Fig. 1. (Color online) (a) Schematic of epitaxial layer structure with GaN contact layer (left) and In_0.15Ga_0.85N contact layer (right). The composition and doping concentration of each layer were labeled. (b) Microscope photo of the fabricated samples with c-TLM pattern. (c) SEM imagine of the Pd/Au = 10/30 nm electrode. Insert of the figure is the SEM imagine of the etched cross section.

Fig. 2. (Color online) (a) I−V characteristic of the sample with In_0.15Ga_0.85N contact layer using the Pd/Au = 10/30 nm electrode annealing in N₂ : O₂ = 4 : 1. Insert of (a) is the total resistance along with the Ln(R/r). R is the outer electrode radius and r is the inner electrode radius. (b) I−V characteristic of the sample with GaN contact layer using the Pd/Au = 10/30 nm electrode annealing in N₂ : O₂ = 4 : 1. Insert of (b) is the total resistance along with the Ln(R/r).

Fig. 3. (Color online) Energy band diagrams at the interface of Pd with p-type (a) GaN; (b) In_0.15Ga_0.85N.

Fig. 4. Barrier level of the interface versus the In composition in the contact layer.

Fig. 5. (Color online) Specific contact resistance of the samples with different contact metal thicknesses including the (a) Pd/Au and (b) Ni/Au electrode groups annealing in N₂ : O₂ = 4 : 1. Au layer is fixed at 30 nm.

Fig. 6. (Color online) Auger electron spectroscopy of (a) Pd/Au = 10/30 nm annealing at N₂ : O₂ = 4 : 1 atmosphere; (b) Pd/Au = 10/30 nm annealing at N₂ atmosphere; (c) Ni/Au = 10/30 nm annealing at N₂ : O₂ = 4 : 1 atmosphere; (d) Ni/Au = 10/30 nm annealing at N₂ atmosphere.