The epitaxial relationship between M-Plane and c-plane GaN grown on gamma-LiAlO$_{2}$

The M-plane and c-plane oriented GaN have been found co-existed in the $\gamma $-LiAlO$_{2}$ substrate grown by molecular beam epitaxy (MBE). Two-step growth with different N/Ga ratios has been used in the experiment at growth temperature 670$^{\circ}$C. By the SEM images, the smooth M-plane surface was observed and the c-plane GaN 3-D structures homogenously spread on it. From the scanning of $\omega $-2$\theta $ X-ray diffraction pattern, the peaks at 32.295$^{\circ}$ and 34.505$^{\circ}$ were attributed to the M-plane and c-plane GaN. The microstructure of the samples was investigated by transmission electron microscopic (TEM) study. It was found that the c-plane and M-plane GaN both were oriented from the substrate with the same growth direction and arranged vertically to the substrate with a periodical pattern. The interfaces between the c-plane and M-plane GaN can be confirmed clearly by one or two monolayer in high resolution TEM images. The stacking faults were found at the edge of c-plane side at the interface that released the dislocation between the M- and c-plane GaN. In summary, we found that the M-plane and c-plane GaN can be assembled on $\gamma $-LiAlO$_{2}$ substrate with a clear phase-transition interface.