Aluminum Nitride Crystals Research Articles

Thermally stimulated luminescence in aluminium nitride crystals

AbstractElectronic levels of point defects in aluminium nitride (AlN) bulk single crystals are evaluated using thermally stimulated luminescence (TSL). As the crystals were grown using AlN source material of different purity levels, the samples show different concentrations of silicon, oxygen, carbon, and (presumably) vacancies and vacancy complexes. In the TSL spectra, broad peaks are detected around 40–70 K, 100 K, 170 K, 220 K, 260 K, and 390 K. The corresponding activation energies were calculated using the Hoogenstraaten method to 8–35 meV, 40–70 meV, 150–215 meV, 130–180 meV, 150–190 meV, 80–170 meV, and 590–770 meV, respectively. TSL peak intensities are discussed in terms of chemical analysis of the samples. The results indicate that oxygen and silicon act as dominant electron traps in AlN. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electronic and magneto‐optic properties of Co‐ and Ni‐doped small quantum dots of indium nitrides

AbstractStructural defects in aluminium nitride (AlN) get visible in panchromatic cathodoluminescence (CL) maps as luminescence at 340 nm and 630 nm is locally enhanced. Low‐angle grain boundaries (LAGBs) and dislocations can be detected as long as they are sufficiently decorated and the defect‐related contrast is higher than local variations in background CL intensity. We applied the method to c ‐plane and a ‐plane cuts of one of our off‐oriented grown AlN crystals and describe occurrence, routes, and dissolution into dislocation bunches of LAGBs. We found that the vast majority of LAGBs are uneven a ‐planes which extend in a preferential m ‐ as well as in c ‐direction. As a consequence, their density decreases only slightly during growth (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Structural defects in aluminium nitride bulk crystals visualized by cathodoluminescence maps

AbstractStructural defects in aluminium nitride (AlN) get visible in panchromatic cathodoluminescence (CL) maps as luminescence at 340 nm and 630 nm is locally enhanced. Low‐angle grain boundaries (LAGBs) and dislocations can be detected as long as they are sufficiently decorated and the defect‐related contrast is higher than local variations in background CL intensity. We applied the method to c ‐plane and a ‐plane cuts of one of our off‐oriented grown AlN crystals and describe occurrence, routes, and dissolution into dislocation bunches of LAGBs. We found that the vast majority of LAGBs are uneven a ‐planes which extend in a preferential m ‐ as well as in c ‐direction. As a consequence, their density decreases only slightly during growth. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of growth direction and polarity on bulk aluminum nitride crystal properties

Aluminum nitride (AlN) single crystals, approximately 25mm in diameter and up to 15mm thickness, were grown by the physical vapor transport (PVT) method in tungsten crucibles. To study the effect of growth direction and polarity, growth was performed using AlN seeds of very different orientations, i.e., Al-polar and N-polar basal and rhombohedral planes. For some growth runs, dual seeds consisting of two halves of switched polarity were used to compare growth under nearly identical settings. Specially roughened seed crystals were prepared to promote large voids in AlN crystals. Stable rhombohedral and (0001) c-plane facets form the Al-polar side of the void, whereas a wavy and less faceted surface occurs at the N-polar side. The growth rate on Al-polar rhombohedral facets is significantly lower compared to the one on the Al-polar basal facet. Regardless of initial seed orientation, the as-grown crystal surface always grows in Al-polar direction and appears to be composed of pyramids bounded mainly by rhombohedral (01−12) and (01−13) surfaces. Finally, the coloration of AlN crystal was found to depend on growth orientation: the material grown on (0001) facet is almost colorless, but when it is grown on rhombohedral facets it demonstrates characteristic amber color. The material grown on N-polar facets appears dark brown and opaque.

Aluminum nitride by microwave-assisted synthesis: Effect of added ammonium chloride

Hexagonal aluminum nitride (AlN) crystals were synthesized by microwave method with ammonium chloride used as an additive. Starting mixtures consisted of Al powder, NH4Cl, and urea as a fuel taken in a 1: 3: 1 ratio. The microwave oven operated at 630 W, the synthesis time was 10, 30, 60, and 120 min. The results showed that the pure AlN powder with regular and fine grains could be obtained in 30 min. The synthesized powders were characterized by TGA/DTA, XRD, FT-IR, UV-VIS, SEM, and TEM.

