PhD Defense: "Material engineering for the next generation of GaN-based high-electron-mobility transistors grown by metal-organic chemical vapor deposition"

Haoran Li

December 1st (Friday), 1:00pm
Elings, Rm 1605

(Al,Ga)N material system have great capacity in high-power, high-frequency applications due to their high breakdown field and electron saturation velocity. One particularly attractive application is the high-electron-mobility transistor (HEMT) with a two-dimensional electron gas (2DEG) formed at the AlGaN/GaN hetero-interface. While the majority of today’s commercial HEMTs are grown in the (0001) metal-polar direction, (000-1) N-polar HEMT structures are advantageous in device scalability, electron confinement and low contact resistance. In this work, we focused on two crucial aspects of N-polar AlGaN/GaN HEMT structures grown by metal-organic chemical vapor deposition (MOCVD), the AlN interlayer and the channel thickness scaling.
The AlN interlayer between the AlGaN barrier and GaN channel is critical to achieve high electron mobility (µ) of the 2DEG by suppressing alloy scattering. However, ~ 50% Ga was previously observed in metal-polar AlN films grown by MOCVD. In this work, both metal- and N-polar AlN films grown under various conditions were investigated. While 40~50% of Ga was again observed in the metal-polar AlN layers, only 1~5% Ga was measured for the N-polar AlN films regardless of the growth conditions, indicating another advantage of N-polar HEMTs compared to the metal-polar ones.
In order to scale down channel thickness of N-polar HEMT structures without sacrificing the sheet charge density (ns) and µ, we proposed a novel design of channel structure and demonstrated it by MOCVD. A thin InGaN layer was inserted between the AlGaN cap and the GaN channel, introducing net positive polarization charge at the InGaN/GaN interface, reducing the electric field in the channel, and therefore increasing ns. The 2DEG also moved away from the AlN/GaN interface, reducing the interface related scattering and improving µ. By replacing the conventional 6 nm thick pure GaN channel with a composite channel of 2 nm In0.1Ga0.9N + 4 nm GaN, the ns increased from 7.5×1012 to 1.2×1013 cm-2 with an improving µ from 606 to 1141 cm2/Vs.

Hosted by: Professor Umesh K. Mishra