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Nondestructive three-dimensional observation of defects in semi-insulating 6H-SiC single-crystal wafers using a scanning laser microscope (SLM) and infrared light-scattering tomography (IR-LST)
ARTICLE IN PRESS Journal of Crystal Growth 311 (2009) 3781–3786
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Nondestructive three-dimensional observation of defects in semi-insulating 6H-SiC single-crystal wafers using a scanning laser microscope (SLM) and infrared light-scattering tomography (IR-LST) Passapong Wutimakun a,, Chumpol Buteprongjit a, Jun Morimoto b a b
Department of Industrial Engineering, Chulachomklao Royal Military Academy, Suwanasorn Rd., Muang, Nakhon–Nayok 26001, Thailand Department of Materials Science and Engineering, National Defense Academy, 1-10-20 Hashirimizu, Yokosuka, Kanagawa 239-8686, Japan
a r t i c l e in fo
abstract
Article history: Received 18 September 2007 Received in revised form 30 April 2009 Accepted 12 May 2009 Communicated by M. Skowronski Available online 30 May 2009
Peripheral and central areas of a semi-insulating 6H-SiC single-crystal wafer were examined using a scanning laser microscope (SLM) and infrared light-scattering tomography (IR-LST). The form and density of the defects in each area were observed by SLM. We reconstructed three-dimensional (3D) IRLST images of scatterers by stacking 2D layer-by-layer IR-LST images on different planes. Using these 3D IR-LST images, variations in the defect distribution with depth were observed for the first time. To study the defect distribution and defect form in detail, we observed the defect configuration in the same volume as for 3D IR-LST images by magnified SLM and merged the images from the two techniques. Information on defects obtained using this approach will be very important in the development of highquality semi-insulating silicon carbide (SiC) substrates. & 2009 Elsevier B.V. All rights reserved.
PACS: 61.72. y 61.72.Ff 81.10. h 81.70.Fy Keywords: A1. Semi-insulating crystal growth A2. Defect and impurities in crystals A2. Direct observation of dislocation B1. Nondestructive testing B1. Optical method
1. Introduction Silicon carbide (SiC) has attracted much attention as a material for future high-temperature, high-power and high-frequency devices, because it has advantages such as a wide bandgap, high breakdown field, high electron velocity saturation, and high thermal conductivity [1]. High-resistivity, semi-insulating SiC single crystals are used as a substrate for high-frequency electronic devices based on both SiC and GaN [2]. However, defects in semi-insulating substrates negatively affect devices because of problems such as heat dissipation. Methods to grow undoped (vanadium-free) semi-insulating SiC single-crystal wafers with drastically decreased defect densities have recently been developed [3]. Thus, detailed evaluation of defects is important. To evaluate conventional defects in SiC substrates, KOH etching [4,5], X-ray topography [6,7], and photoluminescence mapping at room temperature [8,9] are generally used. However, KOH etching is not
Corresponding author.
E-mail address: [email protected] (P. Wutimakun). 0022-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2009.05.024
very effective for the C-face of SiC single crystals [10] and is a destructive method. X-ray topography and photoluminescence mapping cannot reveal variations in the defect distribution with depth. In commercially available SiC wafers, the density and size of defects have been reduced due to improvements in crystal growth. Therefore, detailed observation of defects using conventional methods is difficult. In this study, we used a combination of scanning laser microscopy (SLM) and infrared light-scattering tomography (IRLST) as a new approach for nondestructive, three-dimensional (3D) defect evaluation in SiC. SLM uses confocal optics that have high resolution and a greater depth of focus. The absorption-edge wavelength of 6H-SiC (lg=410 nm) is shorter than that of the light source (l0, He–Ne laser, 633 nm). In other words, SiC is transparent to the laser light. Therefore, buried defects in a SiC single crystal that cannot be detected by conventional microscopy can be observed by SLM. IR-LST was developed to detect bulk microdefects on the surface of semiconductor wafers [9]. In this method, IR light is directed obliquely onto a cleaved surface and is totally reflected at the wafer surface so that the effects of scattering at the surface are
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eliminated. Bulk microdefects are easily detected not only beneath the surface of polished wafers, but also beneath the surface of SiC wafers on which devices are fabricated. Furthermore, we can successfully reconstruct stereoimages of scatterers observed in layer-by-layer IR-LST images using the advantage of a large survey volume not accessible by other methods. The 3D IRLST technique developed in the present study allows observation of the defect distribution with depth. In addition, we used magnified SLM observations of the same sample area as for IRLST images to determine the defect form.
