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Low kinetic energy AED: a tool for the study of Ge epitaxial layers grown on Sb-terminated Si(111) surface
Journal of Electron Spectroscopy and Related Phenomena 83 (1997) 137–142
Low kinetic energy AED: a tool for the study of Ge epitaxial layers grown on Sb-terminated Si(111) surface I. Davoli*, R. Gunnella, R. Bernardini, M. De Crescenzi Dipartimento di Matematica e Fisica and INFM, Universita’ di Camerino, 62032 Camerino, Italy Received 8 November 1995; revised 16 September 1996; accepted 18 September 1996
Abstract The intensity anisotropy of the Si L 2,3VV and of the Ge M 2,3M 4,5M 4,5 Auger lines are used to study the growth of Ge on Si(111) assisted by Sb-surfactant. The very surface sensitivity of the low kinetic energy AED is used to investigate the early stage of the Ge/Si (111) interface formation. We compare the AED data obtained after the 10 ML of Ge deposition on bare Si and on the antimony-terminated Si substrate. In the first case, the structureless of the AED spectrum accounts for the island formation nucleated in random manner, while the marked anisotropy of the Auger line shown in the second case evidences the epitaxial growth of the Ge film. A simplified version of the multiple scattering approximation, developed to describe the low kinetic energy AED of the Si L 2,3VV transition, has been used for the theoretical analysis of the experimental data. q 1997 Elsevier Science B.V. Keywords: Intensity anisotropy; Auger line; Low kinetic energy AED
1. Introduction The mismatch of the lattice parameters is the greatest problem in the heteroepitaxial growth technology. Indeed just a few percent of lattice mismatch is enough to create defects that break the epitaxial growing. This greatly reduces the thickness of the continuous layer of the deposited material. Such a severe constraint can be avoided by surfactant assisted growing techniques [1]. To exploit the idea of the folding of the Brillouin zone, which determines the tailoring of the electronic properties in the artificial semiconductors, it is important to have no constraints on the possible layer thickness. A few years ago it was observed that a (Ge m/Si n) superlattice has a direct optical transition at 0.85 eV, just where the fibre * Corresponding author.
optics have their best optical transmission [1]. Such an observation has attracted a lot of interest because devices based on Si technology are the cheapest and distributed world wide. The current interpretation implies the folding of the Si and Ge Brillouin zone which modifies the natural indirect transition of such semiconductors in an artificial material with a direct optical transition. It is understood that, to obtain a multifold effect, very sharp interfaces are required. Unfortunately the formation of a sharp Ge/Si interface is inhibited by the considerable number of dislocations present on the Si substrate [2]. Le Goues et al. [1] have shown that the microstructure of a Ge film deposited on a Si(111) substrate can be drastically altered by changing the growth mode. The deposition of one monolayer of antimony on the Si(111) surface prevents the island formation and allow the subsequent growing of a large number of Ge strained layers.
0368-2048/97/$17.00 q 1997 Elsevier Science B.V. All rights reserved PII S 0 36 8- 2 04 8 (9 6 )0 3 08 1 -2
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I. Davoli et al./Journal of Electron Spectroscopy and Related Phenomena 83 (1997) 137–142
It is difficult to have a detailed knowledge of the Ge/Si interface and of the Sb/Ge/Si system without the contribution of different electron spectroscopies [3–5]. Among them, Auger electron diffraction (AED) in the low kinetic energy range is particularly interesting, because it allows the study of the early stage of the Ge/Si interfaces. Indeed the very surface sensitivity of electrons leaving the sample with the low kinetic energy and the peculiar atomic selectivity of the Auger line makes this technique unique in probing the interface nature of a heteroepitaxial growth. In this paper we present data collected from the Ge/ Si(111) bare interface and from the Ge overlayer grown on an antimony-terminated Si(111) surface. The experimental curve obtained for a reconstructed Si(111)7 × 7 surface is compared with a theoretical model based on single scattering calculation as a function of the various final state angular momenta of the outgoing electrons. Calculations were also made for the 1 × 1 terminated surface and for the reconstructed Si(111)7 × 7 surface according to DAS (dimer adatom stacking-fault) model [6].
