Reliable Ti/Al and Ti/Al/Ni/Au ohmic contacts to n-type GaN formed by vacuum annealing

Abstract

In this work, we investigated Ti/Al and Ti/Al/Ni/Au ohmic contact metallizations to n-GaN which were formed by a vacuum annealing method. Our effort was focused on minimizing the oxidation of the Al, which limits the performance of the Al-based contacts. We have also investigated the effect of the Ni/Au overlayers on the performance and morphological characteristics of the contacts. Vacuum annealing was found to require temperatures similar to those used in halogen lamp rapid thermal annealing for forming ohmic contacts on n-type GaN, but with minimal oxidation of the Al surface. It was found that the Ni/Au overlayer on the Ti/Al reduces the specific contact resistivity values slightly and at the same time causes an upward shift, by ∼200 °C, in the contact formation temperature. For the Ti/Al and Ti/Al/Ni/Au on n-GaN doped at 1.1×1018 cm−3, a minimum specific contact resistivity of 1.2×10−5 Ω cm2 was achieved after 950 and 1100 °C anneals, respectively. Auger electron spectroscopic studies revealed that obtaining optimum ohmic contact resistance requires the penetration of Al through the Ti layer to reach the GaN surface in addition to N outdiffusing from the GaN surface and into the metal layers. Atomic force microscopy investigations indicated that the introduction of the Ni/Au overlayer on the Ti/Al had the effect of increasing the surface roughness after annealing.

Electrical and structural properties of low-resistance Ti/Al/Re/Au ohmic contacts to n-type GaN

“…However, this is still better than those previously reported for the Ti/Al/Mo/Au and Ti/Al/Ni/Au contacts. 17,21,22 The Ti/Al/Re/Au ohmic contacts were subjected to thermal annealing treatment to ensure suitability for high-temperature and high-power applications.…”

Review of using gallium nitride for ionizing radiation detection

“…For the Ti/Al/Ni/Au contact, Fan et al 127 reported an optimum annealing temperature of 750 C, and the multi-metal layer scheme has been widely used to form Ohmic contact on GaN. [128][129][130][131][132] In Schottky contacts, the degradation varies according to the metal employed. For Pd, Pt, Au, and Ni, degradation starts after exposure to temperatures as low as 300, 400, 575, and 600 C, respectively.…”

Thermally stable, oxidation resistant capping technology for Ti/Al ohmic contacts to n-GaN

“…There was an 8% uncertainty in the microprobe composition due to the difficulties with the Al standard, but the nominal concentration is close to TiAl 3 . A Ti͑20 nm͒/Al͑100 nm͒/Ni͑20 nm͒/Au͑80 nm͒ contact, which is a structure that has been previously studied, 16 was fabricated on the same substrate as a standard for comparison and a control for the contact's performance. We chose to use the same structure as that of Papanicolaou et al due to the fact that it closely matched the Ti:Al layer thickness ratio of our contact.…”

“…A very successful solution has been found with the deposition of a Ni/Au bilayer directly on top of the Ti/Al that addresses the oxidation issue with much success. 6,16 The Ni layer is used to prevent rapid diffusion of Au into the multilayer, but the Ni itself has a very high work function and is a questionable choice for use in an ohmic contact to an n-type semiconductor. Pt has been used in the place of Ni to stop the rapid diffusion of Au into the multilayer, although it really offers no more favorable characteristics than the Ni that it replaces.…”