2.5.1 Ohmic Contacts

Ohmic contacts serve the purpose of carrying electrical current into and out of the semiconductor, ideally with no parasitic resistance. Low resistivity ohmic contacts are essential for high-frequency operation. Additionally, high-temperature and high-power require that the contacts must be reliable under extreme conditions. Once seen as one of the primary impediments to SiC technological development, the ohmic contact area has now matured rapidly. The properties of various ohmic contacts to SiC reported are summarized in [80,81]. While SiC specific ohmic contact resistances at room temperature are generally higher than in contacts to narrow-bandgap semiconductors, they are nevertheless sufficiently low for most envisioned SiC applications.

Lower specific contact resistances are usually obtained to n-type 4H- and 6H-SiC ($\sim$10$^{-4}$ to 10$^{-6}$ $\Omega$ cm$^2$ ) than to p-type 4H- and 6H-SiC ($\sim$10$^{-3}$ to 10$^{-5}$ $\Omega$ cm$^2$ ) [80,81]. Consistent with narrow-bandgap ohmic contact technology, it is easier to make low-resistance ohmic contacts to heavily-doped SiC. While it is possible to achieve ohmic contacts to lighter-doped SiC using high-temperature annealing, the lowest-resistance ohmic contacts are most easily implemented on SiC which is degenerately doped by site competition (Subsection 2.3.3.3) or high-dose ion implantation (Subsection 2.4.2). If the SiC doping is sufficiently degenerate, many metals deposited on a relatively clean SiC surface form ohmic contacts in the as-deposited state [82]. Regardless of doping, it is common practice in SiC to thermally anneal contacts to obtain the minimum possible ohmic contact resistance. Most SiC ohmic contact anneals are performed at temperatures around 1000$~^{\circ}$C in non-oxidizing environments. Depending upon the contact metalization employed, this anneal generally causes limited interfacial reactions (usually metal-carbide or metal-silicide formation) that broaden and roughen the metal-semiconductor interface, resulting in enhanced conductivity through the contact.

Truly enabling harsh-environment SiC electronics will require ohmic contacts that can reliably withstand prolonged harsh-environment operation. Most reported SiC ohmic metalizations appear sufficient for long-term device operation up to 300$~^{\circ}$C [83]. SiC ohmic contacts that withstand heat soaking under no electrical bias at 500 to 600$~^{\circ}$C for hundreds or thousands of hours in non-oxidizing gas or vacuum environments have also been demonstrated. In air, however, there has only been demonstration to date of a contact that can withstand heat soaking (no electrical bias) for 60 hours at 650$~^{\circ}$C [84]. Some very beneficial aerospace systems will require simultaneous high temperature ($T>300$$~^{\circ}$C) and high current density operation in oxidizing air environments. Electromigration, oxidation, and other electrochemical reactions driven by high temperature electrical bias in a reactive oxidizing environment are likely to limit SiC ohmic contact reliability for the most demanding applications. Thus, the durability and reliability of SiC ohmic contacts is one of the critical factors limiting the practical high-temperature use of SiC devices.

T. Ayalew: SiC Semiconductor Devices Technology, Modeling, and Simulation