Dislocations In 4H-SiC Research Articles

The effect of shear on nucleation and movement of basal plane dislocations in 4H-SiC

The effect of shear on nucleation and movement of basal plane dislocations in 4H-SiC

Comparative study of methods for counting of dislocations in 4H-SiC

Three experimental techniques to determine the density of the three relevant dislocation types (basal plane, threading edge and threading screw) in SiC were investigated: etching by molten KOH with and without the addition of Na2O2 and X-ray topography. The applicability to implement these techniques for a reliable full-wafer scale analysis is tested experimentally. The limitations are found to be mainly determined by the feature sizes. Based on simple considerations, the range of acceptable sizes of the dislocation-related features is estimated. Etching with Na2O2 allows to distinguish between all three types of dislocations, but a reliable detection is difficult to set up due to a too large spread of etch pit size. Without the addition of Na2O2, TSD and TED cannot be distinguished, but etch pit sizes become more homogeneous, enabling robust etch pit detection over a wider range of dislocation densities. X-ray topography gives reliable results for TSDs and BPDs whereas the measurement of TEDs is limited.

Nucleation of Threading Dislocations in 4H-SiC at Early Physical-Vapor-Transport Growth Stage

In this work, we identify the nucleation mechanism of threading dislocations (TDs) associated with stacking faults (SFs) and 15R-SiC at the early growth stage of 4H-SiC single crystals grown by the physical vapor transport (PVT) technology. By combining molten KOH etching and photochemical etching, we successfully reveal etch pits of TDs and linear etch patterns of SFs on the (112̅0) surface of 4H-SiC single crystals. Systematic investigations based on transmission electron microscopy (TEM) observations and Raman analysis indicate that the Si–C bilayer stacking sequence of SFs is (3, 2) in Zhdanov’s notation. The accumulation of SFs (3, 2) gives rise to the polymorph fluctuation and thus the formation of 15R-SiC at the early PVT growth stage of 4H-SiC single crystals. Quantitative stress analyses indicate that the strain field distributions along the SFs (3, 2) and the 15R-/4H-SiC interfaces are inhomogeneous, which give rise to the nucleation of TDs and low-angle grain boundaries (LAGBs), respectively. The nucleation of LAGBs releases the high stress at the 15R-/4H-SiC interface, which facilities the following 4H-SiC single-crystal growth. Our work indicates that the avoidance of polymorph fluctuation is important to the reduction of TDs at the early growth stage of PVT-grown 4H-SiC single crystals.

Investigation of dislocations in 4H-SiC by g•b contrast analysis using TEM

Investigation of dislocations in 4H-SiC by g•b contrast analysis using TEM

Discrimination of dislocations in 4H-SiC by inclination angles of molten-alkali etched pits

Discrimination of dislocations is critical to the statistics of dislocation densities in 4H silicon carbide (4H-SiC), which are routinely used to evaluate the quality of 4H-SiC single crystals and homoepitaxial layers. In this work, we show that the inclination angles of the etch pits of molten-alkali etched 4H-SiC can be adopted to discriminate threading screw dislocations (TSDs), threading edge dislocations (TEDs) and basal plane dislocations (BPDs) in 4H-SiC. In n-type 4H-SiC, the inclination angles of the etch pits of TSDs, TEDs and BPDs in molten-alkali etched 4H-SiC are in the ranges of 27°−35°, 8°−15° and 2°−4°, respectively. In semi-insulating 4H-SiC, the inclination angles of the etch pits of TSDs and TEDs are in the ranges of 31°−34° and 21°−24°, respectively. The inclination angles of dislocation-related etch pits are independent of the etching duration, which facilitates the discrimination and statistic of dislocations in 4H-SiC. More significantly, the inclination angle of a threading mixed dislocations (TMDs) is found to consist of characteristic angles of both TEDs and TSDs. This enables to distinguish TMDs from TSDs in 4H-SiC.

Investigation into the Influence of Substrate Dislocations in 4H-SiC on the Subsequent Epitaxy and Resultant Device Performance

As the demand for SiC based devices continues to grow, there is an aggressive push to drive down sources of yield loss and device failures. One route to mitigate this issue is via connection of device yield loss to defects in the bare substrate material. Here, high throughput X-ray topography in conjunction with KOH etching and photoluminescence imaging were utilized to detect and classify various substrate dislocations. Subsequent epitaxial layers, grown on the substrates from various vendors, were analyzed using a confocal microscope to determine which substrate defects propagated into the epi. Data indicates that there is a good correlation between substrate BPDs nucleating into epitaxial stacking faults. This is supported by data from hundreds of different boules demonstrating that there is a relationship between the number of BPDs in the substrate and the number of partials and stacking faults detected in the resultant epitaxial layer. BPDs are not the only substrate dislocation nucleation source as the data also suggests other parallel sources are involved. Thousands of MOSFET and Diode devices were analyzed to understand the influence that substrate defects, propagated to the epitaxial layers, have on device leakage and yield loss. It was determined that MOSFET devices are more sensitive to the stacking faults that nucleate from the substrate dislocations compared to Diodes. While most of the stacking faults and other dislocations that occur in the epi layers do not independently result in device failures or leakage, an increase in the density of these defects within a die, can lead to increased leakage and decreased device performance. The inherent variability of these relationships requires the analysis of very large numbers of wafers to see the clear trends. This work uses a combination of X-ray and photoluminescence detection techniques to trace dislocations in substrates that nucleate into epitaxy layers and result in device leakage and failure over hundreds of wafers and thousands of devices. This correlation between device characteristics and crystal dislocations provides necessary information in determining which defects are most necessary to reduce or eliminate to improve device yield and performance.

