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Ge superlattices
Superlattices and Microstructures, Vol. 9, No. 1, 1991
31
SYMMETRY PROPERTIES OF SHORT PERIOD (001) SI/GE SUPERLATTICES K. Eberl, W. Wegscheider, and G. Abstreiter Walter Schottky Institut, Technische Universi~t Miinchen, D-8046 Garching, FRG H. Cerva and H. Oppolzer Siemens AG, Research Laboratories, Otto Hahn Ring 6 D-8000 Miinchen 83, FRG (Received 13 August 1990 ) Depending on the composition of [001l oriented short period SimGe n superlattices (m monolayers Si/n monolayers Ge) there are six different space groups. In case of m+n odd the microscopic periodicity is doubled to 2(re+n) monolayers due to the tetrahedral bonding. We prepared several superlattices .namely Si22Ge3, Si3Ge 3 and Si4Ge6 with low temperature molecular beam epltaxy. Using selected area diffraciion in the transmission electron microscope we found the microscopic periodicity of 10 monolayers for Si2Ge 3 and Si4Ge6. For Si3Ge 3 the vertical periodicity is six monolayers.
The combination of the group W-elements Si and Ge opens the possibility for bandstructure tailoring with prospects in fast electronics and optoelectronics [1,2]. The scientific interest on extremely short period Si/Ge superlattices is mainly focused on the idea to realize a quasi-direct band-gap semiconductor out of the indirect host materials [3]. The optical, electronical and lattice dynamical properties of S i _ G e , superlattices (sequence of m monolayers Si and n'~on'~layers Ge) are strongly related to the symmetry properties. There are six different space groups for SimGe_ depending on the thickness of the mdiwdual-Iayers [4]. Strain and superperiodicity dSL reduce the symmetry of the host lattice (diamond §ffucture). For m and/or n odd the crystal structure is tetragonal, whereas it is orthorhombic for m,n even. A special situation occurs for the tetragonal S i _ G e , superlattices with m+n odd. Fig. 1 shows the lattice t~odel for Si2Ge 3. There is no lattice position within the fifth atotmc p-lane (see the arrow in Fig. 1) which is equivalent to the origin in that case (re+n=5). The zig-zag bonds to the next atomic plane are aligned in [110] direction, which is 90" rotated to a 1. That means the next appropriate lattice position is ten monolayers (ML) away in growth direction. As a consequence the microscopic translation symmetry for Si2Ge 3 is dSL=10 ML. Orthorhombic s-macfures ~ d periodicity doubling are peculiar to group IV-element superlattices. In the III/Vsuperlattices there are only two different space groups possible which have a tetragonal structure. This is due to the zincblende structure which is the host lattice in that case. •
.
•
II
,
For most of the device structures involving Si/Ge heterostructurcs the interface quality is very important. In order to observe the particular symmetry aspects atomically sharp interfaces are essential. We have used 0749-6036/91/010031 +03 S02.00/0
a special molecular beam epitaxy (MBE) system with in-situ analysis to optimize the growth conditions. To overcome the problems in growth mainly induced by the lattice mismatch of about 4% it was necessary to modify the growth conditions in MBE [5-8]. Detailed investigations of the surface morphology and the interface abruptness by means of low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) have been performed. Short period Si/Ge superlattices with good uniformity and sharpness of the interfaces are achieved at very low growth temperatures and by temperature modulation during deposition of the individual Si and Ge layers in the range of 280"C to 350"C. For further information on the growth technique see Ref. [9]. Fig. 2 shows a cross-sectional bright field image of a Si3Ge3.superlattice. on a. Si (001) substrate. The superlatUce contains 50 penods. Between subs~ate and superlattice there is a partly relaxed 130 ,~, thick Ge buffer• This kind of buffer layer can be used to adjust the lateral lattice constant within the superlattice grown on top of it [10]. In the sample depicted in Fig. 2 the change of the lateral lattice constant is: Aa/ag i = (2.5~0.1)%, which means that the Si layers are late-r/flly extended by 2.5% whereas the Ge layers are compressed by about 1.5%. Fig. 2 demonstrates that exU'emely short period superlattices with different strain distributions can be realized on Si substrates. Remember that the thickness of the individual layers is only about 4.2,~ for Si3Ge 3. However, the also image shows that there is still-a co-nsiderable amount of misfit defects across the whole structure. The best Si/Ge superlattices with very sharp interfaces and without misfit dislocations have so far been achieved for samples which are lattice matched to [001] oriented Ge substrates [11]. © 1991 Academic Press Limited
32
Superlattices and Microstructures, Vol. 9, No. 1, 1991
[ool]l
S'i'zGes
5ML d,y: Fig. 2 Cross-sectional TEM micro-graph [g=(004)l of a 50 period Si3Ge 3 superlattice on Si substrate. Between substrate aiad s-uperlattice there is a partly relaxed 130 /~ Ge layer. The lateral lattice constant within the superlattice is (2.5 0.1)% larger than that of silicon.
