Al-rich Conditions Research Articles

First-Principles Study of Atomic Diffusion by Vacancy Defect of the L12-Al3M (M = Sc, Zr, Er, Y) Phase.

Atomic diffusion by the vacancy defect of L12-Al3M (M = Sc, Zr, Er, Y) was investigated based on a first-principles calculation. The point defect formation energies were firstly evaluated. Then, the migration energy for different diffusion paths was obtained by the climbing-image nudged elastic band (CI-NEB) method. The results showed that Al atomic and M atomic diffusions through nearest-neighbor jump (NNJ) mediated by Al vacancy (VAl) were, respectively, the preferred diffusion paths in Al3M phases under both Al-rich and M-rich conditions. The other mechanisms, such as six-jump cycle (6JC) and next-nearest-neighbor jump (NNNJ), were energetically inhibited. The order of activation barriers for NNJ(Al-VAl) was Al3Zr < Al3Y < Al3Er < Al3Sc. The Al3Sc phase had high stability with a high self-diffusion activation barrier, while the Al3Zr and Al3Y phases were relatively unstable with a low self-diffusion activation energy. Moreover, the atomic-diffusion behavior between the core and shell layers of L12-Al3M was also further investigated. Zr atoms were prone to diffusion into the Al3Y core layer, resulting in no stable core-shelled Al3(Y,Zr), which well agreed with experimental observation.

Fluorine and related complexes in α-Al2O3

Using first-principles calculations based on hybrid-density-functional theory, we examine the energetics and electronic structure of fluorine in α-Al2O3. The F atom can be incorporated as an interstitial (Fi) or substitutional impurity on the oxygen site (FO); the latter tends to be lower in energy, particularly under Al-rich conditions. Fluorine on the oxygen site acts as a donor, but for Fermi-level positions high in the bandgap, a negatively charged DX configuration is lower in energy. Fluorine substituting on the Al site is not energetically stable. We also examine complexes between F and hydrogen or carbon, which can easily be unintentionally incorporated during growth or processing. Our calculated defect levels, combined with band alignments, allow us to assess the impact on Al2O3/semiconductor heterostructures. We find that F can passivate oxygen-vacancy related traps in the Al2O3 dielectric. Complex formation with H or C is either ineffective or could even be detrimental.

Investigation of vacancy defects and substitutional doping in AlSb monolayer with double layer honeycomb structure: a first-principles calculation

The experimental knowledge of the AlSb monolayer with double layer honeycomb structure is largely based on the recent publication (Le Qin et al 2021 ACS Nano 15 8184), where this monolayer was recently synthesized. Therefore, the aim of our research is to consequently explore the effects of substitutional doping and vacancy point defects on the electronic and magnetic properties of the novel hexagonal AlSb monolayer. Besides experimental reports, the phonon band structure and cohesive energy calculations confirm the stability of the AlSb monolayer. Its direct bandgap has been estimated to be 0.9 eV via the hybrid functional method, which is smaller than the value of 1.6 eV of bulk material. The majority of vacancy defects and substitutional dopants change the electronic properties of the AlSb monolayer from semiconducting to metallic. Moreover, the MgSb impurity has demonstrated the addition of ferromagnetic behavior to the material. It is revealed through the calculation of formation energy that in Al-rich conditions, the vacant site of VSb is the most stable, while in Sb-rich circumstances the point defect of VAl gets the title. The formation energy has also been calculated for the substitutional dopants, showing relative stability of the defected structures. We undertook this theoretical study to inspire many experimentalists to focus their efforts on AlSb monolayer growth incorporating different impurities. It has been shown here that defect engineering is a powerful tool to tune the properties of novel AlSb two-dimensional monolayer for advanced nanoelectronic applications.

