Electrical characteristics of Ti/Al ohmic contacts to molecular beam epitaxy-grown N-polar n-type gan for vertical-structure light-emitting diodes

We investigated the electrical properties of Ti(30 nm)/Al(200 nm) contacts to molecular beam epitaxy-grown N-polar n-GaN with different carrier concentrations. Samples with carrier concentration of 1.2 × 10 ¹⁸ cm^-3 showed nonohmic behaviors when annealed at 300 C, but ohmic at 500 C and 700 C. All samples with carrier concentration of 2.0 × 10¹⁹ cm^-3 exhibited ohmic behavior. x-Ray photoemission spectroscopy (XPS) results showed that, for samples with carrier concentration of 1.2 × 10¹⁸ cm^-3, the Ga 2p core levels shift to lower or higher binding energy upon annealing at 300 C or above 500 C, respectively. Scanning transmission electron microscopy (STEM) results showed that, for samples with carrier concentration of 1.2 × 10 ¹⁸ cm^-3, a wurtzite AlN layer (∼2 nm thick) formed at the metal/GaN interface when the samples were annealed at 500 C. An interfacial wurtzite AlN layer also formed upon annealing at 700 C, but its thickness was ∼4 nm. Based on the XPS and STEM results, the ohmic contact formation and degradation mechanisms are described and discussed.