Study of the effect of passivation layers on capacitance of AlGaN/GaN heterostructures

MOCVD grown AlGaN/GaN heterostructures on sapphire and silicon substrates have been studied. The capacity-voltage characteristic have been measured in the 200 Hz-1 MHz frequency range with a planar position of mercury and second probe on the specimen surface. The shape of the typical C-V curves for the heterostructures with upper undoped i-AlGaN and i-GaN layers 1,5-2,5 nm in thickness have been analyzed. The appearance of a typical peak on the C-V curves upon a change from the depletion region to the accumulation region has been registered in some structures with an i-GaN layer thickness of 5 nm and more at low frequencies (f<50-200 kHz). The height of this peak increased with a reduction of frequency. It has been found experimentally that the frequency at which the peak is registered can depend on the dislocation density in heterostructures. Possible explanation of the peak formation and band diagram transformation in these structures under an applied electric field have been presented. We show that the application of a Si3N4 passivation layer results in the formation of additional positive charge.