Single Shockley Stacking Faults Research Articles

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.

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.

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.

Transmission Electron Microscopy Study of Single Shockley Stacking Faults in 4H-SiC Expanded from Basal Plane Dislocation Segments Accompanied by Threading Edge Dislocations on both Ends

Double-rhombic shaped single Shockley stacking faults (1SSFs) were considered to have a converted threading edge dislocation (TED) on the shallower side of the initial basal plane dislocation segments. However, the structural analysis using transmission electron microscopy (TEM) revealed other possible configuration of the double-rhombic 1SSFs expanded from basal plane dislocations (BPDs) of which both ends were connected with two TEDs.

Structural study of single Shockley stacking faults terminated near substrate/epilayer interface in 4H-SiC

Structural analysis is carried out of a single Shockley stacking fault (1SSF) that terminates near the substrate/epilayer interface and originally expanded from a basal plane dislocation (BPD) segment near the epilayer surface of 4H-SiC. The characteristic zigzag structure is found for the partial dislocations (PDs), with microscopic connecting angles of almost 120°. It has been suggested that the microscopic construction of PDs might be limited by the Peierls valley. The termination line near the substrate/epilayer interface was found to have 30° Si-core and 90° Si-core PDs. This combination is the same as that found near the surface of the epilayer in commonly observed 1SSFs. Penetrating BPDs of this kind were also found experimentally for the first time. For the currently proposed charts for the 1SSF expansions, photoluminescence imaging during UV illumination is one of the nondestructive analysis methods that can provide the structural information and expected expansion shapes of the 1SSFs.

Expansion patterns of single Shockley stacking faults from scratches on 4H-SiC

Single Shockley-type stacking faults (1SSFs) in the 4H-SiC epilayer exhibits numerous expansion patterns determined by the relationship between initial basal-plane dislocation (BPD) direction and Burgers vector. In this study, patterns of BPDs and 1SSFs generated by UV illumination near linear scratches on a SiC epilayer were investigated. The correlation between the observed 1SSF patterns and the scratch line directions was discussed by considering shear stress near the scratch lines. This result suggests that it is possible to form 1SSFs with a desired shape by adjusting the components of shear stress through the setting of scribing conditions.

Mechanical-stressing measurements of formation energy of single Shockley stacking faults in 4H-SiC

The formation energy of single Shockley stacking faults (1SSFs) in thermo-equilibrium is crucial for validating the anomalous expansion mechanism of 1SSF induced by forward current injection in 4H-SiC bipolar devices. As a function of variable mechanical stress externally applied to a plate sample of 4H-SiC, we systematically measured the ultraviolet intensity thresholds demarcating photo-induced expansion and contraction of 1SSFs. The experimental results on two types of 1SSF differing in the Schmid factor of mobile partial dislocations achieving 1SSF expansion/contraction showed a value of 4.7 ± 1.6 mJ m−2, significantly smaller than the reported value of 14 ± 2.5 mJ m−2 obtained by transmission electron microscopic measurements.

Direct confirmation of structural differences in single Shockley stacking faults expanding from different origins in 4H-SiC PiN diodes

Structural differences are investigated in partial dislocations that have considerably different threshold current densities for single Shockley-type stacking fault (1SSF) expansions in PiN diodes using transmission electron microscopy and scanning transmission electron microscopy as direct observation techniques. It has been found that when the threshold current density is relatively low, a triangular 1SSF, whose oblique and base sides were formed with a pair of Shockley partials, was found. On the other hand, when the threshold current density is relatively high, a similar-shaped triangular 1SSF, whose three sides were almost formed by a single dislocation of one of the Shockley partials, was found. In the latter case, a conversion point from a basal plane dislocation to a threading edge dislocation was also observed around the deepest part of the epilayer. The realistic model for the formation of the triangular 1SSFs has been proposed.

