Effect of impurity incorporation on crystallization in AlN sublimation epitaxy

Abstract

We have implemented graphite, graphite-tantalum (Ta), and Ta growth environment to the sublimation epitaxy of aluminum nitride (AlN) and have studied development, morphological, and cathodoluminescence emission properties of AlN crystallites. Three apparently different types of crystallites form in the three different types of growth environment, which presumably manifests the relationship between crystallite-habit-type and impurities. Comparison between the cathodoluminescence spectra reveals certain dynamics in the incorporation into AlN of the main residual dopants, oxygen and carbon, when the growth environment changes. At high temperatures, in addition to Al and N2, which constitute the vapor over AlN, vapor molecules of CN, NO, Al2C, and many more can be present in the vapor from which AlN grows and both oxygen and carbon can be incorporated into AlN in varying ratios. Involving calculations of the cohesive energy per atom of such vapor molecules and also of Ta containing molecules, we have considered possible mechanisms how oxygen and carbon get incorporated into AlN and how this kinetics interferes with the growth environment. The positive effect of Ta consists in the marked reduction of residual oxygen and carbon impurities in the vapor from which AlN is growing. However, on the account of this reduction, the overall composition of the vapor changes. We speculate that during AlN nucleation stage small impurity levels may be beneficial in order to provide a better balance between the AlN crystallites development and impurity incorporation issues. We have shown that some impurity containing vapor molecules are acting as essential transport agents and suppliers of nitrogen for the AlN growth.