PhD Defense: "Contributions to the Understanding of (Al,Ga)N – Silicon Nitride Interfaces"

Brian L. Swenson

September 23rd (Friday), 2:00pm
Materials Research Lab, Rm 2053

For the last decade Gallium Nitride and its alloys have been a popular topic in high power and high frequency transistors. GaN’s main enabling material properties are its high breakdown voltage, high electron mobility combined with its high electron density (when combined with AlGaN), and its high temperature stability. In the early years of GaN development, a major breakthrough has been identified to be the introduction of SiNx passivation, which considerably reduced dispersion caused by surface states and led to a significant increase in output power density. The use of a gate insulator is also crucial in power electronics to minimize the leakage current through the gate. Understanding and optimizing these passivation and gate insulators are crucial steps along with optimizing the device design and structure in order to achieve greater performance and higher power densities.

This thesis focuses on minimizing the interface state density of the SiNx / GaN interface grown in situ by MOCVD by exploring the growth parameter space of the SiNx. The SiNx interface state density is studied on Ga-polar (0001), N-polar (000-1), and M-plane (10-10) GaN material. The interface state density is measured by a Photo-assisted CV technique developed in this thesis that takes advantage of the type-II band alignment in the SiNx / GaN system. It was found that growth temperature had the largest impact on interface states for all crystal planes tested, where the highest growth temperature measured (~1200C) also had the lowest interface state density. One the other hand, defect density (from 1E7 to 1E9 /cm^2) had almost no impact on interface state density.

The effect of introducing a thin AlGaN or AlN interlayer between the SiNx and GaN is also studied in an effort to separate the interface states from Fermi level of the device. It is found that, while the interface states still exist, it is possible to use the polarization of the Al(Ga)N on Ga-polar GaN to raise the energy level of the interface states with respect to the bulk GaN so that they are not able to interact with the Fermi level. The opposite effect is shown for N-polar GaN, where the interface states are pushed into the midgap of GaN.

Finally, the charge injection phenomenon, widely observed in SiNx, is preliminarily investigated. The emission and extraction of the injected charge is also studied. Additionally, field enhanced trap ionization was observed and briefly studied.

Hosted by: Professor Umesh K. Mishra