Structure Of Aluminum Nitride Research Articles

Electronic Structure of Aluminium Nitride and Its Solid Solutions with Oxygen and Aluminium

Electronic Structure of Aluminium Nitride and Its Solid Solutions with Oxygen and Aluminium

Laser Processed Aluminum Nitride Structures for Enhanced Two-Phase Cooling

Laser Processed Aluminum Nitride Structures for Enhanced Two-Phase Cooling

Structural and electrical correlation in aluminum nitride thin films grown by plasma enhanced atomic layer deposition as interface insulating layers on silicon carbide (4H-SiC)

(0001) oriented aluminum nitride (AlN) thin films have been grown by plasma enhanced atomic layer deposition (PE-ALD) on silicon carbide (4H-SiC) substrates. During different PE-ALD processes, the ammonia (NH3) plasma pulsing time has been varied and its effect on the microstructure and on the orientation degree of the AlN layers has been monitored. Structural characterization by Transmission Electron Microscopy (TEM) showed that the crystalline structure of the deposited films was strongly dependent on the NH3-plasma pulsing, so that different polymorphic structures were observed. In particular, both processes resulted in wurtzite AlN structure for few nanometers at the interface with the 4H-SiC substrate, while upon increasing thickness a poly-crystalline wurtzite phase was obtained by short-pulse NH3-plasma, whereas longer plasma exposure resulted in a mixture of wurtzite and zincblende defective phases. Phase formation mechanism were discussed and electrical nanoscopic characterization by conductive atomic force microscopy showed a clear correlation between the different AlN crystalline phases and the insulating properties.

Self-propagating high-temperature synthesis and sintering of AlN‒Y<sub>2</sub>O<sub>3</sub>‒Al<sub>2</sub>O<sub>3</sub> compositions

The results of studies on the synthesis of aluminum nitride and AlN‒Al2O3‒Y2O3 compositions with a low oxygen impurity content by the SHS method are presented. The dependence of the lattice parameter c of aluminum nitride on the oxygen impurity content is shown. The conditions for the synthesis of AlN‒Y2O3 compositions with a low oxygen content in the structure of aluminum nitride are determined. The dependence of the composition of oxide phases in compositions on the content of yttrium oxide in the reaction mixture is shown. The optimal content of yttrium oxide in the compositions has been determined. The results of sintering AlN‒Al2O3‒Y2O3 composite powders with different oxygen content in the aluminum nitride structure are shown.

Seeded Growth of AlN Layers on Al‐ and N‐Polar AlN Seeds by the Cu–Al–Ca Solution Growth Method

Herein, the growth behavior of aluminum nitride (AlN) layers on the Al‐polar and N‐polar surfaces of AlN seeds is investigated by the Cu–Al–Ca solution growth method. AlN layers with a thickness of 50–60 μm grow on N‐polar seeds in 5 h, whereas the thickness of the AlN layer on the Al‐polar seeds reaches approximately 35–50 μm. Optical microscopy, X‐ray diffraction, high‐resolution X‐ray diffraction (HRXRD), Raman spectroscopy, and photoluminescence (PL) spectroscopy are performed to investigate the morphology and crystal quality of the grown AlN layers. The N‐polar and Al‐polar surfaces show hillock growth, which is consistent with the hexagonal structure of AlN. However, the surfaces show differences in terms of hillock size (smaller on the N‐polar surface). The HRXRD rocking curves of (002) and (102) show full‐width at half‐maximum (FWHM) values of 160.25 and 33.53 arcsec, respectively. From Raman analysis, the FWHM values of the AlN layers on the N‐polar and Al‐polar seeds are 6.1 and 10.6 cm−1, respectively. The PL results show 310, 460, and 590 nm emission peaks of the AlN layer, which may be caused by oxygen impurity and VN (nitrogen vacancy) and Ali (aluminum gap).

