High mobility 4H-SiC (0001) transistors using alkali and alkaline earth interface layers

Alkali (Rb and Cs) and alkaline earth (Ca, Sr, and Ba) elements have been investigated as interface passivation materials for metal-oxide-semiconductor field-effect transistors (MOSFETs) on 4H-SiC (0001). While the alkali elements Rb and Cs result in field-effect mobility (μFE) values > 25 cm2/V·s, the alkaline earth elements Sr and Ba resulted in higher μFE values of 40 and 85 cm2/V·s, respectively. The Ba-modified MOSFETs show a slight decrease in mobility with heating to 150 °C, as expected when mobility is not interface-trap-limited, but phonon-scattering-limited. With a Ba interface layer, the interface state density 0.25 eV below the conduction band is ∼3 × 1011 cm−2 eV−1, lower than that obtained with nitric oxide passivation. Devices show stable threshold voltage under 2 MV/cm gate bias stress at 175 °C, indicating no mobile ions. Secondary-ion mass spectrometry shows that the Sr and Ba stay predominantly at the interface after oxidation anneals.