2. Experimental 2.1. Sample The sample was a vanadium-free semi-insulating 6H-SiC single-crystal substrate grown by the conventional sublimation method. The seed temperature and pressure were kept at 2100–2300 1C and 10–20 Torr, respectively, during growth. The growth axis was parallel to the /0 0 0 1S face. The sample was cut perpendicular to the c-axis, and both faces were mirror-polished. Vanadium in semi-insulating SiC crystals introduces deep levels that cause high resistivity [10]. However, the presence of vanadium is not desirable in certain types of devices and uniform resistivity in vanadium-doped materials poses a challenge [2]. Crystals of vanadium-free semi-insulating SiC were, thus, developed with enhanced crystalline quality and increased resistivity [3]. However, detailed defect information for vanadium-free semiinsulating SiC wafers has not been reported to date and this information is necessary for improving the performance of highfrequency devices. Therefore, we applied our combined SLM/IRLST technique to examine defect distributions along the growth direction of the 6H-SiC single-crystal substrate. 2.2. Observational methods The configuration of defects in peripheral and central areas of a semi-insulating 6H-SiC single-crystal substrate was determined by SLM (Model 1LM21D, Lasertec). Because of its high contrast and resolution, defects buried underneath the surface can be observed by SLM, so detection of etch pits by KOH etching is not necessary. To demonstrate the capability of SLM to distinguish defect types, we compared conventional SLM images of defects with SLM images of etch pit patterns in the same area. The etching temperature and time were 500 1C and 5 min, respectively. The defect types in each observation area (40 25 mm2) were counted for ten SLM images. The average defect density for each type was calculated as a measure of the crystalline quality. IR-LST (Model EXP-S2, Ratoc System Engineering) was then used for a detailed examination of defects in the semiconductor crystal and to investigate the distribution of defects with depth. The well-shaped beam of an Nd-YAG laser (l=1064 nm) was used to illuminate the bulk of the wafer. This laser light penetrates into 6H-SiC crystals because the wavelength is longer than the absorption-edge wavelength of 6H-SiC (lg=410 nm). The laserbeam diameter was adjusted to 25 mm to produce clear tomograms based on the scatterer size included. We carried out layerby-layer IR-LST observations to determine the 3D structure of defects in the wafer samples. The process for acquiring layer-bylayer tomographic images is schematically represented in Fig. 1. The laser beam scans the first layer and is then moved at intervals of 25 mm along the c-axis of the crystal to take further tomograms of successive internal planes. This yields a set of 3D IR-LST images from the first to the last layer through the depth of the sample.
Fig. 1. Schematic diagram of three-dimensional IR-LST image analysis.
Stereoimages were constructed using data visualization software (AVS/express, Advanced Visual Systems). Finally, interesting defects in the 3D IR-LST images were observed by SLM over the same sample area, and information from the two techniques was merged to study the defect distribution with depth and the plane defect form.
3. Results and discussion 3.1. Detailed SLM observation of structural defects A comparison of conventional SLM image of defects and the SLM image of etch pit patterns is shown in Fig. 2a and b for exactly the same sample area. According to a previous study [11], the large hexagonal pits marked A in Fig. 2b are hollow-core defects aligned along the c-axis, so-called micropipes (MP), and correspond to the large black dots marked A in Fig. 2a. The medium and small hexagonal pits marked B and C in Fig. 2b are closed-core screw dislocations (SD) and threading edge dislocations (TED) and correspond to the medium and small black dots marked B and C, respectively, in Fig. 2a. The results demonstrate that defect types in SiC substrates can be distinguished in SLM images without KOH etching. Fig. 3 shows SLM images of (a) peripheral and (b) central areas of a semi-insulating 6H-SiC single-crystal wafer. The same three types of defects as in Fig. 2b are observed. The dots marked A, B, and C have diameters of 18–25, 10–15, and 5–7 mm and correspond to MP, SD, and TED defects, respectively. The density of each type of dislocation in each area was estimated and the results are listed in Table 1. Several areas without low-angle grain boundaries were selected to obtain representative values of the background dislocation density. The size of each observational area was 40 25 mm2 and the total number of defects in the area ranged between 20 and 140. At the sample periphery, the defect
ARTICLE IN PRESS P. Wutimakun et al. / Journal of Crystal Growth 311 (2009) 3781–3786
a
3783
b B
B
B
B
C
C B
B B
B A
A A
C
A C C
C 20 µm
20 µm
Fig. 2. A comparison of the conventional SLM image of defect forms (a) and the SLM image of etch pit patterns at the same area (b). Etch pits are obtained by KOH etching at 500 1C and 5 min.
B B
B
B
C
C C
C
B B
B C
C C
B
B 100 µm
100 µm
Fig. 3. Observation of defect form by SLM. (a) The periphery part image and (b) the center part image.
Table 1 The densities of each defect types in the periphery and the center parts. Positions
Defect density (cm MP
Periphery part Center part
2.1 10 o102
2
) SD
2
TED 3
2.5 10 1.4 102
1.2 103 3.2 102
density increased in the order MPoTEDoSD. The density of all dislocation types in the center of the sample was lower than the corresponding densities at the periphery. However, in this case the TED density was higher than the SD density. Overall, the defect densities at the periphery were higher than those at the center. The densities of these defect types estimated by SLM correspond to X-ray topography results [12]. 3.2. Observation of defects by 2D and 3D IR-LST A 2D IR-LST image of the periphery is shown in Fig. 4a. Sharp and cloud-like scattering patterns are evident in several places.
