ULTRA HIGH VACCUUM-SCANNING TUNNELLING MICROSCOPY STUDY OF RECONSTRUCTED ATOMIC CELLS GROWN BY MBE ON GaAs (001).
An UHV Omicron AFM-STM system (P<10-10Torr), is coupled in UHV to our MBE Growth System Veeco Gen II. W tunnel tips are annealed at 450 ° C in a specific UHV module. A fully molybdenum sample/tip holder has been designed and fabricated to optimize the structures epitaxy using in situ surface study by RHEED diffraction on all azimuths before transfer to the STM chamber. The specific wire geometry of the GEN II CAR allows avoiding any back opening in the sample holder. So the thermocouple is close to it and maintained nested in molybdenum allowing accurate temperature regulation. The sample remains nearly centred allowing pyrometric measurements. Thanks to the remarkable GEN II CAR thermal shielding, we are able to get epitaxy and surface monitoring at 600°C with 170W heating power only. The remarquable mechanical stability of our coupled MBE-STM system was first demonstrated by a detailed study by D. Martrou (now with CEMES) of the GaAs substrate surface structure in the minimized As overpressure growth regime, which we find since 1988 optimum for the achievement of high mobility modulation doped low 2DEG density heterostructures, because of lowest background impurity content. Fig1 left, displays the corresponding 1/3 RHEED diffraction allowing to monitor this (3x1) RHEED surface. The corresponding STM measurements, (Fig.1 center and right), show that the surface organization is quite inhomogeneous with a surprising organization of different types of surface atomic reconstructions. The usually ubiquitous surface As dimers are now organized in rather scare (6x6)cells, with some short (<50nm) [1 0] straight rows localized in the the vicinity of terraces, and separated by tilted and zigzagging subsurface surface dimmers. These rows are separated by terraces without any dimers displaying either an inhomogeneous mixture of As atoms in (1x1) cells with isolated A(3x2) cells, Fig.3 in ref.(1].In other area these cell organize next to each other along [110], giving a bright row distinctive in STM scans, Fig.2 below. None of these three surface cells follow the Electron Counting Rule (ECR), so successful to explain the 25% missing dimers of the As-rich (2x4) surface The clue for understanding how this complex surface acts to reduce the unintentional impurity background is obtained making a careful statistics of all surface and subsurface atom bonds [2]. Determining their atomic structure, the overall observation allows getting a statistics of subsurface Ga and As atoms sp2 or sp3 hybridizations leading to predict a significant reduction of surface reactivity with contaminants C atoms. This result in a reduction by 27% in background impurity content explaining the increase of electron mobility observed by us long ago using this weakly As-rich growth regime.

Figure 1: Left, RHEED diffraction along [110] on the (1/3) GaAs surface, obtained around Ts=600°C, by decreasing theAs4 flux under RHEED Control. Center and Right, UHV-STM scans by D. Martrou (filled states, Vsub =-2.4V, It =270pA). b), [1 0] As dimer rows on the lower terrace. b) terrace with (3x2) cell rows, structure in [1].

Figure 2: Proposed (3x2) cell structure obtained from high resolution STM scan of isolated cells [1]. Two faulted sp3 As atoms (F) move in the surface towards some neighbouring Ga row. Two Ga atoms are displaced vertically getting a nearly perfect planar sp2 hybridization. This atom pattern explains quite well the STM crux appearance of isolated (3x2) cells. The surface becomes non polar. The occurrence of subsurface Ga dimerization is fully rejected.

A recent diffraction experiment GIXRD done by a research group of the French Synchrotron Soleil have revealed that Si dopant atoms on Gas (001) bind to (3x2) surface cells. The accurate data confirm STM (3x2) atomic pattern [1], with 0.6 Å top As-Ga height difference.

Figure 3: : tructure of the Si-(3x2) GaAs cell accurately determined by GIXRD [3], top view and cross sections. S1 Si dopants are weakly bound to the surface, which explains the strong atomic vertical segregation. With site S1 occupied by Si atoms, ECR becomes fully satisfied.

References
[1] Unreconstructed As atoms mixed with (3x2) cells and (6x6) supercells in low As pressure epitaxy on GaAs (001), David Martrou, Antonella. Cavanna, Franck Natali, Ulf Gennser, et Bernard Etienne, Phys. Rev. B 72, R241307 (2005).
[2] Statistique des liaisons sur la surface (1/3) : ftp://ftp.aip.org/epaps/phys_rev_b/E-PRBMDO-72-R11548/EPAPS-_3x1_.pdf
[3] Large scale atomic ordering on uncovered GaAs(0 0 1) after InAs monolayer capping: Atomic structure of the (12 × 6) reconstruction, A. Ouerghi, A. Cavanna, D. Martrou, Y. Garreau, B. Etienne, Surface Science, 602,1631-1635 (2008).
[4] Atomic structure of the (3 × 2) Si–GaAs (0 0 1) reconstructed surface: A clue to δ doping mechanism derived from in situ grazing incidence X-ray diffraction data, M. Sauvage-Simkin, Y. Garreau, R. Pinchaux, A. Coati, A. Ouerghi, B. Etienne, Surface Science, 604, 415 (2010).