Double Shockley Stacking Faults Research Articles

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.

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).

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).

Formation of Double Shockley Stacking Faults in Heavily Nitrogen Doped 4H-SiC Crystal with Reduction of Residual Stress around Scratch Damage

The scratch damage that caused the generation of double Shockley stacking faults (DSFs) in heavily nitrogen doped 4H-SiC crystal was investigated quantitatively. Scratch tests were carried out on 4H-SiC substrates with a nitrogen concentration of 2.6 × 1019 cm-3. A residual tensile stress of 40 MPa was detected around the scratch loaded at 30 mN with a diamond tip. DSFs were generated from this scratch by annealing at 1100°C for 2 h in Ar atmosphere. After annealing, the residual stress around the scratch was reduced to a tensile stress of 10 MPa. This result suggests that the reduction of residual stress around the scratch coincided with the formation of DSFs.

Temperature dependence of double Shockley stacking fault behavior in nitrogen-doped 4H-SiC studied by in-situ synchrotron X-ray topography

We observed the behavior of double Shockley stacking faults (DSFs) in 4H-SiC crystals with nitrogen concentrations of 1.0 × 1019–2.6 × 1019 cm−3 over an extensive temperature range (1380–1910 K) by in-situ synchrotron X-ray topography. For a nitrogen concentration of 2.6 × 1019 cm−3, the expansion velocity of the DSFs exponentially increased with temperature in the range from 1370 to 1650 K. In contrast, at a nitrogen concentration of 1.0 × 1019 cm−3, this velocity decreased above 1610 K and the DSFs shrank above 1730 K. The DSF energy, the magnitude of which is the driving force on partial dislocation (PD) movement, was quantitatively estimated from the radius of the curvature of bowed-out partial PDs pinned by threading screw dislocations (TSDs), showing a positive temperature dependence and lying in the range from −0.6 to 0.8 mJ/m2 for 1630–1910 K at a nitrogen concentration of 1.0 × 1019 cm−3. The DSF expansion and shrinkage behavior can be understood by the simple temperature and nitrogen concentration dependence of the DSF energy.

Raman scattering microscopy imaging of basal plane stacking faults and associated partial dislocations in 4H-SiC crystals

A novel characterization method using Raman scattering microscopy for basal plane stacking faults (SFs) and associated partial dislocations (PDs) in 4H-SiC crystals is proposed. Atomically thin SFs were successfully detected through the peak shift of the A1t transverse optical phonon mode in 4H-SiC, which could induce an electrical dipole moment within the SFs. The A1t mode was also sensitive to the strain field around the PDs encompassing SFs. Raman scattering microscopy imaging was able to determine the core type of PDs and their line direction within the crystals. Numerical simulation of Raman scattering microscopy images of PDs was also conducted and showed a quantitative agreement with the experimentally obtained images. Finally, the method was applied to the investigation of expansion behaviors of double Shockley SFs (DSSFs) in heavily nitrogen-doped 4H-SiC crystals during high temperature annealing and revealed several important aspects of DSSF expansion.

Temperature Dependence of Double Shockley Stacking Fault Behavior in Nitrogen-Doped 4h-Sic Studied by In-Situ Synchrotron X-Ray Topography

We observed the behavior of double Shockley stacking faults (DSFs) in 4H-SiC crystals with nitrogen concentrations of 1.0×1019-2.6×1019 cm−3 over an extensive temperature range (1380-1910 K) by in-situ synchrotron X-ray topography. For a nitrogen concentration of 2.6×1019 cm−3, the expansion velocity of the DSFs exponentially increased with temperature for temperatures in the range from 1370 to 1650 K. In contrast, at a nitrogen concentration of 1.0×1019 cm−3, this velocity decreased above 1610 K and the DSFs shrank above 1730 K. The DSF energy, considered to be the driving force for DSF expansion, was quantitatively estimated from the radius of the curvature of bowed-out partial dislocations (PDs) pinned by threading screw dislocations (TSDs), showing a positive temperature dependence and lying in the range from −0.6 to 0.8 mJ/m2 at a nitrogen concentration of 1.0×1019 cm−3. The DSF expansion and shrinkage behavior can be understood by the simple temperature and nitrogen concentration dependence of the DSF energy.

