Microstructure of epitaxial GaN films grown on chemomechanically polished GaN(0001) substrates

An organometallic vapor phase epitaxy-based process route has been developed to achieve homoepitaxial deposition of GaN(0001) films via step-flow growth on substrates having <1° off-cut. Atomic force microscopy of the surfaces of 0, 1, 2, 5 and 10 nm thick films revealed steps and terraces as the only features; three-dimensional GaN islands were not observed. Film–substrate interfaces were not present in cross-sectional samples using high-resolution TEM. This indicated that continuous film growth occurred from steps on the substrate without re-nucleation and defect formation on the terraces. This process route also mitigated the generation of additional dislocations, as validated by the exact matches of the density and positions of dislocations that reached the substrate surface and those observed in a subsequently grown 600 nm film. The influence of grain boundaries in the interior of the GaN substrates was manifest in variations in terrace width and step orientation across the substrates and the films. A grain orientation map generated across a representative substrate revealed highly disoriented grains on the periphery. The disorientation angles between these adjacent grains were centered around ∼35°, ∼70° and ∼90°.

► OMVPE process developed to grow GaN film in step-flow mode on CMP GaN substrates.
► No formation of GaN islands at film–substrate interface.
► No generation of new threading dislocations in GaN films.
► Variations of step orientation and terrace width observed on surface.
► Polycrystallinity of GaN substrate illustrated in grain orientation map.