Admittance Spectra Studies of Quantum Well States in AlGaN/AlN/GaN Heterojunctions

Capacitance-voltage (C-V) and capacitance-frequency (C-f) characteristics and admittance spectra were performed in the 85 K–400 K temperature range for AlGaN/AlN/GaN high electron mobility transistor (HEMT) structures with Al composition in the AlGaN barrier changing from 20% Al to 50% Al. AC conductance (G) in the C-V characteristics showed a pronounced peak in G-V dependences for voltages corresponding to partial depletion as did G-f dependences taken at these peak voltages. Admittance spectra measured at these voltages reveal peaks in conductance and steps in capacitance with apparent activation energy 0.11–0.13 eV only weakly dependent on Al composition. Theoretical modeling suggests that these features are most likely due to electron transitions from the ground level in the triangular well near the AlN/GaN interface to the quasi continuum of highly excited states in the well with consequent thermal ionization of electrons. For highest Al composition of 50%, additional traps in admittance with activation energy of 0.2 eV and 0.25 eV were detected and this correlated with partial relaxation of strain in the structure as determined by X-ray analysis.