Stable Low Resistance Ohmic Contacts To p-GaN
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Stable Low Resistance Ohmic Contacts To p-GaN Mi-Ran Park1 and Wayne A. Anderson2 1 Electronics and Telecommunications Research Institute, 161 Kajong-Dong, Yusong-Gu, Taejon 305-600, Korea 2 State University of New York at Buffalo, Department of Electrical Engineering , Buffalo, NY, U.S.A ABSTRACT Stable and low-resistance Ohmic contacts are especially important for laser diodes where high current levels are required. Good contacts are especially difficult on p-type GaN which was the motivation for this study. The GaN was epitaxially grown on (0001) sapphire substrates by MOCVD. Resistivity of this layer was 3.5 Ohm-cm and thickness was 2 microns. After conventional cleaning followed by treatment in boiling HNO3 : HCl (1:3), metallization was by thermally evaporating 40 nm Au / 60 nm Ni or 70 nm Au / 55 nm Pd. Heat treatment in O2 + N2 at various temperatures followed, with best results at 600 °C or 700 °C, respectively. Best values of the contact resistance were 1.8x10-4 Ohm-cm2 for Pd/Au and 2.65x10-4 Ohm-cm2 for Ni/Au contacts. After repetitive cycling from room temperature to 600 °C, the Ni contacts were very stable and more stable than the Pd contacts. X-ray photoelectron spectroscopy depth profiling showed the Ni contacts to be NiO followed by Au at the interface for the Ni/Au contacts whereas the Pd/Au contacts exhibited a Pd : Au solid solution. Some contacts were quenched in liquid nitrogen following sintering. These contacts were much more uniform under atomic force microscopy examination and gave a 3 times lower contact resistance with the Ni/Au design. Current-voltage-temperature analysis revealed that conduction was predominantly by thermionic field emission. INTRODUCTION Great interest exists in the III-nitride semiconductors since the successful development in growth of GaN based materials and operation of electronic and optoelectronic devices such as blue and green light emitting diodes (LEDs) and laser diodes (LDs) [1,2]. The formation of stable and reliable low resistance ohmic contacts to p-type GaN has been a problem in achieving good performance of those devices. For devices with large contact areas such as LEDs and LDs, the specific contact resistance (ρc) between 10-4 to 10-6 Ωcm2 is considered acceptable and for devices with smaller contact areas, values of ρc between 10-5 to 10-7 Ωcm2 are necessary [3]. Bilayer metal schemes such as Ni/Au and Pd/Au were studied by many groups [4,5]. These have been studied due to the stable electrical and thermal properties and the high work function which is one criteria to form low resistance Ohmic contacts to p-type materials. The effects of cryogenic cooling after heat treatment on the formation of Ni/Au and Pd/Au contacts are presented in this paper. We also compare these effects on forming Ni/Au and Pd/Au contacts annealed in a combined O2/N2 gas ambient. High temperature annealing may degrade homogeneity, possibly caused by spiking of metals between themselves or between metal and semiconductor due to the differences in thermodynamic properties of mat
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