The sharp scattering patterns are due to differences in the refractive index of light scattering from strain areas or impurity precipitates around defects, and have a diameter of 20–50 mm. The cloud-like scattering patterns are due to differences in the refractive index of light scattering by impurities in the sample. These scattering patterns are often observed for single crystals such as GaAs [13,14]. Moreover, scatterers of lingering shape and mist shape influenced by carbon inclusion have been observed as a peculiar scattering shape [14]. Fig. 4b shows a 2D IR-LST image of the center of the wafer. The size and number of scatterers in the center were obviously less than those at the periphery. Cloud-like scattering patterns due to impurities were rarely observed. To compare the impurity concentrations in the center and the periphery, photoluminescence (PL) measurements were conducted at 6 K (Fig. 5) using a Xe lamp with a wavelength of 266 nm for excitation. We determined the impurity types from PL spectral data according to the literature [15–17]. The PL intensity for all impurity types at the periphery was higher than that for impurities in the center. This indicates that the impurity concentration was non-uniform and was higher at the periphery than in the center, explaining why cloud-like scattering patterns due to impurities were rarely observed in the center.
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500 µm
500 µm
Fig. 4. Two-dimensional IR-LST image of semi-insulating 6H-SiC single-crystal wafer. (a) The periphery part and (b) the center part.
50 Periphery part Center part 40 PL Intensity (arb. units)
0001
Cr Cr
10 µm
B
[µm] 300
A 10 µm
30
C
Al
20
0 10 µm
Ga
300 µm
N B Be 0001
10 C1
[µm] 300
0 1.5
2.0
2.5 Photon Energy (eV)
3.0
3.5
Fig. 5. Comparison of the impurity concentrations at the center and the periphery parts of semi-insulating 6H-SiC single-crystal wafer by PL measurement (6 K).
10 µm
B1 10 µm
0 300 µm
Fig. 6a shows a 3D IR-LST image of the center for the same area as in Fig. 4a. The 3D distributions of linear scatterers parallel to the c-axis, corresponding to the sharp scattering patterns in Fig. 4a, were observed. The size of the linear scatterers depends on the refractive index of the strain area and impurity precipitates around the defects. Since the defects in SiC single crystals parallel to the c-axis are classified as MP, SD, and TED [11], there is a possibility that the different sizes of linear scatterers in the 3D IRLST image coincide with these three defect types. Thus, we used magnified SLM to observe the three different sizes of linear scatterers to distinguish the defect types. This confirmed that the large continuous scatterer marked A corresponds to an MP in the magnified SLM image. Likewise, the medium continuous scatterer marked B corresponds to an SD and small discontinuous scatterer marked C corresponds to a TED. A 3D IR-LST image of the center for the same area as in Fig. 4b is shown in Fig. 6b. The large and continuous linear scatterer marked A in Fig. 6a was rarely observed. In contrast, medium and continuous scatterers (marked B1) and especially small and discontinuous scatterers (marked C1) were often observed. From
Fig. 6. Three-dimensional IR-LST image of semi-insulating 6H-SiC single-crystal wafer. (a) The periphery part and (b) the center part.
magnified SLM observation we can conclude that B1 and C1 correspond to SD and TED, respectively. The size and number of linear scatterers were considerably less in the center than at the periphery, which supports the 2D IR-LST results and SLM observations. By combining 3D IR-LST and magnified SLM observations, we could identify the types of defects and the defect distribution with depth. Differences between linear scatterers were confirmed by the relationship between the scattering intensity and distance from the surface, as shown in Fig. 7. The variation in scattering intensity is presented as a function of the change in laser position along the sample thickness, where depth=0 mm on the x-axis denotes the first IR-LST layer rather than the top surface of the sample. MPs showed the highest scattering intensity and fluctuation, SDs showed lower scattering intensity and fluctuation, and TEDs showed the lowest and most discontinuous scattering intensity.