Fig. 1 Lattice model of the Si2Ge 3 superlattice. The open circles represent the Si an~ the closed ones the Ge atomic positions, a 1, a2 and a 3 are the primitive unit vectors. The arrows on tlie left mark the positions of the zeroth, fifth and the tenth atomic plane.
Fig. 3 shows the selected area diffraction (SAD) pattern of a Si3Ge 3 superlattice in [110] projection. Two additional diffraction spots appear between the indexed main spots parallel to the growth direction (vertical axis). The diffraction pattern represents a section through a (110) plane of the reciprocal lattice. It is a superposition of diffraction patterns originating from the supeflattice and the substrate. The vertical and the lateral extension of the supeflattice unit cell are inversely related to the separation of the diffraction spots as pointed out in Fig. 3. This has been used to extract detailed information about the lateral strain and the tetragonal deformation in Si/Ge superlattices [9,11]. The vertical separation of the superlattice-spots for Si3Ge3 is exactly 1/3 of the separation of the main spbts,-which is 4 ~ a n. That means 2~/dq L = (1/3).(4rdaa) and consequ-ently dSL = (3/2).a n. Sific-e a o is equal to ~1ML in [001] directio-n the periodicity of the Si3Ge 3 superlattice is 6 ML. Fi~. 4-shows the SAD-patterns for Si4Ge 6 and Si2Ge 3. Both structures have a 10 ML periocqicity in th-e
[1{01 [1101 Fig. 3 SAD-pattern in [110] pro-jection for Si3Ge 3 (140 periods). The arrows mark the position of th6 diffraction spots for 6 ML periodicity.
microscopic lattice model. But they differ in their atomic arrangement within the unit cell and in their space group [4,12]. Si4Ge6 is orthorombic and belongs
Superlattices and Microstructures, Vol. 9, No. 1, 1991
a: Si~Ge s
b: Si2Ge 3
33 The superlattice ordering and the symmetry aspects in particular are also reflected in the lattice dynamic properties. Detailed Raman measurements on the samples discussed here are published elsewhere [ 121. In conclusion we have demonstrated that extremly short period Si/Ge superlattices can be achieved with nearly atomically sharp interfaces by low temperature MBE. SAD measurements in the transmission electron microscope show that the symmetry of a Si2Ge 3 superlattice is not given by the composmonad periodicity of 5 ML but is determined by the microscopic structure giving rise to a 10 ML periodicity in growth direction. From the TEM and from Raman measurements [12] it is shown that Si/Ge superlattices can be realized with a minimum periodicity of only a few monolayers
Acknowledgments This work was finaciaily supported by the Deutsche Forschungsgemeinschaft. In addition, three of us namely K.E., W.W. and G.A. want to thank the Siemens AG for financial support. Fig. 4 SAD-pattern in [110] pro-jection for a) Si4Ge6 (140 periods) and..for b) Si2.Ge,_c3~(100.periods). The arrows mark the posmon of the dlffracUon spots for a superlattice providing 10 ML periodicity.
References: [1]
to the space group D2h 28 0mma). Whereas Si2Ge 3 has a tetragona~,crystai structure and belongs to ~he gpace group D4h l:" (141/amd). In Fig 4b - SAD-pattern of Si2Ge 3 - addititnai spots reflecting the artificial or0erifig are clearly observed. The separation of the superlattice-spots is exacdy the same as for Si4Ge 6 shown in Fig. 4a. It is 1/5 of the separation of the mmn spots as indicated by the arrows. This demonstrates the microscopic 10 ML periodicity in growth direction for Si2Ge a. The intensity distribution of the superlatticesp~ts-for Si .Ge3 and for Si4Ge6 is different, and reflects the ~fferent arrangement of the $1 and Ge atoms within the corresponding unit cell. For Si2Ge~ every second diffraction spot parallel to the growm direction would be forbidden due to a zero kinematic structure factor. A fully dynamic scattering theory has to be applied to discribe the intensity distribution of these diffraction pattern. The fact that the 10 ML periodicity is observed in SAD for Si2Ge3 demonstrates that very sharp interfaces and good-lateral uniformity has been achieved for these samples. However, it does not mean that the Si/Ge interfaces have no monoatomic steps within the whole area of the probing electron beam. The SAD-pattern shows that the atomic interface steps do not change the peridicity. Thickness fluctuations which cause locally four ML or six ML periodicity would lead to a different diffraction pattern. For Si2Ge2, for example, only one additional diffraction-spof- is -expected in the middle between the main spots.
