Expansion Of Stacking Faults Research Articles

Comparison of single Shockley-type stacking fault expansion rates in 4H-SiC under ultraviolet illumination after hydrogen or fluorine ion implantation

The expansion of single Shockley-type stacking faults (1SSFs) was observed in 4H-SiC below the ion-implanted region of hydrogen or fluorine under ultraviolet illumination, and it was found that 1SSF expansion slowed, the expansion angle decreased, and the termination of 1SSF expansion became deeper as the dose of implanted ions was increased. A comparison of implanted ion species revealed that fluorine ion implantation more strongly suppresses 1SSF expansion under ultraviolet illumination than hydrogen ion implantation. The thermal stability of hydrogen and fluorine was also compared by using depth profiles of the implanted species concentrations before and after annealing. Fluorine was found to have superior thermal stability to that of hydrogen.

Study of leakage current degradation based on stacking faults expansion in irradiated SiC junction barrier Schottky diodes

Abstract A comprehensive investigation was conducted to explore the degradation mechanism of leakage current in SiC junction barrier Schottky (JBS) diodes under heavy ion irradiation. We propose and verify that the generation of stacking faults (SFs) induced by the recombination of massive electron--hole pairs during irradiation is the cause of reverse leakage current degradation based on experiments results. The irradiation experiment was carried out based on Ta ions with high linear energy transfer (LET) of 90.5 MeV/(mg/cm2). It is observed that the leakage current of the diode undergoes the permanent increase during irradiation when biased at 20% of the rated reverse voltage. Micro-PL spectroscopy and PL micro-imaging were utilized to detect the presence of SFs in the irradiated SiC JBS diodes. We combined the degraded performance of irradiated samples with SFs introduced by heavy ion irradiation. Finally, three-dimensional (3D) TCAD simulation was employed to evaluate the excessive electron–hole pairs (EHPs) concentration excited by heavy ion irradiation. It was observed that the excessive hole concentration under irradiation exceeded significantly the threshold hole concentration necessary for the expansion of SFs in the substrate. The proposed mechanism suggests that the process and material characteristics of the silicon carbide should be considered in order to reinforcing against the single event effect of SiC power devices.

Suppression of stacking-fault expansion in 4H-SiC diodes by helium implantation

Abstract Bipolar degradation is a critical problem in Silicon carbide devices, caused by the expansion of single Shockley stacking faults (1SSFs) from basal plane dislocations. This paper presents the effects of helium implantation on the suppression of 1SSFs expansion. The fabricated PiN diodes were analyzed using current–voltage characteristics and electroluminescence imaging. The results show that helium implantation can effectively suppress 1SSFs expansion without significantly degrading diode performance. We consider that this suppression is due to the pinning of dislocations by point defects introduced during helium implantation, and the reduction of carrier lifetime may also play a role in suppressing 1SSFs expansion.

Effects of proton implantation into 4H-SiC substrate: Stacking faults in epilayer on the substrate

This study explored proton implantation into 4H-SiC substrates to suppress the expansion of single Shockley stacking faults (SSSFs), which is a source of bipolar degradation in 4H-SiC devices. While previous research has demonstrated the effectiveness of proton implantation in epitaxial layers, concerns about defect generation have persisted. Therefore, we implanted protons into 4H-SiC substrates, followed by epitaxial growth. Then, we fabricate PiN diodes using the epitaxial layer aiming to reduce SSSF expansion and enhance PiN diode reliability. The results indicate that proton implantation has no significant suppression effects on the SSSF expansion, coupled with the undesired induction of double Shockley stacking faults. Thus, proton implantation into the substrates does not enhance the reliability of 4H-SiC devices, emphasizing the need for further investigations into suppression mechanisms.

In-situ investigation of the interaction between hydrogen and stacking faults in a bulk austenitic steel

The interaction between hydrogen (H) atoms and various microstructural defects remains a key to understand the H-induced damage and the subsequent premature failure of high-strength metallic materials. Previous studies on this subject are mainly focused on the in-situ probing of dislocations in a thin foil placed in an environmental transmission electron microscopy (TEM) cell. Here, a three-point bending test coupled with electron channeling contrast imaging (ECCI) has been applied to investigate the interaction of H with stacking faults (SFs) in a bulk high Mn austenitic steel. The expansion of some SFs, in terms of one partial dislocation movement within a partial dislocation pair, was observed on the H pre-charged sample when kept at a constant loading (i.e., a continuous H migration and likely build-up close to the pre-prepared notch tip). A temporal-resolved cross-correlation EBSD (CC-EBSD) measurement shows that the migration of H towards the notch tip region has a minor effect on the internal stress evolution. However, the local shear modulus (μ) and stacking fault energy (γSF) can be reduced by a local H segregation. Further theoretical calculation of SF ribbon width indicates that the reduction of μ by H results in the shrinkage of SF, while the H-induced reduction of γSF results in SF expansion at a lower resolved shear stress. The observed expansion of SF ribbons can be interrupted by a combined reduction of both μ and γSF due to H.

