Interface characteristics of spin-on-dielectric SiO_x-buffered passivation layers for AlGaN/GaN high electron mobility transistors

To reveal the cause for significant enhancement of dc current performance of the AlGaN/GaN high electron mobility transistors (HEMTs) with the spin-on-dielectric (SOD) SiO_x-buffered passivation structure compared to the conventional Si₃N₄ passivation deposited by plasma-enhanced vapor deposition (PECVD), we characterized the passivation interfaces using the cross-sectional transmission electron microscopy, cathodoluminescence, capacitance-voltage (C-V) characterizations, and Hall-effect measurements. The interface state density of PECVD Si₃N₄ passivation was in the range of 10¹²-10¹³ cm^{- 2} eV^{- 1}, which is one-order higher than that of the SOD (10¹¹-10¹² cm^{- 2} eV^{- 1}) as measured by C-V measurements from the metal-insulator-semiconductor capacitors. Higher density of effective oxide charge density (especially dominant contribution of ionic mobile charge) was also derived from the PECVD Si₃N₄ passivation. A well-resolved reduction of the electron Hall mobility of the Si₃N₄ passivation compared to that of the perhydropolysilazane SOD passivation, which can be due to the higher-density interface states and trap charges, can answer the relative dc current collapse of our HEMT devices.