Structure Of AlN Research Articles

Porous AlN with a Low Dielectric Constant Synthesized Based on the Physical Vapor Transport Principle

Porous AlN with low dielectric constant has been synthesized by the sacrificial template method based on the physical vapor transport principle. It is quite different from the traditional method that mixes the matrix with a pore-forming agent and utilizes liquid-phase sintering. The method consists of two parts. Firstly, AlN powder is placed in a graphite crucible. C/AlN composite can be formed by mixing decomposed AlN vapor and volatile carbon originated from a crucible at high temperature. Secondly, pores are formed after removing carbon from the C/AlN composite by an annealing process. The structure, morphology, porosity and properties of porous AlN are characterized. It is shown the obtained porous AlN has a thermal conductivity of 37.3 W/(m K) and a reduced dielectric constant of 5.5–6.1 (at 1 MHz). The porosity measured by a mercury porosimeter is 24.09%. It has been experimentally proved that porous AlN with a sufficiently porous structure and properties can be synthesized based on the vapor-phase principle.

Crystallographic and morphological aspects of AlN precipitation in high Al, TRIP steels

The dendritic AlN associated with poor ductility on straightening during continuous casting is believed to always crystallise out in the hcp structure. The present paper shows that AlN precipitation can also be present as an fcc phase. Because AlN precipitation is sluggish and needs suitable nucleants like MnS to aid its precipitation, the crystal structure of AlN is dependent on the order in which the austenite and MnS precipitate out during solidification. When austenite forms first, as with the 1%Al transformation induced plasticity (TRIP) steel, MnS nucleants are not available and AlN is forced to precipitate at the austenite grain boundaries as fcc so encouraging intergranular failure. With the 1.5%Al TRIP steel, on solidification, MnS is present in abundance, giving rise to the conventional hcp AlN.

Novel high-pressure phases of AlN: A first-principles study

Four novel AlN phases with low density and high hardness are predicted by using a evolutionary methodology on structural search. All four phases are thermodynamically more favourable than the rock salt structure of AlN at ambient pressure and will be transformed to rock salt AlN at certain pressure. All these phases are mechanically and dynamically stable by checking the independent elastic constants and phonon dispersion spectra. The calculated mechanical properties reveal that the proposed AlN phases are hard, and the Vickers hardness of the AlN phases ranges from 13.2GPa to 15.2GPa. The calculated band structures of the novel AlN phases reveal that these AlN phases possess semiconductive properties with wide direct band gaps at G points, and the gaps range from 3.627eV to 3.927eV.

Defect reduction in MBE-grown AlN by multicycle rapid thermal annealing

Multicycle rapid thermal annealing (MRTA) is shown to reduce the defect density of molecular beam epitaxially grown AlN films. No damage to the AlN surface occurred after performing the MRTA process at 1520°C. However, the individual grain structure was altered, with the emergence of step edges. This change in grain structure and diffusion of AlN resulted in an improvement in the crystalline structure. The Raman E2 linewidth decreased, confirming an improvement in crystal quality. The optical band edge of the AlN maintained the expected value of 6.2 eV throughout MRTA annealing, and the band edge sharpened after MRTA annealing at increased temperatures, providing further evidence of crystalline improvement. X-ray diffraction shows a substantial improvement in the (002) and (102) rocking curve FWHM for both the 1400 and 1520°C MRTA annealing conditions compared to the as-grown films, indicating that the screw and edge type dislocation densities decreased. Overall, the MRTA post-growth annealing of AlN lowers defect density, and thus will be a key step to improving optoelectronic and power electronic devices.

Effect of the nand p-type Si(100) substrates with a SiC buffer layer on the growth mechanism and structure of epitaxial layers of semipolar AlN and GaN

A new effect of the n-and p-type doping of the Si(100) substrate with a SiC film on the growth mechanism and structure of AlN and GaN epitaxial layers has been revealed. It has been experimentally shown that the mechanism of AlN and GaN layer growth on the surface of a SiC layer synthesized by substituting atoms on n- and p-Si substrates is fundamentally different. It has been found that semipolar AlN and GaN layers on the SiC/Si(100) surface grow in the epitaxial and polycrystalline structures on p-Si and n-Si substrates, respectively. A new method for synthesizing epitaxial semipolar AlN and GaN layers by chloride–hydride epitaxy on silicon substrates has been proposed.

