Influence of Al2O3 atomic-layer deposition temperature on positive-bias instability of metal/Al2O3/β-Ga2O3 capacitors

Abstract

The influence of Al2O3 atomic-layer deposition (ALD) temperature on the electric characteristics of Al/Al2O3/(2¯01) β-Ga2O3 capacitors was investigated focusing on the positive-bias instability (PBI) of the capacitors. The current in the capacitors increased with ALD temperature, mostly because of the reduced energy barrier height for the electron field emission from the substrate and less negative Al2O3 charge, as revealed by the analysis conducted assuming a space-charge-controlled field emission process. The PBI tests were conducted for cumulative voltage stressing times vastly ranging from 3 × 10−6 to 4 × 105 s. The capacitance–voltage (C–V) characteristics of the capacitors for an ALD temperature of 100 °C displayed negative shifts in the middle of voltage stressing, unlike those for the other ALD temperatures. The bias stability of the capacitors was found to be considerably improved by high-temperature (450 °C) ALD. Additionally, the C–V characteristic shifts caused by the voltage stressing were theoretically reproduced quite accurately, assuming a model proposed in this study. In the simulations, the trap distributions in the Al2O3 films were assumed to be uniform both spatially and energetically. Importantly, the experimental results for various stressing voltages were excellently fitted by the simulations that assumed the same trap distribution. The trap densities in the Al2O3 films thus estimated reduced from 1.2 × 1020 to 2.2 × 1019 cm−3 eV−1 for ALD temperatures of 100–450 °C. This reduction in the trap densities was a major cause of the bias stability enhancement for high-temperature ALD. Moreover, the trap density as a function of ALD temperature qualitatively agreed with the aforementioned Al2O3 charge generated by the current measurements. This agreement provides a strong basis for the validity of the PBI model proposed in this study.