**1. Introduction**

We live in an energy-hungry world in which industrialization and globalization have accelerated the demand for resources that now doubles approximately every 40 years. Today, we consume about 18 TW (18×10<sup>12</sup> Watts) which is equivalent to 97 billion barrels of crude oil yearly. While renewable energy sources offer an environmentally conscious alternative to fossil fuels, they account for only about 10% of this total [64]. In parallel to the advent of clean energy, an effort has to be made to curb consumption, which can in part be achieved by improving system efficiency. In this Section, we will discuss in such terms why high-volume sectors such as transportation, electricity generation, and distribution, can benefit from SiC-based electronics.

First, it should be recognized that the adoption of a new technology will be driven mainly by component cost and end-user benefits. Silicon carbide electronics is no exception and only makes sense if it can deliver on these fronts. A good example is the recent introduction of the pricier fluorescent light sources which make financial sense in the long term since they consume a fraction of the energy of incandescent bulbs and last some 20 times longer, proving that efficiency and reliability can justify the investment. So what are the key parameters that influence SiC device cost and efficiency?

*Cost* - Substrate size and availability have benefited from the boom in LED demand as III-nitride blue diodes can be fabricated on SiC. Indeed, the diameter of commercially available substrates has steadily increased from the release of two inch (50 mm) wafers in September 1997 to the recent unveiling of six inch (150 mm) wafers in August 2012 by Cree, inc., a very fast pace compared to Si evolution [100]. Also, tremendous quality improvements have been achieved together with increased process rate and uniformity. One of the many challenges facing SiC production has been the reduction of extended defects such as micropipes [29, 49]. Today, substrates are virtually free of such defects, optimizing device

Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Rozen; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Rozen, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

©2012 Rozen, licensee InTech. This is an open access chapter distributed under the terms of the Creative

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Tailoring Oxide/Silicon Carbide Interfaces: NO Annealing and Beyond

**2. Breakdown field and device efficiency**

larger than the bulk value, as illustrated in Fig. 3(a).

**(a) (b)**

DMOSFET.

Let us compare vertical double-implanted MOSFETs (DMOSFETs) designed to control the same bias, one Si-based, the other SiC-based, as shown in Fig. 2(b), using the constants of Fig. 2(a). The key differences between the two materials can be traced back to the Si-Si bond and the Si-C bond, respectively. The stronger interaction between silicon and carbon atoms is evidenced by the shorter bond length of 1.89 Å when compared to 2.35 Å for Si-Si. The proximity of atoms in SiC yield a more pronounced splitting of bonding and antibonding levels, which translates into a wider band gap in the periodic crystalline structures. The diatomic base of silicon carbide also explains the better thermal conductivity of the material because its vibration modes, i.e. phonons, are more energetic on average, as reflected in the Debye temperature. Ultimately, it is the phonon distribution that explains the higher critical field of silicon carbide, *ξc*, that can be used to derive a key parameter impacting device efficiency in high power electronics, the drift component of the specific ON resistance.

The breakdown field of a material is indeed not directly related to its band gap *Eg*. While, to first order, the free carriers need to reach a kinetic energy of at least 3/2 *Eg* to induce the cascading impact ionization phenomenon, called avalanche, that multiplies the number of carriers and therefore the conductivity, the limiting factor in the bulk is phonon coupling [84, 86]. If the net velocity of carriers *v*¯, proportional to the current, is smaller than or equal to the thermal velocity *vth* <sup>=</sup> <sup>3</sup>*kbT*/*m*∗, the electron-phonon system is in equilibrium because of the ability of phonons to thermalize the carriers. In that regime, phonon scattering damps the energy gain of free carriers whose distribution in the bands can be visualized as a Fermi sphere slightly shifted in the direction of the electric field. However, if the field increases and reaches *ξc*, the rate at which carriers gain energy becomes too high to allow equilibrium with the lattice vibrations. Hot carriers then achieve phonon runaway. Their motion is no longer damped and they can accelerate freely from *vth* to the critical speed *vc* <sup>≈</sup> <sup>3</sup>*Eg*/*m*<sup>∗</sup> allowing the avalanche process to start. It is worth noting here that in thin films, an additional constraint comes from the time the carriers take to accelerate to *vc*, so that *ξ<sup>c</sup>* can become

**Figure 2.** (a) Properties of 4H-SiC and Si with 1015-10<sup>16</sup> cm−<sup>2</sup> doping at 300 K [30, 65, 66, 84, 121]. (b) Vertical power

**Figure 1.** DC efficiency of SiC-based FETs relative to Si devices at given designed blocking voltages. While commercially available switches using NO-annealed thermal gate oxides have improved efficiency, one suggested route is the use of deposited oxides to achieve optimum properties [109].

yield. Demand and production costs have thus progressively driven down the price of the material, which has translated into cheaper and higher quality components for optoelectronic and high power applications.

*Efficiency* - While investment costs have diminished, SiC-based devices are still more expensive than their Si counterparts. Their efficiency is what can make them attractive in the long run. As illustrated in Fig. 1, the energy consumption of metal-oxide field-effect transistors (MOSFETs) can be orders of magnitude lower when using silicon carbide as a substrate to control high blocking voltages. Industries that would benefit from the widespread of such components include transportation, electricity distribution, grid coupling, high-performance computing, etc. Indeed, automakers have invested heavily in SiC research, targeting the implementation of SiC-based inverters in hybrid vehicles. To get an idea of how single device consumption will translate into system efficiency, let's take the example of photovoltaic (PV) power converters. PV inverters are used to convert the DC current from solar sources to feed it to the AC grid. They are made of power diodes and switches. A typical residential system has a nominal power of 5 kW at 400 V AC. Such Si-based converters can operate at over 95% efficiency but replacing Si components by commercially available SiC Schottky diodes and power MOSFETs can cut the loss by about 50%, yielding a saving of the order of \$100 a year per household [22, 23]. Moreover, they can operate at higher temperature, so that limited cooling and volume requirements go in favor of system prices which can indeed prove beneficial over the years for the consumer choosing to adopt the new technology.

Further improvements in SiC device efficiency will make the case even stronger. Among the key building blocks at the device level is the oxide/semiconductor interface. Figure 1 highlights how it affects consumption, especially at low biases. In this Chapter, we will derive important parameters defining SiC devices from physical properties, and discuss the role and formation of the oxide/semiconductor interface, covering thermal oxides, post-oxidation annealing, and deposited dielectrics.
