Development of Bulk Sic Single Crystal Grown by Physical Vapor Transport

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Optical Materials 23 (2003) 415–420 www.elsevier.com/locate/optmat

Development of bulk SiC single crystal grown by physical vapor transport method Rongjiang Han a, Xiangang Xu a, Xiaobo Hu a,*, Naisen Yu a, Jiyang Wang a, Yulian Tian b, Wanxia Huang b a State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China b Beijing Synchrotron Radiation Laboratory, Institute of High Energy Physics, Beijing 100039, PR China

Abstract

This paper reviews the development of bulk SiC single crystals grown by physical vapor transport method, including the polytype control, defects and doped consideration. In addition, defects in commercial 6H-SiC wafer, such as micropipes, hexagonal voids and subgrain boundaries, are examined by transmission optical microscopy, X-ray synchrotron topography in back-reﬂection geometry and high-resolution X-ray diﬀraction methods. Ó 2003 Elsevier Science B.V. All rights reserved.

PACS: 61.72.Dd; 81.05.Hd; 81.10.Bk Keywords: SiC; Crystal growth; Semiconductor; Defect

1. Introduction crystal is essential to realize the full potential application of this important semiconductor mate- The physical properties of silicon carbide (SiC) rial. In this paper, polytype control, defects and make this wide-band gap semiconductor well sui- doping for SiC crystals grown by physical vapor ted for high-temperature, high voltage, high- transport (PVT) method were reviewed. power, and high frequency electronic devices [1–3]. Furthermore, we examined micropipes, hexag- Many types of SiC-based electronic devices, in- onal voids and subgrain boundaries in commercial cluding power MESFETs, p–i–n diodes, Schottky 6H-SiC wafers using transmission optical micros- diodes, power MOSFETs, thyristor etc, are fabri- copy, X-ray synchrotron topography in back- cated. In addition, SiC will promote the realization reflection geometry and high-resolution X-ray of III-nitride-based optoelectronic devices because diffraction (HRXRD) methods. Optical micros- as a substrate it is considered superior to sapphire copy studies were carried out in the transmission for epitaxy of (Al, Ga, In)Ncompounds [4]. mode of the optical microscope (Opton, Germany). Therefore, the growth of high-quality SiC bulk X-ray back-reflection synchrotron topography experiments were performed at the 4W1A beam line of the topography station in Beijing synchrotron * Corresponding author. Fax: +86-531-8574135. radiation laboratory. HRXRD rocking curves E-mail address: [email protected] (X. Hu). were obtained using a D5005 X-ray diffractometer

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416 R. Han et al. / Optical Materials 23 (2003) 415–420

(Bruker-axs, Germany), in which a four-crystal 1800 °C, producing Si, SiC2 and Si2C vapors. monochromator was used for CuKa1 radiation. Based on this, SiC sublimation growth methods are developed. The Lely method [7], the first method of sublimation technique, is carried out in 2. Polytypism and basic properties of SiC argon ambient at about 2500 °C in a tight graphite container to prevent an escape of sublimation va- A striking structure property of SiC crystal is its pors from the growth chamber. As the SiC source polytypism [5]. SiC is the most prominent of a sublimes, the vapors penetrate a porous graph- family of close packed materials that exhibit a one- ite cylinder to produce lamellar crystals on its dimensional polymorphism called polytypism. So inner surface. The Lely platelets can exhibit far, over 250 SiC polytypes have been identified. good quality with micropipe densities of 1–3 cm2 SiC polytypes are discriminated by the stacking and dislocation densities of 102–103 cm2, respec- sequence of the tetrahedrally bonded Si–C bilay- tively. However, its primary drawback is uncon- ers, such that the individual bond lengths are trollable nucleation and dendrite-like growth. nearly identical, while the overall symmetry of the Also, the reliance on a porous graphite substrate crystal is determined by the stacking periodicity. limits the size of the SiC crystal (on the average, up SiC polytypes are divided into three basic crystal- to 3–5 mm). lographic categories: cubic (C), hexagonal (H) and Hence, a modified-Lely method [8] was devel- rhombohedral (R). 3C for cubic, 4H or 6H for oped in 1978. This method, also called physical hexagonal and 15R for rhombohedral are the vapor transport method or seeded sublimation common polytypes. An appreciation of potential method, has been further refined [6,9] for pro- of SiC for electronic applications can be obtained ducing larger SiC boules. The SiC seed crystal and from Table 1, which compares the relevant prop- the SiC source are placed in close proximity to erties of SiC with Si and GaAs. Despite numerical each other within a crucible. A temperature gra- values of some parameters being slightly different dient is established causing vapor phase transport in different literatures [1,3,6], the magnitude order of SiC species between the source and the seed, of them is consistent respectively. Due to the un- and the process is carried out at a low argon ique combination of high-saturated electron drift pressure. Now, it is successful to produce com- velocity, high breakdown electronic field, high mercial grade 4H- and 6H-SiC for wafer diameters thermal conductivity, wide band gap and excellent up to 100 mm. In addition, 15R-SiC [10] and 3C- stability, SiC is currently emerging as a very po- SiC [11] bulk single crystals had been successfully tential semiconductor material. grown by PVT method under appropriate growth conditions. Recently, Straubinger et al. [12] modified the conventional PVT setup with an addi- 3. SiC bulk crystal grown by sublimation method tional gas pipe running through the lower isolation material and crucible wall into the growth cham- At atmospheric pressure, SiC does not melt but ber (M-PVT setup). High quality 6H- and 4H-SiC it undergoes sublimation at temperature above single crystals with an improved axial and lateral

