Band bending at the InSb-CdTe interface

The band diagrams of InSb/CdTe heterojunctions were calculated. It is shown that it should be relatively easy to create a situation in which there is virtually no band bending at the InSb side of the junction. This provides almost ideal conditions for quantum well devices

InSb/CdTe heterostructures grown by MBE

InSb/CdTe heterostructures were grown by MBE, including a 10 layer “superlattice”. The structures were characterized by X-ray diffraction, van der Pauw measurements, and SNMS depth profiling. The interfaces widen because of interdiffusion and through the formation of an interface compound from the reaction of Te with the InSb surface. The interface compound is identified as strained InTe(II). The interfaces are still too wide for practical devices. Thermochemical analysis indicates that the reaction can be suppressed by applying a Cd overpressure. The diffusion of In and Sb in CdTe is very fast and will necessitate a MEE growth scheme at lower temperatures

A mini-ALE attachment to UHV sufrace analysis equipment

An atomic layer epitaxy (ALE) module has been developed that can be attached to a port of UHV surface analysis equipment. In our case this would, for instance, enable analysis by low energy ion scattering (LEIS), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). This so-called mini-ALE will be used to study and optimize the growth mechanisms of mono-atomic and sub-monatomic metal layers and to prepare model samples for studies of catalysts and small metal clusters. The mini-ALE basically consists of a small (˜ 1 cm3) growth chamber suspended in an UHV environment, with valved inlets for the reaction and purge gases. The sample can be transferred to the analysis chamber via a load-lock. The surface temperature of the sample can be controlled from room temperature to approximately 500°C. The system was tested by attaching it to one of our LEIS set-ups and growing CuO on Al2O3, using Cu(acac)2 and artificial air as reactants. Cu growth was observed, covering about 3% of the surface. Results on growth as a function of surface temperature are given

Surface cleaning and characterization of yttria-stabilized zirconia

Cleaning substrate surfaces and checking the effectiveness of the cleaning method are two very important aspects of reliable surface analysis. The absence of a cleaning process leading to an yttria-stabilized zirconia (YSZ) surface that is free of contaminants and of unchanged structure has inspired us to investigate a new surface cleaning method based on a combination of high vacuum annealing at 300°C and oxidation with atomic oxygen. The efficiency of the cleaning procedure is examined by studying the surface composition after each stage of the process with low-energy ion scattering (LEIS or ISS) and x-ray photoelectron spectroscopy (XPS). The cleaning method results in complete removal of contaminants adsorbed from the ambient atmosphere, such as water and hydrocarbons. No segregation or sputtering effects due to the cleaning procedure are observed. The process also is compared with more conventional methods such as thermal oxidation and plasma etching. Although this study focuses on cleaning polycrystalline YSZ, which is inherently difficult to clean, other oxides and several metals also are investigated. For all the oxides studied, the newly proposed cleaning strategy removes up to 90% more contamination than the commonly used method of thermal oxidation. In the case of metals, the surface carbon is replaced by oxygen, which can be removed much easier. Additional segregation of bulk carbon is avoide

Oxygen exchange and diffusion in the near surface of pure and modified yttria-stabilised zirconia

By studying the oxygen transport through yttria-stabilised zirconia (YSZ), a strategy could be proposed which should lead to a reduction in the operating temperature of the solid oxide fuel cell (SOFC) to the intermediate temperature range without loss in performance. The combination of isotopic exchange depth profiling with low energy ion scattering (LEIS) and elastic recoil detection analysis (ERDA) has shown a complex structure affecting the surface oxygen exchange reaction and self-diffusion in 10 mol% yttria-doped zirconia. Remarkable is the presence of a thin (about 6 nm) layer at the external surface showing resemblance with the monoclinic phase. The results suggest a significant improvement in the surface oxygen exchange with respect to the values reported in literature when impurity oxides are prevented from segregating to the external surface. A possible operating temperature of around 850 °C seems feasible. Improvements in the surface oxygen exchange by addition of a surface catalyst reported in literature are also attributed to the removal of impurities. Further decrease in operating temperature, down to at least 725 °C, should be possible by removing the impurities in the bulk, which should lead to a considerable increase in the grain boundary diffusion and by reduction of the electrolyte thickness

Diffusion of cesium in sodium borosilicate glasses for nuclear waste immobilisation, studied by low-energy ion scattering

Low-energy ion scattering (LEIS) is shown to be a convenient technique for measuring the diffusion coefficient of cesium in sodium borosilicate glass. A 3 keV 4He+ ion beam is first used to create an alkali depletion layer in the outermost 60 nm of the glass. After annealing, to remove the vacancies from the glass matrix, the return of cesium to the surface is monitored. Diffusion coefficients at 815 and 842 K are determined. Both are found to agree, within experimental error, with the values from the concentration couple method. The latter analyzes the cesium profiles by SEM/EDS. The present technique, however, can be applied at lower temperatures, where the concentration couple method would require too long measuring times. This gives the possibility to predict the diffusion coefficient of cesium at actual temperatures in nuclear waste glass cylinders.</p

Surface modification of a fuel cell material by ion implantation

In an attempt to improve the oxygen surface-exchange coefficient of yttria stabilized zirconia, we have implanted the surface with low-energy (10 keV) V and W ions. The surface concentrations of V and W under various annealing and sputtering conditions have been measured by low energy ion scattering . In addition, 18O-isotope tracer measurements have been carried out to examine the oxygen exchange behavior. For certain conditions, the oxygen uptake is enhanced

Diffusion of cesium in sodium borosilicate glasses for nuclear waste immobilisation, studied by low-energy ion scattering

Low-energy ion scattering (LEIS) is shown to be a convenient technique for measuring the diffusion coefficient of cesium in sodium borosilicate glass. A 3 keV 4He+ ion beam is first used to create an alkali depletion layer in the outermost 60 nm of the glass. After annealing, to remove the vacancies from the glass matrix, the return of cesium to the surface is monitored. Diffusion coefficients at 815 and 842 K are determined. Both are found to agree, within experimental error, with the values from the concentration couple method. The latter analyzes the cesium profiles by SEM/EDS. The present technique, however, can be applied at lower temperatures, where the concentration couple method would require too long measuring times. This gives the possibility to predict the diffusion coefficient of cesium at actual temperatures in nuclear waste glass cylinders

Surface-composition of ceramic CeGd-oxide

The atomic composition of the outermost atomic layers of gadolinium-doped ceria, Ce0.8Gd0.2O1.9, was determined with Low Energy Ion Scattering (LEIS). Due to the surface sensitivity of this technique it was possible to determine that the outer 5 monolayers are Gd-enriched. The Ce/Gd ratio changes from 1 at the outermost surface to 4.2 in the ‘bulk'. When this ceria would be used as the electrolyte in a fuel cell, such a surface Gd enrichment could influence its operation. Only in the first monolayer was some Ca observed. No difference in surface composition was observed between samples sintered at 1300 and 1600°C, in spite of large differences in oxygen ionic-conductivity observed previously. At a depth of about 20 monolayers a sudden shift in surface conductivity is observed that might be correlated with a structural change of the material at the grain boundary. The quantification with LEIS was checked with Rutherford Backscattering Spectroscopy (RBS), giving good agreement for the bulk composition