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 x 10(17) to 7.4 x 10(18) cm(-3)) single-crystal GaN (0001) films. Transmission line measurements (TLM) revealed the as-deposited TiN and Au/Ti contacts on it = 1.2 x 10(18) cm-3 to be ohmic with room-temperature specific contact resistivities of 650 and 2.5 x 10(-5) n cm(2), 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 rho(c) 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 rho(c) for n = 1.2 x 10(18) cm(-3) reached 1.2 x 10(-6) Omega cm(2); for n = 4.5 x 10(17) cm(-3), rho(c) = 7.5 x 10(-5) Omega cm(2) after annealing both samples through 900 degrees 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.