The influence of the strain-induced electric field on the charge distribution in GaN-AlN-GaN structure

We show that strongly pronounced piezoelectric properties play a key role in GaN-AlN-GaN semiconductor-insulator-semiconductor (SIS) and related structures. In sufficiently thin AlN layers, the lattice constant mismatch is accommodated by internal strains rather than by the formation of misfit dislocations. These lattice-mismatch-induced strains generate polarization fields. We demonstrate that, in a GaN-AlN-GaN SIS structure with the growth axis along a (0001) crystallographic direction, the strain-induced electric fields can shift the flat band voltage and produce an accumulation region on one side and a depletion region on the other side of the AlN insulator. On which side of the insulator the accumulation region is produced depends on the type of atomic plane at the heterointerface (Ga or N). The surface charge density caused by the piezoeffect is on the order of 1012 cm−2. As a consequence of the asymmetry in the space charge distribution, the capacitance-voltage (C-V) characteristics of the SIS structure become asymmetrical. The asymmetrical shift of the C-V characteristics with respect to the origin is on the order of 1.5 V for a 30 Å AlN film. This asymmetry should vanish in a relaxed film. Hence, capacitance-voltage measurements of GaN-AlN-GaN SIS structures can be used for quantitative characterization as to the degree of AlN film relaxation, depending on the film thickness. This and related techniques should become important tools for the characterization of piezoelectric layered semiconductor films.