Jakub Lis

Inversion layer on the Ge(001) surface from the four-probe conductance measurements

We report four-probe conductance measurements with sub-micron resolution on atomically clean Ge(001) surfaces. A qualitative difference between n-type and p-type crystals is observed. The scaling behavior of the resistance on n-type samples indicates two-dimensional current flow, while for the p-type crystal a three-dimensional description is appropriate. We interpret this in terms of the formation of an inversion layer at the surface. This result points to the surface states, i.e., dangling bonds, as the driving force behind band bending in germanium. It also explains the intrinsic character of band bending in germanium.

Fermi level pinning at the Ge(001) surface—A case for non-standard explanation

To explore the origin of the Fermi level pinning in germanium we investigate the Ge(001) and Ge(001):H surfaces. The absence of relevant surface states in the case of Ge(001):H should unpin the surface Fermi level. This is not observed. For samples with donors as majority dopants the surface Fermi level appears close to the top of the valence band regardless of the surface structure. Surprisingly, for the passivated surface it is located below the top of the valence band allowing scanning tunneling microscopy imaging within the band gap. We argue that the well known electronic mechanism behind band bending does not apply and a more complicated scenario involving ionic degrees of freedom is therefore necessary. Experimental techniques involve four point probe electric current measurements, scanning tunneling microscopy and spectroscopy.

Effect of a skin-deep surface zone on the formation of a two-dimensional electron gas at a semiconductor surface

We acknowledge financial support by Polish NCN (Contract 2011/03/B/ST3/02070). The research was carried out with the equipment purchased thanks to European Regional Development Fund in the framework of the Polish Innovation Economy Operational Program (Contract No. POIG.02.01.00- 12-023/08). L.W. and E.G.M. acknowledge financial support by MINECO (Grant MAT2014-52477-C5-5-P)

Appearance of effective surface conductivity: An experimental and analytic study

Surface conductance measurements on p-type doped germanium show a small but systematic change to the surface conductivity at different length scales. This effect is independent of the structure of the surface states. We interpret this phenomenon as a manifestation of conductivity changes beneath the surface. This hypothesis is confirmed by an analysis of the classical current flow equation. We derive an integral formula for calculating of the effective surface conductivity as a function of the distance from a point source. Furthermore we derive asymptotic values of the surface conductivity at small and large distances. The actual surface conductivity can only be sampled close to the current source. At large distances, the conductivity measured on the surface corresponds to the bulk value.

Electric currents at semiconductor surfaces from the perspective of drift-diffusion equations

Surface sensitive electric current measurements are important experimental tools poorly corroborated by theoretical models. We show that the drift-diffusion equations offer a framework for a consistent description of such experiments. The current flow is calculated as a perturbation of an equilibrium solution depicting the space charge layer. We investigate the accumulation and inversion layers in great detail. Relying on numerical findings, we identify the proper length parameter, the relationship of which with the length of the space charge layer is not simple. If the length parameter is large enough, long-ranged modes dominate the Greens function of the current equation, leading to two-dimensional currents. In addition, we demonstrate that the surface behavior of the currents is ruled by only a few parameters. This explains the fact that simplistic conductivity models have proven effective but makes reconstructions of conductance profiles from surface currents rather questionable.

Atomic wires on Ge(001):H surface

The drive toward miniaturization of electronic devices motivates investigations of atomic structures at semiconductor surfaces. In this chapter, we describe a full protocol of formation of atomic wires on Ge(001):H-(2×1) surface. The wires are composed of bare germanium dimers possessing dangling bonds, which introduce electronic states within the Ge(001):H surface band gap. With a view to the possible applications, we present detailed analysis of the electronic properties of short DB dimer lines and discuss strong electron–phonon coupling observed in STM experiments on single DB dimers. For longer DB dimer wires, this coupling is attenuated making their usage in future nanoelectronic devices feasible.

Imaging of Defects on Ge(001):H by Non-contact Atomic Force Microscopy

The structure of a hydrogenated Ge(001) surface is examined by means of non-contact atomic force microscopy (NC-AFM) at 5 K. Three-dimensional force spectroscopy of a bond defect surrounded by a passivated area shows a qualitatively distinct spatial dependence of the interaction. The force curve over a defect has a range where the short-range attractive force is detected, whereas over the hydrogen-terminated bonds, it is missing: atomic-scale imaging of hydrogens is possible only in the repulsive range. Thus, NC-AFM allows for clear determination of the different chemical activities of the defect compared to the surrounding passivated area.