- Email: [email protected]
SiC single crystal growth by a modified physical vapor transport technique
ARTICLE IN PRESS
Journal of Crystal Growth 275 (2005) e555–e560 www.elsevier.com/locate/jcrysgro
SiC single crystal growth by a modified physical vapor transport technique Peter Wellmanna,, Patrick Desperriera, Ralf Mu¨llera, Thomas Straubingera, Albrecht Winnackera, Francis Bailletb, Elisabeth Blanquetb, Jean Marc Dedulleb, Michel Ponsb a
Materials Department 6, University of Erlangen, Martensstr. 7, 91058 Erlangen, Germany Laboratoire de Thermodynamique et Physico-Chimie Me´tallurgiques, Institut National Polytechnique de Grenoble, 1130 Rue de la Piscine, 38402 St. Martin d’He`res, France
b
Available online 9 December 2004
Abstract We have developed a modified physical vapor transport (M-PVT) growth technique for the preparation of SiC single crystals which makes use of an additional gas pipe in order to control the gas phase composition of the conventional physical vapor transport (PVT) configuration. We discuss the experimental implementation of the extra gas pipe by comparing crystal growth runs under various gas flow conditions with numerical simulations. The potential of the MPVT growth method will be demonstrated by showing the improved doping characteristics when performing nitrogen, phosphorus and aluminum doping of SiC using the additional gas pipe for dopant supply. r 2004 Elsevier B.V. All rights reserved. PACS: 81.10.Bk; 61.72.Ww; 81.05.Je Keywords: A1. Doping; A1. Computer simulation; A2. Growth from vapor; B2. Semiconducting silicon compounds
1. Introduction Today SiC single crystals for commercial applications are grown by the physical vapor transport (PVT) method at elevated temperatures Corresponding author. Tel.: +49 9131 85 27635; fax: +49 9131 85 28495. E-mail address: [email protected] (P. Wellmann).
above 2000 1C in a closed graphite crucible. SiC powder, which is placed in the hot zone of the growth cell (e.g. 2200 1C), sublimes and re-crystallizes in the colder zone (e.g. 2150 1C) at a seed. Control of growth is rather difficult. While temperature and temperature gradient can be set by a proper crucible design and heating procedure, the adjustment of the gas phase composition—e.g. C/Si ratio and/or dopant species concentration—is rather limited. The gas phase composition is
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.11.070
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basically fixed by the temperature field and depends on growth process instabilities as they may occur due to the time-dependent evolution of the SiC crystal and the SiC source morphology. In order to account for the drawbacks of the conventional PVT growth configuration, we have developed a modified PVT (PVT-M-PVT) technique which uses an external gas pipe for the introduction of small values of Si- and C-containing gas species like silane and propane as well as of doping gases [1]. The paper will focus on three issues: (i) We will briefly review the technological implementation of the extra gas pipe [1] and discuss its impact on crystal quality from the experimental point of view. (ii) We will discuss the impact of the extra gas stream on the physical vapor mass transport from a fluid dynamical point of view using numerical modeling. (iii) We will show the significant improvements of the conventional PVT system by the modified setup for doping issues of SiC.
2. Experimental setup Fig. 1 (left) shows a sketch of the PVT growth cell containing SiC powder at the bottom as source material and a SiC wafer at the top for seeded crystallization. In addition, we plotted the corresponding axial temperature field for source sublimation and crystal growth; the temperature field is established by inductive heating and proper setup design. In this conventional PVT configuration, the gas phase composition at the crystal growth interface is given by the temperaturedependent partial pressures of the various Siand C-containing gas species (i.e. Si, Si2C, SiC2) and super-saturation by the axial temperature gradient. At typical growth temperatures of 2200 1C (source) and 2150 1C (seed), the partial pressure of the Si- and C-containing gas species is o10 mbar. Using a typical inert-gas (e.g. argon) pressure of 30–50 mbar, a diffusion-limited mass transport regime is established. In the case of ntype doping, usually nitrogen is used which is chemically inert to carbon and is inserted into the growth cell making use of the graphite wall
PVT setup Doping by ... gas supply ... solid source
M-PVT setup ... additional gas pipe
see Fig. 2
x
Al T T12000˚C)
doping species (i.e. N, P, Al, ...) source depletion
continuous dopant supply
Fig. 1. Sketch of the physical vapor transport (PVT) setup for SiC crystal growth: (Left) Conventional PVT configuration and corresponding temperature field; (center) conventional PVT configuration for doping using solid sources as addition to the SiC source material; (right) modified PVT configuration using an additional gas pipe for the introduction of doping gases and/ or small amounts of C- and Si-containing gases.
