Comparison of the effect of gate dielectric layer on 2DEG carrier concentration in strained AlGaN/GaN heterostructure

摘要

The effect of surface passivation on undoped AlGaN/GaN heterostructures using SiO_2, Al_2O_3, Ta_2O_5 and Si_3N_4 as a function of layer thickness is presented. It is found that all of the oxides caused decreased 2DEG carrier concentration with increasing thickness of the respective oxide layers between the gate and AlGaN layer. On the contrary, the 2DEG carrier concentration increased strongly with increasing Si_3N_4 layer thickness. An elementary polarization model was used to fit the behavior for all materials and thicknesses leading to quantitative results. The fitting suggests that the effect of the oxides and Si_3N_4 on the 2DEG carrier concentration can be explained by the differences between them with respect to charge accumulation at the AlGaN/dielectric interface. High temperature in-situ deposited Si_3N_4 especially shows interesting behavior by bowing measurements as it also adds strain which increases piezoelectric polarization charge in AlGaN layer, so that increases the charge density in the 2DEG.