2. Experiment The measurements were carried out in a UHV LEED (low energy electron diffraction) system equipped with a single-pass CMA (cylindrical mirror analyser) coaxially mounted on a 3 keV electron gun. The electron analyser was modified to an angular detector by screening (354/360)8 of the circular ring detection area. In this way we have built an angular detector having a small detection area with the axis of the acceptance cone positioned at about 428 in the plane of the polar angle scan. The width of the detection cone was of about 0.06 sr [7]. The sample was an optically polished Si(111) ptype crystal (0.1 Q cm) clamped on two rods for direct Joule heating. The atomic cleaning was obtained by a series of successive flashes at 11008C until nor carbon or oxygen were detected within the Auger atomic sensitivity. Sharp LEED pattern of the 7 × 7 reconstruction superimposed to the 1 × 1 bulk terminated geometry was observed. An azimuth rotation of the sample was needed to properly orient the surface with [110] direction perpendicular to the polar scan.
Fig. 1. Auger signal of the cleaned Si(111) 7 × 7 surface (upper panel). After the deposition of 1 ML of Sb substrate temperature was kept at 6008C (middle panel) and after the deposition of 10 ML of Ge with the substrate temperature at 3008C (lower panel).
The surfactant layer was deposited by heating a quartz crucible filled with high purity lumps of antimony, while the Si substrate was kept a little above 6008C. This is a thermodynamic condition to obtain only one monolayer of Sb stacked on the surface [8,9]. The crystalline nature of the adsorbed monolayer was verified by the observation of a sharp 3 × 3 LEED pattern (not reported here). The subsequent Ge layers were deposited by heating a tungsten basket wet with fused Ge and the thickness was measured by a quartz microbalance. In Fig. 1 are shown the Auger signals of the clean Si surface (upper panel), of the antimony-terminated Si ˚ of Ge evapo(middle panel) and of the nominal 10 A rated on antimony-terminated Si surface (lower panel). We note, on the lower panel, that the Si Auger peak is strongly reduced compared to the antimony peak. Actually the antimony peak in the medium panel and in the bottom panel have the same intensity in the absolute scale. This is an evidence for the floating mechanism of the Sb on top of the evaporated Ge. Auger spectrum is the result of a three phase slab formed by Sb(1 ML)/Ge(10 ML)/ Si(bulk), where the intensity of the Auger peak of
p p
I. Davoli et al./Journal of Electron Spectroscopy and Related Phenomena 83 (1997) 137–142
Sb is the same as that measured in the middle panel while the intensity of the Si peak is attenuated by the presence of the Sb and of the 10 layers of Ge. Considering the Auger sensitivity factors for the three elements (Sb = 0.6, Ge = 0.1, and Si = 0.35) [10] we found that the attenuated intensity of the Si peak, with respect to the Sb, is due to the presence of the 10 ML of germanium deposition. The AED in a polar angle mode measurements were performed monitoring the Ge M 2,3M 4,5M 4,5 (51 eV), the Si L 2,3M 4,5M 4,5 (92 eV) and the Sb M 4,5N 4,5N 4,5 (454 eV) transitions. In the AES (Auger electron spectroscopy) measurements we have used a primary electron beam of 2 keV and a modulation of 0.5 V p–p applied to the outer cylinder of the CMA to detect the electron yield distribution dN(E)/dE, in the first derivative mode. Thus anisotropy reported here is the collection of the peak-to-peak first derivative Auger line intensity vs. the polar angle with a step Dv of 28. Such an experimental method was demonstrated to have the same angular behaviour as that obtained by measuring the area under the Auger line in the N(E) spectrum [11].
