A.R. Lang

Cathodoluminescence, optical absorption and x-ray topographic studies of synthetic diamonds

Cathodoluminescence topographs of polished sections of synthetic diamonds reveal very sensitively the distribution of growth sectors belonging to the major forms, {100} and {111}, and the minor forms {110} and {113}. All growth sectors can be clearly distinguished by the characteristic colour and brightness of their cathodoluminescence, and by characteristic differences in the variation of these properties with the current density of the bombarding electrons. Growth sectors of all forms show banding parallel to their respective growth surfaces, such banding being particularly pronounced in the case of cube growth sectors. In some large experimental crystals strong banding was found also in {110} and {113} sectors. The green cathodo-luminescence from cube growth sectors is linearly polarised with direction of the electric vector parallel to the growth surface. No polarisation of emission from {111}, {110} and {113} sectors was detected. Luminescent linear defects, believed to be decorated dislocations, were observed: some gave an orange-pink emission with a prominent yellow spectral band, others gave a bright blue luminescence which was strongly linearly polarised with the plane of the electric vector being determined by, and containing, the direction of the linear defect. Optical absorption at 405 nm, taken as a measure of concentration of dispersed paramagnetic nitrogen impurity, gave relative concentrations in different growth sectors as: {111} rich, {110} poor, {113} rather poor, {100} highly variable. X-ray topographs revealed growth sector boundaries, growth banding and decorated dislocations. Nomarski interference microscopy of polished surfaces indicated abrasion resistances of the different growth sectors to be in the order {110}>{113}>{111}>{100} with relatively very little difference between {113} and {111}.

An optical and X-ray topographic study of giant screw dislocations in silicon carbide

Abstract A 6H SiC crystal plate exhibiting three large growth spirals on its (0001) face was examined. One spiral maintained a simple step structure over an area 0.8 mm in diameter, enabling its step height to be measured interferometrically as 31.7 nm corresponding to 21 repeats of the 6H structure. X-ray topography confirmed the presence of a giant screw dislocation with Burgers vector parallel to [0001] at the centre of each spiral and showed that these dislocations had hollow cores running their lengths. Diameters of the hollow cores within the crystal were measured non-destructively by X-ray section topographs. The values found (∼ 10 μm) were an order of magnitude greater than the diameters of the microscopically visible holes at the surface outcrops of the dislocations. A possible reason for this discrepancy is discussed. Attempts were made to measure the magnitude of the giant screw dislocation Burgers vector by X-ray diffraction methods, firstly from the size of the diffraction-contrast image of the dislocation and secondly from the tilt of the lattice planes close to the dislocation. Both methods were subject to much uncertainty, the former underestimating and the latter overestimating the Burgers vector magnitude by factors of more than two relative to the magnitude inferred from step-height measurements.

Glimpses into the growth history of natural diamonds

Abstract The growth histories of diamonds may be classified crudely as “normal” or “abnormal”. In normal histories, growth continuously in octahedral habit is recorded by concentric, complete {111} growth layers. Very rarely is growth layering not clearly revealed in X-ray diffraction topographs. Dislocations may be many or few; but in diamonds of normal growth and free from inclusions most dislocations originate at the crystals centre of growth and from thence radiate (often with remarkable straightness) out to the crystal faces. Rare occurences are stacking faults, bounded by Frank sessile partial dislocations. Coated diamonds show a transition, often abrupt, from normal facetted growth to a fibrous mode of growth, with simultaneous inclusion of micron-size foreign particles. A much rarer abnormal mode of growth comprises a combination of forms: the normal, facetted {111} form together with non-facetted, generally hummocky, surfaces of mean orientation {100}. Under the latter surfaces (i.e. in the “cuboid” growth sectors) a population of micron-sized X-ray diffraction-contrast-producing bodies may be found. The octahedral growth sectors are generally richer, sometimes much richer, in nitrogen impurity platelet precipitates than their adjacent cuboid growth sectors at corresponding epochs of growth.

