Passivation of GaAs metal–insulator–semiconductor structures by $(NH4)2Sx$ and by evaporation of $SiO2$

$Al–SiO2–GaAs$ structures are prepared and characterized by electrical measurements. The GaAs (100) surfaces are cleaned and passivated by chemical solutions. A passivation treatment based on ammonium polysulfide $(NH4)2Sx$ is applied, then an insulating layer is deposited by electron-beam evaporation of an amorphous $SiO2.$ By adjusting the passivation and the insulator deposition parameters, the accumulation and inversion conditions are observed by a capacitance–voltage $(C–V)$ technique, showing that the Fermi level is unpinned. The interface-states density $Dit$ as calculated using the method of Terman applied to high-frequency $C–V$ characteristics presents a minimum of $1011 cm−2 eV−1.$