AlN Crystals Research Articles

Growth and Characterization of Thick Polycrystalline AlN Layers by HTCVD

Thick polycrystalline AlN layers were grown at low pressure using high temperature chemical vapor deposition (HTCVD). The experimental setup consists of a graphite susceptor heated by an induction coil surrounding a vertical cold wall reactor. The reactants used were ammonia and aluminum chloride species formed in situ via chlorine reaction with high purity aluminum wire. AlN films were deposited on a diameter graphite susceptor between 1200 and . AlN layers have been characterized by scanning electron microscopy, X-ray diffraction, Raman spectroscopy, and electron backscattered diffraction. The influence of temperature on growth rate, surface morphology, grain size, and crystalline structure is presented. Growth rates of up to have been reached. A nonpolar preferred orientation of AlN films is stabilized at a higher temperature. The potential of investigation in this new range of experimental conditions, i.e., high temperature and high growth rate, as well as deposition of nonpolar AlN crystals, is very promising for epitaxial growth and extends the field of applications.

SEM-EDX study of the crystal structure of the condensed combustion products of the aluminum nanopowder burned in air under the different pressures

The experimental results of combustion of aluminum nanopowder (ANP) in air and AlN crystals formation process were studied. The air pressure during the combustion process significantly affected the crystals growth mechanism. Crystals with the different morphology (whiskers, hexagonal crystals, rods) were found in the condensed combustion products.

Dependence of the growth rate of an AlN layer on nitrogen pressure in a reactor for sublimation growth of AlN crystals

The dependence of the layer growth rate on nitrogen pressure in a reactor has been examined in order to analyze the conditions of growth of AlN thick layers and bulk crystals by the sublimation sandwich method. It is shown that the layer growth rate steadily increases as the pressure in the reactor is lowered within the range 1–0.02 bar. This suggests that a key role in the layer growth kinetics is played by the source-to-substrate transfer of the components (Al, N), rather than by their adsorption (desorption) on the substrate surface.

Transmission electron microscopy study of defects in AlN crystals with rough and smooth surface grains

Defects present in (0 0 0 1) textured polycrystalline AlN grown by the sublimation–recombination method were analyzed using transmission electron microscopy (TEM) methods. Grains in the polycrystalline boule had either a smooth or a rough surface. The rough surface grains had mainly edge dislocations, whereas the smooth surface grains had some sub-grain boundaries and were mostly free of dislocations. Dislocations at the grain boundaries were pinned and could not be annihilated.

High-excitation and high-resolution photoluminescence spectra of bulk AlN

Photoluminescence spectra of high-quality bulk AlN crystals are reported. In addition to the expected linear luminescence features like free excitons and donor-bound excitons, nonlinear processes like biexcitons and the exciton-exciton scattering band are seen for higher excitation densities. For temperatures above $\ensuremath{\approx}150\text{ }\text{K}$ the electron-hole plasma becomes clearly visible in the spectra. A detailed analysis of the data yields an exciton binding energy of 55 meV, a biexciton binding energy of 28.5 meV, a band gap of 6.089 eV at low temperature, and a band gap of 6.015 eV at room temperature.

The origin of 2.78 eV emission and yellow coloration in bulk AlN substrates

The yellow color of bulk AlN crystals was found to be caused by the optical absorption of light with wavelengths shorter than that of yellow. This yellow impurity limits UV transparency and hence restricts the applications of AlN substrates for deep UV optoelectronic devices. Here, the optical properties of AlN epilayers, polycrystalline AlN, and bulk AlN single crystals have been investigated using photoluminescence (PL) spectroscopy to address the origin of this yellow appearance. An emission band with a linewidth of ∼0.3 eV (at 10 K) was observed at ∼2.78 eV. We propose that the origin of the yellow color in bulk AlN is due to a band-to-impurity absorption involving the excitation of electrons from the valence band to the doubly negative charged state, (VAl2−), of isolated aluminum vacancies, (VAl)3−/2− described by VAl2−+hν=VAl3−+h+. In such a context, the reverse process is responsible for the 2.78 eV PL emission.

