Formation of basal plane stacking faults on the (0001¯ ) facet of heavily nitrogen-doped 4H-SiC single crystals during physical vapor transport growth

Accepted Manuscript Formation of basal plane stacking faults on the (000 1 ) facet of heavily nitrogendoped 4H-SiC single crystals during physical vapor transport growth Kohei Ohtomo, Nana Matsumoto, Koji Ashida, Tadaaki Kaneko, Noboru Ohtani, Masakazu Katsuno, Shinya Sato, Hiroshi Tsuge, Tatsuo Fujimoto PII: DOI: Reference:

S0022-0248(17)30541-9 http://dx.doi.org/10.1016/j.jcrysgro.2017.09.008 CRYS 24294

To appear in:

Journal of Crystal Growth

Received Date: Revised Date: Accepted Date:

4 July 2017 26 August 2017 8 September 2017

Please cite this article as: K. Ohtomo, N. Matsumoto, K. Ashida, T. Kaneko, N. Ohtani, M. Katsuno, S. Sato, H. Tsuge, T. Fujimoto, Formation of basal plane stacking faults on the (000 1 ) facet of heavily nitrogen-doped 4HSiC single crystals during physical vapor transport growth, Journal of Crystal Growth (2017), doi: http://dx.doi.org/ 10.1016/j.jcrysgro.2017.09.008

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Formation of basal plane stacking faults on the (0001) facet of heavily nitrogen-doped 4H-SiC single crystals during physical vapor transport growth

Kohei Ohtomo1, Nana Matsumoto1, Koji Ashida1, Tadaaki Kaneko1,2, Noboru Ohtani1,2*, Masakazu Katsuno3, Shinya Sato3, Hiroshi Tsuge3, Tatsuo Fujimoto3 1

Kwansei Gakuin University, School of Science and Technology 2-1 Gakuen, Sanda, Hyogo 669-1337, Japan

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Kwansei Gakuin University, R&D Center for SiC Materials and Processes 2-1 Gakuen, Sanda, Hyogo 669-1337, Japan

3

Nippon Steel & Sumitomo Metal Corporation, Advanced Technology Research Laboratories, 20-1 Shintomi, Futtsu, Chiba 293-8511, Japan

Abstract The formation of stacking faults in heavily nitrogen-doped (mid-1019 cm−3) 4H-SiC boules grown by the physical vapor transport (PVT) growth method was investigated by studying _

surface morphologies on the (0001) facet of the boules. Low-voltage scanning electron _

microscopy (LVSEM) observations detected stacking faults on the (0001) facet of heavily nitrogen-doped 4H-SiC crystals. LVSEM and atomic force microscopy (AFM) studies revealed that the stacking faults showed characteristic morphologies, which stemmed from the interaction between stacking faults and surface steps. These observations also revealed that _

heavy nitrogen doping resulted in the nucleation of a number of surface hillocks on the (0001) facet; the hillocks were never observed on the facet of conventionally doped (nitrogen concentration: mid-1018 cm−3) 4H-SiC boules. Furthermore, the hillocks were nucleated only

*

Corresponding author. E-mail address: [email protected] (N. Ohtani)

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on the facet and never observed on the outer regions of the facet. Based on these results, the stacking fault formation mechanism in heavily nitrogen-doped 4H-SiC crystals is discussed.

Keywords: A1. Defects; A1. Surface structure; A2. Growth from vapor; B2. Semiconducting materials

