The surface topological evolution during the growth of indium nitride (InN) by plasma-assisted atomic layer epitaxy (ALEp) on gallium nitride (GaN) (0001) substrates was studied using in situ real-time grazing incidence small-angle x-ray scattering (GISAXS) for 180, 250, and 320 °C growth temperatures. The GISAXS data reveal that the ALEp growth of InN on GaN in this temperature range proceeds in a Stranski–Krastanov mode, in which the 2D–3D transition occurred after 2.3 monolayers for 180 °C, 1 monolayer for 250 °C, and 1.5 monolayers for 320 °C. The corresponding initial island center-to-center distances were 7.4, 11.6, and 11.7 nm. Additionally, island coarsening was observed to increase with temperature. After 200 growth cycles, the mean island diameters were 3.9, 5.6, and 7.0 nm, and the mean island center-to-center distances were 8.6, 13.7, and 17.1 nm for 180, 250, and 320 °C growth temperatures, respectively. For the 320 °C growth, the mean island shape was observed to gradually evolve from relatively mounded to cylindrical. These results are supported by atomic force microscopy and specular x-ray reflectivity.The surface topological evolution during the growth of indium nitride (InN) by plasma-assisted atomic layer epitaxy (ALEp) on gallium nitride (GaN) (0001) substrates was studied using in situ real-time grazing incidence small-angle x-ray scattering (GISAXS) for 180, 250, and 320 °C growth temperatures. The GISAXS data reveal that the ALEp growth of InN on GaN in this temperature range proceeds in a Stranski–Krastanov mode, in which the 2D–3D transition occurred after 2.3 monolayers for 180 °C, 1 monolayer for 250 °C, and 1.5 monolayers for 320 °C. The corresponding initial island center-to-center distances were 7.4, 11.6, and 11.7 nm. Additionally, island coarsening was observed to increase with temperature. After 200 growth cycles, the mean island diameters were 3.9, 5.6, and 7.0 nm, and the mean island center-to-center distances were 8.6, 13.7, and 17.1 nm for 180, 250, and 320 °C growth temperatures, respectively. For the 320 °C growth, the mean island shape was observed to gradually evolve from relati...