**1. Introduction**

Silicon carbide is a wide-bandgap semiconductor that shows high mechanical strength, chemical inertness and thermal conductivity. In particular, 3C-SiC is competitive among the SiC polytypes as it is characterized by high mobility and lower density of states at the 3C-SiC/SiO<sup>2</sup> interface with respect to 4H and 6H-SiC. These properties, arising from the higher symmetry related to lower phonon scattering and lower bandgap (2.5 eV), make 3C-SiC ideal for applications in the field of power electronics [1,2] as they lead to many advantages in metal oxide semiconductor (MOS) devices as well as microelectromechanical systems (MEMS) in harsh environments [3]. Indeed, higher channel mobility and high carrier mobility imply low on-state resistance (RON) for medium-voltage applications working under 1200 V, consequently lowering power dissipation in forward bias [1]. Compared to 4H and 6H hexagonal SiC, 3C-SiC films have the advantage that they can be heteroepitaxially grown through low-temperature CVD as 3C-SiC is the most thermodynamically stable polytype. Growing a high-quality 3C-SiC epilayer on a large-area substrate would be a significant technical and scientific advancement. As a result of these features, silicon is regarded as the most intriguing substrate and crystal seed for thin epitaxial films and/or following bulk 3C-SiC growth [4]. Current technology involves the use of hetero-epitaxial growth on silicon, which inherently entails a 20% lattice parameter mismatch between Si and SiC and contributes to the generation of compressive intrinsic stress, whereas 8% at different degrees of thermal expansion provides a tensile contribution during the cooling

**Citation:** Calabretta, C.; Scuderi, V.; Anzalone, R.; Mauceri, M.; Crippa, D.; Cannizzaro, A.; Boninelli, S.; La Via, F. Effect of Nitrogen and Aluminum Doping on 3C-SiC Heteroepitaxial Layers Grown on 4◦ Off-Axis Si (100). *Materials* **2021**, *14*, 4400. https:// doi.org/10.3390/ma14164400

Academic Editor: Ettore Vittone

Received: 6 July 2021 Accepted: 2 August 2021 Published: 6 August 2021

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period from the growth temperature to room temperature (RT). Such mismatches give rise to misfit dislocations and stacking faults (SFs) growing along the (111) planes of the face-centered cubic (FCC) lattice. Such defectiveness hinders the realization of devices and constitutes considerable leakage sources not compatible with the development of very large-scale integration (VLSI) technology [5].

Both nitrogen (N2) and ammonia (NH3) have been successfully employed as dopant precursors for the n-doping of SiC at temperatures above 1000 ◦C [6–8]. However, nowadays, N<sup>2</sup> is the principal n-type dopant in all SiC polytypes [9]. Highly p-doped 3C-SiC is interesting for emitters in p–n-junction-based devices, because such emitters will have low resistance and can produce a high concentration of injected holes in the base region [10]. Aluminum is a preferred acceptor in SiC due to its lower activation energy (0.24 eV) compared to other acceptors (0.7 eV for B and 0.33 eV for Ga) [11]. Zielinski et al. observed that nitrogen and aluminum atoms are incorporated, respectively, on carbon and silicon sites [12].

The dopant incorporation into the SiC lattice is known to affect the crystallinity, morphology and mechanical properties of the films [13,14]. Crystallinity is very important when applying doped 3C-SiC in electronic devices because the crystal defects directly influence their leakage current and breakdown voltage.

Although 3C-SiC material for semiconductor applications has been investigated for 30 years, the problem of defect formation at the 3C-SiC/Si contact is still far from being solved. Stacking faults (SFs), partial dislocations (PDs) and anti-phase boundaries (APB) or inverted domain boundaries (IDB) are the most important defects [15]. In particular, IDBs are the main defects responsible for the electrical failure of 3C-SiC/Si-based devices [16,17]. Instead, SFs can be considered highly conducting 2D defects, in the energy range where also the bulk material is conductive. Indeed, under forward polarization, SFs were demonstrated to operate as preferred current pathways, causing a decrease in turn-on voltage [17]. In 1987, Shibahara et al. [18] observed the beneficial effect of the growth of 3C-SiC on an off-axis Si substrate with an off-axis angle from 2◦ to 5◦ . In particular, off-axis Si substrates exhibit significant improvements in the anti-phase disorder compared to on-axis ones. Moreover, off-axis growth promotes SF surface propagation lying along the (111) plane, while (−1−11) SFs opposite to the growth step are forbidden.

In this work, we present a comprehensive study of the effects of n-type and p-type doping on hetero-epitaxial 3C growth on different 4◦ off-axis silicon substrates (100). Micro-Raman, photoluminescence (PL) and stacking fault evaluation through molten KOH etching were performed on different doped samples in order to estimate the influence of N and Al incorporation on the morphological and optical properties of the material. The concentrations investigated in this work, 10<sup>18</sup> at/cm<sup>3</sup> and 10<sup>19</sup> at/cm<sup>3</sup> for aluminum and nitrogen, respectively, are compatible with the concentrations necessary for the development of electronic devices on 3C-SiC. In particular, for vertical metal-oxide-semiconductor field-effect transistor (MOSFET) devices, the substrate should have a concentration of approximately 10<sup>19</sup> at/cm<sup>3</sup> to decrease as much as possible the Ron of the device. Instead, for insulated gate bipolar transistors (IGBTs), the p-collector has a doping of 10<sup>18</sup> at/cm<sup>3</sup> [19]. Today, IGBT production is particularly expensive due to the inability to grow 4H-SiC p-doped wafers, while in the case of 3C-SiC bulk grown by CVD, it is possible to grow such types of wafers.
