Ohmic Contacts to n -Type 6H-SiC Without Post-Annealing
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ABSTRACT We formed titanium Ohmic contacts to n-type 6H-SiC epitaxial layer by reducing the Schottky barrier heights. The barrier heights were reduced enough to form the Ohmic contacts by releasing the Fermi level from pinning through making atomically-flat surfaces. The current transport by thermionic emission was dominant at the Ti/SiC interface. Since the Ti contacts were formed without postannealing, surfaces of the Ti electrodes were flat and homogeneous maintaining asdeposited structures. Contact resistivity was (6±1)X10-3 Q-cm 2, which is comparable to that of the annealed Ni contact formed on the SiC epitaxial layer with the same carrier concentration.
INTRODUCTION An Ohmic contact is usually formed by heavy impurity doping to a semiconductor by which a tunneling current is increased due to a thinning of a depletion layer. However, it is difficult to apply this conventional method to SiC because high impurity doping to SiC is difficult. Edmond et ai.' reported that high temperature over 1800 °C is required for making of a heavily doped layer with carrier concentration of over 10"8 cmr3 by thermal diffusion. Another impurity doping method by ion implantation also has a problem of a low electrical dopant activity of about 5 % as estimated by Ruff et al.2 Ni electrodes annealed at high temperatures are widely used as Ohmic electrodes to n-type 6H-SiC crystal instead of the heavily doped electrodes. Even in the Ni electrodes, there are some significant problems. The high-temperature annealing typically at around 1000 'C restricts device fabrication processes and makes morphology of electrodes rougher. Therefore, fabrication of Ohmic contacts without heavy impurity doping and post-annealing at high temperatures is one of the key techniques to develop electronic devices using SiC. In this paper, we propose a new technique for Ohmic contact formation without heavy impurity doping and post-annealing. We also demonstrate an Ohmic contact formation to n-type 6H-SiC epitaxial layer using this new technique.
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Mat. Res. Soc. Symp. Proc. Vol. 423 0 1996 Materials Research Society
NEW TECHNIQUE FOR OHMIC FORMATION According to the Schottky model 3, an Ohmic contact to an n-type semiconductor is in principle formed when the work function of a metal 0 is smaller than the electron affinity of the semiconductor X,, where the Schottky barrier height 0, is negative. However, this methodis of hardly any use4 because high 0 is present at a practical interface even in the case of a low 0y,. It is known to be Fermi level pinning that the dependence of b, on 0,, is weak. Our strategy for Ohmic formation is (1) to release the Fermi level from pinning and (2) to use a metal with a low (high) work function to n-type (p-type) semiconductors. The Ohmic contacts are formed without heavy impurity doping and post-annealing. In the metal/semiconductor system with the Fermi level pinning, p,, is expressed as, tb= so+c
(1)
where C is a constant. The Fermi level is perfectly released from pinning when S' is 1, whereas it is perfectly
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