Methods employed. The superconducting films of NbN were fabricated by the magnetron sputtering of an Nb target in a mixture of nitrogen and argon onto a silicon substrate; the superconducting films of WSi were fabricated by the magnetron sputtering of W and Si targets onto a silicon substrate. The nanostructures of submicron size were patterned with e-beam lithography. The study of the dynamics of nonequilibrium processes was carried out in the temperature range 1.5 – 5 K. The measurements were done in the hot-electron mode, when absorption of radiation by the electrons in films causes a rise of the electron temperature and a change of the film resistivity. Using the method of submillimetre spectroscopy, we performed a direct measurement of the temperature dependence of the resistance relaxation time, which yielded the temperature dependence of the electron-phonon interaction time. The dependence of the resistance relaxation time on the thickness of the film yielded the time of escape of nonequilibrium phonons into the substrate. To estimate the contribution of diffusion to the relaxation process, we measured the electronic diffusion coefficients of NbN and WSi by studying the temperature dependence of the second critical magnetic field. The measurements of the quantum efficiency, defined as the ratio of the number of photon counts to the number of photons incident on the detector, were done with the use of IR semiconducting lasers, IR spectrometers and pulsed IR lasers. For quantum efficiency measurements in the range 0.5 – 1.7 μm, the sample was coupled to a single-mode fibre, and the number of incident photons was deduced from the power of the incident radiation, which was measured with the powermeter Ophir. For quantum efficiency measurements in the range 1.7 – 6 μm, the sample was installed into an optical cryostat, which allowed free-space coupling of radiation. The following characteristics were also measured: the critical temperature, the width of the superconducting transition, the critical-current density at 4.2 K and the counting rate.
Empirical foundation of the research. Currently, there are several groups that study the structure of the resistive state induced by the absorption of a photon, the role of photon-generated of spontaneous vortices in the formation of the resistive state. The objective of the research was to study the dynamics of the energy-relaxation processes and the mechanism of the formation of the resistive state in nanostrustures made of thin superconducting films of NbN and WSi with the aim to find the optimal material for a single-photon detector with a high counting rate and sensitivity. For this, we chose two markedly different low-temperature superconductors: polycrystalline NbN and amorphous WSi with different energy-relaxation times and electronic diffusion coefficients.
The research results:
We have studied the role of vortices in the formation of the resistive state induced by the absorption of photons by thin superconducting films of NbN and WSi. It has been demonstrated that the evolution of the resistive state depends on the position of the hot-spot in the nanowire.
We have developed a technology of fabrication of nanostructures from thin superconducting films of NbN and WSi. We have developed measurement setups and methods for studying the optical response of NbN superconducting nanostructures, and for a direct measurement of the time of electron-phonon interaction.
New empirical knowledge:
- We have studied superconducting properties and measured material parameters of NbN and WSi nanostructures such as: the sheet resistance, the critical temperature, the width of the superconducting transition and the critical-current density.
- We have studied the characteristic energy-relaxation times in WSi in a wide temperature range.
- We have measured the electronic diffusion coefficient in ultrathin superconducting films of NbN and Wsi.
- We have studied characteristics of the optical response of NbN nanostructures in a wide spectral range vs. radiation power: the quantum efficiency, the maximum counting rate, the dark-count rate.
Possibility of practical application, recommendations for practical applications or outcomes practical applications of the research results
One of the fundamental elements of modern electronics is a thin (a few nanometers) layer of a superconducting material where current carriers are driven out of equilibrium. Adequate modelling of such devices (for example highly-sensitive radiation receivers) requires understanding the mechanisms of energy relaxation of current carriers in such devices. In the hot-electron regime, there are no energy losses due to the bolometric heating of the film and the response time of the detector is determined by the electron-phonon interaction time. Devices employing the hot-electron effect are highly sensitive and have a short response time.
Superconducting single-photon detectors based on the hot-electron effect open entirely new possibilities for applied research in areas where detection of weak signals with a high time resolution is required: quantum-cryptography communication channels, single-molecule spectroscopy, analysis of radiation of quantum dots in semiconducting nanostructures and also for registration of weak astronomical signals.