A method for fabricating a vertical gallium nitride (GaN) power device can include providing a GaN substrate with a top surface and a bottom surface, forming a device layer coupled to the top surface of the GaN substrate, and forming a metal contact on a top surface of the vertical GaN power device. The method can further include forming a backside metal by forming an adhesion layer coupled to the bottom surface of the GaN substrate, forming a diffusion barrier coupled to the adhesion layer, and forming a protection layer coupled to the diffusion barrier. The vertical GaN power device can be configured to conduct electricity between the metal contact and the backside metal.