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Formation and multiplication of basal plane dislocations during physical vapor transport growth of 4H-SiC crystals
Accepted Manuscript Formation and multiplication of basal plane dislocations during physical vapor transport growth of 4H-SiC crystals Takahiro Nakano, Naoto Shinagawa, Masahiro Yabu, Noboru Ohtani PII: DOI: Reference:
S0022-0248(19)30203-9 https://doi.org/10.1016/j.jcrysgro.2019.03.027 CRYS 25045
To appear in:
Journal of Crystal Growth
Received Date: Revised Date: Accepted Date:
3 February 2019 27 March 2019 28 March 2019
Please cite this article as: T. Nakano, N. Shinagawa, M. Yabu, N. Ohtani, Formation and multiplication of basal plane dislocations during physical vapor transport growth of 4H-SiC crystals, Journal of Crystal Growth (2019), doi: https://doi.org/10.1016/j.jcrysgro.2019.03.027
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Formation and multiplication of basal plane dislocations during physical vapor transport growth of 4H-SiC crystals
Takahiro Nakano, Naoto Shinagawa, Masahiro Yabu, Noboru Ohtani* Kwansei Gakuin University, School of Science and Technology 2-1 Gakuen, Sanda, Hyogo 669-1337, Japan
Abstract
The distribution of basal plane dislocations (BPDs) in physical vapor transport (PVT) grown 4H-SiC crystals has been investigated using Raman microscopy and x-ray topography. X-ray _
topography observations of (1120) wafers vertically sliced along the growth direction from the crystals revealed that there existed almost periodically arranged layers with a high density of BPDs (bunched BPDs) in PVT-grown 4H-SiC crystals. These layers were characterized using Raman microscopy and high resolution x-ray diffraction, and it was found that large tensile and compressive stresses were associated with the layers. The stresses showed characteristic variations along the basal plane as well as the growth direction. Based on these results, the formation and multiplication processes of BPDs during PVT growth of 4H-SiC crystals are elucidated.
*
Corresponding author. E-mail address: [email protected] (N. Ohtani)
1
1. Introduction Reduction of crystallographic defects, particularly dislocations, in 4H-SiC crystals is still a major issue for the fabrication of reliable SiC power devices. At present, the physical vapor transport (PVT) growth method is the dominant growth method to grow commercially available SiC crystals. Among the dislocations existing in PVT-grown SiC crystals, basal plane dislocations (BPDs) are currently of the most serious concerns since they have negative impacts on the reliability of SiC unipolar devices such as SiC MOSFETs and JFETs [1,2] as well as bipolar devices [3,4]. Therefore, significant effort has been dedicated to the reduction of BPDs in 4H-SiC crystals; however, the formation and multiplication processes of BPDs in SiC crystals during PVT growth are still not well understood. An important difference between BPDs and threading dislocations extending along the c-axis (growth direction) in SiC crystals is in their formation process. Whereas most threading dislocations are inherited from the seed crystal and also often formed at the initial stage of crystal growth, BPDs observed in the crystal portions well distant from the seed crystal could not be formed through such mechanisms because their propagation is confined within the basal plane, normal to the growth direction. Therefore, BPDs existing in the top portion of SiC crystals grown along the c-axis must be nucleated and incorporated into the grown crystals from somewhere well distant from the seed crystal. A major cause of BPDs in SiC crystals is thought to be thermoelastic stress imposed on the growing crystals during PVT growth process. Large resolved shear stress on the (0001) basal plane causes the nucleation and multiplication of BPDs in SiC crystals. Several authors discussed the formation of BPDs using numerical simulation of thermoelastic stress imposed on the growing SiC crystals during PVT growth [5–8], and it has been revealed that when the thermoelastic stress on the basal plane exceeds the critical resolved shear stress in the _
(0001)<1120> slip system (approximately 1 MPa at the typical PVT growth temperatures),
2
BPDs are nucleated and multiplied during PVT growth process. Recently, Sonoda et al. [9] revealed that a large number of BPDs were introduced from _
the shoulder regions of 4H-SiC crystals grown in the [0001] direction by the PVT growth _
method, whereas BPDs were also nucleated at the outcrops of TSDs terminating at the (0001) facet region but hardly multiplied during PVT growth. These phenomena can be explained in terms of a large thermoelastic shear stress imposed on the shoulder regions of the growing crystals during PVT growth of 4H-SiC [8]. In this paper, we tried to reveal the formation and multiplication processes of BPDs during PVT growth of 4H-SiC crystals using Raman microscopy and x-ray topography, through the investigation of distribution of BPDs in PVT-grown 4H-SiC crystals. We observed characteristic distributions of BPDs in 4H-SiC crystals. A number of BPDs were found to be distributed almost periodically along the growth direction. It was also found that BPDs were distributed more densely near threading screw dislocations (TSDs), suggesting that the BPD multiplication preferentially occurs at TSDs propagating along the growth direction.
2. Experimental procedure _
Approximately 50 mm diameter 4H-SiC single crystals were grown on an on-axis (0001) 4H-SiC seed crystal by the PVT growth method. The typical growth temperature was about 2300–2400ºC, and the argon gas pressure was maintained between 1.0–2.0 kPa during growth. The grown crystals were nitrogen-doped, and they contained nitrogen donors in the mid-1018 cm−3 range. The nitrogen concentration in the grown crystals was estimated using Raman microscopy from calibration curve of the peak shift of the longitudinal optical phonon-plasmon coupled (LOPC) mode at ~983 cm−1 versus the nitrogen concentration in 4H-SiC crystals determined by secondary ion mass spectrometry (SIMS). Then, they were
3
_
sliced vertically along the growth direction (c-axis), and (1120) wafers were prepared from the crystals for Raman microscopy, x-ray topography, and high resolution x-ray diffraction (HRXRD). Raman microscopy was conducted at room temperature using a confocal optical microscope with a diode-pumped Nd:YVO4 laser at 532 nm for excitation. We measured the peak shift of the E2 mode at ~776 cm−1 in quasi-backscattering geometry with the polarization vectors of the incident and scattered lights parallel to the basal plane. The peak position of the E2 mode shifts toward higher (lower) wavenumbers when compressive (tensile) stress within the basal plane is imposed on 4H-SiC crystals [10–12]. Transmission x-ray topographs were taken from various portions of the wafers with _
_
symmetric 0008 and 1100 diffractions. The topographs were recorded using a charge-coupled device (CCD) camera with a pixel size of a few micrometers. A computer controlled detection system took multiple several-millimeter-sized topographic images that were stitched together to form topographic images of a few centimeter size. HRXRD rocking curve measurements (-scan mode) were performed using a double-axis diffractometer, where double crystal x-ray optics for Cu K1 radiation was employed, and the footprint size of the incident x-ray beam on the wafer surface was 0.2 mm × 0.7 mm.
