Processing and Characterization of Silicon Carbide ( 6H- and 4H-SiC ) Contacts for High Power and High Temperature Device Applications

  • Lee S
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Abstract

Silicon carbide is a promising wide bandgap semiconductor material for hightemperature, high-power, and high-frequency device applications. However, there are still a number of factors that are limiting the device performance. Among them, one of the most important and critical factors is the formation of low resistivity Ohmic contacts and high-temperature stable Schottky diodes on silicon carbide. In this thesis, different metals (TiW, Ti, TiC, Al, and Ni) and different deposition techniques (sputtering and evaporation) were suggested and investigated for this purpose. Both electrical and material characterizations were performed using various techniques, such as I-V, C-V, RBS, XRD, XPS, LEED, SEM, AFM, and SIMS. For the Schottky contacts to n- and p-type 4H-SiC, sputtered TiW Schottky contacts had excellent rectifying behavior after annealing at 500 oC in vacuum with a thermally stable ideality factor of 1.06 and 1.08 for n- and p-type, respectively. It was also observed that the SBH for p-type SiC (fBp) strongly depends on the choice the metal with a linear relationship fBp = 4.51 – 0.58fm, indicating no strong Fermi-level pinning. Finally, the behavior of Schottky diodes was investigated by incorporation of sizeselected Au nano-particles in Ti Schottky contacts on silicon carbide. The reduction of the SBH is explained by using a simple dipole layer approach, with enhanced electric field at the interface due to the small size of the circular patch (Au nano-particles) and large difference of the barrier height between two metals (Ti and Au) on both n- and p- SiC. For the Ohmic contacts, titanium carbide (TiC) was used as contacts to both n- and ptype 4H-SiC epilayers as well as on Al implanted layers. The TiC contacts were epitaxially deposited using a co-evaporation method with an e-beam Ti source and a Knudsen cell for C60, in a UHV system at low substrate temperature (500 oC). In addition, we extensively investigated sputtered TiW (weight ratio 30:70) as well as evaporated Ni Ohmic contacts on both n- and p-type epilayers of SiC. The best Ohmic contacts to n-type SiC are annealed Ni (> 950oC) with the specific contact resistance of » 8´10-6 Wcm2 with doping concentration of 1.1 ´10-19 cm-3 while annealed TiW and TiC contacts are the preferred contacts to p-type SiC. From long-term reliability tests at high temperature (500 oC or 600 oC) in vacuum and oxidizing (20% O2/N2) ambient, TiW contacts with a platinum capping layer (Pt/Ti/TiW) had stable specific contact resistances for > 300 hours.

Author-supplied keywords

  • Ohmic and Schottky contacts
  • Schottky barrier height lowering
  • and TLM structures
  • capacitance-voltage measurement
  • co-evaporation
  • current- voltage
  • nano-particles
  • power devices
  • silicon carbide

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Authors

  • Sang-kwon Lee

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