Al- and N-polar AlN layers grown on c-plane sapphire substrates by modified flow-modulation MOCVD

We report the growth of N- and Al-polar AlN layers on c-plane sapphire by flow-modulation MOCVD (FM-MOCVD) with some flow sequence modifications. Surface polarities were decided by pre-treatment for sapphire substrates prior to AlN seeding layer growth. Low-temperature nitridation (620 °C) was done for the N-polar samples, and no-nitridation was done for the Al-polar. To avoid strong vapor-phase reaction between TMA and NH3 and to enhance the surface migration of Al atoms, FM-MOCVD technique was used. The flow sequence was modified for optimization of each N- and Al-polar growth. The surface features were completely different between the N- and Al-polar AlN layers in atomic force microscope images. The former had domed structures and the latter atomic steps. The dislocation densities were counted from plane-view transmission microscope images to be about 3×1010 cm−2 for the N-polar layer and about 1×1010 cm−2 for the Al-polar. Secondary-ion mass spectroscopy measurement revealed oxygen- and carbon-incorporation into the layers. Band-edge and impurity-related emissions were observed from only the Al-polar layer by room-temperature cathodoluminescence, whereas the N-polar one did not emit.