AlN bulk crystal growth by sublimation method

AbstractIn this study, we report the growth of aluminum nitride (AlN) bulk single crystals by the sublimation method. The crystals were grown in a newly designed TaC crucible. The crucible has a TaC guard ring around the seed crystal to protect against polycrystal deposition around the seed crystal during the initial growth stage. The ring enhances lateral enlargement growth because it protects against the polycrystal deposition. The (0001) oriented AlN crystals were grown on (0001) SiC and (0001) AlN seed crystals. The largest grown crystal was 43 mm in diameter. The best quality area on the grown AlN crystal had the lowest etch pit density of 1 × 10‐4cm‐2. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Deep‐UV transparent bulk single‐crystalline AlN substrates

AbstractBulk aluminum nitride (AlN) is a very promising substrate material for ultraviolet (UV) and deep‐UV optoelectronics. Bulk AlN crystals grown using multiple sublimation of the source material show plain absorption coefficients below 18 cm–1 in the UV and deep‐UV range of 3.3–5.6 eV (380–220 nm) in the whole sample areas, regardless of sample orientation or light polarization direction. Homogeneity and polarization dependence of optical absorption in the UV and deep‐UV range of such samples is investigated. Finally, cathodoluminescence spectra of samples grown with and without multiple resublimation of the source material are compared. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Hydrostatic deformation potentials and the question of exciton binding energies and splittings in aluminium nitride

By comparing a series of optical experiments performed on bulk aluminum nitride crystals and heteroepitaxial films, we determine the hydrostatic excitonic deformation potentials of AlN. The agreement between the whole available experimental data and our analysis consolidates this determination. Using the previously determined values of the valence-band deformation potentials which account for the strain-induced variation in the crystal-field splitting: ${d}_{3}=\ensuremath{-}8.19\text{ }\text{eV}$ and ${d}_{4}=4.10\text{ }\text{eV}$ we obtain values of $\ensuremath{-}6.04$ and 2.15 eV for the hydrostatic excitonic deformation potentials ${a}_{1}$ and ${a}_{2}$ in the context of the quasicubic approximation. This constitutes the first series of values coherent with the whole set of experimental data. The experimental value of $1s\text{\ensuremath{-}}2s$ splitting disagrees with the theory of excitons in anisotropic semiconductors. This disagreement, we attribute it to our poor knowledge of the valence-band dispersion relations of AlN and to the difficulty we face for including in the calculation plausible values for the anisotropic hole effective mass, dielectric constant.

Approaches for ultrasonic damage detection in aircraft components for structural health monitoring.

Characterization of flaws is an important goal of ultrasonic NDE in aircraft components, especially to meet the new demands for structural health monitoring to achieve retirement for cause. This presentation describes approaches and results for two classes of damage: (1) impact damage in fiber-reinforced composites and (2) cracks in jet engine components. In the application of impact damage on composites the use of non-contact laser based ultrasound with a NdYAG laser as transmitter and Mach–Zehnder interferometer as receiver was complemented by acoustic microscopy. Results are described for both hard and soft impacts on a variety of composites. In the application of crack detection of jet engine components the main emphasis was on the development for in-situ high-temperature piezoelectric sensors. In particular, high temperature operation in excess of 1000 °C is described with sol-gel spray-on coatings, thin AlN films and single crystals of aluminum nitride. Some of the sensors are geared toward wireless retrieval of crack detection in turbine disks and blades. Whereas these approaches in ultrasonic NDE are not routine they hold high promise for near-future applications in the aerospace.

UV transparent single‐crystalline bulk AlN substrates

AbstractBulk aluminum nitride (AlN) is a very promising substrate material for UV optoelectronics, and its UV transparency is of high interest for UV devices designed to emit through the substrate. We report on 500 μm thick bulk AlN substrates with plain UV transmittance exceeding 50% (i.e., absorption coefficients below 14 cm–1) for wavelengths from 220 nm to 380 nm in the main wafer area. Comparing the spectra of different AlN bulk samples, four major below band‐gap absorption bands are identified and interpreted based on a point defect model for AlN crystals. We conclude that a further reduction of oxygen contamination of the source material led to a significant decrease of UV absorption in both colorless and yellowish areas of those substrates. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ring waveguide resonator on surface acoustic waves: First experiments

First results of numerical and physical experiments for the ring waveguide resonator on surface acoustic waves are presented. The structure consists of a gold interdigital transducer in the form of a ring placed on the Z cut of aluminum nitride crystal. Excitation of radial modes is investigated. The electrical admittance frequency dependence of such resonator does not have sidelobes. A high quality factor is achievable.

