Electron microscopy of ‘giant’ platelets on cube planes in diamond

Abstract

Electron microscope studies of relatively large planar defects having linear dimensions up to some 5 μm, which lie in {100} planes in diamond, and which emit a greenish-yellow cathodoluminescence, are reported. Evidence is presented that such ‘giant’ platelets are of the same species as, and merely much larger examples of the more common smaller (〈1000 Å) platelets which cause the anomalous 〈100〉 X-ray spikes in Type Ia diamonds. The majority of the defects observed show pronounced displacement fringe contrast in the electron microscope, and are interstitial (i.e. extrinsic) in character. The contrast behaviour of these fringed defects shows that the matrix displacements at the defects are normal to their planes and are constant from defect to defect in any particular {100} plane. In the same fields as these fringed defects, loops of perfect dislocation, again of interstitial character, and also lying in {100}, have been observed. In thinner regions, transparent to 100 keV electrons, usually only segments of such loops were seen since the loops were truncated by the specimens surfaces, but with thicker specimens and 100 keV electrons, complete loops were often observed. Occasionally such loops were seen to change from one plane to another non-parallel {100} plane. The loops are considered to result from an interaction between fringed defects when two such defects impinge (or come close to impinging) on one another during growth, the end result being a relaxation of the strain-fields at the platelet/matrix interfaces. An immediate consequence of this interpretation is that the {100} platelets in Type Ia diamonds cannot be due to foreign (and in particular, nitrogen) atoms, but must, as has recently been suggested by Evans (1976), be due to aggregations of interstitial carbon atoms in puckered layers parallel to {100} planes.