Pavel N. Dmitriev

Optimization of the phase-locked flux-flow oscillator for the submm integrated receiver

The Superconducting Integrated Receiver (SIR) comprises in one chip a planar antenna integrated with an SIS mixer, a superconducting Flux Flow Oscillator (FFO) acting as Local Oscillator (LO) and a second SIS harmonic mixer (HM) for FFO phase locking. Free-running FFO linewidth well below 10 MHz is required to ensure phase-locked operation of an SIR. Comprehensive experimental study of the Nb-AlOx-Nb FFO linewidth and other main parameters has been carried out in order to achieve this goal. Essential dependence of the FFO linewidth on its width and idle region dimension has been found. It makes possible an optimization of the FFO design and selection of the best FFO parameters for practical operation of the SIR.

High quality Nb-based tunnel junctions for high frequency and digital applications

A number of new fabrication techniques are developed and optimized in order to fit the requirements of contemporary superconducting electronics. To achieve ultimate performance of integrated submm receivers with operational frequency of 1 THz, tunnel junctions with AlN tunnel barrier having a R/sub n/S value as low as 1 /spl Omega//spl mu/m/sup 2/ have been developed. High quality characteristics of Nb/AlN/Nb tunnel junctions with R/sub j//R/sub n/=16 and R/sub n/S=10 /spl Omega//spl mu/m/sup 2/ have been demonstrated. Electron Beam Lithography (EBL) in combination with Chemical Mechanical Polishing (CMP) has been incorporated to produce Nb/AlN/Nb junctions with 0.03 /spl mu/m/sup 2/ area. A new approach to obtain overdamped Nb/AlO/sub x//Nb tunnel junctions has been proposed and realized. The dependencies of the main parameters of novel junctions on the current density and circuit geometry have been studied. These junctions may have a good potential in Josephson junction arrays and Single-Flux-Quantum applications (RSFQ).

Radiation linewidth of flux-flow oscillators

The results of linewidth measurements on flux-flow oscillators (FFOs) of a new design with improved parameters are presented. Extensive measurements of the dependence of the free-running FFO linewidth on the differential resistances associated both with the bias current and the control-line current (applied magnetic field) are taken. A profile of the FFO radiation line is measured in different regimes of FFO operation and compared to the theoretical models. A Lorentzian shape of the FFO line is observed both at Fiske steps (FSs) in the resonant regime and on the flux-flow step (FFS) at high voltages. A phenomenological model of the FFO linewidth taking into account all known noise sources (both internal and external) is used to explain the FFO linewidth dependence on the experimental parameters. Finally, we discuss the feasibility of using an electronic phase-locking loop (PLL) over the entire FFO operational frequency band.

Towards a Phase-Locked Superconducting Integrated Receiver: Prospects and Limitations

Presently a Josephson flux flow oscillator (FFO) appears to be the most developed superconducting on-chip local oscillator for integrated submillimeter-wave SIS receivers. The feasibility of phase locking the FFO to an external reference oscillator at all frequencies of interest has to be proven for practical FFO implementation in radio astronomy and other spectral applications. A linewidth of a phase-locked FFO as low as 1 Hz has been measured relative to an external reference oscillator in the frequency range 270–440 GHz on steep Fiske steps in the low damping regime. The increase of the intrinsic linewidth at higher voltages due to an abrupt increase of the internal damping considerably complicates phase locking of the FFO. Comprehensive measurements of the FFO radiation linewidth have been performed using an integrated harmonic SIS mixer. Results on FFO linewidth and spectral line profile have been compared to theory in order to optimize the FFO design. The influence of FFO parameters on radiation linewidth, particularly the effect of the differential resistances associated both with the bias current and the applied magnetic field, has been studied. Two integrated receiver concepts with phase-lock loop have been developed and experimentally tested. 2002 Published by Elsevier Science B.V.

Superconducting transition metal nitride films for THz SIS mixers

The development of sensitive THz SIS mixers requires a low-loss superconducting strip-line material with a transition temperature above 15 K. In this paper we examine the properties of (Nb,Ti)N, NbN, and (Nb,Zr)N thin films sputtered at ambient substrate temperature on glass wafers. The best properties are observed for the (Nb,Ti)N films, sputtered from an NbTi alloy target with 30 at.% Ti. A similar picture is observed for the epitaxial films deposited on the MgO wafers. We have also examined the homogeneity of the (Nb,Ti)N films versus film thickness and in plane since this factor is clearly important for the microwave behavior of the strip-line. We observe that (Nb,Ti)N films deposited on silicon, sapphire, and glass wafers have much worse homogeneity compared to the films deposited on the MgO wafers.

