Comprehensive (S)TEM characterization of polycrystalline GaN/AlN layers grown on LTCC substrates

Three GaN layers grown on polished AlN buffer films previously sputtered on Low-Temperature Co-fired Ceramic substrates have been studied by X-Ray Diffraction, Atomic Force Microscopy and Electron Microscopy related techniques. This allowed to assess the quality of the whole fabricated III-N/LTCC heterostructures considering factors such as superficial roughness, crystallinity, structural features (e.g.: lattice defects) and chemical homogeneity. All the AlN and GaN films were chemically uniform. However, for different regions and growth conditions, properties like the GaN average grain size or roughness varied significantly, and polycrystalline or partially single-crystalline GaN areas, with an elevated density of lattice defects, were identified. The particular surface structure of the LTCC substrate was regarded as key for the formation of these features, as it notably affected the AlN structure. Additional characteristics, like a different degree of c-axis orientation of the GaN grains or formation of small cubic GaN domains depending on the fabrication conditions, were also detected.