3. Results and discussion
3.1. Cross-sectional x-ray topography observations of the BPD distribution in PVT-grown 4H-SiC crystals Figure 1 illustrates schematic figures of (a) the PVT growth reactor of 4H-SiC crystals _
and (b) the preparation scheme of 4H-SiC (1120) wafers vertically sliced along the growth
4
_
direction from a 4H-SiC single crystal grown on a 4H-SiC (0001)C seed crystal. The _
vertically sliced (1120) wafers consisted of two portions that are classified from the viewpoint of crystal growth. One is associated with the growth front which showed a domed shape (convex toward the growth direction). In this portion, crystal growth occurred during PVT growth. The other portion had side surfaces of the crystal, on which crystal growth did not occur nominally, and thus it had a constant diameter. _
_
Figures 2(a) and 2(b) show transmission x-ray topographs with 0008 and 11 _
00 diffractions, respectively, acquired from the (1120) wafer mentioned above. The area where the topographs were obtained is schematically indicated by a red open square in Fig. 2(c). As seen in the topographs, strong horizontal line contrasts are observed in Fig. 2(b) _
_
(diffraction vector: 1100), which almost diminish in Fig. 2(a) (diffraction vector: 0008). By contrast, in Fig. 2(a), line contrasts extending along the c-axis (growth direction) are observed, which correspond to TSDs. In Fig. 2(a), broad band contrasts are also observed at the positions where the strong horizontal line contrasts are observed in Fig. 2(b). The strong horizontal line contrasts observed in Fig. 2(b) are thought to be caused by BPDs or basal plane stacking faults existing in the crystal. To clarify the origin of the strong horizontal line contrasts in Fig. 2(b), we performed defect selective etching (molten KOH etching) and found that they were caused by a high density of BPDs; we call this type of BPDs “bunched BPDs” in this paper. It is noteworthy here that such bunched BPDs are almost periodically arranged along the growth direction in the grown crystals, as revealed in Fig. 2(b), where some of the bunched BPDs are indicated by closed white triangles.
3.2. Structural characterization of bunched BPDs in PVT-grown 4H-SiC crystals To examine the structure of bunched BPDs in more detail, we conducted Raman microscopy imaging of bunched BPDs. The imaged area is indicated by a red open square in
5
Figs. 2(a) and 2(b), and we plotted the peak position of the E2 mode at ~776 cm−1 across this area with the polarization vectors of incident and scattered lights parallel to the basal plane. The result is shown in Fig. 2(d); the light polarization is indicated by a double headed arrow in the figure, and as seen in the figure, a clear difference in contrast exists between the upper and lower sides of bunched BPDs. In the upper side of bunched BPDs, the peak position of the E2 mode shifts lower, while the peak is positioned at higher wavenumbers in the lower side, implying that tensile (compressive) strain exists in the upper (lower) side of bunched BPDs. Figure 3 shows a variation of the E2 peak position along the growth direction together with that of the a-lattice constant (lattice constant within the basal plane) measured by HRXRD. These variations were measured along a white dashed arrow indicated in Fig. 2(b). As seen in Fig. 3(a), abrupt shifts of the peak position were observed at the points where bunched BPDs existed; the positions of some of bunched BPDs are indicated by arrows in the right-hand side of Fig. 3; those indicated by the numbered arrows from 1 to 4 correspond to the bunched BPDs indicated by closed white triangles in Fig. 2(b). The shifts always occurred toward lower wave numbers at bunched BPDs when the light beam was scanned from the bottom to the top of grown crystals, implying that tensile strain within the basal plane always existed in the upper side of bunched BPDs. The a-lattice constant also varied along the growth direction and became larger in the upper side of bunched BPDs. This is consistent with the variation of the E2 mode peak position, and it can be concluded that the tensile strain within the basal plane existing in the upper side of bunched BPDs resulted in a larger a-lattice constant. Based on these results, we schematically illustrated the atomistic structure of bunched BPDs in Fig. 4, where the tensile and compressive strain sides of bunched BPDs are indicated by blue and pink colored bands. As seen in the figure, the compressive strain side contains extra half-planes pointing toward the [0001] direction (toward the seed crystal),
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which causes tensile strain in the opposite side (upper side) of bunched BPDs and are terminated at bunched BPDs. The pointing direction of the extra half-planes associated with bunched BPDs is reasonably understood if bunched BPDs are introduced by the thermoelastic stress imposed on the growing crystal during PVT growth of SiC crystals. SiC PVT growth process is primarily driven by the temperature gradient along the c-axis (growth direction), and thus a large
shear stress is imposed on the growing crystal during growth, where r and z denote
the radial and the axial directions of the grown crystal (z is parallel to the c-axis). At the typical PVT growth temperatures (>2300ºC), the plastic deformation of SiC crystals occurs and the
shear stress is largely relieved by the introduction of BPDs in the crystal. During
the PVT growth process, a positive temperature gradient toward the growth direction is maintained, and thus, when the thermoelastic stress is relieved, BPDs that have extra-half-planes pointing toward the seed crystal are introduced.
