(Invited) Characteristics of GaN/High-k Capacitors Under Positive Bias Stress
Authors: Nabatame Toshihide , Sawada Tomomi , Irokawa Yoshihiro , Koide Yasuo , Tsukagoshi Kazuhito 
Published: February 2024

Vertical GaN-based trench gate metal-oxide-semiconductor field-effect transistors (MOSFETs) have been widely investigated to suppress leakage currents for applications associated with high power and high frequency. Various materials with high dielectric constant (High-k) such as Al2O3, HfO2, AlSiO
x
, HfSiO
x
, and HfAlOx have been studied as gate insulator to obtain high gm value of GaN MOSFET [1-6]. The structure of High-k gate insulator and interface between GaN and High-k is well known to affect characteristics of GaN MOS capacitor. An amorphous HfAlOx film could be fabricated by forming (HfO2)/(Al2O3) nanolaminate films via plasma-enhanced atomic layer deposition and post-deposition annealing at 800°C. Al2O3 and HfAlOx gate insulators with an amorphous structure found to exhibit superior fundamental electrical properties including no frequency dispersion, a high breakdown voltage and a low interface state density [3,5,6]. However, reliability characteristics under high voltage sweep and positive bias stress (PBS) conditions are not currently well understood.
In this paper, we investigate flatband voltage (Vfb) shift of the Al2O3 and HfAlOx capacitors by varying effective electric field (Eeff) from Vfb under PBS. We also compare Vfb shift behavior among Al2O3, HfAlOx and reported High-k gate insulators.
Here, to understand influence of the thermal budget at 800°C on electrical property, two n-GaN/Al2O3(10nm)/Pt MOS capacitors were prepared: One was fabricated by thermal budget at 300°C (AlO). The other was fabricated by the same capacitor fabrication process, with the GaN substrate annealed at 800°C before Al2O3 film deposition (Pre800C-AlO). To examine interface properties of the n-GaN/HfAlOx, n-GaN/interfacial layer (IL)/HfAlOx(10nm)/Pt capacitor with a 0.5nm-thick-Al2O3-IL (AlO/HfAlO) and a 0.5nm-thick-HfO2-IL (HfO/HfAlO) and without IL (HfAlO) were prepared.
All High-k gate insulators had an amorphous structure. The k-values of Al2O3 and HfAlOx gate insulators estimated from C-V measurement were 8 and 17.2, respectively [3,6]. All capacitors exhibited the smooth J-Eeff properties due to an amorphous structure regardless of the 800°C pre-annealing of the GaN substrate and IL of HfAlOx case. All capacitors also exhibited no frequency dispersion in the range of 1k-1MHz. However, the Pre800C-AlO capacitor showed a large hump due to the electrical defects in the C-V curve. The Vfb shift behavior of the capacitor was examined by varing Eeff from Vfb of PBS. The Eeff was estimated from the applied voltage divided by capacitanc equivalent thickness. The Eeff from Vfb of Al2O3 and HfAlOx capacitors were varied from 4.1-8.7 and 6.5-15.4MVcm-1, respectively. The stress time was varied from 0 to 300s . The sweep voltage range of C-V measurement for obtaining Vfb in each stress time was always applied smaller than the PBS voltage. The positive Vfb shift was observed in all capacitors after PBS, indicating the presence of some electron trapping sites. The positive Vfb increased siginificantly up to a stress time of 10s, gradually increased up to 200s, and saturated above 200s in all Eeff from Vfb regions. The positive Vfb value of all capacitors reached to around 0.2V when the Eeff from Vfb was applied to 6.5MVcm-1 at 300s. Above 6.5MVcm-1, different Vfb shift profiles were observed for two Al2O3 and three HfAlOx capacitors. The positive Vfb value of the Al2O3 capacitors increased drastically to 0.5V as the Eeff from Vfb increased from 6.5 to 8.7MVcm-1. Furthermore, the Pre800C-AlO capacitor became larger Vfb shift than the AlO capacitor because the Pre800C-AlO capacitor is due to the introduction of some defects on the GaN surface by pre-annealing at 800°C. On the other hand, for the HfAlO, AlO/HfAlO and HfO/HfAlO capacitors, the positive Vfb shift value was suppressed to 0.3V even when the Eeff from Vfb was applied to 15.4 MVcm-1. Furthermore the Vfb shift showed similar behavior regardless of Al2O3- and HfO2-IL. Based on these experimental data, we conclude that the HfAlOx capacitors have a high reliability under PBS condition.
This work was supported by the MEXT Program for Creation of Innovative Core Technology for Power Electronics Grant Number JPJ009777. This work was supported in part by JSPS KAKENHI Grant No. JP20H02189.
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