Photochemical (PEC) etching and transmission electron microscopy (TEM) have been used to study the defects in hetero-epitaxial GaN layers. TEM proved that PEC etching reveals not only dislocations but also nanopipes in the form of protruding, whisker-like etch features. It is shown by diffraction contrast techniques that the nanopipes are screw coreless dislocations. An example is shown of the transformation of a normal full-core screw dislocation into a nanopipe. The PEC/TEM experiments indicate the presence of electrically active (recombinative) species in the vicinity of the nanopipes.

Iron nitride layers were formed by a novel low-temperature gaseous nitriding process. Nitriding occurs at a temperature of 325 °C through NH3 decomposition at the surface of Ni (25 nm) coated Fe, followed by N transport through the Ni film into the underlying Fe, where nitride precipitation takes place. The role of Ni is to protect Fe from oxidation by gas impurities and to serve as a catalyst for NH3 decomposition. The precipitation behavior and the development of microstructure were studied by means of elastic recoil detection, cross-sectional transmission electron diffraction (XTEM), and positron annihilation (PA). From PA and XTEM no evidence was found for the occurrence of porosity during nitriding (an effect found at higher temperatures due to the decomposition of the nitrides into Fe and N2). XTEM showed that the original columnar α–Fe grains transform into smaller ′–Fe4N grains which subsequently transform into larger ε–Fe3−xN grains. This microstructural evolution of smaller ′ grains forming in the original columnar α–Fe structure occurs in one of two growth modes of the nitride in the Fe layer, i.e., throughout the entire depth range of the Fe layer, or preferentially at the Ni/Fe interface when an iron oxide layer is present at this interface.

Nitrogen was introduced in an iron layer underneath a top layer of nickel. This was done by ion implantation of N into the Ni layer at a temperature of 200 °C. During implantation and subsequent anneals at 250 and 300 °C, N diffuses from the Ni layer into the Fe layer because of a larger affinity of Fe for N than of Ni for N. The concentration depth profiles of N in the Ni/Fe bilayers, as recorded with the nuclear reaction analysis technique, show at the highest implantation dose a peak below the Ni/Fe interface. From structural analysis techniques (x-ray diffraction and cross-sectional transmission electron microscopy) it was observed that this peak is due to the presence of an ε–Fe3−xN layer below the Ni/Fe interface. It is thus shown that ε –nitride can be formed in Fe at such low temperatures in the absence of radiation damage.

In this paper the structure of a (011)-Σ=3 twist boundary in ordered Cu3Au is analysed geometrically. Wrong nearest-neighbour bonds can be avoided by facetting on an atomic scale along common {112} planes, together with a rigid-body translation parallel to one of the facets as measured in earlier work. In the specimen different translational states were found in different areas of the [165]-Σ=3 boundary by transmission electron microscopy (TEM). One of the translations was in agreement with the model, the other was associated with a presumably energetically more unfavourable structure. The in situ observation at 230 °C of the motion of the dislocation separating the different boundary areas was associated with a transformation of the boundary structure with the higher energy into the more favourable structure. Therefore, it is likely that the driving force of the dislocation motion was a difference in boundary energy.

In recent years, the influence of the establishment of long-range order in cubic alloys on the structure of grain boundaries in L12 alloys has been considered. Thus, for example, for the ⅀=5 (310) tilt boundary the various possible structures have been investigated that are generated upon ordering, starting from plausible structures in the disordered state. However, apart from some rough energy estimates based upon nearest neighbour interactions, no reliable energy calculations have been performed of these different possible structures. In this paper, computer calculations based upon interatomic pair potentials constructed in such a way that the L12 structure is stable with respect to disordering, are reported for the ⅀=5 (310) boundary. The relative stability of various possible structures, with associated different boundary compositions, has been investigated.