3.3. Multiplication of BPDs during PVT growth of 4H-SiC crystals To investigate in more detail the formation and multiplication processes of BPDs during PVT growth of 4H-SiC crystals, we performed more extended Raman microscopy imaging of bunched BPDs in the lateral direction (parallel to the basal plane). The result of Raman microscopy is shown in Fig. 5(a). The image is a two-dimensional mapping of the peak position of the E2 mode at 776 cm−1. Similarly to Fig. 2(d), a clear difference in contrast exists between the upper and lower sides of bunched BPDs. It should be noted in this figure that the contrast difference between the upper and lower sides of bunched BPDs varies along the basal plane. In Fig. 5(a), portions with larger and smaller contrast differences are indicated by closed and open triangles, respectively. We found that the observed variation of the contrast difference (magnitude of the abrupt shift of the E2 mode peak position at bunched BPDs)
7
along the basal plane was correlated with the TSD density in the crystal. Figure 5(b) shows the relationship between the contrast difference and the TSD density; the latter was estimated from the density of the line contrasts extending along the c-axis in x-ray topographs with _
0008 diffraction [e.g., Fig. 2(a)] at the position where the contrast difference was measured by Raman microscopy. As seen in the figure, the magnitude of the abrupt peak shift at bunched BPDs shows a good correlation with the TSD density, and as the TSD density increases, the abrupt peak shift at bunched BPDs becomes larger, which implies that the TSD and BPD densities in PVT-grown 4H-SiC crystals have a positive correlation. Similar positive correlation between the TSD and BPD densities in PVT-grown 4H-SiC crystals was reported by Ohtani et al. [13]. They conducted defect selective etching using molten KOH to estimate the TSD and BPD densities in PVT-grown 4H-SiC crystals and found that the BPD density increases with increase of the TSD density in the crystals. They ascribed this positive correlation to a BPD multiplication process around TSDs. Temperature gradients existing in 4H-SiC crystals during PVT growth process glide BPDs on the basal plane and cause them to cut through TSDs extending along the c-axis in the crystals. After crossing TSDs, BPDs have super jogs parallel to the c-axis, which are immobile and anchored in the crystal. When the BPDs further glide under thermoelastic stress during PVT growth and/or post-growth cooling, the well-known Frank–Read type BPD multiplication occurs and consequently the BPD density significantly increases around TSDs [14,15]. This is the reason that the BPD and TSD densities show a positive correlation in 4H-SiC crystals.
3.4. Formation model of bunched BPDs in PVT-grown 4H-SiC crystals Finally, we discuss the formation mechanism of bunched BPDs observed in PVT-grown 4H-SiC crystals. We found that BPDs showed a characteristic distribution in PVT-grown 4H-SiC crystals; there existed layers with a high density of BPDs (bunched BPDs) arranged
8
almost periodically along the growth direction. As for the BPD formation in PVT-grown 4H-SiC crystals, Sonoda et al. reported that a number of BPDs were introduced from the shoulder regions of grown crystals during PVT growth [9]. In this respect, it is noteworthy that we observed bunched BPDs in the crystal portions relatively distant from the shoulder regions of grown crystals. In view of these results, it becomes an important question where and when bunched BPDs were introduced. One possible mechanism is that bunched BPDs were introduced from the side surfaces of grown crystals. The portions of grown crystals with a constant diameter had side surfaces located very close to the crucible walls, and they would have contacted with the walls under certain growth conditions during PVT growth or cooling process, because of the difference in the coefficient of thermal expansion between SiC and graphite. Therefore, a large thermoelastic stress could have been imposed on the side surfaces of grown crystals during PVT growth process, resulting in the introduction of a number of BPDs from the side surfaces. To verify this scenario, we conducted x-ray topography observations of a crystal portion near the side surface of a grown crystal. Figure 6 shows _
_
x-ray topographs of the portion with (a) 0008 and (b) 1100 diffractions, where the side surface of a grown crystal is indicated by closed red triangles. The location where the _
topographs were taken in a vertically sliced (1120) wafer is schematically indicated by a red open square in Fig. 6(c). As seen in Fig. 6(b), BPDs are observed in the crystal portion near the side surfaces; some of the BPDs are indicated by open triangles in the figure. Their density, however, appears not so high, and bunched BPDs are never observed in this portion. To further examine the origin of bunched BPDs, we performed more extended Raman microscopy analysis in the growth direction. Figure 7 shows variations of the E2 peak position along the growth direction. The variations were measured along two lines: one started from _
the bottom of the vertically sliced wafer and ended near the (0001) facet region at the growth front (denoted by line A), and the other was a line close to the side surface (edge) of a grown
9
crystal (denoted by line B). The locations of these two lines (lines A and B) in a vertically _
sliced (1120) wafer are schematically indicated by dashed arrows in the inset of Fig. 7. As shown in Fig. 7, the abrupt shift of the E2 peak position often occurs along line A; some of the shifts are indicated by open triangles in the figure. On the other hand, such abrupt shifts are not seen along line B, which is consistent with the results of x-ray topography shown in Fig. 6. To be noted here is that the abrupt shift becomes stronger as the measured point approaches the bottom of the vertically sliced wafer, implying that the BPD density in bunched BPDs gradually increases toward the seed crystal. The increase in the BPD density toward the seed crystal is reasonably understood if bunched BPDs are introduced from the growth front (top surface of the growing crystal) during PVT growth. Once bunched BPDs are introduced from the growth front, their BPD density gradually increases as the growth proceeds, since the total duration of thermoelastic stress imposed on the growing crystal becomes longer with increase of the growth time. The discussions mentioned above combine to lead to an important conclusion that bunched BPDs or their nuclei would be introduced at the growth front (domed surface) and hardly introduced from the side surfaces of grown crystals. The most plausible location where bunched BPDs are introduced would be the shoulder regions of growth crystals. Gao and Kakimoto [8] conducted three-dimensional numerical modeling of BPD multiplication in 4H-SiC bulk crystals and found that the domed shape of the growth front gives rise to a large resolved shear stress on the basal plane in the shoulder regions of grown crystals during PVT growth, thus causing a large number of BPDs introduced from these regions. Our results indicate that the shoulder regions of the growth front are a major source of BPDs, including bunched BPDs or their nuclei, in 4H-SiC, and BPDs introduced from these regions would determine the whole distribution of BPDs in PVT-grown 4H-SiC crystals. Our observations of the BPD distribution in 4H-SiC crystals also indicate that the
10
introduction of BPDs from the shoulder regions of grown crystals was almost periodically enhanced during PVT growth, which means that a large thermoelastic stress was almost periodically imposed on the growing crystal during PVT growth. The reason why such a periodicity existed is unknown at this moment, and further investigation is required to clarify this point.
4. Conclusions We have investigated the distribution of BPDs in PVT-grown 4H-SiC crystals using Raman microscopy and x-ray topography. It was found that there existed almost periodically arranged layers with a high density of BPDs (bunched BPDs) along the growth direction, and they accompanied large tensile and compressive stresses in their upper and lower sides, respectively. The stresses showed a characteristic variation along the basal plane, which was well correlated with the TSD density in the crystals. Bunched BPDs were more often observed in the central portion of grown crystals, and their BPD density gradually increased toward the seed crystal. On the basis of these results, we discussed the formation and multiplication processes of BPDs during PVT growth of 4H-SiC crystals.
