Low Thermal Budget Au-Free Hf-Based Ohmic Contacts on InAlN/GaN Heterostructures

Introduction

The InAlN/GaN heterostructures are attractive to researchers for high-power and high-speed applications owing to their unique properties such as high breakdown voltage, large bandgap, and strong polarization effects (thus leading to high 2DEG carrier density). Traditionally, InAlN/GaN high electron mobility transistors (HEMTs) are commonly fabricated with Ti/Al/Ni/Au ohmic contacts. However, Au is not compatible with Si process owing to its high diffusivity in Si. With the commercial availability of large GaN-on-Si wafers, Si CMOS compatible ohmic contacts for InAlN/GaN HEMTs are desirable, as this will allow their fabrication in incumbent Si foundaries, thus containing production costs. Furthermore, Ti/Al/Ni/Au contacts need a high annealing temperature (>800 ^oC) to achieve good ohimc properties on InAlN/GaN substrates [1-3], thus making it difficult to implement gate-first process, which is useful for the self-aligned process development. In this study, we report Au-free Hf/Al/Ta ohmic contacts on InAlN/GaN-on-Si with low contact resistance (R_c) and contact resistivity (ρ_c) achieved using a low annealing temperature (600 ^oC). The contact carrier transportation mechanism and preliminary thermal stability study are also investigated.

Experiment

The InAlN/GaN-on-Si wafer used in the experiment has a 9 nm Al_0.82In_0.18N barrier layer and 1 nm AlN spacer layer, grown on GaN buffer layer. Transfer Line Method (TLM) was used to evaluate the contact electrical properties. After mesa isolation formation by BCl₃/Cl₂ ICP etch, contact window patterning was carried out, followed by Hf/Al/Ta (15/200/20 nm) deposition by sputtering and metal lift-off to form contacts. Annealing of contacts was carried out in vacuum for 1 min at various temperatures (from 550 to 700 ^oC). SIMS and STEM were used to examine the interaction between Hf/Al/Ta and InAlN/GaN, and interface morphology, respectively. R_c, ρ_c and substrate sheet resistance (R_sh) were also measured as a function of temperature to analyze the contact carrier transportation mechanism. In addition, thermal aging test was conducted at 350 ^oC in air for more than 200 hours to assess the Hf/Al/Ta contact thermal stability.

Results and Discussion

Figure 1 shows the variation of R_c and ρ_c of Hf/Al/Ta (15/200/20 nm) contacts on InAlN/GaN-on-Si with annealing temperature. The lowest R_c ~ 0.59 Ω.mm and ρ_c ~ 6.7×10^-6 Ω.cm² are attained after 600 ^oC annealing. SIMS results before and after 600 ^oC annealing indicate that nitrogen out diffusion from substrate could be critical to achieving good ohmic properties for Hf/Al/Ta on InAlN/GaN-on-Si. Fig. 2 shows the R_c, ρ_c and R_sh of Hf/Al/Ta (15/200/20 nm) contacts on InAlN/GaN-on-Si annealed at 600 ^oC as a function of measurement temperature. As shown, R_c has a marginal temperature dependence, as a result of ρc decreases, while R_sh increases with increasing temperature. The latter increase is due to optical phonon scattering. The temperature dependence of ρ_c indicates that electron thermionic field emission (TFE) may take place across Hf/Al/Ta contact on InAlN/GaN-on-Si. On the other hand, the high 2DEG carrier density of our InAlN/GaN-on-Si substrate (1.2×10¹³ cm^-2) suggests that field emission (FE) may also happen. However, as FE is temperature independent, this transport component is unlikely to be dominant. The STEM image shown in Fig. 3 depicts the absence of spikes or contact inclusions in the thin InAlN barrier layer and its smooth interface with Hf/Al/Ta contact (likely as a result of the low annealing temperature at 600 ^oC). This reduces the tunneling mechanism. Hence, it is reasonable to conclude that electron TFE is the dominant transport mechanism between Hf/Al/Ta and InAlN/GaN. Finally, in the thermal aging test, no obvious change in R_c was observed after 200 hours at 350 ^oC in air ambient, indicating a thermal stable property for Hf/Al/Ta contacts.

Conclusion

In summary, Hf/Al/Ta ohmic contacts on AlInN/GaN-on-Si have been realized with R_c ~ 0.59 Ω.mm and ρ_c ~ 6.7×10^-6 Ω.cm2 after 600 ^oC annealing. SIMS results indicate that the ohmic contact formation could be due to nitrogen out diffusion from InAlN/GaN. Decrease of ρ_c with increasing temperature, coupled with the absence of spike formation or contact inclusions in the thin InAlN barrier layer and its smooth interface with Hf/Al/Ta contact, indicates that electron TFE could be the dominant transport mechanism for the Hf/Al/Ta ohmic contacts on InAlN/GaN. Finally, preliminary study has shown that Hf/Al/Ta contacts are stable at 350 ^oC in air.

References

[1] D. S. Lee, X. Gao, S. P. Guo and T. Palacios, 2011 IEEE Electron Device Lett.,32, 617.

[2] F. Medjdoub, M. Alomari, J.-F. Carlin, M. Gonschorek, E. Feltin, M. A. Py, N. Grandjean, and E. Kohn, 2008 IEEE Electron Device Lett., 29, 422.

[3] O. Katz, D. Mistele, B. Meyler, G. Bahir, J. Salzman 2005 IEEE Electron Device Lett., 52, 146.

Figure Captions:

Fig. 1: Variation of R_c and ρ_c of Hf/Al/Ta contacts on InAlN/GaN-on-Si with annealing (in vacuum for 1 min) temperature.

Fig. 2: R_sh, R_c and ρ_c of Hf/Al/Ta (15/200/20 nm) on InAlN/GaN-on-Si annealed at 600 ^oC as a function of measurement temperature.

Fig. 3: STEM image of Hf/Al/Ta (15/200/20 nm) on InAlN/GaN-on-Si after 600 ^oC annealing in vacuum for 1 min.