Study on Effect of Proton Irradiation Energy in AlGaN/GaN Metal-Oxide Semiconductor High Electron Mobility Transistors
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 Al0.24Ga0.76N, 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 (RS), transfer resistance (RT), and contact resistance (RC) 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, IDSS, 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 IDSS, 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.