Aluminum Nitride Bulk Crystal Growth in a Resistively Heated Reactor

Abstract

A resistively heated reactor capable of temperatures in excess of 2300°C was used to grow aluminum nitride (AlN) bulk single crystals from an AlN powder source by physical vapor transport (PVT) in nitrogen atmosphere. AlN crystals were grown at elevated temperatures by two different methods. Self-seeded crystals were obtained by spontaneous nucleation on the crucible walls, while seeded growth was performed on singular and vicinal (0001) surfaces of silicon carbide (SiC) seeds.

During self-seeded growth experiments a variety of crucible materials, such as boron nitride, tungsten, tantalum, rhenium, tantalum nitride, and tantalum carbide, were evaluated. These studies showed that the morphology of crystals grown by spontaneous nucleation strongly depends on the growth temperature and contamination in the reactor. Crucible selection had a profound effect on contamination in the crystal growth environment, influencing nucleation, coalescence, and crystal morphology. In terms of high-temperature stability and compatibility with the growth process, the best results for AlN crystal growth were obtained in crucibles made of sintered tantalum carbide or tantalum nitride. In addition, contamination from the commercially purchased AlN powder source was reduced by pre-sintering the powder prior to growth, which resulted in a drastic reduction of nearly all impurities. Spontaneously grown single crystals up to 15 mm in size were characterized by x-ray diffraction, x-ray topography, glow discharge mass spectrometry, and secondary ion mass spectrometry. Average dislocation densities were on the order of 10³ cm⁻³, with extended areas virtually free of dislocations. High resolution rocking curves routinely showed peak widths as narrow as 7 arcsec, indicating a high degree of crystalline perfection. Low-temperature partially polarized optical reflectance measurements were used to calculate the crystal-field splitting parameter of AlN, Δ[subscript cr] = -230 meV, and from this, a low-temperature (1.7 K) band gap energy of 6.096 eV was obtained for unstrained wurtzite AlN.

Seeded growth of AlN bulk crystals on on-axis and off-axis (0001), Si-face SiC seeds was investigated as a means to scale up maximum single crystal size and pre-define crystal orientation. A two-step deposition process was developed for the growth of thick layers. AlN layers 0.1—3 mm thick were deposited on inch-sized seeds. X-ray diffraction analysis evidenced that the AlN grew in the direction of the seed. A one-dimensional isotropic model was formulated to calculate the thermal stress distribution in AlN/SiC heterostructures. Cracks formed in the AlN layers due to the thermal expansion mismatch between AlN and SiC were observed to decrease with increasing AlN thickness, in agreement with model calculations. Crack-free AlN crystals were obtained from grown layers by evaporating the SiC seed in situ during high-temperature PVT growth. Based on these results, a reproducible seeded growth process was developed for production of crack-free AlN crystals having pre-determined polarity and orientation.