| Cryogenic temperatures below 1 K are of interest both as a source of novel physical phenomena, and as the operating regime for a new generation of superconducting detectors which in recent years have dramatically improved the sensitivity of a variety of analytical techniques. Access to this temperature range, particularly temperatures below approximately 300 mK, can only be attained through the use of relatively complex and costly refrigeration technology such as dilution refrigerators and adiabatic demagnetization refrigerators. In this thesis, we describe experimental investigations of refrigeration based on a recently discovered thermo-electric effect arising in a normal-conductor/insulator/superconductor (NIS) tunnel junction. Because of the energy gap in the excitation spectrum of the superconductor, when properly biased, the junction can be used to selectively remove electrons having energy higher than the Fermi energy from the normal electrode, thus lowering its temperature. Refrigerators based on NIS junctions are extremely compact thin film devices, operate in a continuous mode, dissipate little power, and are easily integrated with cryogenic electronics. However, prior to the investigations described in this report, refrigeration by NIS junctions had only been observed in devices having sub-micron junction dimensions, which greatly limits their practical utility. In this report, experimental results demonstrating electron refrigeration by NIS junctions having areas exceeding 100 mum2 are presented and analyzed. An improved device utilizes a micro-machined silicon nitride membrane for thermal isolation of the junction, enabling the NIS junction to refrigerate both the electrons and the phonons of the thermally active region. Both electron refrigeration and phonon refrigeration devices are well described by a simple theory involving thermal transport though the junction, quasiparticle transport in the superconductor electrode, and thermal conductance to the environment. These devices have demonstrated the largest cooling powers and active volumes for electron and phonon refrigeration by a NIS junction reported to date. |