ARTICLE IN PRESS P. Wutimakun et al. / Journal of Crystal Growth 311 (2009) 3781–3786
Since the scattering intensity estimated from IR-LST images depends on the scatterer size [18], the distributions of linear scatterers in 3D IR-LST were different. In other words, the refractive index of strain areas or impurity precipitates around each defect was not constant along the c-axis (growth direction). For rather small strain areas and impurity precipitates around defects, scatterers were not detected by IR-LST at the same magnification power, and thus the distribution of linear scatterers marked C was not continuous in the 3D IR-LST images. Fig. 8 shows a rotation of the 3D IR-LST image of the periphery. The defect distribution with depth could be observed over 3601 by
Scattered Light Intensity (arb. units)
marked A marked B marked C
3785
changing the 3D IR-LST angle. Linear scatterers were vertically distributed along the c-axis and the distribution changed shape slightly along it. This is due to the change in refractive index of strain areas and impurity precipitates around defects, which supports the scattering intensity results in Fig. 7. The SLM and IR-LST results allow clear differentiation of structural defects in a semi-insulating 6H-SiC single-crystal wafer at the periphery and center. Since the periphery of the wafer is subject to the effects of thermal stress during crystal growth [18], defects occur there and are large in size, and strain areas and impurity precipitates around defects are not constant along the c-axis. Moriya and Ogawa [19] precisely controlled the type of additives and the addition volume in GaAs and observed that a reduction in impurity background during crystal growth of GaAs leads to a decrease in the number of scatterers in IR-LST images. Therefore, it is likely that cloud-like scattering patterns due to impurities or precipitates in IR-LST images were fewer than expected because our sample was not doped (vanadium-free), resulting in lower impurity levels during crystal growth. Information on defects provided by our approach is very important for developing high-quality semi-insulating SiC substrates. Application of this method to other SiC polytypes will be the subject of future work.
4. Conclusions
0
50
100
150 200 Depth (mm)
250
300
Fig. 7. The relationship between scattering light intensity and distance from surface at each line scattering patterns.
We demonstrated differences in structural defects between peripheral and central areas of a semi-insulating 6H-SiC singlecrystal wafer by SLM and IR-LST observations. SLM observations revealed different defect types without the need for KOH etching and the defect density was easily estimated. Scatterers in 2D IRLST observations revealed both defects and impurities, so we could easily confirm differences in the distribution of defects and impurities in peripheral versus central areas of a sample. Defect distributions with depth were clearly observed using 3D IR-LST. The size and shape distribution of line scatterers in 3D IR-LST images depend on the refractive index of the strain area and impurity precipitates around defects. Moreover, the combination of 3D IR-LST and magnified SLM images of the same sample area is an effective method for studying defect distributions with depth and defect types.
Acknowledgement The authors thank Dr. Hiromu Shiomi, Chief Technology Officer of SiXON, Ltd. for valuable discussion. References
Fig. 8. The rotation of three-dimensional IR-LST image of the periphery part.
[1] H. Matsunami, Jpn. J. Appl. Phys. 43 (2004) 6835. [2] M. Bickermann, D. Hofmann, T.L. Straubinger, R. Weinga¨rtner, P.J. Wellmann, A. Winnacker, Appl. Surf. Sci. 184 (2001) 84. [3] H. Shiomi, H. Kinoshita, T. Furusho, T. Hayashi, M. Tajima, E. Higashi, J. Cryst. Growth 292 (2006) 188. [4] S. Wang, E.K. Sanchez, A. Kopec, S. Poplawski, R. Ware, S. Holmes, C.M. Balkas, A.G. Timmerman, Mater. Sci. Forum 389 (2002) 35. [5] S.A. Sakwe, R. Muller, P.J. Wellmann, J. Cryst. Growth 289 (2006) 520. [6] M. Dudley, X.R. Huang, W. Huang, A. Powell, S. Wang, P. Neudeck, M. Skowronski, Appl. Phys. Lett. 75 (1999) 784. [7] M. Sasaki, A. Hirai, T. Miyanagi, T. Furusho, T. Nishiguchi, H. Shiomi, S. Nishino, Mater. Sci. Forum 389 (2002) 411. [8] F. Jun, F. Hisashi, S. Takayuki, Jpn. J. Appl. Phys. 32 (1993) 5178. [9] Sergey A. Reshanov, Diamond Relat. Mater. 9 (2000) 480. [10] M.E. Zvanut, Wonwoo Lee, W.C. Mitchel, W.D. Mitchell, G. Landis, Physica B 376 (2006) 346. [11] J. Takahashi, M. Kanaya, Y. Fujiwara, J. Cryst. Growth 135 (1994) 61. [12] T. Ohno, H. Yamaguchi, S. Kuroda, T. Kojima, T. Suzuki, K. Arai, J. Cryst. Growth 260 (2004) 209.
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[13] J.P. Fillard, P. Gall, A. Baroudi, A. George, J. Bonnafe, Jpn. J. Appl. Phys. 26 (1987) L1255. [14] O. Kordina, C. Hallin, A. Ellison, A.S. Bakin, I.G. Ivanov, A. Henry, R. Yakimova, M. Tuominen, A. Vehanen, E. Janzen, Appl. Phys. Lett. 69 (1996) 1456. [15] M. Ikeda, H. Matsunami, T. Tanaka, Phys. Rev. B 22 (1980) 2842–2854. [16] J.A. Freitas, Prop. Silicon Carbide 13 (1995) 29–41.
