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Polytype stability in nitrogen-doped PVT—grown 2″—4H–SiC crystals
ARTICLE IN PRESS
Journal of Crystal Growth 275 (2005) e451–e454 www.elsevier.com/locate/jcrysgro
Polytype stability in nitrogen-doped PVT—grown 200 —4H–SiC crystals H.-J. Rost, J. Doerschel, K. Irmscher, M. RoXberg, D. Schulz, D. Siche Institute for Crystal Growth, Max-Born-Strasse 2, D-12489 Berlin, Germany Available online 8 December 2004
Abstract The influence of nitrogen on the polytype stability of 4H–SiC single crystals grown by physical vapour transport was investigated for doping levels below 2 1019 cm3. As long as nitrogen doping was maintained no polytype changes were found. However, a certain time after the permanent interruption of nitrogen doping the onset of polytype transformations to 15R and 6H could be observed. The polytype lamellas originate in the vicinity of the transition between the facet and off- facet region and spread along the basal plane in step flow direction. The polytype stabilising effect of nitrogen is assumed to be a result of its influence on the surface energy and step flow. r 2004 Elsevier B.V. All rights reserved. PACS: 81.10.Bk; 61.72.Nn Keywords: A1. Defects; A1. Doping; A2. Growth from vapour; B2. Semiconducting silicon compounds
1. Introduction For most high-power devices low-resistivity 4H–SiC single crystalline substrates are needed which can be obtained by nitrogen doping during the physical vapour transport (PVT) growth. The polytype stability of such crystals strongly depends on the proper choice of growth parameters such as temperature, pressure, temperature gradients, seed polarity and off-orientation. Furthermore, high Corresponding author. Tel.: +49 30 6392 2847;
fax: +49 30 6392 3003. E-mail address: [email protected] (H.-J. Rost).
nitrogen doping levels can influence the formation of planar defects. At nitrogen concentrations above about 2 1019 cm3 the generation of a special type of stacking faults (SFs) observed in 4H–SiC crystals after heat treatment (1100–1200 1C) is drastically enhanced [1–5]. The fault consists of six Si–C bilayers in cubic stacking sequence, can therefore be regarded as a thin 3C lamella and is called double SF. As long as the formation of these double SFs is believed to be hardly avoidable during device processing, 4H substrates of nitrogen doping levels below 2 1019 cm3 should be preferred. In the present paper we have investigated the impact of nitrogen
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.11.018
ARTICLE IN PRESS H.-J. Rost et al. / Journal of Crystal Growth 275 (2005) e451–e454
e452
on the polytype stability in such lower doped material.
2. Experimental procedure 4H–SiC single crystals of 200 in diameter and up to 15 mm in length were grown by PVT in the temperature range from 2100 to 2250 1C. The used 4H–SiC seed crystals had (0 0 0 1)C surfaces offoriented by 7 1 to o1 1 2 04. Crystal growth was always started with nitrogen doping concentrations of about 1 1019 cm3. Doping was stopped at different stages of the growth process. (1 1 0 0) cross sectional wafers of the as-grown crystals were analysed by photoluminescence spectroscopy at room temperature (RTPL), cathodoluminescence at room and low temperatures (RTCL, LTCL) as well as the measurement of selected area channelling patterns (SACP).
3. Results and discussion In Fig. 1a, a (1 1 0 0) plane cut of a 200 - 4H–SiC crystal is shown. The transition between the doped (dark) and the undoped region (bright) is clearly visible and corresponds to the interface at the moment of switching off the nitrogen doping. Inside the doped region a further area is marked which corresponds to the facetted area. It is always located at the rim of the seed and runs to the inner part of the crystal depending on its off-orientation and the real curvature of the phase boundary defined by the growth conditions. In the facet area the nitrogen incorporation is increased by a factor between 1.8 and 1.2 compared to the surrounding area for unintentionally and heavily doped crystals, respectively [1]. Furthermore, it was found that after a certain time delay which corresponds to the time between the nitrogen switch off and the first appearance of polytype lamellas the permanent interruption of
facet area 15R
<000-1> C
6H - lamellas 4H undoped
basal plane
4H N2- doped 3 mm
facet seed
(A)
100 µm
(B)
Fig. 1. (a) Optical transmission micrograph of a (1 1 0 0) cross-sectional wafer showing polytype changes in a 4H–SiC crystal and (b) monochromatic RTCL- images (429 nm) recorded in the transition area between facet and off-facet region (‘‘A’’, ‘‘B’’ see Fig. 2).
