Strain modification of surface electronic states measured with scanning tunnelling spectroscopy

Abstract

Scanning tunnelling microscopy is a surface science tool that allows topographic imaging of a conductive surface on an atomic scale, enabling real space visualisation of surface reconstructions. As it is based on quantum tunnelling to and from electronic states on the surface, it is also useful in mapping local variations in surface electronic structures. In this thesis, we have developed an STM-compatible sample stage capable of applying uniaxial strain to a sample surface in situ. This stage works by clamping a cantilever sample at one end while the other end is deflected by use of a small piezoelectric slip-stick actuator. Deflection of the free end upwards induces compressive strain on the top surface of the sample, while downward deflection induces tensile strain. In order to anneal samples within the ultra-high vacuum (UHV) system to remove native oxide and prepare clean surface reconstructions, the stage was designed to allow direct current heating of the sample while under minimal deflection at the free end. Design challenges included minimising heat transfer to the actuator during annealing and reducing resistance of the contact made at the sample free end to avoid local melting while working within a confined volume afforded by the Createc STM used in this study. Following the development of the strain stage, it was employed to study the effects of strain on the electronic structure of the Si(111) 7?7 surface. The 7?7 cell contains 6 distinct surface sites that each occur in groups of three equivalent sites with a local C3 symmetry. These sites were investigated under both compressive and tensile strain using scanning tunnelling spectroscopy and conductance imaging. We did not observe electronic symmetry breaking between these equivalent sites based on the direction of the applied uniaxial strain at magnitudes of ~0.17%. However, we found evidence of a general shifting of unfilled electronic peaks to higher biases as well as peak broadening under compressive strain and a narrowing and slight reduction of peak energies of those states under tensile strain. This was supported by conductance imaging which showed that successive surface states appeared at higher biases under compressive strain. Beyond observing the strain effects on the 7?7 cell, the high spatial density of the recorded spectra, as well as the high signal-to-noise ratio afforded by the variable height spectroscopy method, enabled us to observe the spectral features of the surface in great detail. We provide further evidence for the splitting of the middle adatom unfilled state into two distinct peaks, previously observed at 7 K. We also found evidence of a high density of states located on the faulted corner adatoms around the Fermi energy. Strain experiments were also performed on Si(100). This surface is composed of parallel rows of dimers which run in perpendicular directions on terraces separated by an atomic step. Preliminary spectroscopic investigation shows a shift in the energy of the D_1^* peak, which is composed of a mixture of states from the dimer antibonds, as well as the backbonds accessible to the STM between dimer rows. The peak shifts are observed primarily over dimer rows, where the backbond contribution is minimal, indicating that the shift is related to the dimer bond. Further investigation is needed to characterise the spatial variation of this peak.