[17] E. Higashi, M. Tajima, N. Hoshino, T. Hayashi, H. Kinoshita, H. Shiomi, S. Matsumoto, Mater. Sci. Semicond. Proc. 9 (2006) 53–57. [18] E. Schmitt, T. Straubinger, M. Rasp, A.D. Weber, Superlattices Microstruct. 40 (2006) 320. [19] K. Moriya, T. Ogawa, Jpn. J. Appl. Phys. 22 (1983) L207.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Nondestructive three-dimensional observation of defects in semi-insulating 6H-SiC single-crystal wafers using a scanning laser microscope (SLM) and infrared light-scattering tomography (IR-LST) Passapong Wutimakun a,, Chumpol Buteprongjit a, Jun Morimoto b a b
Department of Industrial Engineering, Chulachomklao Royal Military Academy, Suwanasorn Rd., Muang, Nakhon–Nayok 26001, Thailand Department of Materials Science and Engineering, National Defense Academy, 1-10-20 Hashirimizu, Yokosuka, Kanagawa 239-8686, Japan
a r t i c l e in fo
abstract
Article history: Received 18 September 2007 Received in revised form 30 April 2009 Accepted 12 May 2009 Communicated by M. Skowronski Available online 30 May 2009
Peripheral and central areas of a semi-insulating 6H-SiC single-crystal wafer were examined using a scanning laser microscope (SLM) and infrared light-scattering tomography (IR-LST). The form and density of the defects in each area were observed by SLM. We reconstructed three-dimensional (3D) IRLST images of scatterers by stacking 2D layer-by-layer IR-LST images on different planes. Using these 3D IR-LST images, variations in the defect distribution with depth were observed for the first time. To study the defect distribution and defect form in detail, we observed the defect configuration in the same volume as for 3D IR-LST images by magnified SLM and merged the images from the two techniques. Information on defects obtained using this approach will be very important in the development of highquality semi-insulating silicon carbide (SiC) substrates. & 2009 Elsevier B.V. All rights reserved.
PACS: 61.72. y 61.72.Ff 81.10. h 81.70.Fy Keywords: A1. Semi-insulating crystal growth A2. Defect and impurities in crystals A2. Direct observation of dislocation B1. Nondestructive testing B1. Optical method
1. Introduction Silicon carbide (SiC) has attracted much attention as a material for future high-temperature, high-power and high-frequency devices, because it has advantages such as a wide bandgap, high breakdown field, high electron velocity saturation, and high thermal conductivity [1]. High-resistivity, semi-insulating SiC single crystals are used as a substrate for high-frequency electronic devices based on both SiC and GaN [2]. However, defects in semi-insulating substrates negatively affect devices because of problems such as heat dissipation. Methods to grow undoped (vanadium-free) semi-insulating SiC single-crystal wafers with drastically decreased defect densities have recently been developed [3]. Thus, detailed evaluation of defects is important. To evaluate conventional defects in SiC substrates, KOH etching [4,5], X-ray topography [6,7], and photoluminescence mapping at room temperature [8,9] are generally used. However, KOH etching is not
Corresponding author.
E-mail address: [email protected] (P. Wutimakun). 0022-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2009.05.024
very effective for the C-face of SiC single crystals [10] and is a destructive method. X-ray topography and photoluminescence mapping cannot reveal variations in the defect distribution with depth. In commercially available SiC wafers, the density and size of defects have been reduced due to improvements in crystal growth. Therefore, detailed observation of defects using conventional methods is difficult. In this study, we used a combination of scanning laser microscopy (SLM) and infrared light-scattering tomography (IRLST) as a new approach for nondestructive, three-dimensional (3D) defect evaluation in SiC. SLM uses confocal optics that have high resolution and a greater depth of focus. The absorption-edge wavelength of 6H-SiC (lg=410 nm) is shorter than that of the light source (l0, He–Ne laser, 633 nm). In other words, SiC is transparent to the laser light. Therefore, buried defects in a SiC single crystal that cannot be detected by conventional microscopy can be observed by SLM. IR-LST was developed to detect bulk microdefects on the surface of semiconductor wafers [9]. In this method, IR light is directed obliquely onto a cleaved surface and is totally reflected at the wafer surface so that the effects of scattering at the surface are
ARTICLE IN PRESS 3782
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eliminated. Bulk microdefects are easily detected not only beneath the surface of polished wafers, but also beneath the surface of SiC wafers on which devices are fabricated. Furthermore, we can successfully reconstruct stereoimages of scatterers observed in layer-by-layer IR-LST images using the advantage of a large survey volume not accessible by other methods. The 3D IRLST technique developed in the present study allows observation of the defect distribution with depth. In addition, we used magnified SLM observations of the same sample area as for IRLST images to determine the defect form.
