INVESTIGATION OF ALUMINUM AND TITANIUM/ALUMINUM CONTACTS TO N-TYPE GALLIUM NITRIDE B. P. LUTHER *, S. E. MOHNEY **, T. N. JACKSON *, M. ASIF KHAN *, Q. CHEN ***, AND J. W. YANG *** *Department of Electrical Engineering, The Pennsylvania State University, University Park, PA 16802 "**Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, mohney@ems.psu.edu ***APA Optics, Inc., 2950 N.E. 84th Lane, Blaine, MN 55449 ABSTRACT We report on a study of Al and Ti/Al contacts to n-type GaN. Al contacts on n-GaN (7x 101 7 cm-3) annealed in forming gas at 600'C reached a minimum contact resistivity of 8x I 0-6cm 2 and had much better thermal stability than reported by previous researchers. Ti/Al (35nm/ 1l5nm) contacts on n-GaN (5x 101 7 cm- 3 ) had resistivities of 7x 10 6 cm 2 and 5 X 10 6 Qcm 2 after annealing in Ar at 400'C for 5min and 600'C for 15sec, respectively. Depth profiles of Ti/Al contacts annealed at 400 0 C showed that low contact resistance was only achieved after Al diffused to the GaN interface. We propose that the mechanism for ohmic contact formation in Ti/Al contacts annealed in the 400-600'C range includes reduction of the native oxide on GaN by Ti and formation of an Al-Ti intermetallic phase in intimate contact with the GaN. Contacts with different Ti/Al layer thicknesses were investigated and those with 5Onm/ I00nm layers had the same low resistance and better stability than 25nm/125nm contacts. INTRODUCTION GaN has received great interest in the past few years for applications in blue/ultraviolet lasers, light emitting diodes, photodetectors, and high temperature/high power electronic devices. High quality ohmic contacts are critical to these applications. Although many researchers have made ohmic contacts to n-GaN with low contact resistivities, the mechanism for ohmic contact formation has not been explained fully. The purpose of this investigation was to determine the mechanism for Al-based ohmic contacts on n-GaN formed by annealing in the temperature range of 400-600'C. Contacts containing Ti/Al layers have displayed the lowest contact resistivity to n-type GaN. , 2 TiN has been observed 3 at the interface of Ti/Al and Ti/Al/Ni/Au contacts annealed at 900 0 C. Researchers have speculated 1 - 3 that due to TiN formation when Ti reacts with GaN, a high concentration of nitrogen vacancies is created near the interface, causing the GaN to be heavily doped n-type. A thin layer of TiN has also been observed at the interface of unannealed samples that were etched by RIE before metal deposition. Without RIE prior to metal deposition, good ohmic contacts have been attained by annealing Ti/Al films on n-GaN between 550 and 750'C for 20sec, 4 however, there are no reported investigations of Ti/Al contacts annealed in this temperature range that would reveal the mechanism for ohmic contact formation. 1097 Mat. Res. Soc. Symp. Proc. Vol. 449 0 1997 Materials Research Society