Room-temperature Growth Of Superconducting NbN Thin Films For LCT Submillimeter Telescope

Terahertz band accounts for nearly half of the space photon energy after cosmic microwave background(CMB),which is of great significance for understanding the state and evolution of the universe.The research of terahertz detectors has become one of the frontier fields of modern astrophysics,which has aroused the enthusiasm of many scientific research institutions in the world.The LCT submillimeter telescope dominated by Our China-Chile International Joint Laboratory for Submillimeter Astronomy is one of the most advanced telescopes in the world,which adopts superconducting SIS coherent detector.In this detector,the growth quality of superconducting material will directly affect the barrier layer of suprconducting SIS tunnel junction and the performance of the mixer.Using NbN with good superconductivity and narrow transition width as superconducting material and developing room-temperature growth technology will help to prepare superconducting SIS tunnel junction mixer with high sensitivity and high operating frequency.Furthermore,it will lay a solid foundation for the preparation of superconducting SIS coherent detector and the upgrade of LCT telescope.In this thesis,ion beam sputtering and magnetron sputtering were respectively adopted to prepare NbN thin films,and the growing conditions were studied.X-ray diffraction(XRD),energy dispersive X-ray spectroscopy(EDS),scanning electron microscopy(SEM),atomic force microscopy(AFM),X-ray photoelectron spectroscopy(XPS)and physical property measurement system(PPMS)were used to characterize the crystal structure,element ratio,surface morphology,cross section morphology,atomic valence state and superconductivity.Next is the main research results:1.The ion beam sputtering technology has the advantages of high-kinetic-energy sputtered particles and compact film surface.The changes of crystal structure,surface morphology,element ratio,atomic valence and superconductivity were investigated by adjusting auxiliary ion source,Ar/N₂gas flow ratio,and sputtering voltage.The physical mechanism was explored to find out the relationship between the sputtering conditions and the microstructure and physical properties of NbN thin films.The results show that the NbN thin films prepared by ion beam sputtering have the-NbN phase structure,and the surface is smooth and compact.Because the ion beam sputtering system is greatly limited in the type of gas,sputtering pressure,gas flow and so on,no superconducting NbN thin films have been achieved.However,the obtained growth parameters and the main factors affecting the quality of NbN thin films provide a reference for the subsequent growth by the magnetron sputtering technology.2.Based on the preparation of NbN thin films by ion beam sputtering,the further fabrication were performed by magnetron sputtering.The magnetron sputtering system used in this experiment has the characteristics of multi-chamber,low substrate temperature and high ion state.The relationship between the crystal structure,surface morphology,cross section morphology,superconductivity and Ar/N₂gas flow ratio,gas flow of Ar and N₂,sputtering power was investigated.According to the relationship between growth conditions and microsturcture and properties,the growth of NbN thin films were further optimized.The results show that the superconducting NbN thin films prepared by magnetron sputtering have good crystal quality with the?-NbN phase structure,and the surface is compact and uniform.The ratio of N to Nb in the thin films is close to the stoichiometric ratio of 1:1,and the superconducting transition temperature reaches13.73 K.The NbN thin films obtained in this thesis process the superconduting properties that can meet the development needs of superconducting SIS coherent detector for the LCT telescope.The results will lay a solid foundation for the further growth of barrier layer,and the preparation of superconducting SIS tunnel junction and mixer.