Table 1 Comparison of several important semiconductor material properties Properties Si GaAs 3C-SiC 6H-SiC 4H-SiC Band gap (eV) (T < 5 K) 1.12 1.43 2.40 3.02 3.26 Saturated electron drift velocity (107 cm/s) 1.0 2.0 2.5 2.0 2.0 Breakdown ﬁeld (MV/cm) 0.25 0.3 2.12 2.5 2.2 Thermal conductivity (W cm1 K1) 1.5 0.5 3.2 4.9 3.7 Dielectric constant 11.8 12.8 9.7 9.7 9.7 Physical stability Good Fair Excellent Excellent Excellent 中国科技论文在线 http://www.paper.edu.cn

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Al-doping homogeneity were grown with the or 3C-SiC as seed crystal had been obtained by M-PVT setup. PVT with 1750 °C for seed crystal and 2250 °C for source materials [11]. A large temperature gradient (about 100 K cm1) and a low pressure (<103 4. Polytype control of SiC bulk crystal growth mbar) were employed for 3C-SiC bulk crystal growth. SiC is a kind of polytypic material. Due to the Recent development of the crystal growth and low stacking fault energy [13], it is difficult to re- device fabrication for SiC crystals requires the strict syntaxy (parasitic polytype formation) dur- polytype identification. Raman scattering mea- ing the bulk crystal growth and thus to grow a surement [21], low temperature photoluminescence single polytype material. It is important to control [9] and infrared reflectivity [22] are all powerful the polytype for application to semiconductor tools to characterize SiC crystal non-destructively. devices because the band gap and electrical property of SiC varies with polytype. SiC crystal growth via sublimation is a complex 5. Defects in bulk SiC crystal process. The influences of growth parameters, such as growth temperature [14,15], supersaturation To meet the challenges of commercialization of [16,17] vapor phase composition and system pres- SiC semiconductors, specific efforts have been sure [16,18] and face polarity of seed crystal [19] on made towards larger diameter high quality SiC the polytype stability had been discussed. Among bulk crystals. Significant advances have been made them, the face polarity of seed crystals is one of in improving the quality of SiC crystals, especially most important growth parameters for controlling in the reduction of micropipe defects. However, the SiC polytype during PVT method. As well there are various defects in PVT-grown SiC crys- known, (0 0 0 1) faces of hexagonal and rhombo- tals, such as dislocations, micropipes, low-angle hedral SiC have different face polarity such as grain boundaries, stacking faults, hexagonal voids, (0 0 0 1)Si and (0 0 0 1)C face. The polytype of 6H- polytype inclusion, etc. The formation of these and 4H-SiC crystal, grown on [0 0 0 1] direction, is defects is related to the quality, the surface orien- mainly determined by the growth temperature as tation and the attachment of the seed [23] or the well as the polytype and face polarity of seed improper growth start conditions [24]. Further- crystal [14,19]. It was reported that 6H polytype more, the defects development depends on the crystal grew on (0 0 0 1)Si face and 4H polytype on growth orientation, possible stresses in the boule, (0 0 0 1)C face regardless of the seed polytype, growth front shape and temperature distribution which was attributed to the different surface free in the crucible. energy of the (0 0 0 1)Si and (0 0 0 1)C face. The Micropipes are hollow defects aligned along the polytype of PVT-grown SiC crystal depends upon c-axis in hexagonal or rhombohedral crystals, the growth direction. If the SiC crystals grew on which can penetrate the whole crystal. Most views [1 1 0 0] or [1 1 2 0] direction, that is, perpendicular on micropipe formation in PVT-grown SiC crystal to the [0 0 0 1] direction [9], the polytype of ob- are based on FrankÕs theory [25] that a micropipe tained crystal succeeds perfectly to the polytype is a hollow core screw dislocation with a huge of seed crystal. Recently, near-thermal equilib- Burgers vector (several times the c-lattice constant rium growth condition by PVT method was em- for 6H-SiC or 4H-SiC) and with the diameter of the ployed, and the influence of face polarity and core having a direct relationship to the magnitude polytype of the seed crystal on polytype of the of its Burgers vector. Synchrotron white-beam X- grown crystal was reported [20]. 15R-SiC bulk ray topography (SWBXT) is a more effective tech- single crystals [10] have been grown on (0 0 0 1) Si nique for examining micropipes due to its high face of 15R-SiC seed with the average temperature strain sensitivity and suitable spatial resolution. gradient of 5 K cm1 or below. In addition, bulk The pure superscrew dislocations nature of mi- 3C-SiC crystal up to 4 mm in length using 6H-SiC cropipes was verified by a series of synchrotron 中国科技论文在线 http://www.paper.edu.cn

