Heavily p-type doping of bulk 6H-SiC and 3C-SiC

grown from Al-Si melts

F.Mercier1,a, I.G. Galben-Sandulache1,b, M. Marinova2,c, G. Zoulis3,d, T. Ouisse1,e, E.K. Polychroniadis2,f and D. Chaussende1,g

1Laboratoire des Matériaux et du Génie Physique CNRS UMR 5628, Grenoble INP Minatec, BP 257, 38016 Grenoble Cedex 01, France

2Departement of Physics, Aristotle University of Thessaloniki, GR 54124 Thessaloniki, Greece 3Groupe d’Etude des Semi-conducteurs CNRS UMR 5650, Université Montpellier 2, cc074-GES,

34095 Montpellier Cedex 5, France afrederic.mercier@grenoble-inp.fr, birina.galben@grenoble-inp.fr, cmarinova@physics.auth.gr,

dzoulis@ges.univ-montp2.fr, ethierry.ouisse@grenoble-inp.fr, fpolychr@auth.gr, gdidier.chaussende@grenoble-inp.fr

Keywords : p-type 3C-SiC, p-type 6H-SiC, solution growth, TEM, Raman

Abstract. We report in this work, the solution growth of heavily p-type doped 3C-SiC and 6H-SiC. Description of the 3C and 6H-SiC crystals in terms of defects and resistivity are presented and discussed with respect to growth conditions such as temperature, Al content in the melt and seed polarity. Crystals and thick layers are investigated by means of TEM, NDIC microscopy and Raman.

Introduction

Specific applications like high power IGBT (insulated gate barrier transistor) devices based on SiC require highly conductive p-type substrates. Such substrates are not commercial products and, even if some works have already been reported on p-type doping during the growth of bulk SiC ingots, only few lead to the adapted resistivity [1]. Liquid phase processes are known to permit an easy in-situ doping of crystals and epilayers. The feasibility of n-type doping (with nitrogen) and p-type doping (with aluminum) has been demonstrated, to get a much broader range than using standard vapor growth techniques [2]. For instance, p-type doping as high as 1021 at.cm-3 was obtained in layers grown by VLS mechanism in Al85Si15 melt at 1100°C [3]. Besides the doping issue, improvement of the substrate quality remains topical. Micropipes are no longer a pronounced issue but the density of basal plane dislocations (BPDs), leading further to formation of stacking faults (SFs), stays relatively high. Once again, the liquid phase approach is best suited since no BPDs were found in homoepitaxial LPE layers grown on 4H-SiC on-axis [4]. We report in this work, the solution growth of heavily p-type doped 3C-SiC and 6H-SiC crystals and thick layers. Samples are investigated by means of TEM, NDIC microscopy and Raman. The structural quality and the evaluation of doping are discussed.

Experimental

The experimental set-up for the crystal growth is a modified Metal ResearchTM puller already described elsewhere [5]. The graphite crucible acts as the container for Si-Al based melt and as the carbon source. The Al content (3N5 purity) varied from 0 at% to 40 at%. Highly pure silicon (9N) is also used. The growth temperature varies from 1650°C to 1800°C. Both self-nucleated and seeded growth experiments are carried out. For the self-nucleated growth experiments, cylindrical crucible is used with a thermal gradient of 25°C/cm. For the seeded growths, the so-called “podium” crucible is used. It ensures a controlled flow patterns in front of the growing crystal [6], giving rise

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to an improved growth front stability. The seeds are on-axis, C- and Si-face PVT grown 6H-SiC substrates and 3C-SiC (001) from Hoya Corp. Size of the seeds are 1 cm x 1 cm. Rotation is only applied to the seed (20 rpm). By sticking multiple seeds, same experimental conditions are applied for the different seeds. The TEM observations are performed on a JEOL 100X conventional TEM operating at 100 kV and JEOL 2011 high resolution TEM operating at 200 kV. Raman spectra were collected using Jobin Yvon/Horiba Labram spectrometer equipped with liquid nitrogen cooled CCD detector. Experiments were conducted in the micro-Raman mode at room temperature, in a backscattering geometry. Using the 514.5 nm line of Ar+ for excitation, the focused spot diameter is less than 1 µm. Spectra are taken on the (0001) and on the (111) faces respectively for 6H-SiC and 3C-SiC.

Results on crystal growth

Spontaneous crystals. SiC crystals grow at the surface of the liquid, starting from the walls of the crucible and extending into the melt. The crystals exhibit a dark blue color with a lateral size of a few millimeters. In the case of 3C-SiC, outstanding growth rates have been measured: 1.6 mm/h for the lateral one and 0.15 mm/h for the growth rate along the c-axis. These values have been achieved in Si60Al40 at 1650°C (Fig. 1a). KOH etching reveals a stacking fault density of 100 cm-1. Occurrence of hexagonal polytype increases with temperature and with Al content. Among all hexagonal polytypes, only 6H-SiC is identified by Raman spectroscopy. 6H-SiC spontaneous crystals are either flat platelets either needles (Fig. 1b).

a) b)a) b)

Figure 1. SiC crystals grown by self-nucleation in Al40Si60 melt at 1650°C. Crystals exhibits dark blue color. a) 3C-SiC. b) 6H-SiC.