Triangular Single Shockley Stacking Fault Analyses on 4H-SiC PiN Diode with Forward Voltage Degradation

The structure of partial dislocations (PDs), which surround triangular single Shockley stacking faults (1SSFs) expanded during electroluminescence observation in a 4H-SiC PiN diode, is investigated by photoluminescence (PL) imaging, PL spectroscopy, plan-view transmission electron microscopy (TEM) imaging, g·b analysis, cross-sectional bright-field scanning TEM (BF-STEM) imaging, and high-resolution high-angle annular dark-field (HAADF)-STEM imaging. The zigzag configuration is found on the upstanding side of the triangular 1SSF by plan-view TEM imaging, while it is observed as a single line by PL imaging. Using g·b analysis, the zigzag line is confirmed to be constructed by 30° and 90° PDs. Cross-sectional BF-STEM observation proved that the expanding 1SSF penetrates through the p–n interface whose position was estimated from secondary-ion mass spectrometry (SIMS) depth profile measurements of the aluminum and nitrogen concentrations. This provides experimental evidence that the 1SSF loses its driving force for recombination-enhanced dislocation glide when it stops somewhere in the p-layer where sufficient electrons no longer exist. Using cross-sectional HAADF-STEM observation, core species of the PDs were analyzed and the Burgers vector for the dislocation loop uniquely determined. Also, the first experimental observation of a 90° silicon-core PD, reported so far, is confirmed. According to these PD analyses, the possible atomic arrangement around the 1SSF is examined and the way tetrahedrons face across each PD is summarized, based on HAADF-STEM analysis of the 1SSF line.

Estimation of the critical condition for expansion/contraction of single Shockley stacking faults in 4H-SiC PiN diodes

The critical condition for expansion/contraction of single Shockley-type stacking faults (1SSFs) was experimentally estimated by monitoring the electroluminescence from 1SSFs in 4H-SiC PiN diodes with different p/n structures while varying the current density and the temperature. The “threshold current density” for expansion/contraction of 1SSF was determined by observing the 1SSF behavior, and it was converted to the “threshold excess carrier density” using the numerical calculation by device simulation. The threshold excess carrier density was almost independent of the diode structures though the threshold current densities were very different among the various diodes, which means that the 1SSF behavior does not depend on the current density but on the excess carrier density. The threshold excess carrier density at room temperature was estimated to be about 4×1014 cm−3, which agrees with the result calculated by a theoretical model previously proposed. In addition, the conduction type- and the doping concentration-dependences of the threshold excess carrier density were discussed.

Numerical study on the suppression of 4H-SiC PiN diodes forward bias degradation due to substrate basal plane dislocations

We propose a calculation model of current density that causes forward bias degradation from substrate basal plane dislocations (BPDs) in 4H-SiC PiN diodes. The hole concentration above which substrate BPDs expand to single Shockley stacking faults (1SSFs) at the buffer/substrate interface was experimentally evaluated from forward-current stress tests of 4H-SiC PiN diodes by comparison with our model results, resulting in 8.0 × 1015 cm−3. We confirmed the dependence of the current density on the dopant concentration and the hole lifetime in the buffer layer numerically. The model was extended to the case where BPD converted to threading edge dislocations (TEDs) in the substrate, and the relational expression between the depth of the BPD-TED conversion position in the substrate and the current density at which BPD expanded to 1SSF was obtained. The model suggested that it will be an effective technique for suppressing forward bias degradation by shorter lifetime and deeper BPD-TED conversion position in the substrate.

Suppressed expansion of single Shockley stacking faults at narrow widths in 4H-SiC

UV-induced expansion of single Shockley stacking faults (1SSFs) in 4H-SiC was found to be suppressed when the width of 1SSFs w is narrower than a certain value of wr. The intensity profiles of 1SSF-originated photoluminescence (PL) show an initial growth with w followed by an intensity ceiling beyond wr. The characteristic width wr is several tens of μm at room temperature, which is close to the range of 1SSF PL reduction near the partial dislocations bounding the 1SSF. A qualitative reasoning is discussed for the larger 1SSF expansion threshold in n+ buffer/substrate interfaces than in n− drift layers.