porosity. In the case of p-type doping using aluminum, the latter process fails due to formation of Al4C3. Hence, an aluminum-containing source needs to be inserted directly into the SiC source material (Fig. 1, center); due to the much higher partial pressure of aluminum than the Si- and Ccontaining gas species, mass transport control is rather poor. To overcome the disadvantages of poor gas phase composition control, we have modified the conventional PVT configuration by introducing a gas pipe into the growth cell to add small amounts of the Si- and C-containing gases silane (SiH4:H2—1:10) and propane (C3H8) as well as doping gases. The phrase ‘‘small amounts’’ points out that we keep the physical vapor process and do not turn to chemical vapor deposition; the additional gases are solely used for ‘‘fine-tuning’’ of the gas phase composition.
3. Numerical modeling We used the CFD-ACE+ software package (CFD Research Corp.) for 2D numerical simulation [2] of the mixing of the additional gas stream with the SiC physical vapor mass transport. For the
ARTICLE IN PRESS P. Wellmann et al. / Journal of Crystal Growth 275 (2005) e555–e560
growth runs and the calculated SiC concentration field in the gas phase (image area corresponds to dashed inset in Fig. 1—right); in Fig. 3, we have plotted the corresponding calculated SiC deposition rate. At high gas fluxes (Fig. 2—left; approx. 10 times the PVT flux) despite some minor polycrystalline deposition, no growth is observed; the additional gas flow basically pushes out the SiC gas stream from the source material as it is also found in numerical modeling for the highest flux (inlet species velocity v ¼ 10v0). In the case of intermediate gas fluxes (Fig. 2—center), crystal
400 10V0
5V0 Growth Rate (µm.h-1)
calculation of inductive heating, heat transfer and gas flow, we used built-in modules; for heat transfer we considered conduction, radiation, convection as well as Stefan flow. To introduce SiC sublimation and recrystallization we developed a user subroutine. The chemical model considers stoichiometric SiC sublimation and crystallization. The source material was treated as a porous medium with a porosity of 70% (given by the initial source material density). Pore size, permeability and effective surface were chosen for initial growth conditions, i.e. granular powder morphology rather than sintered blocks with hollow cores as it would be typical at a later growth stage [3]. In the particular case of phosphorus doping of SiC we performed thermodynamic calculations (0D) using the FactSage software package (GTT-Technologies) in order to understand the decomposition of phosphine (PH3) while being introduced into the gas room of the SiC growth cell. The calculations (i.e. partial pressure of Si-, C- and P-related gas species as well as SiC deposition rate) were carried out assuming thermodynamic equilibrium.
e557
V0
300
200
100
Closed Etching
4. Results and discussion 0 0
4.1. General aspects of the modified physical vapor transport growth 4.1.1. Crystal quality The additional gas flow introduces a convection component to the diffusion-limited growth mode. Fig. 2 shows crystal images of a series of three
SiC crystal
High gas flux
SiC crystal
20mm
v=10.v0
Medium gas flux
10 15 Radius (mm)
20
25
Fig. 3. Calculated SiC deposition rates for various additional gas fluxes in the modified-PVT configuration. ‘‘Closed’’ represents a zero gas flux as in the conventional PVT configuration without additional gas pipe. ‘‘Etching’’ indicates a growth regime of negative deposition: the SiC seed will etch and no new crystal growth will take place in this area.
SiC crystal
20mm
5
SiC conc.
20mm
v=5.v0
Low gas flux
v=1.v0
[a.u.]
Fig. 2. SiC crystal photographs and numerical simulation of the corresponding SiC gas species concentration in the modified-PVT configuration for three different additional gas fluxes. In the case of the calculated SiC gas species concentration of the modified-PVT system, we have plotted the area of the growth cell as pointed out in the inset of Fig. 1.
