The role of Al on Ohmic contact formation on n-type GaN and AlGaN∕GaN

Abstract

A standard metallization scheme for the formation of Ohmic contacts on n-type GaN does exist. It has the following multilayer structure: Ti∕Al∕metal∕Au. Ti is known to extract N out of the GaN. This leaves a high density of N vacancies (donors) near the interface pinning the Fermi level. The created tunnel junction is responsible for an Ohmic contact behavior. Au is deposited as the final metal layer to exclude oxidation of the contact and the metal should limit the diffusion of Au into the layers below and vice versa. Al in the metallization scheme is known to improve the contact resistance, but the reason why has not been reported yet. We studied Ti and Ti∕Al contacts on GaN and AlGaN∕GaN as a function of annealing temperature by transmission electron microscopy. The role of Al in the metal multilayer, and of Al in the AlGaN on the Ohmic contact formation, has been determined. The latter result indicates that the standard metallization scheme for GaN cannot be simply transferred to AlGaN∕GaN structures.

Induced changes in surface band bending of n-type and p-type AlGaN by oxidation and wet chemical treatments

“…In this study, the exact sensitivity factor for p-and n-AlGaN samples was not known, so the observed ratio of samples from groups A and B at a detection angle of 45°was taken as 1. According to the reported result by Daele et al, 10 we find that the enthalpies of formation of GaN and AlN are about −110.9 and −318.1 kJ/ mol. According to the reported result by Kim et al, 11 we find that the Gibbs formation energy ͑⌬G f ͒ of AlO x is about −1582.3 kJ/ mol, which is lower than that ͑⌬G f = −998.3 kJ/ mol͒ of GaO x .…”

Ohmic contact to n-AlGaN through bonding state transition at TiAl interface

“…This phenomenon of N reduction at interface is consistent with previous report for low Al content case. 24,25 Thus, it is convinced that the desired Al-rich bonding state at the interface has been achieved.…”

Investigation of the Ti/Al/Pt/Au and Ti/Au contact materials on the N‐face surface of oxygen doped GaN

“…Becase the binding energy of the N-Ti [13] and N-Al [22] bonds are too close to distinguish. This can simply be explained by considering the enthalpies of formation of GaN, TiN, and AlN (respectively, -110.9, -265.5, and -318.1 kJ/mol) [6]. However, because the Al-N bond is stronger than the Ti-N bond, resulting in the formation of thin AlN layer at the GaN:O interface.…”