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Epitaxial antimony monolayers on III–V(110) surfaces studied by low-energy photoelectron diffraction
s u r f a c e science
ELSEVIER
Surface Science 307-309 (1994) 685-690
ii! i~i!~. .:.:i! i~i~i!~i~iii~i~i~!~:~i~i~iiJ~
Epitaxial antimony monolayers on III-V(ll0) surfaces studied by low-energy photoelectron diffraction C. Nowak
,,a, A. Chass6 b, A. H e m p e l m a n n a, W. Richter a, E. Dudzik c, R. Whittle c, I.T. McGovern c W. Braun d D.R.T. Zahn e
e
a Institut fiir Festk6rperphysik der TU Berlin, Hardenbergstrasse 36, 10623 Berlin, Germany b Fachbereich Physik der MLU Halle, Friedemann-Bach-Platz 6, 06108 Halle, Germany ¢ Department of Pure and Applied Physics, Trinity College Dublin, Dublin 2, Ireland a BESSY Berlin, LentzeaUee 100, 14195 Berlin, Germany Fachbereich Physik der TU Chemnitz-Zwickau, Reichenhainerstrasse 70, PSF964, 09009 Chemnitz, Germany
(Received 20 August 1993)
Abstract
Antimony is known to form well ordered epitaxial monolayers on clean cleaved (110) surfaces of I I I - V semiconductors like InP, GaP, and GaAs. These monolayers provide ideal candidates for photoelectron diffraction (PED) studies. At low photon energies (40-90 eV) the Sb 4d core level emission can be resolved into two chemically shifted spin-orbit split doublets indicative of the two distinct adsorption sites on the surface. The intensity of these two doublets together with that of the In 4d or Ga 3d subtrate emission was monitored as a function of polar angles and photon energy utilizing an ADES 400 photoelectron spectrometer at the BESSY TGM2 beamline. Strong variation of the intensity ratio of the doublets clearly demonstrates the usefulness of resolving chemical shifts. The experimental results are compared to theoretical calculations using the multiple-scattering cluster model including spherical-wave corrections for different adsorption models of the Sb monolayer.
I. Introduction
The growth of ordered antimony (Sb) monolayers (ML) on I I I - V ( l l 0 ) semiconductor substrates and their structural and electronic properties have been extensively studied with experimental techniques like photoemission spectroscopy (PES) [1-3], surface X-ray absorption fine structure (SEXAFS) [4,5], scanning tunneling
* Corresponding author. Fax: +49 (30) 314 21769.
microscopy (STM) [6] and low energy electron diffraction ( L E E D ) [7-9] as well as theoretically using total-energy minimisation calculations [1012]. The Sb atoms are known to form a zig-zag chain bonded to a nearly unrelaxed (110) substrate. However, the precise geometric positions of the Sb atoms are still under discussion. Several atomic structures have been proposed for the Sb monolayer on the I I I - V ( l l 0 ) surface [10] where the epitaxial continued-layer structure (ECLS), which is shown in Fig. 1, provides the best agreem e n t with L E E D analysis of the 1 M L S b / G a A s ( l l 0 ) and S b / I n P ( l l 0 ) systems [7-9].
003%6028/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0039-6028(93)E0745-G
686
C. Nowak et al. / Surface Science 307-309 (1994) 685-690
d12,y I A1,),
Sbl
AI&
__[~
.~ I
I
side view
fop view
Q Sb
02 OZ Fig. 1. Schematic diagram of side and top views of the epitaxial continued layer structure (ECLS)of Sb on III-V(ll0) surfaces and definition of the structural parameters (from Ref. [12]).