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growth is observed; however, a hole is found in the central crystal region. This is confirmed by our calculations (inlet species velocity v ¼ 5v0) that exhibit decreased SiC deposition in the central SiC crystal region. At low gas fluxes (Fig. 2—right; additional gas flow ¼ SiC PVT gas stream), crystal growth with a quality even slightly improved to the conventional setup configuration without a gas pipe is found [1]. This coincides with the calculations that show a homogeneous mixing of the additional gas flux with the SiC PVT gas species and a SiC gas phase concentration field as it is valid for conventional growth without an extra gas pipe (Fig. 3, inlet species velocity v ¼ 1v0 and closed gas pipe). In addition, numerical modeling predicts SiC deposition inside the gas pipe for low gas fluxes (without figure, inlet species velocity v ¼ 0.1v0), which is confirmed by experiments where closure of the pipe took place in the case of low additional gas flux. The wavy profile of the iso-concentration lines of the SiC gas species concentration in Fig. 2— right is caused by a convection roll inside the gas room. The presence of the convection roll was independent of the inlet gas species velocity and even observed in the ‘‘closed’’ inlet configuration. From the convergence behavior of the simulation runs, one could conclude that small additional gas fluxes in the modified-PVT configuration have a stabilizing effect on the gas flow in the growth cell interior compared to the conventional PVT configuration without the gas pipe. The quantitatively rather good agreement of the numerical simulation with experiments point out two issues: (i) At an intermediate flux comparable to the SiC PVT species flux, the additional inlet of gas does not alter fluid dynamics in a growthdestructive way. A minimum gas flux is necessary to establish long-term stable inlet conditions (no closure of the pipe). At the same time, a very high gas flux will push out the SiC species from the central growth cell region. (ii) At intermediate inlet fluxes, a convection roll is present inside the growth cell, which supports a mixture of SiC PVT flux and additional gas flux. The latter is confirmed by absorption mapping measurements [7] showing a rather homogeneous aluminum
doping over the entire crystal diameter, i.e. no increased dopant incorporation was observed in the central crystal area (Fig. 4) where the growth interface is directly exposed to the incoming aluminum vapor from the additional gas pipe. 4.1.2. Suppression of C-inclusions Due to the convection component introduced by the additional gas stream, graphite particles originating from the SiC powder source due to graphitization are pushed out of the gas stream and hence the density of graphite inclusions in the SiC crystal is considerably reduced. In particular, we performed two growth runs with varying ratio of the molar SiC PVT flux and molar additional gas inlet. By doubling the additional gas inlet with respect to the PVT gas flow, we increased the carbon inclusion free area of the grown crystal from 40% to more than 90% (deduced from optical microscopy images, without figure). 4.2. Doping In the case of doping, we observed considerable improvements of the modified PVT setup compared to its conventional counterpart without an additional gas pipe.
Fig. 4. Example of the lateral charge carrier concentration of an aluminum-doped SiC wafer cut from a crystal grown by the modified-PVT method using aluminum vapor feeding through the additional gas pipe [6]. The charge carrier concentration homogeneity was performed using absorption mapping technique [7].
