Impact of GaN cap on charges in Al2O3/(GaN/)AlGaN/GaN metal-oxide-semiconductor heterostructures analyzed by means of capacitance measurements and simulations

Oxide/semiconductor interface trap density (D-it) and net charge of Al2O3/(GaN)/AlGaN/GaN metal-oxide-semiconductor high-electron mobility transistor (MOS-HEMT) structures with and without GaN cap were comparatively analyzed using comprehensive capacitance measurements and simulations. D-it distribution was determined in full band gap of the barrier using combination of three complementary capacitance techniques. A remarkably higher D-it (similar to 5-8 X 10(12) eV(-1) cm(-2)) was found at trap energies ranging from E-C-0.5 to 1 eV for structure with GaN cap compared to that (D-it similar to 2-3 X 10(12) eV(-1) cm(-2)) where the GaN cap was selectively etched away. D-it distributions were then used for simulation of capacitance-voltage characteristics. A good agreement between experimental and simulated capacitance-voltage characteristics affected by interface traps suggests (i) that very high D-it (>10(13) eV(-1) cm(-2)) close to the barrier conduction band edge hampers accumulation of free electron in the barrier layer and (ii) the higher D-it centered about E-C-0.6 eV can solely account for the increased C-V hysteresis observed for MOS-HEMT structure with GaN cap. Analysis of the threshold voltage dependence on Al2O3 thickness for both MOS-HEMT structures suggests that (i) positive charge, which compensates the surface polarization, is not necessarily formed during the growth of III-N heterostructure, and (ii) its density is similar to the total surface polarization charge of the GaN/AlGaN barrier, rather than surface polarization of the top GaN layer only. Some constraints for the positive surface compensating charge are discussed. (C) 2014 AIP Publishing LLC.