N-polar GaN HEMT with HfO2 as gate dielectric for mm-wave applications

Wide bandgap-based RF and power electronics are going to define many crucial “more-electric” transportation and communication systems because of enhanced efficiency in energy conversion afforded by wide bandgap materials from kHz to GHz. In the era of internet of things (IoT), artificial intelligence and autonomous vehicles, there is an urgent need for high-power and high frequency transistors that can facilitate ultra-fast, highly reliable, and low latency wireless networks. To serve these needs, transistors which can provide a combination of high power density and high efficiency at high frequencies are required.

Gallium Nitride has surfaced as a candidate for high power and high frequency applica­tions due to its unique properties, including a large band gap of 3.4 eV, large breakdown electric fields, and high electron mobility and saturation velocity. GaN-based optoelectronic and electronic devices for commercial applications have been mainly developed on Ga-polar GaN templates due to less complexity of epitaxial growth compared with that on N-polar. Nonetheless, N-polar GaN-based HEMTs have several advantages over Ga-polar GaN-based HEMTs that make them a promising candidate for high power RF applications.

To increase frequency of operation while maintaining high power, the gate-to-channel distance and gate length need to be scaled down simultaneously to avoid short-channel effects (SCEs). However, extreme scaling of gate-to-channel distance will result in higher gate leakage current which requires thick dielectrics and in-turn reduces gate-to-channel distance and gate control. This requires high-k dielectric as gate insulator whose small effective oxide thickness (EOT) can suppress gate leakage while maintaining good gate control.

This dissertation primarily is focused on developing HfO₂ as gate dielectric for ultra-thin N-polar GaN HEMT application. In the first part of the dissertation, the impact of various surface cleaning, in-situ atomic layer deposition (ALD) pre-treatment, ALD deposition method and post-deposition annealing ambiance on interface properties of HfO₂/ N-polar GaN was studied. A combination of the BHF and piranha cleaning with UV-ozone pretreatment followed by thermal HfO₂ improved the interface between HfO₂ and N-polar GaN quality significantly. Further annealing in O₂increased the breakdown voltage, and reduced the interface states compared to annealing in N₂. N-polar HEMTs were then fabricated and characterized with HfO₂ as gate dielectric. I showed that the bulk traps present in the gate dielectric cause dispersion which depends on the thickness of the dielectric. Devices with less than 10% DC-RF dispersion were obtained by scaling down the dielectric thickness to only 4.4 nm.

Chair: Professor Elaheh Ahmadi