Development of Low-Resistance Ohmic Contacts with Bilayer NiO/Al-Doped ZnO Thin Films to p-type GaN

The fabrication of low-resistance and thermally stable Ohmic contacts is essential for the realization of reliable GaN power devices. In the particular case of p-type GaN, a thin Ni/Au bilayer is commonly used for Ohmic contacts. However, Au metal contacts are quite expensive, are incompatible with the complementary metal oxide–semiconductor foundries, and also have poor thermal stability. Thus, seeking an alternative that is affordable and thermally stable is crucial. In the present study, we investigate Au-free Ohmic contact formation on p-type GaN using a bilayer Ni/Al-doped ZnO (AZO) thin film. Careful studies were focused on identifying the role of process parameters such as annealing parameters: temperature, time, and atmosphere in order to obtain an excellent Ohmic contact on p-GaN. Our results show that the contact resistance can be significantly reduced using a Ni/AZO bilayer with a suitable rapid thermal process. We demonstrate that the specific contact resistance for Ni/AZO on p-GaN can reach the lowest value of 1.85 × 10 –4 Ω·cm 2 for a sample with a 5 nm Ni layer annealed at 500 °C in air for 5 min. Our work demonstrates that the bilayer Ni/AZO contact could be suitable for efficient GaN power diodes or transistors.