[2] [3] [4]
[5] [6] [7] [8] [9] [10] [11] [12]
E. Kasper in Heterostructures on Sificon: one Step Further with Silicon, eds. Y.I. Nissim and E. Rosencher, Kluwer Academic Publishers, Dordrecht, (1989) Nato ASI Series E, Vol. 160 p 101. S. Luryi and S.M. Sze in Silicon Molecular Beam Epitaxy, eds. E. Kasper and J.C. Bean, CRC Press, Boca Raton, p 182 (1988). U. Gnutzmann and K. Klausecker, Appl. Phys. 3, 9 (1974). M.I. Alonso, M. Cardona and G. Kanellis, Solid State Commun. 69, 479 (1989) and Corrigendum in Solid State Commun. Vol.70, Nr. 7, i (1989). J.C. Bean, L.C. Feldman, A.T. Fiory, S. Nakahara and I.K. Robinson, J. Vac. Sci. Technol. A. 2, 436, (1984). E. Kasper, Surf. Sci. 174, 630 (1986). J. Bevk, A. Ourmazd, L.C. Feldman, T.P PearsaIl, J.M. Bonar, B.A. Davidson and J.P. Mannaerts, Appl. Phys. Lett. 50, 760 (1987). K. Ebefl, G. Kr6tz, R. Zachai and G. Abstreiter, J. de Physique C5, 329 (1987). K. Eberl, E. Friess, W. Wegscheider, U. Menczigar and G. Abstreiter, Thin Solid Films, 183, 95 (1989). E. Kasper, H.J. Herzog, H. Jorke, G. Abstreiter, in Mat. Res. Soc. Symp. Proc. Vol 102, 393 (1988). W. Wegscheider, K. Eberl, H. Cerva, H. Oppolzer, Appl. Phys. Lett. 55,448 (1989). K. Eberl, W. Wegscheider, R. Schorer and G. Abstreiter, submitted for publication.
31
SYMMETRY PROPERTIES OF SHORT PERIOD (001) SI/GE SUPERLATTICES K. Eberl, W. Wegscheider, and G. Abstreiter Walter Schottky Institut, Technische Universi~t Miinchen, D-8046 Garching, FRG H. Cerva and H. Oppolzer Siemens AG, Research Laboratories, Otto Hahn Ring 6 D-8000 Miinchen 83, FRG (Received 13 August 1990 ) Depending on the composition of [001l oriented short period SimGe n superlattices (m monolayers Si/n monolayers Ge) there are six different space groups. In case of m+n odd the microscopic periodicity is doubled to 2(re+n) monolayers due to the tetrahedral bonding. We prepared several superlattices .namely Si22Ge3, Si3Ge 3 and Si4Ge6 with low temperature molecular beam epltaxy. Using selected area diffraciion in the transmission electron microscope we found the microscopic periodicity of 10 monolayers for Si2Ge 3 and Si4Ge6. For Si3Ge 3 the vertical periodicity is six monolayers.
The combination of the group W-elements Si and Ge opens the possibility for bandstructure tailoring with prospects in fast electronics and optoelectronics [1,2]. The scientific interest on extremely short period Si/Ge superlattices is mainly focused on the idea to realize a quasi-direct band-gap semiconductor out of the indirect host materials [3]. The optical, electronical and lattice dynamical properties of S i _ G e , superlattices (sequence of m monolayers Si and n'~on'~layers Ge) are strongly related to the symmetry properties. There are six different space groups for SimGe_ depending on the thickness of the mdiwdual-Iayers [4]. Strain and superperiodicity dSL reduce the symmetry of the host lattice (diamond §ffucture). For m and/or n odd the crystal structure is tetragonal, whereas it is orthorhombic for m,n even. A special situation occurs for the tetragonal S i _ G e , superlattices with m+n odd. Fig. 1 shows the lattice t~odel for Si2Ge 3. There is no lattice position within the fifth atotmc p-lane (see the arrow in Fig. 1) which is equivalent to the origin in that case (re+n=5). The zig-zag bonds to the next atomic plane are aligned in [110] direction, which is 90" rotated to a 1. That means the next appropriate lattice position is ten monolayers (ML) away in growth direction. As a consequence the microscopic translation symmetry for Si2Ge 3 is dSL=10 ML. Orthorhombic s-macfures ~ d periodicity doubling are peculiar to group IV-element superlattices. In the III/Vsuperlattices there are only two different space groups possible which have a tetragonal structure. This is due to the zincblende structure which is the host lattice in that case. •
.