Improved Blue‐Emitting AlN:Eu2+ Phosphors by Alloying with GaN

A method is designed to improve the luminescence of AlN‐based phosphors by tuning the band structure and crystal structure due to alloying with GaN. The pure (Al,Ga)N:Eu phosphors were initially prepared by gas‐phase reaction in an NH3 atmosphere. GaN alloying was used to expand the crystal lattice of AlN due to Ga3+ substituting for smaller Al3+ ions, making dissolution of Eu2+ easier. The dissolution of Ga in the AlN lattice was proven by the result of the Rietveld refinement and the increase in lattice parameters with increasing Ga content. To introduce other energy states mixing with the 5d states of Eu2+, Ga doping was also used to tune the band structure of AlN by acting on Eu2+ ions. The theoretical result was analyzed using the Cambridge Sequential Total Energy Package (CASTEP). According to the calculated total and atom resolved partial density of states, it was observed that the Ga 5p states contribute a large portion to the corresponding Eu2+ absorption band in (Al,Ga)N:Eu phosphors. As a consequence, an enhanced emission intensity at 470 nm and a high quantum efficiency for excitation at 330 nm was obtained despite of stronger thermal quenching of the (Al,Ga)N:Eu phosphors compared with AlN:Eu.

Interactions of h-AlN monolayer with platinum, oxygen, and their clusters

In this paper, we investigate the adsorption properties of single Pt and O atoms, and PtO, Pt2O, and PtO2 clusters on hexagonal AlN monolayer as well as several substitutions in AlN structure. We employ density functional theory to study electronic structure and charge transfers by considering nonmagnetic and ferromagnetic states. PtO and Pt2O adsorbed AlN system has ferromagnetic ground state with 2.00 μB magnetic moment, while PtO2, Pt, and O adsorption lead to nonmagnetic structures. Pt adsorbed AlN system has the lowest adsorption energy with −3.175eV indicating the most stable structure energetically. Oxygen atom largely disrupts the AlN layer due to strong N–O repulsion caused by high electronegativities of N and O atoms. The substitution of AlN monolayer with Pt and O atoms also presents interesting features. The various substitutions are able to yield ferromagnetic structures with semiconducting (AlO), metallic (NPt), or half-metallic (AlPt) ground states. These properties can lead to possible applications in spintronics and nanoelectronics.

Growth of high‐quality AlN layers on sapphire substrates at relatively low temperatures by metalorganic chemical vapor deposition

We report a three‐step method to grow high‐quality AlN heteroepitaxial layers on sapphire substrates at relatively low temperatures by metalorganic chemical vapor deposition (MOCVD) without the use of epitaxial lateral overgrowth (ELO) or pulse atomic layer epitaxy (PALE) method. The three‐layer AlN structure comprises a 15‐nm thick buffer layer, a 50‐nm thick intermediate layer, and a 3.4‐µm thick template layer grown at 930, 1130, and 1100 °C sequentially on the c‐plane sapphire substrate. The resulting AlN layer had smooth surface with well‐defined terraces and low root‐mean square (RMS) roughnesses of 0.50 and 0.07 nm for 20 × 20 and 1 × 1 µm2 atomic force microscopy (AFM) scans. Band‐edge emission was observed at 208 nm by room temperature (RT) photoluminescence (PL) measurements. The total threading dislocation density was 2.5 × 109/cm2 as determined by transmission electron microscopy (TEM), which is comparable to those of some AlN layers recently grown at significantly higher temperatures. Growth evolution was studied and correlated to the TEM results. The residual impurity concentrations were comparable to those of AlN layers grown at higher temperatures, i.e., 1200–1600 °C. This study demonstrates the high quality AlN layers on sapphire substrates can be grown at achievable temperatures for most of the modern MOCVD systems.

Raman spectroscopic assessments of structural orientation and residual stress in wurtzitic AlN film deposited on (0 0 1) Si

In this paper, polarized Raman spectroscopy is applied to quantitatively assess crystallographic alteration and interfacial residual stress with a micron-scale resolution in highly 〈0001〉 oriented (textured) polycrystalline wurtzitic AlN films grown on (001)-oriented Si substrates. Raman selection rules for the wurtzite structure of AlN were explicitly put forward and a set of Raman tensor elements determined from experimentally retrieved angular dependences of Raman band intensities upon in-plane rotation measurements. An appreciably high degree of homogeneity in the AlN film (i.e., with respect to both in-plane and out-of-plane Euler angles, retrieved according to the proposed spectroscopic algorithm) could be observed in spectral line scans randomly selected on the cross-section of the film/substrate system. These characterizations indicated negligible structural alterations, such as grain tilting and twisting during film growth. However, a non-uniform stress distribution in the AlN film along the film thickness direction was found, which remained stored during manufacturing of the AlN film. A quite remarkable magnitude of compressive residual stress (∼−1.5GPa) could be measured at the film/substrate interface. Finally, a Raman (non-destructive) statistical characterization of the film system in terms of micromechanical homogeneity by spectral surface mapping is presented, which provides a prompt overall view of the film quality. The proposed procedure should generally be applicable in crystallographic and micromechanical quality control of electronic film devices exhibiting a Raman spectrum.

