Influence of thickness,width and temperature on critical current density of Nb thin film structures

The density of critical currents j_C in Nb thin films with thickness smaller than 15 nm and width between 100 nm and 10 μm has been measured in a wide temperature range. We have found that the temperature dependencies of j_C in sub-micrometer wide bridges at 0.7T_C < T < T_C are well described by the Ginzburg–Landau de-pairing critical current. In wider bridges already at T < 0.9T_C the j_C value is significantly reduced due to the penetration and de-pinning of magnetic vortices.     

超导纳米线单光子探测器件的制备

超导纳米线单光子探测器件(SNSPD)是超导单光子探测系统的核心器件。文中介绍了成功制备的基于5nm厚的NbN超导超薄薄膜的SNSPD器件。器件核心结构为150nm宽的纳米曲折线结构,纳米线条占空比为75%,面积为20μm×20μm。超导纳米线是利用电子束曝光(EBL)技术和反应离子刻蚀(RIE)等工艺技术制备的。所制备的SNSPD样品,在温度3.5K下的临界电流约12.9μA;在1310nm波长光波辐照,12.5μA的偏置电流下,探测效率约0.016%。     

Proximity effect in Nb/Zr multilayers with variable Nb/Zr ratio

We have investigated the effect of varying the individual layer thickness on the superconducting transition temperature (T_C) of Nb/Zr multilayers. These thin film multilayer structures were deposited using UHV magnetron sputtering with layer thickness ranging from 0.5 to 8 nm. In conformity with the predictions of the de Gennes equations in the Cooper limit (layer thickness small compared to coherence length), we find that the T_C increases with increasing thickness of the Nb layer (when the Zr layer thickness is constant), and decreases with increasing thickness of the Zr layer (when the Nb layer thickness is constant). The possible effect of the existence of an interfacial Nb-Zr layer is discussed. We also point out the marked influence of the in-plane grain dimension on the T_C in these multilayers.     

产生蓝色相干辐射和实现Nd:YAG 1.0642μm倍频90°非临界相位匹配的Nb:KTiPO4

本文报道了用熔盐顶部籽晶法生长Nb浓度(0～13)mol%的Nb:KTP晶体的倍频的Ⅱ型相位匹配的截止波长和Nd:YAG1.0642μm及Nd:YA1O₃1.0795μm激光在这些晶体中倍频的最佳相位匹配角的测量结果.从中可看出,由于Nb⁵⁺的引入使KTP晶体倍频的Ⅱ相位匹配的截止波长有效蓝移,目前已使截止波长蓝移至937nm且有效产生468.5nm的倍频蓝光.同时Nb⁵⁺的引入使Nd:YAG1.0642μm激光和Nd:YA1O₃倍频的最佳相位匹配方向产生很大的变化,目前已使Nb:KTP晶体倍频的最佳相位匹配方向为θ=88.32°、Φ=0°,非常接近于90°非临界相位匹配方向.     

Lithium electrochromic properties of atmospheric pressure plasma jet-synthesized tungsten/molybdenum-mixed oxide films for flexible electrochromic device

An investigation is conducted into the enhanced lithium electrochromic performance of flexible tungsten/molybdenum-mixed oxide (WMo _x O _y C _z ) films deposited onto 40 Ω/□ flexible polyethylene terephthalate/indium tin oxide substrates, using a low temperature (~23 °C) atmospheric pressure plasma-enhanced chemical vapor deposition with an atmospheric pressure plasma jet at various substrate distances. The rapid synthesis of flexible WMo _x O _y C _z films is performed by injecting the mixed hexacarbonyl and precursors W(CO)₆ and Mo(CO)₆ into an air plasma jet and exposing the substrate in the plasmas for short exposure durations (19–34 s) at various substrate distances. The flexible WMo _x O _y C _z films possess the remarkable Li⁺ ion electrochromic performance, even though after being bent 360° around a 2.5-cm diameter rod for 1,000 cycles and tested for 200 cycles of reversible Li⁺ ion intercalation and deintercalation in a 1-M LiClO₄-propylene carbonate electrolyte, respectively, by a potential sweep switching at the scan rates of ±50 mV/s from the potential of ?1 to 1 V and a potential step switching at the potentials of ?1 and 1 V. Significant optical modulation and optical density change of up to 71.2 % and 0.73 at a wavelength of 622 nm are respectively achieved.     

