Ohmic Contacts to GaN

The Schottky-Mott, Bardeen, and metal-induced gap state models of the formation of the Schottky barrier at a metal-semiconductor contact interface and the relative importance of each model to ohmic contact behavior on GaN are described. The underlying principles regarding the development of stable, Ti-based ohmic contacts to n-type GaN for optoelectronic applications has been subsequently addressed. The specific contact resistivity for these contacts has been reported to be as low as 8.9×10^-8 Ω cm² Ω . By contrast, achieving ohmic contacts to p-type GaN with a specific contact resistivity <10^-5 Ω cm² Ω continues to be a challenge due to the inherent difficulties involved in acceptor doping with Mg and the existence of an~2-nm thick tenacious layer of native contamination on the as-grown surface that can add an additional 0.2 eV to the barrier height. The results of various plasma-, directed ion beam- and chemical-based in situ and ex situ surface preparation methods to remove contaminants, and/or modify the surface electronic properties, and/or achieve epitaxy of large grain size contacts with the underlying GaN are detailed. For the majority of studies the mechanism through which the ohmic behavior is improved is complex. The investigators of the present research have used an NH₃-based chemical vapor clean to achieve ordered, stoichiometric, ptype GaN(0001) surfaces without detectable C and a significantly reduced O concentration. The resultant significant reduction in the downward band bending is attributed to the removal of contamination-induced surface states located in the lower portion of the band gap. Ni- and Pd-based contact structures on these cleaned surfaces are significantly less rectifying than identical contact structures on conventional HCl-treated surfaces, which is consistent with the removal of surface contamination and a reduction in the Schottky barrier height. Significant differences in structure and morphology were also observed for contacts on chemical- vapor-cleaned and HCl-treated surfaces following a postmetallization anneal. These results are compared and contrasted with the reported results of electrical, chemical, and structural characterization by numerous other investigators.