Investigations of the spin-Hamiltonian parameters for Er3+ ions in AlN crystal

The spin-Hamiltonian parameters (g factor g //, g ⊥ and hyperfine structure constants A //, A ⊥) for Er3+ ion at the trigonal Al3+ site of AlN crystal are calculated by diagonalising the 52 × 52 energy matrix. The matrix are related to the ground mutiplet 4I15/2 and the first to third excited multiplets 4I13/2, 4I11/2 and 4I9/2 for 4f11 ions in trigonal crystal field under an external magnetic field. The crystal-field parameters used in the matrix are obtained from the superposition model and the local lattice relaxation due to the substitution of Er3+ for Al3+ is considered. The calculated spin-Hamiltonian parameters are in reasonable agreement with the experimental values and the signs of hyperfine structure constants are suggested. The results are discussed.

Materials for photodetectors based on intersubband transitions in GaN/AlGaN quantum dots

The electron states and optical properties of a dense ordered array of quantum dots (QDs) based on GaN and AlN crystals of the wurtzite (w) structure have been studied by the pseudopotential method, with the hexagonal symmetry, deformations, and internal electric fields accurately taken into account. It is shown that the minimum of the first electron miniband at the center of the Brillouin zone of the QD superlattice originates from a state of the central Γ1 valley of the conduction band of the binary crystals, while the higher levels are associated with the states of the U side valleys and the neighborhood of the Γ valley. The first absorption peak of light polarized in the basal plane, e⊥c, is associated with transitions from the lower level with symmetry Γ1 in the quantum Γ well to two close-lying levels with symmetry Γ3. The absorption of light with polarization parallel to the hexagonal axis, e∥c, is weaker, and the peak is shifted toward higher energies. Because of this, an array of small GaN QDs can be used in IR photodetectors with the light incident on the front. A technology for obtaining arrays of small QDs with high density is proposed and developed.

Deep centers in bulk AlN and their relation to low-angle dislocation boundaries

Abstract Structural properties, electrical properties, deep traps spectra, optical properties of bulk 50-mm-diameter AlN crystals prepared by physical vapor transport (PVT) were studied by means of X-ray diffraction, selective etching, admittance spectroscopy, microcathodoluminescence (MCL). The crystals had a low dislocation density with dislocations forming a well defined cellular structure. The dominant electron traps had the ionization energy of 0.26 and 0.65 eV with low concentrations close to some 1013 cm−3 and some 1015 cm−3, respectively. The Fermi level was pinned near the 0.26 eV electron traps. These traps were shown to give rise to a strong persistent photoconductivity and photocapacitance. MCL spectra of the studied crystal were dominated by the 3.3, 4.2 and 5.5 eV luminescence bands. The intensity of the strongest 3.3 eV band was greatly enhanced in the vicinity of low-angle dislocation boundaries.

Directional dependence of AlN intrinsic complex dielectric function, optical phonon lifetimes, and decay channels measured by polarized infrared reflectivity

The directional dependence of AlN intrinsic complex dielectric function, the phonon lifetimes, and decay channels are investigated by means of polarized infrared reflectivity measurements on several facets of self-nucleated wurtzite AlN crystal of high crystalline quality. The measurement technique and the AlN single crystal used have been selected with the purpose to reduce, as much as possible, any instrumental-based effects as well as phonon scattering mechanisms due to defects. The experimental arrangements necessary to detect well-defined crystallographic orientations and phonons are detailed. The dielectric parameters and the phonon lifetimes are precisely determined as functions of the crystallographic direction from a careful Kramers–Kronig and damped Lorentz oscillator analysis. The ordinary (ε∞⊥) and extraordinary (ε∞∥) high frequency dielectric constants for high quality AlN crystal are found to be 3.93 and 4.05, respectively, and the ordinary (ε0⊥) and extraordinary (ε0∥) static dielectric constants are found to be 7.37 and 8.60, respectively. Our values obtained for the pure character phonon lifetimes are in good agreement with Raman measurements when these are obtained with the necessary care to eliminate linewidth broadening due to the finite slit width. The lifetime of a transversal phonon is found to increase with increasing the phonon energy, while that of a longitudinal phonon is found to decrease with increasing the phonon energy. Based on these observations, preferential decay channels for the AlN phonons are estimated. The results show that in the case of AlN, the widely assumed symmetric optical phonon decay into two phonons of lower energy cannot be justified for the zone center longitudinal phonons.