Glide of C-core partial dislocations along edges of expanding double-Shockley stacking faults in heavily nitrogen-doped 4H-SiC

The glide of C-core partial dislocations (PDs) that enclose double-Shockley stacking faults (DSFs) expanding in heavily nitrogen-doped 4H-SiC crystals was studied experimentally. We successively annealed a heavily doped SiC crystal with a nitrogen concentration of 4.9 × 1019 cm−3, and the DSF expansion during high-temperature annealing was investigated with photoluminescence (PL) imaging and synchrotron X-ray topography techniques. The glide velocities of 90° and 30° C-core PDs for DSF expansion in the temperature ranges of 600 °C–725 °C and 975 °C–1150 °C, respectively, were evaluated by collating the PL and synchrotron X-ray topography images. Finally, the activation energies and pre-exponential factors for the gliding of the C-core PDs were obtained from the temperature dependences of the dislocation velocities.

(Invited) Unified Understanding of the Shockley Stacking Fault Formation in 4H-SiC Crystals under Thermal Equilibrium and Non-Thermal Equilibrium Conditions Based on the Quantum Well Action Concept

It is well-known that SiC crystal deficiencies are delaying the market penetration of outstandingly superior SiC power devices. Efforts to date have centred on eradicating extended defects from 4H-SiC bulk crystals and epitaxial thin films. Among the extended defects existing in these materials, basal plane stacking faults (SFs) are one of the most serious concerns in SiC power devices because they are formed during the forward-bias operation of 4H-SiC PiN diodes and detrimental to SiC power devices, such as SiC MOSFETs as well as SiC BJTs and PiN diodes. In this phenomenon, i.e., the so-called “bipolar degradation”, single Shockley-type SFs (SSSFs) expand via the recombination of minority carriers (holes) at the partial dislocations in the Si core, which encompass the SSSFs. Recombination-enhanced dislocation glide (REDG) is widely accepted as the most probable kinematic mechanism of this phenomenon, and several other kinematic and energetic effects have been discussed in conjunction with the REDG mechanism.Another negative effect of SF formation in SiC crystals is the spontaneous formation of double Shockley SFs (DSSFs) in heavily nitrogen-doped 4H-SiC crystals, which largely increases the resistivity of the crystals. When the nitrogen concentration exceeds the critical value (2–3 × 1019 cm−3), DSSFs are spontaneously formed in the crystals during high-temperature annealing (> 1100°C). DSSFs have been shown to heal (shrink) upon annealing at even higher temperatures (1800°C), and the co-doping of aluminum acceptors tends to reduce the density of DSSFs in heavily nitrogen-doped 4H-SiC crystals.The above-mentioned two phenomena can be explained based on the concept of quantum well action (QWA). In the QWA, electrons in the conduction band of 4H-SiC enter the SF-induced quantum well, consequently decreasing the electronic energy of the system. However, from the viewpoint of thermodynamics, the two phenomena differ from each other. Whereas DSSF formation is a thermodynamically equilibrium process, SSSF expansion in SiC PiN diodes occurs under non-equilibrium conditions, such as the forward biasing of the PiN diode or ultraviolet light illumination of the materials. In this respect, it is worthy to note that SSSF expansion only occurs under non-equilibrium conditions and has never been observed in thermal equilibrium even when the nitrogen concentration in 4H-SiC crystals exceeds 1 × 1020 cm−3.In this paper, I will try to clarify the physical origin of the driving force of the SSSF expansion during the forward-biasing of 4H-SiC PiN diodes via theoretical investigation of the energetics of the SF formation in 4H-SiC crystals under non-equilibrium conditions. Unified understanding of the SF expansion under thermal equilibrium and non-thermal equilibrium conditions will also be attempted based on the QWA concept. The similarity and dissimilarity in underlying physics between the SF formation in forward-biased 4H-SiC PiN diodes and that in heavily nitrogen-doped 4H-SiC crystals during high temperature annealing are discussed, taking into account the electron trapping into the SF and the resulting positive charge formation in the materials. The difference in the dimensionality of the electron systems in SFs (two-dimensions) and bulk 4H-SiC (three-dimensions) provides different energy dependencies of the density of states for electrons in the two systems and plays an important role in the temperature dependence of the SF expansion in 4H-SiC crystals.

