2.5.2 Schottky Contacts

Rectifying metal-semiconductor Schottky barrier contacts to SiC are useful for a number of devices including metal-semiconductor field-effect transistors (MESFETs) and fast-switching rectifiers. References [85,84,79,86] summarize electrical results obtained in a variety of SiC Schottky studies to date. Due to the wide bandgap of SiC, almost all unannealed metal contacts to lightly doped 4H- and 6H-SiC are rectifying. Rectifying contacts permit extraction of Schottky barrier heights and diode ideality factors by known current-voltage (IV) and capacitance-voltage (CV) electrical measurement techniques [77]. While these measurements show a general trend that the Schottky junction barrier height does somewhat depend on the metal-semiconductor workfunction difference, the dependence is weak enough to suggest that surface state charge also plays a significant role in determining the effective barrier height of SiC Schottky junctions. At least some experimental scatter exhibited for identical metals can be attributed to cleaning and metal deposition process differences, as well as different barrier height measurement procedures. The work by Shalish et al. [87], in which two different surface cleaning procedures prior to titanium deposition lead to ohmic behavior in one case and rectifying behavior in the other, clearly shows the important role that the process recipe can play in determining SiC Schottky contact electrical properties.

It is worth noting that barrier heights calculated from CV data are often somewhat higher than barrier heights extracted from IV data taken from the same diode. Furthermore, the reverse current drawn in experimental SiC diodes, while small, is nevertheless larger than expected based on theoretical substitution of SiC parameters into well-known Schottky diode reverse leakage current equations developed for narrow-bandgap semiconductors. Bhatnagar et al. [88] proposed a model to explain these behaviors in which localized surface defects, perhaps elementary screw dislocations where they intersect the SiC-metal interface, cause locally reduced junction barriers in the immediate vicinity of the defects. Because current is exponentially dependent on the Schottky barrier height, this results in the majority of measured current flowing at local defect sites instead of evenly distributed over the entire Schottky diode area. In addition to local defects, electric field crowding along the edge of the SiC Schottky barrier can also lead to increased reverse-bias leakage current and reduced reverse breakdown voltage [37,40,77]. Schottky diode edge termination techniques to relieve electric field edge crowding and improve Schottky rectifier reverse properties are discussed in Section 4.4.1. Quantum mechanical tunneling of carriers through the barrier may also account for some excess reverse leakage current in SiC Schottky diodes [89].

The high temperature operation of rectifying SiC Schottky diodes is primarily limited by reverse-bias thermionic leakage of carriers over the junction barrier. Depending on the specific application and the barrier height of the particular device, SiC Schottky diode reverse leakage currents generally grow to excessive levels at around $300-400$$~^{\circ}$C. As with ohmic contacts, electrochemical interfacial reactions must also be considered for long-term Schottky diode operation at high temperatures.

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