For Sb on InP(ll0) and G a P ( l l 0 ) the two distinct bonding sites of the Sb atom to the substrate manifest themselves in two chemically shifted components of the Sb 4d core level emission lines [2]. Due to their ordered arrangement these Sb monolayers seem to be ideal candidates in order to assess the feasibility of using low-energy photoelectron diffraction for geometrical structure determination. In recent years, the photoelectron diffraction technique has become a widely used method for evaluation of the geometric structure of clean crystal surfaces and adsorbed atoms and molecules on crystal surfaces [13]. The experiment involves the excitation of core electrons and the observation of modulations in the resulting photoemission intensities as a function of emission angle and photon energy. These modulations are due to the scattering of the
emitted electron wave at the neighbouring atoms of the emitter and the interference between the direct and scattered waves. Information on the structural parameters of interest such as bond distances, bond angles and interplanar distances is contained in the diffraction pattern. In order to determine the geometrical structure of the Sb monolayers the experimental results are compared with calculations within the multiplescattering cluster approximation (MSCA) [14]. For the calculations two ECLS models which are quite different in their structural parameters have been used initially, namely, one derived from a L E E D study [7] and another from a total-energy minimization calculation [12]. The structural parameters of these two studies are given in Table 1. In this paper we present experimental photoelectron diffraction results for Sb monolayers on InP(ll0) and GaP(ll0). A preliminary comparison of our experimental data with the calculations using the parameters according to the above mentioned geometric models is made for Sb on InP(ll0).
2. Experimental
The experiments were performed at the T G M 2 beamline of the storage ring at BESSY in Berlin using an ADES 400 electron analyzer with an acceptance angle of 4 °. The ordered monolayers of Sb were prepared by depositing 3 - 4 monolayers onto ultra-high vacuum cleaved InP(110) and GaP(110) crystals at room temperature and subsequent annealing up to 620 K. This procedure has been shown to produce well defined Sb monolayers on GaAs(110) and InP(110) and therefore was also applied to the Sb/GaP(110) monolayer preparation [1,2]. Further details of
Table 1 Structural parameters used in the calculation for 1 ML Sb on InP(110) (see Fig. 1) 1 ML Sb/InP(110) Model A I,.A 1,y d12,± A2,± A2,r (,~) (,~) (,~) (,~) (,~) ECLS 0.57 2.02 4.00 0.09 1.47 ECLS 0.13 1.91 4.38 0.03 1.39
(o (deg) 15.8 3.9
Ref. [12] [7]
C. Nowak et al. / Surface Science 307-309 (1994) 685-690
the experiment can be found in Refs. [2,15]. The PED measurements were performed by recording the overlayer (Sb4d) and substrate (In4d and Ga 3d) core level emissions as a function of polar angles and photon energy. The angle of detection was varied in steps of 2.5 ° up to 45° off normal along the [001] and [110] azimuth of the substrate using the three different photon energies 50, 55 and 60 eV, which allows the Sb4d, In4d and Ga 3d photoelectrons to be monitored with high surface sensitivity. In addition, photon energy scans in the range from 40 to 90 eV were performed. The overall resolution (light and electrons) at 55 eV was less than 200 meV. The core level emission spectra were deconvoluted using a computer curve fitting routine described in Ref. [161.
3. Calculations
In this work the intensity of photoelectrons is calculated using a multiple-scattering cluster approximation (MSCA) [14]. The reduced angular momentum expansion (RAME) is applied to take the curvature of the electron wave into account [14,17]. For the excitation of d-electrons both the p and f final states were included. The electron-atom interaction is described by a one-electron muffin-tin potential. The crystal potential is formed from a superposition of overlapping neutral charge densities. The exchange potential is determined from the charge density through the use of the Slater Xa-approximation [18]. Finally, the scattering potentials are represented by energy-dependent scattering phase shifts. The effects of inelastic scattering are included by means of the electron inelastic mean free path (Ae). The energy dependence of h e has been taken from Ref. [19]. The surface potential barrier between vacuum and bulk is approximated by a finite potential step V0, which is set in our calculation as 15 eV. This causes a refraction effect changing the emission angle of the photoelectrons. The analyser response which varies with polar angle 0 has been taken into account by multiply-
687
ing the theoretical intensities by cos2(0) in good agreement with the measured dependence. Lattice vibrations and the finite angular resolution of the spectrometer were neglected in the calculations.