ARTICLE IN PRESS P. Wellmann et al. / Journal of Crystal Growth 275 (2005) e555–e560
4.2.1. n-type doping using nitrogen In the case of n-type of SiC by nitrogen, the main advantage of the modified growth setup is the more efficient use of the high-purity dopant source since the gas is supplied directly in front of the growth interface. Using Hall measurements, we obtained charge carrier concentrations in the mid 1018 cm3 range, which corresponds to a dopant concentration in the low 1019 cm3 range close to the solubility limit of nitrogen on the lattice site as it has been found in doping studies using a conventional PVT configuration [4]. The full potential of the modified PVT growth setup for doping, however, is benefited for doping species being highly reactive with graphite or/and with a partial pressure at growth temperature in the same order or larger than of the Si- and Ccontaining gas species. 4.2.2. n-type doping using phosphorus n-type doping using phosphorus could be of particular interest in the future because implantation experiments indicated that phosphorus has an about 10 times higher solubility limit than the state-of-the-art donor nitrogen [5]. We used phosphine (PH3) as the phosphorus source which is supplied to the crystal growth interface via the additional gas inlet in the M-PVT setup. Currently, we have achieved phosphorus incorporation of approx. 2 1017 cm3 (obtained from glow discharge mass spectroscopy (GDMS) measurements), which is sufficient for fundamental research studies of phosphorus centers in SiC but needs to be increased for real device applications. Our experiments indicate that we have not yet reached kinetic limitation of phosphorus incorporation. Therefore, we currently focus on further increasing the effective phosphorus supply into the growth cell. Thermodynamic studies of PH3 decomposition in a SiC environment suggest that a large relative amount of P-containing species contributing to doping will be available at the growth interface in a low temperature and high inert gas pressure growth regime. 4.2.3. p-type doping using aluminum Using a solid Al or Al4C3 doping source which is added to the SiC powder material (Fig. 1,
e559
center), we usually observed a decreasing dopant incorporation of about factor 50 in the axial direction, which is mainly attributed to source depletion. But it should be pointed out that the increase of temperature at the crystal growth interface which is very typical for (M-)PVT growth also causes a decrease due to kinetics of the dopant incorporation process. Using a continuous dopant species supply (inert carrier gas+aluminum vapor) in the modified-PVT configuration, we improve the axial aluminum incorporation significantly from a variation of factor 50 (PVT) to as low as factor 2 (M-PVT) [6]. The residual incorporation variation of factor 2 is attributed to the above-mentioned temperature effect and could be adjusted in future experiments by a simple increase of the aluminum concentration through the gas inlet. Fig. 4 shows an absorption topogram of the charge carrier concentration of a ptype doped SiC wafer: the lateral homogeneity in the particular case is as low as Dp/p ¼ 10%. It should be mentioned that aluminum vapor was added to the inert carrier gas using a small heated Al-containing reservoir outside the PVT crucible (for details see Ref. [6]). Currently we achieved a conductivity of o0.2 O1 cm1 in aluminum doped 4H-SiC which already meets the requirements for bipolar highpower devices.
5. Summary and outlook We have presented a modified-PVT growth method which allows the direct control of the gas phase composition in front of the SiC growth interface. Growth conditions were established with a crystal defect density as low as in the welldeveloped conventional PVT system. The potential of the M-PVT growth method was demonstrated by showing the improved doping characteristics of SiC using nitrogen, phosphorus and aluminum. Numerical modeling of the gas flow, i.e. a mixture of SiC PVT flux and additional gas inlet, was performed which is able to explain fluid dynamics in the modified setup and which, as an outlook, opens up the possibility to include chemistry of SiH4 and C3H8 in a theoretical and in an experimental approach in order to study the
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impact of gas phase composition (C/Si ratio) on polytypism, defect formation, etc.
Acknowledgements We would like to thank the German Science Foundation for financial support (Contract Number WE2107/3 and WE2107/5). References [1] T.L. Straubinger, P.J. Wellmann, A. Winnacker, Mat. Sci. Forum 353–356 (2001) 33.
[2] D. Chaussande, F. Baillet, L. Charpentier, E. Pernot, M. Pons, R. Madar, J. Electrochem. Soc. 150 (2003) G653. [3] P. Wellmann, Z. Herro, A. Winnacker, R. Pu¨sche, M. Hundhausen, P. Masri, A. Kulik, M. Bogdanov, S. Karpov, M. Ramm, Y. Makarov, ICCG14–1041, submitted for publication. [4] H.-J. Rost, J. Doerschel, K. Irmscher, D. Schulz, D. Siche, J. Crystal Growth 257 (2003) 75. [5] M. Laube, F. Schmid, G. Pensl, G. Wagner, Mater. Sci. Forum 389–393 (2002) 791. [6] T.L. Straubinger, M. Bickermann, R. Weingaertner, P.J. Wellmann, A. Winnacker, J. Crystal Growth 240 (2002) 117. [7] R. Weinga¨rtner, P.J. Wellmann, M. Bickermann, D. Hofmann, T.L. Straubinger, A. Winnacker, Appl. Phys. Lett. 80 (1) (2002) 70.