3. Theoretical background The theoretical calculation of AED in the low energy regime starts from the Fermi golden rule: dN = dkˆ 2p
ˆ i, f to, j ∑ ∑ ∑ i − 1 Yl9, m (k)A l, m l, m, l9, m9 i, f l, m j, l, m
2 exp( − ikRj, o )
1 where the first sum in Eq. (1) acts over all the initial and final states involved in the Auger transition, while the second and the third sum cover the angular momenta of the emitted electron (l,m) and of the scattered electron (l9,m9) and all the atoms of the cluster ( j) [12]. A i,f l,m represents the Auger matrix element where (l,m) is the angular momentum of the emitted electron, j and o the scattered and the absorbing atomic site, respectively. R j,o is the difference vector R j − R o of the atomic coordinate and Y lm are the real spherical harmonics. Finally t o,j l,m,l9,m9 is
139
defined as j to, l, m, l9, m9 j j = tlo − tlo Go, l, m, l9, m9 tl9
+
2
j j … ∑ tlo Gl,o,m,k l0, m0 tl0k Gl0,k, m0, l9, m9 tl0 −
k, l0, m0
and t il is the complex atomic scattering matrix for the ith atom with angular momentum l. G ijlml9m9 is a suitable propagator from site i to site j calculated within a spherical approximation as reported in Ref. [13]. As reported in the literature the importance of multiple scattering is particularly relevant when the emission is along the low index crystallographic direction of the solid [14]. Our recent comparison of AED data with a theoretical calculation showed that the forward scattering is still present even at low kinetic energies [10]. In the calculation of Eq. (2), the series may be truncated to the second term because when no more than two atoms are involved along one of the main emission direction the second scattering perturbation is very small [14]. Computation of the full multiple scattering including the third term has been used to test the second scattering contribution. On the basis of these considerations we conclude that a single scattering approximation is a sufficiently accurate approach for the purpose of the parent AED data.
4. Results and discussion The interpretation of AED in the low kinetic energy range, is still characterised by an intense debate concerning the diffraction effects of the primary electron beam. In Ref. [15] we have shown that the Auger LVV electron diffraction data collected using either an Xray excitation source or an electron beam source give the same diffraction pattern. This is evidence of the negligible role played by the incident beam diffraction. A more detailed discussion is given in Ref. [16] where it was demonstrated that the diffraction effects are mainly due to the outgoing electron as far as AED data are collected by an angular detector. In Fig. 2 we report the Si (111) L 2,3VV Auger polar anisotropy compared with theoretical calculations. The theoretical curves are the calculations for two different surfaces termination models. Both curves
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I. Davoli et al./Journal of Electron Spectroscopy and Related Phenomena 83 (1997) 137–142
Fig. 2. L 2,3VV Auger anisotropy for the Si (111)7 × 7 surface vs. polar angle (crossed curve) compared with theoretical calculations performed for a bulk terminated surface (dashed–dotted line) and for a 7 × 7 reconstructed surface (dotted line) according to the simplified DAS model.
are due to a six-layer slab, but while curve (b) is for a bulk terminated Si(111) surface, curve (c) includes the reconstructed two top layers (the adatom and the first full layer) according to a simplified version of the Si(111)7 × 7 DAS model [6]. In both cases the number ˚ of layers is consistent with the escape depth of 5 A probed by electrons of about 100 eV kinetic energy [10]. We note that there are only slight differences between the two theoretical curves in Fig. 2 and this implies that the surface reconstruction does not introduce meaningful additional features different from those of the bulk terminated surface. Indeed one full layer plus the adatom layer affects very little the remaining five layers used in the calculation. This appears clear from the calculation reported in the next figure where the contributions of the various layers are included. Fig. 3 reports the calculated curves, not corrected by any instrumental function, which are due to the sum of each single layer i plus all the above layers. In the lowest we have the curve A due to the adatoms. It contributes in a negligible way to the features shown in the curve 6 which represent the entire cluster. We note that the peak of the 08 polar angle shows up with the inclusion of the third layer. This indicates that we need three layers to pile up two atoms along
Fig. 3. Theoretical calculations of AED spectra involving different silicon layers. Bottom curve A is due to the adatoms contribution. In the other curves, the numbers indicate the sum of the ith layer plus all the previous ones.
the direction normal to the surface. The same effect appears for the peak around 408 polar angle, which shows up after the formation of the fifth layer. Fig. 4 shows the Si AED spectra of three different
Fig. 4. Si L 2,3VV (92 eV) Auger yield anisotropy vs. polar angle scan. Curve (a) is due to the clean Si(111) along the direction [110] compared: with the same Si line after 1 ML of Sb deposition (curve b); with the same Si line after 10 ML of Sb deposition (curve c) and with the Sb MNN Auger line (454 eV) after the deposition of 1 ML of antimony on the Si(111) bare surface (curve d).
