Favourable growth conditions for the preparation of bulk AlN single crystals by PVT
We report on the growth and characterization of bulk AlN crystals prepared using five different growth conditions (seed temperatures and temperature differences between source and seed) in an otherwise identical growth setup. The experimentally determined growth rates (106–212 μm h−1) are in very good agreement with a simple growth rate model taking into account the supersaturation dependent spiral growth mode at the growth surface. The observed average growth rate increases with both temperature and temperature differences between source and seed. Increasing the temperature at the seed from 2200 °C to 2250 °C, the dislocation density decreases by more than one order of magnitude. At the same time, the deep-UV optical absorption in the 240–300 nm range decreases to well below 50 cm−1, presumably due to a lower oxygen incorporation. Thus, high growth temperatures are favourable to prepare AlN crystals to be used as substrates in AlGaN-based deep-UV emitters, where the light is coupled out through the substrate.