Inhomogeneity of barrier heights of transparent Ag/ITO Schottky contacts on n-type GaN annealed at different temperatures

Abstract

We report on the formation of high barrier-height and transparent Ag/ITO Schottky contacts on n-GaN (nd = 5 × 1018 cm−3) for optoelectronic and transparent electronic devices. Calculations using the thermionic emission model-based current-voltage characteristics of the samples annealed at various temperatures showed small Schottky barrier heights (SBHs) of 0.31–0.37 eV and ideality factors of 1.84–2.19. Conventional activation energy plot showed greatly smaller Richardson constant than the theoretical value. To understand such abnormality, the modified Richardson plot, a model of lateral SBH variation with Gaussian distribution, and capacitance-voltage method were used, where their SBHs were estimated to be in the range 0.74–0.93 eV. Together with the temperature-dependent SBHs and ideality factors, these results imply that SBH behavior could be explained in terms of barrier inhomogeneity at the interfaces. The Ag/ITO samples annealed at 500 °C transmitted 80.9% at 560 nm. The X-ray photoemission spectroscopy (XPS) Ga 2p core level spectra from the interfaces of the samples shifted toward either higher or lower energies. Scanning transmission electron microscopy (STEM)-energy dispersive X-ray spectroscopy mapping results revealed the outdiffusion of Ga atoms from n-GaN when annealed at 500 °C. Based on the electrical, XPS and STEM results, the annealing temperature dependence of the SBHs is described and discussed.