Metastable centers in AlGaN/AlN/GaN heterostructures

A set of AlGaN/AlN/GaN high electron mobility transistor structures with Al composition in the AlGaN barrier changing from 20% Al to 50% Al was grown by metalorganic chemical vapor deposition on sapphire and studied by capacitance-voltage (C-V) measurements, admittance spectroscopy, and deep level transient spectroscopy. C-V and admittance measurements were performed in the dark and after illumination. The results suggest the presence of high concentrations of deep negatively charged traps in the AlGaN barriers, producing shifts of the C-V characteristics to more positive voltages. The density of negatively charged centers can be increased by cooling at high reverse bias. These centers have a high barrier for the capture of electrons. Their thermal activation energy is estimated as 0.85 eV, while the optical ionization energy is similar to 1.7 eV. (C) 2012 American Vacuum Society. [http://dx.doi.org/10.1116/1.4731256]