Low-temperature conductance measurements of surface states in HfO₂-Si structures with different gate materials

Metal-oxide-semiconductor capacitors based on HfO₂ gate stack with different metal and metal compound gates (Al, TiN, NiSi and NiAlN) are compared to study the effect of the gate electrode material on the trap density at the insulator-semiconductor interface. C-V and G-ω measurements were made in the frequency range from 1 kHz to 1 MHz in the temperature range 180-300 K. From the maximum of the plot G/ω vs. ln(ω) the density of interface states was calculated, and from its position on the frequency axis the trap cross-section was found. Reducing temperature makes it possible to decrease leakage current through the dielectric and to investigate the states located closer to the band edge. The structures under study were shown to contain significant interface trap densities located near the valence band edge (around 2×10¹¹ cm^-2eV^-1 for Al and up to (3.5-5.5)×10¹² cm^-2 eV^-1 for other gate materials). The peak in the surface state distribution is situated at 0.18 eV above the valence band edge for Al electrode. The capture cross-section is 5.8×10^-17 cm² at 200 K for Al-HfO₂-Si structure.