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

Two dimensional electron gases (2DEGs) at surfaces and interfaces of semiconductors are described straightforwardly with a 1D self-consistent Poisson-Schr\"{o}dinger scheme. However, their band energies have not been modeled correctly in this way. Using angle-resolved photoelectron spectroscopy we study the band structures of 2DEGs formed at sulfur-passivated surfaces of InAs(001) as a model system. Electronic properties of these surfaces are tuned by changing the S coverage, while keeping a high-quality interface, free of defects and with a constant doping density. In contrast to earlier studies we show that the Poisson-Schr\"{o}dinger scheme predicts the 2DEG bands energies correctly but it is indispensable to take into account the existence of the physical surface. The surface substantially influences the band energies beyond simple electrostatics, by setting nontrivial boundary conditions for 2DEG wavefunctions.Comment: 9 pages, 7 figures, 2 table

How flat is an air-cleaved mica surface?

Ostendorf F, Schmitz C, Hirth S, Kühnle A, Kolodziej JJ, Reichling M. How flat is an air-cleaved mica surface? Nanotechnology. 2008;19(30):305705.Muscovite mica is an important mineral that has become a standard substrate, due to its easy cleavage along the {001} planes, revealing a very flat surface that is compatible with many biological materials. Here we study mica surfaces by dynamic atomic force microscopy (AFM) operated in the non-contact mode (NC-AFM) under ultra-high vacuum (UHV) conditions. Surfaces produced by cleaving in UHV cannot be imaged with NC-AFM due to large surface charges; however, cleavage in air yields much less surface charge and allows for NC-AFM imaging. We present highly resolved NC-AFM images of air-cleaved mica surfaces revealing a rough morphology originating from a high density of nanometre-sized particles. Among these particles, we find regularly shaped structures indicating the growth of crystallites on the surface. The contamination layer cannot be removed by degassing in UHV; even prolonged heating at a temperature of 560 K under UHV conditions does not yield an atomically flat surface

Evidence for Potassium Carbonate Crystallites on Air-Cleaved Mica Surfaces

Ostendorf F, Schmitz C, Hirth S, Kühnle A, Kolodziej JJ, Reichling M. Evidence for Potassium Carbonate Crystallites on Air-Cleaved Mica Surfaces. Langmuir. 2009;25(18):10764-10767.Air-cleaved mica surfaces exhibit a high density of nanometer or micrometer size particles that have been ascribed to potassium carbonate formed as it reaction product of carbonaceous gases with potassium ions. Unambiguous evidence for this assignment has, however, never been presented. We study air-cleaved mica surfaces by high-resolution noncontact atomic force microscopy (NC-AFM) in ultrahigh vacuum to reveal the detailed structure of such precipitates on the surface. Among a large number of irregularly shaped surface structures, we find flat, hexagonally shaped islands exhibiting two different patterns on their surfaces, namely a rectangular atomic corrugation pattern and a hexagonal moire Structure. The unit cell of the rectangular pattern corresponds to the dimensions of the potassium carbonate bulk structure and is found on high crystallites. The moire structure solely appears on very flat islands and is caused by the interference of the potassium carbonate lattice periodicity and the lattice periodicity of the underlying mica substrate. Both results strongly point to the presence of potassium carbonate crystallites on air-cleaved mica surfaces

Architecture of PTCDA molecular structures on a reconstructed InSb(001) surface

An extensive scanning tunneling microscopy (STM) study of adsorption of submonolayer coverages of PTCDA molecules on the c(8 $\times$ 2) reconstructed InSb(001) surface is presented together with ab initio density functional theory (DFT) calculations. Our DFT calculations explain the variety of adsorption sites seen on the experimental STM images. In particular, we prove that the molecules are oriented with their long axes along the [110] direction. Calculated STM images of the molecule agree well with the high-resolution STM images obtained at the temperature of 77 K. We find that molecules form four covalent bonds between edge oxygen atoms of the PTCDA and In atoms of the surface. We also study in detail their diffusion mechanism and explain their ability to form experimentally observed chains along the [110] direction