Estimates of the spontaneous polarization and permittivities of AlN, GaN, InN, and SiC crystals

The values of spontaneous polarization and permittivities of aluminum, gallium, and indium nitrides, as well as silicon carbide, with a wurtzite structure are calculated within the Harrison bond-orbital model. The only fitting parameter γ, which characterizes corrections for a local field, is determined from the fit of the calculated high-frequency permittivity ɛ∞ to the experimental data for compounds with a sphalerite structure. The results obtained are in satisfactory agreement with the available experimental data and calculations performed by other authors.

Spontaneous detachment of a sublimation-Grown AlN layer from a SiC-6H substrate

Growth of thick layers and bulk crystals of AlN is a topical problem for modern science and technology. The main way to solve the problem is to use the sublimation method in which AlN is evaporated at a temperature of about 2000°C and is epitaxially deposited onto a SiC substrate. A severe difficulty in this case is that the coefficients of thermal expansion of these materials are different, which leads to bending, cracking, and pronounced stresses in the AlN layer upon cooling to room temperature. This communication considers the case of a spontaneous detachment of a crack-free AlN layer from a SiC substrate, which points to the real possibility of developing a growth technology in which their separation becomes inevitable. The following reasons for spontaneous separation of the layer and the substrate are probable: (i) formation of a thin Al layer at the interface and (ii) occurrence of the initial growth stage by the previously described scheme, according to which, the layer and substrate are atomically bound only at separate comparatively sparse areas of nucleation of the growing crystal. Upon cooling, these areas disintegrate and the layer is detached from the substrate. It is unclear so far what specific features and anomalies of the growth process give rise to this result.

Advances in Bulk Crystal Growth of AlN and GaN

AbstractAluminum nitride (AlN) and gallium nitride (GaN) play an essential role in modern electronics, particularly in optoelectronics. Highly efficient light-emitting devices covering the ultraviolet to green spectral region are fabricated from these materials. Despite all efforts, the growth of large-size and high-quality AlN and GaN crystals for substrates, which are thermally and lattice-matched to the AlGaN-based device structures, is still in its infancy. This is due to the high equilibrium vapor pressure of nitrogen above these compounds, which requires growth techniques employing either the vapor phase or liquid solutions. The best commercially available GaN substrates show a high dislocation density of >105 per cm2 and strong bowing with a radius of curvature smaller than 10 m. This article reviews current growth techniques that look promising and may become commercially viable.

Freestanding AlN single crystals enabled by self-organization of 2H-SiC pyramids on 4H-SiC substrates

A sublimation-recondensation process is presented for high quality AlN (0001) crystals at a high growth rate by employing 4H-SiC substrates with a predeposited epilayer. It is based on the coalescence of well oriented AlN microrods, which evolve from the apex of 2H-SiC pyramids grown out of hexagonal pits formed by thermal etching of the substrate during a temperature ramp up. This process yields stress-free 120-μm-thick AlN single crystals with a dislocation density as low as 2×106cm−2.

AlN Periodic Multiple-layer Structures Grown by MOVPE for High Quality Buffer Layer

AbstractHigh crystal quality crack-free AlN on sapphire was grown by low pressure metal organic vapor phase epitaxy (MOVPE). Growth experiments combine two recent approaches: the ammonia pulse-flow method and ammonia continuous-flow growth mode by varying the V/III ratio. The detailed aspects of MOVPE, employing the periodic multilayer approach at low, intermediate, and high temperatures are described. This method yields significant reduction of screw dislocation density and provides very smooth surface for thin AlN layers.

