Effect of V/III ratio and growth pressure on surface and crystal quality of AlN grown on sapphire by metal-organic chemical vapor deposition

The epitaxial growth of high-quality AlN on sapphire substrates is challenging due to high lattice and thermal mismatch and low Al-adatom mobility, which cause high dislocation density and rough surface morphology. High-temperature AlN (deposited at 1130 °C) was grown on low-temperature AlN nucleation layers (880 °C) with different V/III ratios and reactor pressures by metal-organic chemical vapor deposition. Surface and crystal quality was optimized using high V/III ratios. Thereby, slow layer-by-layer growth at high V/III laterally overgrows the 3D nucleation layer reducing the dislocation density, twist, and tilt in the crystal. This was as effective as multistep growth with increasing V/III. At high pressure of 95 mbar, step-bunching occurred. This indicates low surface supersaturation due to parasitic reactions in the gas phase. This was suppressed by low growth pressure of 50 mbar, while the crystal quality worsened.