High-temperature structural behavior of Ni/Au Contact on GaN(0001)
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Internet Journal Nitride Semiconductor Research
High-temperature structural behavior of Ni/Au Contact on GaN(0001) Chong Cook Kim1, Jong Kyu Kim1, Jong-Lam Lee1, Jung Ho Je1, Min-Su Yi2, Do Young Noh2, Y. Hwu3 and P. Ruterana4 1Pohang
University of Science and Technology, Institute of Science and Technology, 3Institute of Physics, Academia Sinica, 4Laboratoire d'etude et de recherche sur les materiaux, CNRS, 2Kwangju
(Received Friday, January 19, 2001; accepted Thursday, February 15, 2001)
We investigated the structural evolution 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. GaN decomposition occurred mostly along GaN dislocations at temperature higher than 500°C. The decomposed Ga diffused into Au and Ni substitutional positions, and the decomposed nitrogen reacted with Ni, forming Ni4N. Interestingly, Ni4N was grown epitaxially. The epitaxial relationship of the Ni4N, Au, and Ni was identified as M(111)//GaN(0002) and M[1 -1 0]//GaN[1 1 -2 0] (M= Ni4N, Au, and Ni). At dislocation free regions, however, the atomically smooth interface remained intact up to 700 °C. Remarkable improvement of device reliability is expected in the contact on dislocation free regions compared with the contact on dislocations.
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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], [2]. They also have the potential for electronic devices operating at high temperatures up to 300 °C due to their superior physical properties, such as the wide bandgap, high breakdown electric field, high saturation velocity, and high thermal conductivity [3], [4]. To fabricate reliable, efficient, high-performance devices and circuits, it is essential to develop high quality and thermally stable contacts to GaN-based materials. Many extensive studies have been made for developing optimized ohmic contact systems [5], [6]. Since power dissipation across the p-GaN/metal interface generated Joule heat, failure of LDs was due to indiffusion of the ohmic contact elements along dislocations in the GaN epilayers, leading to the electrical short of the pn junction [7]. 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 sys-
tem. 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 [2], [7]. 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 °C [8]. The decrease of resistivity during annealing was explained due to the creation of Ga vacancies in p-GaN and the formation of Au-Ga solid solution from the x-ray diffraction data of reduced Au layer spacing. Systematic s
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