Combined FEM and phase field method for reliability design of forward degradation in SiC bipolar device

The reliability of 4H-SiC bipolar devices is compromised by the expansion of single Shockley stacking faults (SSFs) during forward-current operation. Because SSF expansion is governed by multiphysical aspects, including electrical, thermal, and stress states, analysis of the mounted structure is important for improving power module design. We propose a practical design method that analyzes the critical condition due to SSF expansion using a combined method with a multiphysical finite element method (FEM) and phase field model based on the time-dependent Ginzburg–Landau equation. In preliminary studies, the thermal deformation of the demonstration module and the variation of threshold current of a bar-shaped SSF were verified from experimental and reference data. Estimating the SSF expansion rate on the constructed response surface under the mutiphysical inputs from FEM, the proposed design method can be used effectively in the design process by changing the various design variables.

Stacking Fault Expansion from an Interfacial Dislocation in a 4H-SiC PIN Diode and Its Expansion Process

Stacking Fault Expansion from an Interfacial Dislocation in a 4H-SiC PIN Diode and Its Expansion Process

Effects of surface roughness and single Shockley stacking fault expansion on the electroluminescence of 4H-SiC

The electroluminescence of a 4H silicon carbide (SiC) bipolar junction transistor was studied using the base-collector junction after a side-wall facet was exposed. This sidewall was ground and polished in sequential stages with increasing grit numbers. After each stage, an electrical stress test under forward bias was performed. Electroluminescence spectra with peaks at 390 nm, 445 nm and 500 nm were initially observed. These peaks were seen to evolve under operation and after changes to the surface condition. Expansion of single Shockley stacking faults (1SSFs) in the device was observed during forward biased operation as evidenced by the growth of the 420nm emission peak, while the broad 500 nm peak was seen to diminish with increasing surface smoothness. Defect-enabled radiative recombination in SiC is a useful pathway for SiC defect characterization and it offers a new opportunity for light emission from SiC.

Suzuki hardening and segregation in Co0.95Cr0.8Fe0.25Ni1.8Mo0.475 high-entropy alloys

A Co0.95Cr0.8Fe0.25Ni1.8Mo0.475 high-entropy alloy was heavily cold-rolled and subsequently aged at 773 K. Aging-induced Suzuki hardening was observed. High-angle annular dark-field scanning transmission electron microscopy imaging and the corresponding energy dispersive X-ray spectroscopy elemental distribution mapping suggested the enrichment of Co atoms at stacking faults (SFs), which pinned down partial dislocations and enhanced the resistance of the alloy to plastic deformation. Furthermore, the enrichment of Co and Mo atoms at SFs was observed in a low dislocated, strain-aged specimen. The significant difference in the SF density of the specimens led to difference in segregation. The local stacking fault energy (SFE) with the segregation of Co and Mo atoms was calculated as −30.6 mJ/m2 at the aging temperature. The negative SFE did not restrict the SF expansion, indirectly resulting in the formation of wide SFs.

Suppression of stacking fault expansion in a 4H-SiC epitaxial layer by proton irradiation

SiC bipolar degradation, which is caused by stacking fault expansion from basal plane dislocations in a SiC epitaxial layer or near the interface between the epitaxial layer and the substrate, is one of the critical problems inhibiting widespread usage of high-voltage SiC bipolar devices. In the present study, we investigated the stacking fault expansion behavior under UV illumination in a 4H-SiC epitaxial layer subjected to proton irradiation. X-ray topography observations revealed that proton irradiation suppressed stacking fault expansion. Excess carrier lifetime measurements showed that stacking fault expansion was suppressed in 4H-SiC epitaxial layers with proton irradiation at a fluence of 1 × 1011 cm−2 without evident reduction of the excess carrier lifetime. Furthermore, stacking fault expansion was also suppressed even after high-temperature annealing to recover the excess carrier lifetime. These results implied that passivation of dislocation cores by protons hinders recombination-enhanced dislocation glide motion under UV illumination.

