HVPE growth of self-aligned GaN nanorods on c-plane, a-plane, r-plane, and m-plane sapphire wafers

Herein, we report the self-aligned growth of GaN nanorods on different orientations of sapphire like c-, a-, r- and m-plane substrates by hydride vapor phase epitaxy. Vertical c-axis orientation of GaN NRs is obtained on c-plane [0001] and a-plane [11 (2) over bar0] sapphire and a skew or inclined NRs on r-plane, and inclined intertwined but self-aligned NR array was formed on m-plane sapphire. GaN (002) and (004) peaks were obtained on c- and a-plane sapphire, whereas (110), (103), and (103) only were observed on r- and m-planes, respectively. In the case of r- and m-plane-grown GaN, A(1) transverse optical mode is dominant, and the A(1) longitudinal optical mode is suppressed. Conversely, in the case of c- and a-plane, it is reversed. The probable reason is the optical mode vibrations difference along the differently inclined NRs surfaces. In addition, the specimen exhibits surface optical modes too. The optical behavior of GaN NR on m-sapphire shows an intensity variation when measured in different angular rotations of the specimen by photoluminescence which is because of the higher area of excitation in the case of axial surfaces and lower area of excitation in radial surface. Their epitaxial crystallographic relationship with the substrates and the reasons for the self-aligned orientations are discussed. The anomalies found in the optical behavior are attributed to Raman antenna effect and so on. The self-aligned intertwined GaN NRs find suitable applications in polarizer.