Three-Step Growth Optimization of AlN Epilayers by MOCVD

A three-step growth process is developed for depositing high-quality aluminium-nitride (AlN) epilayers on (001) sapphire by low pressure metalorganic chemical vapour deposition (LP-MOCVD). We adopt a low temperature (LT) AlN nucleation layer (NL), and two high temperature (HT) AlN layers with different V/III ratios. Our results reveal that the optimal NL temperature is 840--880°C, and there exists a proper growth switching from low to high V/III ratio for further reducing threading dislocations (TDs). The screw-type TD density of the optimized AlN film is just 7.86×10⁶cm^-2, aboutthree orders lower than its edge-type one of 2×10⁹cm^-2 estimated by high-resolution x-ray diffraction (HRXRD) and cross-sectional transmission electronmicroscopy (TEM).