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References [1] A. Agarwal, H. Fatima, S. Haney, R. Sei-Hyung, IEEE Electron Device Letters 28 (2007) 587–589. [2] V. Veliadis, H. Hearne, E. J. Stewart, M. Snook, W. Chang, J. D. Caldwell, H. C. Ha, N. El-Hinnawy, P. Borodulin, R. S. Howell, D. Urciuoli, A. Lelis, C. Scozzie, IEEE Electron Device Letters 33 (2012) 952–954. [3] H. Lendenmann, F. Dahquist, N. Johansson, R. Söderholm, P. Å. Nilsson, P. Bergman, P. Skytt, Mater. Sci. Forum 353–356 (2001) 727–730. [4] P. Bergman, H. Lendenmann, P. A. Nilsson, U. Lindefelt, P. Skytt, Mater. Sci. Forum 353–356 (2001) 299–302. [5] H. McD. Hobgood, M. F. Brady, W. H. Brixius, G. Fechko, R. C. Glass, D. Henshall, J. R. Jenny, R. T. Leonard, D. P. Malta, S. G. Müller, V. F. Tsvetkov, C. H. Carter Jr., Mater. Sci. Forum 338–342 (2000) 3–8. [6] M. Selder, L Kadinski, F. Durst, D. Hofmann, J. Cryst. Growth 226 (2001) 501–510. [7] R.-H. Ma, H. Zhang, M. Dudley, V. Prasad, J. Cryst. Growth 258 (2003) 318–330. [8] B. Gao, T. Kakimoto, Cryst. Growth Design 14 (2014) 1272–1278. [9] M. Sonoda, T. Nakano, K. Shioura, N. Shinagawa, N. Ohtani, J. Cryst. Growth 499 (2018) 24–29. [10] S. Nakashima, H. Harima, Phys. Stat. Sol. (a) 162 (1997) 39–64. [11] R. Sugie, T. Uchida, J. Appl. Phys. 122 (2017) 195703. [12] H. Sakakima, S. Takamoto, Y. Murakami, A. Hatano, A. Goryu, K. Hirohata, S. Izumi, Jpn. J. Appl. Phys. 57 (2018) 106602. [13] N. Ohtani, M. Katsuno, T. Fujimoto, M. Nakabayashi, H. Tsuge, H. Yashiro, T. Aigo, H. Hirano, T. Hoshino, W. Ohashi, Jpn. J. Appl. Phys. 48 (2009) 065503. [14] N. Ohtani, M. Katsuno, H. Tsuge, T. Fujimoto, M. Nakabayashi, H. Yashiro, M.
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Sawamura, T. Aigo, T. Hoshino, Jpn. J. Appl. Phys. 45 (2006) 1738–1742. [15] Y. Chen, G. Dhanaraj, M. Dudley, H. Zhang, R. Ma, Y. Shishkin, S. E. Saddow, Mater. Res. Soc. Symp. Proc. 911 (2006) 151–156.
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Figure Captions Fig. 1. Schematic figures of (a) the PVT growth reactor and (b) the preparation process of _
4H-SiC (1120) wafers vertically sliced along the growth direction from a 4H-SiC _
crystal grown on a 4H-SiC (0001)C seed crystal. _
_
Fig. 2. Transmission x-ray topographs with (a) 0008 and (b) 1100 diffractions of a vertically _
sliced 4H-SiC (1120) wafer, and (c) schematically shows the area in the wafer examined by transmission x-ray topography, which is indicated by a red open square. (d) shows a Raman microscopy image of a layer with a high density of BPDs (bunched BPDs). Fig. 3. Variations of (a) the peak position of the Raman-active E2 mode around 776 cm−1 and (b) the a-lattice constant measured by HRXRD along the growth direction on a _
vertically sliced (1120) wafer. The measured points and direction are indicated by a white dashed arrow in Fig. 2(b). (a) clearly demonstrates that abrupt shifts of the E2 peak position occurred at bunched BPDs, whose positions are indicated by arrows in the right-hand side of the figure. Fig. 4. Schematic figure of the atomic structure of bunched BPDs. The figure shows extra half-planes associated with bunched BPDs pointing toward the seed crystal, as well as the lattice distortion in the upper and lower sides of bunched BPDs. Fig. 5. (a) Extended Raman microscopy image of bunched BPDs along the basal plane. The image shows a variation of the E2 mode peak position around bunched BPDs. The open and closed triangles in the image indicate points which showed smaller and larger peak shifts at bunched BPDs, respectively. (b) shows a positive correlation between the magnitude of the abrupt peak shift of the E2 mode and the TSD density. _
_
Fig. 6. Transmission x-ray topographs with (a) 0008 and (b) 1100 diffractions acquired from a crystal portion near the side surface of a grown crystal with a constant
14
_
diameter. The position where the topographs were taken in a vertically sliced (1120) wafer is schematically indicated by a red open square in (c). Fig. 7. Variations of (a) the E2 mode peak position along the growth direction measured at _
the near-facet (line A) and the edge (line B) portions of a vertically sliced (1120) wafer. The locations of the two measured portions (lines A and B) in the wafer are schematically indicated by dashed arrows in the inset figure.
15
(a)
(b)
Growth front
Potion with a constant diameter
Figure 1
21/36
(a)
𝒈
(d)
(b)
𝟎𝟎𝟎𝟖
0.5 mm
𝒈
𝟏𝟏𝟎𝟎
50 μm
0.5 mm Tensile
(c)
Figure 2
polarization
Compressive
(a)
(b) Tensile strain Compressive strain
Growth direction
1
2
3
0.5 mm
776.76
776.78
776.80
776.82
Raman peak position (cm−1) Figure 3
0.308058 0.308060 0.308062 0.308064
a‐lattice constant (nm)
4
Tensile strain and larger a‐lattice constant
bunched BPDs
Compressive strain and smaller a‐lattice constant
Figure 4
(a)
polarization bunched BPDs
200 μm Low
High
(b)
Peak shift at bunched BPDs (cm−1)
Raman peak position 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0.00 1600
2100
2600
3100
3600
TSD density (cm−2) Figure 5
4100
Growth direction
(a)
𝒈
(b)
𝟎𝟎𝟎𝟖
(c)
Figure 6
0.5 mm
𝒈
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4500
Distance from the bottom of the vertically sliced wafer (μm)
4000 3500 3000
0.02 cm
2500 2000 1500 1000 500 0 line A
E2 mode peak position Figure 7
line B
The distribution of basal plane dislocations (BPDs) in 4H-SiC crystals has been investigated.
Almost periodically arranged layers with a high density of BPDs along the growth direction existed.
The layers accompanied large tensile and compressive stresses in their upper and lower sides.
