (W) substrate by physical vapor transport (PVT) are investigated by below-bandgap-excited photoluminescence (PL) spectroscopy and first-principles calculations. Oxygen (O) is the only impurity in the AlN-on-W crystal grown by PVT. By analyzing the excitation-power-, excitation-photon-energy-, and temperature-dependence of the PL spectra, the emission peaks of defect complexes involving aluminum vacancy (VAl) and substitutional oxygen (ON) with different spatial and atomic configurations, i.e., VAl–ON and VAl–2ON with ON featuring axial or basal configurations, are identified. It is revealed that two different charging states coexist in thermal equilibrium for each configuration of VAl–ON complexes. The optical transitions between the conduction band and (VAl–ON)2− and/or (VAl–2ON)1− contribute the UV emissions and those between the valence band and (VAl–ON)1− or (VAl–2ON)0 contribute the red emissions.

Solute-vacancy clustering in aluminum

We present an extensive first-principles database of solute-vacancy, homoatomic, heteroatomic solute-solute, and solute-solute-vacancy binding energies of relevant alloying elements in aluminum. We particularly focus on the systems with major alloying elements in aluminum, i.e., Cu, Mg, and Si. The computed binding energies of solute-vacancy, solute-solute pairs, and solute-solute-vacancy triplets agree with available experiments and theoretical results in literature. We consider physical factors such as solute size and formation energies of intermetallic compounds to correlate with binding energies. Systematic studies of the homoatomic solute-solute-vacancy and heteroatomic (Cu, Mg, or Si)-solute-vacancy complexes reveal the overarching effect of the vacancy in stabilizing solute-solute pairs. The binding energy database presented here elucidates the interaction between solute cluster and vacancy in aluminum, and it is expected to provide insight into the design of advanced Al alloys with tailored properties.

Photoluminescence efficiency of Al-rich AlGaN heterostructures in a wide range of photoexcitation densities over temperatures up to 550 K

Time-resolved photoluminescence and light-induced transient grating techniques were applied for temperature and excitation dependent internal quantum efficiency and carrier diffusion investigation in silicon-doped epilayer and AlxGa1-xN MQWs (x > 0.6). Decrease of photoluminescence efficiency at high excitations correlated with increase of diffusion coefficient as well as nonradiative recombination rate, indicating excitation and temperature dependent localized exciton density reduction. Excitation-dependent lifetime decrease was less pronounced than the corresponding drop of the time-integrated photoluminescence efficiency dominated by thermal dissociation of excitons. At lowest excitations excitons were found captured to vacancy complexes. With increase of excitation exciton ionization appeared, leading to enhanced nonradiative recombination of holes on aluminum vacancies and simultaneous droop of PL efficiency due to saturated bimolecular free carrier plasma recombination. At initial recombination stages and high excitations diffusive recombination on dislocations was also revealed. Modelling provided free exciton binding energies of 104, 140 meV in the MQWs and in the layer; exciton localization energies in 12-35 meV range; localized exciton lifetimes in 2-4 ns range; free exciton and carrier radiative recombination rate coefficients of rex = (0.6 0.2)109(T/300)-1 1/s and Brad = (7 1)10-10(T/300)-3/2 cm3/s, respectively. Vacancy complex capture cross section for excitons was determined to be = (2 1)10-16(300/T)2 cm2. Electron and hole capture cross sections to aluminum vacancy were modeled (1.5 1)10-13 cm2 and (7 1)10-13 cm2, respectively. Dislocation Coulomb radius for free carrier recombination was found to be 12-15 nm.