Characterization of Structure, Morphology, Optical and Electrical Properties of AlN–Al–V Multilayer Thin Films Fabricated by Reactive DC Magnetron Sputtering

Composite thin films of the AlN–Al–V type, grown by magnetron sputtering, were analyzed by several complementary diagnostic methods. The power of the magnetron was used as a variable parameter, while gas flows, chamber pressure, and substrate temperature remained unchanged during the film fabrication. According to grazing incidence X-ray diffraction (GIXRD) results, in most cases, it was possible to obtain an (002)-oriented aluminum nitride (AlN) layer in the films, although, with an increase in the magnetron power to 800 W, the formation of X-ray amorphous AlN was observed. Similarly, according to the Raman results, the width of the peak of the vibrational mode E1, which characterizes the correlation length of optical phonons, also significantly increased in the case of the sample obtained at 800 W, which may indicate a deterioration in the crystallinity of the film. A study of the surface morphology by atomic force microscopy (AFM) and scanning electron microscopy (SEM) showed that the AlN film grows in the form of vertically oriented hexagons, and crystallites emerge on the surface in the form of dendritic structures. During the analysis of the AFM roughness power spectral density (PSD-x) functions, it was found that the type of substrate material does not significantly affect the surface roughness of the AlN films. According to the energy–dispersive X-ray spectroscopy (SEM-EDS) elemental analysis, an excess of aluminum was observed in all fabricated samples. The study of the current-voltage characteristics of the films showed that the resistance of aluminum nitride layers in such composites correlates with both the aluminum content and the structural imperfection of crystallites.

Investigation of the atomistic behavior in nanofinishing single-crystal aluminium nitride with hydroxyl radical ∙OH environment

The effect of depth of pressing and sliding velocity on chemical reaction, wear and atom removal mechanisms was investigated in nanofinishing single-crystal aluminum nitride with hydroxyl radical ∙OH environment. The simulation results show that aluminum nitride can be oxidized by hydroxyl radical ∙OH from the perspective of free energy change. Only chemical action, the decomposed atoms in the solution form bonds with the Al atoms on the aluminum nitride surface, and the bonds are expressed in the form of O-[Al]n, H-[Al]n and OH–[Al]n. However, no Al atoms were discovered to break away from the substrate. The potential energy of substrate and the number of Al-O bonds are positively correlated with the depth of pressing and sliding velocity, and the degree of chemical reaction can be promoted by strong mechanical behavior. The tangential force relies on interfacial Al-C and AlOC bonds, the more interfacial bonds between substrate and abrasive, the more severe wear of the substrate surface. The sliding of diamond abrasive leads to the amorphous evolution of the crystal structure, and the separation of atoms from the initial position produces defects, which destroys the stable covalent bond structure of aluminum nitride. As a result, the total number of dangling bonds (i.e. unsaturated bond) of surface layer atoms increases, and the greater the depth of pressing and velocity, the more obvious this phenomenon will be, and the more atoms will be removed. In this study, atomic removal behavior of aluminum nitride was systematically analyzed at atomic scale, which will provide theoretical guidance for ultra-precision and low-damage processing of aluminum nitride.

Stimulated emission mechanism of aluminum nitride

Photoluminescence and stimulated emission spectroscopies were performed on transparent aluminum nitride (AlN) substrates grown by hydride vapor-phase epitaxy. The stimulated emission was observed from cryogenic to room temperatures and its origin was assigned on the basis of the spontaneous emission spectra and existing theories. Two stimulated emission mechanism crossovers were also found. One was a temperature-induced crossover from a purely excitonic mechanism to a carrier-involved mechanism, which is a universal behavior in semiconducting materials. The other was an excitation-power--induced crossover from an exciton--exciton scattering mechanism to a phonon-mediated mechanism, which is a unique behavior attributed to the peculiar excitonic structure (negative crystal-field splitting and positive electron--hole exchange interaction energies) of AlN.

