Structural Evolution of Ni/Au Contact on GaN(0001)
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Structural Evolution of Ni/Au Contact on GaN(0001) Chong Cook Kim1, Jong Kyu Kim1, Jong-Lam Lee1, Min-Su Yi2, Jin-Woo Kim2, Do Young Noh2, Yeukuang Hwu3, Pierre Ruterana4, and Jung Ho Je1 1 Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang, Korea. 2 Department of Materials Science and Engineering and Center for Electronic Materials Research, Kwangju Institute of Science and Technology, Kwangju, Korea. 3 Institute of Physics, Academia Sinica, Nankang, Taipei 11529, Taiwan, Republic of China. 4 Equipe Structure et Comportement Thermomécanique des Matériaux (CRISMAT UMR 6508 CNRS), ISMRA 6, Bd Maréchal Juin, Caen Cedex F-14050, France. ABSTRACT We investigated the structural behavior of the Ni/Au contact on GaN(000l) during annealing in N2, using in-situ x-ray diffraction, anomalous x-ray scattering, and high resolution electron microscopy. Thermally activated atomic mobility caused the two metal atoms, Au and Ni, to interdiffuse during annealing and form solid solutions. At temperature higher than 500oC, GaN decomposition and reactions occurred mostly along GaN dislocations. By decomposed nitrogen reacted with Ni, interestingly, epitaxial Ni4N phase was formed. The epitaxial relationship of the Ni4N, Au, and Ni was identified as M(111)//GaN(0002) and M[ 110 ]//GaN[ 1120 ] (M= Ni4N, Au, and Ni). INTRODUCTION The GaN based semiconductors have recently been applied to fabricate optoelectronic devices such as light emitting diodes (LEDs) and laser diodes (LDs) in the blue and violet light region [1-4]. They also have the potential to be fabricated for electronic devices operating at high temperatures up to 300oC due to their superior physical properties, such as the wide bandgap, high breakdown electric field, high saturation velocity, and high thermal conductivity [5,6]. To develop reliable, efficient, high-performance devices and circuits, high quality and thermally stable contacts to GaN-based materials are essential. Many extensive studies have been made for developing optimized ohmic contact systems [7,8]. Since power dissipation across the p-GaN/metal interface generated Joule heat, failure of LDs was reported by Nakamura et al. [9,10] to be due to indiffusion of the ohmic contact elements along dislocations in the GaN epilayers, leading to the electrical short of the pn junction. To employ metal layers as a reliable ohmic contact on GaN, it is essential to understand the thermal stability and metallurgy of metal-GaN contact in addition to developing low-resistance ohmic system. The research on the high temperature behavior, therefore, is the current interest. Ni/Au has been commonly used as ohmic contact to p-type GaN in experimental LDs [10,11]. A low contact resistivity of around 10-3Ωcm2 was obtained for p-GaN after annealing in N2 ambient at temperatures ranging from 500 to 700 oC [12]. The decrease of resistivity during annealing was attributed to the creation of Ga vacancies as a result of the formation of Au-Ga solid solution, which was deduced from the x-ra
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