Chemical and Electrical Mechanisms in Titanium, Platinum, and Hafnium Contacts to Alpha (6H) Silicon Carbide
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CHEMICAL AND ELECTRICAL MECHANISMS IN TITANIUM, PLATINUM, AND HAFNIUM CONTACTS TO ALPHA (6H) SILICON CARBIDE L.M. Porter*, R.C. Glass**, R.F. Davis*, J.S. Bow***, M.J. Kim***, and R.W. Carpenter*** * Department of Materials Science and Engineering, North Carolina State University,
Raleigh, NC 27695-7907. Department of Physics and Measurement Technology, Linkoping University, S-581 83 Linkoping, Sweden. *** Center for Solid State Science, Arizona State University, Tempe, AZ 85287-1704. **
ABSTRACT Thin films (2 A - 1000 A) of titanium, platinum, and hafnium were deposited via UHV electron beam evaporation at room temperature on n-type, (0001) alpha (6H)-SiC and compared in terms of interfacial chemistry, energy barriers to electrical conduction, and macroscopic electrical behavior. Current-voltage measurements have shown that these contacts are rectifying, 2 all with ideality factors between 1.01 and 1.09. The lowest leakage currents (- 5 x 10-8 A/cm at - 10 V) were determined for unannealed Pt contacts and for Hf contacts annealed at 700°C for 20 minutes. Current-voltage (I-V), capacitance-voltage (C-V), and x-ray photoelectron spectroscopy (XPS) were among the techniques used to determine barrier heights, all of which were within a few tenths of an electron volt of 1.0 eV. The narrow range of calculated barrier heights along with the XPS valence spectrum of the chemically prepared SiC surface give evidence that the Fermi level is pinned at the semiconductor surface. INTRODUCTION In metal-semiconductor contacts a critical quantity which describes the relationship between the two materials is the Schottky barrier height (SBH), (I)B. In general it is also this property which best indicates the electrical characteristics of the contact. For an ohmic contact, in which the current is both linear and symmetric for positive and negative voltages, one would expect a small or negative SBH. On the other hand, for a good rectifying contact, in which current flows only under forward bias, one would expect a relatively large SBH. Because the SBH is such an important parameter in terms of defining the ohmic or rectifying characteristics of a particular contact, it is very important to understand how it is determined. In the ideal case the SBH is defined by the Schottky-Mott limit, or (for an n-type semiconductor) the difference between the metal workfunction and the electron affinity of the semiconductor. However, there are many factors which can cause non-ideal relationships, and hence deviations from this rule. J. Pelletier et al. [1] have reported Fermi level pinning in 6H-SiC attributed to intrinsic surface states, suggesting little dependence of barrier height on the workfunction of the metal. In addition, L. J. Brillson [2,3] predicts the pinning rate to be higher for more covalently bonded materials. Alpha (6H)-SiC is a particular polytype of this wide bandgap semiconductor. A variety of high-power, -temperature, -frequency, and radiation hard electronic and opto-electronic devices have been produced in 6H-SiC films for
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