Distribution of Co-dopants in LaFe1-xCoxAsO single crystals

LaFe1-xCoxAsO single crystals have been grown in NaAs flux under ambient pressure. The content and distribution of Co-dopants were studied with wavelength dispersive x-ray spectroscopy. The plate-like single crystals consisit of multilayers with the thickness about 2 $\mu$m. No variation of Co content was observed in one single layer. However, Co content was observed to change gradually along the thickness (or z-) direction, which suggests layer-by-layer growth mechanism. The relation between the Co-content in crystals and starting materials was developed and the distribution coefficient of Co-dopants was estimated to be around 0.60.