Glide velocities of Si-core partial dislocations for double-Shockley stacking fault expansion in heavily nitrogen-doped SiC during high-temperature annealing

We investigated the glide velocities of 30° Si-core partial dislocations for the expansion of double-Shockley stacking faults (DSFs) in heavily nitrogen-doped 4H-SiC crystals at high temperatures of approximately 1000 °C. The heavily doped epilayers grown by chemical vapor deposition were successively annealed. The expansion of DSFs in the heavily doped epilayers was tracked by a photoluminescence (PL) imaging technique. From the PL images obtained after each annealing treatment, the glide velocities of the 30° Si-core partial dislocations were estimated. In particular, temperature dependence and nitrogen-concentration dependence of the dislocation velocities were obtained. We also report the influence of the strain energies of the bounding dislocations on the velocities. Based on the experimental results, the quantitative expression of the dislocation glide is discussed.

Immobilization Phenomenon of Partials Surrounding Double Shockley Stacking Faults in Heavily Nitrogen Doped 4H-SiC Crystal with Thermal Anneal

The expansion behavior of double Shockley stacking faults (DSFs) was investigated in heavily nitrogen doped 4H-SiC crystals at high temperatures up to 1350°C. An immobilization phenomenon of partials surrounding DSFs was discovered by a thermal annealing at temperatures over 1275°C. The electric properties of SiC crystal were maintained after the partial dislocations were immobilized with a high temperature annealing. The mobile partial dislocations extended straight, but the immobile ones bent toward the glide direction. This immobilization phenomenon is significant and useful for achieving low-resistance SiC substrates without DSFs.

Characterization of Strain Due to Nitrogen Doping Concentration Variations in Heavy Doped 4H-SiC

Highly doped 4H-SiC will show a significant lattice parameter difference with respect to the undoped material. We have applied the recently developed monochromatic contour mapping technique for 4H-SiC crystals to a 4H-SiC wafer crystal characterized by nitrogen doping concentration variation across the whole sample surface using a synchrotron monochromatic x-ray beam. Strain maps of 0008 and − 2203 planes were derived by deconvoluting the lattice parameter variations from the lattice tilt. Analysis reveals markedly different strain values within and out of the basal plane indicating the strain induced by nitrogen doping is anisotropic in the 4H-SiC hexagonal crystal structure. The highest strain calculated along growth direction [0001] and along [1-100] on the closed packed basal plane is up to − 4 × 10−4 and − 2.7 × 10−3, respectively. Using an anisotropic elasticity model by separating the whole bulk crystal into numerous identical rectangular prism units, the measured strain was related to the doping concentration and the calculated highest nitrogen level inside wafer crystal was determined to be 1.5 × 1020 cm−3. This is in agreement with observation of double Shockley stacking faults in the highly doped region that are predicted to nucleate at nitrogen levels above 2 × 1019 cm−3.

Studies on Doping Concentration Variations in 4H-SiC Substrates Using X-ray Contour Mapping

Highly doped 4H-SiC will show a significant lattice parameter difference with respect to the undoped material. We have applied the recently developed monochromatic contour mapping technique for 4H-SiC crystals to a 4H-SiC wafer crystal characterized by nitrogen doping concentration variation across the whole sample surface using the synchrotron monochromatic X-ray beam. Strain maps of 0008 and -2203 planes were derived by deconvoluting the lattice parameter variations from the lattice tilt. Analysis reveals markedly different strain values within and out of the basal plane indicating the strain induced by nitrogen doping is anisotropic in the 4H-SiC hexagonal crystal structure. The highest strain calculated along growth direction [0001] and along [1-100] on the closed packed basal plane is up to -4x10-4 and -2.7x10-3 respectively. Using an anisotropic elasticity model by separating the whole bulk crystal into numerous identical rectangular prism units, the measured strain was related to the doping concentration and the calculated highest nitrogen level inside wafer crystal was determined to be 1.5x1020cm-3. This is in agreement with observation of double Shockley stacking faults in the highly doped region that are predicted to nucleate at nitrogen levels above 2x1019 cm-3.

Structural analysis of double-layer Shockley stacking faults formed in heavily-nitrogen-doped 4H-SiC during annealing

We investigated the structures and expansion behavior of double-Shockley stacking faults (DSFs) formed in heavily nitrogen-doped 4H-SiC during annealing. Heavily doped epilayers prepared as specimens were successively annealed. Various types of DSFs showing different shapes and dislocation contrasts were found in photoluminescence and synchrotron X-ray topography images. Taking account of every possible stacking sequence forming DSFs, the structures of various types of DSFs were determined from observations by plan-view transmission electron microscopy (TEM) and cross-sectional high-angle annular dark-field scanning TEM. We found that a bounding dislocation enclosing a DSF splits into two partial dislocations (PDs), and their Burgers vectors are identical, while the distance of the two PDs depended on their core structures (30° Si-, 30° C- or 90° C-core). We also discussed the contrast rule for the dislocation consisting of two PDs in the synchrotron X-ray topography images and the mobile PDs for the DSF expansion in the epilayers with different nitrogen concentrations.

