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Ge multilayers
Thin Solid Films 319 Ž1998. 168–171
HRTEM study of strained SirGe multilayers M. Romeo a , C. Uhlaq-Bouillet a , J.P. Deville a,) , J. Werckmann a , G. Ehret a , R. Chelly b, D. Dentel b, T. Angot b, J.L. Bischoff b a
b
IPCMS, 23 rue du Loess, 67037 Strasbourg, France LPSE, 4 rue des Freres ` Lumiere, ` 68093 Mulhouse, France
Abstract Deformation in SirGe strained multilayers has been characterised by HRTEM. The HRTEM strain profile determined by image treatment has been used to compare two growth techniques: Hot wire assisted gas source and ultra high vacuum molecular beam epitaxy. We have shown that for the first technique, germanium layers are highly strained. This is probably due to the incorporation of atomic hydrogen which would prevent relaxation by stacking faults formation. q 1998 Published by Elsevier Science S.A. Keywords: High-resolution transmission electron microscopy; Multilayers; SiGe; Image treatment; Strain; Relaxation
1. Introduction Recently Chelly et al. w1x opened the question of classifying atomic hydrogen as a surfactant in the growth of GerSi by Hot-Wire assisted Chemical Vapour Deposition ŽHWCVD.. Indeed in this growth technique, atomic H is found to be a sub-product of gaseous source decomposition on the hot wire. Commensurate epitaxial growth is accompanied by a built-in strain energy increasing with the layer thickness, due to the isostructural lattice mismatch Ž4.2%.. However, for thicknesses greater than the critical one w2,3x, strain is relieved by dislocations or a 3D growth. For the germanium growth on silicon the critical thickness is less than 6 monolayers ŽML. w4x. By using a surfactant it was shown that the surface mobility can be decreased to prevent island formation and Ge segregation during Si growth over a Ge layer. It also improves the abruptness of the SirGe interface but with a degradation of its flatness w5x. The present paper is devoted to the HREM characterisation of the local lattice distortion in SirGe strained layer structures prepared by two different methods wHWCVD and hydrogenless Molecular Beam Epitaxy ŽMBE.x to determine by comparison the effect of hydrogen. Bierwolf et al. w6x, have reported the direct measurement of local lattice displacements in semiconductors by HREM. In this
)
Corresponding author. Tel.: q33-3-88-10-70-28; fax: q33-3-88-1072-48
paper we present an alternative technique based on mathematical morphology analysis.
2. Experimental 2.1. Multilayers preparation Two sets of samples have been grown using the two different methods wMBE and HWCVDx. Samples were grown on Si Ž100. B-doped 10 V cm, alternating Ge and Si layers. Prior to the growth of the layered structure, silicon buffer layers were deposited Ž50 nm for MBE, 10 nm for HWCVD.. For HWCVD pure disilane and germane have been used. The hot wire is kept at 15308C during the growth. The total pressure never exceeded 3 = 10y4 mbar insuring heterogeneous decomposition processes at the surface substrate which is resistively heated at 3508C. More details are reported in Ref. w7x. For MBE the total pressure was 10y9 mbar with the substrate resistively heated at 4008C. 2.2. Tripod thinning The study of interfacial strain requires that a great extension of the interfaces can be observed. Cross sections meeting these requirements have been prepared with a tripod polisher w8x. High resolution microscopy was carried out on a TOPCON EM-002B electron microscope Ž Cs s 0.4 mm. operating at 200 kV. All images were taken with the
0040-6090r98r$19.00 q 1998 Published by Elsevier Science S.A. All rights reserved. PII S 0 0 4 0 - 6 0 9 0 Ž 9 7 . 0 1 1 1 5 - 2
M. Romeo et al.r Thin Solid Films 319 (1998) 168–171
w110x zone axis of the Si substrate parallel to the electron beam using a double tilt sample holder. After enlargement at 1.7 = 10 7 magnification, the positive micrographs were digitised using a scanner with a definition of 0.005 nm per pixel in order to analyse the micrographies.
