The disparate polarization at AlGaN/GaN heterostructures engenders a 2D electron gas (2DEG) that effectively reduces on-resistance and power loss, resulting in a high electron mobility ideal for high-frequency, high-temperature, and high-voltage requirements associated with HFETs and HEMTs. Since GaN and AlGaN are characterized by a large spontaneous polarization, there is a large polarization bound surface charge (2.2x1013 and 3.2x1013 charges/cm2) that must be compensated presumably by states near the dielectric-GaN or dielectric-AlGaN interface.
While specific failure mechanisms have yet to be identified, it is clear that these states may play a role in gate leakage and current collapse. Recent efforts to improve performance of high power devices have focused on the role of a high-k dielectric as a gate oxide between the gate metal and AlGaN/GaN and as a surface passivation layer between the gate and source-drain contacts. The use of a specific dielectric requires a large band gap and band offsets that confine carriers in the semiconducting layers. Moreover, the concentration of the 2DEG is directly affected by the presence of interface states and defect states associated with the dielectric.
In this research, we have employed remote plasma enhanced ALD (PEALD) and in-situ photoemission spectroscopy to prepare and characterize different dielectric layers on GaN and AlGaN surfaces. The in-situ x-ray and UV photoemission (XPS and UPS) measurements provide insight into the interface bonding, presence of impurities, band alignment, and band bending, which indicate the presence of defects.
This report will concentrate on the properties of PEALD Al2O3 and HfO2 layers on AlGaN and GaN on both the Ga-face and N-face. Results establish that the band alignment is relatively independent of processing and may be described by the charge neutrality level model. In contrast, the band bending is dependent on processing and can be directly related to the presence of charged defect states at the interface and in the dielectric. The wet chemical and in-situ surface cleaning processes and the PEALD growth affect the band bending, resulting in similar band bending regardless of the surface polarization.
This consistency indicates the presence of interface states that compensate the polarization charge on Ga- and N-face surfaces even though the polarization charge is opposite in sign and for GaN and AlGaN surface where the polarization charge increases by a factor of two. The interplay of polarization and the defect states and the 2DEG properties are discussed for the different dielectric layers and processing approaches.Research is supported by ONR through DEFINE MURI.