Orientation-dependent nitrogen incorporation on vicinals on synthetic diamond cube growth surfaces

Abstract New phenomena significant in relation to optical properties of diamond and to inhomogeneity of impurity distribution in synthetic diamond are reported. Evidence has been collected from (a) optical transmission at 430 nm, (b) Schlieren photography, (c) birefringence, (d) infrared absorption microscopy, (e) differential abrasion resistance patterns, (f) cathodoluminescence topography with analysis of polarisation of emission, and (g) X-ray topography with synchrotron and conventional sources; and is detailed and correlated for one specimen, supplemented by data from a second, similar specimen. The bulk of the specimens comprised material belonging to one cube growth sector, designated (001). They did not contain the seed, which had been polished away. The specimens were slab-shaped, bounded by a pair of polished surfaces parallel to (001), nearer and further from the seed, respectively. The principal feature of interest on both surfaces parallel to (001) was a “Maltese Cross” type of cathodoluminescence feature, radiating from a growth centre that had dominated growth in the direction [001] throughout the thickness of the slab (1 mm), entirely covering the (001) growth surface up to its maximum width of 3 mm exposed on the specimen surface remoter from the seed. The arms of the Maltese Cross brightest in visible emission (peak wavelength ≈ 525 nm) expanded radially in the cube diagonal directions ±[110] and ±[1 1 0]. Their cathodoluminescence emitted in the direction normal to the (001) surface was partially polarised with E-vector parallel to the radial direction in each arm. Less bright emission from arms pointing in the cube directions ±[100] and ±[010] was unpolarised in this [001] view. Application of techniques (a), (b) and (d) showed that the former, diagonal arms were at least 25% to 30% richer than the latter arms in nitrogen impurity present in singly-substituted atomic state, measured at a distance of 600 μm from the cross centre. Differences in impurity incorporation and optical properties are attributed to differences in surface structure on vicinal slopes of a low-elevation growth pyramid. Regarding polarisation of cathodoluminescence emitted in the [001] direction, the diamond structure fourfold screw symmetry normal to (001) provides the foundation for a growth step model that can account for unpolarised emission from the cross arms pointing towards ±[100] and ±[010], and the 90° rotation of partial polarisation direction between arms pointing towards ±[110] and those pointing towards ±[1 1 0].

The growth of nearly perfect hexamethylenetetramine crystals from solution

Single crystals were prepared by unseeded growth from ethanol solutions both by slow cooling and by solvent evaporation at constant temperature. The second method was more successful in yielding crystals free from visible inclusions and with a low dislocation density as assessed by X-ray topography. The latter technique demonstrated that these crystals were free from growth banding and contained regions of mm3 dimensions dislocation-free and highly perfect. The best crystals contained very few dislocations other than those generated at the initial nucleation event and which subsequently were grown in to the crystal as slightly diverging bundles running out in directions roughly perpendicular to the {110} faces. Dislocations with Burgers vectors of both 〈100〉 and12 〈111〉; types were observed.

Structural and chemical inhomogeneities in germanium single crystals grown under conditions of constitutional supercooling

Abstract Solute distributions and dislocation configurations in single crystal of germanium grown with a cellular interface consequent upon the existence of a zone of constitutional supercooling are described. Several solutes which dope the crystal substitutionally have been studied. Dislocations were delineated by chemical etching and by X-ray diffraction topography, whilst chemical inhomogeneities were revealed by X-ray absorption topography, autoradiography and chemical etching. It is shown that the pattern of microsegregation depends strongly on both the interface morphology (which is itself dependent on the crystal orientation) and on the freezing range of the solute-solvent system. The occurrence of very severe microsegregation is due predominantly to the entrapment of liquid droplets in the cell boundary grooves and their subsequent migration in the imposed temperature gradient. The massive increase in dislocation density which occurs after the formation of a cellular interface is shown to be due to stresses introduced during the crystallisation following migration of the trapped liquid droplets.

Occurrences of facetted re-entrants on rounded growth surfaces of natural diamonds

Abstract Some unexpected growth-surface morphologies have been discovered as localised developments within the regions of non-facetted growth which are present (and can be dominant) in diamonds which have had epochs of mixed-habit growth during which crystallisation proceeded simulataneously both on normal octahedral faces and on non-crystallographic, hummocky surfaces having mean orientation {100}. These unexpected minor features are {111}-facetted re-entrants, not involving twinning. They have been observed in two crystallographic settings. In one case pairs of {111} facets develop to form re-entrant notches at the boundaries between adjacent sectors of non-facetted, mean-{100}-orientation growth. In this case growth can propagate on the pair of re-entrant facets, forming a column of octahedral growth inserted within the surrounding matrix of non-facetted growth material. In the second case, {111}-sided pyramidal pits (size range from about 10 μm to 120 μm) develop at points distributed over the area of non-facetted growth surface. There is evidence that they are initiated at particular growth horizons at which an episode of strong inhibition of growth on {111} commences, and that they are preferentially located at dislocation outcrops. In this case no detectable propagation of octahedral growth upon the facets occurs: they are directly overlain by renewed non-facetted growth.

