Boris M. Epelbaum

Ultrasonic Microspectroscopy Characterization of AlN Single Crystals

Basic acoustic properties of AlN single crystal Y-cut and Z-cut plates, grown by the physical vapor transport method, were evaluated using ultrasonic microspectroscopy (UMS) technology. A method of determining the acoustical physical constants with two specimens was developed by measuring the velocities of longitudinal and shear waves and leaky surface acoustic waves (LSAWs). We obtained accurate bulk-wave velocities, LSAW velocities and their distributions in the specimen surfaces, and the density with great differences compared with the calculated ones using the reported constants. We also determined the corresponding constants. This UMS technology will contribute to the further development of AlN crystals.

On the mechanisms of micropipe and macrodefect transformation in SiC during liquid phase treatment

The evolution of hollow-core defects present in vapor phase grown SiC bulk crystals during subsequent liquid phase epitaxial treatment has been investigated in a wide range of supersaturation conditions. Hollow macrodefects were found to decompose into a number of micropipes (MP) already at supersaturations close to zero. The elimination of pure screw-dislocation based MP requires a higher supersaturation. Micropipes were observed to dissociate into individually acting non-hollow core dislocations. After decomposition the activity of growth center based on a MP is usually reduced and a new center may dominate the growing surface. A model for the mechanism of MP transformation is proposed which is based on BCF theory and Chernovs theory of morphological stability.

Orientation-dependent phonon observation in single-crystalline aluminum nitride

In this study, we present a microspectroscopic investigation performed on different facets of a self-nucleated aluminum nitride (AlN) single crystal. We show that, apart from evaluating crystalline quality, Raman and Fourier transform infrared spectroscopy can provide means to detect the orientation of any AlN facet. Such local, nondestructive technique is very useful for selecting and evaluating samples of single crystalline AlN.

Seeded PVT Growth of Aluminum Nitride on Silicon Carbide

Seeded bulk crystal growth of AlN was conducted by physical vapor transport (PVT) using AlN powder as a starting material. SiC substrates with different crystallographic orientation were used as seeds. Materials compatibility was investigated; temperatures higher than 2000°C lead to decomposition of the SiC seed and to degradation of the tungsten heaters. Hexagonal hillocks grow on c-plane SiC seeds leading to growth instabilities and layer quality deterioration, whereas for growth on a-plane seeds millimeter-sized a-oriented areas were obtained showing a smooth morphology typical for step-flow growth mode. A comparison of the natural growth habits of freely nucleated SiC platelets and prismatic elongated AlN can explain this behavior. The best crystal quality was obtained by employing slightly off-oriented a-plane SiC wafers at growth temperatures below 2000°C, but growth rate and surface diffusion necessary for step-flow growth remain insufficient. Introduction Bulk crystals of AlN have a great potential to provide a better substrate for group-III nitride electronic devices than currently available sapphire and silicon carbide [1]. Despite a number of promising crystal growth studies on AlN [3-5] published after the pioneering work of Slack and McNelly [2], available crystals are still to small to allow their further use in seeded growth. Very recently Schlesser, Dalmau and Sitar demonstrated the feasibility of PVT seeded growth on small-sized spontaneously nucleated AlN platelets [6], but for production of larger crystals silicon carbide remains to be the only readily available choice for seeding. Balkas et al. [3] and Sarney et al. [8] grew AlN layers on on-axis 6H-SiC, but deposited layers contain multiple grains of millimeter size. Finally, Liu et al. have achieved an improvement of the AlN layer quality by applying buffer layers of MOCVD grown AlN prior to vapor growth. One critical shortage of SiC seeds in AlN sublimation growth is the problem of poor chemical compatibility of these materials as emphasized in [7]. In our present experimental study we have investigated the dependence of the AlN layer surface morphology and crystal quality on the SiC substrate orientation. Experimental procedure Crystal growth experiments were conducted in a small resistively heated growth reactor provided with tungsten heating elements capable up to 2500°C in the atmosphere of high-purity nitrogen, which was described in details in our previous work [7]. Most of the results discussed below were obtained at temperatures around 2000°C. A flow of 5N purity nitrogen through the reactor volume at 50-70 sccm was established, while the total pressure was kept at 350 mbar, i.e. nearly stagnant gas flow conditions were applied. The source material was AlN powder with 99.5% purity (main residual impurity is oxygen) supplied by H.C. Starck GmbH. Seed plates 10x10 mm2 in size and having different orientations were cut from 6H-SiC crystals grown in our laboratory and mounted inside a graphite holder coated by a protective SiC layer. The utilization of the Materials Science Forum Online: 2003-09-15 ISSN: 1662-9752, Vols. 433-436, pp 983-986 doi:10.4028/www.scientific.net/MSF.433-436.983

Novel solution growth method of bulk AlN using Al and Li3N solid sources

In this work, we developed a solution growth method that uses Li–Al–N solution to epitaxially grow AlN on a self-nucleated, columnar AlN seed crystal. The seed crystal was grown by physical vapor transport, and the solution was obtained by annealing a Li3N–Al mixture. The epitaxial AlN grew ~5 µm in 10 h. Scanning electron microscopy analyses showed that the grown layer had many voids near the epilayer/seed interface, but no evidence of cracks. Using transmission electron microscopy analyses, we found that the growth direction of the AlN was [1100] and the layer had threading dislocation propagating along [1100] with a density of ~4×108 cm-2.

