Microstructure, electrical properties, and thermal stability of Ti-based ohmic contacts to n -GaN
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Microstructure, electrical properties, and thermal stability of Ti-based ohmic contacts to n-GaN L. L. Smitha) and R. F. Davis Materials Research Center, North Carolina State University, Raleigh, North Carolina 27695-7919
R-J. Liu, M. J. Kim, and R. W. Carpenter Center for Solid State Science, Arizona State University, Tempe, Arizona 85287-1704 (Received 24 February 1998; accepted 31 August 1998)
Single Ti layers, single TiN layers, and thin Ti films overlayered with Au were investigated as ohmic contacts to n-type (n 4.5 3 1017 to 7.4 3 1018 cm23 ) single-crystal GaN (0001) films. Transmission line measurements (TLM) revealed the as-deposited TiN and AuyTi contacts on n 1.2 3 1018 cm23 to be ohmic with room-temperature specific contact resistivities of 650 and 2.5 3 1025 V 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 rc was also a function of the carrier concentration in the GaN. The AuyTi contacts exhibited the lowest resistivity values yet observed in these contact studies, particularly for the more lightly doped n-GaN. The rc for n 1.2 3 1018 cm23 reached 1.2 3 1026 V cm2 ; for n 4.5 3 1017 cm23 , rc 7.5 3 1025 V 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. I. INTRODUCTION
The work described in this paper is part of a systematic study of surface cleaning and ohmic contact strategies for GaN. Two main approaches were taken in this study regarding the development of ohmic contacts to GaN. The first approach was similar to that which has resulted in the majority of successful ohmic contacts to the more conventional compound semiconductors, namely, the creation of high carrier concentrations in the semiconductor at the metal interface. The stabilization or “pinning” of the Fermi level at this surface in, for example, GaAs results in a quasi-fixed potential barrier at the metal interface. As such, the approach has generally been to shrink the width of the depletion layer by means of increasing the carrier concentration to the point where carriers tunnel readily through the barrier. Even with optimization of contact composition and annealing times and temperatures, the lowest contact resistivities (rc ) have been obtained only on the most heavily doped semiconductor surfaces.1– 4 Though there are indications that high doping levels and extensive interfacial reactions are not essential for ohmic contact formation in all cases, these processes have frequently proven useful for minimizing rc . However, many otherwise successful a)
Present address: Department of Chemical Engineering, North Ca
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