Structural and electronic properties of SiC/AlN core/shell nanowires: a first-principles study

The structural stabilities and electronic properties of passivated and unpassivated SiC / AlN core/shell nanowires (CSNWs) along [0001] direction are investigated by using first-principles calculations with density functional theory. Our calculations demonstrate that thick AlN shell and small ratio of SiC core make the SiC / AlN CSNWs more stable. The band gaps decrease with the increasing of the CSNWs diameters. After passivation at the surface, type of SiC / AlN heterostructure changes and the mobility can be improved by increasing the CSNWs diameter and the SiC core ratio. These results provide an effective way to modulate the electronic properties of SiC / AlN structure, which is useful for fabrications and applications of CSNWs.

Effects of strong electron–hole exchange and exciton–phonon interactions on the exciton binding energy of aluminum nitride

In this study, the exciton binding energies of AlN are calculated by the anisotropic effective mass theory, which considers the strong electron–hole exchange and Fröhlich-type exciton–phonon interactions. Our calculated results are in good agreement with recent experimental results, indicating that the electron–hole exchange and exciton–phonon interactions play essential roles in describing the excitonic structure of AlN. We estimate that the exciton binding energies of AlN are 53.7 and 67.3 meV, for the spin-singlet and the spin-triplet states, respectively. In addition, a universal correlation has been found in compound semiconductors including AlN, where the effect of the electron–hole exchange and Fröhlich-type exciton–phonon interactions almost compensates each other on the binding energy of a spin-singlet exciton.

Structural and optical properties of C–Si codoped p-type and intrinsic AlN crystal

C–Si codoped p-type and intrinsic AlN crystals were grown by the sublimation method. The structure and optical characterisation of the AlN crystals were examined. X-ray diffraction shows that the quality of AlN crystals does not change obviously with codoping C and Si. Photoluminescence spectra at room temperature reveal two sharp band-edge lines at 309 and 396 nm from the C–Si codoped AlN and a broad peak around 378 nm from the intrinsic AlN. Then, the authors analysed the structure and composition of AlN crystals with X-ray photoelectron spectroscopy measurement in association with an ab initio electronic structure calculation. Combined the structure and composition of the p-type AlN as well as related theoretical results, the two lines are related to the complexes of VN–CN and CN–SiAl, respectively. Meanwhile, the broad peak in the intrinsic AlN is due to O as the native defect.

First-principles calculations of 5d atoms doped hexagonal-AlN sheets: Geometry, magnetic property and the influence of symmetry and symmetry-breaking on the electronic structure

The geometry, electronic structure and magnetic property of the hexagonal AlN (h-AlN) sheet doped by 5d atoms (Lu, Hf, Ta, W, Re, Os, Ir, Pt, Au and Hg) are investigated by first-principles calculations based on the density functional theory. The influence of symmetry and symmetry-breaking is also studied. There are two types of local symmetries of the doped systems: C3v and D3h. The symmetry will deviate from exact C3v and D3h for some particular dopants after optimization. The total magnetic moments of the doped systems are 0μB for Lu, Ta and Ir; 1μB for Hf, W, Pt and Hg; 2μB for Re and Au; and 3μB for Os and Al-vacancy. The total densities of state are presented, where impurity energy levels exist. The impurity energy levels and total magnetic moments can be explained by the splitting of 5d orbitals or molecular orbitals under different symmetries.

Tunable band gap of AlN, GaN nanoribbons and AlN/GaN nanoribbon heterojunctions: A first-principle study

We systematically investigate the electronic structure of AlN, GaN and (AlN)x/(GaN)1−x nanoribbon heterojunctions by employing first-principle calculations. The band gaps of both AlN and GaN ribbons decrease monotonically with the increase of the widths, and the ribbons with armchair edges are direct band gap semiconductors while the zigzag ones are indirect. Interestingly, the band gaps of (AlN)x/(GaN)1−x nanoribbon heterojunctions are closely associated with the AlN/GaN ratios and increase monotonically with the increase of the AlN concentration. The flexible tunability of the band gap in these nanostructures may be widely applied to future optoelectronic devices.

