Temperature Behavior of Pt/Au Ohmic Contacts to p-GaN

Abstract

Ohmic contacts to Mg-doped p-GaN grown by MOCVD {xc[1]} are studied using a circular transmission line model (TLM) to avoid the need for isolation. For samples which use a p-dopant activation anneal before metallization, no appreciable difference in the specific contact resistance, r_c, as a function of different capping options is observed. However, a lower r_c is obtained when no pre-metallization anneal is employed, and the post-metallization anneal simultaneously activates the p-dopant and anneals the contact. This trend is shown for Pt/Au, Pt, Pd/Pt/Au, and Ni/Au contacts to p-GaN. The r_c ‘s for these metal contacts are in the range of 1.4‐7.6 x 10^-3 ohm-cm² at room temperature at a bias of 10mA. No particular metallization formula clearly yields a consistently superior contact. Instead, the temperature of the contact has the strongest influence.

Detailed studies of the electrical properties of the Pt/Au contacts reveal that the I-V linearity improves significantly with increasing temperature. At room temperature, a slightly rectified I-V characteristic curve is obtained, while at 200°C and above, the I-V curve is linear. For all the p-GaN samples, it is also found that the sheet resistance decreases by an order of magnitude with increasing temperature from 25°C to 350°C. The specific contact resistance is also found to decrease by nearly an order of magnitude for a temperature increase of the same range. A minimum r_c of 4.2 x 10^-4 ohm-cm² was obtained at a temperature of 350°C for a Pt/Au contact. This result is the lowest reported r_c for ohmic contacts to p-GaN.