Effect of the Combustion Temperature of Al + AlN Mixtures on the Content of Oxygen Dissolved in the Structure of Aluminum Nitride

Effect of the Combustion Temperature of Al + AlN Mixtures on the Content of Oxygen Dissolved in the Structure of Aluminum Nitride

Ab initio study on electronic structure and magnetic properties of AlN and BP monolayers with Ti doping

The spintronic properties of Aluminum Nitride (AlN) and Boron Phosphide (BP) monolayers with Titanium (Ti) doping have been studied by using first principles calculation. The Ti-doped AlN monolayer will produce the magnetic moment of 1 μ B , which is mainly contributed by d orbital of the impurity. Although Ti P (Phosphorus was replaced by Ti) can lead to semi-metallic properties in BP monolayer, Ti B (Boron was replaced by Ti) has a lower formation energy. The magnetic moment produced by Ti B is 1 μ B , which is also mainly contributed by the Ti- d orbital. The strain studies show that the band gap of AlN monolayer will decrease with the increase of strain, while the pristine BP monolayer increases. Ti B will make the band gap of BP increase first and then decrease when it is 10%. Studies on magnetic coupling indicate that ferromagnetic and antiferromagnetic coupling oscillations will occur at different Ti-Ti distances in the AlN monolayer. However, Ti B -Ti B are always coupled antiferromagnetically at each distance in the BP monolayer. Further study shows that the Néel temperature of the BP system with Ti B may be higher than room temperature. • Applying strain can regulate the band gap of AlN and BP materials regularly. • The magnetic coupling between Ti atoms in AlN monolayer will oscillate regularly. • The results show that Ti P will make BP monolayer produce semi-metallic properties. • The study found that Ti B -doped BP has a Néel temperature above room temperature.

Heat dissipation in 3D printed cellular aluminum nitride structures

The improvement of heat dissipation in electronic and energy devices is a challenge that can be addressed through the use of highly porous materials. Presently, the additive manufacturing of 3D aluminum nitride is described, and different lattice patterns with porosities in the range 45–64 % are achieved by direct ink writing. All the structures are robust and the effective thermal conductivity (keff) for cuboid structures decreases by 50–75 % with the filament separation and shows anisotropic characteristics, since keff along the longitudinal axis of the scaffold is up to six times greater than for the transversal one. Heat transfer during free cooling experiments for cuboid and cylinder scaffolds, after rapid heating at temperatures above 1000 °C, takes place by radiation for temperatures >500 °C and by convection through the complete cooling process. The heat dissipation time constants of both processes decrease almost linearly with the designed scaffold parameters of porosity and rod separation.

Long-range electron-hole exchange interaction in aluminum nitride

To resolve the discrepancies in the exciton fine structure of aluminum nitride (AlN), polarization- and angle-resolved photoluminescence (PL) spectroscopies are performed. The excitonic PL spectra strongly depend on the optical polarization and detection angle. We propose that both the long-range and short-range electron-hole exchange interaction should be used to interpret the luminescence spectra. The theoretical framework fully explains the present and previous experimental results. The large longitudinal-transverse splitting energy obtained in this study suggests that AlN has strong light-matter coupling without quantum-confined structures.

Local Electronic Structure in AlN Studied by Single-Crystal 27Al and 14N NMR and DFT Calculations.

Both the chemical shift and quadrupole coupling tensors for N and Al in the wurtzite structure of aluminum nitride have been determined to high precision by single-crystal NMR spectroscopy. A homoepitaxially grown AlN single crystal with known morphology was used, which allowed for optical alignment of the crystal on the goniometer axis. From the analysis of the rotation patterns of N () and Al (), the quadrupolar coupling constants were determined to kHz, and MHz. The chemical shift parameters obtained from the data fit were ppm and ppm for N, and (after correcting for the second-order quadrupolar shift) ppm and ppm for Al. DFT calculations of the NMR parameters for non-optimized crystal geometries of AlN generally did not match the experimental values, whereas optimized geometries came close for Al with MHz, but not for N with kHz.