The stresses showed characteristic variations along the basal plane and the growth direction.
On the basis of these results, we discussed the formation and multiplication processes of BPDs.
S0022-0248(19)30203-9 https://doi.org/10.1016/j.jcrysgro.2019.03.027 CRYS 25045
To appear in:
Journal of Crystal Growth
Received Date: Revised Date: Accepted Date:
3 February 2019 27 March 2019 28 March 2019
Please cite this article as: T. Nakano, N. Shinagawa, M. Yabu, N. Ohtani, Formation and multiplication of basal plane dislocations during physical vapor transport growth of 4H-SiC crystals, Journal of Crystal Growth (2019), doi: https://doi.org/10.1016/j.jcrysgro.2019.03.027
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Formation and multiplication of basal plane dislocations during physical vapor transport growth of 4H-SiC crystals
Takahiro Nakano, Naoto Shinagawa, Masahiro Yabu, Noboru Ohtani* Kwansei Gakuin University, School of Science and Technology 2-1 Gakuen, Sanda, Hyogo 669-1337, Japan
Abstract
The distribution of basal plane dislocations (BPDs) in physical vapor transport (PVT) grown 4H-SiC crystals has been investigated using Raman microscopy and x-ray topography. X-ray _
topography observations of (1120) wafers vertically sliced along the growth direction from the crystals revealed that there existed almost periodically arranged layers with a high density of BPDs (bunched BPDs) in PVT-grown 4H-SiC crystals. These layers were characterized using Raman microscopy and high resolution x-ray diffraction, and it was found that large tensile and compressive stresses were associated with the layers. The stresses showed characteristic variations along the basal plane as well as the growth direction. Based on these results, the formation and multiplication processes of BPDs during PVT growth of 4H-SiC crystals are elucidated.
*
Corresponding author. E-mail address: [email protected] (N. Ohtani)
1
1. Introduction Reduction of crystallographic defects, particularly dislocations, in 4H-SiC crystals is still a major issue for the fabrication of reliable SiC power devices. At present, the physical vapor transport (PVT) growth method is the dominant growth method to grow commercially available SiC crystals. Among the dislocations existing in PVT-grown SiC crystals, basal plane dislocations (BPDs) are currently of the most serious concerns since they have negative impacts on the reliability of SiC unipolar devices such as SiC MOSFETs and JFETs [1,2] as well as bipolar devices [3,4]. Therefore, significant effort has been dedicated to the reduction of BPDs in 4H-SiC crystals; however, the formation and multiplication processes of BPDs in SiC crystals during PVT growth are still not well understood. An important difference between BPDs and threading dislocations extending along the c-axis (growth direction) in SiC crystals is in their formation process. Whereas most threading dislocations are inherited from the seed crystal and also often formed at the initial stage of crystal growth, BPDs observed in the crystal portions well distant from the seed crystal could not be formed through such mechanisms because their propagation is confined within the basal plane, normal to the growth direction. Therefore, BPDs existing in the top portion of SiC crystals grown along the c-axis must be nucleated and incorporated into the grown crystals from somewhere well distant from the seed crystal. A major cause of BPDs in SiC crystals is thought to be thermoelastic stress imposed on the growing crystals during PVT growth process. Large resolved shear stress on the (0001) basal plane causes the nucleation and multiplication of BPDs in SiC crystals. Several authors discussed the formation of BPDs using numerical simulation of thermoelastic stress imposed on the growing SiC crystals during PVT growth [5–8], and it has been revealed that when the thermoelastic stress on the basal plane exceeds the critical resolved shear stress in the _
(0001)<1120> slip system (approximately 1 MPa at the typical PVT growth temperatures),
2
BPDs are nucleated and multiplied during PVT growth process. Recently, Sonoda et al. [9] revealed that a large number of BPDs were introduced from _
the shoulder regions of 4H-SiC crystals grown in the [0001] direction by the PVT growth _
method, whereas BPDs were also nucleated at the outcrops of TSDs terminating at the (0001) facet region but hardly multiplied during PVT growth. These phenomena can be explained in terms of a large thermoelastic shear stress imposed on the shoulder regions of the growing crystals during PVT growth of 4H-SiC [8]. In this paper, we tried to reveal the formation and multiplication processes of BPDs during PVT growth of 4H-SiC crystals using Raman microscopy and x-ray topography, through the investigation of distribution of BPDs in PVT-grown 4H-SiC crystals. We observed characteristic distributions of BPDs in 4H-SiC crystals. A number of BPDs were found to be distributed almost periodically along the growth direction. It was also found that BPDs were distributed more densely near threading screw dislocations (TSDs), suggesting that the BPD multiplication preferentially occurs at TSDs propagating along the growth direction.
2. Experimental procedure _
Approximately 50 mm diameter 4H-SiC single crystals were grown on an on-axis (0001) 4H-SiC seed crystal by the PVT growth method. The typical growth temperature was about 2300–2400ºC, and the argon gas pressure was maintained between 1.0–2.0 kPa during growth. The grown crystals were nitrogen-doped, and they contained nitrogen donors in the mid-1018 cm−3 range. The nitrogen concentration in the grown crystals was estimated using Raman microscopy from calibration curve of the peak shift of the longitudinal optical phonon-plasmon coupled (LOPC) mode at ~983 cm−1 versus the nitrogen concentration in 4H-SiC crystals determined by secondary ion mass spectrometry (SIMS). Then, they were
3
_
sliced vertically along the growth direction (c-axis), and (1120) wafers were prepared from the crystals for Raman microscopy, x-ray topography, and high resolution x-ray diffraction (HRXRD). Raman microscopy was conducted at room temperature using a confocal optical microscope with a diode-pumped Nd:YVO4 laser at 532 nm for excitation. We measured the peak shift of the E2 mode at ~776 cm−1 in quasi-backscattering geometry with the polarization vectors of the incident and scattered lights parallel to the basal plane. The peak position of the E2 mode shifts toward higher (lower) wavenumbers when compressive (tensile) stress within the basal plane is imposed on 4H-SiC crystals [10–12]. Transmission x-ray topographs were taken from various portions of the wafers with _
_
symmetric 0008 and 1100 diffractions. The topographs were recorded using a charge-coupled device (CCD) camera with a pixel size of a few micrometers. A computer controlled detection system took multiple several-millimeter-sized topographic images that were stitched together to form topographic images of a few centimeter size. HRXRD rocking curve measurements (-scan mode) were performed using a double-axis diffractometer, where double crystal x-ray optics for Cu K1 radiation was employed, and the footprint size of the incident x-ray beam on the wafer surface was 0.2 mm × 0.7 mm.