Structural and superconducting properties of ion beam sputtered Nb thin films and Nb/Cu bilayers

We present the results of a study of structural and superconducting properties of polycrystalline Nb thin films (200 Å, 300 Å, 400 Å, 700 Å and 1000 Å) and Nb/Cu bilayers (300 Å/300 Å and 400 Å/300 Å) prepared on Si substrates by ion beam sputtering at room temperature. The thicknesses, roughnesses at the surfaces and interfaces were determined by X-ray reflectivity whereas the grain sizes were determined from grazing incidence X-ray diffraction and transmission electron microscopic studies. The superconducting transition temperature (T_C) of Nb thin films are smaller than T_C of bulk Nb. The Nb-200 Å sample does not show T_C down to 2.3 K. The average size of the grains varies from 42 Å for Nb-200 Å sample to 69 Å for Nb-1000 Å sample. Our results show that the T_C in these polycrystalline films is not only limited by its thickness but also by the size of the grains. The Nb films deposited in situ on the Cu layer (Nb/Cu) show a marginal increase in average sizes of the grains as compare to their respective values in Nb films of same thicknesses. As a result a marginal increase in T_C of these films is also observed. The maximum decrease in T_C due to oxygen intake during deposition should be about 0.5 K from its bulk value (9.28 K). We have attributed the large decrease in T_C in our case on the basis of decrease in the Debye temperature and density of states at the Fermi level for Nb thin films as compared to their respective values for bulk Nb.     

Physical investigations on pulsed laser deposited nanocrystalline ZnO thin films

The nanocrystalline ZnO thin films were deposited by pulsed laser deposition on quartz and i-Si (100) substrates at different substrate temperatures (473 K–873 K) and at different mixed partial pressures (0.05, 0.01, and 0.5 mbar) of Ar+O₂. The structural studies from XRD spectra reveals that the films deposited at 0.05 mbar and at lower substrate temperatures were c-axis oriented with predominant (002) crystallographic orientation. At 873 K along with (002) orientation, additional crystallographic orientations were also observed in case of films deposited at 0.01 and 0.5 mbar pressures. The composition of Zinc and Oxygen in ZnO films from EDAX reveals that the films deposited at lower partial pressures were have high at.% of O₂ whereas higher partial pressures and substrate temperatures had high at.% Zn. The surface microstructure of the films show that the films deposited at lower partial pressures (0.05 mbar ) and at lower substrate temperatures (473 K) were found to have nanoparticles of size 15 nm where as films deposited at 873 K have nanorods. The length of these nanorods increases with increasing Ar+O₂ partial pressure to 0.5 mbar. The optical energy gap of the film deposited at lower partial pressure and substrate temperature was 3.3 eV and decrease with the increase of substrate temperatures. The films deposited at 0.5 mbar and at 873 K emitted an intense luminescence at a wavelength of 390 nm. The measured thickness of deposited films by spectroscopic ellipsometry is around 456 nm.     

Infrared optical properties of Mn1.56Co0.96Ni0.48O4 thin films prepared by chemical solution deposition

～2 μm thick Mn_1.56Co_0.96Ni_0.48O₄ (MCN) films have been prepared on a Al₂O₃ substrate by the chemical solution deposition method. X-ray diffraction and microstructure analyses show good crystallization and the thickness of the films is 2.12 μm. Mid-infrared optical properties of MCN films have been investigated using transmission spectra and infrared spectroscopic ellipsometry. The optical band gap of the MCN film has been derived to be 0.64 eV by assuming a direct transition between valence and conduction bands. The optical constants and thickness of the thin films have been obtained by fitting the measured ellipsometric parameter data with the classical infrared model. The refractive index n of the MCN films decreases as the wavelength increases, but the extinction coefficient k monotonously increases in the wavelength range of 2–7 μm. The maximal n value is 2.63, and the maximal k value is only 0.024. The above results are instructive for the applications of MCN films in infrared detecting.     