Journal of Crystal Growth 275 (2005) e555–e560 www.elsevier.com/locate/jcrysgro
SiC single crystal growth by a modified physical vapor transport technique Peter Wellmanna,, Patrick Desperriera, Ralf Mu¨llera, Thomas Straubingera, Albrecht Winnackera, Francis Bailletb, Elisabeth Blanquetb, Jean Marc Dedulleb, Michel Ponsb a
Materials Department 6, University of Erlangen, Martensstr. 7, 91058 Erlangen, Germany Laboratoire de Thermodynamique et Physico-Chimie Me´tallurgiques, Institut National Polytechnique de Grenoble, 1130 Rue de la Piscine, 38402 St. Martin d’He`res, France
b
Available online 9 December 2004
Abstract We have developed a modified physical vapor transport (M-PVT) growth technique for the preparation of SiC single crystals which makes use of an additional gas pipe in order to control the gas phase composition of the conventional physical vapor transport (PVT) configuration. We discuss the experimental implementation of the extra gas pipe by comparing crystal growth runs under various gas flow conditions with numerical simulations. The potential of the MPVT growth method will be demonstrated by showing the improved doping characteristics when performing nitrogen, phosphorus and aluminum doping of SiC using the additional gas pipe for dopant supply. r 2004 Elsevier B.V. All rights reserved. PACS: 81.10.Bk; 61.72.Ww; 81.05.Je Keywords: A1. Doping; A1. Computer simulation; A2. Growth from vapor; B2. Semiconducting silicon compounds
1. Introduction Today SiC single crystals for commercial applications are grown by the physical vapor transport (PVT) method at elevated temperatures Corresponding author. Tel.: +49 9131 85 27635; fax: +49 9131 85 28495. E-mail address: [email protected] (P. Wellmann).
above 2000 1C in a closed graphite crucible. SiC powder, which is placed in the hot zone of the growth cell (e.g. 2200 1C), sublimes and re-crystallizes in the colder zone (e.g. 2150 1C) at a seed. Control of growth is rather difficult. While temperature and temperature gradient can be set by a proper crucible design and heating procedure, the adjustment of the gas phase composition—e.g. C/Si ratio and/or dopant species concentration—is rather limited. The gas phase composition is
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.11.070
ARTICLE IN PRESS e556
P. Wellmann et al. / Journal of Crystal Growth 275 (2005) e555–e560
basically fixed by the temperature field and depends on growth process instabilities as they may occur due to the time-dependent evolution of the SiC crystal and the SiC source morphology. In order to account for the drawbacks of the conventional PVT growth configuration, we have developed a modified PVT (PVT-M-PVT) technique which uses an external gas pipe for the introduction of small values of Si- and C-containing gas species like silane and propane as well as of doping gases [1]. The paper will focus on three issues: (i) We will briefly review the technological implementation of the extra gas pipe [1] and discuss its impact on crystal quality from the experimental point of view. (ii) We will discuss the impact of the extra gas stream on the physical vapor mass transport from a fluid dynamical point of view using numerical modeling. (iii) We will show the significant improvements of the conventional PVT system by the modified setup for doping issues of SiC.
2. Experimental setup Fig. 1 (left) shows a sketch of the PVT growth cell containing SiC powder at the bottom as source material and a SiC wafer at the top for seeded crystallization. In addition, we plotted the corresponding axial temperature field for source sublimation and crystal growth; the temperature field is established by inductive heating and proper setup design. In this conventional PVT configuration, the gas phase composition at the crystal growth interface is given by the temperaturedependent partial pressures of the various Siand C-containing gas species (i.e. Si, Si2C, SiC2) and super-saturation by the axial temperature gradient. At typical growth temperatures of 2200 1C (source) and 2150 1C (seed), the partial pressure of the Si- and C-containing gas species is o10 mbar. Using a typical inert-gas (e.g. argon) pressure of 30–50 mbar, a diffusion-limited mass transport regime is established. In the case of ntype doping, usually nitrogen is used which is chemically inert to carbon and is inserted into the growth cell making use of the graphite wall
PVT setup Doping by ... gas supply ... solid source
M-PVT setup ... additional gas pipe
see Fig. 2
x
Al T T1
doping species (i.e. N, P, Al, ...) source depletion
continuous dopant supply
Fig. 1. Sketch of the physical vapor transport (PVT) setup for SiC crystal growth: (Left) Conventional PVT configuration and corresponding temperature field; (center) conventional PVT configuration for doping using solid sources as addition to the SiC source material; (right) modified PVT configuration using an additional gas pipe for the introduction of doping gases and/ or small amounts of C- and Si-containing gases.
