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

Single Ti layers, single TiN layers, and thin Ti films overlayered with Au were investigated as ohmic contacts to n-type (n 4.5 × 10¹⁷ to 7.4 × 10¹⁸ cm⁻³) single-crystal GaN (0001) films. Transmission line measurements (TLM) revealed the as-deposited TiN and Au/Ti contacts on n = 1.2 − 10¹⁸ cm⁻³ to be ohmic with room-temperature specific contact resistivities of 650 and 2.5 × 10^7minus;5 Ω cm², respectively. Single Ti layer contacts had high resistance and were weakly rectifying in the as-deposited condition. The three contact/GaN systems exhibited a substantial decrease in resistivity after annealing; the value of ρc was also a function of the carrier concentration in the GaN. The Au/Ti contacts exhibited the lowest resistivity values yet observed in these contact studies, particularly for the more lightly doped n-GaN. The ρc for n = 1.2 × 10¹⁸ cm⁻³ reached 1.2 × 10²⁶ Ω cm²; for n = 4.5 × 10¹⁷ cm⁻³, ρc = 7.5 × 10²⁵ Ω cm² after annealing both samples through 900 °C. X-ray photoelectron spectroscopy (XPS) and high-resolution cross-sectional transmission electron microscopy (X-TEM) analysis revealed the formation of TiN at the interface of annealed Ti layers in contact with GaN, which is believed to be beneficial for ohmic contact performance on n-GaN.