•
II
,
For most of the device structures involving Si/Ge heterostructurcs the interface quality is very important. In order to observe the particular symmetry aspects atomically sharp interfaces are essential. We have used 0749-6036/91/010031 +03 S02.00/0
a special molecular beam epitaxy (MBE) system with in-situ analysis to optimize the growth conditions. To overcome the problems in growth mainly induced by the lattice mismatch of about 4% it was necessary to modify the growth conditions in MBE [5-8]. Detailed investigations of the surface morphology and the interface abruptness by means of low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) have been performed. Short period Si/Ge superlattices with good uniformity and sharpness of the interfaces are achieved at very low growth temperatures and by temperature modulation during deposition of the individual Si and Ge layers in the range of 280"C to 350"C. For further information on the growth technique see Ref. [9]. Fig. 2 shows a cross-sectional bright field image of a Si3Ge3.superlattice. on a. Si (001) substrate. The superlatUce contains 50 penods. Between subs~ate and superlattice there is a partly relaxed 130 ,~, thick Ge buffer• This kind of buffer layer can be used to adjust the lateral lattice constant within the superlattice grown on top of it [10]. In the sample depicted in Fig. 2 the change of the lateral lattice constant is: Aa/ag i = (2.5~0.1)%, which means that the Si layers are late-r/flly extended by 2.5% whereas the Ge layers are compressed by about 1.5%. Fig. 2 demonstrates that exU'emely short period superlattices with different strain distributions can be realized on Si substrates. Remember that the thickness of the individual layers is only about 4.2,~ for Si3Ge 3. However, the also image shows that there is still-a co-nsiderable amount of misfit defects across the whole structure. The best Si/Ge superlattices with very sharp interfaces and without misfit dislocations have so far been achieved for samples which are lattice matched to [001] oriented Ge substrates [11]. © 1991 Academic Press Limited
32
Superlattices and Microstructures, Vol. 9, No. 1, 1991
[ool]l
S'i'zGes
5ML d,y: Fig. 2 Cross-sectional TEM micro-graph [g=(004)l of a 50 period Si3Ge 3 superlattice on Si substrate. Between substrate aiad s-uperlattice there is a partly relaxed 130 /~ Ge layer. The lateral lattice constant within the superlattice is (2.5 0.1)% larger than that of silicon.
Fig. 1 Lattice model of the Si2Ge 3 superlattice. The open circles represent the Si an~ the closed ones the Ge atomic positions, a 1, a2 and a 3 are the primitive unit vectors. The arrows on tlie left mark the positions of the zeroth, fifth and the tenth atomic plane.
Fig. 3 shows the selected area diffraction (SAD) pattern of a Si3Ge 3 superlattice in [110] projection. Two additional diffraction spots appear between the indexed main spots parallel to the growth direction (vertical axis). The diffraction pattern represents a section through a (110) plane of the reciprocal lattice. It is a superposition of diffraction patterns originating from the supeflattice and the substrate. The vertical and the lateral extension of the supeflattice unit cell are inversely related to the separation of the diffraction spots as pointed out in Fig. 3. This has been used to extract detailed information about the lateral strain and the tetragonal deformation in Si/Ge superlattices [9,11]. The vertical separation of the superlattice-spots for Si3Ge3 is exactly 1/3 of the separation of the main spbts,-which is 4 ~ a n. That means 2~/dq L = (1/3).(4rdaa) and consequ-ently dSL = (3/2).a n. Sific-e a o is equal to ~1ML in [001] directio-n the periodicity of the Si3Ge 3 superlattice is 6 ML. Fi~. 4-shows the SAD-patterns for Si4Ge 6 and Si2Ge 3. Both structures have a 10 ML periocqicity in th-e
[1{01 [1101 Fig. 3 SAD-pattern in [110] pro-jection for Si3Ge 3 (140 periods). The arrows mark the position of th6 diffraction spots for 6 ML periodicity.
