Derivative photoelectron holography of As/Si(001)

Transcription

1 Surface Science 445 (2000) Derivative photoelectron holography of As/Si(001) Paul J.E. Reese a,b, T. Miller a,b, T.-C. Chiang a,b,* a Department of Physics, University of Illinois at Urbana-Champaign, 1110 West Green Street, Urbana, IL , USA b Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, 104 South Goodwin Avenue, Urbana, IL , USA Received 29 July 1999; accepted for publication 18 October 1999 Abstract The new technique of derivative photoelectron holography is used to examine the As-terminated Si(001) surface, which exhibits a two-domain (2 1) reconstruction. Logarithmic derivatives of the photoemission intensity are measured, from which the intensity fine structure function is deduced. Since the logarithmic derivative function is independent of the incident beam intensity and the detection efficiency, most experimental uncertainties are eliminated. The resulting atomic images are in good agreement with expectation Elsevier Science B.V. All rights reserved. Keywords: Adatoms; Arsenic; Photoelectron holography; Photoemission; Silicon; Surface structure, morphology, roughness, and topography 1. Introduction the accuracy of the technique. So far, it has been demonstrated in just one system, As-terminated Photoelectron holography has been demon- Si(111) [18]. To assess the effectiveness of this strated to be a powerful tool for studying the technique, the present study is an application of surface structure of many solid systems [1 17]. the same technique to As-terminated Si(100). The This technique makes use of the diffraction effects structure of the latter involves a different lattice inherent in photoelectrons emitted during illumina- symmetry, and is more complicated due to its twotion of the sample by vacuum ultraviolet and soft domain ( 2 1) dimer reconstruction. As will be X-rays to directly determine the location of a shown, the resulting atomic images are in good single species of emitter atom relative to nearby accord with expectation. This is further evidence subsurface atoms. Conventional photoelectron that the derivative technique is a powerful method holographic techniques rely on direct intensity of general utility for surface atomic structure measurement, and are poorly able to handle the determination. often unavoidable breaks in data acquisition, The As/Si(001) surface has been heavily studied which can result in data uncertainty and image due to its importance as a precursor for GaAs degradation. The new technique of derivative growth and for its corresponding importance in photoelectron holography has been developed as technological development [19]. It has also been a tool to handle such cases and thereby improve studied due to the use of As as a surfactant in heteroepitaxial growth of Ge on Si [ 20,21]. A * Corresponding author. Fax: monolayer of As can first be deposited, before address: t-chiang@uiuc.edu (T.-C. Chiang) growing overlayers of Ge on a Si substrate to /00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S ( 99 )