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1. Introduction Ultra-low resistivity (< 7 mcm) 4H-SiC single-crystal substrates are expected to further reduce the on-resistance of SiC power devices. However, a high density of basal plane stacking faults are introduced into heavily nitrogen-doped (more than 2−3 × 1019 cm−3) 4H-SiC crystals. The formation of stacking faults is classified into two types: annealing-induced formation, which occurs in stacking fault-free crystals in the as-grown state [1], and formation during crystal growth such as physical vapor transport (PVT) growth [2] and solution growth [3]. In the latter case, the resultant 4H-SiC crystals contain a number of stacking faults in the as-grown state. Whether stacking faults are formed during the crystal growth process is determined by the nitrogen doping concentration in the crystals. When the nitrogen concentration ranges from 2−3 × 1019 to 5−6 × 1019 cm−3, the grown crystals do not contain stacking faults in the as-grown state. However, upon high temperature annealing, double Shockley-type stacking faults (DSSFs) are introduced in the crystals. The driving force for the DSSF formation is the so-called quantum well action (QWA) mechanism in which electrons in heavily nitrogen-doped 4H-SiC entering stacking fault-induced quantum wells lower the system energy [4-6]. The DSSF formation not only influences the electrical properties of the substrates [7,8], but also severely degrades their geometrical parameters such as substrate flatness [9]. When the nitrogen concentration reaches approximately 5 × 1019 cm−3, the crystals tend to contain stacking faults in the as-grown state, and as the nitrogen concentration increases, the density of stacking faults in the crystals increases. The annealing-induced formation of stacking faults has been studied by several authors [1,8-15], and the mechanism is relatively well understood [15]; however, the stacking fault formation during the crystal growth of heavily nitrogen-doped 4H-SiC crystals is poorly understood, and the mechanism is yet to be clarified. In this study, we investigate the stacking fault formation during the PVT growth of

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heavily nitrogen-doped 4H-SiC boules through the observations of surface morphologies on _

the (0001) facet of the boules, and we report on the characteristic features of stacking fault formation in heavily nitrogen-doped 4H-SiC crystals. The surface morphology on the {0001} facet of SiC boules has been studied by several authors using atomic force microscopy (AFM) [16-19], and some important morphological features have been reported. Heavy doping of nitrogen donors (mid-1019 cm−3) causes several distinct features from those; some of them we will show are caused by the interaction between the stacking faults and surface steps.

2. Experimental procedure _

4H-SiC single-crystal boules were grown on an on-axis (0001) 4H-SiC seed crystal by the PVT growth method. The growth temperature was controlled to be 2350°C, and the inert (argon) gas pressure was maintained constant at 1.4 kPa during growth. The growth temperature was measured at the crucible outer surface near the seed crystal by a two wavelength pyrometer. The seed surface temperature was estimated to be 2300°C using numerical simulation of the temperature profile in the crucible. The grown boules were nitrogen-doped, and they contained nitrogen donors in the ranges of mid-1019 cm−3 for heavily nitrogen-doped 4H-SiC boules and mid-1018 cm−3 for conventionally doped 4H-SiC boules. _

Macroscopic (millimeter scale) morphology of the (0001) facet of 4H-SiC boules was examined by differential interference contrast (DIC) optical microscopy, and the surface morphology assessments with micrometer- and nanometer-scale resolutions were performed by low-voltage scanning electron microscopy (LVSEM) and AFM. LVSEM observations were performed with two electron detectors: one is an annular detector in the lens column, which is referred to as an In-Lens detector, and the other is an Everhart−Thornley (E−T) detector that is laterally located outside. The latter was used to obtain low-energy electron channeling contrast (LE-ECC) images [20] of step-terrace

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structures on the (0001) facet of 4H-SiC boules. The LE-ECC imaging technique proposed by Ashida et al. [20] uses a tilted low-energy primary electron beam for ECC imaging. They found that ECC images acquired at a primary electron beam energy of ~1 kV and a tilt angle of ~30° from the c-axis of SiC crystals toward _

<1100> show a distinct contrast depending on the atomic structure (stacking sequence) of the _

topmost layer of SiC crystals. It was revealed that (0001) terraces terminated with the atomic columns in the topmost layer inclined toward the primary electron beam direction show a bright contrast, whereas those terminated with the atomic columns rotated 180° around the c-axis exhibit a dark contrast. Raman microscopy was employed to identify the polytype of the grown SiC crystals using a confocal optical microscope in quasi-backscattering geometry with a diode-pumped Nd:YVO4 laser at 532 nm for excitation. AFM observations revealed the step morphology on the facet, and the local electrical conductivity on the facet was measured by tunneling AFM (TUNA) with a conductive tip (PtIr-coated Si) and a current-sensing module at a sample (heavily nitrogen-doped 4H-SiC boule) bias voltage of +1 V.

