**6.3. N implants and radicals**

Following the improvements induced by NO POA, other methods were developed to introduce N at the SiO2/SiC interface. Although they may be more involved, they can yield NO-like properties for oxide-based devices formed on 4H-SiC and bring their own contribution to understanding the role of nitrogen.

One such nitridation technique is implantation. N<sup>+</sup> ions can be inserted in the top semiconductor layer that will subsequently be consumed by thermal oxidation, yielding the presence of nitrogen at the interface. The amount of N atoms can be tuned by implantation dose and energy. Studies have revealed that similarly to NO POA, the higher the nitrogen density at the thermally-formed interface, the lower the *Dit*, and the higher the field-effect mobility [42, 91, 93]. In fact, Poggi *et al.* have reported about an order of magnitude reduction of electrically active defects close to the conduction band edge of 4H-SiC and a room temperature field-effect mobility of up to 42 cm2/V.s in lateral nFETs fabricated on the (0001) surface [94, 95], Fig. 10. While the progressive increase of *µFE* with N dose is consistent with the reduction of Coulomb scattering, Hall mobility measurements reveal that in devices with the higher nitrogen content, *µHall* decreases with temperature. This implies that, unlike for the NO process, another dominant scattering mechanism appears following high implant doses. This has been attributed to induced damages in SiC and residual N interstitials left within the semiconductor [16]. Also, note that the process temperature can be kept at or below 1100 ◦C following implantation, to avoid activation that would convert N atoms into donors in SiC. But activation of a minority of dopants in the tail end of the implant can never be ruled out.

Another elegant way to introduce nitrogen is the exposure of thermal oxides to nitrogen radicals [116, 134, 137]. It can be achieved using a remote plasma generating highly reactive N<sup>+</sup> ions. SIMS measurements have shown that, like NO-POA, this results in nitrogen accumulation strictly at the interface between the oxide and the semiconductor. One advantage being that it can potentially occur without re-oxidation, allowing for N maximization. Studies on devices fabricated on the Si-face of 4H-SiC again show that the D*it* is reduced proportionally to the amount of incorporated nitrogen, in line with results from NO POA. In fact, similar and even better performance in terms of peak field-effect mobility has been demonstrated using that technique. However, prolonged plasma exposure can also reduce the integrity of the gate oxide, implying that this promising nitridation method still requires optimization.
