A custom-built ultra-high-vacuum-low-temperature-scanning-tunneling-microscope (UHV-LT-STM) is used for imaging, manipulation, and spectroscopy to study novel surface properties of Ag(111) and GaN(000-1) surfaces at low temperatures.
The evolution of the surface state and the effect of vacancies on the Ag(111) surface are investigated at an atomic scale by combining scanning tunneling spectroscopy and atom manipulation. Various vacancy sizes, from one atom to hundreds of atoms, on Ag(111) are first created by tip-sample contact in a controlled manner. Using lateral manipulation, smaller vacancies are filled with silver atoms one at a time and the corresponding electronic structure is measured using the dI/dV tunneling spectroscopy after each atom filling. To determine the vacancy depth for the larger vacancies, a cluster model is proposed. It is found that the surface state intensity decreases with the increasing vacancy depth. In addition, the onset of the surface state is gradually shifted towards the Fermi level by increasing vacancy sizes, and it disappears for the larger vacancies. A localized electronic state located at ~ +2.8 eV is observed in the unoccupied s-p band for the larger and deeper vacancies.
A nitrogen polar and gallium rich GaN(000-1) grown on a sapphire substrate using r.f. N-plasma molecular beam epitaxy (MBE) is investigated by using scanning tunneling microscopy and spectroscopy at 77 K and 5 K. Three novel surface reconstructions of the GaN(000-1) have been observed and corresponding atomic models are proposed. The band gaps of these surfaces are measured by using the dI/dV tunneling spectroscopy, which agree well with the bulk-band gap of GaN(000-1). Furthermore, by means of tunneling spectroscopy and voltage dependent STM imaging, it is shown that some of the novel surface features are associated with two of the surface states of the GaN(000-1) surface.