I. Davoli et al./Journal of Electron Spectroscopy and Related Phenomena 83 (1997) 137–142
141
˚ deposition of Ge on the antiAED obtained for 10 A mony terminated Si surface and on the bear Si substrate kept at 3008C. Strong anisotropy for the former indicates a crystalline growth mediated by the Sb film which hampers the formation of the Ge clustering [2]. We may note that the anisotropy features have almost the same angular position of that observed for the Si substrate as shown in Fig. 4 (curve (c)). In contrast, the structureless full curve indicates that the contribution from the Ge atoms is averaged without any well defined anisotropy. In this case the Ge deposition probably forms misoriented droplets.
5. Conclusion
Fig. 5. Ge MMM (51 eV) AED data monitored from the Ge/Si interface obtained after the evaporation of 10 ML of Ge onto an antimony terminated Si surface (crossed curve) compared with the anisotropy of the same Auger line detected from the Ge/Si interface after the Ge deposition on clean Si surface kept at 3008C (full curve).
samples and Sb AED spectrum of the Sb/Si(111) sample. We note that, although the shape appears different, the main structures of curve (b) have the same angular position of the main structures of the curve (a) for the clean Si. In contrast, curve (d) does not show any appreciable diffraction peak. This implies no interdiffusion of the antimony into the Si substrate. Indeed a partial crystalline interdiffusion should produce some diffraction peak. Furthermore a possible island formation of the antimony film should give a peak at 08 polar angle (normal emission). Our results are in agreement with the photoelectron diffraction study reported by Abukawa et al. [8] and with the STM measurements of Ma˚rtenson et al. [17] which present patterns of antimony trimers on the Si (111) surface. Curve c for an antimony terminated surface still shows the anisotropy of the Si L 2,3VV Auger line. This observation indicates that for such high Ge deposition a Stranski–Krastanov growing mode occurs [3,5]. Indeed the formation of Ge islands does not prevent the detection of the AED features ˚ continuous coming from the Si substrate, while a 10 A film of Ge would severely attenuate the signal. Finally in Fig. 5 are shown the Ge M 2,3M 4,5M 4,5
We have shown that AED technique is a versatile tool to investigate the growth mode of Ge on Si(111). It has been confirmed that an antimony layer ˚ Ge film, modifies the deposited, before the 10 A usual Stranski–Krastanov growth mode giving rise to an epitaxial layer. Our technique is atomic sensitive and is able to discriminate whether or not the antimony atoms interdiffuse into the Si substrate. Finally we have shown that the use of low kinetic energy electrons gives information of the same quality of that obtained with the X-ray photoelectron diffraction.
References [1] F.K. Le Goues, M. Horn-Von Hoegen, M. Copel and R.M. Tromp, Phys. Rev. B, 44 (1991) 12894. [2] H. Presting, H. Kibbel, M. Jaros, R.M. Turtou, U. Menczigar, G. Abstreiter and H.G. Grimmeiss, Semicond. Sci. Technol., 27 (1992) 1127, and references cited therein. [3] Y. Shinoda, N. Shimizu, H. Hibino, T. Nishioka, C. Heimlich, Y. Kobayashi, S. Ishizawa, K. Sugii, M. Seki, Appl. Surf. Sci., 60–61 (1992) 112. [4] M. Diani, D. Aubel, J.L. Bischoff, L. Kubler and D. Bolmont, Surf. Sci., 291 (1993) 110. [5] P.M.J. Maree, K. Nakagawa, F.M. Muiders and J.F. Van Der Veen, Surf. Sci., 191 (1987) 305. [6] K. Takayanagi, Y. Tanishiro, M. Takahashi and S. Takahashi, J. Vac. Sci. Technol., A3 (1985) 1502; G.X. Qian and D.J. Chadi, Phys. Rev. B, 35 (1987) 1288. [7] I. Davoli, R. Bernardini, C. Battistoni, P. Castrucci, R. Gunnella and M. De Crescenzi, Surf. Sci., 306 (1994) 144. [8] T. Abukawa, C.Y. Park and S. Kono, Surf. Sci., 201 (1988) L513.