4. Results and discussion
The Sb 4d core level emission lines were deconvoluted into two components using a curve fitting routine. As an example, Fig. 2 shows the evolution of the Sb 4d core level emission intensities at 55 eV photon energy for a limited polar angle range along the [1~0] azimuth. The deconvolution was performed using energy splittings of 1.25 ___0.02 eV for each spin-orbit doublet. As a result of diffraction the intensity branching ratio was found to vary between 1.2 and 2.0. The lineshape of each component is determined by a Lorentzian and a Gaussian halfwidth. The Lorentzian was found to be 0.10 + 0.02 eV while the Gaussian broadening was 0.30 + 0.05 eV. Starting at normal emission (0°) the hatched component in Fig. 2 is more intense than the other one. With increasing angle the intensity of the Sb4d
hv=55eV
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~
,
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. . . . 21 . . . 22 23 24 ~ 19 . .20
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Kinetic Energy Fig. 2. Photoelectron spectra of the Sb 4d core level emission for Sb on I n P ( l l 0 ) recorded for various angles along the [1i0] azimuth at 55 eV photon energy.
C. Nowak et aL / Surface Science 307-309 (1994) 685-690
688
hatched component decreases while the other increases. Similar behaviour of the two components was found for angular variations at the three photon energies used as well as for the photon energy scans. This is demonstrated in Fig. 3 where the evaluated intensities of the Sb 4d core level emission are plotted as a function of polar angle along the [001] azimuth for the three photon energies and both InP(ll0) and GaP(ll0) substrates. The anisotropies (/max -- Imin//Imax ) observed in the experiment were in the range between 20% and 30%. Repetitions of the experiments revealed excellent reproducibility. The individual error bars on the data points of Fig. 3 do not exceed the symbol size. The intensity variations differ for InP and GaP indicating differences in the atomic arrangement of the Sb atoms on the distinct substrates, as confirmed by the calculations assuming an ECLS model with identical structural parameters for the Sb monolayer on both substrates. The Sb 4d emission can be deconvoluted into two components which have to be assigned to the two distinct adsorption sites as sketched in Fig. 1. For the case of Sb on GaAs(ll0) it has been
Sb/lnP(l 10)
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Polar angle along the (li0] azimuth / degrees Fig. 3. Experimental intensity variation of the Sb4d core level along the [110] azimuth for Sb monolayers InP(ll0) and G a P ( l l 0 ) for three different photon energies (50, 55 and 60 eV). The open circles represent the high-kinetic energy component, the closed circles the low-energy component and the solid line the total intensity of the Sb4d core level.
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Photon Energy (eV) Fig. 4. Measured and calculated photoelectron diffraction intensities for the Sb4d core level emission as a function of photon energy. The experiment is labelled with exp. and the open circles represent the high-kinetic energy component and the closed circles the low-kinetic energy component. (a) and (b) are the calculated intensities using the parameters published in Refs. [12] and [7], respectively. In spectra (a) and (b) the open circles are the intensities for the SbI atom and the closed circles are for the SblI atom (see Fig. 1).
suggested that the high-kinetic energy component is associated with Sb atoms bonded to the group III element and the low-kinetic one with the Sb-group V bonding site [1]. It is, however, questionable whether this assignment is also applicable for the systems under study. From the theoretical point of view the assignment of the two adsorption sites is a priori given. A comparison between the whole set of experimental data and the corresponding calculations, an example of which is shown by the photon energy scan of Fig. 4, seems to support the previous assignment. Therefore, the Sb I (Sb II) atoms in Fig. 1 most probably give rise to the high (low) kinetic component in Fig. 2. The polar angle variation along the [001] azimuth, which was chosen as a typical example of the data, is shown in Fig. 5. It reveals that the agreement between experiment and theory is still poor. Consequently, fine tuning of the structural parameters used in the calculations is required in
C. Nowak et al. / Surface Science 307-309 (1994) 685-690
689
found to strongly affect the calculated photoelectron diffraction patterns, as shown in Fig. 6.
