Electrical Characteristics of Ti/Al Ohmic Contacts to Molecular Beam Epitaxy-Grown N-polar n-type GaN for Vertical-Structure Light-Emitting Diodes

Abstract

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 x 10(18) cm(-3) showed nonohmic behaviors when annealed at 300A degrees C, but ohmic at 500A degrees C and 700A degrees C. All samples with carrier concentration of 2.0 x 10(19) cm(-3) exhibited ohmic behavior. x-Ray photoemission spectroscopy (XPS) results showed that, for samples with carrier concentration of 1.2 x 10(18) cm(-3), the Ga 2p core levels shift to lower or higher binding energy upon annealing at 300A degrees C or above 500A degrees C, respectively. Scanning transmission electron microscopy (STEM) results showed that, for samples with carrier concentration of 1.2 x 10(18) cm(-3), a wurtzite AlN layer (similar to 2 nm thick) formed at the metal/GaN interface when the samples were annealed at 500A degrees C. An interfacial wurtzite AlN layer also formed upon annealing at 700A degrees C, but its thickness was similar to 4 nm. Based on the XPS and STEM results, the ohmic contact formation and degradation mechanisms are described and discussed.