2 P.J.E. Reese et al. / Surface Science 445 (2000) try of the Si(001)-( 2 1) surface. These symmetry operations expand the number of emission direc- tions to 128, resulting in a total of 5120 points in k-space within the range 2.8 Å 1<k<5.9 Å 1. allow much thicker Ge adlayers without roughening due to three-dimensional growth. This initial layer structure has been found to consist of As dimers, which break the Si surface dimerization. This leaves a bulk-like Si( 001) substrate structure terminated by As dimer rows, and the resulting surface resembles the original (2 1) reconstructed clean Si surface, but is left passivated due to the saturation of all dangling bonds. The dimer bond length and interlayer relaxation have been explored by techniques including theoretical calculation [22,23] and X-ray standing wave analysis [24]. 2. Experimental details where k is the photoelectron wave vector (deter- mined by the photon energy). One then performs a three-dimensional inverse holographic transform on the x function to return a three-dimensional image of the nearby atomic neighbors. This tech- nique has been described previously [1 18]. Because the photoelectron mean free path is rather short, the resulting holographic images are domi- nated by contributions from nearest neighbors. The holographic image gives a view of the surface centered at the locations of the emitting atoms. One must be able to identify the signal from a set of emitting atoms in specific surface sites in order for the technique to be useful. This can be done using adsorbate atoms of a different species than the bulk and using their unique core level binding energies to identify their emissions, or surface-shifted core levels from surface atoms of the same species as the bulk. If multiple position- ally inequivalent emitters have the same core level binding energy, the images from these will be superimposed. This is the case for As-terminated Si(001). The two As atoms in each dimer give rise to what will be referred to hereafter as paired images, and furthermore, the two-domain config- uration results in two sets of images rotated 90 with respect to each other. A Si sample was cut from a commercially available wafer with (001) orientation. The sample was inserted into an ultrahigh vacuum chamber with a base pressure of better than Torr and was outgassed for 12 h at 600 C. The sample was then heated at 1300 C for 8 s which resulted in a clean two-domain (2 1) diffraction pattern. An effusion cell was used to deposit 10 monolayers (ML) of As on the surface. During this time, the sample was maintained at a temperature of 385 C by passing a current through the sample, and the surface As coverage saturated at 1 ML. The sample was post-annealed for 2 min at the same temperature. Subsequent post-anneals at this temperature were seen to have no effect on the resulting twodomain ( 2 1) diffraction pattern, nor on the shape or normalized intensity of the photoemission spectra of the As 3d and Si 2p core levels. This indicates that the As coverage of the Si substrate remained unchanged by annealing at this temperature. Photoemission spectra were collected at the 1 GeV storage ring Aladdin at the Synchrotron Radiation Center (Stoughton, WI). A total of 16 emission directions was used in data collection, and for each direction 40 As 3d spectra were taken using a hemispherical analyzer with an angular acceptance of ±1.5. The polar emission angles covered a range of 0 <h<70 relative to the surface normal. The azimuthal angles covered a range of 0 <w<45 relative to the [110] direction. The data set was expanded using the fourfold rotation symmetry and appropriate mirror symme- 3. Derivative method of holographic analysis Photoelectron holography involves taking meas- urements of photoemission intensity, I, ink-space at a selected emission angle, kˆ, for numerous photon energies. The measurements are repeated for a number of different emission directions. For each emission direction, a smooth background, I, is 0 subtracted and used for normalization to leave only the oscillatory part of the data. This is the intensity fine structure function x, definedas: x(k, kˆ)= I(k, kˆ) I (k, kˆ) 0, I (k, kˆ) 0

3 402 P.J.E. Reese et al. / Surface Science 445 (2000) The derivative method of photoelectron holography has been introduced recently [18]. This technique uses data collected at a fixed small interval 2d in k. We can then compare these nearby points and calculate the logarithmic derivative function, L: I (k, kˆ) L(k, kˆ) I(k, kˆ) = I(k+d, kˆ) I(k d, kˆ) [I(k+d, kˆ)+i(k d, kˆ)]d = I(k+d, kˆ) I(k d, kˆ) 1 CI(k+d, kˆ) I(k d, kˆ) +1 D d and proceed to extract the x function and to determine the image function as per conventional holography techniques [1 18]. In the present analysis, we use a sweeping cone method in the image transform [2 4,7 10,15,16 ], and the scattering phase shift is simply ignored. This is justified because the scattering phase shift for Si is fairly constant for this energy range, and a numerical simulation suggests that the resulting error in interatomic distance is no more than 0.1 Å [18]. 4. Results A typical set of As 3d core level spectra for a fixed emission direction (h=50 and w=27 ) is. Since this depends on the ratio between two mea- sured intensities, it is independent of the incident beam intensity and the detector efficiency. Changes in experimental settings (such as the analyzer pass energy and monochromator slits and gratings) can have a direct effect on the measured intensity itself, but not on the ratio. Thus, values of logarithmic derivatives can be determined much more precisely, and data taken from different monochromators, different settings, and different runs can be joined together directly. Once the logarithmic derivative function has been determined one can calculate the intensity function itself: I(k, kˆ)=exp L(k, kˆ)dk CP D, Fig. 1. A set of As 3d photoemission spectra taken using different photon energies plotted as a function of photoelectron kinetic energy for the emission direction h=50 and w=27. The dashed curve indicates the integrated photoemission intensities as a function of photoelectron kinetic energy. presented in Fig. 1. A jump in the intensity due to changing the monochromator grating can be seen at a kinetic energy of 38 ev. Analysis using conven- tional photoelectron holography methods would require a renormalization of the measured intensity in order to splice together the two sets of data across the intensity jump. This renormalization tends to introduce error, and the error can propagate if addition intensity renormalization steps are taken. Using the derivative technique, there is no need for such renormalization. It is clear from Fig. 1 that, besides the jump, the measured inten- sity shows a modulation as a function of kinetic energy, as suggested by the dashed curve. This modulation, caused by photoelectron diffraction, is the quantity of interest. Fig. 2a shows a typical logarithmic derivative function for an arbitrarily chosen emission direc- tion (h=50 and w=27 ). The corresponding x function is shown as the solid curve in Fig. 2b. To gauge the experimental uncertainty, we have taken data along a different direction (h=50 and w= 63 ) that is, however, related to the first one by a mirror plane symmetry. The corresponding x function is shown as the dotted curve in Fig. 2b. The two symmetry-related x functions closely match each other in terms of the oscillation periods. This

