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Applied Surface Science 123/124 (1998) 223-227
Sb on GaAs(110) structure studied by direct methods and chemical-shift photoelectron diffraction H. Ascolani a, j. Avila b.c, N. Franco c, M.C. Asensio b,c,, a Centro Atdmico Bariloche and lnstituto Balseiro, Comisidn Nacional de Energ{a Atdmica, 8400 Bariloche, Argentina h lnstituto de Ciencias de Materiales de Madrid (CSIC), 28049 Cantoblanco, Madrid, Spain LURE, B&. 209D, Unicersit6 Paris-Sud, F-91405 Orsay, France
Abstract
We have determined the two unequivalent adsorption sites of Sb in the GaAs(110)p(l × 1)-Sb(l ML) surface by evaluating a simple Fourier transform of scanned-energy photoelectron diffraction data corresponding to chemically shifted Sb 4d core levels. Our results show, in a model-independent procedure, that the atomic geometry of the p(l × 1)Sb overlayer contradicts various models proposed for this surface and is consistent only with the epitaxial continued layer structure (ECLS). © 1998 Elsevier Science B.V. PACS." 68.35.Bs; 79.60.Dp; 6 1 . 1 4 . - x ; 82.65.My Kevwords." Interfaces; Semiconductors; Photoelectron diffraction
1. Introduction
Photoelectron diffraction (PD) in the scanned-energy mode has proven to be a powerful tool for local structural determination of the first few surface layers. This technique involves the measurement of the intensity of photoelectrons emitted from a core level of an adsorbate atom as a function of the incident photon energy for different emission directions. As in other traditional diffraction techniques, a trialand-error procedure is usually applied in order to derive the atomic positions. On the other hand, new methods have recently been developed to obtain
• Corresponding author. LURE, Bat. 209D, Universitd ParisSud, F-91405 Orsay, France. Fax: + 33-1-64464148; e-mail: asensio @lure.u-psud.fr.
structural information directly from the diffraction pattern [1-3]. The most used among these are the Fourier transform method and the projection method, since they do not require too large an experimental data set. They have been successfully applied to determine the adsorption sites of small molecules on metallic substrates. However, they were not used to invert chemically shifted PD data, as the inversion of these kinds of data presents higher technical difficulties imposed by the requirement of a large data set measured with a high energy resolution. The first example wherein a direct inversion method has successfully been used to identify multiple adsorption sites using chemical-shift photoelectron diffraction was presented in our previous paper [4]. In this work, we demonstrate the efficacy of the Fourier transform method for determining the adsorption sites of Sb on the system G a A s ( l l 0 ) p ( l × l ) - S b ( l ML) directly
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H. Ascolani el al. / Applied SutT¢hce Science 123 / 124 (1998) 223-227
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model-independent procedure. The adsorption geometry is obtained without performing any theoretical simulation of the diffraction data, but evaluating a simple Fourier transform of the scanned-energy PD data of chemically shifted Sb 4d core levels.
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Fig. 1. Plan and side views of two adsorption geometries of Sb on Ill-V(110) surfaces: the epitaxial continued layer structure (ECLS) and the epitaxial on-top structure (EOTS) models.
from scanned-energy PD data of chemically shifted Sb 4d core levels. The GaAs(110)p(1 × 1)-Sb(1 ML) surface is considered a prototype system to study adsorption of group-V elements on III-V(110) semiconductor surfaces. Several geometrical models are usually considered in the literature as physically plausible for this surface [5,9]. Among these, the epitaxial continued layer structure (ECLS) is the generally accepted model, as it has been strongly supported by several experimental as well as theoretical studies [6]. However, total energy minimization calculations have shown that the epitaxial on-top structure (EOTS) forms a stable geometry with a total energy similar to that of the ECLS model [7]. Both of them are depicted in Fig. 1. In the ECLS model, the Sb atoms tend to locate in the positions that the As and the Ga atoms would occupy in the next layer of the zincblende structure. On the other hand, the adsorbate atoms in the EOTS model occupy on-top positions. Note that the ECLS model relaxes in the EOTS one by continuously increasing the size Sb zig-zag chains along the (001) direction. In addition, both models saturate all dangling bonds and, as a consequence, the Ga and As atoms in the first substrate layers are expected to occupy approximately bulk positions. In this paper, we determine the registry of the Sb atoms with respect to the substrate by means of a
2. Experimental details The experiments were carried out at the Laboratoire pour l'Utilization du Rayonnement ElectromagnOique (LURE) (Orsay, France), using the Spanish-French Station connected at the SU7 ondulator beamline of the Super-Aco storage ring. Fresh G a A s ( l l 0 ) surfaces were obtained using the cleavage method under ultra-high vacuum conditions (base pressure 10-~0 mbar). The clean substrate was fully oriented within + 1° using Polar Plot PD and LEED. The sample was prepared by evaporating several ML of Sb (5 ML) on the substrate kept at room temperature and, then, annealing the sample at 330 ° for 10 min to remove the Sb in excess of 1 ML. The PD experiment was carried out by recording Sb 4d energy spectra as a function of the photon energy for different emission directions. The measurements spanned a photon energy range of 150-280 eV, which corresponds to an electron kinetic-energy range of 110-240 eV. The polar angle along the [001] and [ 0 0 - 1] directions was scanned from 0 ° to 40 ° with a typical step of 10°. The (110) azimuth was scanned from 0 ° to 30 °, since it is well established that the (001) is a mirror plane of the p(1 × 1)-Sb.