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[9] C.-Y. Park, T. Abukawa, T. Kinoshita, Y. Enta and S. Kono, Jpn. J. Appl. Phys., 27 (1988) 147. [10] L.E. Davis, N.C. Mac Donald, P.W. Palmberg, G.E. Riach and R.E. Weber, Handbook of Auger Electron Spectroscopy, Physical Electronics, Eden Prairie, MN, 1978. [11] Y.U. Idzerda and G.A. Prinz, Phys. Rev. B, 43 (1991) 11460. [12] B. Cieff and W. Mehlorn; J. Phys. B, 7 (1974) 593. [13] T.A. Tyson, K.O. Hdgson, C.R. Natoli and M. Benfatto, Phys. Rev. B, 47 (1992) 5997. [14] C.S. Fadley, in S.G. Davison (ed.), Progress in Surface
Science, Pergamon, New York, 1984; R.Z. Bacharach (ed.), Synchrotron Radiation Research: Advances in Surface Science, Plenum Press, New York, 1990. [15] M. De Crescenzi, R. Gunnella, R. Bernardini, M. De Marco and I. Davoli, Phys. Rev. B, 52 (1995) 1806. [16] I. Davoli, R. Gunnella, P. Castrucci, R. Bernardini, M. De Crescenzi, J. Electron Spectrosc. Relat. Phenom., 76 (1995) 493–497. [17] P. Ma˚rtensson, G. Meyer, N.M. Amer, E. Kaxiras and K.C. Pandey, Phys. Rev. B, 42 (1990) 7230.
Low kinetic energy AED: a tool for the study of Ge epitaxial layers grown on Sb-terminated Si(111) surface I. Davoli*, R. Gunnella, R. Bernardini, M. De Crescenzi Dipartimento di Matematica e Fisica and INFM, Universita’ di Camerino, 62032 Camerino, Italy Received 8 November 1995; revised 16 September 1996; accepted 18 September 1996
Abstract The intensity anisotropy of the Si L 2,3VV and of the Ge M 2,3M 4,5M 4,5 Auger lines are used to study the growth of Ge on Si(111) assisted by Sb-surfactant. The very surface sensitivity of the low kinetic energy AED is used to investigate the early stage of the Ge/Si (111) interface formation. We compare the AED data obtained after the 10 ML of Ge deposition on bare Si and on the antimony-terminated Si substrate. In the first case, the structureless of the AED spectrum accounts for the island formation nucleated in random manner, while the marked anisotropy of the Auger line shown in the second case evidences the epitaxial growth of the Ge film. A simplified version of the multiple scattering approximation, developed to describe the low kinetic energy AED of the Si L 2,3VV transition, has been used for the theoretical analysis of the experimental data. q 1997 Elsevier Science B.V. Keywords: Intensity anisotropy; Auger line; Low kinetic energy AED
1. Introduction The mismatch of the lattice parameters is the greatest problem in the heteroepitaxial growth technology. Indeed just a few percent of lattice mismatch is enough to create defects that break the epitaxial growing. This greatly reduces the thickness of the continuous layer of the deposited material. Such a severe constraint can be avoided by surfactant assisted growing techniques [1]. To exploit the idea of the folding of the Brillouin zone, which determines the tailoring of the electronic properties in the artificial semiconductors, it is important to have no constraints on the possible layer thickness. A few years ago it was observed that a (Ge m/Si n) superlattice has a direct optical transition at 0.85 eV, just where the fibre * Corresponding author.
optics have their best optical transmission [1]. Such an observation has attracted a lot of interest because devices based on Si technology are the cheapest and distributed world wide. The current interpretation implies the folding of the Si and Ge Brillouin zone which modifies the natural indirect transition of such semiconductors in an artificial material with a direct optical transition. It is understood that, to obtain a multifold effect, very sharp interfaces are required. Unfortunately the formation of a sharp Ge/Si interface is inhibited by the considerable number of dislocations present on the Si substrate [2]. Le Goues et al. [1] have shown that the microstructure of a Ge film deposited on a Si(111) substrate can be drastically altered by changing the growth mode. The deposition of one monolayer of antimony on the Si(111) surface prevents the island formation and allow the subsequent growing of a large number of Ge strained layers.