5. Conclusion
0
10 20 30 40
0
10 20 30 40
Polar angle along the [001 ] azimuth / degrees Fig. 5. Measured and calculated photoelectron diffraction intensities for the Sb4d core level emission along the [001] azimuth. The open and closed circles represent the experiment as in Fig. 3, and the lines the calculated intensities. The dotted lines are calculations using the parameters from Ref. [12] and the dashed lines using the parameters from Ref. [7].
order to obtain a good match between experiment and theory. This may in particular be achieved by changing the lateral positions of the Sb atoms with respect to the substrate lattice a n d / o r the angle ~o since these parameters were
The two chemically shifted components of the Sb 4d core emission observed for Sb monolayers on InP(ll0) and GaP(ll0) reveal strong relative intensity variation with photon energy and polar angle induced by photoelectron diffraction effects at low photon energies. Different diffraction patterns were found for Sb/InP(ll0) and Sb/ GaP(ll0) indicating distinct adsorption behaviour. A theoretical modelling of the photoelectron spectra was performed for the Sb on InP(ll0) using two published ECLS models. For Sb on InP(ll0) an assignment of the chemically shifted components was made to the distinct adsorption sites. Full consistency of the calculations with the wealth of experimental data was not achieved. Therefore further refinement of the structural parameters is needed.
6. Acknowledgment Sbl
~
Sb lI
We gratefully acknowledge the financial support by the Bundesministerium ffir Forschung und Technologie under Grant No. 05 5KTCAB 4.
7. References
~~ , i , i , i ,1 i ' fll
0
10
20
30
40
50
0
l0
20
30
40
50
Polar angle along the [001] azimuth / degrees Fig. 6. Sensitivity of the calculated intensities for the Sb I and Sbll atom emission along the [110] azimuth for different structural parameterSoaS denoted in Fig.ol. Parameter d12,,~ was varied by _+0.2 A in steps of 0.05 A, dl2,y by 5:0.3 A (Adlz, y = 0.05 A.) and w from - 4 ° to 16° (Aw = 4°).
[1] F. Sch~iffier, R. Ludeke, A. Taleb-Ibrahimi, G. Hughes and D. Rieger, J. Vac. Sci. Technol. B 5 (1987) 1048. [2] N. Esser, D.R.T. Zahn, C. Miiller, W. Richter, C. Stephens, R. Whittle, I.T. McGovern, S. Kulkarni and W. Braun, Appl. Surf. Sci. 56-58 (1992) 169. [3] R. Whittle, I.T. McGovern, D.R.T. Zahn, C. Miiller, C. Nowak, A. Cafolla and W. Braun, Appl. Surf. Sci. 56-58 (1992) 218. [4] J.C. Woicik, T. Kendelewicz, K.E. Miyano, P.L. Cowan, C.E. Bouldin, B.A. Karlin, P. Pianetta and W.E. Spicer, Phys. Rev. B 44 (1991) 3475. [5] K.E. Miyano, J.C. Woicik, T. Kendelewicz, W. Spicer, M. Richter and P. Pianetta, Phys. Rev. B 47 (1993) 6444. [6] P. M~rtonsson and R.M. Feenstra, Phys. Rev. B 39 (1989) 7744.
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C. Nowak et al. / Surface Science 307-309 (1994) 685-690
[7] W.K. Ford, T. Guo, K.-J. Wan and C.B. Duke, Phys. Rev. B 45 (1992) 11896. [8] C.B. Duke, A. Paton, W.K. Ford, A. Kahn and J. Carelli, Phys. Rev. B 26 (1982) 803. [9] W.K. Ford, T. Guo, D.L. Lessor and C.B. Duke, Phys. Rev. B 42 (1990) 8952. [10] J.P. LaFemina, C.B. Duke and C. Mailhiot, J. Vac. Sci. Technol. B 8 (1990) 888. [11] G.P. Srivastava, Phys. Rev. B 46 (1992) 7300. [12] C. Mailhiot, C.B. Duke and D.J. Chadi, Phys. Rev. B 31 (1985) 2213. [13] C.S. Fadley, in: Synchrotron Radiation Research: Advances in Surface Science, Ed. R.Z. Bachrach (Plenum, New York, 1990).
