| Summary: | 碩士 === 國立臺灣大學 === 天文物理研究所 === 106 === Niobium nitride compounds have rich physical properties due to their complexities in stoichiometry and structures. Among these NbxNy compounds, some of them show superconductivity. The δ-NbN phase, with a cubic crystalline symmetry, has a high superconducting transition temperature (TC), up to 17 K. Ultra-thin superconducting δ-NbN film has been widely applied on ultrasensitive devices due to its prominent physical properties, such as having a high superconducting transition temperature, a short intrinsic electron-phonon interaction time, and a high superconducting critical current density. Especially, hot-electron-bolometer (HEB) mixers using -NbN ultra-thin film demonstrate excellent performance on terahertz detection and are used on several astronomical telescopes.
In my thesis study, I focused on the fabrication of high quality ultra-thin superconducting -NbN films and characterize their physical properties with the help of Dr. Hsiao-Wen Chang and other members in Dr. Ming-Jye Wang’s research group. We have realized the epitaxial growth of ultra-thin δ-NbN films on (100)-oriented 3C-SiC/Si substrates at temperature around 760°C by DC reactive magnetron sputtering. The deposition rate is about 0.05 nm/s. The δ-NbN films show superconductivity even with a thickness of 2.14 ± 0.03 nm (∼ 5 unit cells). The high-resolution transmission electron microscope images of films confirm excellent epitaxy and are used for estimating films’ thickness and lattice constant.
We have investigated the magnetotransport properties of ultra-thin NbN films with the thickness ranging from 2.14 ± 0.03 nm to 4.95 ± 0.03 nm under external magnetic field up to 9 Tesla. From the measured Hall resistances, the carrier concentration, n, of film can be calculated, for example n ~ (4.13 ± 0.04) × 1028 m-3 for 3.84 ± 0.02 nm film. Generally, the TC of film decreases as the film thickness is reduced. The degradation of TC can be explained by the scaling law which describes the competition between disorder and superconductivity. Surprisingly, some distinct oscillations of both TC and ρ20K as a function of film thickness are observed with a period near 0.5 nm, where ρ20K is the resistivity of film at 20 K. The transition temperature of NbN film is suppressed under external magnetic field. The upper critical field at zero temperature, μ0HC2(0), was estimated by using the empirical equation of μ0HC2(T) = μ0HC2(0)(1-t2)/(1+t2), where t = T/TC(μ0H = 0). For example, the μ0HC2(0) of 2.14 ± 0.03 nm film is 8.13 ± 0.16 Tesla. Similar to TC and ρ20K, the μ0HC2(0) of film is also oscillating in thickness. The oscillation of TC, ρ20K, and μ0HC2(0) might be explained by the quantum size effect which was used to explain the oscillation of TC in thickness in other superconducting ultra-thin films.
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