Characterization of bulk AlN with low oxygen content

Abstract

We have established a PVT growth process for preparation of polycrystalline bulk AlN with low impurity content, especially in regard to oxygen. With this process at hand, we compare crystals grown at different temperatures in order to investigate the impact of growth temperature on impurity incorporation and crystal properties. Bulk AlN samples grown at temperatures of 2125°C and 2225°C were investigated. Chemical analysis shows that oxygen contamination in the crystals equals to a concentration of about 1×10 19 cm −3 near the final as-grown surface. This is the lowest value in bulk AlN reported up to date. No change of oxygen contamination with growth temperature is observed. The samples are electrically insulating at room temperature. Resistivity measurements on a sample grown at 2225°C showed a thermal activation energy of about 860 meV in the 500–650 K range. In optical absorption, a band at around 2.8 eV was detected in all of our samples, while different absorption bands show up in the 3.0–6.0 eV energy range. The bands are presumably ‘oxygen-related’, but as changes in growth temperature lead to significant variation in the optical properties of AlN even in samples with comparable oxygen content, we suggest that these bands are also influenced by temperature-dependent formation of intrinsic defects. Additional bands at 5.0 and at 5.6 eV, which are observed only in samples grown at lower temperatures, are attributed to the presence of nitrogen vacancies. Finally, thermal conductivity λ of the samples was found to be mainly limited by phonon–phonon scattering in the investigated temperature range of T=293–1473 K. In the resulting relation λ∝ β/ T, the variation of β is caused by scattering on aluminum vacancies and reflects differences in growth temperature as well as sample position. Presumably, aluminum vacancy formation does not only depend on oxygen contamination, but also on growth temperature. We conclude that although further reduction of oxygen contamination is considered a key prerequisite for enhancing crystal quality, variations in optical, electrical, and thermal properties, as observed in samples grown at different temperatures, can be attributed to vacancy formation rather than to oxygen alone. Thus, vacancy formation control is another key issue for further optimization in AlN bulk crystal growth.