Analysis of Defect Structures During the Early-Stages of PVT Growth of 4H-SiC Crystals

. To better understand the effects of various growth parameters during the early-stages of PVT growth of 4H-SiC on resulting defect structures, multiple short duration growths have been carried out under varying conditions of seed quality, nucleation rate, thermal gradients


Introduction
Silicon Carbide (SiC), typically 4H-SiC, due to its excellent electrical and thermal properties, has been replacing conventional silicon materials, which are considered to have reached their limit, for high power and high frequency applications [1].The reliability of 4H-SiC power devices is highly dependent on the quality of the bulk crystals and epitaxial layers.Over the past decade, considerable progress has been made to enhance the quality of the crystals grown by the physical vapor transport (PVT) method and defect densities have decreased dramatically [2].So far, PVT-grown 4H-SiC substrates with diameter of 150mm are commercially available and 200mm diameter SiC has been demonstrated [3].Nevertheless, improvements in quality of SiC wafers and further lowering of defect densities are still in urgent need for growing automotive and energy saving applications.
Various types of defect existing in 4H-SiC, such as micropipes (MPs), threading screw/mixed dislocations (TSDs/TMDs), threading edge dislocations (TEDs), and basal plane dislocations (BPDs), have varying detrimental effects on the performance of 4H-SiC power devices [4].Behavior and formation of these defects in substrates have been extensively studied [5].A recent approach to lowering defect densities is to closely examine the early stages of crystal growth when many defects are nucleated.Nucleation of TEDs and TSDs at the initial growth stage is revealed by Sanchez et al. [6].Ohtani et al. [7] observed convex bending of (0001) basal plane in the growth direction near the seed/boule interface.Ailihumaer et al. [8] observed dislocations with unique configurations at the initial growth stage, which is associated with deflection of TMDs and TEDs.Recently, we have reported on the expansion of Shockley stacking faults in the facet region during the early growth stage [9].Continuing these early-stage studies, we seek to better understand the effects of various growth parameters on resulting defect structures.Multiple short duration growths have been carried out under varying conditions of nucleation temperature, thermal gradients, and N incorporation.Synchrotron xray topography techniques, in combination with TEM and various microscopic and spectroscopic methods, are employed to evaluate defect characteristics during growth, which provide more insights on dislocation nucleation and multiplication forming the basis for the subsequent growth of the full crystal boule.

Experimental
Three 4°off-axis (0001) PVT-grown SiC short crystals, which have newly grown layers with thicknesses of only several hundred microns grown on the seed under varying conditions, were imaged by synchrotron monochromatic beam x-ray topography (SMBXT) in grazing incidence geometry at beamline 1-BM, Advanced Photon Source at Argonne National Laboratory.X-ray topographic images of 112 � 8, 224 � 16 and 22 � 0 10 reflections were recorded on the Agfa Structurix D3-SC films.Based on . = 0 invisibility criterion, TSDs, TEDs and BPDs are visible in 112 � 8 and 224 � 16 reflections.The 224 � 16 reflection provides a penetration depth of about 40µm compared to around 15µm for 112 � 8. Stacking faults are visible in 22 � 0 10 reflection based on the g.R criteria [10].Cross-sectional transmission electron microscopy (TEM) was conducted on specimens encompassing stacking faults by FEI Talos 200x.Specimens were prepared by FEI HELIOS 600 Dual Beam focused ion beam (FIB) perpendicular to the step-flow [112 � 0] direction.High-angle annular dark-field (HAADF)-scanning transmission electron microscopy (STEM) was used to examine the stacking sequence of the TEM samples at atomic level.Macroscopic morphology of the newly grown surface was examined by Nomarski differential interference contrast (DIC) optical microscopy.Individual dislocation images on x-ray topographs were simulated by ray tracing simulation method [11], which was conducted by Python3 software on personal computer.

