Cubic AlxGa1-xN/GaN heterostructures were grown by plasma-assisted molecular beam epitaxy on free-standing 3C-SiC(001). The samples consist of an unintentionally doped 600nm thick c-GaN buffer and a 30 nm c-Al0.3Ga0.7N layer. Capacitance-voltage measurements were performed on metal-oxide-semiconductor heterojunction structure using SiO 2 as an insulator. A depth profile of the net donor concentration NCV of the grown sample was measured, demonstrating a clear carrier accumulation at the heterojunction. By electron holography in a transmission electron microscope the potential profile was measured and a free electron concentration of 5.1 × 1011 cm-2 was estimated at the c-Al xGa1-xN/ GaN interface. Using a one-dimensional (1D) Poisson simulator the results of both techniques are compared and a conduction-to-valence band offset ratio of about 4 : 1 for the cubic Al xGa1-xN/GaN interface is estimated, which promotes the electron accumulation. Our results demonstrate that the two-dimensional electron gas (2DEG) in cubic AlxGa1-xN/GaN heterostructures can be achieved without the need of polarization effects and is due to the residual background doping in the AlxGa1-xN and GaN. © 2013 The Japan Society of Applied Physics.