Observation of double Shockley stacking fault expansion in heavily-nitrogen-doped 4H-SiC using PL technique

The expansion of double-Shockley stacking faults (DSFs) in 4H-SiC was investigated by a photoluminescence (PL) imaging technique. To observe DSFs nondestructively, heavily-nitrogen-doped epilayers were prepared to be used as specimens and the PL technique was applied for the observation. The size and shape of the DSFs in the epilayer were clearly distinguished in the two dimensional PL images, although it is normally difficult to observe luminescence from DSFs in substrates grown by the sublimation method. The nucleation and expansion of the DSFs were tracked in the course of successive high-temperature annealing. Dislocation velocities of Shockley partials along the edge of the DSFs were evaluated from the PL observations. Activation energy for the dislocation glide was estimated from the temperature dependence of the velocities. The nucleation sites of the DSFs are also discussed.

(Invited) Study of Minority Carrier Lifetime Killer by Synchrotron X-Ray Topography

The lifetime of minority carrier is an important property of a semiconductor material that controls the performance of high voltage/ high power and frequency devices. For instance, in high power device applications, a longer carrier lifetime is desirable to mitigate power loss under forward bias. By comparison, a shorter lifetime is more suitable in high frequency devices as it help reduce power loss during switching cycles. Despite recent studies linking structural defects, growth parameters and material thickness to variation in carrier lifetime in silicon carbide epitaxial layers [1-3], little is known about these lifetime limiting defects/parameters and their influence on carrier recombination because of variability in measurement. Therefore, systematically identifying these factors will shed light into ways to mitigate/annihilate their effects. In this paper, we first report on the influence of structural defects on minority carrier lifetime in n-type 4H-SiC epitaxial layers. The lifetime maps for a 2” quarter wafer (sample 1) and a 4” diameter half wafer (sample 2) were recorded using microwave photoconductive decay (μPCD) measurements and correlated with the synchrotron X-ray topography (SXRT) map of structural defects. The lifetime map of sample 1 revealed a drastic reduction in carrier lifetime along the edges and inside one of its facets (facet 1), which was correlated with the presence of microcracks, BPD loops, overlapping triangular defects, high density of multi-layer Shockley stacking faults (SFs) and low angle grain boundaries (LAGBs). Like sample 1, sample 2 showed a reduced carrier lifetime along the edges, which corresponded to regions of high density of overlapping triangular defects, microcracks and BPD loops in the SXRT map. Furthermore, a shorter lifetime was observed in the middle region of sample 2. Synchrotron X-ray topography map correlated this shorter lifetime to networks of interfacial dislocations (IDs) and half loop arrays (HLAs) originating from 3C inclusions that were generated during epilayers growth. Following further analysis of the SXRT maps, it was observed that the stacking faults in facet 1 of sample 1 were double Shockley stacking faults (DSSFs) that have likely nucleated from scratches present on the substrate surface and LAGBs present in that region, which propagated during epilayer growth. The mechanism of formation of these DSSFs is discussed in light of an electronic model proposed by Kuhr [4] and observed by Pirouz [5]. The mechanism of formation of the IDs and HLAs is also discussed. We discuss the mitigated influence of numerous morphological defects observed on the surface of both epilayer samples. Finally, the implication of these observations on the selection and processing of silicon carbide substrates for epilayer growth is addressed in light of the lifetime maps. 1. J. Hassan and J. P. Bergman, J. Appl. Phys. 105 (12), 2009 2. Louisa Lija, J. Hassan, I. Booker, P. Bergman and E. Janzen, Mat. Sci. Forum 740-742, 2013 3. B. Kallinger, P. Berwian, J. Friedrich, M. Rommel, M. Azizi, C. Hecht and P. Friedrichs, Mat. Sci. Forum 740-742, 2013 [4] P. Pirouz, et al., Phil. Mag. 86, 2006. [5] T. A. Kuhr et al., J. Appl. Phys. 92, 2002.

