First-Principles Study on Structure and Anisotropy of High N-atom Density Layer in 4H-SiC

Abstract

A nitridation annealing process is well employed to reduce interface trap states that degrade the channel mobility of 4H-SiC/SiO 2 metal-oxide-semiconductor field-effect transistor. In recƒent experiments, the existence of high N-atom density layers at the annealed interface is reported and their concentrations are known to be anisotropic in the crystal planes. Until now, the role of atomic structure and the electronic states surrounding the N atoms incorporated by the nitridation annealing process on the origin of anisotropy is not well understood. In this work, we propose a simplified atomic-scale model structure of 4H-SiC with the a high N-atom density layer (∼ 10 15 atom/cm 2 ), which is of the order of the experimental observation. We use bulk 4H-SiC as host crystal and consider several sets of the atomic configurations of the N-atom incorporated structure at the quasi cubic-(k-) and hexagonal-(h-)sites on a-, m-, and Si-(C-)faces. Based on the density functional theory calculations, we investigate the influence of the energy stability on the distribution directions. Although our bulk model is simplified compared to the realistic interface structures, we confirm significant difference among models and observe that the incorporation of N atoms on the a-face is stable. Furthermore, from the analysis of the electronic states, we suggest that this anisotropy of the formation energy originates from the change of the coordinating number due to the difference in geometric configurations of the N-atom incorporated structures.

Density functional theory calculations for investigation of atomic structures of 4H-SiC/SiO$_2$ interface after NO annealing

“…The nitride layers growing along the a face are the most stable among those along the a, m, C, and Si faces. These results are consistent with the conclusion obtained for the nitride layers in bulks; that is, the nitride layer preferentially grows along the a face from the viewpoint of thermodynamics, 15 and the dependence of the areal N-atom density on the crystal plane reported in Ref. 10 is caused by the kinetic factor.…”

“…We consider the cases in which V Si s are arranged parallel to the surface to evaluate the anisotropy of the formation energy of the nitride layer. As reported in our previous paper, 15 the areal N-atom density is on the order of 10 14 cm −1 . Although there are two inequivalent lattice sites of 4H-SiC, i.e., h (hexagonal) and k (quasi-cubic) sites, our previous studies have shown that nitride layers tend to grow at the k site.…”

“…To explore the most preferable crystal plane of 4H-SiC for N-atom incorporation, we study the stability and electronic states of the nitride layers on a 4H-SiC surface. We find that the nitride layers grow along the a face, which is consistent with the conclusion obtained using the bulk model, 15 and NO bonds are hardly generated at the interface. Any defect states in the bandgap of 4H-SiC are not generated in our structure.…”

“…Although the anisotropy mechanism has been investigated so far, the atomic-scale structures of the interface after annealing have been unclear for arbitrary crystal planes. In our previous study, 15 we proposed the interface atomic structures in which N atoms are incorporated by replacing C atoms, and we investigated the total energies and electronic structures of the nitride layers by density functional theory (DFT) calculation. 16 It was found that the incorporation at the k site on the a face is energetically most stable, and the areal density of N atoms in our model (∼ 10 14 atom/cm 2 ) agrees well with experimental results.…”

Density functional theory study on the effect of NO annealing for SiC(0001) surface with atomic-scale steps

“…30) To set up a similar structure for the nitrided layers, four C atoms adjacent to a Si vacancy are replaced by N atoms. The interface structures have been proposed in our previous paper 31) and the areal N atom density of 1.2 × 10 15 atoms/cm 2 corresponds to that observed in experiments. 32,33) We have also reported that the insertion of the nitrided layers is an exothermic reaction and the nitrided layers are preferentially formed immediately below the SiO 2 region.…”