Research On Growth Of NbN/AlN Thin Films Based On Terahertz Superconducting SIS Tunnel Junction

After the cosmic microwave background radiation,nearly half of the photon energy in the universe is concentrated in the terahertz band,so the astronomical observation and research of this band has become one of the frontier fields of modern astrophysics.Submillimeter waves belong to the terahertz band,corresponding to the wavelength range of 0.1?1 mm.In recent decades,the rapid development of high-sensitivity detection technology based on low-temperature superconducting devices has promoted the remarkable progress of submillimeter wave astronomy.The LCT submillimeter wave telescope,led by the China-Chile Joint International Laboratory for Submillimeter Astronomy,is one of the most advanced telescopes in the world.Its core detection component is a superconducting SIS mixer.In the mixer,the NbN thin film with high transition temperature and good stability is selected for the superconducting layer,while the AlN thin film with low dielectric constant and low band width is selected for the barrier layer.The growth quality of the superconducting layer and the barrier layer will directly affect the performance of the mixer,so the development of the growth technology for high-quality NbN and AlN thin films is the key to enhance the mixer performance.It will lay a foundation for the preparation of the next superconducting SIS tunnel junction and the independent research,development and upgrading of the mixer.In this paper,the RF magnetron sputtering method was used to grow the NbN and AlN thin films,and the best growth conditions were explored.The crystal structure,surface morphology,cross section morphology,element valence state,superconducting properties and electrical properties of thin films were characterized by using X-ray diffraction(XRD),scanning electron microscope(SEM),atomic force microscope(AFM),X-ray photoelectron spectroscopy(XPS),physical property measurement system(PPMS)and semiconductor parameter analyzer.The main results are as follows:1.First,the substrate material was changed to grow the superconducting NbN thin films,respectively on magnesium oxide(MgO),silicon dioxide(SiO₂),sapphire(Al₂O₃)and silicon(Si).Sapphire substrate was selected as the best substrate for the growth of NbN thin films by analyzing the hydrophilicity of substrate with contact Angle analyzer,combining with the characterization of microstructure and superconductivity.Then,the influence of substrate temperature and sputtering time on the structure and properties of NbN thin films was studied by the control variable method.The results show that the superconducting NbN thin films can grow at low temperature on sapphire substrate by optimizing the growth conditions.The prepared NBN thin films have a preferred growth orientation along the(111)direction.The surface is uniform and dense,and the roughness is 0.42-5.49 nm.The Nb and N ions exist in the valence states of Nb³⁺and N^3-,respectively.When the substrate temperature is 150?,the superconducting transition temperature of NbN thin film reaches 16.58 K.2.On the basis of the above NBN thin films,AlN thin films were further grown at room temperature as the barrier layer of superconducting SIS tunnel junction.By the control variable method,the effects of gas flow ratio and AlN film thickness on the microstructure and electrical properties of AlN thin films were investigated respectively,and the corresponding variation rules of structural and properties were obtained.The results show that when the gas flow ratio is Ar:N₂=15:4,the AlN thin film exhibits a good c-axis preferred orientation.The surface roughness is in the range of0.39-0.60 nm.The Al ion exists in the valence state of Al³⁺.The insulating properties of AlN thin films are easily affected by the film thickness.When the film thickness is15.9 nm,the AlN thin film shows good insulating properties,with the leakage current as low as about 1?10^-11 A.In summary,the NbN and AlN thin films obtained in this study have good superconducting and insulating properties,respectively.They are expected to be used as superconducting layer and barrier layer materials in the superconducting SIS tunnel junction.