Microstructure, electrical properties, and thermal stability of Al ohmic contacts to n-GaN

Abstract

As-deposited Al contacts were ohmic with a room-temperature contact resistivity of 8.6 × 10⁻⁵ Ω • cm² on Ge-doped, highly n-type GaN (n = 5 × 10¹⁹ cm⁻³). They remained thermally stable to at least 500 °C, under flowing N₂ at atmospheric pressure. The specific contact resistivities (ρ_c) calculated from TLM measurements on as-deposited Al layers were found to range from 8.6 × 10⁻⁵ Ω • cm² at room temperature and 6.2 × 10⁻⁵ Ω • cm² at 500 °C. Annealing treatments at 550 °C and 650 °C for 60 s each under flowing N₂ resulted in an overall increase of contact resistivity. Cross-sectional, high-resolution electron microscopy (HREM) revealed that interfacial secondary phase formation occurred during high-temperature treatments, and coincided with the degradation of contact performance. Electron diffraction patterns from the particles revealed a cubic structure with lattice constant a = 0.784 nm, and faceting occurring on the {100} faces. Spectroscopic analysis via electron energy loss spectroscopy (EELS) revealed the presence of nitrogen and small amounts of oxygen in the Al layer, but no appreciable amounts of Ga. The results of microstructural and crystallographic characterization indicate that the new interfacial phase is a type of spinel Al nitride or Al oxynitride.