Abstract
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.
Highlights
The quality of the electrical contact between a semiconductor and a metal electrode is one of the key process elements in building high performance microelectronic circuits[1,2]
It was anticipated that the Schottky barrier could be free from the unwanted leakage and pinhole effects that were present in large scale, high transparency normal metal-insulator-superconductor (NIS) junctions
Our results are strongly linked to the physics of semiconductor-metal junctions and one of the key observations is that a metal electrode in the superconducting state acts as a sensitive probe to the metal-semiconductor surface states, which often dictate the junction resistance through the Fermi level pinning effect
Summary
The quality of the electrical contact between a semiconductor and a metal electrode is one of the key process elements in building high performance microelectronic circuits[1,2]. One example is a Schottky junction in the tunnelling limit with temperature below the superconducting critical temperature of the metal electrode (see Fig. 1a,b) Such a semiconductor-superconductor (Sm-S) cooler junction introduces strong energy filtering for the tunnelling electrons due to the superconducting gap and the sharp peaks in the quasiparticle density-of-states (DOS) around the gap (see Fig. 1b). It was anticipated that the Schottky barrier could be free from the unwanted leakage and pinhole effects that were present in large scale, high transparency NIS junctions. It turned out that the high transparency (low resistance) Sm-S junctions that are needed for efficient coolers, did not behave according to the expectations[16,17] They suffered from significant sub-gap leakage, which can be described phenomenologically by smearing of the ideally sharp density of states in the superconductor (see Fig. 1c). Our results are strongly linked to the physics of semiconductor-metal junctions and one of the key observations is that a metal electrode in the superconducting state acts as a sensitive probe to the metal-semiconductor surface states, which often dictate the junction resistance through the Fermi level pinning effect
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