3. Results and discussion

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3.1. Morphological features of stacking faults on the (0001) facet _

Figure 1 shows (a) a DIC optical image of the whole (0001) facet and (b) an LVSEM image of the central region of the facet of a heavily nitrogen-doped (mid-1019 cm−3) 4H-SiC boule, where the LVSEM image [Fig. 1(b)] was acquired from the facet region marked by a dashed line rectangle in Fig. 1(a). The LVSEM image was obtained with the In-Lens detector, and step-terrace structures on the facet surface were clearly detected in the image. AFM measurements revealed that the typical height of the steps on the facet was 0.5 nm (half

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unit-cell height), although bunched-steps (macrosteps) of several nanometer height were occasionally observed. LVSEM observations also revealed white contrast bands along some of the step edges; examples are marked by white open ellipses in the image. The width of the contrast bands was approximately 10 m and extended over 100 m along the step edges. They faded out toward the facet center, which is the up-step direction on the facet. They were not detected when the image was obtained with the E −T detector. Based on these results, we attributed them to basal plane stacking faults [21]. _

Wider-area LVSEM observations of the step-terrace structure on the (0001) facet revealed that white contrast bands (basal plane stacking faults) were concentrically distributed around the facet center, suggesting that they have an origin in the up-step direction and extend toward the down-step direction. Furthermore, white contrast bands on the facet of heavily nitrogen-doped 4H-SiC boules were almost uniformly distributed on the facet, indicating that stacking fault formation occurred almost constantly during the PVT growth of the boules. Difference in the crystal shape and nitrogen concentration between the facet and non-facetted regions would result in an enhanced thermoelastic stress near the boundary between the two regions; however, the uniform distribution of stacking faults on the facet indicates that the stress is not a major cause of stacking faults. For comparison, we also performed LVSEM _

observations of the (0001) facet of conventionally nitrogen-doped (mid-1018 cm−3) 4H-SiC boules and found no white contrast bands on the facet. Thus, it can be concluded that the stacking fault formation is pertinent to the growth process of heavily nitrogen-doped 4H-SiC crystals and would be caused by their growth kinetics and/or energetics. A magnified LVSEM image of a white contrast band is shown in Fig. 2(a), which reveals characteristic morphologies of the white contrast band: straight edge (marked by a white arrow) together with a highly wavy morphology as marked by a white open ellipse in Fig. 2(a). Figure 2(b) shows a corresponding AFM image of the same area of the facet surface

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shown in Fig. 2(a). The image shows a train of steps of half unit-cell height on the surface that _

propagated roughly in the [1100] direction (to the right-hand side of the image). As seen in Fig. 2(b), most of the edge of the white contrast band does not coincide with the step edges, which implies that the stacking fault causing the white contrast band was buried under the surface, and surface steps lay above the stacking fault. The figure also shows that the edge of _

the buried stacking fault was fairly straight and extended along a crystallographic <1120> direction, which is energetically the most favorable direction for partial dislocations in 4H-SiC crystals. The edge causes a faint white line in the AFM image [marked by a white arrow in Fig. 2(b)]. This corresponds to a small bump of approximately 0.1 nm height along the edge of the stacking fault; the small bump is likely to be caused by an extra half-plane of atoms pointing toward the surface, introduced at the boundary between the faulted and unfaulted regions of the stacking fault. By contrast, the wavy part of the white contrast band coincides well with the step edge morphology on the surface, suggesting that the buried stacking fault is exposed to the atmosphere at this part and forms surface steps. These observations reasonably suggest a behavior of the buried stacking faults in heavily nitrogen-doped 4H-SiC crystals during the PVT growth wherein they would expand during the high temperature growth process and often eventually reach the vicinal growing surface of the crystals, forming wavy steps on the surface. The above-mentioned structural model of stacking faults was experimentally supported by TUNA measurements conducted on the facet surface of heavily nitrogen-doped 4H-SiC boules. Figure 3 shows (a) AFM and (b) TUNA images of the step-terrace structure on the facet, where a buried stacking fault was imaged. In Fig. 3(a), a faint white line vertically extending in the figure (marked by a white arrow) corresponds to the edge of the buried stacking fault, and the faulted plane extends on the left side of the white line. Figure 3(b) is the corresponding TUNA image of the same area as that in Fig. 3(a), and in Fig. 3(b), the

