Study on Effect of Proton Irradiation Energy in AlGaN/GaN Metal-Oxide Semiconductor High Electron Mobility Transistors

AlGaN/GaN high electron mobility transistors (HEMTs) showed its promising features in high power and high frequency applications such as inverter units in hybrid electric vehicles, advanced radar systems, and satellite-based communication networks with its high density of sheet carrier concentration, high electron mobility, and radiation hardness. However, the high gate leakage and current collapse limits the reliability and performance of the HEMTs. In the other hand, using oxide as gate dielectric solves the problems at the same time not degrading the device performances. In this paper, we investigated the effect of the proton energy dependency on dc characteristics of atomic layer deposition (ALD) deposited Al₂O₃ gate dielectric AlGaN/GaN MOS-HEMTs. AlGaN/GaN MOS-HEMTs were irradiation with fixed proton dose of 5 × 10¹⁵ cm^-2 at 5MeV, 10MeV, and 15MeV. The MOS-HEMTs’ dc characteristics were compared before and after the irradiation to assess the damage done by irradiation.

AlGaN/GaN epilayer were grown on sapphire substrate by metal-organic chemical vapor deposition (MOCVD), consisting of 5 μm GaN buffer, 21 nm of undoped Al_0.24Ga_0.76N, and a 5 nm undoped GaN cap. On-wafer Hall measurement was used to measure sheet resistance, sheet carrier concentration, and mobility of 479 Ω/□, 9.55 × 10¹² cm^-2, and 1363 cm²/V·s respectively. Mesa isolation process was achieved by Unaxis Shuttlelock Reactive Ion Etcher with Inductively Coupled Plasma Module (ICP) with Cl₂/Ar plasma. Consisting of Ti/Al/Ni/Au (25 nm/125 nm/45 nm/100 nm), 5 μm distance of source to drain Ohmic metallization was achieved by lift-off process of electron-beam evaporator deposited Ohmic metal. Rapid thermal annealing at 850ºC with N₂ ambient for 45 seconds was followed by Ohmic metallization. A 10 nm Al₂O₃ layer was deposited by ALD at 300ºC. Ni/Au (20nm/80nm) gate metal with 100 μm width and 1 μm length was deposited by liftoff of e-beam evaporator. After fabricating MOS-HEMT, proton irradiation was projected at the Korean Institute of Radiological & Medical Sciences (KIRAMS) using a MC 50 (Scanditronix) cyclotron with proton fluence of 5 × 10¹⁵ cm^-2 at 5MeV. MOS-HEMT’s dc characteristics were measured by 4156 HP parameter analyzer. Transmission line measurement (TLM) was used to measure sheet resistance (R_S), transfer resistance (R_T), and contact resistance (R_C) before and after proton irradiation. Stopping and range of ions in matter simulations (SRIMS) was used to simulate the penetration depth of the protons into AlGaN/GaN MOS-HEMTs at different proton energies.

Figure 1 shows the effect of 5 × 10¹⁵ cm^-2 proton irradiation at 10MeV on drain I-V characteristics before and after. The saturation current, I_DSS, was reduced by 68.3% and mobility, which was calculated from linear region of drain I-V curve using charge control model, was reduced by 69.3%. These reductions were due to the displacement damage induced by ion bombardment from proton irradiation. Table 1 shows the summary of MOS-HEMTs device degradation in I_DSS, transconductance, threshold voltage shift, mobility, and sheet carrier concentration.

In conclusion, irradiation-induced damage causes the reductions of carrier mobility and carrier concentration. The severe degradation in lower irradiation energy is observed due to the more acceptor type defects generated near the 2 dimensional electron gas channel.