*2.2. Survey of Literature Data on Schottky Contacts to n-Type 4H-SiC*

In literature, many studies on the metal/n-type 4H-SiC systems have been reported and focused on the choice of the metal and its evolution in the Schottky contact formation [27–33]. A collection of literature results related to some of the most diffused metal/n-type 4H-SiC contacts is reported in Table 1, including as deposited (unannealed) Schottky contacts or contacts subjected to thermal annealing treatments. The reported barrier height values were determined by I–V measurements on Schottky diodes.

**Figure 2.** Schematic energy band diagrams for the metal/4H-SiC contact under forward bias *VF*, according to the predominance of the (**a**) thermionic emission (TE) or (**b**) thermionic field emission (*TFE*) current transport mechanism.

**Table 1.** Schottky barrier height for metal/n-type 4H-SiC system for different metals. The values were determined by I–V measurements on Schottky diodes.


As can be seen, a large variety of barrier height values is found, depending on the metals and post-metallization thermal treatments. Especially, by reporting the barrier height values *φ<sup>B</sup>* versus the metal work function *WM*, it is possible to determine the correlation between *φ<sup>B</sup>* and *WM*, which represents the so-called "interface index" S = d*φB*/d*φ*<sup>m</sup> [45]. Typically, for real 4H-SiC-based Schottky contacts, a linear correlation is found, with S values between the Bardeen limit (i.e., S = 0, indicating interface properties independent of the metal) and the ideal Schottky–Mott behavior (S = 1) [4,46], suggesting the occurrence of a partial Fermi level pinning at the interface. Figure 3 displays this kind of plot for the unannealed and low-temperature annealed metal/4H-SiC contacts, with the values taken from Table 1. The slope values confirm an intermediate behavior, with a slight improvement towards the Schottky–Mott behavior from 0.46 to 0.54 for the low-temperature annealed contacts.

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*Φ* **Figure 3.** Experimental dependence of the barrier height *φ<sup>B</sup>* on the metal work function *W<sup>M</sup>* in unannealed and low-temperature annealed metal/n-type 4H-SiC systems. All the reported barrier values were determined by I–V characterization of Schottky diodes. Data are taken from Refs. [21–44].

*Φ* Throughout the years, titanium (Ti) and nickel silicide (Ni2Si) have come out as widely diffused barrier metals for 4H-SiC Schottky diodes in different applications. Ti- and Ni2Sibased metallization schemes are currently well-established technology, offering a high level of reproducibility for the Schottky barrier height values. The representative forward I–V characteristics of the 4H-SiC Schottky diodes, employing Ti and Ni2Si barrier metals and acquired at three different temperatures (173, 298 and 373 K), are reported in Figure 4a,b, respectively [23,47]. All the curves were analyzed according to the TE model, by fitting the linear region in a semilog plot of the forward I–V curve according to Equation (2) approximated for the linear region. From this analysis, the extrapolation of an ideality factor *n* very close to 1 for both contacts confirms the predominance of the TE mechanism in the current transport through the metal/semiconductor interface. Specifically, the values of the Schottky barrier height typically obtained at room temperature were *φ<sup>B</sup>* = 1.27 eV for the Ti/4H-SiC and 1.60 eV for the Ni2Si/4H-SiC contacts [23,47]. Consequently, Ti is used as the Schottky barrier material in power electronics applications, for which a low barrier height is desired [48], whereas Ni2Si is preferred for sensing or detection applications [49,50], where the higher barrier permits a low leakage current to be obtained. Moreover, the possibility of a "self-aligned" process given by the Ni2Si formation is used for the fabrication of semi-transparent interdigit electrodes in UV-detectors [51].

**Figure 4.** Semilog plot of the temperature-dependent forward-voltage characteristics of 4H-SiC Schottky diodes based on (**a**) Ti and (**b**) Ni2Si. Adapted with permission from Ref. [23]. Copyright 2021 AIP Publishing.