Effect of oxygen vacancies on the electronic structure and dielectric properties of SrAl2O4: A first-principles study

In this study, both the generalized gradient approximation (GGA) and the GGA-1/2 methods were applied to examine the effect of oxygen vacancies on the electronic and optical properties of bulk monoclinic SrAl2O4. Since the monoclinic structure of SrAl2O4 has two non-equivalent sites for oxygen atoms, we can distinguish between two kinds of oxygen vacancies (VO) in this material. These two kinds of VO are labeled VOI and VOII. Our results show that although these two types of vacancies have comparable formation energies, their electronic and optical properties are significantly different. The formation energy of the VO defect under Sr-rich, Al-rich, and O-poor conditions was less than 1 eV, which means that it could exist at high concentrations. Moreover, the results of density of states (DOS) analysis show that the oxygen vacancy had a deep donor defect featuring a fully occupied electronic level located deep in the band gap of SrAl2O4. This electronic level was located at Ev+2.9eV in case of VOI and at Ev+3.3eV in case of VOII. This defect induced a noticeable change in the spectrum of optical absorption of SrAl2O4. Our results show that the formation of oxygen vacancies can significantly enhance the UV absorption-related properties of this material.

Eutectic Formation, V/III Ratio and Controlled Polarity Inversion in Nitrides on Silicon[1

The crystallographic polarity of AlN grown on Si(111) by plasma assisted molecular beam epitaxy is intentionally inverted from N-polar to Al-polar at a planar boundary. The position of the inversion boundary is controlled by a two-step growth process that abruptly changes from Al-rich to N-rich growth conditions. The polarity inversion is induced by the presence of Si, which is incorporated from an Al-Si eutectic layer that forms during the initial stages of AlN growth and floats on the AlN surface under Al-rich growth conditions. When the growth conditions change to N-rich the Al and Si in the eutectic react with the additional N-flux and are incorporated into the solid AlN film. Relatively low levels of Al-Si eutectic formation combined with lateral variations in the Si incorporation lead to nonuniformity in the polarity inversion and formation of surprisingly narrow, vertical inversion domains. The results suggest that intentional incorporation of uniform layers of Si may provide a method for producing polarity engineered nitride structures.

First-principles study of the structural and electronic properties of the clean and O-deficient ZnAl2O4(111) surfaces

In this study, the GGA-PBE method was used to study the structural and electronic properties of the ZnAl2O4(111) surface with and without oxygen vacancies. The ZnAl2O4(111) surface has six possible surface terminations called Zn(O2), Zn(Al), Al(Zn), Al(O2), O2(Zn), and O2(Al). The results obtained in this work show that the surface energy, electronic structure and work function of the ZnAl2O4(111) surface depend greatly on the nature of atoms in the top surface layer. The calculated surface energies indicate that the Zn(O2) termination is the most stable surface termination in O-rich and Al-poor environments, while the Al(Zn) termination is the most stable termination in Al-rich and O-poor environments. On the other hand, our results show that the formation of oxygen vacancies can greatly affect the stability of the ZnAl2O4(111) surface. We found that the O-deficient surface is more stable than the perfect surface in the case of O2(Zn) and O2(Al) surface terminations. In the case of other surface termination, the formation energy of VO is positive under both O-rich and O-poor conditions except the case of the Zn(O2) termination under O-poor and Al-rich conditions. Moreover, we found that the formation of oxygen vacancies can greatly affect the stability, electronic structure, and work function of the ZnAl2O4(111) surface.

Orbital-free density functional theory characterization of the β′−Mg2Al3 Samson phase