Formation mechanism of horizontal-half-loop arrays and stacking fault expansion behavior in thick SiC epitaxial layers

The formation mechanism of half-loop arrays (HLAs) that form parallel (horizontal) to the step-flow direction in 120 μm thick 4H-silicon carbide (SiC) epitaxial layers was investigated using ultraviolet photoluminescence (UVPL) imaging and x-ray topography (XRT). The horizontal-HLAs are generated by the multiplication and glide of basal plane dislocation (BPD) loops that are created within the epitaxial layer. The BPD loops were initiated after ∼40–50 μm of growth from a small BPD segment, which glides toward the surface as well as the substrate interface. BPD multiplication occurs and several loops are generated. Some of these loops are terminated by the growth front and create HLAs due to the 4° offcut of the wafer. XRT images show that successive BPD loops interact with previously generated HLA segments. Successive loops also interact with the moving growth front and create new HLAs that are spatially displaced from the previous HLA segments. These appear as a string of horizontal-HLAs in the UVPL images. The expansion of stacking faults (SFs) from these horizontal-HLAs was investigated, and we show that they all lie on the same basal plane. The complex defect structure is created in the epitaxial layer from a single BPD loop but extends over a large (∼5 × 0.5 cm2) region of the SiC wafer during epitaxial growth. The high density of HLAs and BPDs would generate several SFs upon device operation leading to severe device degradation.

Cathodoluminescence and EBIC investigations of stacking fault expansion in 4H-SiC due to e-beam irradiation at fixed points

The effect of e-beam irradiation in the local and scan modes on the stacking fault expansion in 4H-SiC has been studied. It is shown that the distance, at which the e-beam affects the glide of partial dislocations driving the stacking fault expansion, does not exceed 10–12 μm. The dislocations were found to glide as straight lines with a velocity independent of their length, even when this length essentially exceeds the size of excitation volume. The irradiation at fixed points allows to separate the excess carrier effects on the kink formation and kink migration. The results obtained were explained under an assumption that the irradiation is necessary only to stimulate the kink pair formation and then the kinks can migrate without any excitation. That could mean that the barrier for the kink migration along Si-core 30° partial dislocations in 4H-SiC is very small.

Formation and Anisotropic Mechanical Behavior of Stacking Fault Tetrahedron in Ni and CoCrFeNiMn High-Entropy Alloy

Stacking fault tetrahedron (SFT) is a kind of detrimental three-dimensional defect in conventional face-centered cubic (FCC) structural metals; however, its formation and anisotropic mechanical behavior in a CoCrFeNiMn high-entropy alloy (HEA) remain unclear. In this work, we first performed molecular dynamics simulations to verify the applicability of the Silcox-Hirsch mechanism in the CoCrFeNiMn HEA. The mechanical responses of the SFT to shear stress in different directions and that of the pure Ni counterpart were simulated, and the evolutions of the atomic structures of the SFTs during shear were analyzed in detail. Our results revealed that the evolution of the SFT has different patterns, including the annihilation of stacking faults, the formation and expansion of new stacking faults, and insignificant changes in stacking faults. It was found that the effects of SFT on the elastic properties of Ni and HEA are negligible. However, the introduction of SFT would reduce the critical stress, while the critical stress of the CoCrFeNiMn HEA is much less sensitive to SFT than that of Ni.

Immobilization of partial dislocations bounding double Shockley stacking faults in 4H-SiC observed by in situ synchrotron X-ray topography

The expansion of double Shockley stacking faults (DSFs) in an n-type 4H-SiC substrate with a nitrogen concentration of 3.9 × 1019 cm−3 was investigated using in situ synchrotron X-ray topography. DSF expansion was observed to be suppressed and immobilized above 1590 K, along with the partial dislocation (PD) shape being changed from a straight to zig-zag configuration. For a different heating process (higher heating rate), the PDs could continue to expand, even above 1590 K. Ex situ topography experiments revealed that the DSFs close to the specimen surface expanded widely, although those expanding toward the specimen interior became immobile. One possible mechanism for this immobilization was proposed, where the core structural changes from a Si-core to the C-core by non-conservative motion induced by the interaction between the PDs and point defects (C interstitials).

In-operando x-ray topography analysis of SiC metal–oxide–semiconductor field-effect transistors to visualize stacking fault expansion motions dynamically during operations

We developed an in-operando x-ray topography method for dynamically visualizing single Shockley-type stacking fault (1SSF) expansion motions in silicon carbide (SiC) metal–oxide–semiconductor field-effect transistors (MOSFETs) during their operations and investigated the effect of the operating condition applied to the body diodes in SiC MOSFETs on dislocation glide velocity. In-operando x-ray topography observations were carried out in reflection geometry, and a high-resolution x-ray camera was used as a detector to record topographies dynamically. The sequence of 1SSF expansion motions in the SiC MOSFETs was observed at a high resolution of 1 s in x-ray topographies, which is sufficient to analyze the dislocation glide velocity of a 1SSF expansion. The observation results of changing the forward current density applied to the body diodes in SiC MOSFETs revealed that each triangular and bar-shaped 1SSF expands at different forward current densities. The 1SSF expansion timings also differed, even in the same chip under the same current density. The dislocation glide velocity of each expanded 1SSF in SiC MOSFETs was extracted, and it increased with the forward current density. Our method enables the dynamic visualization of bipolar degradation in SiC MOSFETs during their operations, and we can accurately obtain the information of when, where, and which 1SSF expands in a SiC MOSFET.