ARTICLE IN PRESS H.-J. Rost et al. / Journal of Crystal Growth 275 (2005) e451–e454
nitrogen doping was reproducibly correlated to the generation of polytype changes to 15R and/or 6H. In Fig. 1b this is shown in more detail. Monochromatic RTCL-images recorded in the transition area between the facet and off-facet region show the generation and further spreading of single lamellas of different polytypes in the basal plane in the originally 4H-matrix. Leaving the facet area, their number and extension quickly increases towards the crystal rim. Sometimes they form wide ribbons. Later they often transform to homogeneous areas of a single foreign polytype. LTCL spectra taken from the same region are shown in Fig. 2. The normalised LTCL intensity values vary in dependence on the polytype and status during growth. Intensity peaks are found at wavelengths of 414 and 418 nm, which correspond the starting points of 6H and 15R lamellas, respectively, compared with the intensities at a later point, where a 6H ribbon and a compact 15R region are formed. Whereas at the starting points for the different lamellas the intensity peak of the 4H matrix (384 nm) still appears because of large beam focus in comparison with the thin lamellas, this peak is absent in a ribbon and compact areas.
e453
Polytype lamellas are spreading along the basal plane in step flow direction and are inclined to the seed surface corresponding to the off-orientation angle. Within the delay period of about 2.5 h the crystal length increases at about 475 mm for our growth conditions. This is a long time compared to other defect generation processes like for example stress reduction by dislocation generation. If the doping is continued within the delay period polytype changes will be prevented. They arise in the transition region between the facet and offfacet area. As long as nitrogen doping was maintained no polytype changes were found. Additionally, no local cubic polytype based SF could be revealed. Although the relatively high off-orientation should promote the step flow and therefore stabilise a polytype stable crystal growth polytype changes appeared under the absence of nitrogen. A local discontinuity at the edge of the facet area was revealed. According to this behaviour the following is suggested. The actual shape of the interface will be determined by the temperature distribution described by the isotherms. That part of the real
6H (414 nm) 1.2x105 15R (418 nm) 2
LTCL-intensity (a.u.)
9.0x104
4H (384 nm) 4 1 2 3 4
6.0x104
lamella A : starting point 6H-ribbon lamella B : starting point 15R-compact
3.0x104
1
3
0.0 380
400
420
wave length (nm) Fig. 2. Low-temperature cathodoluminescence spectra (T ¼ 16 K) taken from the lamellas marked. ‘‘A’’ and ‘‘B’’ in Fig. 1b originating from the transition area between facet and off-facet region.
ARTICLE IN PRESS e454
H.-J. Rost et al. / Journal of Crystal Growth 275 (2005) e451–e454
growth interface which is parallel to the basal plane, as it is the case for the facetted area, there is no further preferential orientation for the incorporation of adatoms on permanently present growth steps and their lateral spreading along the basal plane (step flow) with the advancing growth interface. On the facet region which is characterised by large terrace widths adatoms may randomly form single islands or can escape before they enter the next step. Therefore, the growth of the desired polytype will not longer be stabilised. This corresponds to the 3D-growth mode according the epitaxial growth on on-axis substrates. Outside the facetted area the growth boundary always includes an angle with the basal plane direction. As a consequence the generation of new polytypes will be prevented. Once a foreign polytype is generated in the transition region between facetted and off-facetted area after switching off the nitrogen it will be more and more stabilised with increasing angle. The reason why no other polytypes are formed in the facetted area which is always present during each growth can be attributed to the presence of nitrogen. The growth stabilising effect of nitrogen can be regarded as surface catalytic effect under consideration of its influence on the surface energy and on the step flow [6]. Nitrogen doping effectively prevents the 3D-growth on the facet because it changes the surface kinetics and morphology by reduction of the microscopic step height and width. Additionally, the polytype generation on the facet after switching off the nitrogen is preferred at its rim because off the decreasing nitrogen concentration in the transition region compared to the central part. Furthermore, additional stresses caused by the change of the doping level can be excluded as a reason for the polytype switching within the investigated doping range. It could be shown earlier [1] that only at higher nitrogen doping levels noticeable stress-induced changes like wafer warp and/or stacking fault generation after thermal treatment may occur.