2. Experimental 2.1. Sample The sample was a vanadium-free semi-insulating 6H-SiC single-crystal substrate grown by the conventional sublimation method. The seed temperature and pressure were kept at 2100–2300 1C and 10–20 Torr, respectively, during growth. The growth axis was parallel to the /0 0 0 1S face. The sample was cut perpendicular to the c-axis, and both faces were mirror-polished. Vanadium in semi-insulating SiC crystals introduces deep levels that cause high resistivity [10]. However, the presence of vanadium is not desirable in certain types of devices and uniform resistivity in vanadium-doped materials poses a challenge [2]. Crystals of vanadium-free semi-insulating SiC were, thus, developed with enhanced crystalline quality and increased resistivity [3]. However, detailed defect information for vanadium-free semiinsulating SiC wafers has not been reported to date and this information is necessary for improving the performance of highfrequency devices. Therefore, we applied our combined SLM/IRLST technique to examine defect distributions along the growth direction of the 6H-SiC single-crystal substrate. 2.2. Observational methods The configuration of defects in peripheral and central areas of a semi-insulating 6H-SiC single-crystal substrate was determined by SLM (Model 1LM21D, Lasertec). Because of its high contrast and resolution, defects buried underneath the surface can be observed by SLM, so detection of etch pits by KOH etching is not necessary. To demonstrate the capability of SLM to distinguish defect types, we compared conventional SLM images of defects with SLM images of etch pit patterns in the same area. The etching temperature and time were 500 1C and 5 min, respectively. The defect types in each observation area (40 25 mm2) were counted for ten SLM images. The average defect density for each type was calculated as a measure of the crystalline quality. IR-LST (Model EXP-S2, Ratoc System Engineering) was then used for a detailed examination of defects in the semiconductor crystal and to investigate the distribution of defects with depth. The well-shaped beam of an Nd-YAG laser (l=1064 nm) was used to illuminate the bulk of the wafer. This laser light penetrates into 6H-SiC crystals because the wavelength is longer than the absorption-edge wavelength of 6H-SiC (lg=410 nm). The laserbeam diameter was adjusted to 25 mm to produce clear tomograms based on the scatterer size included. We carried out layerby-layer IR-LST observations to determine the 3D structure of defects in the wafer samples. The process for acquiring layer-bylayer tomographic images is schematically represented in Fig. 1. The laser beam scans the first layer and is then moved at intervals of 25 mm along the c-axis of the crystal to take further tomograms of successive internal planes. This yields a set of 3D IR-LST images from the first to the last layer through the depth of the sample.
Fig. 1. Schematic diagram of three-dimensional IR-LST image analysis.
Stereoimages were constructed using data visualization software (AVS/express, Advanced Visual Systems). Finally, interesting defects in the 3D IR-LST images were observed by SLM over the same sample area, and information from the two techniques was merged to study the defect distribution with depth and the plane defect form.
3. Results and discussion 3.1. Detailed SLM observation of structural defects A comparison of conventional SLM image of defects and the SLM image of etch pit patterns is shown in Fig. 2a and b for exactly the same sample area. According to a previous study [11], the large hexagonal pits marked A in Fig. 2b are hollow-core defects aligned along the c-axis, so-called micropipes (MP), and correspond to the large black dots marked A in Fig. 2a. The medium and small hexagonal pits marked B and C in Fig. 2b are closed-core screw dislocations (SD) and threading edge dislocations (TED) and correspond to the medium and small black dots marked B and C, respectively, in Fig. 2a. The results demonstrate that defect types in SiC substrates can be distinguished in SLM images without KOH etching. Fig. 3 shows SLM images of (a) peripheral and (b) central areas of a semi-insulating 6H-SiC single-crystal wafer. The same three types of defects as in Fig. 2b are observed. The dots marked A, B, and C have diameters of 18–25, 10–15, and 5–7 mm and correspond to MP, SD, and TED defects, respectively. The density of each type of dislocation in each area was estimated and the results are listed in Table 1. Several areas without low-angle grain boundaries were selected to obtain representative values of the background dislocation density. The size of each observational area was 40 25 mm2 and the total number of defects in the area ranged between 20 and 140. At the sample periphery, the defect
ARTICLE IN PRESS P. Wutimakun et al. / Journal of Crystal Growth 311 (2009) 3781–3786
a
3783
b B
B
B
B
C
C B
B B
B A
A A
C
A C C
C 20 µm
20 µm
Fig. 2. A comparison of the conventional SLM image of defect forms (a) and the SLM image of etch pit patterns at the same area (b). Etch pits are obtained by KOH etching at 500 1C and 5 min.
B B
B
B
C
C C
C
B B
B C
C C
B
B 100 µm
100 µm
Fig. 3. Observation of defect form by SLM. (a) The periphery part image and (b) the center part image.
Table 1 The densities of each defect types in the periphery and the center parts. Positions
Defect density (cm MP
Periphery part Center part
2.1 10 o102
2
) SD
2
TED 3
2.5 10 1.4 102
1.2 103 3.2 102
density increased in the order MPoTEDoSD. The density of all dislocation types in the center of the sample was lower than the corresponding densities at the periphery. However, in this case the TED density was higher than the SD density. Overall, the defect densities at the periphery were higher than those at the center. The densities of these defect types estimated by SLM correspond to X-ray topography results [12]. 3.2. Observation of defects by 2D and 3D IR-LST A 2D IR-LST image of the periphery is shown in Fig. 4a. Sharp and cloud-like scattering patterns are evident in several places.
