Interfacial Engineering of Semiconductor-Superconductor Junctions for High Performance Micro-Coolers.

The control of electronic and thermal transport through material interfaces is crucial for numerous micro and nanoelectronics applications and quantum devices. Here we report on the engineering of the electro-thermal properties of semiconductor-superconductor (Sm-S) electronic cooler junctions by a nanoscale insulating tunnel barrier introduced between the Sm and S electrodes. Unexpectedly, such an interface barrier does not increase the junction resistance but strongly reduces the detrimental sub-gap leakage current. These features are key to achieving high cooling power tunnel junction refrigerators, and we demonstrate unparalleled performance in silicon-based Sm-S electron cooler devices with orders of magnitudes improvement in the cooling power in comparison to previous works. By adapting the junctions in strain-engineered silicon coolers we also demonstrate efficient electron temperature reduction from 300 mK to below 100 mK. Investigations on junctions with different interface quality indicate that the previously unexplained sub-gap leakage current is strongly influenced by the Sm-S interface states. These states often dictate the junction electrical resistance through the well-known Fermi level pinning effect and, therefore, superconductivity could be generally used to probe and optimize metal-semiconductor contact behaviour.

Mobility spectrum computational analysis using a maximum entropy approach.

A method to calculate a smooth electrical conductivity versus mobility plot ("mobility spectrum") from the classical magnetoconductivity tensor in heterogeneous structures with the help of a "maximum entropy principle" has been developed. In this approach the closeness of the fit and the entropy of the mobility spectrum are optimized. The spectrum is then the most probable one with the least influence of the personal bias of the investigator for any given set of experimental data and is maximally noncommittal with regard to the unmeasured data. The advantages of the maximum entropy mobility spectrum analysis as compared to the conventional mobility spectrum analysis are demonstrated using a synthetic dataset.