AlN Crystal Growth Research Articles

Tantalum carbide coating via wet powder process: From slurry design to practical process tests

TaC coatings were deposited on graphite substrates via wet powder forming and sintering to potentially achieve both a highly reliable and low-cost process. Non-aqueous solvent mixtures of TaC slurries were optimized through characterization of raw materials and a novel design guide based on Hansen solubility parameters. The optimized TaC slurries enabled the formation of high-quality TaC powder compact films with ultrahigh powder packing densities of ≥70% (ca. 85% at maximum), which contributed to prevent defect formation in the TaC coatings after sintering. The TaC-coated components were tested in practical high-temperature processes, such as AlN and SiC bulk single crystal growth processes, and a SiC device fabrication process, which confirmed sufficiently low-levels of impurity incorporation and surface contamination from the TaC layers. The novel TaC-coated graphite components will contribute to higher quality and lower cost for bulk crystal growth, device fabrication, and epitaxial film growth processes of group-III nitrides and SiC.

6H-SiC 종자 결정을 사용하여 PVT법으로 성장된 AlN 결정 연구

The effect of process parameters such as the growth pressure and temperature on the AlN crystal growth has been investigated. AlN crystal was grown onto 6H-SiC seed crystal using PVT (Physical Vapor Transport) method. Crystal properties and morphology of AlN crystal was changed with growth pressure and temperature. Raman analysis confirmed that AlN crystals with different orientation were successfully grown on SiC seed crystal.

Growth of bulk AlN crystals by vapor-phase epitaxy from atomic Al and NH3

A new approach to obtaining bulk AlN single crystals by vapor-phase epitaxy has been tested. NH3 and Al vapor were used as growth reagents. The following ranges of growth parameters were admissible for the laboratory equipment (experimental growth installation): temperatures of 1050–1500°C at ammonia flow rates of up to 50 sccm and pressures on the order of 10–5–10–4 bar, growth rates of up to 200 μm h–1. At a temperature of 1450°C, samples of strained bulk block AlN crystals with thicknesses of up to 200 μm were obtained in the wurtzite phase in the [0001] direction on MBE templates based on sapphire substrates with a diameter of 2″.

Substrates and epitaxial deposition processes for Group III-nitride thin films and power device heterostructures

Abstract

溶鉄-固体アルミナ間の界面物性を利用した窒化アルミニウム結晶成長

溶鉄-固体アルミナ間の界面物性を利用した窒化アルミニウム結晶成長

Native seeding and silicon doping in bulk growth of AlN single crystals by PVT method

AbstractHomo‐epitaxial growth of AlN crystals using native seeds prepared from our hetero‐epitaxial AlN/SiC template crystals has been successfully carried out. Single crystals of acceptable structural properties with XRD‐RC FWHM of 150 arcsec, Raman E2(high) phonon mode FWHM of 9 cm‐1 and with low concentrations of background impurities (below the detection limit of EPMA) were obtained. An effort has been made to obtain n‐type conductivity in the PVT grown AlN single crystals through intentional Si‐doping during bulk growth. The investigations using EPMA and micro‐Raman measurements confirm that it is possible to incorporate intentionally the Si atoms in the bulk AlN host lattice without any kind of Si inclusions or precipitates. Structural quality of the Si‐doped crystals was almost same as that of the native seed crystal. Si‐doping induces a compressive strain in the doped AlN crystals as seen from the 5 cm–1 shift (blue shift) in the Raman E2(high) peak position. EPR measurement gives first evidence of the presence of shallow level Si donor in the grown crystal, though the free‐carrier concentrations are quite low for a semiconductor doping. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A study of the step-flow growth of the PVT-grown AlN crystals by a multi-scale modeling method

A multi-scale model taking into account surface reactions and material transport in vapor was developed for PVT growth of AlN.

The Influence of ZrO2 Additions on Al2 O3 Evaporation and AlN Crystal Growth by Thermal Nitridation of Al2 O3 -ZrO2 Pellets

The effects of ZrO2 additions to Al2O3 were investigated to improve the evaporation rate of Al2O3 for bulk AlN crystal growth. The evaporation rate of Al2O3 increased concomitantly with increasing ZrO2 concentration under a nitrogen gas stream at 2223 K. The ZrO2 was predominantly nitrided. The nitridation of ZrO2 kept the local oxygen partial pressure high at the pellet surface, which suppressed the nitridation of Al2O3. The nitridation of ZrO2 caused the outward diffusion of ZrO2 (Zr4+ and O2−) in the pellet, which was accelerated further by the presence of Al2O3–ZrO2 liquid phase in grain boundaries, leading to the prompt formation of ZrN porous layer on the pellet surface. The suppressed nitridation of Al2O3 and the formation of porous ZrN layer were the reasons for the enhanced evaporation of Al2O3, leading to enhanced bulk AlN growth.

