Al–SiO2GaAs 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 1011cm−2eV−1.

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