Defects in natural diamonds: Recent observations by new methods

Abstract The observations presented concern both grown-in defects and those which develop after growth. They will deal principally with impurity platelets precipitated on {100} planes and with dislocations. The former defects are a notable feature in the most common variety of natural diamond (Type Ia.) The experimental techniques used are of the direct-imaging, topographic type and include ultraviolet absorption topography, X-ray topography, cathodoluminescence topography (both with visible and near infrared wavelengths) and transmission electron microscopy at 100 kV, 120 kV and 1 MeV. Spectrography and colour photography are informative in cathodoluminescence studies. The cathodolumisnecent images of individual dislocation lines can be photographed, whether the dislocations be grown-in or arise from plastic deformation post growth, provided that they lie in matrices sufficiently free from point defects acting as strong electron-hole recombination centres. In practice this requirement implies a low concentration of nitrogen impurity much of which is distributed in “small clusters” which give rise to the bright blue cathodoluminescent emission characteristic of Type Ia diamonds. The luminescence from individual dislocation lines may be either a violet-coloured emission, or an emission which is dominantly a system (known as “H3”) having a zero-phonon line at 2.46 eV and stretching from green to orange in the visible spectrum. The violet dislocation emission is strongly polarised with E vector parallel to the dislocation line. New observations on platelets have been mainly concerned with those attaining diameters of 1 μm or more which can be observed individually by X-ray topography and cathodoluminescence topography. However, evidence from electron and X-ray diffraction contrast indicates that these “giant platelets” produce the same matrix lattice displacements as the more familiar submicrometre-sized platelets in Type Ia diamond and may be presumed to have essentially the same structure and composition. Near infrared cathodoluminescence has been recorded from individual large platelets and it has been discovered that this emission is highly polarised with the E vector in the platelet plane. Available evidence on the structure and properties of the platelets weighs against a currently mooted hypothesis that they are composed of interstitial carbon, and does not conflict with an older idea, that they are composed of nitrogen impurity.

Linear decorations defining edges of an internal octahedron within a natural diamond: observations and an explanation

Abstract A 2.13 carat weight natural diamond, a slightly flattened octahedron with roughly-pitted surfaces, exhibits an internal framework of dense black “lines” located a short distance below the present surface. Most of the lines lie closely parallel to 〈110〉 and connect up to delineate edges of an octahedron concentric with and slightly smaller than the present body. The lines are agglomerations of thin black discs (thickness too small to be measured in situ by optical microscopy), diameters ranging from 1 to 40 μm (but mainly in the 5 to 20 μm range), equantly distributed on diamond {111}, and consistent with graphite. Special optical techniques were developed for examining this specimen and they provided control of relative visibility of internal versus superficial features. Optical micrographs taken in all eight 〈111〉 directions are discussed. Synchroton X-ray topography detected the internal linear framework with good contrast, and helpfully displayed spatial relations between its segments. Findings from cathodoluminescence topography are described. An explanation of the phenomenon is outlined. It proposes a sequence of pressure and temperature conditions and of resulting stresses between diamond and encasing solid matrix whereby graphitization was caused to occur locally along edges of an octahedron, which was subsequently enclosed by further diamond growth. Implications for the origin of the edge grooves frequently exhibited by diamond octahedra are suggested.

Study of a platelet-free infilling of a crack in natural diamond: evidence for a late growth event

Abstract In the cores of coated diamonds, healed cracks may be detected electron-microscopically. At these, the parted crack surfaces have reunited coherently over a substantial fraction of the crack surface, which now contains dislocations and sometimes trapped non-diamond material as well. A healed crack has been discovered that is accompanied on either side by an extremely sharply-defined zone of absence of the electron-microscopically observable {100} platelet defects that populated the rest of the diamond core. (Such platelets are characteristic of nitrogen-impurity-containing natural diamonds that have undergone long storage at high pressure and temperature.) The maximum overall width of the platelet-free zone along the observable trajectory of the crack was about 0.5 μm. It narrowed as the crack was followed into the core. Experiments are described that led to interpretation of the platelet-free zone as consisting of a sheet of coherent diamond regrowth upon incompletely closed crack surfaces, filling the gap between them, and representing a late event in the diamonds growth history.