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.氮化镓（GaN）因其本征高电子迁移率与宽禁带特性，是后端制程（BEOL）存取晶体管极具应用前景的沟道材料，可实现优异的开关电流比与极低的漏电流。然而，制备高质量结晶态氮化镓通常需要远高于BEOL兼容上限的生长温度。采用传统两步生长法在450℃下生长的氮化镓，其传输性能较差，阻碍了其在先进CMOS架构中的集成。本研究探究了低温下氮化镓电子性能的退化机制，并开发了可使其应用于BEOL制程的策略。研究首先考察了生长温度对氮化镓化学计量比与结晶度的影响，识别出低温下的两大主要退化因素：一是氮前驱体分解不完全导致的过量镓掺入，二是吸附原子迁移率受限引发的结晶度下降。当生长温度高于750℃时，氮化镓呈六方相结构；而低温生长则会形成嵌有立方晶粒的无定形网络，二者各自表现出不同的退化行为。为克服上述局限，本研究降低了镓前驱体的流速以减少过量镓掺入，并采用生长-退火超循环（Growth-Anneal Super Cycle）策略，通过促进吸附原子迁移来提升结晶度。进一步的优化（包括蓝宝石衬底预处理）首次实现了在BEOL兼容温度下生长纯无定形氮化镓，其电子迁移率优于10 cm²/(V·s)。研究对该材料的结构与成分进行了全面表征。此外，本研究还采用多种接触金属对金属/无定形氮化镓界面开展了研究。X射线光电子能谱（XPS）结果显示，金（Au）与氮化镓形成了惰性的类范德瓦尔斯界面；而镍（Ni）、铂（Pt）与钯（Pd）则会催化氮化镓的分解。由于无定形结构、过量镓掺入与界面氧化物的存在，观测到了与金属功函数无关的强费米能级钉扎现象。包括化学钝化与超薄插入层在内的初步缓解策略已取得部分成效，为未来优化无定形氮化镓器件的接触工程指明了方向。