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magnitude of the local TUNA current, which measures the local electrical conductivity across the basal plane, is plotted over the area. As seen in the figure, the TUNA current is lower on the left side of the white line, indicating that the buried basal plane stacking fault hinders the electron transport across it. We also measured the local electrical conductivity of the wavy part of the stacking fault (the result is not shown), where the edge of the stacking fault was exposed to the air and could come in direct contact with the TUNA tip. It was found that the local TUNA current was enhanced at the edge of the wavy part of stacking faults. These results are reasonably explained by the fact that stacking faults in SiC crystal act as a quantum well [4-6], which hinders electron transport across the stacking fault, whereas it shows a high electrical conductivity when electron transport occurs within the stacking fault [7].

3.2. Structure of as-grown stacking faults in heavily nitrogen-doped 4H-SiC crystals To shed more light on the nature of as-grown stacking faults in heavily nitrogen-doped 4H-SiC crystals, we conducted photoluminescence measurements using a UV light excitation. Photoluminescence is a strong tool to identify the types of basal plane stacking faults in 4H-SiC [22,23]; however, we failed to detect stacking faults by this method. The reason would be that heavily nitrogen-doped 4H-SiC crystals contained a large number of recombination centers for minority carriers (holes). Therefore, we needed to adapt different methods to measure or deduce the types of basal plane stacking faults. In this study, we adapted a method using LE-ECC imaging. Figure 4(a) shows an LE-ECC image of a facet region that contains a stacking fault. As shown in the figure, long narrow strips with different LE-ECCs were observed; they correspond to atomically flat terraces on the facet terminated with different stacking sequences in the topmost layer. Such contrast formation is caused by different electron channeling probabilities because of the inclined atomic column structure in SiC crystals [20]. Terraces terminated with the atomic columns in the topmost layer inclined

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toward the primary electron beam direction show bright contrast, whereas those terminated with the atomic columns rotated 180° around the c-axis exhibit dark contrast. Thus, vicinal 4H-SiC (0001) surface covered with steps of half unit-cell height exhibits alternate dark and bright contrast structures [14]. If a stacking fault exists in near surface layer, terraces with longer atomic columns in the underlying layer are formed, which show brighter or darker contrast than terraces of the perfect 4H-SiC crystal. Figure 4(b) shows a contrast analysis of the LE-ECC image shown in Fig. 4(a), where the intensity of secondary and reflected electrons detected by the E−T detector is plotted as a function of the distance from point A to B on the facet shown in Fig. 4(a). As Fig. 4(b) shows, a terrace marked as (ix) shows the brightest LE-ECC, whereas in its up-step direction, the contrast pattern of a perfect 4H-SiC step-terrace structure, i.e., step-terrace structure with steps of half-unit cell height, appears [regions (i) to (v)]. The result indicates that the brightest LE-ECC terrace has a cubic-type stacking sequence of more than two Si-C bilayers (half-unit cell thickness). To identify the number of Si-C bilayers in the cubic-type stacking sequence (stacking fault) in heavily nitrogen-doped 4H-SiC crystals, we adapted a method using the relative magnitude of LE-ECC for SiC {0001} terraces [24]. The relative magnitude of LE-ECC is defined as

I 

IB  ID , I av

(1)

where I B is the E−T detector current when the incident primary electron beam is parallel to the atomic column in the topmost layer of SiC {0001} terrace (bright contrast geometry), whereas I D is the detector current when the primary electron beam is incident from the direction 180° around the c-axis (dark contrast geometry). I av is the arithmetic mean of I B and I D (  ( I B  I D ) / 2 ). Ashida measured the relative magnitude of LE-ECC ( I ) for SiC (0001) terraces of various polytype crystals such as 3C, 6H, and 4H-SiC and found that under a certain energy and tilt angle condition of the primary electron beam, I monotonically 9

increases with the number of SiC bilayers directed in the same atomic column direction in the topmost layer of SiC (0001) terraces [24]. We conducted the same experiment for the SiC _

