N-polar GaN HEMT with HfO2 as Gate Dielectric for mm-wave Applications
Creators: name: Mohanty, Subhajit
Publisher: name: My University
Year: 2023
Subjects: subject: GaN
subject: HEMT
subject: HfO2
subject: ALD
subject: N-polar
subject: Electrical Engineering
subject: Engineering

description: N-polar GaN-based HEMTs have shown tremendous promises for high-power high-frequency (30-110 GHz) applications due to several advantages such as better pinch-off, low ohmic contact and better gate control. To further achieve higher 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 via UV-assisted C-V and current-voltage characterization methods. 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 O2 increased the breakdown voltage, and reduced the interface states compared to annealing in N2. The lowest average interface trap density was achieved to be 1.64 × 1012 cm-2 eV-1 with breakdown field of 3.14 MV/cm. xv All the processes developed previously were used for fabrication of planar N-polar GaN MIS-HEMTs. The dual pulsed-IV measurements suggest that there are two competing mechanisms at play during device operations resulting in anti-dispersion and dispersion at the same time. Additionally, to reduce the DC-RF dispersion, N-polar deep recess (NPDR) HEMTS with HfO2 as gate dielectric have been explored. BCl3/SF6 selective dry etch was developed to realize deep-recess structure with thin AlGaN cap as etch stop layer. The HEMTs exhibited a peak saturation current of 1.1A/mm with fT=18.1GHz and fmax = 66.1 GHz. The HEMTs biased in class AB at VDS=9 V and IDS=100 mA/mm at 4 GHz frequency demonstrated a max output power of 1.53 W/mm with associated PAE of 45.4%. Pulsed IV measurements reveal that the deep recess structure did not exhibit DC-RF dispersion from surface traps present in drain access region. Rather the traps present in the gate dielectric cause dispersion which has been shown to depend on the thickness of the dielectric. In fact, the dispersion is less than 10% at a very thin dielectric of 4.4 nm. It is reported that interfacial nitrogen vacancies at negative polarization interface (NPI) were the source of these hole/donor traps. Using the Silvaco TCAD simulation platform, for the first time, the effect of hole traps density and energy level on 2DEG density in N-polar HEMT structures was studied. Comparing the calculated 2DEG density with that of experimental results, hole trap density and activation energy were estimated. The results showed that Si doping in both the barrier and at barrier-buffer interface effectively neutralizes the traps present throughout the epi-structure while simultaneously modulating the 2DEG charge density. Moreover, an epi-structure was designed to enable aggressive scaling of the channel (5nm-thick) in N-polar GaN HEMTs while maintaining a large 2DEG density of approximately 2.6×1013 cm-2 with minimal parasitic.