For most of our group's scientific work, it is normally important to prepare a clean and well ordered crystal surface and then perform a deposition experiment, or, in the case of epitaxially grown samples, follow an appropriate cleaning / depassivation procedure. For experiments at central facilities, where time is often pressing, these surface preparation recipes can be perfected in the Warwick labs for replication during the experiment.
Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) are extremely useful for routine chemical analysis of a surface. Typically, it is important to check for surface cleanliness (have all the contaminants such as oxygen and carbon been removed?) and in a deposition experiment it is important to check that the right quantity of material has been adsorbed. Low energy electron diffraction (LEED) provides a simple visual method for assessing the symmetry of any surface reconstruction, which is crucial in nearly all of our experiments.
Further applications of LEED and XPS
XPS is also useful in its own right to analyse surface stoichiometry, oxidation, segregation and similar phenomena in epitaxial electronic materials. For a complex heteroepitaxial combination with no common element or crystal structure (e.g. MnSb grown on InP or GaAs) the growth behaviour can be far from ideal. In the field of next-generation electronic materials such as InN and ZnO, where epitaxial growth has not been perfected, the surface region can deviate strongly from the bulk material. Even for very perfect semiconductor materials, we expect to observe band bending due to charged surface states; in the case of highly mismatched alloys, this can often be estimated directly from XPS. For all these measurements, we generally use the Science Cities high resolution XPS facility here at Warwick.
As well as providing the symmetry of the surface structure directly, LEED can also be used to measure quantitatively the actual surface atomic structure. This relies on measuring the intensity of diffraction spots as a function of electron energy and is known as LEED I-V. The group has a LEED I-V data acquisition system and the technique is used to support other quantitative structure determination methods.