Microstructure, electrical properties, and thermal stability of Al ohmic contacts to n -GaN
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Microstructure, electrical properties, and thermal stability of Al ohmic contacts to n-GaN L. L. Smith and R. F. Davis Materials Research Center, North Carolina State University, Raleigh, North Carolina 27695-7919
M. J. Kim and R. W. Carpenter Center for Solid State Science, Arizona State University, Tempe, Arizona 85287-1704
Y. Huang Argonne National Laboratory, Argonne, Illinois 60439 (Received 19 January 1996; accepted 18 March 1996)
As-deposited Al contacts were ohmic with a room-temperature contact resistivity of 8.6 3 10–5 V? cm2 on Ge-doped, highly n-type GaN (n 5 3 1019 cm–3 ). They remained thermally stable to at least 500 ±C, under flowing N2 at atmospheric pressure. The specific contact resistivities (rc ) calculated from TLM measurements on as-deposited Al layers were found to range from 8.6 3 10–5 V? cm2 at room temperature and 6.2 3 10–5 V? 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 h100j 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.
I. INTRODUCTION
The formation of ohmic contacts with semiconductor materials and devices is a fundamental component of solid state device architecture. As device size has diminished and the scale of integration has increased, the quality of these interfaces has become an increasingly important concern. The presence of parasitic resistances and capacitances, such as those existing at contact interfaces, becomes more detrimental at higher operating powers and higher oscillation frequencies. For many devices, the losses that occur at the contact interfaces account for a large fraction of the total losses, and, as such, have a significant impact on device performance. The majority of successful ohmic contact systems implemented with the more conventional compound semiconductors have relied upon alloying (liquidphase reaction) or sintering (solid-phase reaction) via post-deposition annealing treatments, and/or the presence of high carrier concentrations near the interface.1–4 However, many otherwise successful ohmic contact systems have only limited thermal stability and are subject to degradation— usually in the form of extensive interdiffusion, interfacial reaction, and interphase growth, accompanied by increase in contact J. Mater. Res., Vol. 11, No. 9, Sep 1996
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