4. Summary Low-resistivity 4H–SiC single crystalline substrates for high-power applications can be obtained by nitrogen doping during the growth. The influence of nitrogen on the polytype stability in low doped material was investigated. It is shown that after a certain time delay the permanent interruption of nitrogen doping was reproducibly correlated to the generation of polytype changes to 15R and/or 6H. Polytype lamellas are spreading along the basal plane in step flow direction, inclined to the growth direction corresponding to the off-orientation angle. The origin of the polytype changes could be located in the transition region between the facet and off-facet area. Their generation mechanism was discussed. As long as nitrogen doping was maintained no polytype changes were found. Additionally, no local cubic polytype-based SF could be revealed. The polytype stabilising effect of nitrogen can be regarded as surface catalytic effect under consideration of its influence on the surface energy and on the step flow.
Acknowledgements The authors would like to acknowledge the support by Si Crystal AG and by the Bundesministerium fu¨r Bildung und Forschung under the Project (No.) 01BM071.
References [1] H.-J. Rost, J. Doerschel, K. Irmscher, D. Schulz, D. Siche, J. Crystal Growth 257 (2003) 75. [2] R.S. Okojie, M. Xhang, P. Pirouz, S. Tumakha, G. Jessen, L.J. Brillson, Appl. Phys. Lett. 79 (2001) 3056. [3] B.J. Skromme, K. Palle, C.D. Poweleit, L.R. Bryant, W.M. Vetter, M. Dudley, K. Moore, T. Gehoski, Mater. Sci. Forum 389–393 (2002) 455. [4] T. Kuhr, J.Q. Liu, H.J. Chung, M. Skowronski, J. Appl. Phys. 92 (2002) 5863. [5] J.Q. Liu, H.J. Chung, T. Kuhr, Q. Li, M. Skowronski, Appl. Phys. Lett. 80 (2002) 2111. [6] D. Schulz, M. Lechner, H.-J. Rost, D. Siche, J. Wollweber, Mater. Sci. Forum 433–436 (2003) 17.
Journal of Crystal Growth 275 (2005) e451–e454 www.elsevier.com/locate/jcrysgro
Polytype stability in nitrogen-doped PVT—grown 200 —4H–SiC crystals H.-J. Rost, J. Doerschel, K. Irmscher, M. RoXberg, D. Schulz, D. Siche Institute for Crystal Growth, Max-Born-Strasse 2, D-12489 Berlin, Germany Available online 8 December 2004
Abstract The influence of nitrogen on the polytype stability of 4H–SiC single crystals grown by physical vapour transport was investigated for doping levels below 2 1019 cm3. As long as nitrogen doping was maintained no polytype changes were found. However, a certain time after the permanent interruption of nitrogen doping the onset of polytype transformations to 15R and 6H could be observed. The polytype lamellas originate in the vicinity of the transition between the facet and off- facet region and spread along the basal plane in step flow direction. The polytype stabilising effect of nitrogen is assumed to be a result of its influence on the surface energy and step flow. r 2004 Elsevier B.V. All rights reserved. PACS: 81.10.Bk; 61.72.Nn Keywords: A1. Defects; A1. Doping; A2. Growth from vapour; B2. Semiconducting silicon compounds
1. Introduction For most high-power devices low-resistivity 4H–SiC single crystalline substrates are needed which can be obtained by nitrogen doping during the physical vapour transport (PVT) growth. The polytype stability of such crystals strongly depends on the proper choice of growth parameters such as temperature, pressure, temperature gradients, seed polarity and off-orientation. Furthermore, high Corresponding author. Tel.: +49 30 6392 2847;
fax: +49 30 6392 3003. E-mail address: [email protected] (H.-J. Rost).
nitrogen doping levels can influence the formation of planar defects. At nitrogen concentrations above about 2 1019 cm3 the generation of a special type of stacking faults (SFs) observed in 4H–SiC crystals after heat treatment (1100–1200 1C) is drastically enhanced [1–5]. The fault consists of six Si–C bilayers in cubic stacking sequence, can therefore be regarded as a thin 3C lamella and is called double SF. As long as the formation of these double SFs is believed to be hardly avoidable during device processing, 4H substrates of nitrogen doping levels below 2 1019 cm3 should be preferred. In the present paper we have investigated the impact of nitrogen
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.11.018
ARTICLE IN PRESS H.-J. Rost et al. / Journal of Crystal Growth 275 (2005) e451–e454
e452
on the polytype stability in such lower doped material.
