Flat band voltage modulation of AlGaN/GaN MOS capacitor with stacked Al2O3/La2O3 high dielectric structures

AlGaN/GaN metal-oxide-semiconductor (MOS) capacitor structures using atomic layer deposited high-dielectric-constant (High-k) Al2O3/La2O3 bilayer films as dielectric have been investigated using high-frequency capacitance-voltage measurement. The stable thickness and uniform surface morphology of the bilayer films with different La/Al deposition cycle ratio (La/Al ratio) were observed after rapid thermal annealing by spectroscopic ellipsometry and atomic force microscopy, respectively. We have found that with a decrease of the La/Al ratio, the dipole layer observed by X-ray photoelectron spectroscopy at Al2O3/La2O3 interfaces is close to the surface of semiconductor and the flat band voltage shifts to the negative direction. Furthermore, the dramatic drop in dielectric constant of the films as La/Al ratio decrease was caused by the formation of La(OH)3 in La2O3. Finally, the reason for the flat band voltage shifts, which is based on the dielectric constant of Al2O3 and La2O3 comprising the position of dipole layer in the dielectric films, is proposed.