Nearly all intermetallic compounds in the Mg-Al phase diagram have been characterized using Kohn-Sham density functional theory (KSDFT), providing insight into their properties. Two compounds with a common chemical formula of $\mathrm{M}{\mathrm{g}}_{2}\mathrm{A}{\mathrm{l}}_{3}$, known as the \ensuremath{\beta} and \ensuremath{\beta}\ensuremath{'} Samson phases, remain challenging for KSDFT to interrogate due to their large unit cells containing partially occupied atomic sites and many hundreds of atoms. The much less expensive orbital-free DFT (OFDFT) is as accurate as KSDFT for Al-Mg alloys, provided one employs a nonlocal kinetic energy density functional. Here, we use nonlocal OFDFT to evaluate properties of the relatively simpler ${\ensuremath{\beta}}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{M}{\mathrm{g}}_{2}\mathrm{A}{\mathrm{l}}_{3}$ phase with a unit cell of 879 atoms, as a first step toward full DFT characterization of these Samson phases. We employ the virtual crystal approximation to treat the potentials associated with the partially occupied atomic sites in the ${\ensuremath{\beta}}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{M}{\mathrm{g}}_{2}\mathrm{A}{\mathrm{l}}_{3}$ unit cell. OFDFT lattice and elastic constants, with pair distribution functions (PDFs) of ${\ensuremath{\beta}}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{M}{\mathrm{g}}_{2}\mathrm{A}{\mathrm{l}}_{3}$ computed via OFDFT molecular-dynamics simulations, are consistent with available experimental data; the PDF is a prediction, as it has not yet been measured. The predicted OFDFT formation energy of ${\ensuremath{\beta}}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{M}{\mathrm{g}}_{2}\mathrm{A}{\mathrm{l}}_{3}$ (derived from the phonon density of states) increases with temperature, consistent with the experimental observation that this phase becomes increasingly unstable at higher temperatures. Using two common metrics of ductility (Pugh's ratio and brittleness), OFDFT-derived ideal tensile and shear strains/stresses of ${\ensuremath{\beta}}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{M}{\mathrm{g}}_{2}\mathrm{A}{\mathrm{l}}_{3}$ and $\mathrm{M}{\mathrm{g}}_{17}\mathrm{A}{\mathrm{l}}_{12}$ (precipitates present at Al/Mg interfaces) reveal that ${\ensuremath{\beta}}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{M}{\mathrm{g}}_{2}\mathrm{A}{\mathrm{l}}_{3}$ should be more ductile than $\mathrm{M}{\mathrm{g}}_{17}\mathrm{A}{\mathrm{l}}_{12}$. The superior ductility of ${\ensuremath{\beta}}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{M}{\mathrm{g}}_{2}\mathrm{A}{\mathrm{l}}_{3}$ suggests that welding of Al to Mg should be done at temperatures where this alloy remains stable and under Al-rich conditions to favor its formation.

First-principles study of the structural, electronic, and optical properties of the clean and O-deficient ZnAl2O4(110) surfaces

In this work, the stability, structural, electronic, and optical properties of the ZnAl2O4(110) surface have been investigated by using the first-principles method based on the density functional theory. The obtained results show that the AlO2-terminated surface is more stable than the ZnAlO2-terminated surface under O-rich and Al-poor conditions, while the ZnAlO2 surface termination is the most stable under O-poor and Al-rich conditions. The results of structural relaxation show that, for both surface terminations, the most noticeable change in the interlayer distances occurs in the four outermost layers only. In addition, our results also show that the electronic structure and properties of these two surfaces are very different. The work function of the ZnAlO2-terminated surface was found to be 2 times smaller than that of the AlO2 surface. Moreover, the effect of oxygen vacancies on the properties of the ZnAl2O4(110) polar surfaces was investigated in details. We find that the formation energy of VO in the case of the AlO2-terminated surface is lower than that of the ZnAlO2-terminated surface. Our results show that the formation of oxygen vacancies affects greatly the electronic and optical properties of the ZnAl2O4(110) surface. We found also that, at high temperatures, the O-deficient AlO2-terminated surface is more stable than the clean surface.

First-principles calculations of threading screw dislocations in AlN and InN

The threading screw dislocation in AlN and InN, two technologically important nitride-based materials, has been studied using first-principles calculations. By performing a comprehensive search for the most stable core configuration, several original core structures have been obtained depending on experimental conditions. In Al-rich and In-rich conditions, screw dislocation cores exhibit a metalliclike character, predicted to provide pathways to leakage currents in III-N based electronic devices. Specifical to AlN, most stable core geometries in N-rich conditions are shown to contain nitrogen atoms organized like nitrogen molecules. Finally, our investigations revealed that in InN dislocation cores are associated to partial or complete closure of the gap and to the injection of electrons into the bulk conduction band.