porosity. In the case of p-type doping using aluminum, the latter process fails due to formation of Al4C3. Hence, an aluminum-containing source needs to be inserted directly into the SiC source material (Fig. 1, center); due to the much higher partial pressure of aluminum than the Si- and Ccontaining gas species, mass transport control is rather poor. To overcome the disadvantages of poor gas phase composition control, we have modified the conventional PVT configuration by introducing a gas pipe into the growth cell to add small amounts of the Si- and C-containing gases silane (SiH4:H2—1:10) and propane (C3H8) as well as doping gases. The phrase ‘‘small amounts’’ points out that we keep the physical vapor process and do not turn to chemical vapor deposition; the additional gases are solely used for ‘‘fine-tuning’’ of the gas phase composition.
3. Numerical modeling We used the CFD-ACE+ software package (CFD Research Corp.) for 2D numerical simulation [2] of the mixing of the additional gas stream with the SiC physical vapor mass transport. For the
ARTICLE IN PRESS P. Wellmann et al. / Journal of Crystal Growth 275 (2005) e555–e560
growth runs and the calculated SiC concentration field in the gas phase (image area corresponds to dashed inset in Fig. 1—right); in Fig. 3, we have plotted the corresponding calculated SiC deposition rate. At high gas fluxes (Fig. 2—left; approx. 10 times the PVT flux) despite some minor polycrystalline deposition, no growth is observed; the additional gas flow basically pushes out the SiC gas stream from the source material as it is also found in numerical modeling for the highest flux (inlet species velocity v ¼ 10v0). In the case of intermediate gas fluxes (Fig. 2—center), crystal
400 10V0
5V0 Growth Rate (µm.h-1)
calculation of inductive heating, heat transfer and gas flow, we used built-in modules; for heat transfer we considered conduction, radiation, convection as well as Stefan flow. To introduce SiC sublimation and recrystallization we developed a user subroutine. The chemical model considers stoichiometric SiC sublimation and crystallization. The source material was treated as a porous medium with a porosity of 70% (given by the initial source material density). Pore size, permeability and effective surface were chosen for initial growth conditions, i.e. granular powder morphology rather than sintered blocks with hollow cores as it would be typical at a later growth stage [3]. In the particular case of phosphorus doping of SiC we performed thermodynamic calculations (0D) using the FactSage software package (GTT-Technologies) in order to understand the decomposition of phosphine (PH3) while being introduced into the gas room of the SiC growth cell. The calculations (i.e. partial pressure of Si-, C- and P-related gas species as well as SiC deposition rate) were carried out assuming thermodynamic equilibrium.
e557
V0
300
200
100
Closed Etching
4. Results and discussion 0 0
4.1. General aspects of the modified physical vapor transport growth 4.1.1. Crystal quality The additional gas flow introduces a convection component to the diffusion-limited growth mode. Fig. 2 shows crystal images of a series of three
SiC crystal
High gas flux
SiC crystal
20mm
v=10.v0
Medium gas flux
10 15 Radius (mm)
20
25
Fig. 3. Calculated SiC deposition rates for various additional gas fluxes in the modified-PVT configuration. ‘‘Closed’’ represents a zero gas flux as in the conventional PVT configuration without additional gas pipe. ‘‘Etching’’ indicates a growth regime of negative deposition: the SiC seed will etch and no new crystal growth will take place in this area.
SiC crystal
20mm
5
SiC conc.
20mm
v=5.v0
Low gas flux
v=1.v0
[a.u.]
Fig. 2. SiC crystal photographs and numerical simulation of the corresponding SiC gas species concentration in the modified-PVT configuration for three different additional gas fluxes. In the case of the calculated SiC gas species concentration of the modified-PVT system, we have plotted the area of the growth cell as pointed out in the inset of Fig. 1.
