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Evaluation of SrTiO₃ single crystal substrate by high-resolution X-ray reciprocal space mapping


Single crystal SrTiO₃ is an ideal substrate material for the epitaxial growth of many functional oxide thin films, including superconducting, magnetic, ferroelectric, pyroelectric, and piezoelectric oxides. The quality of the epitaxial films depends highly on the crystal quality of the substrate. Conventionally, X-ray rocking curve width is often used as a quantitative measure to the crystal quality. Unfortunately rocking curve alone cannot distinguish between the effects of lattice constant variation due to strain or surface damage caused by processing, and defects, such as dislocations, formed during crystal growth. These features however can be distinguished by high-resolution X-ray reciprocal space mapping. With a reciprocal space map, it is possible to determine what kinds of structural imperfection affect the crystal quality.


Shown in Figure 1 below is a (006) reciprocal space map of a commercially available SrTiO3 substrate that has been widely accepted by film growers. The peak, acquired using Rigaku's SmartLab multipurpose diffractometer, is quite sharp, with a full-width at half maximum of just 18 arc seconds in the omega direction. In addition to the sharp (006) peak, a crystal truncation rod structure along the 2θ/ω direction is clearly visible, indicating high surface flatness and low processing damage. In addition a broadened, weak intensity halo is observed mainly along the ω direction. This diffuse scattering is caused mainly by low-density dislocations inside the crystal.

Single crystal SrTiO3
Figure 1: (006) reciprocal space map of a commercially available SrTiO<sub>3</sub> substrate that has been widely accepted by film growers


Although the (006) peak width is quite narrow, defects clearly exist, which may become dislocation nucleation centers during epitaxial film growth and may severely affect the quality of the deposited film.