Seeded growth. At 1650°C the growth rate is 12 µm/h in Si86.5Al13.5 melt for 6H-SiC on both polarities. At 1800°C, for the same Al content, the average growth rate is strongly increased. On the C-face, c-axis growth rate is 65 µm/h and few solvent inclusions are detected (Fig. 2b). On the Si-face, c-axis growth rate is lower (40 µm/h). This reduction could be attributed to a higher lateral growth rate (Fig. 2a). No lateral growth rate is detected on the C-face. Observations of the surfaces by NDIC microscopy show two different morphologies according to the polarity. On the Si-face, the strong faceting results from a difference in growth rate between <11-20> and <1-100> directions (the higher growth rate is observed for the <11-20> direction). Such strong anisotropy is not observed for growths in pure Si melt. C-face exhibits smooth surface (Fig. 2d). On the other hand, we did not succeed in stabilizing growth front on 3C-SiC (001) at 1650°C in Si85Al15. Cross section of the 3C-SiC crystal reveals numerous solvent inclusions and polytype transition (3Cà6H).

Structural quality and evaluation of doping

On the Si-face of 6H-SiC, increasing of growth temperature to 1800°C leads to a dramatic decrease of crystalline quality including stacking order. Stabilization of several short and long period polytypes like 4H, 15R, 27R, 69R or 108R for thicknesses higher than 1 µm are observed by TEM.

60 Silicon Carbide and Related Materials 2009

Figure 2. Homoepitaxial growth on 6H-SiC (0001) on-axis. Si polarity (a and c) and C polarity (b and d) at 1800°C. a) and b) Cross section of 6H-SiC crystal observed with transmission optical microscope. c) and d) As-grown surface observed with NDIC microscope.

Additionally, locally different stacking disturbances are also noticed. An example is given in Fig. 3(a). On the other hand, the growth front is stable on the C-face and no polytype transition is observed. The defect density is comparatively high close to the substrate/layer interface and strongly decreases with layer thickness. Main defects that are formed are dislocations, Fig. 3(b). After a few tens of microns dislocations and stacking faults density decreases below the TEM detection limit. The strong density of defects close to the interface is attributed to the difference of doping between the substrate (n-type) and the p-type grown layer. However, few Al-based inclusions are detected for both polarities. These inclusions do not seem to give additional defects.

Figure 3. (a) HRTEM image from the 6H-SiC layer grown on Si-face substrate. Locally the stacking sequence is changed to 3432; (b) XTEM image from the near substrate/layer region of the 6H-SiC layer grown on C-face substrate. It reveals enhanced dislocation density combined with stacking faults.

Table 1 gives estimation of hole concentration in 6H-SiC thick layers grown at 1650°C and

1800°C in Si86.5 Al13.5. Values of table 1 are based on the line width of the linear optical phonon–plasmon coupled (LOPC) mode. The calibration curve is given on Ref. [7]. P-doping is lower on the C-face and follows the site-competition theory. Moreover, increasing growth temperature leads to higher p-doping. It results from the Al solubility in SiC which is temperature-dependent.

C-face Si-face

1650°C 1710]8.26.2[ ×− 1810]5.22.2[ ×− 1800°C 1810]3.56.4[ ×− 1910]3.23.1[ ×−

Table 1. Charge carrier concentration (at.cm-3) in p-type 6H-SiC. Solvent: Si86.5Al13.5. Determination from the FWHM of the LOPC mode. Ref.[7] used as calibration curve.

We have also investigated evolution of line shape of LO mode for 3C-SiC. Each LOPC mode has

been fitted by a Lorentzian law. By increasing Al content in the melt, LOPC is enlarged and shifted

Materials Science Forum Vols. 645-648 61

towards low frequency side (Fig. 4). Based on 6H-SiC tendencies [7], hole concentration is increased with temperature and with Al content in the melt. As for p–type 6H-SiC, site-competition doping is also observed. Difference in doping between the both polarities seems to decrease when Al content increases. Correlation between LO evolution and hole concentration in 3C-SiC crystal is under investigation.

Figure 4: P-type spontaneous 3C-SiC crystals, evolution of LOPC peak with aluminum content in the melt for both polarities. a) FWHM of LOPC (cm-1). b) shift of LOPC peak (cm-1) towards low frequency side with respect to the standard position of 972 cm-1.

Conclusion

P-doped 3C-SiC and 6H-SiC thick layers and bulk crystals has been studied in terms of doping and evaluation of structural quality. Low stacking fault densities as well as the high values of doping make the solution growth worthwhile to consider. TEM investigation reveals unstable growth front on the Si-face with polytype transition. For identical growth conditions, growth front on the C-face remains stable.

