Effects of Pre-Metallization on the MOCVD Growth and Properties of Ge-doped AlGaN on AlN/Sapphire Templates

The effects of pre-metallization of the growth surface on film stress and structural properties of undoped and Ge-doped Al_xGa_1−xN (x ~ 0.5–0.6) epilayers grown by metal–organic chemical vapor deposition (MOCVD) on 500 nm-thick hydride vapor-phase epitaxy (HVPE) AlN/sapphire templates were investigated. Al_xGa_1−xN typically grows under compressive stress on the AlN templates due to its larger lattice parameter, which can lead to increased surface roughness and V-pits in undoped and Ge-doped Al_xGa_1−xN. The introduction of the group III sources in the growth ambient for a short period of time (5 s) prior to the addition of NH₃ induced a tensile growth stress in the Al_xGa_1−xN, as measured by in situ wafer curvature measurements, which correlated with an improvement in the surface morphology. However, the pre-metallization was also observed to result in the deposition of a carbon-rich layer at the Al_xGa_1−xN/AlN interface and an increased density of screw-type dislocations as measured by post-growth x-ray diffraction. By utilizing a pre-metallization step with a lower Al_xGa_1−xN growth rate, it was possible to eliminate the carbon interfacial layer and maintain low surface v-pitting and threading dislocation density in Ge-doped Al_xGa_1−xN. The results provide insight into the impact of pre-metallization on the Al_xGa_1−xN/AlN interface and the structural properties of the layers.