H. H. Chang

Crystal orientation and thickness dependence of the superconducting transition temperature of tetragonal FeSe1-x thin films.

Superconductivity was recently found in the tetragonal phase FeSe. A structural transformation from tetragonal to orthorhombic (or monoclinic, depending on point of view) was observed at low temperature, but was not accompanied by a magnetic ordering as commonly occurs in the parent compounds of FeAs-based superconductors. Here, we report the correlation between structural distortion and superconductivity in FeSe(1-x) thin films with different preferred growth orientations. The films with preferred growth along the c axis show a strong thickness dependent suppression of superconductivity and low temperature structural distortion. In contrast, both properties are less affected in the films with (101) preferred orientation. These results suggest that the low temperature structural distortion is closely associated with the superconductivity of this material.

TSV process using bottom-up Cu electroplating and its reliability test

TSV (through silicon via) is a core technology in 3D IC package. The micro vias can be made by etching or laser drilling. Standard processes for TSV filling begin with seed layer deposition, followed by blind vias copper electroplating. If the aspect ratio of the TSV is higher than 5:1, the costly MOCVD process needs to be used to deposit the seed layer with good step coverage. A special designed electroplating machine and solution for high aspect ratio copper electroplating is needed. Some researchers even use PRP (periodic reverse plating) for void free copper electroplating instead of traditional DC power supply.

Transport properties in FeSe0.5Te0.5 nanobridges

FeSeTe nanobridges of different widths have been fabricated on MgO substrates using focused ion beams. These nanobridges exhibit the Josephson effects. The current-voltage curves of junctions with 248–564 nm wide follow the resistively and capacitatively shunted junction model. Shapiro steps under microwave radiation were clearly observed in these nanobridges. The products of the critical current and normal state resistance (IcRn) are remarkably high. The temperature dependence of IcRn product followed the Ambegaokar-Baratoff (A-B) relation. The value of energy gap of FeSeTe calculated from the A-B relation is 3.5kBTc. The nanobridge junctions have a strong potential for high frequency applications.

Heterojunction of Fe(Se1−xTex) superconductor on Nb-doped SrTiO3

We report the fabrication of heterojunctions formed by the FeSe0.5Te0.5 (FeSeTe) superconductor and Nb-doped SrTiO3 semiconducting substrate and their properties. At high temperature when FeSeTe is in its normal state, the forward bias I−V 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 I−V 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.

Weak localization in FeSe1-xTex superconducting thin films

We have investigated the magneto-resistivity (MR) of FeSe1 xTex superconducting films on MgO substrate. The MR of pure FeSe and slightly Te-substituted films demonstrates regular Lorentz-type magnetic field dependence, MR B 2 . In highly Te-substituted samples, however, negative MR contribution due to the weak-localization effect gradually dominates at low temperature, which is consistent with the evolution of the temperature dependence of resistivity from a metallic to a weakly semiconductor-like behavior. Furthermore, the negative MR weakens and turns positive as temperature approaches the superconducting transition temperature, which is evidence for the Maki‐Thompson correction in the weak-localization regime. The experimental data can be described very well by the weak-localization theory with the existence of scattering by some magnetic moments. The fitting parameters demonstrate that disorder most likely comes from the excess iron. (Some figures may appear in colour only in the online journal)

The vortex state of FeSe1 − xTex superconducting thin films

We report the vortex dynamics of tellurium substituted FeSe1-xTex superconducting thin films. The electric field versus current density (E-J) curve for films with low Te substitution is still governed by the thermally activated flux flow model at temperatures as low as 0.5T(C)(offset). In contrast, we clearly observed a vortex liquid-glass transition in films with high Te substitution. The E-J curves of these samples fit nicely to the scaling relations based on the 3D vortex glass theory. Our results reveal an enhancement of the vortex pinning as more Te content is introduced, which probably originates from the excess Fe at the interstitial site.