(Invited) Study of HfO2-Based High-k Gate Insulators for GaN Power Device

Abstract

Gallium nitride (GaN) high-electron-mobility transistor and vertical GaN devices with metal-oxide-semiconductor (MOS) structure have been widely investigated to suppress leakage currents for applications associated with high power and high frequency [1-3]. Metal oxide (High-k) with a high dielectric constant (k)-value have been employed instead of SiO2 from the viewpoint of increasing the physical thickness [4, 5]. HfO2-based materials including HfO2, HfSiOx and HfAlOx are the candidate materials because of its relatively large bandgap (~6.5 eV) and a high k value (> 13) [6-9]. These films for transistor applications are typically fabricated using atomic layer deposition (ALD) because ALD is compatible with semiconductor fabrication process and has a big advantage of conformal film fabrication on three-dimensional structure. In this study, we report comparison of characteristics among HfO2, HfSiOx and HfAlOx gate insulators of GaN MOS capacitors.Pt-gated n-GaN MOS capacitors with HfO2, HfSiOx and HfAlOx gate insulators were prepared as follows. A single HfO2 layer, (HfO2)m/(SiO2)n and (HfO2)m/(Al2O3)n nanolaminate layers were deposited on n+-GaN/n-GaN epilayer by plasma-enhanced ALD at 300°C. The m/n ratio was kept at 2/1. Next, PDA was carried out at 800°C in N2. HfSiOx and HfAlOx films were formed from (HfO2)m/(SiO2)n and (HfO2)m/(Al2O3)n nanolaminate after PDA, respectively. The thicknesses of these films were ~24 nm. Finally, Pt gate electrode was deposited on gate insulator and Pt/Ti ohmic contact was deposited on the n-GaN substrate.The HfSiOx and HfAlOx films had an amorhous structure while HfO2 film had a polycrystalline structure after PDA at 800°C using transmission electron microscopy and electron diffraction pattern analysis. The HfO2 capacitor exhibited a small breakdown electric field (EBD) value of 2.6 MV/cm, defined at a current density of 1.0 × 101 A/cm2, because leakage currents easily flow along grain boundaries of the polycrystalline structure. On the other hand, the EBD values of the HfSiOx and HfAlOx capacitors increased up to 8.5 MV/cm. From capacitance-voltage measurements, the estimated k values of the HfO2, HfSiOx and HfAlOx films were 17.7, 17.2 and 13.5, respectively, as expected. The flatband voltage hysteresis (620 mV) of the HfO2 capacitor was much larger than those (~10 mV) of the HfSiOx and HfAlOx capacitors, indicating that the number of charge trapping/detrapping sites and frequency responsive defects was significantly decreased in the HfSiOx and HfAlOx capacitors. No frequency dispersion of the HfSiOx and HfAlOx capacitors were observed while the HfO2 capacitor showed a large frequency dispersion. The interface state density (Dit) values at Ec - E = 0.25 eV which estimated using the conductance method of the HfSiOx (~1 × 1011cm-2V-1) and HfAlOx (4.4 × 1011cm-2V-1) capacitors were one magnitude lower than that of the HfO2 capacitor (3.1 × 1012cm-2V-1), indicating that the electrical defects at n-GaN/HfSiOx and n-GaN/HfAlOx interfaces were much less than that of the n-GaN/HfO2 interface.We conclude that the HfSiOx and HfAlOx were the attractive materials as gate insulator because of amorphous structure, higher k, relatively low Dit, and high Ebd.This work was supported by JSPS KAKENHI (Grant No. JP20H02189). A part of this work was also supported by the MEXT “Program for research and development of next-generation semiconductor to realize energy-saving society.” Program Grant Number JPJ005357. The authors wish to thank Dr. A. Ohi, Dr. N. Ikeda, and the members of the nanofabrication group of the National Institute for Materials Science for their support during this study.[1] M. Kodama et al., Appl. Phys. Express 1, 021104 (2008).[2] Z. Yatabe et al., J. Phys. D: Appl. Phys. 49, 393001 (2016).[3] T. Oka et al., Appl. Phys. Express 8, 054101 (2015).[4] T. Hashizume et al., Appl. Phys. Express 11, 124102 (2018).[5] T. Kikuta et al, J. Vac. Sci. Technol. A 35, 01B122 (2017).[6] T. Nabatame et al., Appl. Phys. Express 12, 011009 (2019).[7] E. Maeda et al., Microelectron. Eng. 216, 111036 (2019).[8] R. Ochi et al., AIP Advances 10, 065215 (2020).[9] S. Miyazaki et al., ECS Trans. 92, 11(2019).