Impact of UV annealing on the hole effective mobility in SnO pFET

Using ultraviolet (UV) annealing through wide energy bandgap HfO₂/SiO₂ gate dielectric, nanosheet SnO pFET achieved hole effective mobility (µ_eff) from 55 cm²/V-s at low hole density (Q_h) to 13.38 cm²/V-s at 5 × 10¹² cm^-2 Q_h, compared to that of 9.03 cm²/V-s at 5 × 10¹² cm^-2 Q_h for SnO device without UV annealing. This is the highest µ_eff among oxide semiconductor pFETs at high Q_h, which is required to realize low-power high-density monolithic 3D CMOS logic. This requires excellent surface roughness, good uniformity and free-from grain boundaries that is beyond the thermally-annealed poly-Si. Excellent on-current/off-current (I_ON/I_OFF) value of 1.05 × 10⁵ were measured simultaneously in the UV-annealed SnO pFET, which is due to the ultra-thin 8 nm thick SnO nanosheet channel to pinch off the channel leakage. From X-ray photoelectron spectroscopy (XPS) analysis, the 48% µ_eff improvement by UV irradiation is due to increased Sn²⁺ and decreased Sn⁰. Such high µ_eff at high Q_h, large I_ON/I_OFF, smooth surface, good uniformity and low thermal budget process are the enabling technologies for monolithic 3D CMOS.