Nonpolar (112¯0) a-plane gallium nitride thin films grown on (11¯02) r-plane sapphire: Heteroepitaxy and lateral overgrowth

Nonpolar (112¯0) a-plane GaN films with planar surfaces were grown on (11¯02) r-plane sapphire substrates via metalorganic chemical vapor deposition by depositing a low temperature nucleation layer prior to the high temperature epitaxial growth. The film/substrate epitaxial relationship was defined by X-ray diffraction measurements to be [0001]GaN [1¯101]sapphire and [1¯100]GaN [112¯0]sapphire while the polarity of the GaN c-axis was confirmed using convergent beam electron diffraction. The film morphology and microstructure were studied using atomic force microscopy and transmission electron microscopy (TEM), respectively. Threading dislocations (TDs), which dominated the heteroepitaxial a-plane GaN microstructure (density ∼2.6 × 1010 cm-2), were reduced via lateral epitaxial overgrowth from mask stripe openings. The overgrowth from stripes aligned along [1¯100] was dislocation-free, as observed using TEM and cathodoluminescence, and dependent upon crystal polarity. For all other stripe orientations, TDs were observed to propagate into the overgrown regions.