(Invited) Characteristics of Several High-k Gate Insulators for GaN Power Device
Authors: Nabatame Toshihide , Maeda Erika , Inoue Mari , Hirose Masafumi , Kiyono Hajime , Irokawa Yoshihiro , Shiozaki Koji , Koide Yasuo 
Published: February 2020

Introduction

Vertical GaN metal-oxide-semiconductor (MOS) devices on freestanding GaN substrates have been widely investigated as a means of reducing the dimensions of devices intended for applications associated with high power and high frequency [1]. Inserting a gate insulator in such as MOS structures has been found to result in low leakage currents and high breakdown strengths. Oxide (High-k) with a high dielectric constant (k)-value have been studied instead of SiO2 from the viewpoint of increasing the physical thickness. Numerous High-k gate insulators, including Al2O3, HfO2, Al2O3-HfO2 bilayers, AlSiOx and HfSiOx have been studied for use in MOS units [2, 3]. These films are typically fabricated using atomic layer deposition (ALD) because ALD has a big advantage of conformal film fabrication on three-dimensional structure, which is required when producing vertical GaN devices.
To improve electrical properties of these gate insulators and GaN/insulator interface, annealing process was carried out after High-k deposition (post-deposition annealing: PDA) and post-metal deposition (post-metal deposition annealing: PMA). Actually, GaN capacitors with Al2O3 reduced significantly the interface state density (Dit) at GaN/Al2O3 interface by PMA at low temperarure ~ 300 °C [4]. On the other hand, Ga diffusion and additional electrical defects were also found to be easily introduced into GaN/High-k stack structure by PDA at a high temperarure above 700 °C. Here, we have an interesiting about how the PDA and PMA processes affect to characteristics of GaN devices with several High-k gate insulators.
In this paper, we present influence of the PDA and PMA processes on characteristics of GaN capacitors with Al2O3, HfO2, and HfSiOx gate insulators.

Experiment

Pt-gated n-GaN MOS capacitors were fabricated as follows: Firstly, ~20-nm-thick High-k gate insulators such as Al2O3, HfO2, and HfO2/SiO2 laminate were deposited on n+-GaN/n-GaN epilayer by plasma-enhanced ALD at 300 °C. Next, PDA was carried out at 800 °C in N2. HfSiOx was formed from HfO2/SiO2 laminate after PDA. Finally, Pt gate electrode and Pt/Ti ohmic cntact were deposited to fabricate n-GaN/High-k/Pt MOS capacitors. PMA was performed at 300 °C in N2.

Results and discussion

The Al2O3 and HfO2 films had polycrystalline structure of g phase and monoclinic phase, respectively, while the HfSiOx film had an amorphous structure after PDA at 800 °C. The k values for the Al2O3, HfO2, and HfSiOx films which esgtimated from capacitance (C)-voltage (V) data were 8, 17.6, and 15.1, respectively. The Al2O3 capacitor exhibited a large frequency dispersion while the HfO2 and HfSiOx capacitors were negligible small, indicating that the charging state of the interface traps occur at GaN/Al2O3 interface. The frequency dispersion of the Al2O3 capacitor significantly decreases after PMA at 300 °C. To better understand the characteristics of the GaN/High-k interface, the Dit was also estimated using a conductance method. The Dit values of all capacitors before PMA at the energy level of Ec-E = 0.4 V was decreased in the following order: Al2O3 (5 × 1012 cm-2eV-1) > HfO2 (1.4 × 1012 cm-2eV-1) > HfSiOx (4 × 1011 cm-2eV-1). As expected from the C-V characteristics, the Al2O3 capacitor had a large Dit value due to the electron traps.
From the leakage current density (J)-electric field (E) characteristics, The J values of the HfSiOx capacitor was significantly reduced compared to those of the Al2O3 and HfO2 capacitors. It is thought that the difference in the J properties between the HfSiOx and other Al2O3, HfO2 can be primarily attributed to the presence of amorphous or polycrystalline structures, because grain boundaries in polycrystalline structures can act current leakage paths. Considering the bias application for the same amount of charge accumulation at the interface, effective breakdown electric field (Ebd) values, which was estimated from the applied voltage divided by capacitance equivalent thickness, were 15, 14, and 35 MV/cm in Al2O3, HfO2, and HfSiOx capacitors, respectively.

Summary

We conclude that the HfSiOx film is an attractive material as gate insulator because of amorphous structure, higher k, relatively low Dit, and high effective Ebd.

Acknowledgement

The authors wish to thank Prof. T. Hashizume of Hokkaido University for helpful discussion. This work was supported in part by the MEXT Program for the Research and Development of Next-generation Semiconductors to Realize an Energy-saving Society.

References

[1] M. Kodama et al., Appl. Phys. Express 1, 021104 (2008).
[2] T. Kikuta et al, J. Vac. Sci. Technol. A 35, 01B122 (2017).
[3] T. Nabatame et al., Appl. Phys. Express 12, 011009 (2019).
[4] T. Hashizume et al., Appl. Phys. Express 11, 124102 (2018).