3. Results and discussion
3.1. Cross-sectional x-ray topography observations of the BPD distribution in PVT-grown 4H-SiC crystals Figure 1 illustrates schematic figures of (a) the PVT growth reactor of 4H-SiC crystals _
and (b) the preparation scheme of 4H-SiC (1120) wafers vertically sliced along the growth
4
_
direction from a 4H-SiC single crystal grown on a 4H-SiC (0001)C seed crystal. The _
vertically sliced (1120) wafers consisted of two portions that are classified from the viewpoint of crystal growth. One is associated with the growth front which showed a domed shape (convex toward the growth direction). In this portion, crystal growth occurred during PVT growth. The other portion had side surfaces of the crystal, on which crystal growth did not occur nominally, and thus it had a constant diameter. _
_
Figures 2(a) and 2(b) show transmission x-ray topographs with 0008 and 11 _
00 diffractions, respectively, acquired from the (1120) wafer mentioned above. The area where the topographs were obtained is schematically indicated by a red open square in Fig. 2(c). As seen in the topographs, strong horizontal line contrasts are observed in Fig. 2(b) _
_
(diffraction vector: 1100), which almost diminish in Fig. 2(a) (diffraction vector: 0008). By contrast, in Fig. 2(a), line contrasts extending along the c-axis (growth direction) are observed, which correspond to TSDs. In Fig. 2(a), broad band contrasts are also observed at the positions where the strong horizontal line contrasts are observed in Fig. 2(b). The strong horizontal line contrasts observed in Fig. 2(b) are thought to be caused by BPDs or basal plane stacking faults existing in the crystal. To clarify the origin of the strong horizontal line contrasts in Fig. 2(b), we performed defect selective etching (molten KOH etching) and found that they were caused by a high density of BPDs; we call this type of BPDs “bunched BPDs” in this paper. It is noteworthy here that such bunched BPDs are almost periodically arranged along the growth direction in the grown crystals, as revealed in Fig. 2(b), where some of the bunched BPDs are indicated by closed white triangles.
3.2. Structural characterization of bunched BPDs in PVT-grown 4H-SiC crystals To examine the structure of bunched BPDs in more detail, we conducted Raman microscopy imaging of bunched BPDs. The imaged area is indicated by a red open square in
5
Figs. 2(a) and 2(b), and we plotted the peak position of the E2 mode at ~776 cm−1 across this area with the polarization vectors of incident and scattered lights parallel to the basal plane. The result is shown in Fig. 2(d); the light polarization is indicated by a double headed arrow in the figure, and as seen in the figure, a clear difference in contrast exists between the upper and lower sides of bunched BPDs. In the upper side of bunched BPDs, the peak position of the E2 mode shifts lower, while the peak is positioned at higher wavenumbers in the lower side, implying that tensile (compressive) strain exists in the upper (lower) side of bunched BPDs. Figure 3 shows a variation of the E2 peak position along the growth direction together with that of the a-lattice constant (lattice constant within the basal plane) measured by HRXRD. These variations were measured along a white dashed arrow indicated in Fig. 2(b). As seen in Fig. 3(a), abrupt shifts of the peak position were observed at the points where bunched BPDs existed; the positions of some of bunched BPDs are indicated by arrows in the right-hand side of Fig. 3; those indicated by the numbered arrows from 1 to 4 correspond to the bunched BPDs indicated by closed white triangles in Fig. 2(b). The shifts always occurred toward lower wave numbers at bunched BPDs when the light beam was scanned from the bottom to the top of grown crystals, implying that tensile strain within the basal plane always existed in the upper side of bunched BPDs. The a-lattice constant also varied along the growth direction and became larger in the upper side of bunched BPDs. This is consistent with the variation of the E2 mode peak position, and it can be concluded that the tensile strain within the basal plane existing in the upper side of bunched BPDs resulted in a larger a-lattice constant. Based on these results, we schematically illustrated the atomistic structure of bunched BPDs in Fig. 4, where the tensile and compressive strain sides of bunched BPDs are indicated by blue and pink colored bands. As seen in the figure, the compressive strain side contains extra half-planes pointing toward the [0001] direction (toward the seed crystal),
6
which causes tensile strain in the opposite side (upper side) of bunched BPDs and are terminated at bunched BPDs. The pointing direction of the extra half-planes associated with bunched BPDs is reasonably understood if bunched BPDs are introduced by the thermoelastic stress imposed on the growing crystal during PVT growth of SiC crystals. SiC PVT growth process is primarily driven by the temperature gradient along the c-axis (growth direction), and thus a large
shear stress is imposed on the growing crystal during growth, where r and z denote
the radial and the axial directions of the grown crystal (z is parallel to the c-axis). At the typical PVT growth temperatures (>2300ºC), the plastic deformation of SiC crystals occurs and the
shear stress is largely relieved by the introduction of BPDs in the crystal. During
the PVT growth process, a positive temperature gradient toward the growth direction is maintained, and thus, when the thermoelastic stress is relieved, BPDs that have extra-half-planes pointing toward the seed crystal are introduced.
3.3. Multiplication of BPDs during PVT growth of 4H-SiC crystals To investigate in more detail the formation and multiplication processes of BPDs during PVT growth of 4H-SiC crystals, we performed more extended Raman microscopy imaging of bunched BPDs in the lateral direction (parallel to the basal plane). The result of Raman microscopy is shown in Fig. 5(a). The image is a two-dimensional mapping of the peak position of the E2 mode at 776 cm−1. Similarly to Fig. 2(d), a clear difference in contrast exists between the upper and lower sides of bunched BPDs. It should be noted in this figure that the contrast difference between the upper and lower sides of bunched BPDs varies along the basal plane. In Fig. 5(a), portions with larger and smaller contrast differences are indicated by closed and open triangles, respectively. We found that the observed variation of the contrast difference (magnitude of the abrupt shift of the E2 mode peak position at bunched BPDs)
7
along the basal plane was correlated with the TSD density in the crystal. Figure 5(b) shows the relationship between the contrast difference and the TSD density; the latter was estimated from the density of the line contrasts extending along the c-axis in x-ray topographs with _
0008 diffraction [e.g., Fig. 2(a)] at the position where the contrast difference was measured by Raman microscopy. As seen in the figure, the magnitude of the abrupt peak shift at bunched BPDs shows a good correlation with the TSD density, and as the TSD density increases, the abrupt peak shift at bunched BPDs becomes larger, which implies that the TSD and BPD densities in PVT-grown 4H-SiC crystals have a positive correlation. Similar positive correlation between the TSD and BPD densities in PVT-grown 4H-SiC crystals was reported by Ohtani et al. [13]. They conducted defect selective etching using molten KOH to estimate the TSD and BPD densities in PVT-grown 4H-SiC crystals and found that the BPD density increases with increase of the TSD density in the crystals. They ascribed this positive correlation to a BPD multiplication process around TSDs. Temperature gradients existing in 4H-SiC crystals during PVT growth process glide BPDs on the basal plane and cause them to cut through TSDs extending along the c-axis in the crystals. After crossing TSDs, BPDs have super jogs parallel to the c-axis, which are immobile and anchored in the crystal. When the BPDs further glide under thermoelastic stress during PVT growth and/or post-growth cooling, the well-known Frank–Read type BPD multiplication occurs and consequently the BPD density significantly increases around TSDs [14,15]. This is the reason that the BPD and TSD densities show a positive correlation in 4H-SiC crystals.