3. Image treatment and analysis As the point resolution of the TOPCON microscope is 0.18 nm, the structure of Si is not resolvable. Indeed, each dark spot in our micrographs will correspond to Si–Si, Si–Ge, Ge–Si or Ge–Ge dumbbells. Therefore, we restrict ourselves to the determination of the distance between the centres of the dumbbells. Image treatment and analysis have been performed using the Khoros software development system w9,10x. Each 8-bit grey level image has been filtered and binarised using MMACH w11x in order to obtain the contour of each white dot present on the image. The main advantage of using mathematical morphology operators to filter an image, instead of the more classical way of Fourier filtering, is that type casting to complex image is not necessary. This implies that the memory and disk space taken by images are at least 8 times less than the corresponding space taken by a Fourier transformed image. For our images Ž4096 = 4096, 8-bit grey level images., this would require 128 Mbytes instead of the 16 Mbytes required by our method. Another important point is the gain of speed with morphological operators. Filtering in the direct space has been performed using a sequential filter of Closing ŽOpeningŽ.. type w12x. The structuring element was a disk of about 0.1 nm in size Ždiameter equal to 21 pixels.. This low-pass filter allowed us to get a noiseless image suitable for the determination of the contour of white dots. This determination has been carried out using the watershed transformation w13x. After binarisation, a routine developed under Khoros in our laboratory allowed us to shrink the images into a file of only 70 Kbytes. This function simply looks for the centre of mass of each < feature4 found by the watershed transformation and writes its coordinates into a file. Naturally, as the contrast of white dots may be neither isotropic nor Gaussian, the centre of mass determined using the latter procedure may not exactly correspond to real position of Si–Si or Ge–Ge dumbbells. Indeed, slight bending of the sample andror thickness variations may cause shifts of intensity maxima. However, the error that affects this determination is the same for the columns of dumbbells of the same type and does not cumulate throughout the lattice. This means that the error induced by these shifts on the lattice distortions determination is very small. A set of other routines, also developed in our laboratory, performing simple vector calculus, allowed us to determine the first neighbours for each dumbbell, their distance along one specific crystallographic direction, and the mean value of these distances along the
169
w110x direction. The latter procedure has been applied to the two images presented on Fig. 1. These images correspond to two regions of about 350 nm2 taken from the experimental HREM micrographs of w110x cross sections of MBE and HW-CVD samples, respectively. The direction of growth goes from the top of image to its bottom Žw001x.. Both images contain two Ge multilayers Ž7 and 10 monolayers, from top to bottom..
4. Interpretation and discussion of results In Figs. 2 and 3, we present a plot of the mean values along the w110x direction for the three crystallographic directions w001x, w112x and w112x. Each curve represents the
Fig. 1. HRTEM micrographs of w110x cross sections. Crystallographic directions w110x, w001x, and the projections over the plane Ž110. of w112x and w122x are shown. Ža. MBE sample; Žb. HWCVD sample.
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M. Romeo et al.r Thin Solid Films 319 (1998) 168–171
Fig. 2. Plot of the distance mean values along the w110x direction. MBE sample. X-axis is the Ž002. layer number. Y-axis: each division corresponds to 1% variation as compared to bulk Si.
ratio of distances taken between two adjacent dumbbell columns and the average distance calculated, for the same directions, of an unperturbed area of the sample. The standard deviations determined on pure Si from experimental micrographs for the previous crystallographic directions are less than 0.31% and 0.69% for HW-CVD and MBE samples, respectively. This means that variations of distance shown on Figs. 2 and 3 are significant. We can notice that, for the MBE sample ŽFig. 2., this variation is
the same, in shape and magnitude, for the three crystallographic directions. Moreover, two maxima can be detected; one corresponding to the 7 Ge layers, with a dilation of parameter of about 2%, and another one corresponding to the 10 Ge layers, with a dilation of 4%. This is congruous with a tetragonal deformation of the lattice along the w001x direction. For what concerns the HWCVD sample ŽFig. 3. the situation is the same if we just look at the variations along the w001x direction Ž2% for 7 Ge layers
Fig. 3. Plot of the distance mean values along the w110x direction. HWCVD sample. X-axis is the Ž002. layer number. Y-axis: each division corresponds to 1% variation as compared to bulk Si.
