Large-scale alkali-assisted growth of monolayer and bilayer WSe₂ with a low defect density

The development of p-type WSe₂ transistors has lagged behind n-type MoS₂ because of challenges in growing high-quality, large-area WSe₂ films. This study employs an alkali-assisted CVD (AACVD) method by using KOH to enhance nucleation on sapphire substrates, effectively promoting monolayer growth on c-plane sapphire and enabling controlled bilayer seeding on miscut surfaces with artificial steps. With AACVD, we achieve 2-inch monolayer and centimeter-scale bilayer WSe₂ films with defect densities as low as 1.6 × 10¹²cm^-2 (monolayer) and 1.8 × 10¹²cm^-2 (bilayer), comparable to exfoliated WSe₂. Bilayer WSe₂ transistors exhibit hole/electron mobilities of 119/34 cm²/Vs, while monolayers achieve 105/22 cm²/Vs with suitable metal contacts. Additionally, bilayer WSe₂ demonstrates lower contact resistance for both n-type and p-type transistors, making it highly promising for future high-performance electronic applications.