2. Experimental procedure 4H–SiC single crystals of 200 in diameter and up to 15 mm in length were grown by PVT in the temperature range from 2100 to 2250 1C. The used 4H–SiC seed crystals had (0 0 0 1)C surfaces offoriented by 7 1 to o1 1 2 04. Crystal growth was always started with nitrogen doping concentrations of about 1 1019 cm3. Doping was stopped at different stages of the growth process. (1 1 0 0) cross sectional wafers of the as-grown crystals were analysed by photoluminescence spectroscopy at room temperature (RTPL), cathodoluminescence at room and low temperatures (RTCL, LTCL) as well as the measurement of selected area channelling patterns (SACP).
3. Results and discussion In Fig. 1a, a (1 1 0 0) plane cut of a 200 - 4H–SiC crystal is shown. The transition between the doped (dark) and the undoped region (bright) is clearly visible and corresponds to the interface at the moment of switching off the nitrogen doping. Inside the doped region a further area is marked which corresponds to the facetted area. It is always located at the rim of the seed and runs to the inner part of the crystal depending on its off-orientation and the real curvature of the phase boundary defined by the growth conditions. In the facet area the nitrogen incorporation is increased by a factor between 1.8 and 1.2 compared to the surrounding area for unintentionally and heavily doped crystals, respectively [1]. Furthermore, it was found that after a certain time delay which corresponds to the time between the nitrogen switch off and the first appearance of polytype lamellas the permanent interruption of
facet area 15R
<000-1> C
6H - lamellas 4H undoped
basal plane
4H N2- doped 3 mm
facet seed
(A)
100 µm
(B)
Fig. 1. (a) Optical transmission micrograph of a (1 1 0 0) cross-sectional wafer showing polytype changes in a 4H–SiC crystal and (b) monochromatic RTCL- images (429 nm) recorded in the transition area between facet and off-facet region (‘‘A’’, ‘‘B’’ see Fig. 2).
ARTICLE IN PRESS H.-J. Rost et al. / Journal of Crystal Growth 275 (2005) e451–e454
nitrogen doping was reproducibly correlated to the generation of polytype changes to 15R and/or 6H. In Fig. 1b this is shown in more detail. Monochromatic RTCL-images recorded in the transition area between the facet and off-facet region show the generation and further spreading of single lamellas of different polytypes in the basal plane in the originally 4H-matrix. Leaving the facet area, their number and extension quickly increases towards the crystal rim. Sometimes they form wide ribbons. Later they often transform to homogeneous areas of a single foreign polytype. LTCL spectra taken from the same region are shown in Fig. 2. The normalised LTCL intensity values vary in dependence on the polytype and status during growth. Intensity peaks are found at wavelengths of 414 and 418 nm, which correspond the starting points of 6H and 15R lamellas, respectively, compared with the intensities at a later point, where a 6H ribbon and a compact 15R region are formed. Whereas at the starting points for the different lamellas the intensity peak of the 4H matrix (384 nm) still appears because of large beam focus in comparison with the thin lamellas, this peak is absent in a ribbon and compact areas.
e453
Polytype lamellas are spreading along the basal plane in step flow direction and are inclined to the seed surface corresponding to the off-orientation angle. Within the delay period of about 2.5 h the crystal length increases at about 475 mm for our growth conditions. This is a long time compared to other defect generation processes like for example stress reduction by dislocation generation. If the doping is continued within the delay period polytype changes will be prevented. They arise in the transition region between the facet and offfacet area. As long as nitrogen doping was maintained no polytype changes were found. Additionally, no local cubic polytype based SF could be revealed. Although the relatively high off-orientation should promote the step flow and therefore stabilise a polytype stable crystal growth polytype changes appeared under the absence of nitrogen. A local discontinuity at the edge of the facet area was revealed. According to this behaviour the following is suggested. The actual shape of the interface will be determined by the temperature distribution described by the isotherms. That part of the real
6H (414 nm) 1.2x105 15R (418 nm) 2
LTCL-intensity (a.u.)