Superconducting Integrated Receiver Based on Nb-AlN-NbN-Nb Circuits

The superconducting integrated receiver (SIR) comprising in one chip a superconductor-insulator-superconductor (SIS) mixer and a phase-locked superconducting flux flow oscillator (FFO) is under development for the international project TELIS. To overcome temperature constraints and extend operation frequency of the SIR we have developed and studied Nb-AlN-NbN-Nb circuits with a gap voltage Vg up to 3.7 mV and extremely low leakage currents . Based on these junctions integrated microcircuits comprising FFO and harmonic mixer have been designed, fabricated and tested; the radiation from such circuits has been measured at frequencies up to 700 GHz. Employment of NbN electrode does not result in the appearance of additional noise. For example, FFO linewidth as low as 1 MHz was measured at 600 GHz, that allows us to phase lock up to 92% of the emitted by FFO power and realize very low phase noise about 90 dBc. Preliminary results demonstrated uncorrected DSB noise temperature of the Nb-AlN-NbN SIR below 250 K at frequencies around 600 GHz.

Source optimization for magnetron sputter-deposition of NbTiN tuning elements for SIS THz detectors

NbTiN is one of the most promising materials for use in the tuning circuits of Nb-based SIS mixers for operating frequencies above the gap frequency of Nb (700 GHz). Device development requires stable and reproducible film properties. In this manuscript we compare the properties of NbTiN films obtained with a sputtering source using balanced and unbalanced magnetic trap configurations. This experiment shows that reducing the effectiveness of the magnetic trap by changing the magnet configuration is equivalent to reducing the sputtering pressure. We also show that it is possible to optimize the configuration of the magnetron magnets to produce stable and reproducible NbTiN films under the same gas pressure and applied power throughout the target lifetime.

Properties of DC magnetron sputtered Nb and NbN films for different source conditions

We have compared the quality of Nb and NbN films obtained by DC magnetron sputtering from a new and a fully eroded Nb target. Since current superconducting electronic devices such as SIS mixers, RSFQ digital circuits and hot electron bolometers are produced by reactive sputtering, we are interested in optimum source operating conditions over the target lifetime. We find that stress-free Nb films can, at any state of the target, be obtained under the same Ar pressure and DC power applied to the sputtering source. We show that this approach also works for NbN reactive sputtering if the nitrogen flow rate is maintained proportional to the deposition rate of Nb. In both cases the zero-stress point shifts to lower cathode DC voltage as the target erodes. Additionally we find that the effectiveness of the magnetic trap of the magnetron influences the normal state resistivity of the NbN films.

Submillimeter superconducting integrated receivers: Fabrication and yield

Fabrication procedure and yield analysis of superconducting integrated receivers is reported. These chip receivers, apart from the quasi-optical SIS mixers, contain internal local oscillators and associated rf and dc interfaces. Due to both complexity and design requirements of the integrated circuit, certain restrictions are applied to the standard Nb/Al/Al/sub x/O/sub y//Nb SNEAP process. To obtain accurate area for micron-size SIS junctions and thickness for multi-layer SiO/sub 2/ insulation, a few solutions and modifications were developed. The possibility of transferring this fabrication process worldwide has been proven experimentally.

Cryogenic Phase Detector for Superconducting Integrated Receiver

New superconducting element, a cryogenic phase detector (CPD) has been proposed and preliminary tested. The CPD is based on a superconductor-insulator-superconductor junction and initially intended for phase locking of a flux-flow oscillator in a superconducting integrated receiver. First results of the CPD development and study are very encouraging; a sinusoidal response of the CPD has been measured at the variation of the phase shift between input signals. Dependences of the output signal and phase response on the CPD bias voltage have been studied; main parameters of this new device are estimated. Important that the CPD output current well above 10 has been measured at the input signal provided by the harmonic mixer of the integrated receiver and amplified by the existing HEMT-amplifier. Due to the large conversion coefficient this current being supplied to the flux-flow oscillator (FFO) control line is sufficient to directly tune FFO frequency. Obtained data demonstrate that the CPD intrinsically could operate with effective bandwidth more than 100 MHz. Preliminary results of the CPD implementation for the FFO phase locking are presented; possible advantages of such combination are discussed.