First principle calculations study of AlN surface terminal structure evolution under different conditions

The surface structure and properties of aluminum nitride (AlN) play an important role in many applications. Using the first principle calculations method, we analyzed the surface terminal structure of AlN and its evolution under different conditions by determining the surface energy, adsorption energy, and evaporation energy of the Al and N terminals on the AlN(0001) surface. Our results show that the reason why the N terminal is less stable than the Al terminal is not only because of its high surface energy but also because its adsorption performance is extremely sensitive to the adsorption position. The surface N atoms combine to form N2 molecules that escape during the evaporation process at high temperature. After surface N atoms escape, the AlN surface structure reconstitutes to form a hexagonal closest packing (HCP)–like structure, and the energy barrier for the reconstructing process is 3.2 eV. This shows that the structure and form of the AlN(0001) terminals depend on the environmental conditions.

Theoretical study of AlN mechanical behaviour under high pressure regime

Aluminum nitride (AlN) is a very important industrial and technological material due to its properties, e.g. high melting point, thermal conductivity, electrical resistivity, mechanical strength, and corrosion resistance. This work represents detailed study of mechanical and elastic properties of AlN structures under compression. Theoretical modeling has been performed using quantum mechanical calculations and computed values were compared with the experimental results when available. Structural properties, volume change and strain (dilatation) under high pressure has been investigated for various AlN phases. Elastic constants Cij for wurtzite, sphalerite and rock salt structure were calculated under pressure. Important mechanical properties were investigated; bulk modulus B, shear modulus K, Young's modulus E, Vickers hardness Hv, anisotropy, stiffness, Poisson's ratio, brittleness/ductility, in order to investigate influence of pressure on wurtzite, sphalerite and rock salt based AlN materials. Detailed analysis of anisotropic mechanical properties under compression has been performed, as well as relationship between B, K, E and Hv, in order to offer novel technological and industrial applications of AlN.

First-Principles Study of Electronic Structure and Optical Properties of La-Doped AlN

As a wide-bandgap semiconductor material with small dielectric constant and good thermal stability, aluminium nitride (AlN) can theoretically emit light in the deep ultraviolet wavelength region, so it is important in expanding the response of AlN in the visible region. In order to study the influences of La doping on the photoelectric properties of wurtzite AlN crystal, the first-principle plane-wave pseudopotential method and generalized gradient approximation were used to study the lattice constants, electronic structure and optical properties of undoped and La-doped AlN. The calculation results indicate that the intrinsic AlN is a direct-bandgap semiconductor material which both conduction band bottom and valence band top being at the G point, but La-doped AlN forms an indirect-bandgap semiconductor in which the conduction band bottom is at the G point and the valence band top is at the F point. The peak of the density of states is reduced near Fermi energy, and the electronic localization features are significantly diminished. The La doping makes the forbidden band width of AlN narrow, which reduces the photon energy needed for the electron transition, and shows a red shift phenomenon, which expands the influence on visible light. Therefore, this provides a theoretical basis for the study of the photoelectric properties of rare-earth-metal La-doped AlN.

Microstructural Characterization and Formation Mechanism of Nitrided Layers on Aluminum Substrates by Thermal Plasma Nitriding

Nitrided layers on 6082 aluminum alloy substrates and 1060 aluminum substrates are formed at atmospheric pressure using thermal nitrogen plasma, which only takes seconds to form a millimeter-level layer. The nitrided layers are composed of aluminum nitride (AlN) and aluminum solid solution phases. Microstructures in these nitrided layers can be divided into three regions from bottom to top: the transition region, the dendrite region, and the lamella region. These regions are formed in sequence. The formation mechanisms and processes of the three regions are discussed in detail. Furthermore, we found that Al melt is transported upward through the voids and the capillaries in the AlN structures, and reacts with N plasma in the melt surface. The growth of the AlN structures promotes this transport. With the increase of N2 flow rates from 1 L/min to 7.5 L/min, both the hardness and the wear resistance of the nitrided layers are improved, and the nitrided layer becomes thicker.

