Structural study of GaN(As,P) layers grown on (0001) GaN by gas source molecular beam epitaxy

Transmission electron microscope (TEM) and transmission electron diffraction (TED) examination has been performed to investigate microstructural properties of gas source molecular beam epitaxial GaN(As,P) layers grown on (0001) GaN/sapphire at temperatures in the range 500 - 760°C. As for the GaNAs, we report the observation of ordering with a space group P3ml in the layer grown at 730°C. The layers grown at temperatures below 600°C are polycrystalline, whist the 730°C GaNAs layer has epitaxial relation to the GaN substrate. It is also shown that the GaNAs layers experience a structural change from a zinc-blende phase to a wurtzite phase, as the growth temperature increases. As for the GaNP, it is shown that the layers grown temperatures ≤ 600°C experience phase separation resulting in a mixture of GaN-rich and GaP-rich GaNP with zinc-blende structure. However, the layers grown at temperatures ≥ 730°C are found to be binary zinc-blende GaN(P) single crystalline materials. The layers grown at temperatures ≥ 730°C consist of two types of micro-domains, i.e., GaN(P)_I and GaN(P)_II; the former having twin relation to the latter.