Mn induced 1 × 2 reconstruction in the τ-MnAl(0 0 1) surface

We report on first principles total energy calculations to describe the structural, electronic and magnetic properties of MnAl(0 0 1) surfaces. We have concentrated in structural models having 1 × 1 and 1 × 2 periodicities, since recent experiments of the similar MnGa(0 0 1) surface have found 1 × 1 and 1 × 2 reconstructions. Our calculations show the existence of two stable structures for different ranges of chemical potential. A 1 × 1 surface is stable for Al-rich conditions, whereas a Mn-induced 1 × 2 reconstruction appears after increasing the Mn chemical potential up to Mn-rich conditions. It is important to notice that experimentally, Mn rich conditions are important for improved magnetic properties. The Mn layers in both structures have ferromagnetic arrangements, but they are aligned antiferromagnetically with the almost no magnetic Al atoms. Moreover, the on top Mn atoms, which produce the 1 × 2 reconstruction, align antiferromagnetically with the second layer Mn atoms. These findings are similar to those obtained experimentally in MnGa thin films grown by molecular beam epitaxy. Therefore, this method could also be used to grow the proposed MnAl films.

Charged vacancy defects in an AlN nanosheet: A first-principles DFT study

Charged vacancy defects in an AlN nanosheet are studied by first principles calculations. Formation energies values at valence band maximum indicate that the nitrogen vacancy is more stable than the aluminum vacancy in both Al and N rich conditions. Under Al-rich and N-rich conditions, aluminum vacancy presents two transition levels in the band gap: from neutral state to a negative charge-state (0/−1), and from a negative charge-state to a double negative charged state (−1/−2). Nitrogen vacancy at low Fermi energies prefers a positive charged state, then it adopts a neutral charged state, and finally it is stabilized in a negative charge-state. Bond lengths of atoms in vicinity of vacancy and electronic structure are affected by charge states. AlN sheet with vacancy defects could present spin-polarized charge states. Magnetism originated by charged vacancy defects in AlN sheets can serve for potential applications in new spintronic devices.

Structural and electronic properties of low-index surfaces of NbAl3 intermetallic with first-principles calculations

The structural, electronic and surface properties of low-index surfaces of tetragonal NbAl3 have been studied with first-principles plane-wave ultrasoft pseudo-potential method based on density functional theory. The atomic relaxations, surface energies and work functions are reported. The calculated atomic relaxations and surface energies suggest that the (111) surface is the most stable stoichiometric surface. Furthermore, the Al-terminated (110) surface is thermodynamically stable than other surfaces in both Al-rich and Nb-rich conditions.

Growth of aluminum nitride on flat and patterned Si (111) by high temperature halide CVD

The aim of the present study is to evaluate the possibility of improving crystalline quality of AlN deposited on Si (111) substrates by implementation of substrate patterning. Growth on flat Si (111) substrates was conducted to investigate influence of deposition parameters on the growth behavior and quality of resulting AlN films. This investigation showed that SiC buffer layer is required to suppress disorientation of the AlN layer, optimum growth temperature is about 1260°C, Al-rich conditions favor more rapid coalescence of the nucleation island while N-rich conditions lead to the formation of a smooth surface. Transposition of the resulting AlN growth recipe on to patterned Si (111) substrates revealed reduction of the stress in AlN, bending of the threading dislocations and formation of a dislocation free area in the overgrown region. For the formation of a continuous layer on silicon pillars, we propose a novel technique based on the decrease of the mean free path of gaseous species leading to localized growth of AlN on the top of the pillars. Such continuous layer exhibit lower crack density compared to that on a flat substrate. Thus, the growth of AlN on pillar patterned Si substrate is seen to be a promising way for the further improvement of the AlN films quality.

First-Principles Thermodynamics Study of Spinel MgAl2O4 Surface Stability

The surface stability of all possible terminations for three low-index (100, 110, 111) structures of spinel MgAl2O4 was studied using a first-principles-based thermodynamic approach. The surface Gibbs free energy results indicate that the 100_AlO2 termination is the most stable surface structure under ultrahigh vacuum at T = 1100 K regardless of an Al-poor or Al-rich condition. With increasing oxygen pressure, the 111_O2(Al) termination becomes the most stable surface in the Al-rich condition. The oxygen vacancy formation is thermodynamically favorable over the 100_AlO2, 111_O2(Al), and (111) structures with Mg/O connected terminations. On the basis of the surface Gibbs free energies for both perfect and defective surface terminations, 100_AlO2 and 111_O2(Al) are the most dominant surfaces in Al-rich conditions under atmospheric conditions. This is also consistent with our previously reported experimental observation.

