(Invited) Contacts to Group III Nitride Semiconductors

In this presentation, I will review recent work on contacts to group III nitride semiconductors, including two examples from our lab. Conductive metal nitrides are candidates for high-temperature, high-power Schottky diodes to n-type GaN. One of the metal nitrides in thermodynamic equilibrium with GaN that also has a high work function is Mo2N. Using bis (tertbutylimido) bis (dimethylamino) molybdenum, either a remote N2-H2 plasma1 or NH3 was used as the co-reactant to deposit a conductive metallization. ALD offers an advantage over sputter deposition because deep levels can be created in GaN from the direct plasma during sputter deposition. ALD also has an edge over electron-beam evaporation because it is difficult to control the stoichiometry of nitride compounds during e-beam evaporation. Both types of ALD processes resulted in layers that contained some carbon, but the layers were good conductors, and there is significant solubility of MoC in Mo2N. Thermal ALD resulted in diodes with a Schottky barrier height of 0.69 eV and an ideality factor of 1.06 before annealing, while the remote-plasma process resulted in non-ideal diodes with high reverse currents. However, after annealing in N2 at 600 °C, diodes prepared by both methods offered similar Schottky barrier heights (0.84 eV by thermal ALD and 0.87 eV by remote-plasma ALD). Inspired by prior work to “deconstruct” Ti/Al-based contacts to n-GaN,2,3 we investigated the previously reported Zr/Al/Mo/Au4–6 contact on AlxGa1-xN with x = 0.66–0.83 using the circular transfer length method and also examined the metallurgy of contacts to AlN. Zirconium is an interesting metal for contacts to AlN-rich compositions because Zr reacts with AlN at lower temperatures than many other metals, while Al, Mo, and Au are predicted to be unreactive with AlN based on thermodynamics. Preparing Zr/Al stacks without the Mo/Au cap, we found that Al-rich contacts (with a 1:4 ratio of Zr:Al) became ohmic on Al x Ga1-x N with x = 0.67 after annealing at 800 °C, while Zr-rich contacts did not become ohmic even after annealing as high as 1150°C, highlighting the continuing importance of Al in the contacts to AlxGa1-xN with high x. Moreover, the thickness of Au in the capping layer also played a role in both the contact morphology and the required temperature to form ohmic contacts AlxGa1-xN with x = 0.66–0.67. An annealing temperature of 700 °C was optimal for the Au-rich contact, while a higher temperature of 950 °C was needed when the Au cap was thin, despite both contacts having similar specific contact resistance once they became ohmic (3–4 x 10-4 Ohm-cm2). Based on an examination of the contact morphology and relevant phase diagrams, we have formed a hypothesis that contacts that undergo partial melting can be prepared using lower annealing temperatures. References A. Molina et al., Appl. Phys. Lett. 119 102102 (2021) Luther, B. P. et al. Appl Phys Lett. 71, 3859–3861 (1997) Kwak, J. S. et al. Semicond. Sci. Technol. 15, 756–760 (2000) Hu, X. et al. IEEE Electron Device Lett. 39(10), 1568–1571 (2018) Yafune, N. et al. Electron Lett. 50(3), 211–212 (2014) Douglas, E. A. et al. physica status solidi (a) 214(8) 1600842 (2017) The authors gratefully acknowledge the support of ONR through N00014-22-1-2462 and N00014-23-1-2028. DISTRIBUTION STATEMENT A. Approved for public release distribution unlimited (DCN# 0543-1697-24 and DCN# 0543-1699-24).