microscopic lattice model. But they differ in their atomic arrangement within the unit cell and in their space group [4,12]. Si4Ge6 is orthorombic and belongs
Superlattices and Microstructures, Vol. 9, No. 1, 1991
a: Si~Ge s
b: Si2Ge 3
33 The superlattice ordering and the symmetry aspects in particular are also reflected in the lattice dynamic properties. Detailed Raman measurements on the samples discussed here are published elsewhere [ 121. In conclusion we have demonstrated that extremly short period Si/Ge superlattices can be achieved with nearly atomically sharp interfaces by low temperature MBE. SAD measurements in the transmission electron microscope show that the symmetry of a Si2Ge 3 superlattice is not given by the composmonad periodicity of 5 ML but is determined by the microscopic structure giving rise to a 10 ML periodicity in growth direction. From the TEM and from Raman measurements [12] it is shown that Si/Ge superlattices can be realized with a minimum periodicity of only a few monolayers
Acknowledgments This work was finaciaily supported by the Deutsche Forschungsgemeinschaft. In addition, three of us namely K.E., W.W. and G.A. want to thank the Siemens AG for financial support. Fig. 4 SAD-pattern in [110] pro-jection for a) Si4Ge6 (140 periods) and..for b) Si2.Ge,_c3~(100.periods). The arrows mark the posmon of the dlffracUon spots for a superlattice providing 10 ML periodicity.
References: [1]
to the space group D2h 28 0mma). Whereas Si2Ge 3 has a tetragona~,crystai structure and belongs to ~he gpace group D4h l:" (141/amd). In Fig 4b - SAD-pattern of Si2Ge 3 - addititnai spots reflecting the artificial or0erifig are clearly observed. The separation of the superlattice-spots is exacdy the same as for Si4Ge 6 shown in Fig. 4a. It is 1/5 of the separation of the mmn spots as indicated by the arrows. This demonstrates the microscopic 10 ML periodicity in growth direction for Si2Ge a. The intensity distribution of the superlatticesp~ts-for Si .Ge3 and for Si4Ge6 is different, and reflects the ~fferent arrangement of the $1 and Ge atoms within the corresponding unit cell. For Si2Ge~ every second diffraction spot parallel to the growm direction would be forbidden due to a zero kinematic structure factor. A fully dynamic scattering theory has to be applied to discribe the intensity distribution of these diffraction pattern. The fact that the 10 ML periodicity is observed in SAD for Si2Ge3 demonstrates that very sharp interfaces and good-lateral uniformity has been achieved for these samples. However, it does not mean that the Si/Ge interfaces have no monoatomic steps within the whole area of the probing electron beam. The SAD-pattern shows that the atomic interface steps do not change the peridicity. Thickness fluctuations which cause locally four ML or six ML periodicity would lead to a different diffraction pattern. For Si2Ge2, for example, only one additional diffraction-spof- is -expected in the middle between the main spots.
[2] [3] [4]
[5] [6] [7] [8] [9] [10] [11] [12]
E. Kasper in Heterostructures on Sificon: one Step Further with Silicon, eds. Y.I. Nissim and E. Rosencher, Kluwer Academic Publishers, Dordrecht, (1989) Nato ASI Series E, Vol. 160 p 101. S. Luryi and S.M. Sze in Silicon Molecular Beam Epitaxy, eds. E. Kasper and J.C. Bean, CRC Press, Boca Raton, p 182 (1988). U. Gnutzmann and K. Klausecker, Appl. Phys. 3, 9 (1974). M.I. Alonso, M. Cardona and G. Kanellis, Solid State Commun. 69, 479 (1989) and Corrigendum in Solid State Commun. Vol.70, Nr. 7, i (1989). J.C. Bean, L.C. Feldman, A.T. Fiory, S. Nakahara and I.K. Robinson, J. Vac. Sci. Technol. A. 2, 436, (1984). E. Kasper, Surf. Sci. 174, 630 (1986). J. Bevk, A. Ourmazd, L.C. Feldman, T.P PearsaIl, J.M. Bonar, B.A. Davidson and J.P. Mannaerts, Appl. Phys. Lett. 50, 760 (1987). K. Ebefl, G. Kr6tz, R. Zachai and G. Abstreiter, J. de Physique C5, 329 (1987). K. Eberl, E. Friess, W. Wegscheider, U. Menczigar and G. Abstreiter, Thin Solid Films, 183, 95 (1989). E. Kasper, H.J. Herzog, H. Jorke, G. Abstreiter, in Mat. Res. Soc. Symp. Proc. Vol 102, 393 (1988). W. Wegscheider, K. Eberl, H. Cerva, H. Oppolzer, Appl. Phys. Lett. 55,448 (1989). K. Eberl, W. Wegscheider, R. Schorer and G. Abstreiter, submitted for publication.