Effects of phosphoric acid on the surface morphology and reflectance of AlN grown by MBE under Al-rich conditions

Aluminium nitride (AlN) films grown by molecular beam epitaxy (MBE) were exposed to phosphoric acid (H3PO4) heated to 70°C for 10min. The H3PO4 treatment removed excess aluminium (Al) from the surface of AlN grown under Al-rich conditions. A side effect of the H3PO4 process was the formation of hexagonal etch pits up to 180nm in size. Reflectance measurements were performed before and after the removal of excess Al from the film surface. Excess surface Al in the form of droplets was found to strongly influence the reflectance of the AlN films. Consequently the Al surface droplets (AlSD) introduced an error into film thickness values derived from Lorentz oscillator model fitting of the specular reflectance. The film thickness measurement error due to excess surface Al was quantified as a function of the AlSD surface coverage percentage. The effect of AlSD on reflectance-based film thickness measurements must be taken into account when using AlN grown in the Al droplet regime to characterize the nitrogen (N) growth rate.

Structure and energetics ofLaAlO3(001) surfaces

Reconstructions of the ${\mathrm{LaAlO}}_{3}$ (001) surface are studied using hybrid density functional calculations. We calculate surface energies as a function of O and Al chemical potentials for both the LaO- and ${\mathrm{AlO}}_{2}$-terminated surfaces. We find that the $(3\ifmmode\times\else\texttimes\fi{}2)$ Al adatom is the most stable reconstruction on the ${\mathrm{AlO}}_{2}$ termination, with the $(2\ifmmode\times\else\texttimes\fi{}2)$ O vacancy slightly higher in energy. On the LaO termination, the $(3\ifmmode\times\else\texttimes\fi{}2)$ La vacancy is the most stable reconstruction. Overall, the LaO termination with the $(3\ifmmode\times\else\texttimes\fi{}2)$ La vacancy has the lowest surface energy, in the absence of impurities. However, our calculations also indicate that the presence of hydrogen alters the stability of the LaO versus the ${\mathrm{AlO}}_{2}$ termination. Under Al-rich conditions, H binds to an O atom forming a $(2\ifmmode\times\else\texttimes\fi{}1)$ H-adatom reconstruction on the ${\mathrm{AlO}}_{2}$-terminated surface, whereas under La-rich conditions a $(2\ifmmode\times\else\texttimes\fi{}1)$ OH adsorbate reconstruction forms on the LaO-terminated surface. These reconstructions are found to be the most stable terminations over a wide range of H and O chemical potentials.

Growth model investigation for AlN/Al(Ga)InN interface growth by plasma-assisted molecular beam epitaxy for high electron mobility transistor applications

Heterostructures with lattice matched Al(Ga)InN barriers have been widely investigated as alternative to standard AlGaN/GaN based high electron mobility transistor structures for high power applications. Mostly these heterostructures comprise a thin AlN based spacer between GaN channel and lattice matched barrier. One key issue for high quality plasma-assisted molecular beam epitaxy (PA-MBE) of these structures is the control of the AlN–Al(Ga)InN interface since optimal growth conditions for high quality AlN differ significantly from those for growth of indium containing material. In this paper, a detailed analysis and a deduced model of the interface growth is presented. The Al/N ratio during AlN spacer growth is likely to influence the subsequent growth of quaternary Al(Ga)InN. Ideal Al/N ratio leads to high performance heterostructures, while slightly Al-rich conditions lead to the formation of Al residues on the substrate surface, which hinder subsequent epitaxial growth. Al/N ratios below unity lead to the deposition of ternary AlGaN instead of binary AlN spacers and to increased alloy scattering. An insertion of a thin GaN layer between spacer and barrier can hinder the formation of Al residues and leads to improved wafer homogeneity.