Results and Discussion
Early-stage growth 4H-SiC wafers are characterized by a distribution of TEDs, TSDs/TMDs, as well as Frank dislocations as shown in Fig. 1(a) where most TSDs/TMDs and TEDs are replicated from the seed while pairs of TEDs and TSD/TMD are also observed across the wafers likely nucleated at the seed/newly grown layer interface.Similar observations have been reported by Ailihumaer et al. [8].Of particular significance is the observation of stacking fault formation in these early-stage growth wafers as revealed in the 22 � 0 10 reflection [12] in Fig. 1(b), where multiple area contrast features indicative of different types of stacking faults are observed.
Stacking Fault Type 1 and Type 2. In the wafer grown under high nitrogen partial pressure condition, as shown in Fig. 2(a), a stacking fault region (enclosed in yellow rectangle) shows area contrast at the down step side where it intersects the wafer surface.This fault is bound by two Frank partial dislocations with black/white contrast.The one bounding the top side of the fault has white contrast at the top and black contrast at the bottom and the one bounding the bottom side has black contrast at the top and white contrast at the bottom.This stacking fault labeled as Type 1 is also observed in a wafer grown under low nitrogen partial pressure growth condition (marked by yellow rectangle) as shown in Fig. 2(b).

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Defects of Solid Semiconductor Structures Stacking Fault Type 3. A DIC optical image of a morphological defect labeled as a 'carrot' defect (due to similarity to carrot defects in epilayers [16,17]) observed on early-stage growth wafers is shown in Fig. 3(a).This carrot defect is bounded by Frank partial dislocations as shown in the 112 � 8 grazing incident topograph in Fig. 3  To further investigate the origin of this carrot defect, TEM and STEM experiments were conducted.The TEM sampling position is at the right side of the carrot defect as indicated by the solid blue line in Fig. 4(a).The cross-sectional TEM and STEM image is adjusted to that viewed from the down-step side towards the up-step side as indicated by the black arrow in Fig. 4   To identify the stacking fault vector for the basal plane stacking fault, the method developed by Wu et al. [18] was adopted.The method includes the following steps: 1) A one-way path across the faulted region is drawn in the distorted material lattice, starting from a perfect 4H region and ending in a perfect 4H region; 2) This path is repeated in the perfect lattice.Evidently, the path will deviate from the perfect lattice at some point.Each time the path deviates from the perfect lattice a vector (as small as possible) is used to correct the path; 3) After completing the path, we simply sum all the vectors used to correct the path to get the fault vector of the stacking fault.Based on this method, the one-way path drawn for the basal plane stacking fault associated with carrot defect is represented by red line in Fig. 4(d).Two vectors marked by green arrows are used to correct the path, which are S and 3c/4.The fault vector for the basal plane stacking fault from carrot defect is determined as 3c/4+S, which is 1/12[44 � 09].
To investigate further, an additional TEM samples was prepared for the other side of the carrot defect to investigate the nature of the prismatic stacking fault.The positions of the TEM samples are indicated by the white dashed rectangles in Fig. 5(a).From low magnification TEM images for sample 1 and sample 2 shown by Fig. 5(b) and (d), prismatic stacking faults are observed at both sides of the carrot defect.The prismatic stacking faults exhibit a peculiar zigzag morphology with planar segments inclined by about 28° from the {11 � 00} plane, allowing identification of habit planes to be the first order pyramidal {11 � 02} planes.Fig. 5(c) and (e) shows the atomic resolution HAADF-STEM image for prismatic stacking fault in sample 1 and sample 2, respectively.In Fig. 5(c), the lattice on the left side shifts 3c/4 to the top with respect to the lattice on the right side across the prismatic stacking fault region in sample 1, which was taken from the left side of the carrot defect.Meanwhile in Fig. 5(e), the lattice on the left side shifts c/4 to the top with respect to the lattice on the right side across the prismatic stacking fault region in sample 2, which was taken from the right side of the carrot defect.Such lattice shift could be caused by the overgrowth of spiral step from a TMD/TSD which separates into 3c/4 and c/4 increments.

Fig. 1 .
Fig. 1.Grazing-incidence synchrotron monochromatic beam x-ray topographs recorded from selected regions of a 4H-SiC early-stage growth wafer in (a) 224 � 16 reflection showing the distribution of TEDs, TSDs/TMDs and Frank dislocations; and (b) 22 � 0 10 reflection showing the area contrast from different types of stacking faults as marked by yellow dashed box.
(b).Comparing with ray tracing simulated images of opposite sign Frank dislocations in Fig. 2(c), the Frank partials bounding the carrot defect have the same sign, implying the carrot defect formation is related to the overgrowth of spiral steps from TSDs/TMDs which separate into c/2 or c/4 increments.Area contrast from stacking fault is observed at down step Defect and Diffusion Forum Vol.434 side of the carrot defect in the 22 � 0 10 reflection topograph.This stacking fault related to carrot defects is labeled as Type 3 stacking fault.