0368-2048/97/$17.00 q 1997 Elsevier Science B.V. All rights reserved PII S 0 36 8- 2 04 8 (9 6 )0 3 08 1 -2
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I. Davoli et al./Journal of Electron Spectroscopy and Related Phenomena 83 (1997) 137–142
It is difficult to have a detailed knowledge of the Ge/Si interface and of the Sb/Ge/Si system without the contribution of different electron spectroscopies [3–5]. Among them, Auger electron diffraction (AED) in the low kinetic energy range is particularly interesting, because it allows the study of the early stage of the Ge/Si interfaces. Indeed the very surface sensitivity of electrons leaving the sample with the low kinetic energy and the peculiar atomic selectivity of the Auger line makes this technique unique in probing the interface nature of a heteroepitaxial growth. In this paper we present data collected from the Ge/ Si(111) bare interface and from the Ge overlayer grown on an antimony-terminated Si(111) surface. The experimental curve obtained for a reconstructed Si(111)7 × 7 surface is compared with a theoretical model based on single scattering calculation as a function of the various final state angular momenta of the outgoing electrons. Calculations were also made for the 1 × 1 terminated surface and for the reconstructed Si(111)7 × 7 surface according to DAS (dimer adatom stacking-fault) model [6].
2. Experiment The measurements were carried out in a UHV LEED (low energy electron diffraction) system equipped with a single-pass CMA (cylindrical mirror analyser) coaxially mounted on a 3 keV electron gun. The electron analyser was modified to an angular detector by screening (354/360)8 of the circular ring detection area. In this way we have built an angular detector having a small detection area with the axis of the acceptance cone positioned at about 428 in the plane of the polar angle scan. The width of the detection cone was of about 0.06 sr [7]. The sample was an optically polished Si(111) ptype crystal (0.1 Q cm) clamped on two rods for direct Joule heating. The atomic cleaning was obtained by a series of successive flashes at 11008C until nor carbon or oxygen were detected within the Auger atomic sensitivity. Sharp LEED pattern of the 7 × 7 reconstruction superimposed to the 1 × 1 bulk terminated geometry was observed. An azimuth rotation of the sample was needed to properly orient the surface with [110] direction perpendicular to the polar scan.
Fig. 1. Auger signal of the cleaned Si(111) 7 × 7 surface (upper panel). After the deposition of 1 ML of Sb substrate temperature was kept at 6008C (middle panel) and after the deposition of 10 ML of Ge with the substrate temperature at 3008C (lower panel).
The surfactant layer was deposited by heating a quartz crucible filled with high purity lumps of antimony, while the Si substrate was kept a little above 6008C. This is a thermodynamic condition to obtain only one monolayer of Sb stacked on the surface [8,9]. The crystalline nature of the adsorbed monolayer was verified by the observation of a sharp 3 × 3 LEED pattern (not reported here). The subsequent Ge layers were deposited by heating a tungsten basket wet with fused Ge and the thickness was measured by a quartz microbalance. In Fig. 1 are shown the Auger signals of the clean Si surface (upper panel), of the antimony-terminated Si ˚ of Ge evapo(middle panel) and of the nominal 10 A rated on antimony-terminated Si surface (lower panel). We note, on the lower panel, that the Si Auger peak is strongly reduced compared to the antimony peak. Actually the antimony peak in the medium panel and in the bottom panel have the same intensity in the absolute scale. This is an evidence for the floating mechanism of the Sb on top of the evaporated Ge. Auger spectrum is the result of a three phase slab formed by Sb(1 ML)/Ge(10 ML)/ Si(bulk), where the intensity of the Auger peak of
p p
I. Davoli et al./Journal of Electron Spectroscopy and Related Phenomena 83 (1997) 137–142
Sb is the same as that measured in the middle panel while the intensity of the Si peak is attenuated by the presence of the Sb and of the 10 layers of Ge. Considering the Auger sensitivity factors for the three elements (Sb = 0.6, Ge = 0.1, and Si = 0.35) [10] we found that the attenuated intensity of the Si peak, with respect to the Sb, is due to the presence of the 10 ML of germanium deposition. The AED in a polar angle mode measurements were performed monitoring the Ge M 2,3M 4,5M 4,5 (51 eV), the Si L 2,3M 4,5M 4,5 (92 eV) and the Sb M 4,5N 4,5N 4,5 (454 eV) transitions. In the AES (Auger electron spectroscopy) measurements we have used a primary electron beam of 2 keV and a modulation of 0.5 V p–p applied to the outer cylinder of the CMA to detect the electron yield distribution dN(E)/dE, in the first derivative mode. Thus anisotropy reported here is the collection of the peak-to-peak first derivative Auger line intensity vs. the polar angle with a step Dv of 28. Such an experimental method was demonstrated to have the same angular behaviour as that obtained by measuring the area under the Auger line in the N(E) spectrum [11].