The sharp scattering patterns are due to differences in the refractive index of light scattering from strain areas or impurity precipitates around defects, and have a diameter of 20–50 mm. The cloud-like scattering patterns are due to differences in the refractive index of light scattering by impurities in the sample. These scattering patterns are often observed for single crystals such as GaAs [13,14]. Moreover, scatterers of lingering shape and mist shape influenced by carbon inclusion have been observed as a peculiar scattering shape [14]. Fig. 4b shows a 2D IR-LST image of the center of the wafer. The size and number of scatterers in the center were obviously less than those at the periphery. Cloud-like scattering patterns due to impurities were rarely observed. To compare the impurity concentrations in the center and the periphery, photoluminescence (PL) measurements were conducted at 6 K (Fig. 5) using a Xe lamp with a wavelength of 266 nm for excitation. We determined the impurity types from PL spectral data according to the literature [15–17]. The PL intensity for all impurity types at the periphery was higher than that for impurities in the center. This indicates that the impurity concentration was non-uniform and was higher at the periphery than in the center, explaining why cloud-like scattering patterns due to impurities were rarely observed in the center.
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500 µm
500 µm
Fig. 4. Two-dimensional IR-LST image of semi-insulating 6H-SiC single-crystal wafer. (a) The periphery part and (b) the center part.
50 Periphery part Center part 40 PL Intensity (arb. units)
0001
Cr Cr
10 µm
B
[µm] 300
A 10 µm
30
C
Al
20
0 10 µm
Ga
300 µm
N B Be 0001
10 C1
[µm] 300
0 1.5
2.0
2.5 Photon Energy (eV)
3.0
3.5
Fig. 5. Comparison of the impurity concentrations at the center and the periphery parts of semi-insulating 6H-SiC single-crystal wafer by PL measurement (6 K).
10 µm
B1 10 µm
0 300 µm
Fig. 6a shows a 3D IR-LST image of the center for the same area as in Fig. 4a. The 3D distributions of linear scatterers parallel to the c-axis, corresponding to the sharp scattering patterns in Fig. 4a, were observed. The size of the linear scatterers depends on the refractive index of the strain area and impurity precipitates around the defects. Since the defects in SiC single crystals parallel to the c-axis are classified as MP, SD, and TED [11], there is a possibility that the different sizes of linear scatterers in the 3D IRLST image coincide with these three defect types. Thus, we used magnified SLM to observe the three different sizes of linear scatterers to distinguish the defect types. This confirmed that the large continuous scatterer marked A corresponds to an MP in the magnified SLM image. Likewise, the medium continuous scatterer marked B corresponds to an SD and small discontinuous scatterer marked C corresponds to a TED. A 3D IR-LST image of the center for the same area as in Fig. 4b is shown in Fig. 6b. The large and continuous linear scatterer marked A in Fig. 6a was rarely observed. In contrast, medium and continuous scatterers (marked B1) and especially small and discontinuous scatterers (marked C1) were often observed. From
Fig. 6. Three-dimensional IR-LST image of semi-insulating 6H-SiC single-crystal wafer. (a) The periphery part and (b) the center part.
magnified SLM observation we can conclude that B1 and C1 correspond to SD and TED, respectively. The size and number of linear scatterers were considerably less in the center than at the periphery, which supports the 2D IR-LST results and SLM observations. By combining 3D IR-LST and magnified SLM observations, we could identify the types of defects and the defect distribution with depth. Differences between linear scatterers were confirmed by the relationship between the scattering intensity and distance from the surface, as shown in Fig. 7. The variation in scattering intensity is presented as a function of the change in laser position along the sample thickness, where depth=0 mm on the x-axis denotes the first IR-LST layer rather than the top surface of the sample. MPs showed the highest scattering intensity and fluctuation, SDs showed lower scattering intensity and fluctuation, and TEDs showed the lowest and most discontinuous scattering intensity.
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Since the scattering intensity estimated from IR-LST images depends on the scatterer size [18], the distributions of linear scatterers in 3D IR-LST were different. In other words, the refractive index of strain areas or impurity precipitates around each defect was not constant along the c-axis (growth direction). For rather small strain areas and impurity precipitates around defects, scatterers were not detected by IR-LST at the same magnification power, and thus the distribution of linear scatterers marked C was not continuous in the 3D IR-LST images. Fig. 8 shows a rotation of the 3D IR-LST image of the periphery. The defect distribution with depth could be observed over 3601 by
Scattered Light Intensity (arb. units)
marked A marked B marked C
3785
changing the 3D IR-LST angle. Linear scatterers were vertically distributed along the c-axis and the distribution changed shape slightly along it. This is due to the change in refractive index of strain areas and impurity precipitates around defects, which supports the scattering intensity results in Fig. 7. The SLM and IR-LST results allow clear differentiation of structural defects in a semi-insulating 6H-SiC single-crystal wafer at the periphery and center. Since the periphery of the wafer is subject to the effects of thermal stress during crystal growth [18], defects occur there and are large in size, and strain areas and impurity precipitates around defects are not constant along the c-axis. Moriya and Ogawa [19] precisely controlled the type of additives and the addition volume in GaAs and observed that a reduction in impurity background during crystal growth of GaAs leads to a decrease in the number of scatterers in IR-LST images. Therefore, it is likely that cloud-like scattering patterns due to impurities or precipitates in IR-LST images were fewer than expected because our sample was not doped (vanadium-free), resulting in lower impurity levels during crystal growth. Information on defects provided by our approach is very important for developing high-quality semi-insulating SiC substrates. Application of this method to other SiC polytypes will be the subject of future work.