Mechanical Load-Induced Atomic-Scale Deformation Evolution and Mechanism of SiC Polytypes Using Molecular Dynamics Simulation.

Silicon carbide (SiC) is a promising semiconductor material for making high-performance power electronics with higher withstand voltage and lower loss. The development of cost-effective machining technology for fabricating SiC wafers requires a complete understanding of the deformation and removal mechanism. In this study, molecular dynamics (MD) simulations were carried out to investigate the origins of the differences in elastic–plastic deformation characteristics of the SiC polytypes, including 3C-SiC, 4H-SiC and 6H-SiC, during nanoindentation. The atomic structures, pair correlation function and dislocation distribution during nanoindentation were extracted and analyzed. The main factors that cause elastic–plastic deformation have been revealed. The simulation results show that the deformation mechanisms of SiC polytypes are all dominated by amorphous phase transformation and dislocation behaviors. Most of the amorphous atoms recovered after completed unload. Dislocation analysis shows that the dislocations of 3C-SiC are mainly perfect dislocations during loading, while the perfect dislocations in 4H-SiC and 6H-SiC are relatively few. In addition, 4H-SiC also formed two types of stacking faults.

Doping-dependent nucleation of basal plane dislocations in 4H-SiC

Basal plane dislocations (BPDs) are one of the most harmful dislocations in 4H silicon carbide (4H-SiC). Understanding the nucleation of BPDs is the basis of reducing the density of BPDs in 4H-SiC. In this work, we investigate the nucleation mechanism of BPDs, as well as the effect of doping on the nucleation of BPDs in 4H-SiC using nanoindentation. It is found that the shear stress plays a dominant role in the nucleation of BPDs in undoped 4H-SiC. This indicates that the shear component of the thermal stress during the growth of 4H-SiC single crystals and that of the mechanical stress during the processing of 4H-SiC wafers both give rise to the nucleation of BPDs. Nitrogen (N) doping is found to facilitate the nucleation of BPDs and decrease the shear stress required for the nucleation of BPDs. In contrast, vanadium (V) doping hinders the nucleation of BPDs, which promotes the polymorph transition from 4H-SiC to 3C-SiC.

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.

Discrimination of Dislocations in 4H-SiC by Inclination Angles of Molten-Alkali Etched Pits

Discrimination of Dislocations in 4H-SiC by Inclination Angles of Molten-Alkali Etched Pits

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.

Direct evaluation of threading dislocations in 4H-SiC through large-angle convergent beam electron diffraction

ABSTRACTIn this study, the structures of threading dislocations (TDs) in a 4H-SiC epilayer were directly characterised using large-angle convergent beam electron diffraction (LACBED) via transmission electron microscopy (TEM), by examining the splitting of reflections due to the presence of dislocation lines. Previously, X-ray topography (XRT) mapping in combination with ray-tracing simulations has been shown to be an efficient method for imaging and differentiating common TD types and variants in 4H-SiC. In this work, the validity of XRT-based results was verified by LACBED through direct evaluation and differentiation of the Burgers vectors and physical crystallographic features of all threading edge, threading screw, and threading mixed dislocations variants. The LACBED results agreed with the XRT-based results with respect to TD type and variant. The screw-sense or half-plane orientation and Burgers vector determined from LACBED analysis for each TD were self-consistent. For the case of -TEDs, atomic-resolution analysis of the core structure, through high-angle annular dark field scanning TEM, directly revealed two half-planes, each with a Burgers vector component, and the sum of these components was confirmed to be . Finally, an algorithm for efficient LACBED analysis of TDs in 4H-SiC was discussed.

Initiation of Shockley Stacking Fault Expansion in 4H-SiC P-i-N Diodes

To understand the effects of temperature and injection current density on expansion of Shockley stacking faults (SSFs) from basal-plane dislocations in 4H-SiC p-i-n diodes, the threshold current density for SSF expansion was investigated at eight temperatures by electroluminescence image observation. The threshold injection current density was found to decrease at lower temperatures and to increase at higher temperatures. We identified the origin of this temperature dependence and found that the limiting factor for expansion differed depending on the temperature.