(Invited) Study of Minority Carrier Lifetime Killer by Synchrotron X-Ray Topography

The lifetime maps for a 2” n-type 4H-SiC quarter wafer (sample 1) and a 4” diameter half wafer (sample 2) were recorded using microwave photoconductive decay (μPCD) measurements and correlated with the synchrotron X-ray topography (SXRT) map of structural defects. The lifetime map of sample 1 revealed a drastic reduction in carrier lifetime along the edges and inside one of its facets (facet 1), correlated with microcracks, BPD loops, overlapping triangular defects, high density of multi-layer Shockley stacking faults (SFs) and low angle grain boundaries (LAGBs). Similarly, sample 2 showed a reduced carrier lifetime along the edges corresponding to regions of high density of overlapping triangular defects, microcracks and BPD loops in the SXRT map. Shorter lifetime observed in the middle region of sample 2 correlated with networks of interfacial dislocations (IDs) and half loop arrays (HLAs) originating from 3C inclusions that were generated during epilayers growth. Further analysis of the SXRT maps revealed the SFs in facet 1 were double Shockley stacking faults (DSSFs) that have likely nucleated from scratches present on the substrate surface and LAGBs present in that region, which propagated during epilayer growth. We discuss the mitigated influence of numerous morphological defects observed on the surface of both epilayer samples.

Correlation of Lifetime Mapping of 4H-SiC Epilayers with Structural Defects Using Synchrotron X-Ray Topography

Lifetime maps for two 4H-SiC epi-wafers (samples 1 and 2) were recorded using microwave photoconductive decay (μPCD) measurements and correlated with the type and distribution of structural defects mapped by synchrotron X-ray topography (white beam and monochromatic). Sample 1 showed lower lifetime inside one of its higher doped facet regions and along its edges. The low lifetime in the facet region was associated with the presence of a high density of multi-layered Shockley stacking faults (SFs) and low angle grain boundaries (LAGBs). These stacking faults are likely double Shockley stacking faults (DSSFs) and probably nucleated from scratches present on the substrate surface and LAGBs present in that region, propagating during epilayer growth. In contrast, sample 2 showed a reduced carrier lifetime in the middle region associated with a network of interfacial dislocations (IDs) and half loop arrays (HLAs) originating from 3C inclusions that are generated during epilayer growth. Along the edges of both samples, overlapping triangular defects, microcracks and BPD loops lowered lifetime.

Experimental verification of the model for formation of double Shockley stacking faults in highly doped regions of PVT-grown 4H–SiC wafers

We recently reported on the formation of overlapping rhombus-shaped stacking faults from scratches left over by the chemical mechanical polishing during high temperature annealing of PVT-grown 4H–SiC wafer. These stacking faults are restricted to regions with high N-doped areas of the wafer. The type of these stacking faults were determined to be Shockley stacking faults by analyzing the behavior of their area contrast using synchrotron white beam X-ray topography studies. A model has been proposed to explain the formation mechanism of the rhombus shaped stacking faults based on double Shockley fault nucleation and propagation. In this paper, we have experimentally verified this model by characterizing the configuration of the bounding partials of the stacking faults on both surfaces using synchrotron topography in back reflection geometry. As predicted by the model, on both the Si and C faces, the leading partials bounding the rhombus-shaped stacking faults are 30° Si-core and the trailing partials are 30° C-core. Using high resolution transmission electron microscopy, we have verified that the enclosed stacking fault is a double Shockley type.

Double Shockley Stacking Fault Formation in Higher Doping Regions of PVT-Grown 4H-SiC Wafers

In this study of 4H-SiC wafers, we report the observation of overlapping Shockley stacking faults generated from residual surface scratches in regions of higher doping concentration after the wafers have been subjected to heat treatment. The stacking faults are rhombus shaped and are bound by partial dislocations. The process is driven by the fact that a faulted crystal containing double Shockley faults is more stable than perfect 4H–SiC crystal at the high temperatures (>1000 C) that our samples have been subjected to during heat treatment. We have developed a model for the formation mechanism of the rhombus shaped stacking faults and the partial dislocation configuration has been verified using back reflection synchrotron topography. Our studies show that double Shockley faults can be generated in regions where dislocation sources are presents (eg. scratches or low angle boundaries) and when the nitrogen doping concentration exceeds a certain level when the crystals are subjected to heat treatment.