超导纳米线单光子探测器

利用磁控溅射、电子束光刻和反应离子刻蚀等微加工技术,开展了超导纳米线单光子探测器(SNSPD)的研究.通过对SNSPD的设计和制备工艺参数的优化,成功制备出了高质量的SNSPD.单光子检测实验表明,制备的SNSPD对660 nm波长的光信号,系统检测效率可达30%,对1550 nm波长光信号,最大系统检测效率为4.2%.在平均暗计数小于10 c/s的情况下,系统检测效率大于20%(660 nm)和3%(1550 nm). 关键词： 单光子 氮化铌 纳米线 探测器     

Compatibility of glass textures with E-beam evaporated polycrystalline silicon thin-film solar cells

For polycrystalline silicon thin films on glass, E-beam evaporation capable of high-rate deposition of amorphous silicon (a-Si) film precursor up to 1 μm/minute is a potentially low-cost solution to replace the main stream a-Si deposition method—plasma enhanced chemical vapour deposition (PECVD). Due to weak absorption of near infrared light and a target of 2 μm Si absorber thickness, glass substrate texturing as a general way of light trapping is vital to make E-beam evaporation commercially viable. As a result, the compatibility of e-beam evaporation with glass textures becomes essential. In this paper, glass textures with feature size ranging from ～200 nm to ～1.5 micron and root-mean-square roughness (Rms) ranging from ～10 nm to 200 nm are prepared and their compatibility with e-beam evaporation is investigated. This work indicates that e-beam evaporation is only compatible with small smooth submicron sized textures, which enhances J _sc by 21 % without degrading V _oc of the cells. Such textures improve absorption-based J _sc up to 45 % with only 90 nm SiN _x as the antireflection and barrier layer; however, the enhancement degrades to ～10 % with 100 nm SiO _x +90 nm SiN _x as the barrier layer. The absorption-based J _sc is abbreviated by J _sc(A), which is deduced by integrating the multiplication product of the measured absorption and the AM1.5G spectrum in the wavelength range 300–1050 nm assuming unity internal quantum efficiency at each wavelength. This investigation is also relevant to other thin-film solar cell technologies which require evaporating the absorber onto textured substrate/superstrate.     

Optical excitation and emission processes of Si-QD/SiO2 multilayer films with different SiO2 layer thicknesses

Si-rich oxide/SiO₂ multilayer films with different SiO₂ layer thicknesses have been deposited by the plasma enhanced chemical vapor deposition technique, and crystallized Si quantum dot (Si-QD)/SiO₂ multilayer films are obtained after annealing at 1100 ^°C. The photoluminescence (PL) intensity of the multilayer films increases significantly with increasing SiO₂ layer thickness, and the PL peak shifts from 1.25 eV to 1.34 eV. The PL excitation spectra indicate that the maximal PL excitation intensity is located at 4.1 eV, and an excitation–transfer mechanism exists in the excitation processes. The PL decay time for a certain wavelength is a constant when the SiO₂ thickness is larger than 2 nm, and a slow PL decay process is obtained when the SiO₂ layer is 1 nm. In addition, the PL peak shifts toward high energy with decreasing temperature only when the SiO₂ layer is thick enough. Detailed analyses show that the mechanism of PL changes from the quantum confinement effect to interface defects with decreasing SiO₂ layer thickness.     

Study of critical exponents in doped La2/3Ca1/3Mn1-y Fe y O3 (y = 0, 0.03) manganite films

In this work, we investigated close to the Curie temperature T _C the critical exponents of the magnetization of doped manganite La_2/3Ca_1/3Mn_0.97Fe_0.03O₃ (LCMFO) thin films, as well as undoped La_2/3Ca_1/3MnO₃ (LCMO). Using a T _C distribution given by the intrinsic magnetic inhomogeneities in these ferromagnets enables the determination of β and δ critical exponents [corresponding to M(T) and M(H) respectively], average Curie temperature < T _C > , and the T _C distribution width, ΔT _C. Additionally, we extracted the critical exponent η = βδ from the fits of ΔT _C as a function of the external applied magnetic field. We found a value of 1.74 ± 0.09 for this exponent, close to that reported in undoped La_2/3Ca_1/3MnO₃ thin films. Even though the substitution effects of the Mn ions by Fe affect the magnetotransport and structural properties of LCMO system, these results suggest that around T _C, the magnetic phase transition, governed by the critical exponents, is similar in both magnetic systems, and belongs to the same universality class.     