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growth is observed; however, a hole is found in the central crystal region. This is confirmed by our calculations (inlet species velocity v ¼ 5v0) that exhibit decreased SiC deposition in the central SiC crystal region. At low gas fluxes (Fig. 2—right; additional gas flow ¼ SiC PVT gas stream), crystal growth with a quality even slightly improved to the conventional setup configuration without a gas pipe is found [1]. This coincides with the calculations that show a homogeneous mixing of the additional gas flux with the SiC PVT gas species and a SiC gas phase concentration field as it is valid for conventional growth without an extra gas pipe (Fig. 3, inlet species velocity v ¼ 1v0 and closed gas pipe). In addition, numerical modeling predicts SiC deposition inside the gas pipe for low gas fluxes (without figure, inlet species velocity v ¼ 0.1v0), which is confirmed by experiments where closure of the pipe took place in the case of low additional gas flux. The wavy profile of the iso-concentration lines of the SiC gas species concentration in Fig. 2— right is caused by a convection roll inside the gas room. The presence of the convection roll was independent of the inlet gas species velocity and even observed in the ‘‘closed’’ inlet configuration. From the convergence behavior of the simulation runs, one could conclude that small additional gas fluxes in the modified-PVT configuration have a stabilizing effect on the gas flow in the growth cell interior compared to the conventional PVT configuration without the gas pipe. The quantitatively rather good agreement of the numerical simulation with experiments point out two issues: (i) At an intermediate flux comparable to the SiC PVT species flux, the additional inlet of gas does not alter fluid dynamics in a growthdestructive way. A minimum gas flux is necessary to establish long-term stable inlet conditions (no closure of the pipe). At the same time, a very high gas flux will push out the SiC species from the central growth cell region. (ii) At intermediate inlet fluxes, a convection roll is present inside the growth cell, which supports a mixture of SiC PVT flux and additional gas flux. The latter is confirmed by absorption mapping measurements [7] showing a rather homogeneous aluminum
doping over the entire crystal diameter, i.e. no increased dopant incorporation was observed in the central crystal area (Fig. 4) where the growth interface is directly exposed to the incoming aluminum vapor from the additional gas pipe. 4.1.2. Suppression of C-inclusions Due to the convection component introduced by the additional gas stream, graphite particles originating from the SiC powder source due to graphitization are pushed out of the gas stream and hence the density of graphite inclusions in the SiC crystal is considerably reduced. In particular, we performed two growth runs with varying ratio of the molar SiC PVT flux and molar additional gas inlet. By doubling the additional gas inlet with respect to the PVT gas flow, we increased the carbon inclusion free area of the grown crystal from 40% to more than 90% (deduced from optical microscopy images, without figure). 4.2. Doping In the case of doping, we observed considerable improvements of the modified PVT setup compared to its conventional counterpart without an additional gas pipe.
Fig. 4. Example of the lateral charge carrier concentration of an aluminum-doped SiC wafer cut from a crystal grown by the modified-PVT method using aluminum vapor feeding through the additional gas pipe [6]. The charge carrier concentration homogeneity was performed using absorption mapping technique [7].