M. Romeo et al.r Thin Solid Films 319 (1998) 168–171
171
Fig. 4. HRTEM micrograph on a thick area of the MBE sample.
and 4% for 10 Ge layers.. However, when looking at the w112x Žresp. w112x. directions one can notice that the projected distances on the Ž110. plane suffer of a strong dilation Žresp. contraction. when entering the Ge layers and of a strong contraction Žresp. dilation. when leaving the Ge layers. These dilations Žresp. contractions. are as high as 9% of the parameter, for the 10 Ge layers, and as high as 6%, for the 7 Ge layers.
5. Concluding remarks It is known that there is a critical size of 6 monolayers of Ge in Si beyond which the strain field is relaxed giving rise to dislocations and stacking faults w2,3x. Furthermore, when preparing the sample for HRTEM observations, on thin areas of the sample dislocations and stacking faults tend to migrate to the surface according to a process of imaging force. This is the main limitation of HRTEM technique to study dislocations and stacking faults. Indeed, even though the thickness of Ge layers is greater than the critical one Ž7 and 10 monolayers. on the area suitable for our calculations, no dislocation is observed Žsee Fig. 1.. However, on thicker areas of the MBE sample ŽFig. 4., we can observe the presence of dislocations. This is not the case for the HWCVD sample. Thus, the latter is strongly strained. This is coherent with the measure of atomic displacements observed with our method. We can then infer from this analysis that atomic hydrogen is presumably kept in the lattice dwindling mobility of Ge atoms. Even after the thinning procedure necessary for TEM
observations, the HWCVD samples keep the memory of the strain field.
References w1x R. Chelly, J. Werckmann, T. Angot, P. Louis, D. Bolmont, J.J. Koulmann, Thin Solid Films, to be published Ž1997.. w2x J.H. VanderMerve, C.A.B. Ball, in: J.W. Matthews ŽEd.., Epitaxial Growth, Part B, Academic Press, New York Ž1975. 493. w3x J.W. Matthews, J. Vac. Sci. Technol. 12 Ž1975. 126. w4x H.J. Gossmann, G.J. Fisanick, Surf. Sci. 244 Ž1991. L117. w5x K. Sakamoto, K. Miki, T. Sakamoto, H. Matsuhata, K. Kyoya, J. Cryst. Growth 127 Ž1993. 392. w6x R. Bierwolf, M. Hohenstein, F. Phillipp, O. Brandt, G.E. Crook, K. Ploog, Ultramicroscopy 49 Ž1993. 273. w7x R. Chelly, T. Angot, D. Bolmont, J.J. Koulmann, Appl. Phys. Lett. 67 Ž1995. 1773. w8x J. Ayache, P.H. Albarede, ` Ultramicroscopy 60 Ž1995. 195. w9x J. Rasure, C. Williams, D. Argiro, T. Sauer, Int. J. Imaging Syst. Technol. 2 Ž1990. 183. w10x M. Young, D. Argiro, S. Kubica, Comput. Graphics 29 Ž2. Ž1995. 22. w11x J. Barrera, J.F. Banon, R.A. Lotufo, Mathematical Morphology Toolbox for the Khoros System, Conf. on Image Algebra and Morphological Image Processing V, International Symposium on Optics, Imaging and Instrumentation, SPIE’s Annual Meeting, 24–29 July, 1994, San Diego, USA. w12x H.J.A.M. Heijmans, IEEE Workshop Nonlinear Signal Image Processing 1 Ž1995. 30. w13x S. Beucher, F. Meyer, Mathematical Morphology in Image Processing Ž1993. 433.
