Article pubs.acs.org/JPCC
Water-Induced, Spin-Dependent Defects on the Silicon (001) Surface Phillip V. Smith,† Daniel R. Belcher,† Marian W. Radny,*,†,‡ Leszek Jurczyszyn,§ Steven R. Schofield,∥ and Oliver Warschkow⊥ †
School of Mathematical and Physical Sciences, University of Newcastle, Callaghan 2308, Australia Institute of Physics, Poznan University of Technology, 62-965 Poznan, Poland § Institute of Experimental Physics, University of Wroclaw, pl. Maksa Borna 9, 50-204 Wroclaw, Poland ∥ London Centre for Nanotechnology, University College London, London WC1H OAH, U.K. ⊥ Centre for Quantum Computation and Communication Technology, School of Physics, The University of Sydney, Sydney, Australia, 2006 ‡
ABSTRACT: It is now well established that an isolated C-defect on the Si(001) surface corresponds to a dissociated water molecule with the H and OH species bonded to silicon atoms on the same side of two adjacent surface dimers. In this paper we show that there are two distinct types of such C-defects at both low temperature (∼100 K) and room temperature. These defects differ in the buckling orientation of the Si−Si dimers adjacent to the defect and/or the distribution of their localized electron spins. Changes in the buckling configuration of the bare Si−Si dimers neighboring the defect modifies the interaction between its dangling bonds and the surface and leads to significant variations in the defect charge density distributions.
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INTRODUCTION The ultimate goal of silicon-based semiconductor fabrication technologies is to develop atomic scale devices in which the charge and/or spin of individual atoms can be manipulated. Such manipulation is necessary, for example, for the development of a Si-based solid-state quantum computer1 or the functionalization of Si surfaces using organic molecules for molecular electronics.2 It has been shown that charge ordering on the technologically important Si(001) surface can be achieved by controlling the network of free surface dangling bonds on the hydrogenated surface.3 In this paper we report local changes in the charge density and associated spin ordering within the dangling bond structures that are formed on the clean Si(001) surface by the dissociative adsorption of an isolated water molecule. The adsorption of a single H2O molecule on the dimerized Si(001) surface leads to a commonly observed feature in scanning tunneling microscopy (STM) that is known as a Cdefect.4,5 This feature has been shown to correspond to a H and OH bonded to silicon atoms on the same side of two neighboring surface dimers,5−7 as shown schematically in Figure 1. Choi and Cho8 recently reported density functional calculations of a single C-defect on the Si(001) surface and showed that this defect can exhibit three different spin configurations which they labeled as NM (nonmagnetic), FM (ferromagnetic), and AFM (antiferromagnetic). These configurations were shown to be characterized by different distributions of the electrons and their spins across the dangling bond orbitals of the H and OH chemisorbed dimers. In the NM case, one of these orbitals was doubly occupied, and © 2015 American Chemical Society
Figure 1. Schematic of a C-defect on the Si(001) surface showing the Si, oxygen, and hydrogen atoms as gray, red, and white circles, respectively.
the other empty, while in the other two cases both orbitals were singly occupied with parallel (FM) or antiparallel (AFM) spins. In this paper we report STM images of isolated C-defects on the Si(001) surface and show that these appear as two distinct types at both room temperature and low temperature (∼100 K). It should be noted that these different C-defect types are not a result of either the H or OH moving to neighboring dangling bond sites as has previously been observed9−13 but correspond to different charge and spin density distributions with the H and OH remaining bonded to two adjacent silicon dimer atoms on the same side of the dimer row. Using density functional theory (DFT), and following the work of Choi and Cho,8 we identify these C-defect types as arising from variations in the magnetic spin ordering of the localized electrons within the defect and/or variations in the static buckling orientations of the bare Si−Si dimers Received: February 12, 2015 Revised: April 28, 2015 Published: May 13, 2015 11612
DOI: 10.1021/acs.jpcc.5b01493 J. Phys. Chem. C 2015, 119, 11612−11618
Article
The Journal of Physical Chemistry C
2. In the empty-state STM images, both the room temperature and 115 K defects appear as bright asymmetric, teardrop-
immediately adjacent to the defect. We also describe how the buckling of these bare Si−Si dimers mediates the interaction between the dangling bonds of the defect and the bare surface.
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METHODOLOGY STM data were acquired at room temperature and low temperature using an ultrahigh-vacuum (UHV) Omicron variable temperature STM (VT-STM) and a UHV Omicron low temperature STM (LT-STM), respectively. Both instruments have base pressures below 5 × 10−11 mbar. The STM tips were etched from polycrystalline tungsten wire and outgassed for 2 h at 420 K prior to use. Samples were n-type (antimony-doped) Si(001) crystal wafers prepared by degassing overnight in UHV by heating the sample to 550 °C, followed by several cycles of rapid annealing to 1100 °C by direct current sample heating. Samples prepared in this way exhibited a surface defect concentration