[14] A. Chass6, Surf. Sci. 269/270 (1992) 22, and references therein. [15] C. Stephens, D.R.T. Zahn, K. Fives, R. Cimino, W. Braun and I.T. McGovern, J. Vac. Sci. Technol. B 8 (1990) 674. [16] W.G. Wilke and K. Horn, J. Vac. Sci. Technol. B 6 (1988) 1211. [17] V. Fritzsche, J. Phys.: Condensed Matter 2 (1990) 1413, and references therein. [18] J.C. Slater, Quantum theory of Molecules and Solids, The self-consistent Field for Molecules and Solids, Vol. 4 (McGraw-Hill, New York, 1974). [19] M.P. Seah and W.A. Dench, Surf. Interf. Anal. 1 (1979) 2.
ELSEVIER
Surface Science 307-309 (1994) 685-690
ii! i~i!~. .:.:i! i~i~i!~i~iii~i~i~!~:~i~i~iiJ~
Epitaxial antimony monolayers on III-V(ll0) surfaces studied by low-energy photoelectron diffraction C. Nowak
,,a, A. Chass6 b, A. H e m p e l m a n n a, W. Richter a, E. Dudzik c, R. Whittle c, I.T. McGovern c W. Braun d D.R.T. Zahn e
e
a Institut fiir Festk6rperphysik der TU Berlin, Hardenbergstrasse 36, 10623 Berlin, Germany b Fachbereich Physik der MLU Halle, Friedemann-Bach-Platz 6, 06108 Halle, Germany ¢ Department of Pure and Applied Physics, Trinity College Dublin, Dublin 2, Ireland a BESSY Berlin, LentzeaUee 100, 14195 Berlin, Germany Fachbereich Physik der TU Chemnitz-Zwickau, Reichenhainerstrasse 70, PSF964, 09009 Chemnitz, Germany
(Received 20 August 1993)
Abstract
Antimony is known to form well ordered epitaxial monolayers on clean cleaved (110) surfaces of I I I - V semiconductors like InP, GaP, and GaAs. These monolayers provide ideal candidates for photoelectron diffraction (PED) studies. At low photon energies (40-90 eV) the Sb 4d core level emission can be resolved into two chemically shifted spin-orbit split doublets indicative of the two distinct adsorption sites on the surface. The intensity of these two doublets together with that of the In 4d or Ga 3d subtrate emission was monitored as a function of polar angles and photon energy utilizing an ADES 400 photoelectron spectrometer at the BESSY TGM2 beamline. Strong variation of the intensity ratio of the doublets clearly demonstrates the usefulness of resolving chemical shifts. The experimental results are compared to theoretical calculations using the multiple-scattering cluster model including spherical-wave corrections for different adsorption models of the Sb monolayer.
I. Introduction
The growth of ordered antimony (Sb) monolayers (ML) on I I I - V ( l l 0 ) semiconductor substrates and their structural and electronic properties have been extensively studied with experimental techniques like photoemission spectroscopy (PES) [1-3], surface X-ray absorption fine structure (SEXAFS) [4,5], scanning tunneling
* Corresponding author. Fax: +49 (30) 314 21769.
microscopy (STM) [6] and low energy electron diffraction ( L E E D ) [7-9] as well as theoretically using total-energy minimisation calculations [1012]. The Sb atoms are known to form a zig-zag chain bonded to a nearly unrelaxed (110) substrate. However, the precise geometric positions of the Sb atoms are still under discussion. Several atomic structures have been proposed for the Sb monolayer on the I I I - V ( l l 0 ) surface [10] where the epitaxial continued-layer structure (ECLS), which is shown in Fig. 1, provides the best agreem e n t with L E E D analysis of the 1 M L S b / G a A s ( l l 0 ) and S b / I n P ( l l 0 ) systems [7-9].
003%6028/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0039-6028(93)E0745-G
686
C. Nowak et al. / Surface Science 307-309 (1994) 685-690
d12,y I A1,),
Sbl
AI&
__[~
.~ I
I
side view
fop view
Q Sb
02 OZ Fig. 1. Schematic diagram of side and top views of the epitaxial continued layer structure (ECLS)of Sb on III-V(ll0) surfaces and definition of the structural parameters (from Ref. [12]).