Effect of indium on the physical vapor transport growth of AlN

AlN bulk single crystals were grown by spontaneous nucleation on the side wall of a hot pressed BN crucible, using both aluminum metal and AlN powder as source materials in an open system. Optimization of reactor pressure, temperature profile, and crucible design yielded AlN crystals with an a-plane platelet morphology. The crystals had low (1 1 2¯ 0) ω rocking curve full-width at half-maximum values of 17–60 arcsecs indicative of their high crystalline quality. Further, the addition of 1% indium by weight to the source material was found to significantly influence growth. When Al metal was used as a source material, the addition of indium to the source material resulted in an increase in the average crystal size by roughly an order of magnitude. When AlN powder was used as a source material, the indium addition resulted in a decrease in the total number of nucleation sites. The results show that the addition of indium is a promising method for improving the physical vapor transport growth of AlN.

Different optical absorption edges in AlN bulk crystals grown in m- and c-orientations

AlN single crystals were grown on m-plane (101¯0) and c-plane (0001¯) AlN seeds under identical growth conditions. The m-plane AlN crystals exhibited substantially lower oxygen incorporation, ∼1018cm−3, than the c-plane crystals, ∼1019cm−3. By investigating optical transmission spectra, m-plane AlN had absorption bands at 4.05 and 4.35eV, while c-plane AlN had an absorption band edge at 4.85eV. These below bandgap absorption bands strongly correlate with the reported transitions related to Al vacancy-impurity complexes, such as the complex of an Al vacancy and two oxygen atoms, (VAl–2ON)1− and the complex of an Al vacancy and one oxygen atom, (VAl–ON)2−, becoming the major cause for the poor, below bandgap optical transparency (α>200cm−1) of these crystals.

Role of additives in enhancing the thermal conductivity of AlN ceramics

The thermal conductivity of AlN ceramics with different amounts of Y2O3 as sintering additive has been calculated at low, intermediate and high temperatures by applying the Callaway theory and using a detailed account of three-phonon scattering processes. By making a quantitative analysis of the results, it is found that the increase in the room-temperature conductivity due to the addition of Y2O3 is largely attributed to the decrease in the oxygen impurities level in the AlN crystal. In addition to matching the experimental measurements (available in a limited temperature range) our theoretical work predicts the resulting thermal conductivity over a large temperature range.

High-temperature growth of thick AlN layers on sapphire (0 0 0 1) substrates by solid source halide vapor-phase epitaxy

High-temperature growth of thick AlN layers was performed by solid source halide vapor-phase epitaxy (HVPE) with local heating system. It was found that crystalline quality of AlN layers grown on sapphire (0 0 0 1) substrates improved with increase of growth temperature until 1500 °C. Degradation of crystalline quality occurred over 1550 °C, which is thought to be due to the decomposition of AlN crystal or sapphire substrate during the epitaxial growth. The full-width at half-maximum (FWHM) values of (0 0 0 2) and ( 1 0 1 ¯ 0 ) diffraction from the AlN layer grown at 1500 °C were 103 and 828 arcsec, respectively. Secondary ion mass spectrometry (SIMS) measurement showed a high concentration of oxygen incorporated into the AlN layer through the interface between sapphire and AlN at 1550 °C. The near band-edge emission peaking at 208.2 nm was also observed by cathodoluminescence measurement at room temperature.

Sublimation growth of 2 inch diameter bulk AlN crystals

AbstractThe technology of sublimation growth of 15 mm diameter bulk single AlN crystals is scaled to grow similar 2‐inch diameter crystals. The best results are currently achieved with the two‐stage technique including 1) seeding and initial growth of 2‐3 mm thick single‐crystal AlN layers on 2‐inch diameter 6H‐SiC wafers in pre‐carbonized Ta crucibles in graphite equipment and 2) growth of bulk AlN crystals on the above AlN layers in tungsten crucibles and equipment. The initial AlN layers prove of good crystallographic quality but may contain up to 6 at.% of Si impurities. The eventual bulk AlN crystals consist of about 40 mm diameter round single‐crystal core and polycrystalline rim. No impurities in concentration higher than 0.01 at.% are found in the bulk crystals. The bulk AlN crystal may be separated from the initial AlN layer to use the latter repeatedly for growth of a new crystal. Such “secondary” crystals were found to be semi‐insulating. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)