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

摘要

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 300 deg C, but ohmic at 500 deg C and 700 deg 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 300 deg C or above 500 deg 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 A1N layer (approx2 nm thick) formed at the metal/GaN interface when the samples were annealed at 500 deg C. An interfacial wurtzite A1N layer also formed upon annealing at 700 deg C, but its thickness was approx4 nm. Based on the XPS and STEM results, the ohmic contact formation and degradation mechanisms are described and discussed.