Low-resistance and thermally stable Pd/Ru ohmic contacts to p-type GaN

Journal of Electronic Materials, Sep 2002 by Jang, Ja-Soon, Lee, Chang-Won, Park, Seong-Ju, Seong, Tae-Yeon, Ferguson, I T

We report on low-resistance and thermally stable Pd/Ru ohmic contacts to surface-treated p-GaN (3 x 10^sup 17^ cm^sup -3^). It is shown that annealing at 500 deg C for 2 min in a N^sub 2^ ambient improves ohmic contact properties. Specific contact resistance is measured to be 9.2(+/- 0.2) x 10^sup -4^ and 2.4(+/- 0.2) x 10^sup -5^ (Omega)cm^sup 2^ for the as-deposited and annealed samples, respectively. Atomic force microscopy results show that the surfaces of both the contacts are remarkably smooth with a root-mean-square (rms) roughness of about 0.6 nm. The current-voltagetemperature (I-V-T) and calculation results indicate that, for the as-deposited contact, thermionic field emission is dominant, while for the annealed contact, field emission dominates the current flow.

Key words: Gallium nitride, ohmic contact, Schottky barrier, specific contact resistance, surface Fermi level

INTRODUCTION

Gallium nitride (GaN)-based compound semiconductors are of great technological importance for optoelectronic and electronic devices, such as blue light emitting diodes, laser diodes, metal semiconductor field effect transistors, and high electron mobility transistors.1-5 High-quality ohmic contacts, which have thermal stability and excellent electrical properties, are essential to the improvement of the performance of such devices. Actually, the high contact resistance of p-GaN is one of the major obstacles to the realization of the long-lifetime operation of GaN-- based optical devices. Thus, in order to develop highquality ohmic contacts to p-GaN (and, hence, to improve the device performance), various metallization schemes, such as Ni/Au,6-9 Ni/Pt/Au,10,11 Pd/Au,12-14 and Pt/Au,15 have been widely investigated.

Recently, surface treatments have been employed to obtain low-resistance ohmic contacts to p-- GaN.13,14,16,17 It was shown that the surface treatments were effective in removing native oxide and resulted in a decrease of band bending (by the shift of surface Fermi level toward valence band maximum).18,19 Consequently, the treatments gave rise to a remarkable improvement of the electrical and thermal properties of the contacts to p-GaN.16,20 In this article, we report on low-resistance Pd/Ru ohmic contacts on p-GaN, which was two-step surface-treated using a buffered oxide etch solution. It is shown that annealing improves the contact properties. Based on the current-voltage-temperature (I-V-T) and x-ray photoemission spectroscopy (XPS) results, the carrier flow mechanisms and ohmic mechanisms are described and discussed.

ACKNOWLEDGEMENTS

This work was supported in part by the Korea Institute of Industrial Technology Evaluation and Planning and the U.S. AFOSR/AOARD.

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