Thermal Diffusion of Dopants in Silicon Carbide

Abstract

Despite considerable advancement in SiC material and device technology, there are still vital issues to be resolved such as metallization and doping. In SiC, ohmic contacts require high concentration doping. Commonly used doping methods in SiC device processing include in-situ doping during epitaxial growth, thermal diffusion, and ion implantation. These methods require specialized equipment and elaborate processing protocols involving high temperature annealing in excess of 1900 K. High temperature processes are detrimental to the material and device performance. Therefore, a low temperature process is needed. The successful development of a low temperature technique called vacancy assisted impurity doping (VAID) is presented. The objective of VAID is to create vacancies, or vacancy injection, at the surface to allow impurities to diffuse more easily into the surface region. The implementation of this technique using phosphorus for n-type doping of SiC will be described. Phosphorus is an important n-type dopant for both silicon and silicon carbide. While solid-state diffusion of phosphorus in silicon is an experimentally proven method, solid-state diffusion of phosphorus in silicon carbide is relatively unproven, especially at lower temperatures. When subjected to favorable thermodynamic conditions, the presence of metal oxides in contact with SiC can lead to the introduction of impurities in to SiC at optimal temperature annealing. A detailed thermodynamic study confirmed the possibility of phosphorus and boron diffusion in silicon carbide using phosphorus oxide and boron oxide as sources of phosphorus and boron dopants. Another variation of the VAID technique is to use the formation of silicides to assist in the vacancy creation process. Ternary phase diagrams of stability have been used to study nickel as a potential catalyst for the process. In this study, a silcidation assisted doping process has been described using nickel. Experimental results show successful phosphorus impurity incorporation and activation in 4H-SiC at temperatures below 1400oC. Phosphorus was incorporated in SiC at temperatures as low as 900oC using the silicidation assisted VAID technique, which is significant progress in thermal diffusion in SiC. SIMS results are presented, showing dopant concentrations in the range of 1019 cm-3 while EDX analyses have also confirmed the presence of phosphorus. Specific contact resistivity values in the range of 10-5 ~ 10-6 Ωcm2 resulted from Ni93%V7% contacts to phosphorus doped 4H-SiC. A different variation of the VAID process that uses silicidation of metal without oxidation was demonstrated. Electroless nickel plating was investigated as an ohmic contact to n-type 4H-SiC. Electroless nickel film contains 5-14% phosphorus by weight. Due to its high concentration of phosphorus atoms, electroless nickel can be a useful and convenient source of phosphorus dopant in the fabrication of n-type ohmic contact for SiC. Specific contact resistivity on lightly-doped samples with carrier concentration of 2.5  1016 cm-3 is 5.3  10-6 Ωcm2 without any need for ion implantation. This metallization technique is especially useful in broad area ohmic contact formation on the highly doped layer at the back of an n-type SiC substrate.