Fig. 2 .
Fig. 2. SMBXT images (g = 22 � 0 10 ) showing area contrast for Type 1 (enclosed in yellow rectangles) and Type 2 (enclosed in red rectangles) stacking faults bounded by Frank partial dislocations for samples grown under (a) high nitrogen partial pressure condition and (b) low nitrogen partial pressure condition.(c) Ray tracing simulated images of Frank dislocations with Burgers vector of +c and -c.(d) Topographic image and ray tracing simulated image of c/2 Frank partial dislocation.Formation mechanism for Type 1 stacking fault is illustrated by (e): overgrowth of a 2D nucleated 6H-SiC by macrostep forming Type 1 stacking fault, and formation mechanism for Type 2 stacking fault is shown by (f): overgrowth of the spiral step of a TMD which separates into c/2+S increments forming Type 2 stacking fault.
(a).In Fig.4(b), the TEM image confirms that the carrot defect consists of a basal plane fault and a prismatic fault whereby the prismatic fault is connected on the right side of the basal plane fault.Fig. 4(c) shows an atomic resolution cross-sectional STEM image of the junction point of the basal plane fault and prismatic stacking fault as marked by the white rectangle in Fig. 4(b).The left side of the image where the basal plane stacking fault is located at has a stacking sequence of AB/A'C'B'/CA/C'B'(2322) while the right side of the image still shows AB/A'C'/AB/A'C' perfect 4H-SiC stacking sequence as indicated by the canted triangles representing tetrahedrons (twinned and untwinned).The prismatic stacking fault is located near the center of the STEM image.

Fig. 4 .
Fig. 4. (a) DIC optical image for one carrot defect.(b) The corresponding low-magnification darkfield cross-sectional TEM image with g=11 � 00 for the carrot defect at the position marked by solid blue line in (a), showing basal plane stacking fault connecting with prismatic stacking fault.(c) Atomic resolution cross-sectional HAADF-STEM image of the area marked by white dashed rectangular in (b), showing the stacking sequence of basal plane stacking fault is (2322).(d) Schematic showing that the fault vector for basal plane stacking fault is identified as 1/12[44 � 09] [18].

Fig. 5 (
f) schematically shows the separation of spiral step into 3c/4 and c/4 that can cause the lattice shift shown by Fig. 5(c) and 5(e).Each red box represents a layer of lattice with one unit cell high and consists of four bilayers.The green box marks where the spiral step separates into 3c/4 and c/4 increments with 3c/4 increment on the left side and c/4 increment on the right side.Prismatic stacking faults are formed at the edge of both 3c/4 and c/4 increments.The lattice shift across the prismatic stacking fault at the 3c/4 increment position is the same as what is observed in Fig. 5(c) while the lattice shift across the prismatic stacking fault at the c/4 increment position is the same as what is observed in Fig. 5(e).

Fig. 5 .
Fig. 5. (a) DIC optical image for a carrot defect.(b)(d) Low resolution TEM image, and corresponding (c)(e) Atomic resolution HAADF-STEM images for sample 1 and sample 2, respectively.(f) Schematic showing the separation of 3c/4 and c/4 spiral step increments can cause the lattice shift observed in (c) and (e).Red boxes represent perfect 4H SiC lattice with four bilayers while green box represents the separated 3c/4 and c/4 increments from a TMD.

Fig. 6 .
Fig. 6.Schematics showing formation mechanism of Type 3 stacking faults.(a) A vicinal step advancing towards the terrace formed by separated 3c/4 and c/4 spiral steps from a TMD.(b) The vicinal step trying to overgrow the terrace.(c) The view direction and location of the schematics in (d).(d) The cross-sectional image of the overgrowth process at the position marked by the red box in (c).(e)STEM image from the view marked in (c).The vicinal step slips onto the terrace and form prismatic stacking faults on the two sides of the terrace connecting to the stacking fault on the basal plane.The stacking sequence across the basal plane stacking fault obtained from the model, which is ABCB'A', correlates to the stacking sequence observed in STEM image shown in (e), in which the left side lattice is perfect 4H SiC lattice while the right side lattice includes the basal plane stacking fault.The red dashed line marks the same row of the lattice to make the observation of different stacking sequence at the two sides clearer.