AlN Crystals Research Articles

Boron as an anti-surfactant in sublimation growth of AlN single crystals

We study the influence of B evaporating from BN crucible walls on the nucleation and growth of AlN by a physical vapor transport. An experimental analysis by means of electron microscopy and optical spectroscopy shows that B incorporation and surface segregation depends on the orientation of the respective surface facet. Surface segregation is found to be predominant on {11-20} facets. This leads to the formation of wurtzite BN nuclei on these facets which extend over the whole surface and may form closed layers. Both boron surface segregation and growth of BN nuclei influence Al and N adsorption and surface diffusion, and in consequence reduce the growth rate. The differences in surface segregation of B on different facets can be understood in terms of incorporation kinetics of boron adatoms into the AlN crystal. B thus acts as a facet selective antisurfactant.

PVT Growth at High Temperatures

Physical vapor transport (PVT) growth method was used to grow bulk crystals of AlN and thick epitaxial SiC films on on-axis Si-face 4H-SiC(0001) substrates from the powder source materials. Both bulk and thick film crystals were characterized by scanning electron microscopy (SEM), which showed a surface morphology did not have any observable growth defects or cracking. We observed nice hexagonal growth morphology a typical characteristic of diffusion-limited growth. River-patterns free from micro bubbles and micro voids were observed on the as grown surface. An XRD rocking curve measured the FWHM as 51.0 arc seconds and a reciprocal space map showed a singular intense region that is attributed to the high quality homoepitaxial SiC film. A similar value was observed for small AlN crystals also.

Ferromagnetism inAl1−xCrxNthin films by density functional calculations

We report the results of a theoretical study of magnetic coupling between Cr atoms doped in bulk AlN as well as AlN $(11\overline{2}0)$ thin films having wurtzite structure. The calculations are based on density fuctional theory with the generalized gradient approximation to the exchange and correlation potential. In the thin film, modeled by a slab of finite thickness, Cr atoms are found to cluster around N on the surface layer and couple ferromagnetically. The results for the Cr-doped AlN crystal are similar, namely, Cr atoms cluster around N and couple ferromagnetically. In the thin film, the preference of Cr to occupy surface sites over the bulk sites is shown to be due to reduced coordination of the surface atoms. As the distance between the Cr atoms increases, both the ferro- and antiferromagnetic states become energetically degenerate and this degeneracy may account for the observed low magnetic moment per Cr atom.

Temperature dependence of Raman-active modes in AlN

Micro-Raman scattering and FT-IR reflectivity measurements were performed on bulk wurtzite AlN crystals grown by PVT method over a temperature range from 10 K to the room temperature after identifying the symmetry of each of the Raman-active zone-center optic modes. The empirical relationship previously introduced for diamond for the temperature dependence of the frequency was verified on AlN. The damping of the Raman-active zone-center optic modes was measured in the same temperature range. The experimentally obtained damping is modeled by a theory taking into account a symmetric decay of the optical phonons into two and three phonons of lower energy. The results show that the decay into two phonons is the most probable channel for the Raman-active zone-center optic modes A 1(LO), E 1(TO), E 2 2(high frequency), and A 1(TO). A comparison between the temperature dependence of the phonons of AlN and other III-Nitride (GaN) was made.

Thermodynamic Analysis of Impurities in the Sublimation Growth of AlN Single Crystals

ABSTRACTThe vapor phase species responsible for the transport of impurities in the sublimation-recondensation growth of bulk AlN crystals was predicted by thermodynamic analysis. AlN powder containing oxygen was investigated in Al-O-N system for an inert reactor. Dialuminum monoxide (Al2O) is strongly favored over all other possible oxygen containing species including NO and NO2. For AlN crystal growth in a graphite furnace, the Al-O-C-N system was studied. CO is the main species containing carbon and oxygen, and has a partial pressure more than one hundred times higher than all other carbon or oxygen containing species. Its partial pressure even exceeds that of Al vapor. Pure AlN growth on SiC seed was represented in the Al-N-Si-C system. SiC is not stable at high temperatures, the presence of nitrogen accelerates the decomposition of the SiC, and the most probable volatile silicon and carbon species originating from the SiC seed are Si, CN and C2N2.

High Resolution Transmission Electron Microscopy Study of Thermal Oxidation of Single Crystalline Aluminum Nitride

ABSTRACTThe impact of process conditions and crystal properties on the structure of thermal oxides formed on AlN were determined by high resolution transmission electron microscopy (HRTEM). Oxidation for 2 hours at both 800 ° and 1000 °C produced mostly amorphous oxide layers whereas oxidation for 4 and 6 hours at 1000 °C produced partly crystalline and epitaxial oxide layers. The crystalline oxide was mostly single phase á-Al2O3 except at the surface where it was a mixture of γ-Al2O3 and á-Al2O3. The amorphous oxide layer first transformed to γ-Al2O3 and then to the stable á-Al2O3 as evidenced by the non-uniform thickness of the oxide and the existence of the γ-Al2O3 at the surface. The AlN crystal contained a high density of defects at the interface at 800 °C but it was nearly defect- and oxygen-free at 1000 °C. This could be due to the rapid diffusion of the nitrogen and aluminum interstitials at high temperatures leading to a point defect equilibrium throughout the nitride. A faceted interface between Al2O3 and AlN could be attributed to non-uniform out diffusion of aluminum.

