Effects of Annealing Pressure and Ambient on Thermally Robust RuOx Schottky Contacts on InAlN/AlN/GaN-on-Si(111) Heterostructure
Authors: Kyaw Lwin Min , Liu Yi , Lai Mei Ying , Bhat Thirumaleshwara N. , Tan Hui Ru , Lim Poh Chong , Tripathy Sudhiranjan , Chor Eng Fong 
Published: February 2020

We have shown that RuOx 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 RuOx Schottky contacts depend on the annealing ambient and/or pressure, namely RuOx 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 RuOx to RuO2 in N2 annealing and the reduction of RuOx 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 RuOx 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 RuOx Schottky diodes by conducting annealing in vacuum, N2 and Ar, with annealing in N2 and Ar performed at the same pressure.

RuOx 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 600oC in vacuum for 1 minute. After Schottky contact patterning (RuOx) was sputter-deposited in Ar and O2 plasma with 2:1 flow-rate ratio. During RuOx deposition, the substrates were not heated. The Schottky contacts were also formed by means of the lift-off process, and were annealed at 800oC 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 RuOx Schottky contacts.

RuOx 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 RuOx 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 RuOx Schottky diodes without annealing. Since the electrical characteristics of RuOx 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 RuOx Schottky contact without annealing is observed to be amorphous. With vacuum annealing at pressure of 6.67×10-3Pa and temperature of 800oC, only Ru phases are formed and no RuOx phase is observed, owing possibly to the dissociation of Ru and O bond in RuOx. 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 800oC is ~0.255Pa, which is much higher (~40 times) than the pressure of our vacuum annealing, RuOx 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 RuOx to RuO2 with Ar annealing and the reduction of RuOx to Ru with vacuum annealing (at the temperature of 800oC) 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 RuOx and only pressure plays a critical role in the formation of RuO2 in RuOx annealed at 800oC.

In conclusion, based on our studies, the electrical and material characteristic differences observed in RuOx RuOx 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).

Figure 1