For Sb on InP(ll0) and G a P ( l l 0 ) the two distinct bonding sites of the Sb atom to the substrate manifest themselves in two chemically shifted components of the Sb 4d core level emission lines [2]. Due to their ordered arrangement these Sb monolayers seem to be ideal candidates in order to assess the feasibility of using low-energy photoelectron diffraction for geometrical structure determination. In recent years, the photoelectron diffraction technique has become a widely used method for evaluation of the geometric structure of clean crystal surfaces and adsorbed atoms and molecules on crystal surfaces [13]. The experiment involves the excitation of core electrons and the observation of modulations in the resulting photoemission intensities as a function of emission angle and photon energy. These modulations are due to the scattering of the
emitted electron wave at the neighbouring atoms of the emitter and the interference between the direct and scattered waves. Information on the structural parameters of interest such as bond distances, bond angles and interplanar distances is contained in the diffraction pattern. In order to determine the geometrical structure of the Sb monolayers the experimental results are compared with calculations within the multiplescattering cluster approximation (MSCA) [14]. For the calculations two ECLS models which are quite different in their structural parameters have been used initially, namely, one derived from a L E E D study [7] and another from a total-energy minimization calculation [12]. The structural parameters of these two studies are given in Table 1. In this paper we present experimental photoelectron diffraction results for Sb monolayers on InP(ll0) and GaP(ll0). A preliminary comparison of our experimental data with the calculations using the parameters according to the above mentioned geometric models is made for Sb on InP(ll0).
2. Experimental
The experiments were performed at the T G M 2 beamline of the storage ring at BESSY in Berlin using an ADES 400 electron analyzer with an acceptance angle of 4 °. The ordered monolayers of Sb were prepared by depositing 3 - 4 monolayers onto ultra-high vacuum cleaved InP(110) and GaP(110) crystals at room temperature and subsequent annealing up to 620 K. This procedure has been shown to produce well defined Sb monolayers on GaAs(110) and InP(110) and therefore was also applied to the Sb/GaP(110) monolayer preparation [1,2]. Further details of
Table 1 Structural parameters used in the calculation for 1 ML Sb on InP(110) (see Fig. 1) 1 ML Sb/InP(110) Model A I,.A 1,y d12,± A2,± A2,r (,~) (,~) (,~) (,~) (,~) ECLS 0.57 2.02 4.00 0.09 1.47 ECLS 0.13 1.91 4.38 0.03 1.39
(o (deg) 15.8 3.9
Ref. [12] [7]
C. Nowak et al. / Surface Science 307-309 (1994) 685-690
the experiment can be found in Refs. [2,15]. The PED measurements were performed by recording the overlayer (Sb4d) and substrate (In4d and Ga 3d) core level emissions as a function of polar angles and photon energy. The angle of detection was varied in steps of 2.5 ° up to 45° off normal along the [001] and [110] azimuth of the substrate using the three different photon energies 50, 55 and 60 eV, which allows the Sb4d, In4d and Ga 3d photoelectrons to be monitored with high surface sensitivity. In addition, photon energy scans in the range from 40 to 90 eV were performed. The overall resolution (light and electrons) at 55 eV was less than 200 meV. The core level emission spectra were deconvoluted using a computer curve fitting routine described in Ref. [161.
3. Calculations
In this work the intensity of photoelectrons is calculated using a multiple-scattering cluster approximation (MSCA) [14]. The reduced angular momentum expansion (RAME) is applied to take the curvature of the electron wave into account [14,17]. For the excitation of d-electrons both the p and f final states were included. The electron-atom interaction is described by a one-electron muffin-tin potential. The crystal potential is formed from a superposition of overlapping neutral charge densities. The exchange potential is determined from the charge density through the use of the Slater Xa-approximation [18]. Finally, the scattering potentials are represented by energy-dependent scattering phase shifts. The effects of inelastic scattering are included by means of the electron inelastic mean free path (Ae). The energy dependence of h e has been taken from Ref. [19]. The surface potential barrier between vacuum and bulk is approximated by a finite potential step V0, which is set in our calculation as 15 eV. This causes a refraction effect changing the emission angle of the photoelectrons. The analyser response which varies with polar angle 0 has been taken into account by multiply-
687
ing the theoretical intensities by cos2(0) in good agreement with the measured dependence. Lattice vibrations and the finite angular resolution of the spectrometer were neglected in the calculations.