3. Theoretical background The theoretical calculation of AED in the low energy regime starts from the Fermi golden rule: dN = dkˆ 2p
ˆ i, f to, j ∑ ∑ ∑ i − 1 Yl9, m (k)A l, m l, m, l9, m9 i, f l, m j, l, m
2 exp( − ikRj, o )
1 where the first sum in Eq. (1) acts over all the initial and final states involved in the Auger transition, while the second and the third sum cover the angular momenta of the emitted electron (l,m) and of the scattered electron (l9,m9) and all the atoms of the cluster ( j) [12]. A i,f l,m represents the Auger matrix element where (l,m) is the angular momentum of the emitted electron, j and o the scattered and the absorbing atomic site, respectively. R j,o is the difference vector R j − R o of the atomic coordinate and Y lm are the real spherical harmonics. Finally t o,j l,m,l9,m9 is
139
defined as j to, l, m, l9, m9 j j = tlo − tlo Go, l, m, l9, m9 tl9
+
2
j j … ∑ tlo Gl,o,m,k l0, m0 tl0k Gl0,k, m0, l9, m9 tl0 −
k, l0, m0
and t il is the complex atomic scattering matrix for the ith atom with angular momentum l. G ijlml9m9 is a suitable propagator from site i to site j calculated within a spherical approximation as reported in Ref. [13]. As reported in the literature the importance of multiple scattering is particularly relevant when the emission is along the low index crystallographic direction of the solid [14]. Our recent comparison of AED data with a theoretical calculation showed that the forward scattering is still present even at low kinetic energies [10]. In the calculation of Eq. (2), the series may be truncated to the second term because when no more than two atoms are involved along one of the main emission direction the second scattering perturbation is very small [14]. Computation of the full multiple scattering including the third term has been used to test the second scattering contribution. On the basis of these considerations we conclude that a single scattering approximation is a sufficiently accurate approach for the purpose of the parent AED data.
4. Results and discussion The interpretation of AED in the low kinetic energy range, is still characterised by an intense debate concerning the diffraction effects of the primary electron beam. In Ref. [15] we have shown that the Auger LVV electron diffraction data collected using either an Xray excitation source or an electron beam source give the same diffraction pattern. This is evidence of the negligible role played by the incident beam diffraction. A more detailed discussion is given in Ref. [16] where it was demonstrated that the diffraction effects are mainly due to the outgoing electron as far as AED data are collected by an angular detector. In Fig. 2 we report the Si (111) L 2,3VV Auger polar anisotropy compared with theoretical calculations. The theoretical curves are the calculations for two different surfaces termination models. Both curves
140
I. Davoli et al./Journal of Electron Spectroscopy and Related Phenomena 83 (1997) 137–142
Fig. 2. L 2,3VV Auger anisotropy for the Si (111)7 × 7 surface vs. polar angle (crossed curve) compared with theoretical calculations performed for a bulk terminated surface (dashed–dotted line) and for a 7 × 7 reconstructed surface (dotted line) according to the simplified DAS model.