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imaging techniques together with corresponding simulations [26]. In addition, the elementary screw dislocations (also called close-core screw dislocations) in a network structure in SiC crystals was examined by SWBXT using transmission topography [27]. At present, there are high densities (103–104 cm2) of elementary dislocations in SiC crystal. The detrimental effects of screw dislocations have been clearly demonstrated for SiC-based devices [28]. Therefore, SiC bulk crystal growth research has focused on the reduction of elementary dislocations. Micropipe defects differ from dislocations in SiC in the sense that any micropipe existing in the Fig. 2. Back-reflection synchrotron X-ray topograph (negative device area limits high voltage operation. Even one film) of 6H-SiC wafer recorded from (0 0 0 1) plane showing the micropipe in the active area of a high voltage SiC images of superscrew dislocations. Diffraction conditions: dif- device will lead to electric breakdown. Therefore, fraction vector g ¼ 00018, diffraction wavelength k ¼ 1:54 AA, it is generally conceded that micropipes are the the sample-to-film distance L ¼ 14 cm. major obstacles to the production of high-performance SiC-based devices [29], especially at high- pearing as black spots) were clearly discernible in voltages or for high current power devices. Over spite of the black spots slightly moving in the field the past several years, a significant reduction in of vision. The single wavelength X-ray back- micropipe density has been achieved at Cree and reflection synchrotron topograph recorded from a SiC substrates with a micropipe density <1cm2 6H-SiC (0 0 0 1) wafer was shown in Fig. 2 (nega- [30]. tive film). The black spots represent the images of Fig. 1 shows a transmission optical micrograph superscrew dislocations in SiC wafer. The diame- of micropipes in a 6H-SiC (0 0 0 1) wafer. In the ters of black spots are related to the Burgers vec- transmission mode of optical microscope, focusing tors of the superscrew dislocations. The larger the at different depths showed that micropipes (ap- diameter of a spot is, the bigger the Burgers vector of the superscrew dislocation. In addition, subgrain boundaries (low angle grain boundaries) bordered by regions of high dislocation density are commonly observed in the modified-Lely SiC crystals. If SiC crystals with subgrain structure were used as substrates, the subgrains would be replicated in the device epit- axial layers, and consequently has a negative im- pact on the device performance. Katsuno et al. [31] pointed out that the main cause of subgrain boundary formation in PVT-grown SiC crystals is the inclusion of foreign polytypes. HRXRD rocking curves are a powerful and effective tool to identify the existence of subgrain boundaries in SiC crystal. Fig. 3 shows a HRXRD rocking curve using 0 0 0 1 2 reflection of 6H-SiC wafer. The split- Fig. 1. Transmission optical micrograph of 6H-SiC (0 0 0 1) peaks indicate the existence of subgrains slightly wafer (image size is 300 lm 250 lm). misoriented to each other. Because the X-ray in- 中国科技论文在线 http://www.paper.edu.cn

