AlON insulated-gate structure on AlGaN prepared by ECR-CVD using an aluminum-ethoxide source

Mizue, Chihoko; Matsuyama, Tetsuya; Kotani, Junji; Miczek, Marcin; Hashizume, Tamotsu Japan

An insulated-gate structure is very attractive for GaN-based high-power devices, because it can reduce gate leakage and enhance the dynamic range of device operation even at the forward bias at high temperatures. To realize such devices, it is inevitable to achieve insulator-semiconductor interfaces with low density of electronic states. In addition, the insulator should have a large energy gap (E_G) and a large dielectric constant (ε). In this paper, for the first time, we report on electrical characterization of the AlON insulated-gate on AlGaN prepared by electron-cyclotron-resonance assisted chemical vapor deposition (ECR-CVD), especially focusing on electronic-state properties of AlON-AlGaN interfaces. The values of E_G and ε for AlON are expected to be 6 ~ 8 eV and 8~9, respectively. We used Al_xGa_1-xN (0<x<0.26) layers grown on a sapphire substrate by metal organic vapor phase epitaxy. Typical values of electron concentration and mobility at room temperature (RT) were 1 x 10¹⁷ cm^-3 and 100 cm²/Vs, respectively. For the deposition of AlON on AlGaN, we used diethyl-aluminum ethoxide [(C₂H₅)₂ AlOC₂H₅] as a source in which the Al-O bond is originally included. N₂ was used as a carrier gas to introduce the source into the ECR-CVD chamber. The deposition was performed in ECR-N₂ plasma with a microwave power of 100 W at RT. After the deposition, we fabricated metal-insulator-insulator (MIS) diodes using Ti/Al/Ni/Au and Ni/Au as the ohmic and the gate-metal electrodes, respectively. From x-ray photoelectron spectroscopy analysis, we found that the N composition in AlON could be changed from 5 to 10 % by changing the N₂ flow rate in the range of 5-20 sccm. The bandgap of the AlON layer was 6.7 eV for the N composition of 10 %. The capacitance-voltage (C-V) measurements were carried out on the MIS diodes. At RT, we obtained the C-V curve very close to the theoretical one. A relatively low density of 5 x 10¹¹ cm^-2eV^-1 was obtained for the interface states at Ec-0.5 eV, which is much less than that of the SiO₂/AlGaN interface. In addition, no change in the C-V curve was observed at high temperatures up to 300^oC. The results obtained indicate that the AlON insulated gate is promising for realizing GaN-based devices with stable operation at high temperatures.

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