Properties of heterojunctions-high TC cuprates superconducting p-n junction and super-Schottky diode of Fe-based superconductor

Abstract

We report the fabrication and the properties of YBa2Cu3O7-δ (YBCO)/Nd1.85Ce0.15CuO4-δ (NCCO) high temperature superconducting p-n junction on (100) SrTiO3 substrate. The bilayer thin films were c-axis oriented and both layers are superconducting. The superconducting p-n junction shows a resistive-shunted-junction (RSJ) like characteristics with extremely low critical current density, ~6.5 . Shapiro steps were observed under microwave irradiation. Atomically sharp interface was observed by high resolution tunneling electron microscope (HRTEM). The geometric phase analysis (GPA) of the HRTEM image was applied to study the local variation of lattice parameters of YBCO and NCCO near the interface. Furthermore, the junction reveals a strongly aging effect, attributed to the inter-diffusion of oxygen at the YBCO and NCCO interface. We demonstrate that oxygen diffusion at the interface is a possible origin for the barrier formation. On the other hand, we also report the fabrication of epitaxial heterojunctions formed by the FeSe0.5Te0.5 (FeSeTe) superconductor and Nb-doped SrTiO3 (NSTO) semiconducting substrate and their properties. At high temperature when FeSeTe is in its normal state, the forward bias curves behave like a Metal-Semiconductor junction with a low Schottky barrier. Direct tunneling through the thin depletion layer of the junction dominates the reverse bias curves. When FeSeTe film becomes superconducting at low temperature, we observed that the Schottky barrier height of the junction increased but was suppressed by an external magnetic field. This deviation provides an estimate of the superconducting energy gap of the FeSeTe film is about 2.06 meV. A dip associated with information about superconducting energy gap has been observed from differential conductance characteristics at low temperatures. We build a Superconductor/Semiconductor tunneling theory to fit the experimental curve and theoretical curve then extract energy gap from fitting results.