Ti/AlNi/W Ohmic Contacts to P-Type SiC

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0911-B11-03

Ti/AlNi/W Ohmic Contacts to P-Type SiC Bang-Hung Tsao1, Jacob Lawson1, and James Scofield2 1 Metals and Ceramics, University of Dayton Research Institute, 300 College Park, Dayton, OH, 45469-0179 2 AFRL/PRPE, WPAFB, OH, 45433

ABSTRACT Improved AlNi-based ohmic contacts to p-type 4H-SiC have been achieved using low energy ion (Al+) implantation, the addition of a thin Ti layer, and a novel two-step implant activation anneal process. Resistivities sometimes as low as 5x10-5 Ω-cm2 were reached by doping the surface region of lightly p-doped 4H-SiC epilayers via low energy Al+ implantation. Acceptor activation was achieved by annealing the samples with a 1400+1700oC two-step sequence in an Ar atmosphere, which also yielded improved surface morphology when implanted samples were capped with photo resist during the anneals. In this study, Ti/AlNi/W contacts on implanted layers were compared to Ti/AlNi/Au contacts. Even though the resistivities are higher than those of the Ti/AlNi/W system, the reduced anneal temperature, 650°C for Ti/AlNi/Au compared to 950°C for Ti/AlNi/W implies that Ti/AlNi/Au is a promising stacking configuration. Furthermore, the effects of a longer 30 minute anneal time at 600 – 700oC, in atmospheric pressure Ar ambients was observed. Namely, the 2 minute annealing cycle used for the Ti/AlNi/W study resulted in higher anneal temperatures before ohmic characteristics were seen. This same anneal time was not sufficient for the Ti/AlNi/Au samples, whereas increasing the cycle time to 30 minutes resulted in ohmic behavior at a much lower temperature. Increasing the anneal time however, had little or no impact on reducing the required anneal temperature of the Ti/AlNi/W. INTRODUCTION SiC is an excellent candidate for modern power electronics due to its superior breakdown voltage, thermal conductivity, and saturated electron velocity parameters, in addition to its inherent resistance to radiation and chemical attack [1]. A persistent problem plaguing SiC device development has been the realization of low-resistivity, thermally stable ohmic contacts to p-SiC. In previous experiments, we have successfully demonstrated improved thermal stability of the basic Al-contact metallurgy by using a binary AlNi compound on p-SiC [2]. Prior results had shown significant potential for the AlNi system but inconsistency and data scatter had created difficulties in practice when employing these ohmics for device work. Contact resistivities in the range of 10-4 Ω-cm2 were routinely achieved, but data was typically scattered over a wide range. We began our investigation by comparing our base AlNi metallization scheme to structures with Ti as an adhesion enhancing and resistivity reducing layer [3]. Tungsten was initially selected as a protective cap layer for diffusion reduction and thermal stability. Gold was later chosen as the capping material to both prevent the oxidation of AlNi and to promote wire bonding to the contact. This paper reports the results of our efforts aimed at improving the repeatability a