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1600 microscope, it was possible to bring the voids to 1400 focus, by adjusting the focal depth to a distance below the surface of the wafer. The above obser- 1200 vation indicated that the hexagonal voids are not 1000 located on the surface, and that it is indeed a bulk 800 defect embedded inside the wafer. Optical trans- 600 mission micrographs of hexagonal voids are

counts (a.u.) 400 shown in Fig. 4. A relatively thick black border 200 surrounding the voids may be attributed to the slanted sidewalls at the top or bottom of the void. 0 37.0 37.1 37.2 37.3 37.4 37.5 ω (degree) 6. Doped SiC crystals Fig. 3. High-resolution X-ray rocking curve of 0 0 0 1 2 reflection of 6H-SiC wafer. SiC is one of the IV–IV compound semiconductors in the Mendeleyev periodic system. For cidence plane is parallel to the [0 0 0 1] direction, it realizing the full potential application of SiC ma- was shown that subgrains are separated by low terials, n-, p-type and semi-insulating SiC crystals angle grain boundaries extending in {0 0 0 1} are required by incorporation of different electri- plane. Experiments also showed that low angle cally active dopants. grain boundaries appear preferentially at the pe- Nitrogen and aluminum are the main n- and p- riphery of SiC wafer. type dopants because they create relatively shallow Another prevalent defect in the modified-Lely donor and acceptor levels within SiC bandgap, SiC wafer is hexagonal voids (also called planar respectively. For SiC, nitrogen is the most com- defects). Stein [32] provided an explanation for the mon donor impurity, while aluminum is favored formation of hexagonal voids through secondary for p-type doping. In situ n-type doping can be evaporation of the crystal surface. The evidence of easily achieved by the introduction of gaseous ni- void formation as well as their axial and lateral trogen during sublimation growth. SiC can be motion during SiC sublimation growth was pre- heavily doped with nitrogen, and donor concen- sented [33]. When the 6H-SiC (0 0 0 1) wafer was trations as high as 1020 cm3 are achievable. P-type observed in the transmission mode of the optical doping is usually performed by the adding Al- containing species to SiC source material. A major drawback of Al-doping is the quick exhaustion of doping reservoir at higher growth temperature, which results in a strongly decreasing Al incorporation in SiC crystal. In order to overcome the inhomogeneity of Al-dopant distrubution, SiC crystals with an improved axial and lateral Al- doping homogeneity [12] had been grown by the M-PVT setup. In addition, there is significant an- isotropy for nitrogen and aluminum incorporation in sublimation boule growth for the crystal face (0 0 0 1)Si, (0 0 0 1)C, (1 1 0 0) and (1 1 2 0). For nitrogen there is an increase in incorporation from (0 0 0 1)Si to (1 100) to (0001)C, while for aluminum it is the reverse [30]. Fig. 4. Transmission optical micrograph of hexagonal voids in Vanadium in SiC occupies silicon substitutional 6H-SiC wafer (image size is 500 lm 360 lm). site and acts as an amphoteric dopant providing 中国科技论文在线 http://www.paper.edu.cn

