Direct measurement of the polarization charge in AlGaN/GaN heterostructures using capacitance-voltage carrier profiling

The polarization charge at AlxGa1-xN/GaN heterostructure interfaces arising from differences in spontaneous polarization between AlxGa1-xN and GaN and the presence of piezoelectric polarization in strained layers has been directly measured using capacitance-voltage carrier profiling in GaN/AlxGa1-xN/GaN heterostructures with varying Al composition grown by molecular-beam epitaxy. The measured polarization charge densities (2.36+/-0.30x10(12) e/cm(2), 6.79+/-0.48x10(12) e/cm(2), and 6.92+/-0.74x10(12) e/cm(2) for 5%, 12%, and 16% AlxGa1-xN/GaN interfaces, respectively) reveal substantial bowing in the polarization charge as a function of Al composition, and are in reasonable agreement with those calculated using a model that accounts for the nonlinearity of the spontaneous and piezoelectric polarizations as functions of Al composition. Our results yield an explicit expression for polarization charge as a function of Al composition at an AlxGa1-xN/GaN interface. (C) 2002 American Institute of Physics.