(Invited) Recent Progress in 4H-SiC CVD Growth for High-Voltage Power Devices

This paper reports recent progress in 4H-SiC CVD growth, aiming to obtain high-quality epilayers and bulk crystals with a high productivity for high-voltage power devices. The production of 150 mm-diameter 4H-SiC wafers has become the industry standard and the practical use of 4H-SiC unipolar devices such as Schottky barrier diodes and MOSFETS with a blocking voltage from 600 to 3,300 V started in various fields, including power conditioners for photo voltaic systems, air conditioning equipment, power supplies for IT and manufacturing industries, and inverters for electric cars and trains. Although bipolar devices such as PiN diodes and IGBTs may be more suitable for very high-voltage classes over ten kilo volts, the practical application of the bipolar devices is hindered because of the degradation phenomenon (“bipolar degradation”) in which forward on-state voltage of the devices increases after current stress applied due to the expansion of stacking faults in the crystal during current conduction. Based on the current understanding on the bipolar degradation phenomenon, basal plane dislocations (BPDs) are responsible for the nuclei of expanding single Shockley stacking faults (1SSFs) and the expansion of the 1SSFs is activated by minority carrier injection exceeding a certain level. Although epilayers with low-density BPDs (< 0.1 cm-2) were obtained by enhancing the conversion of BPDs in the substrate to threading edge dislocations (TEDs) at the beginning of epitaxial growth, the expansion of 1SSFs from the BPD segments below the conversion points was confirmed in the high current density operation [1]. This can be understood due to the injection of minority carriers into a region below the BPD-TED conversion points located near the epilayer and substrate interface or in the substrate where high-density BPDs (102 cm-2) are existing. Meanwhile, inserting a buffer layer with a short carrier lifetime between the substrate and drift epilayer has been proposed to prevent minority carrier injection into a region where BPDs remain. At this time, a thinner buffer layer is more preferable and a technique to obtain shorter carrier lifetimes compatible with the practical epitaxial growth and device fabrication processes will be promising. High nitrogen (N) doping yielding high-density free electrons has been shown as a promising method to enhance direct and Auger recombination and reduce carrier lifetimes [2, 3]. Short carrier lifetimes of 66-120 ns were obtained at RT-250 ºC for an epilayer doped with N at 5×1018 cm-3. We also attempted intentional vanadium (V) doping during epitaxial growth of a buffer layer to realize much shorter carrier lifetimes. The highly N+V-doped epilayer (N=5×1018, V=7×1014 cm-3) shows very short carrier lifetimes of ~20 ns or less at RT-250 ºC. Inserting a N- or N+V-doped buffer layer between a substrate and n- drift epilayer in the PiN diodes was confirmed to be effective to prevent the formation of 1SSFs from BPDs in the substrate. Besides continuous improvements in the quality of wafers, the strong demand to cut the cost of 4H-SiC devices from the application side may request to promote wafer productivity. Meanwhile, the development of 200 mm-diameter wafers has been demonstrated by sublimation growth in past conferences. Apart from enlarging wafer diameter, massively enhancing the growth rate or ingot length may pose the ultimate challenge in 4H-SiC bulk growth and trials featuring solution and high-temperature CVD growth methods are now underway. For high-temperature CVD growth, we attempted fast bulk crystal growth of 4H-SiC at a seed temperature of 2350-2550ºC using an H2-SiH4-C3H8-N2 gas system in a vertical CVD reactor [4]. By adjusting process parameters to enhance growth rates and improve crystal quality, fast 4H-SiC crystal growth at a growth rate exceeding 3 mm/h was achieved while maintaining the low dislocation density (threading screw dislocations: ~2.5×102 cm-2) of the seed crystal. A part of this work was supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “Next-generation power electronics/Consistent R&D of next-generation SiC power electronics” (funding agency: NEDO). [1] A. Tanaka et al., J. Appl. Phys. 119, 095711 (2016), [2] T. Tawara et al., J. Appl. Phys. 120, 115101 (2016), [3] T. Tawara et al., J. Appl. Phys. 123, 025707 (2018), [4] N. Hoshino et al., J. Cryst. Growth 478, 9 (2017).