4 P.J.E. Reese et al. / Surface Science 445 (2000) Fig. 2. (a) The logarithmic derivative function for the emission direction h=50 and w=27 ; (b) the corresponding x function (solid curve) as well as a x function for h=50 and w=63 (dotted curve) which is related to the former by a mirror symmetry. is expected, and the differences are a measure of the experimental uncertainty. Fig. 3 shows planar slices of the image function using a gray scale representation. Included in the figure are a number of + symbols that correspond to the expected positions of intensity maxima. These intensity maxima correspond to either atomic positions or saddle points between nearby paired atomic positions (one in front of the page and the other behind the page). Because the holographic data represent discrete sampling over a limited region in k space, each atom is represented by a broadened intensity maximum. A saddle point between two nearby atomic positions is an intensity maximum in a plane that bisects the interatomic axis. A schematic model of the dimer reconstruction is shown in Fig. 4 that will aid in the discussion of the images in Fig. 3. Each As adatom is bonded below by two Si atoms in an xz plane that is offset either in the +y or the y direction by 0.65 Å due to the dimerization [22 24]. The holographic image should be a linear combination of these two cases, and the result is a set of paired Si atomic images at ±0.65 Å lined up along the y direction. This is for just one of the two possible domain orientations. The other domain and the corresponding atomic images are rotated by 90. Fig. 3a is a vertical xz planar cut ( y=0) through the As emitter position. None of the Si atoms in the first layer below the As emitter is within this plane, and the black crosses represent the saddle points associated with the paired Si images as discussed above. These correspond very well to the main intensity maxima in the image. The Si atoms in the second layer below the emitter (z= 2.7 Å) are far from this plane and therefore not visible here. The white crosses in Fig. 3a represent the expected atomic positions in the third layer involving two sets of paired images lying in this plane. There may be a hint of their contribution in the image, but the positions are noticeably off and the contrast is low. Detecting distant neighbors accurately is difficult partly due to the very rapid attenuation of the photoelectron diffraction signal as a function of depth below the surface. For the third layer atoms, the attenuation is so great that their contribution to the diffraction signal is of the same order as the uncertainty seen in Fig. 2b. Fig. 3b is an xy planar cut at z= 1.4 Å. This plane should contain four sets of paired Si images in the first layer below the As emitter as indicated by the black crosses. These crosses correspond well to the observed main intensity maxima although the pairs in each set are not resolved. As mentioned above, the finite data range and discrete sampling in k space lead to broadening of the atomic images. An estimate based on the angular resolution of the data set gives a spatial resolution of about ±0.6 Å at the first neighbor positions. This is insufficient to resolve the paired atomic images separated by just ±0.65 Å. This spatial resolution is expected to get proportionally worse as we move farther away from the emitter, which explains the progressive diffusiveness of the image in Fig. 3a away from the origin. Fig. 3c is a vertical xz cut at y=1.9 Å. This plane should cut through one of the four sets of pairs in the first layer below the emitter. The black crosses indicate the expected positions of the pairs. Again, they are in good agreement with the