HRTEM study of strained SirGe multilayers M. Romeo a , C. Uhlaq-Bouillet a , J.P. Deville a,) , J. Werckmann a , G. Ehret a , R. Chelly b, D. Dentel b, T. Angot b, J.L. Bischoff b a
b
IPCMS, 23 rue du Loess, 67037 Strasbourg, France LPSE, 4 rue des Freres ` Lumiere, ` 68093 Mulhouse, France
Abstract Deformation in SirGe strained multilayers has been characterised by HRTEM. The HRTEM strain profile determined by image treatment has been used to compare two growth techniques: Hot wire assisted gas source and ultra high vacuum molecular beam epitaxy. We have shown that for the first technique, germanium layers are highly strained. This is probably due to the incorporation of atomic hydrogen which would prevent relaxation by stacking faults formation. q 1998 Published by Elsevier Science S.A. Keywords: High-resolution transmission electron microscopy; Multilayers; SiGe; Image treatment; Strain; Relaxation
1. Introduction Recently Chelly et al. w1x opened the question of classifying atomic hydrogen as a surfactant in the growth of GerSi by Hot-Wire assisted Chemical Vapour Deposition ŽHWCVD.. Indeed in this growth technique, atomic H is found to be a sub-product of gaseous source decomposition on the hot wire. Commensurate epitaxial growth is accompanied by a built-in strain energy increasing with the layer thickness, due to the isostructural lattice mismatch Ž4.2%.. However, for thicknesses greater than the critical one w2,3x, strain is relieved by dislocations or a 3D growth. For the germanium growth on silicon the critical thickness is less than 6 monolayers ŽML. w4x. By using a surfactant it was shown that the surface mobility can be decreased to prevent island formation and Ge segregation during Si growth over a Ge layer. It also improves the abruptness of the SirGe interface but with a degradation of its flatness w5x. The present paper is devoted to the HREM characterisation of the local lattice distortion in SirGe strained layer structures prepared by two different methods wHWCVD and hydrogenless Molecular Beam Epitaxy ŽMBE.x to determine by comparison the effect of hydrogen. Bierwolf et al. w6x, have reported the direct measurement of local lattice displacements in semiconductors by HREM. In this
)
Corresponding author. Tel.: q33-3-88-10-70-28; fax: q33-3-88-1072-48
paper we present an alternative technique based on mathematical morphology analysis.
2. Experimental 2.1. Multilayers preparation Two sets of samples have been grown using the two different methods wMBE and HWCVDx. Samples were grown on Si Ž100. B-doped 10 V cm, alternating Ge and Si layers. Prior to the growth of the layered structure, silicon buffer layers were deposited Ž50 nm for MBE, 10 nm for HWCVD.. For HWCVD pure disilane and germane have been used. The hot wire is kept at 15308C during the growth. The total pressure never exceeded 3 = 10y4 mbar insuring heterogeneous decomposition processes at the surface substrate which is resistively heated at 3508C. More details are reported in Ref. w7x. For MBE the total pressure was 10y9 mbar with the substrate resistively heated at 4008C. 2.2. Tripod thinning The study of interfacial strain requires that a great extension of the interfaces can be observed. Cross sections meeting these requirements have been prepared with a tripod polisher w8x. High resolution microscopy was carried out on a TOPCON EM-002B electron microscope Ž Cs s 0.4 mm. operating at 200 kV. All images were taken with the
0040-6090r98r$19.00 q 1998 Published by Elsevier Science S.A. All rights reserved. PII S 0 0 4 0 - 6 0 9 0 Ž 9 7 . 0 1 1 1 5 - 2
M. Romeo et al.r Thin Solid Films 319 (1998) 168–171
w110x zone axis of the Si substrate parallel to the electron beam using a double tilt sample holder. After enlargement at 1.7 = 10 7 magnification, the positive micrographs were digitised using a scanner with a definition of 0.005 nm per pixel in order to analyse the micrographies.
