Model-Independent Structure Determination of the InSb(111)2×2 Surface with Use of Synchrotron X-Ray Diffraction

Abstract

We have measured accurate x-ray crystallographic data from the InSb(l 11)2x2 reconstructed surface using synchrotron radiation from the DORIS storage ring at DESY. We have analyzed these independently of all models and find a structure with seven atoms in the unit cell, implying that one site is unoccupied. The bonding configuration is reasonable for a III-V semiconductor surface, and is topologically identical (in projection) to the "vacancy-buckling" model proposed for the GaAs(lll)2x 2 surface. The differences between the InSb and GaAs structures are significant and reflect chemical trends in the periodic table.PACS numbers: 68.20.+t, 61.10.FrOrbital rehybridization is currently considered to be a major determinant of the atomic geometry in compound-semiconductor surfaces. The III-V(llO) surfaces, well understood from low-energy electrondiffraction (LEED) studies, 1 buckle under this influence to reduce the bond angles at the surface group-V element and increase those at the group-Ill element, demonstrating ;>-type and sp 2 -type bonding tendencies, respectively. 2 A buckling model has also been proposed by Haneman 3 to allow similar rehybridization in the reconstructed GaAsO 11)2x2 surface, but Chadi 4 has shown that the buckled arrangement is unstable to the removal of one surface Ga atom. The vacancy soformed has been identified in a recent LEED structure analysis of GaAs(lll), 5 which shows dramatic inplane buckling of the remaining seven atoms in the surface unit cell. Because glancing-incidence x-ray diffraction is highly sensitive to in-plane surface structures, 6 it is well-suited to this problem, and so we applied it to the InSb(lll)2x 2 reconstructed surface and determined the atomic arrangement. Our analysis includes no prior knowledge of these models and derives directly a seven-atom structure of the projected unit cell. We are able even to identify the atom types and conclude that it is an In atom missing, implicating "vacancy buckling" in InSb(lll) also.Precisely cut, oriented, and polished InSb crystals were cleaned in 2x 10~1 0 mbar by repeated cycles of sputtering with 500-eV Ar + ions and annealing at 420 °C for several hours. The (111) surface gave a sharp 2x2 LEED pattern, while the (111) surface was 3x3; the latter is discussed elsewhere and not considered further here. 7 Valence-band energy distribu-tions curves were recorded at /zv = 30 eV with use of synchrotron light from the storage ring DORIS at Deutsches Elektronen-Synchrotron in Hamburg; these were found to be much more sensitive to surface contamination than conventional Auger spectra, and showed the surface to be clean. The sample was then transferred in UHV to a small, ion-pumped diffraction cell with a 360° beryllium window and was mounted on a diffractometer on the adjacent beam line.The white synchrotron beam was vertically monochromated by parallel Ge(lll) crystals and filtered by total reflection from a horizontal flat Au-coated mirrror. The diffraction took place in the horizontal plane of an air table which was tilted wit...

Structure Factor Determination in Surface X-ray Diffraction

“…The former is usually constant and can be ignored for in-plane measurements, while the sin 28 dependence of the latter acts like a second Lorentz factor. The procedure implicit in equation (3) has been used in several in-plane surface structure determinations with success (Bohr et al 1985;Feidenhans'l et al 1987;Robinson et al 1988). Agreement between calculated and observed intensities is in the 5-10% range.…”

X-ray structure analysis of the InSb ()-(3 × 3) reconstruction

“…In contrast to the In terminated (1 1 1) surface which has a (2 x 2) reconstruction [1] little is known about the atomic structure of this polar surface. From total energy calculations [2] no vacancies are expected which have been observed for InSb (1 1 1) (2 × 2) and GaAs (1 1 1) (2 × 2) [3].…”

“…This is in correspondence to the (2 x 2) reconstruction. However, in this case rehybridization is made possible by creation of a vacancy and inward relaxing of the InSb double layer [1].…”

Ultrahigh-Vacuum Facility for High-Resolution Grazing-Angle X-ray Diffraction at a Vertical Wiggler Source of Synchrotron Radiation

“…The application of these instruments and others has been reviewed by Zegenhagen (1993). There is no facility, however, to the authors' knowledge, that allows both SXD and XSW measurements, except the baby chamber designed by Bohr et al (1985). Sakata & Hashizume (1995a) fabricated a compact chamber for grazing-angle XSW (GAXSW) experiments on beamline 14B at the Photon Factory using vertically polarized X-rays and horizontal scattering geometry.…”

“…Actually, this was not so compact since two sputterion pumps were used. A rather compact chamber, 15 kg in weight, was designed and used to measure an InSb(111) 2 Â 2 surface by Bohr et al (1985).…”