3. Results and conclusions Fig. 2 shows a typical Sb 4d core-level photoelectron emission spectrum measured at 150 eV photon energy. The two well-resolved peaks correspond to 4ds/: and 4d3/2 spin-orbit split states. The peak clearly displays two components which are interpreted in the literature as due to a chemical shift between two inequivalent adsorption sites for the Sb atoms [8]. The continuous and dashed lines correspond to the low- and high-binding energy components (LBE and HBE) obtained by fitting the experimental spectrum with gaussian line shapes. The relative intensity of these two components varies with
H. Ascolani et al. / Applied Surface Science 123 / 124 (1998) 223 -227 both the photon energy and the emission direction. All fittings yielded an energy separation between the two components of 0.45 eV, in good agreement with previous results [8]. To isolate the diffraction effects associated with the atomic structure we define the modulation functions:
Xa(~i,k)= (I°~(~i'k)-lO(~i'k)) where U ( k i, k) is the intensity of the LBE or the HBE component o f the Sb 4d5/2 signal measured along the direction k i, and lo(k i, k) is a background intensity. Fig. 3 shows the X nBE and X LBE curves corresponding to different emission directions contained in the plane (110), i.e. the mirror plane of the surface. Positive polar angles indicate emission along the [001] azimuth, while the negatives indicate emission along the [001] azimuth. As seen in the figure, the diffraction effects contained in the LBE and HBE components are clearly different, indicating that they originate in Sb atoms in different atomic environments. It is known that, at low kinetic energies, the strongest modulations in scanned-energy PD occur for those emission directions in which a substrate atom lies exactly behind the emitter [1-3]. Therefore, the strong modulation of X nBE at 2 0 - 3 0 ° suggests that the Sb atoms which contribute to this component have a Ga or As atom below them for this geometry detection. Similarly, the strong modulation of X I~BE at - 2 0 to - 2 5 ° suggests that the Sb atoms which contribute to this component have a Ga or As atom below them in line with the e m i t t e r - d e tector direction. The emission direction corresponding to a backscattering geometry can be determined on a more objective basis by evaluating the Fourier Transform (FT), u'~(kl, p ) of t h e )(e~p(ki, k) curves. The FT will present strong maxima in positions (kN, PN), where kN is an emission direction parallel to an internuclear axis passing through the emitter and through a near-neighbor atom located at a distance R N = P N / 2 from the emitter [1-3]. In fact, PN gives approximately the path length difference between the wave which goes directly from the emitter to the detector and the single-scattering wave centered on
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Kinetic Energy (eV) Fig. 2. Sb 4d core-level spectra of GaAs(110)p(l × 1)-Sb(IML) obtained at 150 eV photon energy alter background subtraction. The circles are data points and the curves are results of a fitting to the data using two Gaussian line-shapes.