3. Image treatment and analysis As the point resolution of the TOPCON microscope is 0.18 nm, the structure of Si is not resolvable. Indeed, each dark spot in our micrographs will correspond to Si–Si, Si–Ge, Ge–Si or Ge–Ge dumbbells. Therefore, we restrict ourselves to the determination of the distance between the centres of the dumbbells. Image treatment and analysis have been performed using the Khoros software development system w9,10x. Each 8-bit grey level image has been filtered and binarised using MMACH w11x in order to obtain the contour of each white dot present on the image. The main advantage of using mathematical morphology operators to filter an image, instead of the more classical way of Fourier filtering, is that type casting to complex image is not necessary. This implies that the memory and disk space taken by images are at least 8 times less than the corresponding space taken by a Fourier transformed image. For our images Ž4096 = 4096, 8-bit grey level images., this would require 128 Mbytes instead of the 16 Mbytes required by our method. Another important point is the gain of speed with morphological operators. Filtering in the direct space has been performed using a sequential filter of Closing ŽOpeningŽ.. type w12x. The structuring element was a disk of about 0.1 nm in size Ždiameter equal to 21 pixels.. This low-pass filter allowed us to get a noiseless image suitable for the determination of the contour of white dots. This determination has been carried out using the watershed transformation w13x. After binarisation, a routine developed under Khoros in our laboratory allowed us to shrink the images into a file of only 70 Kbytes. This function simply looks for the centre of mass of each < feature4 found by the watershed transformation and writes its coordinates into a file. Naturally, as the contrast of white dots may be neither isotropic nor Gaussian, the centre of mass determined using the latter procedure may not exactly correspond to real position of Si–Si or Ge–Ge dumbbells. Indeed, slight bending of the sample andror thickness variations may cause shifts of intensity maxima. However, the error that affects this determination is the same for the columns of dumbbells of the same type and does not cumulate throughout the lattice. This means that the error induced by these shifts on the lattice distortions determination is very small. A set of other routines, also developed in our laboratory, performing simple vector calculus, allowed us to determine the first neighbours for each dumbbell, their distance along one specific crystallographic direction, and the mean value of these distances along the
169
w110x direction. The latter procedure has been applied to the two images presented on Fig. 1. These images correspond to two regions of about 350 nm2 taken from the experimental HREM micrographs of w110x cross sections of MBE and HW-CVD samples, respectively. The direction of growth goes from the top of image to its bottom Žw001x.. Both images contain two Ge multilayers Ž7 and 10 monolayers, from top to bottom..
4. Interpretation and discussion of results In Figs. 2 and 3, we present a plot of the mean values along the w110x direction for the three crystallographic directions w001x, w112x and w112x. Each curve represents the
Fig. 1. HRTEM micrographs of w110x cross sections. Crystallographic directions w110x, w001x, and the projections over the plane Ž110. of w112x and w122x are shown. Ža. MBE sample; Žb. HWCVD sample.
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M. Romeo et al.r Thin Solid Films 319 (1998) 168–171
Fig. 2. Plot of the distance mean values along the w110x direction. MBE sample. X-axis is the Ž002. layer number. Y-axis: each division corresponds to 1% variation as compared to bulk Si.
ratio of distances taken between two adjacent dumbbell columns and the average distance calculated, for the same directions, of an unperturbed area of the sample. The standard deviations determined on pure Si from experimental micrographs for the previous crystallographic directions are less than 0.31% and 0.69% for HW-CVD and MBE samples, respectively. This means that variations of distance shown on Figs. 2 and 3 are significant. We can notice that, for the MBE sample ŽFig. 2., this variation is
the same, in shape and magnitude, for the three crystallographic directions. Moreover, two maxima can be detected; one corresponding to the 7 Ge layers, with a dilation of parameter of about 2%, and another one corresponding to the 10 Ge layers, with a dilation of 4%. This is congruous with a tetragonal deformation of the lattice along the w001x direction. For what concerns the HWCVD sample ŽFig. 3. the situation is the same if we just look at the variations along the w001x direction Ž2% for 7 Ge layers
Fig. 3. Plot of the distance mean values along the w110x direction. HWCVD sample. X-axis is the Ž002. layer number. Y-axis: each division corresponds to 1% variation as compared to bulk Si.
