Effect of annealing atmosphere (CO and N₂ gas flow) on surface morphology and crystal quality of AlN buffer layer

In this study, the effect of the annealing atmosphere (N₂–CO and pure N₂ atmosphere) on the crystal quality, surface morphology, and surface O content of the annealed AlN buffer layers was studied. The FWHM of the (0002) plane of an XRD rocking curve showed that the screw dislocation density of the as-grown AlN film can be reduced by annealing in both N₂–CO and pure N₂ atmosphere. The AlN film showed an improvement in crystal quality under pure N₂ annealing atmosphere. As annealing the AlN buffer layer improves the crystal quality, surface roughness, and surface O content of the AlN film, it should be well controlled for the subsequent growth of III-nitride layers. The surface oxidation that occurred on the AlN films during annealing in N₂–CO atmosphere resulted in an increase in surface O content and surface roughness of the AlN films, which is undesirable for the subsequent growth of III-nitride layers. We observed that the surface O content (in the depth of 2-nm) of all annealed AlN films was reduced in the N₂ annealing atmosphere, which could be due to the O outgassing from the AlN surface to the pure N₂ annealing atmosphere. The O outgassing of the surface O content in the AlN surface layer was related to the volume of space in the annealing setups. However, a large volume of annealing space under pure N₂ gas flow resulted in higher thermal decomposition on the annealing AlN films, leading to pits forming on the annealed AlN surface and a smearing of the original step-terrace structure. Therefore, we found that there is an optimal annealing atmosphere space in maintaining the original surface morphology and achieving low surface O content in the annealed AlN film.