報告人 : 楊庭維班級 : 碩研機械二甲學號 :M9910217

20120109

高等物理冶金 期末報告

•前言

•實驗步驟

•結果與討論

•結論

•參考資料

Outline

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前言

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． Silicon Carbide has excellent physical, mechanical and electro- nic properties, which make it a promising material for high-power, high-temperature and high-frequency electronic devices [1–3]. Sublimation method (modified Lely method) [4,5] is a successful method for growing large diameter SiC bulk single crystals. Recently, the diameter and quality of SiC substrate have made tremendous progress [6]. However, SiC wafers still have high densities of structural defects, such as dislocations and LAGBs. ． LAGBs can extend to the epitaxial layer and degrade device performance [7–9]. Glass et al. [10] investigated the structural defects in 6H–SiC wafers by HRXRD and observed multiple peaks of symmetric or asymmetric reflections from the rocking curves of SiC (0001) wafers.They explained that X-ray reflections occurred at different Omega angles for several misoriented domains and each domain at two sides of grain boundaries was of high quality and contained few defects. Glass et al. proposed that LAGBs originated from a spiral growth mechanism. Amelinckx and Strumane [11] and Tuominenetal. [12] observed the morphology of KOH etched (0001) SiC wafer. They found that the domain boundaries were arranged by high-density etch pits, while domain centers exhibited few dislocation etch pits. Takahashi et al. [13] studied the grain boundaries by X-ray topography. They noticed that the rows of each pits were aligned almost in <1-100> direction. Ha et al. [14] studied the structure of grain boundaries using a combination of transmission electron microscopy (TEM), HRXRD and KOH etching. They confirmed that LAGBs could be formed by poly- gonization of the threading edge dislocations. However, the formation and evolution mechanism of LAGBs in growth process are still not fully understood. ． In many experiments, we found that the distributions of LAGBs were related with temperature field in 200 6H–SiC growth. Inthis work, we studied themorphologies and structures of LAGBs in 6H–SiC single crystals grown in different radial temperature gradients (RTGs) by acombination of KOH etching, optical scanner and HRXRD. The evolution process of LAGBs was confirmed and the relationship between the structure of LAGBs and RTGs was revealed.

實驗步驟

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The on-axis 2’’ undoped 6H–SiC bulk crystals were grown by sublimation method.Two kinds of temperature fields with different RTG were designed. According to thermal field simula- tion results, the large RTG was 6 /cm while the small one was 1.2 /cm. In two cases, seed temperature was ℃ ℃controlled at 2250 , which was calculated based on the measured tempera- ture of the crucible lid ℃surface. The growth rate was controlled at about 200 µm/h and the length of crystal center was 20mm. The crystal grown in large RTG had a 5.3mm convexity growth interface and the one grown in small RTG had a 1.3mm convexity growth interface.The (0001) wafers,with distances of 1,8 and 14 mm from the seed, were cut from crystal ingot as representa- tives of initial, middle and late stages, respectively, and then double-side polished. Raman spectroscopy results indicated that 6H–SiC polytype was uniform. Chemical wet etching in molten KOH was used to reveal the defect structure on (0001) Si faces of the wafers. The wafers were etched at 450–500 for 5–10min. ℃

Optical scanner was used to assess the dislocation etch pits distribution on the entire wafer. The etch pits were observed by Nomarski - contrastoptical microscope. X-ray diffraction analysis was conducted on an X-Pert Pro ( PANalytical ) X-ray diffractometer with the CuKα1 radiation source operated at 40kV and 40 mA. The angular resolution of HRXRD was better than 12 arcsec. The ω rocking curves of 0006 symmetric reflection and 10 –12 a symmetric reflection of (0001) wafers were used to characterize the structure of LAGBs.

結果與討論

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Figs. 1(a, b and c) are the optical scanning images of three (0001) wafers after etching, which correspond to initial, middle and late stages, respectively in the same crystal. The crystal was grown in a large RTG with a convex growth interface. As seen in Fig. 1(a) for the initial stage wafer, the white dots (etch pits) are scattered.

In the middle stage wafer as shown in Fig.1(b), many straight white lines radiate from the center to the edge approximately along the <1-100> direction. In the late stage wafer inFig.1(c), the straight lines along the <1-100> direction are more obvious and have an apparent six-fold symmetry.

結果與討論

Fig. 2. Optical micrographs of etched 6H-SiC (0001) wafers.(a) Optical micrograph of the area I of Fig.1(a);(b)optical micro graph of the area II of Fig.1(c); (c)magnified micrograph of representative area III of Fig.2(a);(d)magnified micrograph of representative area IV of Fig.2(b).

Figs. 2(a) and (d) are the Nomarski-contrast images of area I in Fig. 1(a) and area II in Fig.1(c), respectively. Etch pits are shown as black dots in Nomarski-contrast image. As shown in Fig.2(c) and (d), all the etch pits have round shapes, with the diameters between 8 and13 mm. According to the shape of etch pits [15–17], round etch pits are caused by threading dislocations (TDs, screw or edge dislocations extending approximately normal to the basal plane and penetrating the entire wafer). On the early stage wafer in Fig.2(a), TDs distribute uniformly with the density of 1.0 × 105 cm-2. On the late stage wafer in Fig.2(b), the homogeneous distribution of etch pits changes.