Electronic Band Structure of Aluminium Nitride and Gallium Nitride Crystallizing in the Wurtzite and Zinc Blende Structures

We have studied the electronic band structure of aluminium nitride and gallium nitride crystallizing in the wurtzite and zinc blende structures. We have presented theoretical study of atom and orbital projected partial density of state for the group third nitrides. We have compared the energy distribution of electronic states in valence and conduction bands as calculated by means of linear muffintin orbital method. The direct comparison with the appropriate density of state was presented to indicate the extent to which we got direct information about allowed electron states by considering the positions of the maxima and minima of the intensity in spectra. The influence of the core level width and spectrometer broadening was also considered. The good agreement between structures observed in spectra and structures in the calculated density of state allowed for consistent analysis of results. We have compared the amounts of bonding and antibonding states near the band edges for different choices of cations and crystal structures. Since the device applications are based mainly on wurtizite type nitride, particular attention was given to this phase. We examined for wultzite structure the level of anisotropy in the formed chemical bonds. Two kinds of bonds π and σ are connected directly with bond lengths. We examined the amount of electronic states available for optical transition as a function of crystal structure, crystallographic direction and presence of cation semicore states. The hydridization between d and p states for different cations and nitrogen were studied. We found that d-p interaction affected the valence band edge and influenced magnitude of the fundamental gap. The obtained results were compared with previously obtained results of theoretical and experimental work and were found in good agreement.

Piezoelectric and structural properties of c-axis textured aluminium scandium nitride thin films up to high scandium content

Partial substitution of aluminium by scandium in the wurtzite structure of aluminium nitride (AlN) leads to a large increase of the piezoelectric response by more than a factor of 2. Therefore, aluminium scandium nitride (ASN) thin films attracted much attention to improve piezoelectric MEMS devices such as RF filters, sensors, micro actuators and energy harvesting devices. In this work, process-microstructure-property relationships of ASN thin films containing up to 42% Sc were investigated. Like AlN thin films, ASN films are sputter deposited at 300–350°C with pulsed DC powered magnetrons. The influence of the process parameters on the film structure, the intrinsic stress and the piezoelectric response was investigated in order to achieve optimal piezoelectric coefficients up to high Sc concentrations. X-Ray diffraction (XRD) and transmission electron microscopy (TEM) were used to analyse the quality of c-axis texture. The films showed exclusively (002) texture with rocking-curve widths in the range of 1.3–2° (FHWM). The films were further analysed by scanning electron microscopy (SEM). The Sc content was determined by energy-dispersive X-ray spectroscopy (EDX). A good compositional homogeneity in the range of 0.5–1at.% was achieved between border and centre of 200-mm wafers. So far, we obtained ASN films with transversal piezoelectric coefficients of up to e31,f=−2.77C/m2, which is a factor 2.6 higher than in pure AlN thin films.

Facile synthesis of powder-based processing of porous aluminum nitride

The use of aluminum nitride (AlN) has recently begun to be explored for advanced functional applications. For some particular applications, it is more advantageous to use porous-structured AlN, simply because it can provide a greater surface area and higher permeability. However, porous or bulk AlN is very difficult to achieve due mainly to its high melting point (2200°C). This study proposes a new novel processing method to synthesize porous AlN through a complete transformation from porous aluminum (Al) using a remarkably low nitriding/sintering temperature (620°C) as opposed to only the surface nitride AlN-Al core composite systems reportedly at or above 1000°C in the literature. A uniform microporous bead structure of porous AlN with a mean pore size of 74.0±27.7μm was obtained that also contained nanoparticles ranging from 80 to 230nm that covered the surface.