3.4. Formation model of bunched BPDs in PVT-grown 4H-SiC crystals Finally, we discuss the formation mechanism of bunched BPDs observed in PVT-grown 4H-SiC crystals. We found that BPDs showed a characteristic distribution in PVT-grown 4H-SiC crystals; there existed layers with a high density of BPDs (bunched BPDs) arranged
8
almost periodically along the growth direction. As for the BPD formation in PVT-grown 4H-SiC crystals, Sonoda et al. reported that a number of BPDs were introduced from the shoulder regions of grown crystals during PVT growth [9]. In this respect, it is noteworthy that we observed bunched BPDs in the crystal portions relatively distant from the shoulder regions of grown crystals. In view of these results, it becomes an important question where and when bunched BPDs were introduced. One possible mechanism is that bunched BPDs were introduced from the side surfaces of grown crystals. The portions of grown crystals with a constant diameter had side surfaces located very close to the crucible walls, and they would have contacted with the walls under certain growth conditions during PVT growth or cooling process, because of the difference in the coefficient of thermal expansion between SiC and graphite. Therefore, a large thermoelastic stress could have been imposed on the side surfaces of grown crystals during PVT growth process, resulting in the introduction of a number of BPDs from the side surfaces. To verify this scenario, we conducted x-ray topography observations of a crystal portion near the side surface of a grown crystal. Figure 6 shows _
_
x-ray topographs of the portion with (a) 0008 and (b) 1100 diffractions, where the side surface of a grown crystal is indicated by closed red triangles. The location where the _
topographs were taken in a vertically sliced (1120) wafer is schematically indicated by a red open square in Fig. 6(c). As seen in Fig. 6(b), BPDs are observed in the crystal portion near the side surfaces; some of the BPDs are indicated by open triangles in the figure. Their density, however, appears not so high, and bunched BPDs are never observed in this portion. To further examine the origin of bunched BPDs, we performed more extended Raman microscopy analysis in the growth direction. Figure 7 shows variations of the E2 peak position along the growth direction. The variations were measured along two lines: one started from _
the bottom of the vertically sliced wafer and ended near the (0001) facet region at the growth front (denoted by line A), and the other was a line close to the side surface (edge) of a grown
9
crystal (denoted by line B). The locations of these two lines (lines A and B) in a vertically _
sliced (1120) wafer are schematically indicated by dashed arrows in the inset of Fig. 7. As shown in Fig. 7, the abrupt shift of the E2 peak position often occurs along line A; some of the shifts are indicated by open triangles in the figure. On the other hand, such abrupt shifts are not seen along line B, which is consistent with the results of x-ray topography shown in Fig. 6. To be noted here is that the abrupt shift becomes stronger as the measured point approaches the bottom of the vertically sliced wafer, implying that the BPD density in bunched BPDs gradually increases toward the seed crystal. The increase in the BPD density toward the seed crystal is reasonably understood if bunched BPDs are introduced from the growth front (top surface of the growing crystal) during PVT growth. Once bunched BPDs are introduced from the growth front, their BPD density gradually increases as the growth proceeds, since the total duration of thermoelastic stress imposed on the growing crystal becomes longer with increase of the growth time. The discussions mentioned above combine to lead to an important conclusion that bunched BPDs or their nuclei would be introduced at the growth front (domed surface) and hardly introduced from the side surfaces of grown crystals. The most plausible location where bunched BPDs are introduced would be the shoulder regions of growth crystals. Gao and Kakimoto [8] conducted three-dimensional numerical modeling of BPD multiplication in 4H-SiC bulk crystals and found that the domed shape of the growth front gives rise to a large resolved shear stress on the basal plane in the shoulder regions of grown crystals during PVT growth, thus causing a large number of BPDs introduced from these regions. Our results indicate that the shoulder regions of the growth front are a major source of BPDs, including bunched BPDs or their nuclei, in 4H-SiC, and BPDs introduced from these regions would determine the whole distribution of BPDs in PVT-grown 4H-SiC crystals. Our observations of the BPD distribution in 4H-SiC crystals also indicate that the
10
introduction of BPDs from the shoulder regions of grown crystals was almost periodically enhanced during PVT growth, which means that a large thermoelastic stress was almost periodically imposed on the growing crystal during PVT growth. The reason why such a periodicity existed is unknown at this moment, and further investigation is required to clarify this point.
4. Conclusions We have investigated the distribution of BPDs in PVT-grown 4H-SiC crystals using Raman microscopy and x-ray topography. It was found that there existed almost periodically arranged layers with a high density of BPDs (bunched BPDs) along the growth direction, and they accompanied large tensile and compressive stresses in their upper and lower sides, respectively. The stresses showed a characteristic variation along the basal plane, which was well correlated with the TSD density in the crystals. Bunched BPDs were more often observed in the central portion of grown crystals, and their BPD density gradually increased toward the seed crystal. On the basis of these results, we discussed the formation and multiplication processes of BPDs during PVT growth of 4H-SiC crystals.