Sublimation Growth of AlN and GaN Bulk Crystals on SiC Seeds

Growth techniques of high quality AlN and GaN bulk crystals on SiC seeds by sublimation sandwich method are presented. GaN crystals were grown in the temperature range of 1100-1250 °C and with addition of ammonia (NH3) to prevent GaN decomposition. GaN powder or metallic Ga was used as the source. AlN crystals up to 2 inch diameter have been grown on SiC seeds in the temperature range of 1950 -2050 0С. Kinetic mechanisms and transport features in the sandwich cell are discussed. The achieved high crystal quality has allowed producing semiconductor devices on their basis, in particular, ultraviolet LEDs

Specific features of sublimation growth of bulk AlN crystals on SiC wafers

AbstractThe features of 2” AlN bulk crystal growth by the sublimation sandwich method (SSM) on SiC seeds in a graphite‐heated reactor are investigated. AlN growth conditions and crystal properties are compared with the results of SiC sublimation growth in the same reactor. The specifics of AlN sublimation growth, including insufficient reproducibility of results, are believed to be caused by the low sticking coefficient of molecular nitrogen and the high reactivity of Al vapors. The formation of pores in the AlN crystal is explained by selective sublimation etching of the growing surface. The negative effect of non‐optimal growth conditions and impurities on AlN crystal quality is discussed. The analysis of container materials, inert to Al vapors was carried out. Various types of TaC crucibles suitable for growth of bulk AlN crystals on SiC seeds were developed. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Characteristics of AlN crystal growth depending on m‐ and a‐axis oriented off‐angle of c‐sapphire substrate

AbstractFor the achievement of high‐efficiency AlGaN‐based deep‐ultraviolet (DUV) light‐emitting diodes (LEDs), we investigated the characteristics of AlN crystal growth depending on m‐ and a‐axis oriented off‐angle of c‐sapphire substrate. In order to reduce threading dislocation density (TDD) in AlN crystal growth, we used ammonia (NH3) pulse‐flow multilayer (ML) growth technique in the metal‐organic chemical vapor phase deposition (MOCVD). We found that a macrostep geometry (large step‐bunching) is generated on the surface of AlN template grown on m‐axis oriented (0001) sapphire substrate. On the other hand, macrostep‐free surface can be obtained for AlN grown on a‐axis oriented sapphire. We also found that cracks are easier generated in AlN grown on a‐axis oriented sapphire. It is considered that step‐flow growth is easily obtained for the growth of a‐axis oriented case due to that the atom site of the step edge in the growth is more stable. On the other hand, for m‐axis oriented case, step‐bunching is considered to occur because atoms are easily drifted during the growth due to that the atom site of the edge steps are more unstable. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The impact of pressure and temperature on growth rate and layer uniformity in the sublimation growth of AlN crystals

To effectively design a large furnace for producing large-size AlN crystals, a fully coupled compressible flow solver was developed to study the sublimation and mass transport processes in AlN crystal growth. Compressible effect, buoyancy effects, flow coupling between aluminum gas and nitrogen gas, and Stefan effect are included. Two sets of experimental data were used to validate the present solver. Simulation results showed that the distributions of Al and N 2 partial pressures are opposite along the axial direction due to constant total pressure and Stefan effect, with the Al and nitrogen partial pressures being highest at the source and seed crystals positions, respectively. The distributions of species inside the growth chamber are obviously two-dimensional, which can curve a flat crystal surface. Simulation results also showed that AlN crystal growth rate can be increased by reducing total pressure or by increasing seed temperature or by increasing source-seed temperature difference. High nitrogen pressure causes decrease in growth rate, but it is beneficial for obtaining uniform growth rate in the radial direction. Results of simulation also showed that there is an optimized temperature difference (40 °C) in the present furnace for obtaining good homogeneity of growth rate.

Comparison of the homoepitaxial growth of AlN crystals on Al and N surfaces

A special experimental technique capable of excluding the influence of differences between sources, substrate crystals, and growth process parameters has been employed to correctly compare the quality of homoepitaxial AlN layers grown by sublimation on Al and N surfaces of the substrate crystal with 〈0001〉 orientation. It has been found that, in most cases, the quality of layers grown on the N surface is somewhat higher; however, no differences have been observed in separate cases. Hence the conclusion follows that the range of growth process parameters is substantially wider for the N side compared with the Al side.

