Rotational twins in heteroepitaxial CuInSe2 layers on Si(111)

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Abstract
The microstructure of (112) oriented CuInSe2 heteroepitaxial layers grown with molecular beam epitaxy on (111) oriented Si wafers was investigated by transmission electron microscopy. Experimental and calculated diffraction patterns of different zone axes were compared. Extra spots are caused by rotational twins on (112) growth planes. Six twin variants rotated by ±120°, ±60° and 180° about the [221] axis were identified in the layers. The tetragonal chalcopyrite structure of CuInSe2, the crystal symmetry of the substrate and variations of the growth conditions during the growth are responsible for the formation of these rotational twins. Coherent twin boundaries as well as partly coherent boundaries of twin variants rotated by ±60° and 180° were imaged by high resolution transmission electron microscopy. The boundaries can be formed by inserting partial dislocations with Burgers vectors a/6〈111̄〉 into the CuInSe2 structure. This suggests that the annealing of the samples induces the annihilation of these partial dislocations and consequently reduces the density of twins in the CuInSe2 layers.