Temperature-driven crystalline ordering and Ohmic contact formation of PdAu layers onp-type GaN

Abstract

We have investigated the effect of thermal annealing on the electrical characteristics of Pd/Au contact layer to p-GaN and on its crystalline ordering. While an as-deposited Pd/Au layer on p-GaN showed Schottkycontact characteristics, a thermally annealed Pd/Au layer yielded Ohmic characteristics, accompanying singlecrystalline ordering with an abrupt interface with GaN. The Ohmic contact characteristics and crystalline ordering of PdAu layer were attributed to the substantial elimination of oxidation layers at the interface during thermal annealing.Recently, the thin films of transition metals have drawn much attention due to increasing demands on various engineering applications such as heat-and corrosion-resistant coatings or metallization layers of microelectronic devices. 1,2 In particular, their roles in III-nitrides research have become undeniably significant in relation to the fact that a large work function of transition metals can improve Ohmic-contact characteristics to wide band-gap p-GaN.III-nitrides and related ternary alloys have been extensively investigated owing to their wide applicability to highpower electronic devices and short-wavelength optical devices. 3-5 Currently, one of the main concerns in this area pertains to accomplishing a reliable Ohmic contact to p-GaN with a low resistance and thermal stability. So far, many experiments on the electrical characteristics of metal contacts to p-GaN have been carried out, mainly focusing on the methodology of surface treatments. Various surface-cleaning treatments are known to effectively reduce a barrier height between a contact layer and p-GaN. 6,7 In this work, we have investigated the contact characteristics of the Pd/Au system to p-GaN, focusing on the effect of thermal annealing on the elimination of supposedly buried oxide layers at the interface. The removal of oxidation layers had a significant influence on the formation of an abrupt interface, accompanying a subsequent ordering of an alloyed Pd/Au layer and yielding an Ohmic-contact formation.Two-micrometers-thick p-GaN was grown by metalorganic chemical-vapor deposition on ͑0001͒ sapphire substrate. The hole concentration and resistivity were 3.0 ϫ10 17 cm Ϫ3 and 1.67 ⍀ cm, respectively. Prior to metal deposition, surface cleaning with acetone and methanol followed by etching in HF was performed to remove contaminants. Upon surface cleaning and rinsing, samples were placed in an e-beam evaporation chamber. The vacuum level and the deposition rate were maintained constant during operation at 7ϫ10 Ϫ7 Torr and 1 Å/sec, respectively. Two samples were prepared by depositing Pd and Au in sequence and the nominal thickness for each layer was 50 Å. One of the samples was thermally annealed at 550°C for 20 min in nitrogen ambience.Current-voltage characteristics of our samples are plotted in Fig. 1. For the as-deposited and the annealed sample, Schottky-and Ohmic-contact characteristics were observed, respectively. While some other groups reported that a nonalloyed Ohmic contact could be obtaine...

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