Acknowledgements

The authors thank the French ANR-Jeunes Chercheurs program (contract number ANR-05-JCJC-0207-01) and the MANSiC – Marie Curie Research and Training Network (contract number MRTN-CT-2006-035735) for their financial supports.

References

[1] P.J. Wellmann et al.: Surf. Coat. Technol. Vol. 201 (2006), p. 4026

[2] V.A. Dmitriev: Physica B Vol. 185 (1993), p. 440

[3] C. Jacquier et al.: Mater. Sci. Forum Vol. 483-485 (2005), p. 125

[4] K. Kusunoki et al.: Mater. Sci. Forum Vol. 615-617 (2009), p. 137

[5] F. Mercier et al.: Mater. Sci. Forum Vol. 615-617 (2009), p. 41

[6] H. Harima et al.: Mater. Sci. Forum Vol. 338–342 (2000), p. 607

[7] R. Müller et al.: Chem. Vap. Deposition Vol. 12 (2006), p. 557

[8] D.J. Larkin et al.: phys. stat. sol. (b) Vol. 202 (1997), p. 305

62 Silicon Carbide and Related Materials 2009

Silicon Carbide and Related Materials 2009 10.4028/www.scientific.net/MSF.645-648 Heavily p-Type Doping of Bulk 6H-SiC and 3C-SiC Grown from Al-Si Melts 10.4028/www.scientific.net/MSF.645-648.59

DOI References

[1] P.J. Wellmann et al.: Surf. Coat. Technol. Vol. 201 (2006), p. 4026

doi:10.1016/j.surfcoat.2006.08.033 [2] V.A. Dmitriev: Physica B Vol. 185 (1993), p. 440

doi:10.1016/0921-4526(93)90276-C [3] C. Jacquier et al.: Mater. Sci. Forum Vol. 483-485 (2005), p. 125

doi:10.4028/www.scientific.net/MSF.483-485.125 [4] K. Kusunoki et al.: Mater. Sci. Forum Vol. 615-617 (2009), p. 137

doi:10.4028/www.scientific.net/MSF.615-617.137 [5] F. Mercier et al.: Mater. Sci. Forum Vol. 615-617 (2009), p. 41

doi:10.4028/www.scientific.net/MSF.615-617.41 [6] H. Harima et al.: Mater. Sci. Forum Vol. 338–342 (2000), p. 607

doi:10.4028/www.scientific.net/MSF.338-342.607 [7] R. Müller et al.: Chem. Vap. Deposition Vol. 12 (2006), p. 557

doi:10.1002/cvde.200606474 [8] D.J. Larkin et al.: phys. stat. sol. (b) Vol. 202 (1997), p. 305

doi:10.1002/1521-3951(199707)202:1<305::AID-PSSB305>3.0.CO;2-9 [3] C. Jacquier et al.: Mater. Sci. Forum Vol. 483-485 (2005), p. 125

doi:10.4028/www.scientific.net/MSF.483-485.125 [4] K. Kusunoki et al.: Mater. Sci. Forum Vol. 615-617 (2009), p. 137

doi:10.4028/www.scientific.net/MSF.615-617.137 [5] F. Mercier et al.: Mater. Sci. Forum Vol. 615-617 (2009), p. 41

doi:10.4028/www.scientific.net/MSF.615-617.41 [6] H. Harima et al.: Mater. Sci. Forum Vol. 338–342 (2000), p. 607

doi:10.4028/www.scientific.net/MSF.338-342.607 [7] R. Müller et al.: Chem. Vap. Deposition Vol. 12 (2006), p. 557

doi:10.1111/j.1574-6968.2006.00499.x [8] D.J. Larkin et al.: phys. stat. sol. (b) Vol. 202 (1997), p. 305

doi:10.1002/1521-3951(199707)202:1<305::AID-PSSB305>3.0.CO;2-9 [3] C. Jacquier et al.: Mater. Sci. Forum Vol. 483-485 (2005), p. 125

doi:10.4028/www.scientific.net/MSF.483-485.125 [4] K. Kusunoki et al.: Mater. Sci. Forum Vol. 615-617 (2009), p. 137

doi:10.4028/www.scientific.net/MSF.615-617.137 [5] F. Mercier et al.: Mater. Sci. Forum Vol. 615-617 (2009), p. 41

doi:10.4028/www.scientific.net/MSF.615-617.41 [6] H. Harima et al.: Mater. Sci. Forum Vol. 338–342 (2000), p. 607

doi:10.4028/www.scientific.net/MSF.338-342.607

[7] R. Müller et al.: Chem. Vap. Deposition Vol. 12 (2006), p. 557

doi:10.1111/j.1574-6968.2006.00499.x [8] D.J. Larkin et al.: phys. stat. sol. (b) Vol. 202 (1997), p. 305

doi:10.1002/1521-3951(199707)202:1<305::AID-PSSB305>3.0.CO;2-9