Ohmic Contact Formation Mechanism of Pd-based Contact to p-GaN
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Ohmic Contact Formation Mechanism of Pd-based Contact to p-GaN Dae-Woo Kim, Joon Cheol Bae, Woo Jin Kim, Hong Koo Baik, Chong Cook Kim1 and Jung Ho Je1 and Chang Hee Hong2 Thin Film Materials Laboratory, School of Materials Science & Engineering, Yonsei University, 134 Shinchon-Dong, Seodaemun-Gu, Seoul 120-749, Korea 1 Department of Materials Science and Engineering, Pohang University of Science and Engineering, Pohang, Korea 2 Department of Semiconductor Science and Technology, Semiconductor Physics Center, Chonbuk National University, Chonju 561-756, Korea ABSTRACT We have investigated surface treatment effect on the interfacial reaction of Pd/p-GaN interface and also room temperature ohmic contact formation mechanism of Pd-based ohmic contact. In order to examine room temperature ohmic behavior, various metal contact systems were deposited and current-voltage measurements were carried out. In spite of large theoretical Schottky barrier height between Pd and p-GaN, Pd-based contact showed perfect ohmic characteristic even before annealing. According to the results of synchrotron X-ray radiation, the closed-packed atomic planes (111) of the Pd film were quite well ordered in surface normal direction as well as in the in-plane direction. The effective Schottky barrier height of Au/Pd/Mg/Pd/p-GaN was 0.47eV, which was estimated by Norde method. This discrepancy between theoretical barrier height and the measured one might be due to the epitaxial growth of Pd contact metal and so the room-temperature ohmic characteristic of Pd-based ohmic contact was related strongly to the in-plane epitaxial quality of metal on p-GaN. INTRODUCTION In recent years, - nitrides such as GaN, InN etc, have drawn much interest as a promising material for the fabrication of efficient blue emitting devices, laser diode, and high temperature/high power electronic devices because they have direct wide band gap (3.4eV at R.T.) and a high saturation electron velocity (3×107 cm/s)[1-3]. As GaN device technology advances, more stringent demands will be made on the reproducibility, uniformity, thermal stability, and high temperature operation of the ohmic contacts to GaN-based devices[4]. For nGaN, Al/Ti-based ohmic contacts have been used widely as an n-type ohmic contact and they showed extremely low contact resistance[4,5]. On the other hand, no satisfying ohmic contacts to p-GaN have been developed because the maximum doping concentration of p-GaN was restricted under 1018cm-3 and there is no metal with a high work function comparable to the electron affinity of p-GaN. In order to obtain low resistance p-type ohmic contact, various metal contacts were applied to p-GaN ohmic contact, such as Au, Ni, Ti, Pd, Pt, Au/Ni, Au/Pt, Au/Cr, Au/Pd, Au/Mg/Au, Au/Pt/Pd, Au/Cr/Ni, Au/Pt/Ni, Au/Ni/Pt, Au-Zn/Ni, Si/Ni/Mg/Ni etc[6]. Among those reports, Au/Pd contact system showed very interesting results that Au/Pd contact system showed ohmic characteristic even before annealing in spite of large theoretical Schottky barrier height[7]. In this study, in order
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