In recent years remarkable progress has been made in solution growth of 4H-SiC crystals [1-4]. However, slow growth rate and surface roughening during growth are serious problems to be solved for applications of this method for industrial production of very-low-dislocation-density 4H-SiC wafers. Especially, step bunchings of several-hundred-nanometers height are always formed, and which often lead to macroscopic surface defects with incorporation of solvent and parasitic polytypes. Generally, growth rate and surface morphology are strongly influenced by supersaturation of solute elements and also by elemental compositions of alloy solvents. In order to find adequate conditions for high speed bulk growth with keeping a smooth surface, we have investigated the feature of the solution growth with SiCr and SiCrAl solvents. Since solubility of carbon in Si_0.6Cr_0.4 melt becomes larger than that in pure-Si melt by approximately 20 times, Si_0.6Cr_0.4 melt can satisfys the conditions of high C solubility and low supersaturation. In this paper, we present the growth rate and surface morphology of 4H-SiC grown from Si-Cr-C based solutions under various temperature gradient conditions. In addition, according to our preliminary growth experiments with Si_xAl_1-x (x=0.01~0.2) solvents, the decrease in average step-width was observed with the addition of Al to Si solvent. Since the surface modification by Al is expected to promote step-flow growth and possibly reduce macroscopic surface defects, the changes in surface morphology for SiCr_0.4Al_x (x=0.01~0.05) solvents were also investigated.
  A schimatic illustration of the growth cell is shown in Fig. 1. Temperatures at seed crystals (T_S) and crucibles (T_C) were measured by pyrometers. Solution growth was carried out with Si_0.6Cr_0.4 and SiCr_0.4Al_x (x=0.01~0.05) at 2050 ºC for T_C. The crucibles were used both as a container for the solvent and as a carbon source. Figure 2 (a) shows the dependence of growth rate on temperature difference (ΔT) between T_S and T_C (ΔT=T_S-T_C). The dependence of growth rate on ΔT in Si_0.6Cr_0.4 melt is approximately 15 times larger than that in pure-Si melt. In Fig. 2 (b), growth rate is shown as a function of supersaturation of C. The supersaturation was calculated from the temperature difference between T_S and T_C refering liquidus curve for Si-C and Si-Cr-C phase diagrams. Figure 2 (b) shows that the growth rate in Si_0.6Cr_0.4 melt is much higher than that in pure-Si melt even for small increase in supersaturation. This indicates that the increase of C concentration in Si-Cr-C solution gives significant contribution on the growth rate. Though the solution growth was carried out under the conditions of higher C solubility and lower supersaturation, as seen in Fig. 3 (a) the surface roughening is still observed in Si_0.6Cr_0.4 melt. By the addition of Al the surface was improved. In Si_0.56Cr_0.4Al_0.04 solvent, high growth rate with smooth surface is achieved as shown in Fig. 3 (b). Step-flow growth was promoted very much by the addition of Al. Mechanism of surface modification by Al is now under investigation. Based on the results shown in Figs. 2 and 3, growth of 4H-SiC crystal in 2" diameter has been performed. 4H-SiC crystals in 3 mm-thick were succesfully grown with smooth step-flow surface from Si-Cr-Al-C solution.

Acknowledgement: This work is supported by the Novel Semiconductor Power Electronics Project Realizing Low Carbon Emission Society under the New Energy and Industrial Technology Development Organization (NEDO).

[1] Ujihara et al., Mater. Sci. Forum 717-720 (2012) 351. [2] Yamamoto et al., Appl. Phys. Express 5 (2012) 115501. [3] Daikoku et al., Mater. Sci. Forum 717-720 (2012) 61. [4] Kado et al., Mater. Sci. Forum 740-742 (2013) 73. [5] Komatsu et al. Mater. Sci. Forum 740-742 (2013) 23.

Fig. 1: A schimatic illustration of the growth cell.
Fig. 2: Dependence of growth rate on (a) temperature difference between T_S and T_C (ΔT=T_S-T_C) and (b) supersaturation of C for the growth in Si, SiCr, and SiCrAl solvents.
Fig. 3: Micrographs of 4H-SiC crystals grown in (a) Si_0.6Cr_0.4 and (b) Si_0.56Cr_0.4Al_0.04 solvents. Diameters of crystals were 18 mm.

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