Self-organization of palladium nanoislands on GaN and AlxGa1−xN/GaN heterostructures

Abstract

We report on systematic study on the dewetting process of thin Pd layer and self-organized Pd nano-islands on SiO2, GaN and AlxGa1−xN/GaN heterostructures with various Al content. The influence of factors such as the thickness of metal layer, type of top layer of AlGaN/GaN heterostructures, temperature and time of annealing process on the dimensions, shapes and density of Pd islands was analyzed. Comparing the behavior of self-organization of Pd islands on Al0.25Ga0.75N/GaN and SiO2 we can conclude that solid-state dewetting process on SiO2 occures much faster than on Al0.25Ga0.75N. For substrates with SiO2 this process requires less energy and can arise for thicker layer. On the Al0.25Ga0.75N surface the islands take more crystalline shape which is probably due to surface reconstruction of Pd-Ga alloy thin layer on interface. For thin metal layer the coalescence of islands into larger islands similar to Ostwald ripening mechanism was observed. Greater surface roughness of AlxGa1−xN/GaN heterostructures with higher Al content causes an increase of surface density of islands and the reduction of their sizes which improves the roundness. In case of GaN and AlxGa1−xN layers with Al content lower than 20%, the surface degradation caused by annealing process was observed. Probably, this is due to the decomposition of layers with gallium droplet formation on catalytic metal islands.