Evaluation of Effect of Mechanical Stress on Stacking Fault Expansion in 4H-SiC P-i-N Diode

Single Shockley stacking faults (SSFs) expand from basal plane dislocations (BPDs) under forward current operation of 4H-SiC bipolar devices, giving rise to a reliability deterioration mode called “bipolar degradation”. Several groups have proposed models for the expansion of SSFs, in which the SSFs expand when electron-hole pair recombination takes place at BPDs. Maeda proposed a formulation of SSF expansion that includes stacking fault energy. However, the mechanisms by which mechanical stress affects the expansion of SSFs are unclear. In this paper, we evaluated the “expansion threshold current” of bar-shaped SSFs in a mechanical stress field using a p-i-n diode fabricated on 4H-SiC. To confirm the effect of mechanical stress on the threshold current for bar-shaped SSF expansion, a SiC-p-i-n diode was evaluated by the four-point bending method. Experimental results show that the threshold current of SSFs decreases or increases by more than 100 A/cm2 depending on the direction of the applied stress of SSFs. This result indicates that mechanical stress is an important factor for SiC bipolar device design.

Dynamics Analysis of Single Shockley Stacking Fault Expansion in 4H-SiC P-i-N Diode Based on Free Energy

Expansion of single Shockley stacking faults (SSFs) during forward current operation is an important issue, because it decreases the reliability of 4H-SiC bipolar devices. In this paper, we propose a method for analyzing SSF dynamics based on free energy under current conduction, temperature, and resolved shear stress conditions. The driving force for dislocation dissociation reactions and formation of SSFs is incorporated into the free energy function, including chemical potential, stacking fault energy, crystallographic energy, gradient energy and elastic strain energy. The net energy gain of the chemical potential was calculated as a function of temperature and current conduction through use of the a TCAD device simulator based on the Boltzmann equation, Poisson equation and the current continuity equation concerning electron and hole distributions with self-consistency. It was confirmed that SSF dynamics can be simulated by the proposed method. It was also found that SSF formation can be attributed to quantum well variation in which electrons in n-type 4H–SiC enter SSF-induced quantum well states to lower the energy of the dislocation system.

Expansion and contraction of single Shockley stacking faults in SiC epitaxial layer under ultraviolet irradiation

Behaviors such as expansion and contraction of single Shockley stacking faults (1SSFs) in a 4H-SiC epitaxial layer under various ultraviolet (UV) irradiation conditions were systematically investigated. We demonstrate that the area of the 1SSF has a key role in its behavior during UV light exposure. Contraction immediately progresses after a relatively weak UV irradiation at a high-temperature. The contraction velocity gradually decreases and eventually becomes zero at a certain area, which is dependent on the UV irradiation.

Quantum well action model for the formation of a single Shockley stacking fault in a 4H-SiC crystal under non-equilibrium conditions

The formation of single Shockley stacking faults (SSSFs) in 4H-SiC crystals under non-equilibrium conditions (e.g., the forward biasing of PiN diodes and ultraviolet light illumination) is a key phenomenon in the so-called bipolar degradation of SiC power devices. This study theoretically investigated the physical mechanism of this phenomenon based on the concept of quantum well action. As a first approximation describing the non-equilibrium state of the material, we employed quasi-Fermi level approximation. We then made improvements by considering several physical effects governing the carrier distribution near and in the SSSF. The improved model accounts well for the excitation threshold and the temperature dependence of SSSF expansion. Thus, the model provides useful insights into the driving force of SSSF expansion under non-equilibrium conditions.

Observation of carrier recombination in single Shockley stacking faults and at partial dislocations in 4H-SiC

Because the expansion of single Shockley stacking faults (1SSFs) is an important problem for the viability of SiC bipolar devices, there is a need to suppress it during device operation. The expansion mechanism, however, is still unclear. Therefore, the method to suppress the expansion has never been established. An important factor for the expansion could be carrier recombination in 1SSFs because the expansion has only been observed during bipolar operation or light illumination. In this study, we characterized carrier recombination by observing the photoluminescence from 1SSFs and partial dislocations (PDs). The luminescence from 1SSFs and PDs showed a fast decay component compared with that from the band edge. This result indicates that the carrier recombination in 1SSFs and at PDs was faster than that in regions without 1SSFs in 4H-SiC. In addition, because of the slower recombination at Si-core PDs compared with that in 1SSFs and at C-core PDs, the velocity of 1SSF expansion would be limited by the carrier recombination at Si-core PDs. The temperature dependence of the decay time implies that the recombination at the Si-core PD was enhanced on increasing the temperature.