Lattice-plane curvature and small-angle grain boundaries in SiC bulk crystals

SiC crystals grown by the physical vapour transport process along the [001] direction show a curvature of the crystal growth front in correspondence with the shape of the isotherms. A large radius for the curvature of the isotherms enhances the formation of an extended facet. Under the facet, the lattice planes are flat with a high crystal quality as expressed by rocking-curve half widths of 0.022°. In the non-faceted region, the lattice planes become bent, following the shape of the isotherms with a radius of typically 0.5 to 0.8 m and an increased rocking-curve half width of 0.3°. A reduction of the growth rate from 300 μm h -1 to 70 μm h -1 does not affect this behaviour significantly. The lattice-plane curvature and the development of the facet are predominantly affected by the shape of the isotherms. For crystals grown in the [015] direction, the lattice planes adjust only in a one-dimensional manner to the isotherms. In all cases, the lattice-plane curvature results from the formation of a high density of small-angle grain boundaries. They are generated by the condensation of dislocations with Burgers vectors in the ab plane.

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.

Flux Growth of SiC Crystals from Eutectic Melt SiC-B4C

Well known obstacles in solution growth of SiC are very small growth rate limited by low solubility of carbon in silicon-based melts used up to now in flux growth and poor growth stability. The emphasis of our work was the search for alternative high-temperature solvents for SiC taking the example of SiC-B4C eutectic. The main problem of using this eutectic system is the choice of a practical crucible material. To solve this problem we have prepared polycrystalline SiC crucibles of high-purity and of theoretical density using the approach of PVT growth on graphite mandrels. Crystal growth experiments have been conducted in SiC crucibles at temperature of 23002350°C. Both self-nucleated SiC platelets up to 5 mm in diameter and epitaxial layers on PVT grown 6H-SiC substrates have been grown.

Sublimation Growth of Bulk AlN Crystals: Process Temperature and Growth Rate

This study was made to estimate the temperature range for AlN growth by sublimation and to evaluate the rate-limiting step in order to achieve bulk growth at practically useful rates. Growth experiments have been conducted in a proprietary designed reactor capable up to 2500°C in vertical transport geometry. PVT transport of AlN was found to be feasible in a wide range of temperature starting from 1850°C, but stable growth of well-faceted crystals was possible only at temperatures exceeding 2100°C. Quick material transport in combination with insufficient crucible integrity at said temperatures is a crucial problem and the main size-limiting factor in PVT growth of AlN. Polycrystalline boules up to two inch in diameter and up to 15 mm produced at growth rates 0.3-3.0 mm/h have been demonstrated. Grown material is a theoretically dense, highly pure (below 100 ppm of oxygen), textured bulk containing single-crystalline areas larger than 5x5 mm 2 . Introduction Recently the first devices grown and processed on bulk AlN substrates have been reported. They include AlGaN/AlN quantum wells (MQW) emitting in deep UV region (shortest stimulated emission of 258 nm reported up to date in semiconductor materials) [1] as well as high-power AlGaN/GaN/AlN HFETs being superior than analogous devices made on SiC [2]. However, the potential of AlN in these and also other applications (in particular in surface acoustic wave devices) has been disadvantaged by the lack of sufficiently large bulk single crystals. The largest substrates prepared up to now are less than 10x10 mm 2 in size [3] and it makes device processing difficult due to sample edge effects. The reason for small crystal size remains a subject of much controversy. One common opinion is that AlN can grow only very slowly at realistic PVT conditions because of small sticking coefficient of nitrogen, therefore the interface kinetics is the limiting factor in the growth rate [5,9]. Another known obstacle in growth technology is the deficiency of sufficiently inert and tight crucible material [4,6]. AlN is a strong-bonded covalent compound, and limited atomic mobility prevents single-crystalline growth of AlN at T<1850°C as reported in [7], however no systematic study has been published up to now on PVT growth temperature of AlN. Evaluation of growth temperature in evaporation experiments It is a well-known observation that a solid material may be transported via the gas phase and deposited at the cold end of the crucible over a range of temperatures. However the sublimation product will be a bulk single crystal only when (i) it is produced at certain elevated temperature (i.e. sufficient surface mobility of adsorbed atoms is achieved) and (ii) the rate of material transport towards growing interface does not exceed the limit set by maximal rate of morphologically stable growth at given temperature. Otherwise the process of sublimation-recondensation leads to formation of powder, porous ceramic-like material or imperfect (twinned, dendritic) crystals. In real high-temperature PVT experimental conditions it is often difficult to distinguish between kinetic and transport effects, since both are temperature dependent. In order to evaluate growth temperature Materials Science Forum Online: 2004-06-15 ISSN: 1662-9752, Vols. 457-460, pp 1537-1540 doi:10.4028/www.scientific.net/MSF.457-460.1537

Polarization-dependent below band-gap optical absorption of aluminum nitride bulk crystals

The polarization dependence of the below band-gap optical absorption of aluminum nitride (AlN) is investigated in detail using cuts of bulk single crystals grown by physical vapor transport. We show that optical absorption at 445nm (2.8eV) features a polarization-dependent transition which is strongest for P⊥c, while optical absorption in the range of 250–320nm (4–5eV) features a transition which is strongest for P∥c. Such information may aid in understanding the nature of the underlying electronic transitions and subsequently decreasing unwanted blue/UV optical absorption in AlN.