Lateral variations in threshold voltage of an Al_xGa_1-xN/GaN heterostructure field-effect transistor measured by scanning capacitance spectroscopy

Local dC/dV spectroscopy performed in a scanning capacitance microscope (SCM) was used to map, quantitatively and with high spatial resolution (∼ 50nm), lateral variations in the threshold voltage of an Al_xGa_1-xN/GaN heterostructure field-effect transistor epitaxial layer structure. Scanning capacitance and the associated threshold voltage images show small round features less than 150 nm in diameter with a corresponding shift in threshold voltage of about 1.5-2 V, and larger features several microns in size with a corresponding shift in threshold voltage of approximately 1 V. The small features in the SCM and threshold voltage images are consistent with the presence of charged threading dislocations, while the variations in threshold voltage over large areas could be a result of thickness and/or composition variations in the Al_xGa_1-xN layer.