4. Results and discussion
The Sb 4d core level emission lines were deconvoluted into two components using a curve fitting routine. As an example, Fig. 2 shows the evolution of the Sb 4d core level emission intensities at 55 eV photon energy for a limited polar angle range along the [1~0] azimuth. The deconvolution was performed using energy splittings of 1.25 ___0.02 eV for each spin-orbit doublet. As a result of diffraction the intensity branching ratio was found to vary between 1.2 and 2.0. The lineshape of each component is determined by a Lorentzian and a Gaussian halfwidth. The Lorentzian was found to be 0.10 + 0.02 eV while the Gaussian broadening was 0.30 + 0.05 eV. Starting at normal emission (0°) the hatched component in Fig. 2 is more intense than the other one. With increasing angle the intensity of the Sb4d
hv=55eV
12.5o
10"0o
.a"
~
,
7.50
~
. . . . 21 . . . 22 23 24 ~ 19 . .20
0°
,2~5, 26J ,217, 28
Kinetic Energy Fig. 2. Photoelectron spectra of the Sb 4d core level emission for Sb on I n P ( l l 0 ) recorded for various angles along the [1i0] azimuth at 55 eV photon energy.
C. Nowak et aL / Surface Science 307-309 (1994) 685-690
688
hatched component decreases while the other increases. Similar behaviour of the two components was found for angular variations at the three photon energies used as well as for the photon energy scans. This is demonstrated in Fig. 3 where the evaluated intensities of the Sb 4d core level emission are plotted as a function of polar angle along the [001] azimuth for the three photon energies and both InP(ll0) and GaP(ll0) substrates. The anisotropies (/max -- Imin//Imax ) observed in the experiment were in the range between 20% and 30%. Repetitions of the experiments revealed excellent reproducibility. The individual error bars on the data points of Fig. 3 do not exceed the symbol size. The intensity variations differ for InP and GaP indicating differences in the atomic arrangement of the Sb atoms on the distinct substrates, as confirmed by the calculations assuming an ECLS model with identical structural parameters for the Sb monolayer on both substrates. The Sb 4d emission can be deconvoluted into two components which have to be assigned to the two distinct adsorption sites as sketched in Fig. 1. For the case of Sb on GaAs(ll0) it has been
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Polar angle along the (li0] azimuth / degrees Fig. 3. Experimental intensity variation of the Sb4d core level along the [110] azimuth for Sb monolayers InP(ll0) and G a P ( l l 0 ) for three different photon energies (50, 55 and 60 eV). The open circles represent the high-kinetic energy component, the closed circles the low-energy component and the solid line the total intensity of the Sb4d core level.
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8eOo~ooeeoeeol 80
90
Photon Energy (eV) Fig. 4. Measured and calculated photoelectron diffraction intensities for the Sb4d core level emission as a function of photon energy. The experiment is labelled with exp. and the open circles represent the high-kinetic energy component and the closed circles the low-kinetic energy component. (a) and (b) are the calculated intensities using the parameters published in Refs. [12] and [7], respectively. In spectra (a) and (b) the open circles are the intensities for the SbI atom and the closed circles are for the SblI atom (see Fig. 1).
suggested that the high-kinetic energy component is associated with Sb atoms bonded to the group III element and the low-kinetic one with the Sb-group V bonding site [1]. It is, however, questionable whether this assignment is also applicable for the systems under study. From the theoretical point of view the assignment of the two adsorption sites is a priori given. A comparison between the whole set of experimental data and the corresponding calculations, an example of which is shown by the photon energy scan of Fig. 4, seems to support the previous assignment. Therefore, the Sb I (Sb II) atoms in Fig. 1 most probably give rise to the high (low) kinetic component in Fig. 2. The polar angle variation along the [001] azimuth, which was chosen as a typical example of the data, is shown in Fig. 5. It reveals that the agreement between experiment and theory is still poor. Consequently, fine tuning of the structural parameters used in the calculations is required in
C. Nowak et al. / Surface Science 307-309 (1994) 685-690
689
found to strongly affect the calculated photoelectron diffraction patterns, as shown in Fig. 6.