4. Conclusions
0
50
100
150 200 Depth (mm)
250
300
Fig. 7. The relationship between scattering light intensity and distance from surface at each line scattering patterns.
We demonstrated differences in structural defects between peripheral and central areas of a semi-insulating 6H-SiC singlecrystal wafer by SLM and IR-LST observations. SLM observations revealed different defect types without the need for KOH etching and the defect density was easily estimated. Scatterers in 2D IRLST observations revealed both defects and impurities, so we could easily confirm differences in the distribution of defects and impurities in peripheral versus central areas of a sample. Defect distributions with depth were clearly observed using 3D IR-LST. The size and shape distribution of line scatterers in 3D IR-LST images depend on the refractive index of the strain area and impurity precipitates around defects. Moreover, the combination of 3D IR-LST and magnified SLM images of the same sample area is an effective method for studying defect distributions with depth and defect types.
Acknowledgement The authors thank Dr. Hiromu Shiomi, Chief Technology Officer of SiXON, Ltd. for valuable discussion. References
Fig. 8. The rotation of three-dimensional IR-LST image of the periphery part.
[1] H. Matsunami, Jpn. J. Appl. Phys. 43 (2004) 6835. [2] M. Bickermann, D. Hofmann, T.L. Straubinger, R. Weinga¨rtner, P.J. Wellmann, A. Winnacker, Appl. Surf. Sci. 184 (2001) 84. [3] H. Shiomi, H. Kinoshita, T. Furusho, T. Hayashi, M. Tajima, E. Higashi, J. Cryst. Growth 292 (2006) 188. [4] S. Wang, E.K. Sanchez, A. Kopec, S. Poplawski, R. Ware, S. Holmes, C.M. Balkas, A.G. Timmerman, Mater. Sci. Forum 389 (2002) 35. [5] S.A. Sakwe, R. Muller, P.J. Wellmann, J. Cryst. Growth 289 (2006) 520. [6] M. Dudley, X.R. Huang, W. Huang, A. Powell, S. Wang, P. Neudeck, M. Skowronski, Appl. Phys. Lett. 75 (1999) 784. [7] M. Sasaki, A. Hirai, T. Miyanagi, T. Furusho, T. Nishiguchi, H. Shiomi, S. Nishino, Mater. Sci. Forum 389 (2002) 411. [8] F. Jun, F. Hisashi, S. Takayuki, Jpn. J. Appl. Phys. 32 (1993) 5178. [9] Sergey A. Reshanov, Diamond Relat. Mater. 9 (2000) 480. [10] M.E. Zvanut, Wonwoo Lee, W.C. Mitchel, W.D. Mitchell, G. Landis, Physica B 376 (2006) 346. [11] J. Takahashi, M. Kanaya, Y. Fujiwara, J. Cryst. Growth 135 (1994) 61. [12] T. Ohno, H. Yamaguchi, S. Kuroda, T. Kojima, T. Suzuki, K. Arai, J. Cryst. Growth 260 (2004) 209.
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[13] J.P. Fillard, P. Gall, A. Baroudi, A. George, J. Bonnafe, Jpn. J. Appl. Phys. 26 (1987) L1255. [14] O. Kordina, C. Hallin, A. Ellison, A.S. Bakin, I.G. Ivanov, A. Henry, R. Yakimova, M. Tuominen, A. Vehanen, E. Janzen, Appl. Phys. Lett. 69 (1996) 1456. [15] M. Ikeda, H. Matsunami, T. Tanaka, Phys. Rev. B 22 (1980) 2842–2854. [16] J.A. Freitas, Prop. Silicon Carbide 13 (1995) 29–41.
[17] E. Higashi, M. Tajima, N. Hoshino, T. Hayashi, H. Kinoshita, H. Shiomi, S. Matsumoto, Mater. Sci. Semicond. Proc. 9 (2006) 53–57. [18] E. Schmitt, T. Straubinger, M. Rasp, A.D. Weber, Superlattices Microstruct. 40 (2006) 320. [19] K. Moriya, T. Ogawa, Jpn. J. Appl. Phys. 22 (1983) L207.