ARTICLE IN PRESS P. Wellmann et al. / Journal of Crystal Growth 275 (2005) e555–e560
4.2.1. n-type doping using nitrogen In the case of n-type of SiC by nitrogen, the main advantage of the modified growth setup is the more efficient use of the high-purity dopant source since the gas is supplied directly in front of the growth interface. Using Hall measurements, we obtained charge carrier concentrations in the mid 1018 cm3 range, which corresponds to a dopant concentration in the low 1019 cm3 range close to the solubility limit of nitrogen on the lattice site as it has been found in doping studies using a conventional PVT configuration [4]. The full potential of the modified PVT growth setup for doping, however, is benefited for doping species being highly reactive with graphite or/and with a partial pressure at growth temperature in the same order or larger than of the Si- and Ccontaining gas species. 4.2.2. n-type doping using phosphorus n-type doping using phosphorus could be of particular interest in the future because implantation experiments indicated that phosphorus has an about 10 times higher solubility limit than the state-of-the-art donor nitrogen [5]. We used phosphine (PH3) as the phosphorus source which is supplied to the crystal growth interface via the additional gas inlet in the M-PVT setup. Currently, we have achieved phosphorus incorporation of approx. 2 1017 cm3 (obtained from glow discharge mass spectroscopy (GDMS) measurements), which is sufficient for fundamental research studies of phosphorus centers in SiC but needs to be increased for real device applications. Our experiments indicate that we have not yet reached kinetic limitation of phosphorus incorporation. Therefore, we currently focus on further increasing the effective phosphorus supply into the growth cell. Thermodynamic studies of PH3 decomposition in a SiC environment suggest that a large relative amount of P-containing species contributing to doping will be available at the growth interface in a low temperature and high inert gas pressure growth regime. 4.2.3. p-type doping using aluminum Using a solid Al or Al4C3 doping source which is added to the SiC powder material (Fig. 1,
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center), we usually observed a decreasing dopant incorporation of about factor 50 in the axial direction, which is mainly attributed to source depletion. But it should be pointed out that the increase of temperature at the crystal growth interface which is very typical for (M-)PVT growth also causes a decrease due to kinetics of the dopant incorporation process. Using a continuous dopant species supply (inert carrier gas+aluminum vapor) in the modified-PVT configuration, we improve the axial aluminum incorporation significantly from a variation of factor 50 (PVT) to as low as factor 2 (M-PVT) [6]. The residual incorporation variation of factor 2 is attributed to the above-mentioned temperature effect and could be adjusted in future experiments by a simple increase of the aluminum concentration through the gas inlet. Fig. 4 shows an absorption topogram of the charge carrier concentration of a ptype doped SiC wafer: the lateral homogeneity in the particular case is as low as Dp/p ¼ 10%. It should be mentioned that aluminum vapor was added to the inert carrier gas using a small heated Al-containing reservoir outside the PVT crucible (for details see Ref. [6]). Currently we achieved a conductivity of o0.2 O1 cm1 in aluminum doped 4H-SiC which already meets the requirements for bipolar highpower devices.
5. Summary and outlook We have presented a modified-PVT growth method which allows the direct control of the gas phase composition in front of the SiC growth interface. Growth conditions were established with a crystal defect density as low as in the welldeveloped conventional PVT system. The potential of the M-PVT growth method was demonstrated by showing the improved doping characteristics of SiC using nitrogen, phosphorus and aluminum. Numerical modeling of the gas flow, i.e. a mixture of SiC PVT flux and additional gas inlet, was performed which is able to explain fluid dynamics in the modified setup and which, as an outlook, opens up the possibility to include chemistry of SiH4 and C3H8 in a theoretical and in an experimental approach in order to study the
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impact of gas phase composition (C/Si ratio) on polytypism, defect formation, etc.
Acknowledgements We would like to thank the German Science Foundation for financial support (Contract Number WE2107/3 and WE2107/5). References [1] T.L. Straubinger, P.J. Wellmann, A. Winnacker, Mat. Sci. Forum 353–356 (2001) 33.
[2] D. Chaussande, F. Baillet, L. Charpentier, E. Pernot, M. Pons, R. Madar, J. Electrochem. Soc. 150 (2003) G653. [3] P. Wellmann, Z. Herro, A. Winnacker, R. Pu¨sche, M. Hundhausen, P. Masri, A. Kulik, M. Bogdanov, S. Karpov, M. Ramm, Y. Makarov, ICCG14–1041, submitted for publication. [4] H.-J. Rost, J. Doerschel, K. Irmscher, D. Schulz, D. Siche, J. Crystal Growth 257 (2003) 75. [5] M. Laube, F. Schmid, G. Pensl, G. Wagner, Mater. Sci. Forum 389–393 (2002) 791. [6] T.L. Straubinger, M. Bickermann, R. Weingaertner, P.J. Wellmann, A. Winnacker, J. Crystal Growth 240 (2002) 117. [7] R. Weinga¨rtner, P.J. Wellmann, M. Bickermann, D. Hofmann, T.L. Straubinger, A. Winnacker, Appl. Phys. Lett. 80 (1) (2002) 70.