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References [1] A. Agarwal, H. Fatima, S. Haney, R. Sei-Hyung, IEEE Electron Device Letters 28 (2007) 587–589. [2] V. Veliadis, H. Hearne, E. J. Stewart, M. Snook, W. Chang, J. D. Caldwell, H. C. Ha, N. El-Hinnawy, P. Borodulin, R. S. Howell, D. Urciuoli, A. Lelis, C. Scozzie, IEEE Electron Device Letters 33 (2012) 952–954. [3] H. Lendenmann, F. Dahquist, N. Johansson, R. Söderholm, P. Å. Nilsson, P. Bergman, P. Skytt, Mater. Sci. Forum 353–356 (2001) 727–730. [4] P. Bergman, H. Lendenmann, P. A. Nilsson, U. Lindefelt, P. Skytt, Mater. Sci. Forum 353–356 (2001) 299–302. [5] H. McD. Hobgood, M. F. Brady, W. H. Brixius, G. Fechko, R. C. Glass, D. Henshall, J. R. Jenny, R. T. Leonard, D. P. Malta, S. G. Müller, V. F. Tsvetkov, C. H. Carter Jr., Mater. Sci. Forum 338–342 (2000) 3–8. [6] M. Selder, L Kadinski, F. Durst, D. Hofmann, J. Cryst. Growth 226 (2001) 501–510. [7] R.-H. Ma, H. Zhang, M. Dudley, V. Prasad, J. Cryst. Growth 258 (2003) 318–330. [8] B. Gao, T. Kakimoto, Cryst. Growth Design 14 (2014) 1272–1278. [9] M. Sonoda, T. Nakano, K. Shioura, N. Shinagawa, N. Ohtani, J. Cryst. Growth 499 (2018) 24–29. [10] S. Nakashima, H. Harima, Phys. Stat. Sol. (a) 162 (1997) 39–64. [11] R. Sugie, T. Uchida, J. Appl. Phys. 122 (2017) 195703. [12] H. Sakakima, S. Takamoto, Y. Murakami, A. Hatano, A. Goryu, K. Hirohata, S. Izumi, Jpn. J. Appl. Phys. 57 (2018) 106602. [13] N. Ohtani, M. Katsuno, T. Fujimoto, M. Nakabayashi, H. Tsuge, H. Yashiro, T. Aigo, H. Hirano, T. Hoshino, W. Ohashi, Jpn. J. Appl. Phys. 48 (2009) 065503. [14] N. Ohtani, M. Katsuno, H. Tsuge, T. Fujimoto, M. Nakabayashi, H. Yashiro, M.
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Sawamura, T. Aigo, T. Hoshino, Jpn. J. Appl. Phys. 45 (2006) 1738–1742. [15] Y. Chen, G. Dhanaraj, M. Dudley, H. Zhang, R. Ma, Y. Shishkin, S. E. Saddow, Mater. Res. Soc. Symp. Proc. 911 (2006) 151–156.
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Figure Captions Fig. 1. Schematic figures of (a) the PVT growth reactor and (b) the preparation process of _
4H-SiC (1120) wafers vertically sliced along the growth direction from a 4H-SiC _
crystal grown on a 4H-SiC (0001)C seed crystal. _
_
Fig. 2. Transmission x-ray topographs with (a) 0008 and (b) 1100 diffractions of a vertically _
sliced 4H-SiC (1120) wafer, and (c) schematically shows the area in the wafer examined by transmission x-ray topography, which is indicated by a red open square. (d) shows a Raman microscopy image of a layer with a high density of BPDs (bunched BPDs). Fig. 3. Variations of (a) the peak position of the Raman-active E2 mode around 776 cm−1 and (b) the a-lattice constant measured by HRXRD along the growth direction on a _
vertically sliced (1120) wafer. The measured points and direction are indicated by a white dashed arrow in Fig. 2(b). (a) clearly demonstrates that abrupt shifts of the E2 peak position occurred at bunched BPDs, whose positions are indicated by arrows in the right-hand side of the figure. Fig. 4. Schematic figure of the atomic structure of bunched BPDs. The figure shows extra half-planes associated with bunched BPDs pointing toward the seed crystal, as well as the lattice distortion in the upper and lower sides of bunched BPDs. Fig. 5. (a) Extended Raman microscopy image of bunched BPDs along the basal plane. The image shows a variation of the E2 mode peak position around bunched BPDs. The open and closed triangles in the image indicate points which showed smaller and larger peak shifts at bunched BPDs, respectively. (b) shows a positive correlation between the magnitude of the abrupt peak shift of the E2 mode and the TSD density. _
_
Fig. 6. Transmission x-ray topographs with (a) 0008 and (b) 1100 diffractions acquired from a crystal portion near the side surface of a grown crystal with a constant
14
_
diameter. The position where the topographs were taken in a vertically sliced (1120) wafer is schematically indicated by a red open square in (c). Fig. 7. Variations of (a) the E2 mode peak position along the growth direction measured at _
the near-facet (line A) and the edge (line B) portions of a vertically sliced (1120) wafer. The locations of the two measured portions (lines A and B) in the wafer are schematically indicated by dashed arrows in the inset figure.
15
(a)
(b)
Growth front
Potion with a constant diameter
Figure 1
21/36
(a)
𝒈
(d)
(b)
𝟎𝟎𝟎𝟖
0.5 mm
𝒈
𝟏𝟏𝟎𝟎
50 μm
0.5 mm Tensile
(c)
Figure 2
polarization
Compressive
(a)
(b) Tensile strain Compressive strain
Growth direction
1
2
3
0.5 mm
776.76
776.78
776.80
776.82
Raman peak position (cm−1) Figure 3
0.308058 0.308060 0.308062 0.308064
a‐lattice constant (nm)
4
Tensile strain and larger a‐lattice constant
bunched BPDs
Compressive strain and smaller a‐lattice constant
Figure 4
(a)
polarization bunched BPDs
200 μm Low
High
(b)
Peak shift at bunched BPDs (cm−1)
Raman peak position 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0.00 1600
2100
2600
3100
3600
TSD density (cm−2) Figure 5
4100
Growth direction
(a)
𝒈
(b)
𝟎𝟎𝟎𝟖
(c)
Figure 6
0.5 mm
𝒈
𝟏𝟏𝟎𝟎
0.5 mm
4500
Distance from the bottom of the vertically sliced wafer (μm)
4000 3500 3000
0.02 cm
2500 2000 1500 1000 500 0 line A
E2 mode peak position Figure 7
line B
The distribution of basal plane dislocations (BPDs) in 4H-SiC crystals has been investigated.
Almost periodically arranged layers with a high density of BPDs along the growth direction existed.
The layers accompanied large tensile and compressive stresses in their upper and lower sides.
The stresses showed characteristic variations along the basal plane and the growth direction.
On the basis of these results, we discussed the formation and multiplication processes of BPDs.