Transport Mechanisms in Focused Ion Beam Assisted Ohmic Contacts to p-Type 6H-SiC
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Transport Mechanisms in Focused Ion Beam Assisted Ohmic Contacts to p-Type 6H-SiC Agis A. Iliadis Electrical and Computer Engineering Department University of Maryland, College Park, MD 20742
ABSTRACT The current transport mechanism in non-annealed Ohmic contact metallizations on p-type 6H-SiC formed by using focused ion beam (FIB) surface-modification and direct-write metal deposition is reported, and the properties of such focused ion beam assisted non-annealed contacts are discussed. The process uses a Ga focused ion beam to modify the surface of the semiconductor with different doses, and then introduces an organometallic compound in the Ga ion beam, to effect the direct-write deposition of a metal on the modified surface. Contact resistance measurements by the transmission line method produced values in the low 10-4 Ω cm2 range for surface-modified and direct-write Pt and W non-annealed contacts, and mid 10-5 Ω cm2 range for surface-modified and pulse laser deposited TiN contacts. The current transport mechanism of these contacts was examined and found to proceed mainly by tunneling through the metalmodified-semiconductor interface layer. I. INTRODUCTION SiC is one of the wide band-gap semiconductors suitable for applications in high temperature, high power electronics. Such applications have severe requirements for Ohmic contact electrodes, and the development of stable and reliable Ohmic metallizations is one of the main goals in improving SiC device performance. For a high quality Ohmic contact metallization the targets are to achieve as low a contact resistance value as possible, high stability under high current, high temperature, harsh environment operation, high uniformity over the area of the contact, and a reliable long-life operation. The development of such Ohmic contact is a challenging task, especially in the case of SiC p-type contacts. Aluminum has been the most common approach for Ohmic contact formation in p-type SiC, but low melting point (660°C), oxidation, pitting and deep spiking problems, make Al contacts inappropriate for high temperature applications. To lessen these problems, Al/Ti contacts were investigated and found to produce contact resistance values between 10-3 and mid 10-4 Ohms cm2 for moderately to highly doped (1.4x1018 cm-3) p-6H-SiC respectively, after high temperature annealing (1650°C) [1], while on very highly doped (2x1019 cm-3) p-6H-SiC, a value of 1.5x10-5 Ohms cm2 has been reported [2]. Promising approaches using sputter deposited metal borides [3] annealed at 1100 °C, with contact resistance values in the mid 10-5 Ohms cm2 range for a doping level of 1.3x1019 cm-3, and CoSi2 with contact resistance values in the 10-6 Ohms cm2 range for a doping level of 2x1019 cm-3[4], were also reported. Co-implantation of C and Al has also produced promising results [5], while other systems [6] based on Ti and W with some Pt and Au combinations that are expected to have better stability under extended high temperature operation, showed higher contact resistance values, tha
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