Growth and Characterization of Nb3Al Thin Films for Low-Noise Terahertz Electronics

Demand on ultimately sensitive heterodyne receivers for, e.g., radio astronomy and environmental science instrumentation, has driven large research activities on novel materials and devices for extending the frequency range into the THz domain. The useful RF band of traditionally used niobium-based superconductor-insulator-superconductor (SIS) mixers is limited by the niobium superconducting gap frequency to 700 GHz. Therefore, for mixing applications at higher frequencies, high-energy gapsuperconductors are needed. As an alternative mixing component, the hot electron bolometer (HEB) device can also offer a compromise solution at THz frequencies and new materials could be an useful extension of this technology as well.This thesis presents the work on deposition and characterization of Nb3Al superconducting high-energy gap material thin films. The focus of the study aims at Nb3Al as a promising material for SIS tunnel junctions and HEB devices. A heteroepitaxial growth on a thin niobium underlayer has been applied to the preparation of the Nb3Al thin films by means of dc magnetron co-sputtering. Subsequent ex-situ rapid heat treatment of the deposited Nb3Al films has been proved to enhance the crystal orderingand thus, the superconducting properties of the film. The grown films have been studied by XRD, AFM, TEM, etc. techniques. The highest critical temperature observed in this work was 15.7 K and the values of all major superconducting parameters, such as penetration depth, coherence length etc. were consistent with previously reported results on A15 materials, hence, confirming the prospective to use such films in SIS tunnel junctions. In addition, an epitaxial growth of Nb3Al films on yttriastabilized zirconia substrate showed substantial possibility to grow monocrystalline Nb3Al ultra thin films.It was also found that the studied Nb3Al thin films possess the ability to carry substantially high current densities in the range of above 10E7 A/cm2 and can possibly find appropriate application where high magnetic fields are to be used

Thin Films and Component Technologies for Low-Noise Terahertz Electronics

This thesis deals with several distinct subjects such as thin film deposition, design, fabrication and characterization of components for low-noise THz electronics.High energy-gap superconducting materials are of specific importance for low-noise THz mixer applications. Extending the upper operation frequency of superconductor-insulator-superconductor (SIS) mixers above 1.5 THz is of particular interest. Here, a novel approach of producing Nb3Al thin film based on DC magnetron co-sputtering using heteroepitaxial, as well as epitaxial growth techniques, has been employed. As a result, a superconducting transition temperature, as high as 15.7 K has been observed. Structural and electrical characterization techniques have been utilized to evaluate the set of material and superconducting parameters of the deposited Nb3Al thin films. The superconducting properties of the obtained Nb3Al films have revealed strong potential for using them in practical applications. It has been also demonstrated that Nb3Al can be epitaxially grown on yttria-stabilized-zirconia substrates, thus opening a possibility for hot-electron bolometer (HEB) applications.Another substantial part of this work deals with the in-house development of phonon-cooled HEB mixers. A reliable deposition process for high-quality ultra-thin NbN films has been established. Epitaxial ultra-thin NbN films grown on 3C-SiC/Si substrate with state-of-the-art performance have shown a strong potential to be used in THz HEB mixers with a purpose for improving the intermediate frequency (IF) bandwidth. A novel membrane-like layout for 1.6-2.0 THz HEB mixer has been suggested, developed and implemented for use with micromachined all-metal waveguides.Being the most used in mm- and submillimeter mixer technology for radio astronomy, Nb/Al-AlOx/Nb tunnel junction stability properties are of great importance. In this thesis, the behaviour of a large number high-quality Nb/Al-AlOx/Nb tunnel junctions, subjected to long-term room temperature aging and thermal annealing is systematically studied and explanations of the observed phenomena are suggested.Finally, the work on development of a planar superconducting IF/biasing circuit for use with a SIS mixer is presented. The circuit was integrated into a 385-500 GHz DSB SIS mixer currently under operation at the APEX telescope. Circuit design methodology, measurement results and overall mixer performance with the integrated circuit are shown

Growth and Characterization of Nb3Al Thin Films for Low-Noise Terahertz Electronics

Demand on ultimately sensitive heterodyne receivers for, e.g., radio astronomy and environmental science instrumentation, has driven large research activities on novel materials and devices for extending the frequency range into the THz domain. The useful RF band of traditionally used niobium-based superconductor-insulator-superconductor (SIS) mixers is limited by the niobium superconducting gap frequency to 700 GHz. Therefore, for mixing applications at higher frequencies, high-energy gapsuperconductors are needed. As an alternative mixing component, the hot electron bolometer (HEB) device can also offer a compromise solution at THz frequencies and new materials could be an useful extension of this technology as well.This thesis presents the work on deposition and characterization of Nb3Al superconducting high-energy gap material thin films. The focus of the study aims at Nb3Al as a promising material for SIS tunnel junctions and HEB devices. A heteroepitaxial growth on a thin niobium underlayer has been applied to the preparation of the Nb3Al thin films by means of dc magnetron co-sputtering. Subsequent ex-situ rapid heat treatment of the deposited Nb3Al films has been proved to enhance the crystal orderingand thus, the superconducting properties of the film. The grown films have been studied by XRD, AFM, TEM, etc. techniques. The highest critical temperature observed in this work was 15.7 K and the values of all major superconducting parameters, such as penetration depth, coherence length etc. were consistent with previously reported results on A15 materials, hence, confirming the prospective to use such films in SIS tunnel junctions. In addition, an epitaxial growth of Nb3Al films on yttriastabilized zirconia substrate showed substantial possibility to grow monocrystalline Nb3Al ultra thin films.It was also found that the studied Nb3Al thin films possess the ability to carry substantially high current densities in the range of above 10E7 A/cm2 and can possibly find appropriate application where high magnetic fields are to be used

