Crystallinity enhancement of AlN thin films on Si via nucleation layer engineering and ceramic target sputtering

Aluminium nitride (AlN) sputter-deposited directly on a bare silicon (Si) substrate suffers from significant lattice mismatch due to the presence of an amorphous interfacial layer at the AlN–Si interface and a considerable difference in in-plane lattice constant, leading to the growth of highly amorphous or poorly crystalline AlN. The introduction of titanium (Ti) and molybdenum (Mo) seed layers, along with AlN ceramic sputtering substantially reduced the lattice mismatch between AlN and the Si substrate from 45.4 % to 25.9 % and 3.7 %, respectively, leading to improved c-axis (002)-oriented growth and reduced inhibited local intrinsic strain in AlN. Although Ti seeding on the AlN–Si interface did not effectively suppress the undesired interfacial layer influence, the Mo seed layer facilitated the formation of a highly crystalline AlN layer with minimal amorphous content, attributed to its close lattice match and stable interface. In contrast, AlN grown on bare Si exhibited increased amorphosity and favoured a-axis (100)-oriented growth. The abstraction of disruptive amorphous interfacial phase layer through the combination of refractory molybdenum nucleation layer insertion and AlN ceramic target sputtering has enhanced the crystal quality of the grown AlN layer, which exhibited up to two orders of magnitude lower leakage current and a 1.5 × enhancement in piezoelectric amplitude. These findings underscore the critical role of nucleation layers and sputtering target chemistry in tailoring the structural and functional performance of AlN thin films for advanced insulating and piezoelectric applications.