(0001) surface and obtained a similar result; the relative magnitude of LE-ECC from SiC _

(0001) terraces also monotonically increases as the number of SiC bilayers in the cubic _

stacking sequence in the topmost layer of the SiC (0001) surface increases (4H to 6H to 3C), as shown in Fig. 5. The above-mentioned method using the relative magnitude of LE-ECC was applied to analyze the stacking sequence of the cubic-type basal plane stacking fault shown in Fig. 4(a). As mentioned above, the terrace region (ix) shows the brightest LE-ECC, and thus, in this terrace region, the cubic stacking sequence in the basal plane stacking fault is assumed to lie in the topmost layer of the surface; namely, the cubic stacking sequence is exposed to the air. To estimate the thickness of the cubic stacking sequence, we measured the relative magnitude of LE-ECC from the terrace region (ix), and the result is shown in Fig. 5 in comparison with _

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_ _ _

the relative magnitude of LE-ECC from 4H-SiC (0001), 6H-SiC (0001), and 3C-SiC (111) surfaces; they were terminated with a cubic stacking sequence consisting of two, three, and infinite (extremely thick) Si-C bilayers in the topmost layer, respectively. As the figure shows, the relative magnitude of LE-ECC from the terrace region (ix) is larger than those from 4Hand 6H-SiC and smaller than that from 3C-SiC, which implies that as-grown basal plane stacking faults in heavily nitrogen-doped 4H-SiC crystals are those containing at least four SiC bilayers of cubic (3C) stacking sequence.

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3.3. Surface hillock formation on the (0001) facet of the crystals One important question yet to be addressed is how the nuclei of expanding stacking faults are brought into the grown crystals. A plausible answer to this question is that they _

nucleate on the growing surface (on the (0001) terraces) as 3C-SiC nuclei or those containing

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a stacking fault of cubic stacking sequence; once the nuclei of a certain size and height are formed on the terrace, they are subsequently incorporated into growing crystals during PVT growth. To confirm this assumption, we performed AFM observations over a wide range of surface areas on the facet; however, no direct evidence of the nucleation was obtained. Instead, we observed a number of surface hillocks on the facet. An example of surface hillocks is _

shown in Fig. 6(a), where a DIC optical microscope image of the hillocks on the (0001) facet of a heavily nitrogen-doped 4H-SiC boule is shown. The fact that deserves attention here is that they were observed solely on heavily nitrogen-doped ([N]: mid-1019 cm−3) boules and never observed on conventionally doped 4H-SiC boules ([N]: mid-1018 cm−3). Furthermore, _

the hillocks were only observed on the (0001) facet and never observed on the outer regions _

of the facet. Figure 7 shows the hillock and step densities on the (0001) facet and its outer (non-facetted) regions of a heavily nitrogen-doped 4H-SiC boule as a function of the distance _

from the facet center. As seen in the figure, surface hillocks were observed only on the (0001) facet plane and never observed on the outer regions of the facet, indicating that their formation (nucleation) is deeply related to the surface structure of the growing crystal and would be controlled by the growth kinetics on the surface. The figure also shows that both the densities of surface hillocks and steps increase toward the edge of the facet, implying that the terrace width between surface steps on the facet would greatly influence the formation of surface hillocks. On the other hand, the size of hillocks became smaller toward the facet edge (the result is not shown), and the total volume of surface hillocks at each point was not much different across the facet. Near the facet center, larger terrace widths are likely to induce the merger of surface hillocks (Oswald ripening), resulting in larger diameter hillocks. We observed three types of surface hillocks in terms of their heights. They are classified into the following three categories: hillocks of a few micrometer height, those of ~100 nm height, and those of a few nanometer height. The former two types of hillocks were revealed