Electron energy loss near edge structure of InxAl1−xN alloys

We present a systematic computational study of electron energy loss near edge structure (ELNES) spectra of the N–K edge of InxAl1−xN with various indium contents. Density functional theory calculations are implemented by means of WIEN2k code. Supercell size as well as core–hole effects are investigated in order to establish the optimum parameters. Based on the known electronic structure of bulk wurtzite AlN, the fine structure of N–K ionization edge is investigated in order to study the effect of Indium in the electronic structure and the bonding environment of Nitrogen in InAlN alloys. The unintentional oxygen contamination in MOVPE In0.24Al0.76N and the variation of the indium content on defected areas of the structure are investigated.

Negative spin-exchange splitting in the exciton fine structure of AlN

The exact energy position of the free exciton transition and thus the lowest band gap in bulk wurtzite AlN are still under discussion. By combined high resolution optical emission and absorption experiments on a sample with (11¯00) surface, we resolve the fine structure of the lowest energy free exciton and determine an electron-hole spin-exchange interaction constant of j=−4 meV. This results in a low energy Γ1 exciton at 6.032 eV and a high energy Γ5 component at 6.040 eV. Only the latter one is observable for (0001) oriented AlN films due to selection rules.

Enhancement of piezoelectric response of diluted Ta doped AlN

The authors have investigated the variation of microstructure and piezoelectric response of TaxAl1−xN films with Ta concentrations. The results indicate that moderate Ta doping facilitates the c-axis orientation, crystallinity and enlargement of lattice constants. When the doping content is about 5.1at.%, the c-axis orientation is optimized in the best with c constant of about 0.5086nm. The Raman spectra of TaxAl1−xN films further determine the wurtzite structure of AlN after Ta doping. The high resolution transmission electric microscopy demonstrates that the column single crystal was obtained with presence of nano distortion domains. The Ta 4f7/2 binding energy of TaxAl1−xN shows a shift toward lower binding energy side, indicating that the coordination configuration of Ta with N changed as Ta contents increased. A significant enhancement in piezoelectric response was obtained from 4.2pC/N to 8.2pC/N, which is enhanced by ca. 100%, when the Ta content arrived to 5.1at.%. The expansion of unit cell volume and the competition between Ta and Al atoms about the coordination of nitrogen atoms, which will enhance the internal nitrogen displacement under electric field, are proposed to be the main microscopic origin of the enhancement of piezoelectric response.

碳硅共掺杂p型AlN的光电性能研究

p-type AlN crystals by C and Si codoping were grown on SiC substrates by the sublimation method.The structural,optical and electronic properties of the samples were investigated by XRD,XPS,PL and Hall-effect measurement.The XPS analysis,especially about binding energies of Si 2p and C 1s peaks,reveals that in AlN crystals,C replaces N as an accepter and Si replaces Al as a donor.In PL spectroscopy,two main emission peaks are observed.Combined the structure and composition of AlN and related theoretical results,the two peaks are attributed to the complexes of V N-C N and C N-Si Al,respectively.

The Evaluation of the Effect of Thermal Annealing on Local Structure of AlN:Gd Film by XAFS

The Evaluation of the Effect of Thermal Annealing on Local Structure of AlN:Gd Film by XAFS

Hybrid density functional theory studies of AlN and GaN under uniaxial strain

The structural stability, spontaneous polarization, piezoelectric response, and electronic structure of AlN and GaN under uniaxial strain along the [0001] direction are systematically investigated using HSE06 range-separated hybrid functionals. Our results exhibit interesting behavior. (i) AlN and GaN share the same structural transition from wurtzite to a graphite-like phase at very large compressive strains, similarly to other wurtzite semiconductors. Our calculations further reveal that this well-known phase transition is driven by the transverse-acoustic soft phonon mode associated with elastic instabilities. (ii) The applied tensile strain can either drastically suppress or strongly enhance the polarization and piezoelectricity, based on the value of the strain. Furthermore, large enhancements of polarization and piezoelectricity close to the phase-transition regions at large compressive strains are predicted, similar to those previously predicted in ferroelectric fields. Our calculations indicate that such colossal enhancements are strongly correlated to phase transitions when large atomic displacements are generated by external strains. (iii) Under the same strain, AlN and GaN have significantly different electronic properties: both wurtzite and graphite-like AlN always display direct band structures, while the the bandgap of wurtzite GaN is always direct and that of graphite-like GaN always indirect. Furthermore, the bandgap of graphite-like AlN is greatly enhanced by large compressive strain, but that of wurtzite GaN is not sensitive to compressive strain. Our results are drastically different from those for equibiaxial strain (Duan et al 2012 Appl. Phys. Lett. 100 022104).