Impact of Growth Temperature and Al/N Ratio on AlN Films Grown by Radio‐Frequency Molecular Beam Epitaxy on GaN Templates

Aluminum nitride (AlN) is a promising candidate for surface passivation of AlGaN/GaN high electron mobility transistors to mitigate current collapse. High crystalline quality is essential to minimize the interface traps and bulk defects that cause current collapse and degrade device reliability. The quality of this passivation layer is highly dependent on growth conditions, particularly temperature and the Al/N flux ratio. This study systematically investigates the impact of these parameters on AlN films grown by radio‐frequency molecular beam epitaxy on GaN templates. We compare two growth regimes: one at a high temperature (700°C) and another at a low temperature (300°C). At 700°C, Al‐rich conditions (Al/N > 1) promote two‐dimensional growth, yielding smooth surfaces and high crystalline quality, although excessive Al results in surface droplet formation. In contrast, at 300°C, limited adatom mobility results in a three‐dimensional growth morphology, regardless of the Al/N ratio. Under Al‐rich conditions at 300°C, excess Al is incorporated into the film as a metallic layer, forming a strained Al/AlN structure that degrades crystalline quality, rather than forming surface droplets. These findings reveal fundamentally different growth mechanisms and defect incorporation pathways, which are dictated by temperature, establishing a critical baseline for developing low‐temperature AlN passivation strategies.