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two mid gaps levels: the deep acceptor and the References deep donor [34], for the electrical compensation of residual impurities. High resistivity, semi-insulat- [1] J.B. Casady, R.W. Johnson, Solid-State Electron. 39 (1996) ing SiC crystals resulting from vanadium doping 1409. [2] K. Rottner et al., Mater. Sci. Eng. B 61–62 (1999) 330. are important as substrate for high frequency [3] S. Sriram, R.R. Siergiej, R.C. Clarke, A.K. Agarwal, C.D. electronic devices because of their low dielectric Brandt, Phys. Stat. Sol. (a) 162 (1997) 441. losses in the GHz range. N-type [15], p-type [35] [4] L. Liu, J.H. Edgar, Mater. Sci. Eng. R 37 (2002) 61. and semi-insulating [15,35] 6H- and 4H-SiC have [5] P.T.B. Shaffer, Acta Cryst. B 25 (1969) 477. been grown by PVT method. For vanadium dop- [6] D.L. Barrett et al., J. Cryst. Growth 109 (1991) 17. [7] J.A. Lely, Ber. Deut. Keram. Ges. 32 (1955) 229. ing, the solubility limit of vanadium in SiC of [8] Yu.M. Tairov, V.F. Tsvetkov, J. Cryst. Growth 43 (1978) 17 3 3 10 cm [36] must not be exceeded, so, 209. elimination of residual impurities at least to the [9] J. Takahashi, M. Kanaya, Y. Fujiwara, J. Cryst. Growth level of 1016 cm3 is necessary. In short, future 135 (1994) 61. focus should be toward the residual impurity [10] N. Schulze, D. Barrett, G. Pensl, Phys. Stat. Sol. (a) 178 (2000) 645. control and improvement of doping homogeneity [11] K. Furukawa, Y. Tajima, H. Saito, Y. Fujii, A. Suzuki, S. in order to obtain the high quality doping SiC Nakajima, Jpn. J. Appl. Phys. 32 (1993) L645. crystals. [12] T.L. Straubinger et al., J. Cryst. Growth 240 (2002) 117. [13] Momeen, M.Y. Khan, Cryst. Res. Technol. 30 (1995) 1127. [14] M. Kanaya, J. Takahashi, Y. Fujiwara, A. Moritani, Appl. 7. Summary Phys. Lett. 58 (1991) 56. [15] G. Augustine et al., Phys. Stat. Sol. (b) 202 (1997) 137. Bulk SiC crystals with single polytype, low de- [16] Y.M. Tairov, V.F. Tsvetkov, Prog. Cryst. Growth Cha- ract. 7 (1983) 111. fect density and different electrically active dopants [17] R. Yakimova et al., J. Cryst. Growth 217 (2000) 255. should be grown to realize its full potential appli- [18] H.-J. Rost et al., Mater. Sci. Eng. B 61-62 (1999) 68. cation. The polytype control, defects and doped [19] R.A. Stein, P. Lanig, J. Cryst. Growth 131 (1993) 71. consideration in PVT-grown SiC crystals is re- [20] N. Schulze, D.L. Barrett, G. Pensl, S. Rohmfeld, M. viewed. The defects in the commercial 6H-SiC Hundhausen, Mater. Sci. Eng. B 61–62 (1999) 44. [21] S. Nakashima, H. Harima, Phys. Stat. Sol. (a) 162 (1997) wafers had been examined by transmission optical 39. microscopy, back-reflection synchrotron X-ray [22] J.M. Bluet et al., Mater. Sci. Eng. B 61–62 (1999) 212. topography and HRXRD methods. It is of im- [23] M. Tuominen, R. Yakimova et al., Mater. Sci. Eng. B 57 portance to produce high quality SiC bulk crystals (1999) 228. for accelerating the development of SiC semicon- [24] M. Tuominen et al., Diamond Relat. Mater. 6 (1997) 1272. [25] F.C. Frank, Acta Cryst. 4 (1951) 497. ductor technology. [26] X.R. Huang et al., Appl. Phys. Lett. 74 (1999) 353. [27] M. Dudley et al., J. Phys. D 28 (1995) A63. [28] P.G. Neudeck, Mater. Sci. Forum 338–342 (2000) 1161. [29] V. Dmitriev et al., Mater. Sci. Eng. B 61–62 (1999) 446. Acknowledgements [30] C.H. Carter Jr. et al., Mater. Sci. Eng. B 61–62 (1999) 1. [31] M. Katsuno et al., J. Cryst. Growth 216 (2000) 256. The authors would like to thank the 863 Pro- [32] R.A. Stein, Physica B 185 (1993) 211. gram (grant no. 2001AA311080) and National [33] T.A. Kuhr et al., J. Appl. Phys. 89 (2001) 4625. [34] J.R. Jenny et al., J. Appl. Phys. 78 (1995) 3839. Natural Science Foundation (grant no. 60025409) [35] M. Bickermann et al., J. Cryst. Gworth 233 (2001) 211. of China for financial support. [36] J.R. Jenny et al., Appl. Phys. Lett. 68 (1996) 1963.