Characterization of dislocations in gadolinium gallium garnet single crystals by transmission X-ray topography

This paper describes results of characterization of dislocations in gadolinium gallium garnet (GGG) single crystals using transmission X-ray topography. The crystals used in this investigation had been grown from the melt by the Czochralski technique. Wafers cut parallel to {111}, {011} and {211} have been investigated. Growth bands resulting from compositional variations due to thermal fluctuations during growth are observed in almost all the crystal wafers. Growth facets were found to be the traces of (211), (121) and (112) lattice planes. In some (111) wafers dislocations loops of dodecagon shapes are quite prominent. The sides of the dodecagons are parallel to 〈112〉 and 〈011〉. These loops are associated with iridium inclusions and were found to be of the interstitial kind. These dislocation loops have been characterized regarding their Burgers vector by recording topographs with different diffraction vectors. They lie in (111) and have Burgers vectors parallel to [111]. Some dislocation loops were observed lying on (001) and (111) planes. Interstitial kind of loops have also been observed in (011) wafers. In these wafers the dislocation loops had Burgers vectors parallel to 〈211〉, 〈111〉 and 〈100〉. A few helices having [211] as the axis were also observed. Isolated dislocations with a line vector parallel to [111] have been observed in (211) wafers; The contrast produced by these dislocations in different reflections showed that they have Burgers vector lying in (211) and may possibly be along the low indexed directions 〈011〉 or 〈211〉.