Phase field model of single Shockley stacking fault expansion in 4H-SiC PiN diode

Expansion of a single Shockley stacking fault (SSF) during forward-current operation decreases the reliability of 4H-SiC bipolar devices. We propose a practical method for analyzing the defect evolution of SSF expansion based on free energy according to current density, temperature, and resolved shear stress conditions. The free energy includes chemical potential and elastic strain energy. Specifically, the chemical potential is related to the driving force for the formation of SSFs by temperature and current, and the elastic strain energy corresponds to the driving force for dislocations that form SSFs under the applied stress. It was confirmed that the proposed multiphysics method could well simulate SSF evolution when stress and current were applied. Furthermore, the results suggest that quantum well action, in which electrons in n-type 4H-SiC enter SSF-induced quantum well states to lower the energy of the dislocation system, affects the driving force of SSF formation.

Immobilization of Partial Dislocations Bounding Double Shockley Stacking Faults in 4h-Sic Observed by in Situ Synchrotron X-Ray Topography

The expansion of double Shockley stacking faults (DSFs) in an n-type 4H-SiC substrate with a nitrogen concentration of 3.9×10 19 cm −3 was investigated using in situ synchrotron X-ray topography. DSF expansion was observed to be suppressed and immobilized above 1590 K, along with the partial dislocation (PD) shape being changed from a straight to zig-zag configuration. For a different heating process (higher heating rate), the PDs could continue to expand, even above 1590 K. Ex situ topography experiments revealed that the DSFs close to the specimen surface expanded widely, although those expanding toward the specimen interior became immobile. One possible mechanism for this immobilization was proposed, where the core structural changes from a Si-core to the C-core by climb motion induced by the interaction between the PDs and point defects (C interstitials).

Growth of 4H-SiC recombination-enhancing buffer layer with Ti and N co-doping and improvement of forward voltage stability of PiN diodes

“Bipolar degradation” phenomenon has severely impeded the development of 4H-SiC bipolar devices. Their defect mechanism is the expansion of Shockley-type stacking faults from basal plane dislocations under the condition of electron-hole recombination. To suppress the “bipolar degradation” phenomenon, not only do the basal plane dislocations in the 4H-SiC drift layer need eliminating, but also a recombination-enhancing buffer layer is required to prevent the minority carriers of holes from reaching the epilayer/substrate interface where high-density basal plane dislocation segments exist. In this paper, Ti and N co-doped 4H-SiC buffer layers are grown to further shorten the minority carrier lifetime. Firstly, the dependence of Ti doping concentration on TiCl<sub>4</sub> flow rate in 4H-SiC epilayers is determined by using single-dilution gas line and double-dilution gas line. Then the p<sup>+</sup> layer and p<sup>++</sup> layer in PiN diode are obtained by aluminum ion implantation at room temperature and 500 ℃ followed by high temperature activation annealing. Finally, 4H-SiC PiN diodes with a Ti, N co-doped buffer layer are fabricated and tested with a forward current density of 100 A/cm<sup>2</sup> for 10 min. Comparing with the PiN diodes without a buffer layer and with a buffer layer only doped with high concentration of nitrogen, the forward voltage drop stability of those diodes with a 2 μm-thick Ti, N co-doped buffer layer (Ti: 3.70 × 10<sup>15</sup> cm<sup>–3</sup> and N: 1.01 × 10<sup>19</sup> cm<sup>–3</sup>) is greatly improved.

Single Shockley stacking fault expansion from immobile basal plane dislocations in 4H-SiC

Some combinations of immobile partial dislocations (PDs) that constitute basal plane dislocations (BPDs) have not previously been considered as sources for single Shockley stacking fault expansion. We searched for and found this type of BPD and investigated its structure. The realistic reason for immobile C-core PDs being converted into mobile Si-core PDs is speculated from the results obtained by plan-view transmission electron microscopy (TEM) and cross-sectional scanning TEM. A model is proposed from a dynamic viewpoint for interpreting the mechanism of core-species change by step-flow motion during epitaxial crystal growth in 4H-SiC. Moreover, all possible combinations of immobile PDs are summarized and the necessary condition for immobile BPDs to change to include mobile PDs is discussed.

Modeling the effect of mechanical stress on bipolar degradation in 4H-SiC power devices

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.