Mapping the electrostatic potential across AlGaN∕AlN∕GaN heterostructures using electron holography

Electron holography has been used to study the properties of a nominally undoped AlGaN∕AlN∕GaN heterostructures. Important characteristics such as the electrostatic potential and the two-dimensional electron gas (2DEG) distribution have been determined with high spatial resolution across the thin film structure. The origin of 2DEG electrons is directly probed by analyzing the charge distribution in the AlGaN layer. It is shown that the contribution of residual donors is trivial and the ionized donorlike surface states are the major source of the 2DEG electrons.