Effect of impurity trapping on the capacitance-voltage characteristics of n-GaAs/N-AlGaAs heterojunctions

We have studied the capacitance-voltage (C-V) characteristics of Schottky barriers on inverted n-GaAs/N-AlGaAs and normal N-AlGaAs/n-GaAs heterojunctions. Impurities introduced during film growth produced a negative sheet charge of 6.0×1011 cm−2 at the interface of the inverted n-GaAs/N-AlGaAs heterojunction. The effectiveness of GaAs quantum wells in trapping these impurities was investigated. GaAs quantum wells 20 Å wide were placed in intervals of 2500 Å for the first 0.75 μm of the AlGaAs layer; in the last 0.25 μm, the periodicity of the quantum wells was progressively decreased by half with the last quantum well placed at about 160 Å from the GaAs/AlGaAs interface. The resulting measured interface charge concentration of 4.4×1010 cm−2 is more than a magnitude lower than measured before the use of the quantum wells and is essentially at the limit of the accuracy of the C-V technique for this structure.