Microstructures of GaN_1-xP_x layers grown on (0001)GaN substrates by gas source molecular beam epitaxy

Transmission electron microscope (TEM), transmission electron diffraction (TED), and synchrotron x-ray diffraction (XRD) studies have been performed to investigate microstructural behavior of gas source molecular beam epitaxial GaN_1-xP_x layers grown on (0001)GaN/sapphire at temperatures (T_g) in the range 500-760°C. TEM, TED, and XRD results indicate that the samples grown at T_g≤600°C undergo phase separation resulting in a mixture of GaN-rich and GaP-rich GaNP with zinc-blende structure. However, the samples grown at T_g≥730°C are found to be binary zinc-blende GaN(P) single crystalline materials. As for the 500°C layer, the two phases are randomly oriented and distributed, whereas the 600°C layer consists of phases that are elongated and inclined by 60°-70° clockwise from the [0001]_α-GaN direction. The samples grown at T_g ≥730°C are found to consist of two types of microdomains, namely, GaN(P)_I and GaN(P)_II; the former having twin relation to the latter.