Effects of Annealing Pressure and Ambient on Thermally Robust RuOx Schottky Contacts on InAlN/AlN/GaN-on-Si(111) Heterostructure

We have shown that RuO_x based Schottky contacts have good potential as the anode and gate electrode in III-nitrides high power diodes and high electron mobility transistors, respectively, owing to their low leakage current¹. We have also found that the electrical characteristics of RuO_x Schottky contacts depend on the annealing ambient and/or pressure, namely RuO_x Schottky diodes annealed in N₂ exhibit better thermal stability and lower leakage current compared to those annealed in vacuum. This is believed to be related to the oxidation of RuO_x to RuO₂ in N₂ annealing and the reduction of RuO_x to Ru in vacuum annealing. Between N₂ and vacuum annealing, there are differences of pressure and ambient. Hence, it is not clear if the formation of RuO₂ or the reduction to Ru from RuO_x is related to the annealing pressure or ambient, or both. This motivates us to investigate the effects of annealing pressure and ambient on the electrical and material characteristics of RuO_x Schottky diodes by conducting annealing in vacuum, N₂ and Ar, with annealing in N₂ and Ar performed at the same pressure.

RuO_x Schottky diodes fabrication process began with Ohmic contact patterning on InAlN/AlN/GaN-on-Si(111) substrates with a sheet resistance of 490Ω/□. The Ohmic contacts (Hf/Al/Ta) were then deposited and formed by the lift-off process, followed by annealing at 600^oC in vacuum for 1 minute. After Schottky contact patterning (RuO_x) was sputter-deposited in Ar and O₂ plasma with 2:1 flow-rate ratio. During RuO_x deposition, the substrates were not heated. The Schottky contacts were also formed by means of the lift-off process, and were annealed at 800^oC under different pressures and ambients (in vacuum at a pressure of 6.67 × 10^-3Pa, in N₂ and Ar at the same pressure of 101.3kPa) for 1 minute to study their effects on the electrical and material characteristics of RuO_x Schottky contacts.

RuO_x Schottky diodes without annealing have a reverse leakage current of ~18.0±2.1mAcm^-2 at the bias voltage of -20V (see Figure 1(a)) and Schottky barrier height (SBH) of ~0.754±0.072eV. With vacuum annealing at the pressure of 6.67×10^-3Pa, the reverse leakage current increases by one order of magnitude and SBH drops to 0.546±0.050eV. In contrast, the reverse leakage currents of both RuO_x Schottky diodes annealed in N₂ and Ar at the same pressure (~101.3kPa) decrease by 2 orders of magnitude and SBH increases by ~0.177eV compared to those of RuO_x Schottky diodes without annealing. Since the electrical characteristics of RuO_x Schottky diodes with either N₂ or Ar annealing are similar, and different from those with vacuum annealing, it may be deduced that the annealing pressure, and not the ambient, is the reason for the differences observed. To substantiate this, we conduct X-ray Diffraction (XRD) characterisations of our samples, as shown in Figure 1(b), where RuO_x Schottky contact without annealing is observed to be amorphous. With vacuum annealing at pressure of 6.67×10^-3Pa and temperature of 800^oC, only Ru phases are formed and no RuO_x phase is observed, owing possibly to the dissociation of Ru and O bond in RuO_x. This agrees well with the relationship between the dissociation pressure and temperature of RuO₂ bond proposed by Brunetti et al.². Since the dissociation pressure of RuO₂ bond at 800^oC is ~0.255Pa, which is much higher (~40 times) than the pressure of our vacuum annealing, RuO_x reduces to Ru. On the other hand, with Ar annealing at the pressure of 101.3kPa, which is more than 5 orders of magnitude higher than the dissociation pressure of RuO₂ bond, only RuO₂ phases are formed and no Ru phase is observed. Since Ar is a noble gas, the oxidation of RuO_x to RuO₂ with Ar annealing and the reduction of RuO_x to Ru with vacuum annealing (at the temperature of 800^oC) are most likely due to the difference in the annealing pressure. In addition, with N₂ annealing at the same pressure and temperature as Ar annealing, the I-V characteristic and XRD spectrum are similar to those of Ar annealed sample (see Figure 1). Hence, N₂ does not seem to react with RuO_x and only pressure plays a critical role in the formation of RuO₂ in RuO_x annealed at 800^oC.

In conclusion, based on our studies, the electrical and material characteristic differences observed in RuO_x RuO_x Schottky diodes annealed in N₂, Ar and vacuum are most likely due to the annealing pressure and not the annealing ambient. 

References

1. L. M. Kyaw et al., Phys. Status Solidi C 11, 883(2014).

2. B. Brunetti, et al., Matls. Chem. Phys. 83, 145(2004).