High-resolution X-ray topography of threading edge dislocations in 4H-SiC using a novel nuclear emulsion film improved special resolution and sensitivity

We developed a nuclear emulsion film in order to improve the spatial resolution of X-ray topography (XRT). The film allowed detection of threading edge dislocations (TEDs) in a 4H-SiC single-crystal substrate over a large area by grazing-incidence monochromatic XRT at a practical level of sensitivity. The directions of the Burgers vectors of the observed TEDs were also determined.

Two-photon-excited, three-dimensional photoluminescence imaging and dislocation-line analysis of threading dislocations in 4H-SiC

This paper describes the three-dimensional imaging of threading edge dislocations (<a>TEDs), pure c threading screw dislocations (<c>TSDs), and c + a threading mixed dislocations (<c + a>TMDs) in 4H silicon carbide (4H-SiC) epilayers by two-photon-excited photoluminescence (2PPL), where inclinations of dislocations in epilayers with different offcut angles are investigated. A numerical model based on carrier diffusion explains the mechanism of the dark-contrast 2PPL imaging provided by band edge emission. More than 450 threading dislocations in 4H-SiC epilayers are visualized three-dimensionally and are classified into six <a>-type TEDs, two <c>-type TSDs, and twelve <c + a>-type TMDs according to the directions of extra half-planes and the chiral geometries (right- or left-handed screw). The 2PPL images reveal that all threading dislocation types incline in the step-flow direction, the extent of which is more prominent for a larger offcut angle. The <a>TEDs also incline in the directions of their extra half-planes. The inclinations of <c>TSDs perpendicular to the step-flow direction are influenced by chirality, while those of <c + a>TMDs are influenced by both the directions of the extra half-planes and the chirality. The origins of the dislocation inclinations are discussed taking into account the kinetic interactions between the dislocations and the advancing steps on the growing surface.

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.

Determination of observable depth of dislocations in 4H-SiC by X-ray topography in back reflection

We performed X-ray topography using the asymmetric back-reflection by changing the incident X-ray energy (E) in order to evaluate the observable depth (tobs) of dislocations in 4H-SiC(0001) wafers with a 4° off-axis angle toward the []-direction. Since the incident angle (ω) of the X-rays satisfying the Bragg condition changed along with E, the observable length of the straight segment of the basal plane dislocation (BPD) on the X-ray topographs also changed. The tobs values were determined from the observable length of the straight BPD segment, and consequently, plots of tobs versus E, and tobs versus ω were obtained. We discussed the relationships between tobs and the absorption depth (tμ), between tobs and the extinction depth (tΛ), and between tobs and tΛ/tμ as criteria of the dislocation visibility, and we found that the plot of tobs versus tμ was described as tobs ∼ 0.75tμ with good linearity.

Elementary Screw and Mixed-Type Dislocations in 4H-SiC Characterized by X-Ray Topography Taken with Six Equivalent 11-28 <i>g</i>-Vectors and a Comparison to Etch Pit Evaluation

We have studied threading dislocations (TDs) in 4H-SiC by means of X-ray topography (XRT) taken under 6 equivalent g-vector of 11-28 and two different chemical etching methods. Threading screw and mixed-type dislocations (TSDs and TMDs) can be distinguished and the direction of the a-components of TMDs can be determined by XRT. Efforts have been made to examine if there are features of etch pits that can be used to distinguish TMDs from TSDs.

4H-SiCエピ層中の転位

Properties and behavior of dislocations in 4H-SiC are presented based on classical theory of dislocations. 1) A new symbol which is an extension of the famous Frank symbol is introduced to explain complex stacking sequence in SiC polytypes. 2) A basal plane dislocation (BPD) is dissociated into two Shockley partials. Polarity of BPD is described. 3) Principles to assign Burgers vectors and polarity to the individual Shockley partials are derived. 4) Threading screw dislocations (TSD) can be dissociated into four partials, thereby reducing the total energy considerably. 5) Deviation of threading dislocations (TD) from the exact [0001] orientation is reasoned from the viewpoint of energy consideration. 6) Transition from BPD to threading edge dislocations (TED) is explained by taking into account effects of the crystal surface. All of behavior can be explained quite well in terms of classical theory of dislocations.

Strain energy analysis of screw dislocations in 4H-SiC by molecular dynamics

We simulated screw dislocations with the Burgers vector parallel to the [0001] direction in 4H-SiC by a classical molecular dynamics method. A stable structure of an extended dislocation generated by the dissociation of a screw dislocation was identified by calculating the strain energy caused by dislocation cores and stacking faults. As a result, we conclude that the most expected structure of the extended dislocation is made of partial dislocations with the Burgers vector b = 1/2c + 1/2c (c is equal to the thickness of one period in the c-axis direction of 4H-SiC) and the stacking fault that is parallel to the a-plane, and that the distance between the dislocation cores is less than about 44 Å.