H. Nörenberg

Visualization of precipitation induced crystallographic shear planes as one-dimensional structures on surfaces: an STM and RHEED study on TiO2(110)

Abstract Surface sensitive techniques [scanning tunneling microscopy (STM), reflection high-energy electron diffraction (RHEED), and Auger electron spectroscopy (AES) have been used to study the effects of changes in the bulk on the surface structure in TiO 2 . Annealing of TiO 2 (110) at elevated temperatures leads to straight steps on the surface with lengths in excess of 500 A. The structure of these steps has been investigated in detail by voltage dependent STM and electron diffraction. The steps are caused by crystallographic shear (CS) during annealing creating areas of different crystallographic structure. STM and RHEED studies showed that the dominant directions of the steps are along [110], [111] and [111]. From these experimental finding and the tetragonal symmetry of rutile it was concluded that the CS planes belong to the close packed family {112} of planes. CS is correlated to Ca precipitation. CaO diffusion to the surface may be an important process in the creation of oxygen vacancies. The calcium precipitates induce a (1 × 3) surface reconstruction which is visible both in STM images and electron diffraction patterns.

The Si(001) c(4×4) surface reconstruction: a comprehensive experimental study

Abstract We have carried out a comprehensive experimental study of the Si(001) c(4×4) surface reconstruction by scanning tunneling microscopy (STM) (at room temperature and elevated temperatures), Auger electron spectroscopy (AES), reflection high-energy electron diffraction (RHEED) and low-energy electron diffraction (LEED). Si(001) samples were kept under ultra-high vacuum (UHV) at around 550°C until the c(4×4) reconstruction appeared. STM contrast of the c(4×4) reconstruction is strongly influenced by electronic effects and changes considerably over a range of bias voltages. The c(4×4) surface reconstruction is a result of stress which is caused by incorporation of impurities or adsorbates in sub-surface locations. The resulting c(4×4) reconstruction in the top layer is a pure silicon structure. The main structural element is a one-dimer vacancy (1-DV). At this vacancy, second layer Si-atoms rebond and cause the adjacent top Si-dimers to brighten up in the STM image at low bias voltages. At higher bias voltage the contrast is similar to Si-dimers on the (2×1) reconstructed Si(001). Therefore, besides the 1-DV and the two adjacent Si-dimers, another Si-dimer under tensile stress may complete the 4× unit cell. This is a refinement of the missing dimer model.

Surface structure of CeO2(111) studied by low current STM and electron diffraction

Abstract We have studied the surface structure of CeO 2 (111). The surface morphology can be described as stacks of disk-like appearance. On the atomic scale a (1×1) terminated surface is visible in STM. The top layer of oxygen is imaged as can be concluded from the bias-voltage range during observation and previous band structure calculations. The electron diffraction experiments confirm the (1×1) terminated surface structure.

Network-like surface reconstruction on rutile TiO2(001) by non-equilibrium self-organization

A self-organized network structure has been discovered after annealing TiO2(001) rutile single crystals under non-equilibrium conditions. This novel structure appears after annealing under ultrahigh vacuum conditions to above 1000°C and subsequent quenching at a rate of ∼100°C s−1. The surface structure was investigated by scanning tunnelling microscopy (STM) and reflection high-energy electron diffraction (RHEED). We observed one-dimensional structures running along the [110] and [110] directions, thus forming a two-dimensional network of ridges separated by 4 nm. From atomically resolved STM images and RHEED observations, a two-domain (72×2)R45° surface reconstruction is concluded. The network formation requires a massive rearrangement of atoms starting from the initially epi-polished surface, suggesting that kinetic processes play a crucial role in establishing the network structure. After further annealing to 1500–1600°C the structure remains but the ridges appear fragmented, presumably as result of stress which is induced by deformation of the crystal after high-temperature annealing. Several factors such as readily available oxygen diffusing from the bulk to the surface, titanium diffusing from the surface to the bulk and a high surface energy of the (001) surface of TiO2 seem to favour the appearance of the network structure.