Crystal Growth and Defect Characterization of AlN Single Crystals

In this paper, we report results from seeded AlN PVT growth experiments carried out using SiC seeds. The purpose of the experiments was to understand the morphology and crystalline quality of PVT AlN crystals grown under the crystal growth environments investigated. AlN single crystal films of 120-650μm in thickness were grown and freestanding AlN single crystal pieces up to 4×5mm2 were obtained. Surface morphologies and crystal defects in these AlN single crystals were studied using optical microscopy. Selected AlN single crystals were studied using a high-resolution triple-axis X-ray diffraction technique and a Synchrotron White Beam X-ray Diffraction Topography technique. Defects identified in AlN crystals are cracks, dislocations, grain boundaries and crystallite inclusions.

AlN nanorings

Aluminum nitride nanorings with hexangular crystal structure were first synthesized by evaporating of aluminum alloy in ammonia/nitrogen atmosphere at 1100 °C. The products were characterized by transmission electron microscopy, scanning electron microscopy, FT-IR absorption and Raman scattering. Nanorings with the diameter of about 3 μm are formed by bending nanowires. HRTEM and electron diffraction studies revealed that nanorings are single crystal and the c-axis of the AlN crystal directs along the radial direction of the nanoring. The electrostatic polar charge model is employed in the explanation of the AlN nanoring formation.

Study on strain and piezoelectric polarization of AlN thin films grown on Si

The strain and piezoelectric polarization of AlN thin films grown on Si(111) substrates by metalorganic chemical-vapor deposition were investigated by using x-ray diffraction and Raman measurements. The stress and piezoelectric polarization of the AlN films were analyzed with E2 (high) phonon-mode frequency shifts in Raman spectra. The result indicates that the biaxial compressive stress is about 4.3GPa, and the piezoelectric polarization is about 1.91×10−2C∕m2. Phonons of Si–N bonds were also observed accompanying phonons of AlN crystal in Raman spectra, which indicate the interdiffusion of Si and N atoms during growth.

Structural properties of AlN crystals grown by physical vapor transport

Structural properties of different AlN material grown by physical vapor transport (freestanding single crystals vs. bulky material) are compared. In polycrystalline samples, void formation leading to dark and opaque crystallites and grain boundaries is restricted to high growth temperatures and to grains grown with N-polarity. Single-crystalline freestanding AlN exhibits a zonal structure as growth takes place on multiple faces simultaneously. As a conclusion, optical transmission of different grains varies regardless of void formation: Grains grown with N-polarity appear darker and more opaque than grains grown in other directions. Presumably, impurity incorporation and/or intrinsic defect formation depends on the crystallographic orientation of the facet where growth occurs. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Seeded growth of bulk AlN crystals and grain evolution in polycrystalline AlN boules

Large AlN crystals were grown by powder sublimation in a nitrogen atmosphere at low supersaturation and growth rates of 0.1–0.3 mm/h. The starting deposition surface was a sintered TaC disc. An appropriate adjustment of the system pressure and source-seed temperature gradient during the early stages of growth allowed epitaxial re-growth on AlN seeds that had been exposed to air. Single-crystalline AlN grains of 1 cm in size were achieved through multiple sublimation growth runs conducted at P = 500 Torr and growth temperatures of 2050–2150 °C. Elemental analysis of impurities in the grown AlN boules confirmed low oxygen contamination levels of ∼10 19/cm 3. No discontinuities were introduced in the structural defect distribution in the individual single-crystalline grains by the multiple re-growth steps. Absence of preferred growth directions of grains suggest the epitaxial re-growth process is suitable for seeded single-crystal growth in any orientation.

Excitonic structure of bulk AlN from optical reflectivity and cathodoluminescence measurements

Reflection measurements in the near-band-edge region of bulk AlN crystals have been performed as a function of temperature. The optical reflectance spectra of an $a$-face AlN sample show a feature at about 6.029 eV, which we assign to the free exciton $A$. The observation of the free exciton $A$ first excited state yields the estimated values of the direct band gap and the limit of the electron effective mass. Optical reflectance measurements performed on AlN samples with two different crystallographic orientations allow the observation of transitions associated with the optical selection rules related to the so-called $A,B$, and $C$ excitons. It was not possible to fully resolve the $B$- and $C$-excitonic transitions, which were observed at about 6.243 and 6.268 eV, respectively. Using the measured exciton energies and a quasicubic model developed for the wurzite crystal structure we estimate the spin-orbit splitting $\ensuremath{\delta}=36\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ and the crystal-field splitting $\ensuremath{\Delta}=\ensuremath{-}225\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$.

