Bipolar degradation, which is caused by the expansion of stacking faults (SFs) during operation, has been a serious issue in 4H-SiC power devices. To evaluate the threshold minority carrier density of SF expansion, ρth, Maeda et al. proposed a theoretical model based on quantum well action and dislocation theory. This model includes SF energy variations, electronic energy lowering due to carrier trapping, and resolved shear stress applied to partial dislocations, τrss. Though the SF energy and the electric energy lowering were quantitatively established, the effect of τrss has not been discussed well yet. In this study, we first conducted theoretical predictions of the effect of τrssonρth. Then, based on our previous experiment on the dependence of threshold current density on mechanical external stress, we investigated the dependence of ρthonτrss. We conducted submodeling finite element analysis to obtain τrss induced by both residual stress due to the fabrication process and experimentally applied external stress. Finally, we obtained ρth at the origin of SF expansion from the experimentally measured threshold current density using device simulation. It was found that the dependence of ρthonτrss was almost linear. Its gradient was −0.04 ± 0.01 × 1016 cm−3/MPa, which well agrees with the theoretical prediction of −0.03 ± 0.02 × 1016 cm−3/MPa. Our study makes possible a comprehensive evaluation of the critical condition of bipolar degradation.

4H-SiC has gained attention as a material for advanced power devices. In this paper, we investigate the surface effect on the conversion from screw-type basal plane dislocation (BPD) to threading edge dislocation (TED) using reaction pathway analysis. We find that the constriction of a partial dislocation pair easily occurs in the vicinity of the surface and that the constriction in the Si-face substrate is easier than that in the C-face one. Also, we find that the cross slip of a perfect screw BPD easily occurs in the vicinity of the surface and that the cross slip in the Si-face is easier than that in the C-face. In addition, we reveal that the rate-limiting step of the cross slip is the glide to shuffle-glide mix transition. We also perform molecular dynamics simulations of a perfect screw BPD-TED conversion in an off-cut substrate and confirm that spontaneous conversion occurs even at low temperature (500 K).

We detected all components of the deformation potential constants of 4H-SiC by first-principles calculations and developed a method to estimate the stress distribution in 4H-SiC power devices by Finite Element Method (FEM) and Raman spectroscopy. The values of bA1, aE2, and bE2 obtained by calculations agreed well with experimental results, while those of aA1, bE1, and cE2 were about 45% larger. The relationship between phonon frequency and stress was nonlinear as shear stress increased. The multistep FEM analysis reproducing the manufacturing process is also conducted. The stress distribution was converted to the Raman shift and compared with results of micro-Raman spectroscopy. Except for the interface between SiC and the electrode, the analysis results agreed well with the experimental results. It was found that a compressive stress of about 200 MPa at the SiC/electrode interface and a resolved shear stress of about 20 MPa at the epilayer/substrate interface were generated.

The stacking fault (SF) energy of 4H-SiC around room temperature is important for the quantitative investigation of bipolar degradation, which is a serious issue in 4H-SiC bipolar power devices. However, the experimental measurement of SF energy around room temperature is very difficult. We have theoretically estimated the dependence of 4H-SiC SF energy on temperature using a calculation of the free energy of phonons based on ab initio calculations. Calculations using both the harmonic vibration approximation and quasi-harmonic approximation are performed in order to account for the effects of thermal expansion. The SF energies of a single Shockley-type stacking fault (SSSF) at room temperature and at 1500 K are 14.5 mJ/m2 and 12.8 mJ/m2, respectively. The SF energy of an SSSF is not sensitive to temperature and at a high temperature agrees with the experimental value. The SF energy of a double Shockley-type stacking fault is about 8 mJ/m2 at room temperature, and the energy increases with temperature, reaching about 11 mJ/m2 at 1500 K. The critical minority carrier density at which SFs expand in bipolar degradation is estimated by applying the obtained SF energy to the quantum well action model. The estimated critical minority carrier density is 1.0 × 1016–1.0 × 1017 cm−3, which is consistent with the previous experimental value. Our estimated SF energy enables us to accurately estimate the critical conditions of SF expansion in bipolar degradation.

Van der Waals (vdW) interactions have recently been demonstrated to have a non-negligible effect on the theoretical polytype stability and stacking fault energies of SiC. Calculations with density functional theory have been demonstrated to reproduce polytype stability consistent with experimental results when vdW interactions are considered. The effect of vdW interactions on stacking fault energies in SiC is an important engineering issue; however, it has not been studied in detail. Since previous studies used vdW correction methods that are rather simple and semi-empirical, the application of more sophisticated correction strategies and comparison among several proposed methods is required. In this study, we examined the dependence of polytype stability on the vdW correction method. While most methods could reproduce the polytype stability order, the extensively used DFT-D3 and its variants could not since the computed dependence of vdW interaction energy on hexagonality of SiC was small. Then, we examined the stacking fault energies considering vdW interactions. The vdW interactions were found to have a significant effect on the stacking fault energies only when the insertion of stacking faults changes the local hexagonality. The vdW interactions were found to cause negative energy for double Shockley-type stacking faults (DSSFs) in 4H-SiC. This negative energy is inconsistent with the electric energy model for the spontaneous expansion of DSSFs, which assumes that the stacking fault energy is inherently positive. Our results indicate that previous theoretical models may require being modified.

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Since the wear of front-rod bearings of railroad switches induce the switch failure, wear evaluation is indispensable. On the other hand, the various parts of railroad switches are adjusted so as to avoid derailment. For example, the attachment position of the adjustment nut of switch adjuster rod could enlarge the contact force between tongue rail and stock rail. The extension of front rod is also used to eliminate the gap between the toe of tongue rail and stock rail. In this paper, the bearing wear during the switching operation is estimated by multi-body dynamics simulation. The effects of the attachment position of adjustment nut and the extension of front rod are also evaluated. When the switch moves rightward, the driving force of electric point machine is transmitted to switch rod through switch adjuster rod. Then, the force is transmitted to right tongue rail through right switch rod bracket which connects switch rod and right tongue rail. Although the attachment position of adjustment nut controls the contact force between tongue rail and stock rail, that does not change the path of force transmission and the force acting on the bearing. Therefore, the attachment position does not affect the bearing wear. On the other hand, the extension of front rod changes the path of force transmission. The most of driving force of electric point machine is transmitted to the left switch rod bracket, front rod and right tongue rail in order. Therefore, the increase in the force acting on the bearing increases its wear. In the case of 6 mm extension of front rod, the estimated amount of wear is 3.44 μm, which is 3 times larger than the case of no extension.