Most of the dislocations gather in the lines parallel to <1 -1 0 0> and form the dislocation walls as shown in Fig.2(d). According to previous studies [11–14], these dislocation walls are defined as low-angle grain boundaries (LAGBs) .In Fig.2 (d), there are two parallel LAGBs with different line etch pit densities. One is 5 × 105 cm-1 (right) and the other is 1×105 cm-1 (left). Between the two boundaries, the quality of domain is high with few dislocations. The distance between neighboring boundaries is estimated to be 90 µm.

結果與討論

Fig. 3 shows optical scanning images of three KOH etched (0001) wafers. The wafers were selected corresponding to early, middle and late stages from the same crystal In got grown in small RTG with a near flat interface. There are also no LAGBs at the initial stage of crystal growth. The LAGBs are introduced after a period of crystal growth.There are many irregular white lines on (0001) Si faces of middle and late stage wafers after etching. Comparing Fig.1 with Fig.3, we knowth at the morphologies of LAGBs grown in different RTGs are different.

LAGBs formed in large RTG are straight and long,while the ones formed in small RTG are short and in consistent. Compared with Fig.1(b) and (c), the LAGBs in Fig.3(b) and (c) have no apparent six-fold symmetry along the <1-100> directions.

結果與討論

Fig. 4. Optical micro graphs of etched 6H–SiC (0001)wafers.(a) Optical micro graph of theareaI of Fig.3(a);(b)optical micro graph of the area II of Fig.3(c); (c)magnified micrograph of representative area III of Fig.4(a);(d)magnified micro graph of representative area IV of Fig.4(b).

Figs. 4(a) and (c) are the Nomarski-contrast images of the area I of Fig. 3(a) and area II of Fig. 3(c), respectively. It could be seen that the dislocations in the initial stage wafer are also dispersed with the density of 1.3 × 105 cm-2, but at the last stage, the dislocations are arrayed in inconsistent lines. The LAGBs are also formed by arraying TDs as shown in Fig.4(b) and (d). Many LAGBs in Fig.4(b) still have the tendency to line up along the <1 -1 0 0> directions.

結果與討論

It is clear that dislocations slip along the <11 - 20> direction and terminate eventually in the LAGBs. By analyzing the entire growth process, outcrops of threading dislocation lines existing at the initial growth-stage were dispersed and threading dislocation lines in the slip band were not completely parallel to the c-axis. This behavior can be explained by reducing the entire energy of the system under the thermal stress.

結果與討論

Ha etal. [14], Takahashi et al. [13] and Seitz et al. [19] believed that there were three fundamental grain boundary configurations as seenin Fig.6.Two are distinguished by a rotation axis perpendicular to the c-axis, either parallel or perpendicular to the boundary plane as shown in Fig. 6(a) and (b). The third configuration shows a rotation parallel to the c-axis and the boundary plane as shown in Fig.6(c). Under previous two cases, LAGBs could result in multiple peaks of the basal plane 0 0 0 6 reflection, but the third case cannot be detected by 000 n type reflection. {10 -1 2} face inclines about 70.5℃ with respect to the (0001) plane of 6H–SiC. The tilting component with a rotation parallel to the c-axis results in misorientation of {10 - 12} faces and can be detected by the rocking curve of 10–12 reflection.

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結果與討論

Fig. 7(a) is the rocking curve of the basal plane 0 0 0 6 reflection at area II of Fig.1(c).The size of beam footprint on the sample was 1.1 mm wide and 10 mm high for both 0 0 0 6 reflection and 1 0–12 reflections. The ω rocking curve includes multiple diffraction peaks reflecting from misorientating domains delineated by etching images. These diffraction peaks locate in a 285 arcsec range.The diffraction plane normal of 0 0 0 6 reflection is the c-axis and multiple peaks result from the misorientation in the basal plane (twist or/and tilt component as shown in Fig.6(a) and (b)).Fig.7(b) is the asymmetric reflections result of {1 0 -1 2} face at area II of Fig.1(c).The rocking curvein Fig. 7(b) also includes many independent peaks in a 553 arcsec range.

The HRXRD results confirm that the LAGBs formed in high RTG include two/three rotation components.Fig.7(c) is the rocking curve of 0006 reflection at area II of Fig.3(c).The resulting ω rocking curve consists of a single diffraction peak with full-width at half-maximum (FWHM) of 50 arcsec.Such result indicates that the LAGBs formed in small RTG have no misorientation in the basal plane.Fig.7(d) is the rocking curve of 10–12 reflection at area II of Fig.3(c).The rocking curve shows several well-defined peaks in 233 arcsec range.This result indicates that the LAGBs only include tilt components with rotations axis parallel to the c-axis (the case of Fig.6(c)).

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結論

KOH etching morphology of different growth-stage wafersfrom the same crystalin got was observed on (0001) Si faces of6H – SiC crystal. The LAGBs along the <1 1 0 0 > direction wereintroduced by slip of high-density threading dislocations in thegrowth process rather than formed at the initial stage of growth.

By optical scanner and X-ray diffraction,it has been confirmedthat themorphology and structure of LAGBs formed in differentRTGs were different.The thermal stress in troduced by RTG wasconsidered as the driving force of dislocations slip.Small RTGcould reduce the distortion components of LAGBs.

參考資料

Evolution and structure of low-angle grain boundaries in 6H–SiC single crystals grown by sublimation method - Yuqiang Gao, Xiaobo Hu, Xiufang Chen, Xiangang Xu n, Yan Peng, Sheng Song, Minhua Jiang. State Key Laboratory of Crystal Materials, Shandong University, Jinan250100, China