Oscillatory behavior of the resistance of three-layer heteroepitaxial Mo/Nb/Mo (001) films

Electrical resistance of a three-layer heteroepitaxial Mo(35 nm)/Nb(d _Nb)/Mo(35 nm) (001) film was found to exhibit oscillatory behavior as a function of thickness d _Nb of the ultrathin Nb interlayer. The oscillation period is equal to one Nb monolayer (0.16 nm). The resistance minimum of such a film corresponds to the interlayer thickness with a half-completed monolayer and is equal to the resistance of an individual Mo film of thickness 70 nm equivalent to the total thickness of the three-layer film. In this case, the three-layer film conducts as if it is a unified layer. The resistance of a three-layer film with completed Nb monolayers is maximum and equal to the resistance of two individual parallel-connected Mo films of half thickness (35 nm) each. These results are explained by the strong influence of the built-in potential, whose magnitude and scattering properties are periodic functions of the interlayer thickness.     

Band gap control and photoluminescence properties of Ba(Co2x Ti1−x )O3 thin films prepared by Sol–gel method

This paper reports on the preparation, characterization and optical properties of transparent Ba(Co_2x Ti_1?x )O₃ (0 ≤ x ≤ 0.06) thin films prepared by sol–gel method and deposited on fused quartz substrate by spin-coating technique. Their formation is confirmed by X-ray diffraction patterns, energy dispersive X-ray spectrometry and Fourier transformed infrared measurements. Hitherto unreported near-band-gap photoluminescence in ultraviolet, at 378 nm (3.28 eV), of exciton origin is observed which remains unaffected with change in excitation wavelength from 320 to 350 nm. A weak defect emission appears in green region. For larger excitation wavelength, i.e., 488 nm, emission arising from localized states again occurs in green region but with lower energy. The occurrence of efficient violet–blue PL emission is related to ‘direct’ band gap and shallow levels with high optical band gap values. Analysis of band gap variation with dopant concentration, determined using Tauc’s plot assuming them both of ‘direct’ and ‘indirect’ nature, also indicates the ‘direct’ nature. Co⁺² ions as dopants promote a decrease of band gap of films linearly. Scanning electron micrographs show the granular and flakes-like surface growth. Atomic force microscopy images show the presence of ribbon-like nanostructured grains throughout the surface of the films which is smooth with small values of surface roughness.     

High-speed planar laser-induced fluorescence of the CH radical using the C^{ 2} \varSigma ^{ + } {-}X^{ 2} \varPi \left( {0,0} \right) band

The potential for kHz-rate or high-speed planar laser-induced fluorescence (PLIF) of the Methylidyne (CH) radical using its \(C^{ 2} \varSigma ^{ + } {-}X^{ 2} \varPi\) (v′ = 0, v′′ = 0) band was investigated. We show that due to its strength and the excitation wavelength (~314 nm), which is conveniently generated by a dye laser operating with a red dye, the C–X(0,0) band is a good choice for CH LIF studies wherein suppression of background scattering is not required. While interference from polycyclic aromatic hydrocarbons is small, that caused by hydroxyl (OH) can be significant. Nonetheless, the OH lines can be avoided, and we observe good CH image fidelity. Most importantly, we show that due to the favorable properties of the CH C–X band (i.e., good signal strength and convenient wavelength), it can be used for PLIF at kHz acquisition rates using a continuously pulsing laser system. This is demonstrated in laminar and turbulent CH₄-air flames with a laser system operating at 10 kHz and delivering ~0.2 mJ/pulse at 314 nm.     

Photon-counting chirped amplitude modulation lidar system using superconducting nanowire single-photon detector at 1550-nm wavelength

We demonstrate a photon-counting chirped amplitude modulation(CAM) light detection and ranging(lidar) system incorporating a superconducting nanowire single-photon detector(SNSPD) and operated at a wavelength of 1550 nm.The distance accuracy of the lidar system was determined by the CAM bandwidth and signal-to-noise ratio(SNR) of an intermediate frequency(IF) signal. Owing to a short dead time(10 ns) and negligible dark count rate(70 Hz) of the SNSPD, the obtained IF signal attained an SNR of 42 d B and the direct distance accuracy was improved to 3 mm when the modulation bandwidth of the CAM signal was 240 MHz and the modulation period was 1 ms.     

