Anisotropic chemical etching is an important means for characterizing the polarity and defect density of single crystals. In this letter, we present the results of our studies on the etching of bulk AlN crystals in aqueous potassium hydroxide solution. The nitrogen polarity (0001) basal plane initially etched rapidly, while the aluminum polarity basal plane, and prismatic (1100) planes were not etched. The etch rate of the nitrogen polarity basal plane eventually decreased to zero, as the surface became completely covered with hexagonal hillocks which were bounded by {1101} planes. The hillock density for the self-seeded AlN crystals studied was typically in the range of 5 × 10 7 cm -2 to 10 9 cm -2 . From our analysis of etched AlN crystals, we infer that freely nucleated crystals predominately have the nitrogen to aluminum direction pointing out from the nucleation surface, that is the ends of the AlN crystals facing the source are aluminum polarity.
A global model accounting for both the surface kinetics and transport in the vapor phase for AlN sublimation growth is described in this paper. The model parameters for the sticking coefficient of N 2 were identified from the experimental data. The refined model more accurately predicts the growth rate over a wider pressure range than the model considering transport only. Above 200 Torr the growth rate is determined by the transport, while below 100 Torr it is controlled by surface kinetics in the current temperature range ͑1700-1900°C͒.Aluminum nitride has attracted much attention as the terminal composition for Al x Ga 1Ϫx N-based electronic and optoelectronic devices due to its excellent properties. 1 The sublimation technique is the most promising method to grow AlN bulk crystals. 2-4 Due to the high thermal stability and low reactivity of nitrogen, the low growth rate of AlN is a major problem of this method. Therefore, understanding the AlN growth rate dependence on the temperature and pressure is critical for improving the performance for this technique.Previously, we formulated a detailed two-dimensional model which included the thermal convection and Stefan flow for predicting the AlN sublimation growth rate. 5 The model assumed that there are no adsorption or nucleation barriers at the source and seed interfaces. A detailed numerical investigation of the transport effect on the sublimation growth was carried out. The theoretical predictions agreed well with the experimental data at pressures above 100 Torr and an operating temperature ͑seed temperature͒ ranging from 1700 to 1900°C. The activation energy of the growth rate, 681 kJ/mol, was close to the sublimation heat of aluminum nitride. However, at low pressure ͑less than 100 Torr͒, the model predicted considerably higher growth rates than experimentally observed, since the dissociation of N 2 became important and consequently, the assumptions were no longer valid. Actually, at low pressure the pressure of N 2 is no longer thousands of times that of aluminum. Due to the extremely low sticking coefficient of N 2 at the crystal surface, 6 only about 10 Ϫ4 at 1800°C, the availability of nitrogen atoms at the growing surface is the rate-controlling step. Consequently, a surface kinetic model considering the adsorption/desorption of Al and N 2 is necessary to predict the growth rate more accurately at low pressure.Dryburgh 7 estimated the maximum growth rate for AlN crystal grown by sublimation utilizing only thermodynamics. But in practice, crystal growth is not strictly controlled by thermodynamics; vapor transport and surface kinetics are of greater importance. Recently Segal et al. 8 developed a model for the sublimation of AlN considering both transport effects and surface kinetics. The model predictions compared well with the experimental data. In this paper, we establish a global model considering both the vapor transport and surface kinetics effect over wider temperature and pressure operation ranges. In contrast to their model, here a simple trans...
Single crystalline platelets of aluminum nitride (AlN) were successfully grown by a new technique. It consists of (1) depositing an AlN buffer layer on a SiC substrate by metal organic chemical vapor deposition (MOCVD) below 1100°C, (2) forming an (AlN)x(SiC)1−x alloy film on the AlN film by condensing vapors sublimated at a temperature of 1800°C from a source mixture of AlN-SiC powders, followed by (3) condensing vapors sublimated from a pure AlN source (at 1800°C). The necessity of the first two steps for the successful AlN sublimation growth on SiC substrate was illustrated by the initial nucleation studies of alloys on SiC substrates with and without MOCVD AlN buffer layers: an AlN MOCVD buffer layer leads to continuous, single grain growth mode; The (AlN)x(SiC)1−x alloy film reduces the crack density because its thermal expansion coefficient is intermediate between SiC and AlN. X-ray diffraction (XRD) and Raman spectroscopy studies indicated the high quality of the AlN single crystal.
The effect of substrate preparation on the sublimation growth of AlN at about 1800 °C and 400 torr on (0001) 6H-SiC was investigated. The AlN grew in the step flow growth mode on an off-axis 6H-SiC substrate with a 6H-SiC epilayer, an island growth mode on as-received substrates, and a 2-D growth mode on substrates first coated with an AlN epitaxial layer by MOCVD. Cracks in the deposited AlN crystal due to the lattice and thermal expansion coefficient mismatches were always observed by SEM and optical microscopy.
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