The Fabrication and Study of GaN Schottky Barrier Ultraviolet Photodetectors with Nanorod Template

UV PDs can be used in numerous civilian and military applications such as flame detection, furnace control, water purification, ozone layer monitoring, missile threat warning, and space-to-space communications [1]. Currently, most GaN-based devices are prepared by metalorganic chemical vapor deposition (MOCVD) on a sapphire substrate. However, the crystal quality of the GaN epitaxial layers prepared on a sapphire substrate is poor due to the large mismatches in lattice constant and thermal expansion coefficient between GaN and sapphire. In this paper, we report the preparation of nanorod templates with a simplified NRELOG [2] method and the fabrication of GaN Schottky barrier PDs on the nanorod template. Schematic diagram of the fabricated PDs are shown in Fig. 1. As shown in Fig. 1, an inductively coupled plasma (ICP) etcher was used to etch the GaN layer using the Ni nanoparticles as the hard mask. It was found that these GaN nanorods were vertically aligned with an average diameter of 350 nm. It was also found that density of the GaN nanorods was around 3x10 cm. Finally, a GaN layer was regrown on top of the nanorod template. For the fabrication of these Schottky barrier PDs, we deposited ohmic Ti/Al outer-ring with an inner diameter of 420 μ m. Ni/Au Schottky contact with a diameter of 400 μ m was subsequently deposited inside the ring opening. Fig 1. Schematic diagram of the fabricated PDs. Figure 2 shows bias-dependent spectral responses measured from the PDs prepared on nanorod template. The cutoff clearly occurred at around 360 nm, which corresponds to the bandgap energy of GaN. With an applied bias of -2 V, the responsivities measured from the PDs prepared on nanorod template were 0.009 A/W. Figure 3 shows measured noise power spectra of the PDs prepared on nanorod template. It was also found that both devices follow Hooge-type equation [2]