Intrinsic mechanical properties of ultra-thin amorphous carbon layers

In this work, we extracted the film's hardness (H_F) of ultra-thin diamond-like carbon layers by simultaneously taking into account the tip blunting and the substrate effect. As compared to previous approaches, which did not consider tip blunting, this resulted in marked differences (30-100%) for the H_F value of the thinner carbon coatings. We find that the nature of the substrate influences this intrinsic film parameter and hence the growth mechanisms. Moreover, the H_F values generally increase with film thickness. The 10 nm and 50 nm thick hydrogenated amorphous carbon (a-C:H) films deposited onto Si have H_F values of, respectively, ∼26 GPa and ∼31 GPa whereas the 10 nm and 50 nm thick tetrahedral amorphous carbon (t-aC) films deposited onto Si have H_F values of, respectively, ∼29 GPa and ∼38 GPa. Both the a-C:H and t-aC materials also show higher density and refractive index values for the thicker coatings, as measured, respectively by X-ray reflectometry and optical profilometry analysis. However, the Raman analysis of the a-C:H samples show bonding characteristics which are independent of the film thickness. This indicates that in these ultra-thin hydrogenated carbon films, the arrangement of sp² clusters does not relate directly to the hardness of the film.     

Nitrogen-doped and gold-loaded TiO2 photocatalysts synthesized by sequential reactive pulsed laser deposition

Nitrogen-doped titanium oxide thin films covered by gold metal nanoparticles were grown on (001) SiO₂ quartz substrates by pulsed laser deposition. A KrF* excimer laser source (λ = 248 nm, τ _FWHM ≤ 25 ns, ν = 10 Hz) was used for the irradiation of TiO₂ and gold metal targets. The experiments were performed in controlled reactive oxygen or nitrogen atmosphere. The layers were grown for photocatalytic applications. Evaluation of photocatalytic activity was performed by photodegradation of methyl orange under near-UV light irradiation. Our results show that nitrogen doping and addition of gold nanoparticles have complementary effects, photoactivity being significantly improved as compared to that of pure titanium oxide.     

Ferroelectric (Na1/2Bi1/2)TiO3 thin films showing photoluminescence properties

Polycrystalline lead-free (Na_1/2Bi_1/2)TiO₃ (NBT) ferroelectric thin films doped with 1 mol% of rare earth (RE) elements are processed on Pt-terminated silicon substrates using a solution deposition method. The thin films that exhibit single-phase perovskite structure show photoluminescence properties with highest intensities in the wavelength range between 700 and 850 nm, depending on RE element. The ferroelectric properties of the pure NBT film (P _r: 20.5 µC cm^?2, E _c: 150 kV cm^?1) are somewhat decreased for the doped films, which is ascribed to decreasing of the number of Bi lone pairs through the substitution of Bi with RE elements in the perovskite lattice.     

Analysis of correlation between electrical and infrared optical properties of anatase Nb doped TiO2 films

Optical and electrical analysis are required to clarify infrared reflection behavior of anatase Nb doped TiO₂ (NTO) films, a novel transparent conductive oxide (TCO). In this paper, UV–vis-near infrared transmission spectrum, Fourier transformed infrared (FTIR) reflection and ellipsometry analysis are conducted on anatase NTO films (Nb doping concentration: 6, 4 and 2 at%) deposited by pulse laser deposition (PLD). NTO samples show good transparency from 400 nm to 1000 nm, but no obvious plasmon induced high IR reflection even when wavelength >3000 nm. Assuming inhomogeneous resistivity (ρ) and mean scattering time (τ) along film thickness, dielectric constants (ε_r, ε_i) and optical constants (n, k) are extracted by fitting spectroscopic ellipsometry using Code-Lorentz (CL) & Drude exponential model. It is shown that unique optical constants and small τ (1.1–2.6 fs) intrinsically prevent NTO to be high infrared reflective TCO.