9.0x104
4H (384 nm) 4 1 2 3 4
6.0x104
lamella A : starting point 6H-ribbon lamella B : starting point 15R-compact
3.0x104
1
3
0.0 380
400
420
wave length (nm) Fig. 2. Low-temperature cathodoluminescence spectra (T ¼ 16 K) taken from the lamellas marked. ‘‘A’’ and ‘‘B’’ in Fig. 1b originating from the transition area between facet and off-facet region.
ARTICLE IN PRESS e454
H.-J. Rost et al. / Journal of Crystal Growth 275 (2005) e451–e454
growth interface which is parallel to the basal plane, as it is the case for the facetted area, there is no further preferential orientation for the incorporation of adatoms on permanently present growth steps and their lateral spreading along the basal plane (step flow) with the advancing growth interface. On the facet region which is characterised by large terrace widths adatoms may randomly form single islands or can escape before they enter the next step. Therefore, the growth of the desired polytype will not longer be stabilised. This corresponds to the 3D-growth mode according the epitaxial growth on on-axis substrates. Outside the facetted area the growth boundary always includes an angle with the basal plane direction. As a consequence the generation of new polytypes will be prevented. Once a foreign polytype is generated in the transition region between facetted and off-facetted area after switching off the nitrogen it will be more and more stabilised with increasing angle. The reason why no other polytypes are formed in the facetted area which is always present during each growth can be attributed to the presence of nitrogen. The growth stabilising effect of nitrogen can be regarded as surface catalytic effect under consideration of its influence on the surface energy and on the step flow [6]. Nitrogen doping effectively prevents the 3D-growth on the facet because it changes the surface kinetics and morphology by reduction of the microscopic step height and width. Additionally, the polytype generation on the facet after switching off the nitrogen is preferred at its rim because off the decreasing nitrogen concentration in the transition region compared to the central part. Furthermore, additional stresses caused by the change of the doping level can be excluded as a reason for the polytype switching within the investigated doping range. It could be shown earlier [1] that only at higher nitrogen doping levels noticeable stress-induced changes like wafer warp and/or stacking fault generation after thermal treatment may occur.
4. Summary Low-resistivity 4H–SiC single crystalline substrates for high-power applications can be obtained by nitrogen doping during the growth. The influence of nitrogen on the polytype stability in low doped material was investigated. It is shown that after a certain time delay the permanent interruption of nitrogen doping was reproducibly correlated to the generation of polytype changes to 15R and/or 6H. Polytype lamellas are spreading along the basal plane in step flow direction, inclined to the growth direction corresponding to the off-orientation angle. The origin of the polytype changes could be located in the transition region between the facet and off-facet area. Their generation mechanism was discussed. As long as nitrogen doping was maintained no polytype changes were found. Additionally, no local cubic polytype-based SF could be revealed. The polytype stabilising effect of nitrogen can be regarded as surface catalytic effect under consideration of its influence on the surface energy and on the step flow.
Acknowledgements The authors would like to acknowledge the support by Si Crystal AG and by the Bundesministerium fu¨r Bildung und Forschung under the Project (No.) 01BM071.
References [1] H.-J. Rost, J. Doerschel, K. Irmscher, D. Schulz, D. Siche, J. Crystal Growth 257 (2003) 75. [2] R.S. Okojie, M. Xhang, P. Pirouz, S. Tumakha, G. Jessen, L.J. Brillson, Appl. Phys. Lett. 79 (2001) 3056. [3] B.J. Skromme, K. Palle, C.D. Poweleit, L.R. Bryant, W.M. Vetter, M. Dudley, K. Moore, T. Gehoski, Mater. Sci. Forum 389–393 (2002) 455. [4] T. Kuhr, J.Q. Liu, H.J. Chung, M. Skowronski, J. Appl. Phys. 92 (2002) 5863. [5] J.Q. Liu, H.J. Chung, T. Kuhr, Q. Li, M. Skowronski, Appl. Phys. Lett. 80 (2002) 2111. [6] D. Schulz, M. Lechner, H.-J. Rost, D. Siche, J. Wollweber, Mater. Sci. Forum 433–436 (2003) 17.