the neighbor atom at A'N =RN~:N. Fig. 4 shows contour plots of the f u n c t i o n s [b/LBE(]£i, /3)[ and [UHBE(~:i, p)l. Those in panels (a) and (b) correspond to emission directions in the plane (110) of the surface. As seen in these figures, the LBE compo-
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226
H. Ascolani et al./ Applied Surface Science 123 / 124 (1998) 223-227
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Fig. 4. Contour plots of the Fourier transforms [/ALBE(ki, p)] (a, C) and [uHBE(kl, p)] (b, d) of the experimental modulation functions corresponding to the low- and high-binding energy components, respectively. The emission directions in the upper panels of the figure are contained in the plane (110) (the mirror plane of the surface). On the other hand, in panels (c) and (d) the emission directions are in the plane (001) (perpendicular to the mirror plane of the surface). The lower part of the figure shows top views of geometric arrangements deduced from the contour plots in (a) and (b). More details in the text.
nent has a strong maximum at 25 ° off normal along the [001] azimuth, while the HBE one has a maximum at 30 ° off normal but along the [001]. A path length difference of approximately 5 A, i.e. an interatomic distance of ~ 2.5 A, indicates that both maxima are originated by substrate atoms in the first surface layer, i.e. Ga or As atoms. Thus, from Fig. 4(a) and (b) it follows that: (i) the Sb atoms which contribute to the LBE component have a substrate near-neighbor atom located in the direction (25 °, [00~]), and (ii) the Sb atoms which originate the HBE component have a substrate near-neighbor atom
located in the direction (30 °, [001]). The two possibilities of accommodating the Sb atoms on the G a A s ( l l 0 ) surface which are consistent with the contour plots in Fig. 4(a) and (b) are in Fig. 4(e) and (f). The structure in Fig. 4(e) corresponds to the assumption that the LBE (HBE) component originates in Sb atoms bonded to Ga (As) atoms, while the structure in Fig. 4(0 implies the inverse relationship. On the other hand, Fig. 4(c) and (d) show contour plots of the FT corresponding to emission in the plane (001), which is perpendicular to the mirror
H. Ascolani et al. / Applied Surface Science 123/124 (1998) 223 227
plane of the surface. The low FT intensities clearly indicate that the diffraction effects contained in both the LBE and HBE components are very small along this azimuth. However, although the structure in Fig. 4(e) is compatible with this observation, the one in Fig. 4(f) is not. In the latter, the Sb atoms bonded to Ga (As) are located practically at a bridge position between two As (Ga) atoms and, as a consequence, there should be important diffraction effects along the [110] azimuth. We therefore conclude that the geometrical arrangement shown in Fig. 4(e) is the only one compatible with the contour plots in Fig. 4(a-d). Very interestingly, this structure corresponds to the ECLS model illustrated in Fig. l(a). Furthermore, we can associate unequivocally LBE ~ (SbGa) and HBE -~ (Sb-As). Finally, we complete the analysis by comparing the contour plots in Fig. 4 with the other models proposed for this surface. These are the EOTS, the Epitaxial Overlapping Chain Structure (EOCS), the p3 and the dimer models. The EOTS model (illustrated in Fig. l(b)) is clearly incompatible with the GaAs(110)p(1 × 1)-Sb(1 ML) surface. According to this model the Sb atoms occupy on-top positions; however, as seen in Fig. 4(a) and (b), the directions of the bonds (Sb-Ga) and (Sb-As) are at least 15° off normal, i.e. clearly away from the on-top position. Similarly, the EOCS model, which corresponds to the structure in Fig. 4(f), is evidently inconsistent with the surface. The p3 model is also clearly incompatible with the contour plots in Fig. 4(a) and (b), since according to this model both maxima should appear at the same side of the surface normal. The same conclusion is valid for the dimer model. Therefore, we conclude that the EOTS, the EOCS, the p3 and the dimer models must be discarded. In conclusion, evaluating a simple Fourier transform of the scanned-energy PD data corresponding to both chemically-shifted components of the Sb 4d
227
peak, we have unambiguously determined that the ECLS model is the only one consistent with the GaAs(110)p(l X 1)-Sb(1 ML) surface. The simplicity of the procedure and the clarity of the results prove the utility of direct methods for studying the geometry of interfaces with multiple adsorption sites.
Acknowledgements The authors are pleased to acknowledge stimulating discussions with P.D. Woodruff and A.M. Bradshaw. This work has been supported by the DGICYT under grant PB-94-0022-C02.01 and the Large-Scale Installation Program at LURE. H. Ascolani acknowledges a post-doctoral grant from the Spanish Ministry of Education and Science. He is a member of the Consejo Nacional de lnvestigaciones Cien6ficas y T~cnicas (CONICET) of Argentina.
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