5. Conclusion
0
10 20 30 40
0
10 20 30 40
Polar angle along the [001 ] azimuth / degrees Fig. 5. Measured and calculated photoelectron diffraction intensities for the Sb4d core level emission along the [001] azimuth. The open and closed circles represent the experiment as in Fig. 3, and the lines the calculated intensities. The dotted lines are calculations using the parameters from Ref. [12] and the dashed lines using the parameters from Ref. [7].
order to obtain a good match between experiment and theory. This may in particular be achieved by changing the lateral positions of the Sb atoms with respect to the substrate lattice a n d / o r the angle ~o since these parameters were
The two chemically shifted components of the Sb 4d core emission observed for Sb monolayers on InP(ll0) and GaP(ll0) reveal strong relative intensity variation with photon energy and polar angle induced by photoelectron diffraction effects at low photon energies. Different diffraction patterns were found for Sb/InP(ll0) and Sb/ GaP(ll0) indicating distinct adsorption behaviour. A theoretical modelling of the photoelectron spectra was performed for the Sb on InP(ll0) using two published ECLS models. For Sb on InP(ll0) an assignment of the chemically shifted components was made to the distinct adsorption sites. Full consistency of the calculations with the wealth of experimental data was not achieved. Therefore further refinement of the structural parameters is needed.
6. Acknowledgment Sbl
~
Sb lI
We gratefully acknowledge the financial support by the Bundesministerium ffir Forschung und Technologie under Grant No. 05 5KTCAB 4.
7. References
~~ , i , i , i ,1 i ' fll
0
10
20
30
40
50
0
l0
20
30
40
50
Polar angle along the [001] azimuth / degrees Fig. 6. Sensitivity of the calculated intensities for the Sb I and Sbll atom emission along the [110] azimuth for different structural parameterSoaS denoted in Fig.ol. Parameter d12,,~ was varied by _+0.2 A in steps of 0.05 A, dl2,y by 5:0.3 A (Adlz, y = 0.05 A.) and w from - 4 ° to 16° (Aw = 4°).
[1] F. Sch~iffier, R. Ludeke, A. Taleb-Ibrahimi, G. Hughes and D. Rieger, J. Vac. Sci. Technol. B 5 (1987) 1048. [2] N. Esser, D.R.T. Zahn, C. Miiller, W. Richter, C. Stephens, R. Whittle, I.T. McGovern, S. Kulkarni and W. Braun, Appl. Surf. Sci. 56-58 (1992) 169. [3] R. Whittle, I.T. McGovern, D.R.T. Zahn, C. Miiller, C. Nowak, A. Cafolla and W. Braun, Appl. Surf. Sci. 56-58 (1992) 218. [4] J.C. Woicik, T. Kendelewicz, K.E. Miyano, P.L. Cowan, C.E. Bouldin, B.A. Karlin, P. Pianetta and W.E. Spicer, Phys. Rev. B 44 (1991) 3475. [5] K.E. Miyano, J.C. Woicik, T. Kendelewicz, W. Spicer, M. Richter and P. Pianetta, Phys. Rev. B 47 (1993) 6444. [6] P. M~rtonsson and R.M. Feenstra, Phys. Rev. B 39 (1989) 7744.
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C. Nowak et al. / Surface Science 307-309 (1994) 685-690
[7] W.K. Ford, T. Guo, K.-J. Wan and C.B. Duke, Phys. Rev. B 45 (1992) 11896. [8] C.B. Duke, A. Paton, W.K. Ford, A. Kahn and J. Carelli, Phys. Rev. B 26 (1982) 803. [9] W.K. Ford, T. Guo, D.L. Lessor and C.B. Duke, Phys. Rev. B 42 (1990) 8952. [10] J.P. LaFemina, C.B. Duke and C. Mailhiot, J. Vac. Sci. Technol. B 8 (1990) 888. [11] G.P. Srivastava, Phys. Rev. B 46 (1992) 7300. [12] C. Mailhiot, C.B. Duke and D.J. Chadi, Phys. Rev. B 31 (1985) 2213. [13] C.S. Fadley, in: Synchrotron Radiation Research: Advances in Surface Science, Ed. R.Z. Bachrach (Plenum, New York, 1990).
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