5 404 P.J.E. Reese et al. / Surface Science 445 (2000) Fig. 3. Cuts through various planes of the holographic image of As/Si(001). The tick marks indicate distances from the emitter in angstroms: (a) through the emitter in a vertical (100) plane; (b) through a horizontal (001) plane 1.4 Å below the emitter, at the expected first Si layer position; (c) through a vertical (100) plane offset in y by 1.9 Å from the emitter; (d) through a (001) plane 2.9 Å below the emitter, at the expected second Si layer position. + indicates the expected positions of intensity maxima. The same linear gray scale is used for all images. observed main intensity maximum. However, the two members are not resolved as discussed above. The white crosses indicate the expected atomic positions of the two sets of pairs in the second layer below the emitter. The images become quite diffuse, and the contrast is diminished for reasons already discussed above. Fig. 3d is a horizontal xy cut at z= 2.9 Å. A total of 16 atomic images in the second layer below the emitter is expected as indicated by the white crosses. Although the exper- imental results are consistent with the structure, the image is too diffuse for a precise atomic position determination. 5. Summary and conclusions The results of our photoelectron holography study illustrate that the derivative technique is useful for avoiding intensity normalization prob-

6 P.J.E. Reese et al. / Surface Science 445 (2000) this evidence is derived from indirect methods. The main advantage of holography is the direct approach involving no a priori assumption of the structure, and the present results provide a useful confirmation of the established structure. Acknowledgements This work was supported by the US Department of Energy (Division of Materials Sciences, Office of Basic Energy Sciences) under Grant No. DEFG02-91ER The Synchrotron Radiation Center of the University of Wisconsin-Madison is supported by the US National Science Foundation Grant No. DMR An acknowledgement is also made to the Donors of the Petroleum Research Fund, administered by the American Chemical Society, and to the US National Science Foundation Grant Nos. DMR and for partial personnel and equipment support in connection with the synchrotron beamline operation. Fig. 4. A model of the As/Si(001) dimer system. Only one domain orientation is shown. References lems caused by changes in beam intensity and [1] J.J. Barton, Phys. Rev. Lett. 67 (1991) [2] H. Wu, G.J. Lapeyre, H. Huang, S.Y. Tong, Phys. Rev. detection efficiency. This technique is applied to Lett. 71 (1993) 251. As/Si(100)-( 2 1). Due to the dimerization of the [3] H. Wu, G.J. Lapeyre, Phys. Rev. B 51 (1995) As and the presence of two domain orientations, [4] G.J. Lapeyre, Nucl. Intrum. Meth. A 347 (1994) 17. the holographic images for this system are signifi- [5] S.Y. Tong, C.M. Wei, T.C. Zhao, H. Huang, H. Li, Phys. cantly more complicated than the simple (1 1) Rev. Lett. 66 (1991) 60. [6] S.Y. Tong, H. Li, H. Huang, Phys. Rev. B 46 (1992) unreconstructed As/Si(111) surface that is the only [7] J.G. Tobin, G.D. Waddill, Surf. Rev. Lett. 1 (1994) 297. other system studied so far using the derivative [8] S.Y. Tong, H. Li, H. Huang, Surf. Rev. Lett. 1 (1994) 303. technique. The images presented in this work are [9] K. Heinz, H. Wedler, Surf. Rev. Lett. 1 (1994) 319. dominated by first nearest neighbors, and the [10] C.M. Wei, I.H. Hong, Y.C. Chou, Surf. Rev. Lett. 1 positions of the observed intensity maxima are in (1994) 335. [11] M.T. Sieger, J.M. Roesler, D.-S. Lin, T. Miller, T.-C. good agreement with the known structure. Images Chiang, Phys. Rev. Lett. 73 (1994) of more distant atomic neighbors become more [12] J.M. Roesler, M.T. Sieger, T. Miller, T.-C. Chiang, Surf. diffuse, and the contrast diminishes rapidly. The Sci. 380 (1997) L485. reasons are a rapid attenuation of the photoemis- [13] J.G. Tobin, G.D. Waddill, H. Li, S.Y. Tong, Phys. Rev. sion signal below the surface and a finite and Lett. 70 (1993) [14] L.J. Terminello, J.J. Barton, D.A. Lapiano-Smith, Phys. discrete sampling in k and in emission angles. Rev. Lett. 70 (1993) 599. Although the structure of As/Si(100) is considered [15] G.R. Harp, D.K. Saldin, B.P. Tonner, Phys. Rev. Lett. 65 known based on overwhelming evidence, much of (1990) 1012.

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