Improved Low-Temperature Growth of Gallium Nitride for Back-End-of-Line Technologies

Gallium Nitride (GaN) is a promising channel material for back-end-of-line (BEOL) access transistors, owing to its inherently high electron mobility and wide bandgap, which enable a high on/off current ratio and low leakage current. However, achieving high-quality crystalline GaN typically requires growth temperatures far exceeding BEOL-compatible limits. GaN grown at 450?°C using conventional two-step growth strategies exhibits poor transport properties, hindering its integration into advanced CMOS architectures. This research investigates the degradation mechanisms of GaN electronic properties at reduced growth temperatures and develops strategies to enable its use in BEOL applications. The study first examines how growth temperature impacts GaN stoichiometry and crystallinity. It identifies two primary degradation factors at low temperatures: excess gallium incorporation from incomplete nitrogen precursor decomposition, and reduced crystallinity due to limited adatom mobility. While GaN grown above 750?°C adopts a hexagonal phase, low-temperature growth results in an amorphous network with embedded cubic grains, each exhibiting distinct degradation behaviors. To overcome these limitations, the Ga precursor flow rate was lowered to minimize excess Ga incorporation, while a Growth-Anneal Super Cycle strategy was implemented to improve crystallinity by facilitating adatom migration. Further optimization—including sapphire substrate pretreatment—led to the first demonstration of purely amorphous GaN grown at BEOL-compatible temperatures with electron mobility exceeding 10?cm²/V·s. The structure and composition were thoroughly characterized. Additionally, the metal/amorphous GaN interface was studied using various contact metals. XPS revealed inert, van der Waals-like interfaces with Au, while Ni, Pt, and Pd catalyzed GaN decomposition. Strong Fermi level pinning was observed, independent of metal work function, due to the amorphous structure, excess Ga, and interfacial oxides. Initial mitigation strategies, including chemical passivation and ultrathin insertion layers, showed partial success, suggesting future pathways for improving contact engineering in amorphous GaN devices.<p></p>