Admittance of Al/GaAs Schottky contacts under forward bias as a function of interface preparation conditions

Admittance measurements have been made under forward bias on two types of Al/n-GaAs contacts. The Al and n-GaAs layers have been prepared by molecular beam epitaxy on an n⁺-GaAs wafer. In the first type, which shows ideal current-voltage characteristics, only an inductive effect was observed and no effect attributable to interface states was detected. In the second type, differing only in the n-GaAs surface which was prepared under inferior vacuum conditions, nonideality of current-voltage characteristics and excess capacitances were both seen. The back contact being the same in both types of samples, the onset of excess capacitances must be related to the change in surface characteristics. A model which assumes a U-shaped density of extrinsic interface states accounts simultaneously for the behavior of conductance, capacitance, and ideality factor as functions of temperature, voltage, and frequency.