Structural and optical properties of AlxGa1−xN (0.33 ≤ x ≤ 0.79) layers on high-temperature AlN interlayer grown by metal organic chemical vapor deposition

High-Al-content AlxGa1−xN films with x varying from 0.33 to 0.79 were grown on GaN templates with the high temperature AlN (HT-AlN) interlayer by metal organic chemical vapor deposition (MOCVD). The best crystalline quality, among these AlxGa1−xN alloys, can be obtained for an AlN mole fraction x = 0.55, where the full-width at half-maximum of the Al0.55Ga0.45N (0002) diffraction peak was measured to be 259 arcsec by high resolution X-ray diffraction (HRXRD). The screw threading dislocation (TDs) density was 2 × 108 cm−2 evaluated by transmission electron microscope (TEM), which agreed with the calculations from Williamson-Hall plots. Moreover, cross-sectional TEM indicated that the HT-AlN interlayer could sufficiently reduce the threading dislocations (TDs) through generation of V trenches in the HT-AlN interlayer, since the TDs propagated along the V trenches, then bent into basal planes and annihilated with other dislocations. The study of optical properties indicated that obvious S-shape of temperature dependence on emission energy was observed for Al0.55Ga0.45N layers, which was attributed to exciton localization with energy (Eloc) ∼14.95 meV at 10 K resulting from potential fluctuation and band tail states. The time-resolved photoluminescence (TRPL) curves showed a bi-exponential decay at low temperature. The fast decay time implied the presence of the localized excitons enhancing radiative recombination, while the quite slow one was due to the dominance of trapping mechanisms originating from cation vacancy complexes and the VIII-related complexes.