Low Thermal Budget Au-Free Hf-Based Ohmic Contacts on InAlN/GaN Heterostructures
Authors: Liu Yi , Kyaw Lwin Min , Bera Milan Kumar , Singh Sarab Preet , Ngoo Yi Jie , Lo Guo Qiang , Chor Eng Fong 
Published: February 2020

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 (Rc) and contact resistivity (ρc) 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 Al0.82In0.18N 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. Rc, ρc and substrate sheet resistance (Rsh) 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 Rc and ρc of Hf/Al/Ta (15/200/20 nm) contacts on InAlN/GaN-on-Si with annealing temperature. The lowest Rc ~ 0.59 Ω.mm and ρc ~ 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 Rc, ρc and Rsh 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, Rc has a marginal temperature dependence, as a result of ρc decreases, while Rsh increases with increasing temperature. The latter increase is due to optical phonon scattering. The temperature dependence of ρc 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 Rc 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 Rc ~ 0.59 Ω.mm and ρc ~ 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 ρc 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 Rc and ρc of Hf/Al/Ta contacts on InAlN/GaN-on-Si with annealing (in vacuum for 1 min) temperature.
Fig. 2: Rsh, Rc and ρc 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.