Nb/Al-AlOx/Nb Junction Properties\u27 Variations Due to Storage and Mounting

We report studies of room temperature aging and annealing of Nb/Al-AlO x /Nb tunnel junctions with a 2. . . 3 sq.\ub5msize. We observed a noticeable drop of the junction normal resistance Rn unusually combined with increase of subgap resistance Rj as a result of aging. Changes of Rj occur at sufficiently shorter time scale than that of Rn. Variation of both Rn and Rj depend on the junction size. An effect of aging history on the junction degradation after consequent annealing was discovered. We suggest that the observed junction aging andannealing behavior could be explained by diffusional ordering and structural reconstruction in the tunnel AlOx barrier. The diffusion driving such structural ordering and reconstruction of the AlO x tunnel layer is enhanced due to the intrinsic stress relaxation (creep) processes in the underlying Al layer. Also, we discuss the influence of dicing the wafer into the single mixer chip on the junction aging behavior

Aging- and annealing-induced variations in Nb/Al-AlOx/Nb tunnel junction properties

In this paper, we present studies of room temperature aging and annealing of Nb/Al–AlOx/Nb tunnel junctions with the size of 2–3 μm2. We observed a noticeable drop of junction normal resistance Rn unusually combined with increase in subgap resistance Rj as a result of aging. Variation in both Rn and Rj are subject to the junction size effect. An effect of aging history on the junction degradation after consequent annealing was discovered. Discussion and interpretation of the observed phenomena are presented in terms of structural ordering and reconstruction in the AlOx layer, driven by diffusion flows enhanced due to stress relaxation processes in the Al layer interfacing the AlOx layer

Nb3Al thin film deposition for low-noise terahertz electronics

Higher energy gap superconducting materials were always interesting for low-noise mixer applications such as superconductor-insulator-superconductor tunnel junctions (SIS) and hot-electron bolometer (HEB) used in sub-millimeter and terahertz parts of electro-magnetic spectrum. Here, we report a novel approach for producing Nb3Al thin film by co-sputtering from two confocally arranged Nb and Al dc-magnetrons onto substrate heated up to 830\ub0C. Characterization of the deposited films revealed presence of the A15 phase and measured critical temperature was up to 15.7 K with the transition width 0.2-0.3 K for a 300 nm thick film. We measured the film critical magnetic field and studied influence of annealing on the film properties. We have investigated compositional depth profile of the deposited films by spectroscopy of reflected electrons

Nb3Al thin film deposition for low-noise terahertz electronics

Higher energy gap superconducting materials were always interesting for low-noise mixer applications such as superconductor-insulator-superconductor tunnel junctions (SIS) and hot-electron bolometer (HEB) used in sub-millimeter and terahertz parts of electro-magnetic spectrum. Here, we report a novel approach for producing Nb3Al thin film by co-sputtering from two confocally arranged Nb and Al dc-magnetrons onto substrate heated up to 830\ub0C. Characterization of the deposited films revealed presence of the A15 phase and measured critical temperature was up to 15.7 K with the transition width 0.2-0.3 K for a 300 nm thick film. We measured the film critical magnetic field and studied influence of annealing on the film properties. We have investigated compositional depth profile of the deposited films by spectroscopy of reflected electrons

Towards Multi-Pixel Heterodyne Terahertz Receivers

Terahertz multi-pixel heterodyne receivers introduce multiple challenges for their implementation, mostly due to the extremely small dimensions of all components and even smaller tolerances in terms of alignment, linear dimensions and waveguide component surface quality. In this manuscript, we present a concept of terahertz multi-pixel heterodyne receiver employing optical layout using polarization split between the LO and RF. The frontend is based on a waveguide balanced HEB mixer for the frequency band 1.6 – 2.0 THz. The balanced HEB mixer follows the layout of earlier demonstrated APEX T2 mixer. However for the mixer presented here, we implemented splitblock layout offering minimized lengths of all waveguides and thus reducing the associated RF loss. The micromachining methods employed for producing the mixer housing and the HEB mixer chip are very suitable for producing multiple structures and hence are in-line with requirements of multi-pixel receiver technology. The demonstrated relatively simple mounting of the mixer chip with self-aligning should greatly facilitate the integration of such multi-channel receiver

A 211-275 GHz Sideband Separating SIS Mixer for APEX

We present the results of the development and characterization of the sideband separating (2SB) SIS mixer for the APEX band 1, 211-275 GHz.All mixer components, except the IF hybrid, are integrated into a single mixer block. The sideband separation is achieved by using a quadrature scheme where a local oscillator (LO) pumps two identical SIS mixers. The RF power is divided using a waveguide branch line coupler and directed with 900 phase difference to the ends of the substrate, where each path is coupled to the mixer chip through a waveguide to microstrip transition. Preliminary tests of this 2SB mixer show a sideband suppression ratio of about 12 dB and a typical SSB noise temperature of 80K