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to contain a 3C-SiC inclusion or a 3C-type (cubic-type) stacking fault by using micro Raman microscopy and LE-ECC imaging. It was confirmed that there was no locational correlation between the hillock formation and crystallographic defects existing in the crystals, such as threading screw dislocations terminating at the facet surface of heavily nitrogen-doped 4H-SiC boules. Figure 6(a) shows a DIC image of surface hillocks on the facet. A hillock marked by a white open square is an example of hillock of ~100 nm height, and Figs. 6(b), 6(c), and 6(d) show AFM, LE-ECC (dark contrast geometry), and LE-ECC (bright contrast geometry) images of the hillock, respectively. The LE-ECC images shown in Figs. 6(c) and 6(d) were obtained with the tilted incident primary electron beam of the opposite azimuthal angles ( = 0° and 180°). As shown in the figure, the AFM image shows that a spiral step exists on the hillock, whose height was 0.25 nm; the step height is apparently different from _

that on the (0001) surface of perfect 4H-SiC crystals, which is usually half unit-cell size (0.5 nm). The LE-ECC images shown in Figs. 6(c) and 6(d) exhibit no contrast difference among the terraces associated with the spiral steps, although they are terminated at different positions in the stacking sequence along the c-axis, implying that the hillock is 3C-SiC or has a relatively thick 3C inclusion. Figure 8 shows the differential AFM image of a hillock of a few nanometer height. In the figure, paired 0.5 nm height steps (half unit-cell height steps) are clearly seen, and a very flat hillock lies over the steps. One of the surface steps on the hillock shows 0.75 nm height, indicating that the hillock contains a stacking fault. The steps under the hillock are very uniform and show no indication of hindrance of step propagation. This fact would suggest that the observed hillock extended over the steps after the step flow growth on the facet ceased. It is very possible that such extension of surface hillocks occurs during the early stage of the cooling process, in which the vapor from the source material is preferentially incorporated into surface nuclei because 3C-SiC is a more stable polytype than 4H-SiC at low temperatures

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(< 2000°C). Based on these results, we assume that the observed hillocks are closely related to the stacking fault formation during the PVT growth of heavily nitrogen-doped 4H-SiC boules; the hillocks originated from very small 3C nuclei or those containing a basal plane stacking fault nucleated on the facet during crystal growth. The formed small nuclei are incorporated into the crystals as the growth proceeds, becoming the origin of the observed buried basal plane stacking faults, which expand in the crystal during the PVT growth process, whereas at the early stage of the cooling process, they stay on the facet surface and evolve into large 3C hillocks or those containing cubic type stacking faults. The mechanism by which the heavy nitrogen doping stabilizes 3C nuclei on the facet surface is yet to be clarified; however, we believe that the formation of 3C nuclei is driven by an electronic effect, i.e., the QWA effect [4−6] rather than the chemical influence of nitrogen donors. The QWA mechanism would be effective not only for cubic inclusions in heavily nitrogen-doped 4H-SiC crystals but also for _

surface 3C nuclei on the (0001) facet of the crystals, where the vacuum side of the nuclei acts as an almost infinity energy barrier for electrons in the quantum well (3C nuclei). As shown in Section 3.2, basal plane stacking faults incorporated in the grown crystals had a relatively long cubic stacking sequence such as DSSFs, and this fact reasonably suggests that they were introduced by the QWA mechanism.

4. Conclusions The formation of basal plane stacking faults in heavily nitrogen-doped (mid-1019 cm−3) 4H-SiC crystals was investigated using LVSEM and AFM. Stacking faults were detected on _

the (0001) facet of the crystals by LVSEM as white contrast bands, which showed characteristic morphologies on the facet. The morphologies were interpreted in terms of the interaction between the stacking faults and surface steps, suggesting that stacking faults

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expand in the grown crystals during the PVT growth process. The LE-ECC analysis revealed that the basal plane stacking faults contain a thick (at least four) SiC bilayers of cubic (3C) stacking sequence. Surface hillocks observed on the facet of the crystals provide great insight into how and where the nuclei of stacking faults are formed during growth. The surface hillocks were never observed on conventionally doped crystals nor on the non-facetted region of heavily nitrogen-doped crystals, suggesting that the origin of basal plane stacking faults observed in the crystals is small 3C nuclei or those containing a basal plane stacking fault _

nucleated on the (0001) facet during PVT growth.