Point defect content and optical transitions in bulk aluminum nitride crystals

AbstractDifferent zones of nominally undoped AlN bulk single crystals are investigated using optical absorption and cathodoluminescence. Incorporation of impurities and formation of intrinsic defects during growth strongly depend on the facet which forms the zone. Following first principles calculations and earlier observations on AlN epilayers, we assign the endpoints of the observed optical transitions to ionization levels of oxygen, VAl2–/3–, and (VAl–ON)–/2–. According to the strength of the respective optical transitions in different zones, we find that oxygen contamination increases in the order Al‐polar (0001), rhombohedral {10$ \bar 1 $2} and prismatic {10$ \bar 1 $0}, N‐polar (000$ \bar 1 $). We conclude that the low UV transmittance typically observed in bulk AlN is caused by the formation of VAl and (VAl–ON). These deep levels form in the presence of oxygen, which is the major contaminant in bulk AlN. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Seeded Growth of AlN on (0001)-Plane 6H-SiC Substrates

Aluminum nitride (AlN) is a promising substrate material for epitaxy of Al-rich III-nitrides to be employed, e.g., in deep-UV optoelectronic and high-power microwave devices. In this context, preparation of bulk AlN crystals by physical vapor transport (PVT) appears to be of most importance. In this work, seeded growth of AlN on (0001)-plane 6H-SiC substrates was investigated. SiC substrates with a diameter of 15 mm were used. AlN layers with thicknesses up to 3 mm were deposited at growth rates in the range of 10 to 40 μm/hour. Such templates provide large-area seeds, but they are often cracked, especially at thicknesses below 1mm. Besides cracks, other defects from the SiC seed propagate into the AlN layer and subsequently into the bulk AlN crystal. That is why, the aim of this work is to assess structural quality and defect content in thick AlN templates grown on (0001) plane SiC substrates. An optimum thickness-quality, the most appropriate growth stage for further use of the AlN template as a seed for subsequent PVT growth of bulk AlN growth, will be provided. We found that low growth rates mitigate crack propagation; slow cooling as well as optimization of the thermal field inside the crucible can prevent formation of new cracks after growth.

Electron paramagnetic resonance of Er3+ ions in aluminum nitride

An electron paramagnetic resonance (EPR) spectrum from Er3+ ions has been observed in a bulk single crystal of aluminum nitride (AlN). These Er3+ ions were introduced into the crystal during growth and had a concentration of approximately 2×1016 cm−3. The Er3+ EPR signal, monitored at 4.5 K, exhibits axial symmetry (the unique axis is parallel to the c axis in this wurtzite lattice) and shows well-resolved hyperfine splittings due to E167r nuclei. An absence of site splittings in the EPR angular dependence indicates that these erbium ions, replacing aluminum ions in the AlN crystal, have no nearby defects. Principal values for the g and hyperfine matrices are g∥=4.337, g⊥=7.647, |A∥|=454 MHz, and |A⊥|=796 MHz. Forbidden transitions, appearing in the low-field portion of the hyperfine spectrum when the magnetic field is rotated a few degrees away from the c axis, give |P|=7.8 MHz for the nuclear electric quadrupole parameter.

Characterization of bulk AlN crystals with positron annihilation spectroscopy

We have applied positron annihilation spectroscopy to study in-grown vacancy defects in bulk aluminium nitride (AlN) crystals grown by physical vapor transport. We interpret the lowest lifetime value of about 155 ps, measured at low temperatures, to represent the annihilations from the free state of the positron in the crystal lattice. The increased lifetime at high temperatures is an indication of positrons annihilating as trapped at vacancy defects, and a second lifetime component could be separated from the lifetime spectra at temperatures above 400 K. The same lifetime component τ 2=210±10 ps was found in all AlN samples. This component can be attributed to positrons annihilating at Al vacancies, possibly complexed with impurities such as oxygen, present at concentrations in the 10 17 cm −3 range in the samples. In addition to Al vacancies, negatively charged non-open volume defects acting as shallow hydrogenic traps for positrons were detected, with concentrations of about 10 18 cm −3 or higher. These defects are the dominant negative acceptor defects in these samples.