Effect of the seed crystallographic orientation on AlN bulk crystal growth by PVT method

AbstractThe growth of AlN crystals by PVT method was investigated using TaC crucible in the temperature range of 2250‐2350 °C. AlN boules with 30 mm in diameter were successfully grown on the crucible lid by self‐seeded growth. The AlN boules consist of the spontaneously nucleated AlN single crystal grains with the {1010} natural crystalline face. The fast growth rate of more than 1 mm/h was achieved. AlN crystals grown on (11$ \bar 2 $0)‐, (10$ \bar 1 $0)‐, and (0001)‐face AlN seeds were investigated. Different experimental phenomena have been observed under particular condition. The crystal grown on (11$ \bar 2 $0)‐face seed has different natural crystalline face from the seed. For the crystal grown on (10$ \bar 1 $0) or (0001) seed, the crystal natural crystalline face is same as the crystallographic orientation of the seed. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

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.

Native oxide and hydroxides and their implications for bulk AlN crystal growth

Oxygen degrades the properties of AlN, thus producing bulk single crystals with low oxygen concentrations is an important goal. Most of the oxygen in bulk AlN single crystals grown by the sublimation–recondensation method originates from the hydroxides and oxides that spontaneously form on the surfaces of the AlN source powder. For a typical AlN powder with an average particle size of 1–2 μm, a 1–3 nm thick oxide and/or hydroxide can account for most of its oxygen (generally in the order of 1.0 wt%) and hydrogen (200–300 ppm). Heating the AlN powder source at 1950 °C for 10 h in a nitrogen atmosphere reduced its surface area by a factor of 160 (due to sintering), the oxygen concentration by 16 and the hydrogen concentration by 67. The difference in these reduction factors suggests some of the oxygen is dissolved into the bulk AlN with this heat treatment. Firstly annealing the AlN powder at a low temperature (950–1000 °C) for several hours before sintering at 1950 °C, the oxygen and hydrogen concentrations were reduced to lower levels. The low-temperature treatment is effective in eliminating oxygen and hydrogen from the surface of the powder, while high-temperature sintering reduces the specific surface area of the source. The combination of heat treatments produced a source with oxygen and hydrogen concentrations as low as 0.015 wt% O (1.9×10 19 atoms O cm −3) and 1.7 ppm H (3.4×10 18 atoms H cm −3). Annealing becomes less effective at removing oxygen and hydrogen with longer heat treatments, suggesting there is a minimum oxygen concentration that can be produced by this method.

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)

Sublimation growth of aluminum nitride on silicon carbide substrate with aluminum nitride–silicon carbide alloy transition layer

The advantages of depositing AlN–SiC alloy transition layers on SiC substrates before the seeded growth of bulk AlN crystals were examined. The presence of AlN–SiC alloy layers helped to suppress the SiC decomposition by providing vapor sources of silicon and carbon. In addition, cracks in the final AlN crystals decreased from ∼5 × 106/mm2 for those grown directly on SiC substrates to less than 1 × 106/mm2 for those grown on AlN–SiC alloy layers because of the intermediate lattice constants and thermal expansion coefficient of AlN–SiC. X-ray diffraction confirmed the formation of pure single-crystalline AlN upon both AlN–SiC alloys and SiC substrates. X-ray topography (XRT) demonstrated that strains present in the AlN crystals decreased as the AlN grew thicker. However, the XRT for AlN crystals grown directly on SiC substrates was significantly distorted with a high overall defect density compared to those grown on AlN–SiC alloys.

The Effect of Aluminum Nitride-Silicon Carbide Alloy Buffer Layers on the Sublimation Growth of Aluminum Nitride on SiC (0001) Substrates

The benefits of depositing AlN-SiC alloy transition layers on SiC substrates before the seeded growth of bulk AlN crystals were determined. The presence of the AlN-SiC alloy layer helped to suppress the SiC decomposition by providing vapor sources of silicon and carbon. It enabled a higher growth temperature, and hence a higher growth rate. In addition, cracks in the final AlN crystals can be decreased because of the intermediate lattice constants and thermal expansion coefficient of AlN-SiC alloy. AlN-SiC alloys were first grown on off-axis SiC substrates by the sublimation-recondensation method. Then pure AlN crystals were grown upon those. For comparison, AlN crystals were directly grown on SiC substrates under similar conditions. X-ray diffraction (XRD) confirmed the formation of a pure single crystalline AlN layer upon the AlN-SiC alloy on SiC substrate. The presence of an AlN-SiC transition layer effectively inhibited the appearance of cracks in the resultant AlN crystals. X-ray topography (XRT) demonstrated that the thick AlN layer effectively released the strain present.