Trap states in AlGaN/GaN metal-oxide-semiconductor structures with Al2O3 prepared by atomic layer deposition

Abstract

Atomic layer deposition (ALD) of ${\text{Al}}_2{\text{O}}_3$ was used to prepare metal-oxide-semiconductor (MOS) devices on two different_AlGaN/GaN heterostructures, with and without a thin GaN cap layer. Their trapping effects were evaluated by the frequency dependent conductance measurement. The trap state density decreased sharply from $\sim 1\times {10}^{12}{\text{cm}}^{-2}{\text{eV}}^{-1}$ at the energy of 0.27 eV to $\sim 3\times {10}^{10}{\text{cm}}^{-2}{\text{eV}}^{-1}$ at 0.45 eV. The low trap state density and exactly exponential dependence of the trap state time constant on the gate voltage show a good quality of the gate oxide. The trap state density in the structure with a GaN cap is about 2–3 times lower than that in the structure without a cap, which might be due to the different ${\text{Al}}_2{\text{O}}_3/\text{GaN}$ and ${\text{Al}}_2{\text{O}}_3/\text{AlGaN}$ interface properties. The trap state density in the structures investigated is lower than those reported for the devices with the metal-organic chemical vapor deposition and Al-oxidized ${\text{Al}}_2{\text{O}}_3$ gate oxide. This shows an importance of the ALD technique for the preparation of high-performance AlGaN/GaN MOS transistors.