are due to a six-layer slab, but while curve (b) is for a bulk terminated Si(111) surface, curve (c) includes the reconstructed two top layers (the adatom and the first full layer) according to a simplified version of the Si(111)7 × 7 DAS model [6]. In both cases the number ˚ of layers is consistent with the escape depth of 5 A probed by electrons of about 100 eV kinetic energy [10]. We note that there are only slight differences between the two theoretical curves in Fig. 2 and this implies that the surface reconstruction does not introduce meaningful additional features different from those of the bulk terminated surface. Indeed one full layer plus the adatom layer affects very little the remaining five layers used in the calculation. This appears clear from the calculation reported in the next figure where the contributions of the various layers are included. Fig. 3 reports the calculated curves, not corrected by any instrumental function, which are due to the sum of each single layer i plus all the above layers. In the lowest we have the curve A due to the adatoms. It contributes in a negligible way to the features shown in the curve 6 which represent the entire cluster. We note that the peak of the 08 polar angle shows up with the inclusion of the third layer. This indicates that we need three layers to pile up two atoms along
Fig. 3. Theoretical calculations of AED spectra involving different silicon layers. Bottom curve A is due to the adatoms contribution. In the other curves, the numbers indicate the sum of the ith layer plus all the previous ones.
the direction normal to the surface. The same effect appears for the peak around 408 polar angle, which shows up after the formation of the fifth layer. Fig. 4 shows the Si AED spectra of three different
Fig. 4. Si L 2,3VV (92 eV) Auger yield anisotropy vs. polar angle scan. Curve (a) is due to the clean Si(111) along the direction [110] compared: with the same Si line after 1 ML of Sb deposition (curve b); with the same Si line after 10 ML of Sb deposition (curve c) and with the Sb MNN Auger line (454 eV) after the deposition of 1 ML of antimony on the Si(111) bare surface (curve d).
I. Davoli et al./Journal of Electron Spectroscopy and Related Phenomena 83 (1997) 137–142
141
˚ deposition of Ge on the antiAED obtained for 10 A mony terminated Si surface and on the bear Si substrate kept at 3008C. Strong anisotropy for the former indicates a crystalline growth mediated by the Sb film which hampers the formation of the Ge clustering [2]. We may note that the anisotropy features have almost the same angular position of that observed for the Si substrate as shown in Fig. 4 (curve (c)). In contrast, the structureless full curve indicates that the contribution from the Ge atoms is averaged without any well defined anisotropy. In this case the Ge deposition probably forms misoriented droplets.
5. Conclusion
Fig. 5. Ge MMM (51 eV) AED data monitored from the Ge/Si interface obtained after the evaporation of 10 ML of Ge onto an antimony terminated Si surface (crossed curve) compared with the anisotropy of the same Auger line detected from the Ge/Si interface after the Ge deposition on clean Si surface kept at 3008C (full curve).
samples and Sb AED spectrum of the Sb/Si(111) sample. We note that, although the shape appears different, the main structures of curve (b) have the same angular position of the main structures of the curve (a) for the clean Si. In contrast, curve (d) does not show any appreciable diffraction peak. This implies no interdiffusion of the antimony into the Si substrate. Indeed a partial crystalline interdiffusion should produce some diffraction peak. Furthermore a possible island formation of the antimony film should give a peak at 08 polar angle (normal emission). Our results are in agreement with the photoelectron diffraction study reported by Abukawa et al. [8] and with the STM measurements of Ma˚rtenson et al. [17] which present patterns of antimony trimers on the Si (111) surface. Curve c for an antimony terminated surface still shows the anisotropy of the Si L 2,3VV Auger line. This observation indicates that for such high Ge deposition a Stranski–Krastanov growing mode occurs [3,5]. Indeed the formation of Ge islands does not prevent the detection of the AED features ˚ continuous coming from the Si substrate, while a 10 A film of Ge would severely attenuate the signal. Finally in Fig. 5 are shown the Ge M 2,3M 4,5M 4,5
We have shown that AED technique is a versatile tool to investigate the growth mode of Ge on Si(111). It has been confirmed that an antimony layer ˚ Ge film, modifies the deposited, before the 10 A usual Stranski–Krastanov growth mode giving rise to an epitaxial layer. Our technique is atomic sensitive and is able to discriminate whether or not the antimony atoms interdiffuse into the Si substrate. Finally we have shown that the use of low kinetic energy electrons gives information of the same quality of that obtained with the X-ray photoelectron diffraction.
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