Doping and Defect Chemistry in LiCoO2 By First-Principles Calculation

Introduction In order to improve the performance of lithium-ion batteries, doping to electrode active materials is often carried out. The doped ions are extrinsic point defects in the active materials, and thus the doping exerts an influence on native point defects in the materials. Since the point defects are associated with many properties of the active materials, information on the doped ions as well as the native defects is of importance to control the materials. In this study, we made comprehensive first-principles calculations to estimate solubility of various species doped into LiCoO2and to discuss effects of the doping on the defect chemistry. Computation method As the doped species to LiCoO2, several metals such as Na, Mg, and Al, were selected. Both the Li and Co sites were considered as the substitution positions of the doped ions. Vacancies, interstitial cations and antisite cations were examined as the native defects. Defect formation energy was evaluated by first-principles calculations using a supercell consisting of 144 atoms with a single defect. The first-principles calculations were carried out using the plain-wave basis PAW method and the GGA+U exchange correlation functional. Equilibrium defect concentrations were estimated by the Boltzmann distribution under given chemical conditions and charge neutrality. Results and discussion Figure 1(a) illustrates equilibrium defect concentrations in Al-doped LiCoO2 as a function of temperature when LiCoO2 coexists with LiAlO2 and Li5AlO4 under an oxygen pressure of 0.2 atm. Al preferentially substitutes for Co (AlCo) as shown in the figure, and thus this condition can be considered as a nominal composition of Li(Al x Co1-x )O2 with a small amount of excess Li. At this chemical condition, one can expect high Al solubility of ~50% with low concentrations (<1%) of the native defects such as CoLi under usual synthetic conditions (800-900 °C in air). Since defect formation energy and equilibrium defect concentration depend on the chemical condition, more Li-rich and Al-rich conditions were also examined. Figure 1(b) illustrates equilibrium defect concentrations at the more Li-rich condition, where LiCoO2 coexists with Li2O and Li5AlO4. At this condition, the concentration of AlCo is less than 10%. This is because excess Li reacts with Al, resulting in suppression of the Al-doping to LiCoO2. On the other hand, when LiCoO2 coexists with LiAlO2 and CoAl2O4 as the Al-rich condition, the concentration of CoLi becomes ~6% at 1100 K as shown in Fig. 1(c). The Al-rich condition means Li-poor condition, and thus Li deficiency in LiCoO2 is increased. These calculations suggest that the chemical conditions are of great importance to achieve both the increase in the Al solubility and the suppression of the Li deficiency. The solubility and effects on the defect chemistry of other species are discussed at the meeting. Acknowledgement This work was supported by RISING project of NEDO.

Temperature- and Al/N ratio-dependent AlN seed layer formation on (110) Si substrates by using plasma-assisted molecular beam epitaxy

AlN seed layers with a thickness of 50 nm were grown by using nitrogen plasma-assisted molecular beam epitaxy on (110) Si substrates with different V/III ratios in the temperature range from 850 °C to 940 °C. In varying the Al/N ratio and the growth temperature, distinct surface morphologies emerge, which are quite different from those observed in AlN growth on (111) Si substrates. Under N-rich conditions, AlN films exhibits randomly distributed islands with different sizes ranging from 10 nm to 1 m. In Al-rich conditions, two distinct surface morphologies, (1) closely-packed islands preferentially aligned along the [1120]AIN/[110]Si azimuth and (2) smooth flat surfaces, are observed at various growth temperatures. The observed morphology transition is attributed to the asymmetric strain distribution between hexagonal symmetric AlN layers and rectangular (110) Si substrates and to varying surface adatom migration rates present at different growth temperatures.

Stability and electronic properties of carbon in α-Al2O3

The stability and electronic properties of carbon in α-Al2O3 are investigated using density functional theory. In the host lattice, the substitutional C prefers the Al site under the O-rich conditions, whereas the O site is preferred by carbon under the Al-rich conditions. The calculated results predict a direct relationship between the thermodynamic and optical transition levels with the degree of the local distortion induced by C in the alumina lattice. We also find C at the O site acts as a charge compensator to stabilize the F+ center, thereby enhancing the TL signal at 465K. Also, C at Al site can serve as electron traps for TL emission process in α-Al2O3.