Thickness dependence of grain growth orientation in MgB2 films fabricated by hybrid physical-chemical vapor deposition

We have investigated the effect of thickness of the MgB2 film on the grain growth direction as well as on their superconducting properties. MgB2 films of various thicknesses were fabricated on c-cut Al2O3 substrates at a temperature of 540 degrees C by using hybrid physical-chemical vapor deposition (HPCVD) technique. The superconducting transition temperature (T-c) was found to increase with increase in the thickness of the MgB2 film. X-ray diffraction analysis revealed that the orientation of grains changed from c-axis to a-axis upon increasing the thickness of the MgB2 film from 0.6 to 2.0 mu m. MgB2 grains of various orientations were observed in the microstructures of the films examined by scanning electron microscopy. It is observed that at high magnetic fields the 2.0-mu m-thick film exhibit considerably larger critical current density (J(c)) as compared to 0.6-mu m-thick film. The results are discussed in terms of an intrinsic-pinning in MgB2 similarly as intrinsic-pinning occurring in high-T-c cuprate superconductors with layered structure.