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Figure Captions _

Fig. 1. (a) DIC optical microscopy image of the (0001) facet of a heavily nitrogen-doped (mid-1019 cm−3) 4H-SiC boule and (b) an LVSEM image of the central region of the _

(0001) facet [the region marked by a dashed line rectangle in (a)]. The LVSEM image was obtained using the In-Lens detector, which is sensitive to low-energy secondary electrons. White contrast bands seen in the LVSEM image (some of them are marked by white open ellipses) correspond to basal plane stacking faults buried in the near surface layer of the facet. Fig. 2. (a) Magnified LVSEM and (b) AFM images of a white contrast band (basal plane _

stacking fault) on the (0001) facet. The white open ellipse indicates the wavy part of the white contrast band, where the buried stacking fault is exposed to the atmosphere and forms surface steps. _

Fig. 3. (a) AFM and (b) TUNA images of the step-terrace structure on the (0001) facet, under which a buried basal plane stacking fault exists. The TUNA image shows a map of the local electrical conductivity across the basal plane. The faint white line (marked by a white arrow) in (a) corresponds to the boundary between the faulted and unfaulted regions of the stacking fault. Fig. 4. (a) LE-ECC image of a facet region that contains a basal plane stacking fault, and (b) contrast analysis of the LE-ECC image shown in (a). The cubic stacking sequence of Si-C bilayers in the stacking fault is assumed to lie in the topmost layer of the brightest terrace region (ix). _

Fig. 5. Relative magnitudes of LE-ECC [defined as Eq. (1)] from 6H-SiC (0001), 4H-SiC _

_ _ _

(0001), and 3C-SiC (111) surfaces together with that from the terrace region (ix) in Fig. 4(a). _

Fig. 6. (a) DIC image of surface hillocks on the (0001) facet. The hillock marked by a white

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open square is a hillock of ~100 nm height. (b), (c), and (d) are AFM and LE-ECC images of the hillock, and the LE-ECC images shown in (c) and (d) were taken with the tilted incident primary electron beam of the opposite azimuthal angles ( = 0° and 180°). _

Fig. 7. Hillock and step densities on the (0001) facet and its outer (non-facetted) regions of a heavily nitrogen-doped 4H-SiC boule as a function of the distance from the facet center. _

Fig. 8. Differential AFM image of a hillock of a few nanometer height on the (0001) facet of a heavily nitrogen-doped 4H-SiC boule.

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(a)

(b)

1 mm 100 m

Figure 1

(a)

(b) 1120

1100

1 m

Figure 2

1 m

(a)

500 nm

(b)

0

Figure 3

Current (pA)

12

Electron intensity [arb. unit]

(a)

125

(b)

(ix)

115 105

(vii) 95

(iii)

(i)

(viii)

(v)

85

(iv)

(ii)

75

(vi)

65 0

1

2

3

4

Distance [μm]

Figure 4

5

6

7

4H

Figure 5

6H terrace (ix) 3C

⊿I (=(IB－ID) /Iav)  [arb. unit]

(a)

100μm

Figure 6

(b)

(c)

2μm

= 0° (d)

2μm

= 180°

2μm

Facet region

Non‐Facet region

25

2500

20 15

1500

10

1000

5

500

0

0 0

Figure 7

2000

particle density

2 4 6 8 10 Distance from the facet center [mm]

12

Particle density [mm‐2]

Step density [μm‐1]

step density

Down step direction

10 μm

Figure 8

The stacking fault (SF) formation in heavily N-doped 4H-SiC boules was investigated. _

SFs detected on the (0001) facet of the boules showed characteristic morphologies.

The morphologies stemed from the interaction between the SFs and surface steps.

Heavy N-doping induced surface particles on the facet of the boules.

Based on these results, the SF formation mechanism is discussed.