Buckling Type, Domain Boundaries and Donor Atoms: Atomic Scale Characterization of the Si(111)-2x1 Surface
by Karolin Löser
Date of Examination:2013-01-31
Date of issue:2013-03-01
Advisor:Dr. Martin Wenderoth
Referee:Prof. Dr. Rainer G. Ulbrich
Referee:Prof. Dr. Michael Rohlfing
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Description:Dissertation
Abstract
English
In this thesis, three different aspects of the Si(111)-2x1 surface are investigated in more detail: positive and negative buckling of the pi-bonded chains, mobility of domain boundaries and signatures of phosphorus (P) atoms, which are present due to the n-type doping of the samples. In the first part, domain boundaries are utilised in order to determine whether two Si(111)-2x1 domains are buckled differently considering distances between the ‘up’-atoms of frontally meeting pi-bonded chains of the two domains. dI/dV measurements at 6K of domains with long, defect-free pi-bonded chains of both buckling types do not only show the band gap of pi-bonded chains with positive and negative buckling, but permit also the determination of the relative energetic positions of the surface bands of the two buckling types. The tip induced movement of domain boundaries between positively and negatively buckled domains is treated in the second part of this thesis. At low positive and negative voltages, the position of a domain boundary may be reversibly altered by up to 8nm in favour of the domain with negative buckling. The movement is largest in voltage regions where surface states are available for pi- bonded chains with negative buckling but not for positively buckled pi-bonded chains. The extent of movement is not influenced by the amount of tunnelling current or the tip sample distance. As a comparison of measurements with the tip scanning parallel to the pi-bonded chains to measurements with a perpendicular scan direction indicates, the trigger for the movement is transmitted along the pi-bonded chains and suppressed perpendicular to them. The last and major part of this thesis is concerned with the signatures of the dopant phosphorus atoms. These atoms are located, statistically distributed, at substitutional sites in bulk and surface. The Si(111)-2x1 surface offers four different sites, and all signatures of P atoms in the surface layer can be assigned to one of these sites. P atoms induce the same contrast pattern for positive and negative buckling, except for the voltage values at which the contrast patterns occur. These depend on the surface states which are at different energetic positions for the two buckling types. We investigated pi-bonded chains with a length from 10nm up to more than 1 mm. In these limits, the chain length does not affect the topographic contrast patterns of the signatures induced by P atoms at the different surface sites. Signatures of P atoms beneath the Si(111)-2x1 surface are attributed to three subsurface layers. This is supported by counts of signatures which show even numbers for signatures of P atoms in the surface layer and in the each of the three subsurface layers, as can be expected from a large number of statistically distributed donor atoms. The origin of the contrast patterns at low negative voltages, which extend over 8nm in pi-bonded chain direction, cannot be the defect state of the P atoms alone, as this defect state is highly localised within less than one surface unit cell. The comparison of dI/dV measurements of P induced signatures to dI/dV spectra of single metal atoms on metal surfaces shows that the origin of these signatures are bound states which split-off from the surface states. As electrons at the band edges of the surface states must have a minimal wavelength of l = 7.4nm in pi-bonded chain direction, the bound states also show this spatial extension. We can distinguish between three cases where a bound state (or resonance) occurs: The potential of P atoms located in the surface layer of free pi-bonded chains causes a bound state which splits-off from the SCB and is located directly beneath the surface band minimum. The potential of subsurface P atoms is weaker and results in a surface state resonance in the lower part of the SCB. The third case are P atoms in the surface layer of short pi-bonded chains. Their potential also induces a bound state, but this bound state is split-off from the SVB instead of the SCB, due to the presence of a Coulomb gap at EF which prevents the coupling between the potential of the P atom and the SCB.
Keywords: Scanning Tunneling Microscopy; Si(111)-2x1 surface reconstruction; Donor Atoms; Buckling Type; Domain boundary