(Invited) Non-Destructive Three-Dimensional Imaging of Extended Defects in 4H-SiC

Grazing-incidence X-ray topography and photoluminescence (PL) imaging are utilized to understand the nature and behavior of extended defects in 4H-SiC. Although these methods are useful for non-destructive defect analysis, they provide two-dimensional (2D) images with low depth resolution. Advanced three-dimensional (3D) imaging methods are thus expected to be used for characterizing extended defects. In particular, very high-voltage SiC bipolar devices with a thick epilayer require defect imaging as deep as 100-200 μm. We established a 3D imaging technique using an X-ray microbeam and a novel fine slit, which was successfully applied to characterizing threading screw dislocations (TSDs) [1, 2], threading edge dislocations (TEDs), and basal-plane dislocations (BPDs) [3, 4] in 4H-SiC. However, this method requires a large-scale synchrotron-radiation facility. In this study, we show how second-harmonic generation (SHG) and two-photon-excited photoluminescence (2PPL) imaging techniques are powerful tools for 3D analysis of extended defects in 4H-SiC epilayers [5, 6]. The SHG and 2PPL methods can be performed by a multi-photon microscope in individual laboratories. The SHG method provides clear 3D images of 3C-inclusions because 3C-SiC is SHG-active, but not 4H-SiC host crystal in c-axis incidence (Fig. 1). The 2PPL method yields 3D images; not only of 3C-inclusions but also 8H stacking faults (Fig. 2), and single Shockley stacking faults in the epilayers. The 2PPL method also achieved 3D imaging of TSDs, TEDs, and BPDs using band-edge emission [7]. Since band-edge emission quenches near defects, these dislocations can be visualized as dark contrasts on a bright background. We obtained 3D images of TSDs and TEDs extending ~200 μm from the surface (Fig. 3). Unlike 2D PL imaging with uniform illumination, the dark-contrast imaging is governed by the diffusion and transport of excess carriers injected from a scanning focal point. A simulation study was conducted to reveal the mechanism of the dark-contrast imaging. Each of the dislocations provided 2D dark-contrast images stacked depthwise, from which the tilt angles of dislocations were determined. Figure 4 shows the plots of φ versus θ for 89 TSDs in the 4º off epilayer, where θ denotes the angle of the dislocation line from the c-axis, and φ the counter-clockwise angle of (0 0 0 1) projected dislocation lines from the step-flow [1 1 -2 0] direction. Their dislocation lines incline in the step-flow direction. It is also shown that the regions of left- and right-handed (LH and RH) dislocations clearly differ. The plots in Fig. 4 include those of 1c and c+a dislocations. We found that 1c dislocations had very similar tilt angles, whereas those of c+a dislocations were largely spread out. The tilt angles were also investigated for 105 TSDs in the 8º off epilayer, which exhibits larger θ angles and a smaller φ range than the 4º off-cut case. We examined the tilt angles of more than 300 TEDs and found that the TEDs not only inclined in the step-flow direction, but also the direction of the extra half planes [7]. The results of the tilt-angle analyses cannot be explained by the energy minimization model in bulk materials. We consider that the directions of TEDs and TSDs depend upon the interactions between dislocations and advancing steps on a growing surface. [1] R. Tanuma, T. Kubo, F. Togoh, T. Tawara, A. Saito, K. Fukuda, K. Hayashi, and Y. Tsusaka, Phys. Status Solidi A 204, 2706 (2007). [2] R. Tanuma, T. Tamori, Y. Yonezawa, H. Yamaguchi, H. Matsuhata, K. Fukuda, and K. Arai, Material Sci. Forum 615-617, 251 (2009). [3] R. Tanuma, D. Mori, I. Kamata, and H. Tsuchida, Appl. Phys. Express 5, 061301 (2012). [4] R. Tanuma, D. Mori, I. Kamata, and H. Tsuchida, J. Appl. Phys. 114, 023511 (2013). [5] R. Tanuma and H. Tsuchida, Appl. Phys. Express 7, 021304 (2014). [6] R. Tanuma and H. Tsuchida, Mat. Sci. Forum 778-780, 338 (2014). [7] R. Tanuma, M. Nagano, I. Kamata, and H. Tsuchida, Appl. Phys. Express 7, 121303 (2014). Figure 1