Atom-Probe Field-Ion Microscopy of Electronic Materials
Doctoral thesis, 1995
This thesis presents work in which atom-probe field-ion microscopy (APFIM) has been applied to two types of electronic materials. In the case of metal/GaAs contacts, the purpose was to characterise the microstructure of the contact interface, particularly the chemical composition variation across the interface. For high-Tc superconductors, YBa2Cu3O6+.delta. (Y-123), the purpose was to detect any oxygen ordering and boundary depletion occurring on a nanometer length scale.
A large part of the work has been devoted to developing methods for preparing needle-shaped APFIM specimens from GaAs and from Y-123. A synthesis procedure was optimised to prepare Y-123 ceramics suitable for APFIM study. The grand canonical ensemble representation of the atom-probe data was devised to discern any pattern of chemical concentration variation in the Y-123 materials.
The metal/GaAs contacts were formed by thermally depositing metals on the tips of the needles. Ag and Au were used to form the Schottky contacts on clean as well as air-exposed GaAs surfaces. Au-Ge was used to form ohmic contacts on air-exposed GaAs.
Atom-probe analysis showed that the interface between Ag and a clean GaAs substrate was abrupt while intermixing occurred for Au on clean GaAs. The exposure of the GaAs tip to air before metal deposition resulted in oxidation of the surface. The oxide seemed to block the intermixing effectively in the case of Au, while in the case of Ag, oxygen seemed necessary for Ag to diffuse into the substrate. The result of AuGe was similar to Au. However, annealing of the AuGe/GaAs contacts resulted in diffusion of Ge into GaAs, forming a layer a few nanometres thick containing 3 at.% Ge.
In order to make APFIM of Y-123, reliable experimental conditions were first determined. APFIM data obtained from materials with different oxygen contents were subjected to statistical analysis. The result provided direct evidence of oxygen ordering on a nanometer scale in .delta.=0.9, 0.7, 0.6 materials, but not in d=0.5 materials. Composition profiles of twin boundaries were obtained by APFIM. Oxygen depletion was found in d=0.6 material, but not in .delta.=0.9 material. This provided the first direct evidence of oxygen depletion at twin boundaries in polycrystalline Y-123 ceramics.