Surface structure of the most oxygen deficient Magnéli phase – an STM study of Ti4O7

Abstract Non-stoichiometric TiO2 forms a range of crystalline structures known as Magneli phases TinO2n−1. TiO2(110) single crystals have been annealed and the surface structure was investigated by scanning tunnelling microscopy (STM). From the surface topography seen in the STM images the existence of large Ti4O7 regions are established. They are formed by crystallographic shear and lead to a stepped surface. The contrast in the STM images indicates a possible anisotropy of conductivity.

Arsenic adsorption on GaAs(001)

Abstract A method for accurate determination of the arsenic coverage of reconstructed GaAs(001) surfaces prepared by molecular beam epitaxy (MBE) is presented. The time of gallium supply to the reconstructed surface until a halo appears in the reflection high energy electron diffraction (RHEED) pattern is taken as measure for the arsenic coverage. (2 × 4), c(4 × 4), (2 × 3) and also intermediate GaAs(001) surface structures were investigated at a substrate temperature of 200°C. The RHEED results were verified by high resolution scanning electron microscopy (HRSEM). Dependent on the surface reconstruction, arsenic coverages between 0.76 and 1.22 monolayer (ML) were observed. The relation between RHEED pattern and arsenic coverage is discussed quantitatively.

Pressure-dependent permeation of noble gases (He, Ne, Ar, Kr, Xe) through thin membranes of oriented polypropylene (OPP) studied by mass spectrometry.

Abstract A mass spectrometric method has been used to study transmission of noble gases (He, Ne, Ar, Kr, Xe) through thin polymer membranes of OPP and PET. The transmission of gas originating from a confined volume in a gas cell through the membrane is measured by a mass spectrometer. Due to depletion of gas inside the gas cell, transmission decreases as a function of time, which is taken as a measure for permeation. For the smaller noble gases helium and neon gas transmission is found to be an exponential function of time indicating a pressure independent permeability for PET and OPP. For the permeation of heavier noble gases xenon, krypton and — to a lesser extent — argon through OPP a pressure dependent permeability was found. A model takes into account the higher solubility for bigger gas atoms as they have a higher probability to condense in cavities of the free volume of the polymer. There is good agreement between experimental and calculated data. Sensitivity of the mass spectrometric method is discussed for argon isotopes 40 Ar, 38 Ar and 36 Ar and shows an instrumental detection limit of less than 0.1%.

Formation of highly ordered Ca-overlayers on TiO2(110) surfaces studied by scanning tunneling microscopy and atomistic simulation

Abstract An ordered surface layer of Ca on TiO 2 (110) has been prepared and studied after annealing of a TiO 2 -rutile crystal. During the annealing process Ca impurities from the bulk segregate to the surface. A well ordered p(3×1) reconstruction has been observed by scanning tunneling microscopy (STM). Calculations have shown that the p(3×1) reconstruction has the lowest energy. Every third Ca ion in the [001] direction is elevated by around 1 A. Therefore, the STM contrast is mainly due to calcium and caused by geometry rather than electronic effects.

Influence of carbon on the formation of the Si(001) c(4×4) surface reconstruction

We have carried out a comprehensive experimental study of the Si(001) c(4×4) surface reconstruction by STM, AES and RHEED. The Si(001) samples were kept in UHV at a temperature of 600°C until the c(4×4) reconstruction was evident in RHEED. The reconstruction is related to carbon as can be seen in AES and the formation of β-SiC after further annealing. Carbon originating from hydrocarbon compounds in the vacuum background is cracked after adsorption on the surface at these elevated temperatures. STM shows a number of different structures within the c(4×4) unit mesh indicating that the surface structure is influenced by defects. Imaging the c(4×4) reconstructed surface at high bias voltages shows dark features that are attributed to screening effects by subsurface defects.

Permeation of gases through polymer membranes investigated by mass spectroscopy

Abstract A new method to estimate permeation coefficients for small sized polymer samples is introduced. It uses a mass spectrometer to measure the partial pressure of gas permeating from an enclosed volume of gas as a function of time. In agreement with predictions from a simple model the partial pressure varies exponentially with time. From the time constant and the geometry of the gas cell it is possible to calculate the permeation coefficient for individual species in gas mixtures. The mass spectrometric estimated permeation coefficients are in reasonable agreement with control experiments and data from the literature.