**1. Introduction**

Nowadays, the wide bandgap semiconductors SiC and GaN are considered as the basis of a huge advancement in power electronics, enabling the definition of a game-changing generation of devices with superior performance if compared with that currently achieved by traditional Si-based devices [1]. This is due to the outstanding physical properties of this class of materials, such as wide bandgap, high critical electrical field and high saturation velocity, that push forward the limits reached by Si-based power electronics [2]. In addition to a more efficient performance, the superior properties of wide-band gap semiconductors also translate into devices able to operate in high temperature, high power and high frequency regimes, with the possibility of extending the field of applications for power electronics [3].

Among the wide bandgap semiconductors, one of the hexagonal polytypes of silicon carbide, i.e., 4H-SiC, plays a pivotal role in power electronics, owing to the excellent quality achieved from the commercially available substrates and epilayers and the high level of compatibility with the existing Si-based device manufacturing facilities and technology. Quantitatively, 4H-SiC features a wide bandgap of 3.26 eV, high critical electric field >2 MV/cm, high thermal conductivity of 4.9 WK−<sup>1</sup> cm−<sup>1</sup> and saturated drift velocity higher than 2 <sup>×</sup> <sup>10</sup><sup>7</sup> cm s−<sup>1</sup> [4], making it the material of choice for a variety of power devices operating in the medium/high voltage range (600–3000 V) [3].

Along the lines of Si-based technology, several 4H-SiC power devices have been developed, with a mature technology level in terms of processing implementation and real-world applications [5]. Among them, n-type 4H-SiC-based Schottky barrier diodes (SBDs) are currently commercialized on a large scale in the power electronics market and

**Citation:** Vivona, M.; Giannazzo, F.; Roccaforte, F. Materials and Processes for Schottky Contacts on Silicon Carbide. *Materials* **2022**, *15*, 298. https://doi.org/10.3390/ ma15010298

Academic Editor: Alexander A. Lebedev

Received: 12 November 2021 Accepted: 22 December 2021 Published: 31 December 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

have penetrated our daily lives in various fields, e.g., consumer electronics, electric and hybrid vehicles, industrial processes, energy conversion from renewable sources, sensors, photodetectors and so on [6–8].

A schematic cross-section view of a typical SBD is depicted in Figure 1. In this device, the drift layer is given by a lightly doped n-type 4H-SiC epitaxial layer (doping density *N<sup>D</sup>* ~ 10<sup>16</sup> cm−<sup>3</sup> ) grown onto a heavily doped n-type 4H-SiC substrate (*N<sup>D</sup>* > 5 <sup>×</sup> <sup>10</sup><sup>18</sup> cm−<sup>3</sup> ). The connection with external components is given by metal electrodes consisting of a front-side Schottky contact to the 4H-SiC epitaxial layer and a back-side Ohmic contact to the 4H-SiC substrate. In addition, the Figure schematically illustrates a p-type implanted edge termination needed to optimize the breakdown behavior. For the sake of completeness (not reported in the scheme), a few µm thick front-side metallization (typically Al based) acting as device bonding pad and a polymeric passivation finalize the top part of the device, while a thick metal layer, soldered to the back-side and consisting of a gold-plated metal frame wrapped in ceramic, operates as connection to the package. A schematic fabrication process flow, with the sequential steps typically adopted for the Schottky diode fabrication, can be found in [9]. − − μ

**Figure 1.** Schematic cross-section view of a 4H-SiC Schottky barrier diode (SBD).

The core of an SBD is the metal/semiconductor junction (the so-called Schottky contact) and the properties of this system must be carefully investigated to assess and optimize the electrical performance of the whole device [9].

Over the years, different approaches, ranging from the choice of materials for the Schottky barrier formation to semiconductor treatments or even considering the device layout, have been developed to improve and gain control on the Schottky contact properties. Moreover, unconventional methods were also explored as alternative solutions in improving and controlling the Schottky barrier systems.

In this paper, after a brief discussion on the fundamentals of the metal/4H-SiC Schottky barrier formation and its electrical characterization, we will give an overview on the current materials and processing solutions for the fabrication of Schottky contacts to 4H-SiC. Afterwards, besides the consolidated approaches, we will present a variety of the nonconventional methods proposed in literature to control the Schottky barrier properties for specific applications.