M. Romeo et al.r Thin Solid Films 319 (1998) 168–171
171
Fig. 4. HRTEM micrograph on a thick area of the MBE sample.
and 4% for 10 Ge layers.. However, when looking at the w112x Žresp. w112x. directions one can notice that the projected distances on the Ž110. plane suffer of a strong dilation Žresp. contraction. when entering the Ge layers and of a strong contraction Žresp. dilation. when leaving the Ge layers. These dilations Žresp. contractions. are as high as 9% of the parameter, for the 10 Ge layers, and as high as 6%, for the 7 Ge layers.
5. Concluding remarks It is known that there is a critical size of 6 monolayers of Ge in Si beyond which the strain field is relaxed giving rise to dislocations and stacking faults w2,3x. Furthermore, when preparing the sample for HRTEM observations, on thin areas of the sample dislocations and stacking faults tend to migrate to the surface according to a process of imaging force. This is the main limitation of HRTEM technique to study dislocations and stacking faults. Indeed, even though the thickness of Ge layers is greater than the critical one Ž7 and 10 monolayers. on the area suitable for our calculations, no dislocation is observed Žsee Fig. 1.. However, on thicker areas of the MBE sample ŽFig. 4., we can observe the presence of dislocations. This is not the case for the HWCVD sample. Thus, the latter is strongly strained. This is coherent with the measure of atomic displacements observed with our method. We can then infer from this analysis that atomic hydrogen is presumably kept in the lattice dwindling mobility of Ge atoms. Even after the thinning procedure necessary for TEM
observations, the HWCVD samples keep the memory of the strain field.
References w1x R. Chelly, J. Werckmann, T. Angot, P. Louis, D. Bolmont, J.J. Koulmann, Thin Solid Films, to be published Ž1997.. w2x J.H. VanderMerve, C.A.B. Ball, in: J.W. Matthews ŽEd.., Epitaxial Growth, Part B, Academic Press, New York Ž1975. 493. w3x J.W. Matthews, J. Vac. Sci. Technol. 12 Ž1975. 126. w4x H.J. Gossmann, G.J. Fisanick, Surf. Sci. 244 Ž1991. L117. w5x K. Sakamoto, K. Miki, T. Sakamoto, H. Matsuhata, K. Kyoya, J. Cryst. Growth 127 Ž1993. 392. w6x R. Bierwolf, M. Hohenstein, F. Phillipp, O. Brandt, G.E. Crook, K. Ploog, Ultramicroscopy 49 Ž1993. 273. w7x R. Chelly, T. Angot, D. Bolmont, J.J. Koulmann, Appl. Phys. Lett. 67 Ž1995. 1773. w8x J. Ayache, P.H. Albarede, ` Ultramicroscopy 60 Ž1995. 195. w9x J. Rasure, C. Williams, D. Argiro, T. Sauer, Int. J. Imaging Syst. Technol. 2 Ž1990. 183. w10x M. Young, D. Argiro, S. Kubica, Comput. Graphics 29 Ž2. Ž1995. 22. w11x J. Barrera, J.F. Banon, R.A. Lotufo, Mathematical Morphology Toolbox for the Khoros System, Conf. on Image Algebra and Morphological Image Processing V, International Symposium on Optics, Imaging and Instrumentation, SPIE’s Annual Meeting, 24–29 July, 1994, San Diego, USA. w12x H.J.A.M. Heijmans, IEEE Workshop Nonlinear Signal Image Processing 1 Ž1995. 30. w13x S. Beucher, F. Meyer, Mathematical Morphology in Image Processing Ž1993. 433.