Polarity Analysis of Self-Seeded Aluminum Nitride Crystals Grown by Sublimation

Self-seeded aluminum nitride (AlN) crystals were successfully grown in a tungsten crucible by the sublimation method. The polarities along the growth direction of these AlN samples were characterized by chemical etching, combined with high-resolution transmission electronic microscopy (HRTEM) and Raman spectroscopy. It has been proven by our experimental results that etching in KOH solution and in molten KOH-NaOH can be a reliable method to determine the surfaces with different polarities. Chemically active N-polar (0001) AIN surfaces were removed appreciably, while only etch pits related to threading dislocations were observed on Al-polar surfaces after chemical etching. HRTEM images revealed that there are noticeable inversion domain boundaries in the samples. Raman spectrum measurements showed the vibration mode change before and after etching in the same geometrical arrangement.

Seeded growth of AlN on SiC substrates and defect characterization

In this study, seeded sublimation growth of aluminum nitride (AlN) on SiC substrates was investigated. Large diameter (15–20 mm) and thick (1–2 mm) AlN layers were demonstrated on Si-face, 3.5° off-axis 6H-SiC (0 0 0 1). A c-axis growth rate of 15–20 μm/h was achieved at 1830 °C, and the surface morphology was highly textured: step features were formed with a single facet on the top of the layer. High-resolution X-ray diffraction (HRXRD), X-ray photoelectron spectroscopy (XPS), and molten KOH/NaOH etching were employed to characterize the AlN layers. The AlN crystals grew highly orientated along the c-axis, however, the impurities of Si (3–6 at%) and C (5.9–8 at%) from the SiC changed the lattice constants of AlN and shifted the AlN (0 0 .2) 2 θ value from pure AlN toward SiC. All the growth surfaces had Al-polarity and the dislocation density decreased from 10 8 to 10 6 cm −2 as the film thickness increased from 30 μm to 2 mm.

Effect of Crucibles on Qualities of Self-Seeded Aluminium Nitride Crystals Grown by Sublimation

Self-seeded aluminium nitride (AIN) crystals are grown in tungsten and hot pressed boron nitride (HPBN) crucibles with different shapes by a sublimation method. The qualities of the AIN crystals are characterized by high-resolution transmission electronic microscopy (HRTEM), scanning electron microscopy (SEM) and Micro-Raman spectroscopy. The results indicate that the better quality crystals can be collected in conical tungsten crucible.

Fabrication of free-standing AlN crystals by controlled microrod growth

The aim of this study was to propose a growth procedure for preparation of crack-free thick aluminum nitride (AlN) layers that can be easily separated from the substrate. The overall process is based on the physical vapor transport method employing a seed and a source material. In this case, the substrate is an epitaxial 4H-SiC layer and the growth of AlN is initiated at etch pits formed during the ramp up time prior to establishing growth temperature. Development of hexagonal pyramids on which arrays of microrods are formed is the core of the growth procedure. Free-standing wafers having 10 mm diameter and about 120 μm thick have been fabricated.

Experimental and theoretical analysis of sublimation growth of AlN bulk crystals

The current status of sublimation growth of aluminum nitride (AlN) bulk crystals is discussed. Growth of AlN single-crystal layers on silicon carbide (SiC) seeds in pre-carbonized tantalum crucibles in graphite equipment and of AlN bulk crystals on the AlN layers in tungsten crucibles and equipment is considered. All stages of the technology, including pre-growth processing (preparation of durable crucibles, high-purity AlN sources, and high-quality seeds), seeding on SiC and AlN, growth of bulk AlN crystals, and post-growth processing (calibration, slicing, lapping, polishing, and characterization of the crystals) are described. Special attention is given to “scaling” the technology to grow large-diameter (up to 2 in) AlN crystals, in which connection seeding on large-diameter SiC substrates and lateral overgrowth of the crystals are considered.