Electronic Energy Structure and X-ray Spectra of Wide-Gap AlN and BN Crystals and B[sub x]Al[sub 1 – ][sub x]N Solid Solutions

The electronic energy structure of 2H and 3C AlN and BN crystals and BxAl1−x N solid solutions is calculated on the basis of the local coherent potential method using the cluster version of the MT approximation and the theory of multiple scattering. The features of the electronic structure of 2H-AlN crystals are compared with x-ray K and L absorption and emission spectra of aluminum and nitrogen. An interpretation of these features is given. The concentration dependences of the width of the upper subband of the valence band and the band gap in BxAl1−x N solid solutions (x = 0.25, 0.5, 0.75) are investigated. Charge transfer from aluminum to nitrogen atoms is shown to occur and increase with boron doping in both crystallographic modifications.

Growth of AlN, GaN and InN from the solution

Thermodynamic and kinetic properties of group III metal nitrides Me(Al,Ga,In)N-Me(l)-N2 systems are strongly influenced by the high binding energy of molecular nitrogen which favours the decomposition of these compounds into the constituents, severely limiting the growth possibilities of these crystals. From all three nitrides, the AlN sizeable crystals have been grown from the vapour only. In contrast, the best GaN and InN crystals were grown from solution using different nitrogen sources: ammonia (NH3), plasma-activated nitrogen, or molecular nitrogen (N2) under high pressure. AlN crystals grown from solution have low crystallographic quality and needle-like unstable morphology. They are electrical insulators. GaN hexagonal plate-like crystals, grown under high nitrogen pressure, have high crystallographic quality, sufficient for epitaxy. GaN undoped crystals, are heavily n-type whereas those doped by Mg or Be are usually semi-insulating. InN crystals, obtained by the plasma method, are similar to undoped GaN crystals - they possess high concentration of free electrons.

Study on the Seeded Growth of AlN Bulk Crystals by Sublimation

Seeded growth of AlN single crystals on 6H-SiC substrates by sublimation has been investigated. Pyrocarbon coated graphite crucibles were used. Temperature profile and source-substrate distance have been found to be the most influencing parameters of crystal growth. AlN crystals of maximum dimension 9 mm (length) × 5 mm (width) × 300 µm (thickness) were grown on SiC substrates and the best crystal showed an XRD omega rocking curve FWHM of 4.81 arcmin. AlN nucleated as independent hexagonal islands and coalesced as growth progressed on. Growth rate of AlN grown on C-face SiC has been found to be higher than that on Si-face SiC. Pyrocarbon coated crucibles have been found to be better suited for AlN growth as the impurity incorporation in to crystals due to crucible was less.

Growing ledge structures of AlN crystals in a Fe–Mn–Al–C alloy

Abstract The Fe–30.4Mn–8.7Al–1.0C (wt.%) alloy was nitridized at 1000 °C. The AlN formed into a Widmanstatten side-plate shape. The side-plate of the AlN is parallel to the basal plane of the HCP crystal. There are three possible growing riser planes; namely ( 01 1 ¯ 0 ) AlN , ( 1 1 ¯ 00 ) AlN , and ( 10 1 ¯ 0 ) AlN planes. The angle for growing riser planes met on the (0 0 0 1) terraces is 120°.

Interface structures of AlN∕MgB2 thin films sputtered on sapphire c- and r-plane

Structures of interfaces between AlN and MgB2 thin films were investigated by cross-sectional transmission electron microscopy. These layers were deposited on sapphire substrate with different orientations by a conventional method. A thick amorphous layer was present between films in the case of r-plane substrate, whereas many AlN crystals were grown epitaxially in the case of c-plane substrate. Then NbN∕AlN∕MgB2 trilayers were intentionally fabricated on sapphire c-plane and showed Josephson and quasiparticle tunneling property with small subgap leakage current.

Size-Induced Reduction of Transition Pressure and Enhancement of Bulk Modulus of AlN Nanocrystals

An in situ X-ray-diffraction study of AlN nanocrystals under hydrostatic (or quasihydrostatic) conditions was performed to pressures of 36.9 GPa, using an energy dispersive synchrotron-radiation technique in a diamond-anvil cell (DAC). Hexagonal AlN nanocrystals have a particle size of 10 nm on average, and display an apparent volumetric expansion as compared to the bulk AlN polycrystals. Upon compression to 14.5 GPa, AlN nanocrystals start to transform to a rocksalt structure phase. This pressure is significantly lower than the transition pressure of 22.9 GPa observed from the bulk AlN by using the same technique. The nanosized wurtzite phase has a bulk modulus (B0) of 321 ± 19 GPa, larger than that of the bulk AlN crystals with a B0 of 208 GPa. There is a large volumetric decrease of 20.5% upon the wurtzite-to-rocksalt phase transformation. Combination of the size-induced volumetric expansion and resulting softening of the Poisson ratio and shear modulus may explain the reduction of transition pressure ...

Structural, Optical and Electrical Properties of Bulk AlN Crystals Grown by PVT

Structural, Optical and Electrical Properties of Bulk AlN Crystals Grown by PVT

In-Situ Monitoring of AlN Crystal Growth on 6H-SiC by the Use of a Pyrometer

In-Situ Monitoring of AlN Crystal Growth on 6H-SiC by the Use of a Pyrometer