A CRYOGENIC LOCAL OSCILLATOR BASED ON DISTRIBUTED TUNNEL JUNCTION FOR A SUBMILLIMETER INTEGRATED RECEIVER WITH A PLL

SYSTEM

TECHNICAL FIELD

The invention relates to the general field of superconducting electronics. More specifically, it concerns high-sensitivity submillimeter-range receiving devices featuring high spectral resolution. It can be used in developing on-board and stationary systems designed to be used in radio astronomy, Earth atmosphere monitoring, biomedical research, and safety systems.

BACKGROUND ART

Conventional submillimeter integrated receivers are known to be used for reconstructing spectral properties of received signals (V.P. Koshelets, et al., "Superconducting Integrated Receiver for TELIS", "IEEE Trans, on Appl. Supercond. ", vol. 15, pp. 960-963, 2005). An integrated receiver typically comprises a receiving unit consisting of a superconductive planar antenna and a superconductive mixer based on superconductor-insulator-superconductor (SIS) tunnel junction. A long Josephson junction (a Flux Flow Oscillator, FFO) is used as a local oscillator; see e.g. patents US5339457, Kawasaki, et al., 1994, and US6331805, Gupta, et al., 2001. However, the autonomous line width of FFOs may exceed tens of megahertzes, and an efficient operation of the receiver requires using frequency control systems that could ensure the FFO synchronization with a reference oscillator. There exist conventional systems that provide frequency stabilization in cryogenic oscillators due to e.g. using a delay line based on a high-temperature superconductor microstrip line (US2005040902, Beylor et al., 2005). However, such systems do not provide a fundamental solution to the problem of FFO synchronization with a reference oscillator and do not allow for phase-locked loop operation.

We have earlier proposed a design for an integrated superconductive receiver with a phase-locked loop (FLL) frequency control system (V.P. Koshelets, S. V. Shitov, "Integrated Superconducting Receivers", Superconductor Science and Technology, vol. 13, pp. R53- R69, 2000). This design used integrated superconductive microcircuits manufactured using the conventional technique based on Nb-AlO _x -Nb tunnel structure. Superconductive oscillators based on Nb-AlO _x -Nb distributed Josephson junction have been successfully

tested in use as integrated local oscillator operating in the frequency range of 100 to 700 GHz. They provided sufficient power for pumping a SIS mixer (about 1 μW at a frequency of 500 GHz). Both frequency and output power of a FFO may be widely varied without any mechanical adjustment.

Frequency resolution of a receiver is (along with the noise temperature and the beam pattern) one of the main parameters from the point of view of radio astronomy and atmosphere monitoring. In order to obtain the required frequency resolution, a superconducting local oscillator should be synchronized to a reference oscillator. A harmonic mixer based on a SIS junction is used for stabilizing the FFO frequency (Koshelets, et al., "Linewidth of Submillimeter Wave Flux-Flow Oscillators"; Appl. Phys. Lett., vol. 69, pp. 699-701, 1996). The mixer is used to lower the frequency of the FFO from a few hundred gigahertzes down to 400 MHz. The FFO signal frequency down-converted to 400 MHz is used for phase-locked loop (PLL) system operation.

One of the main characteristics of an oscillator operating in PLL mode is its spectral ratio (SR), which is equal to the share of the oscillator emission power concentrated in a narrow band surrounding its central frequency. In order to ensure a PLL mode with a spectral ratio above 50%, the autonomous line width of the FFO must not exceed 5 MHz. When the line width is about 10 MHz, spectral ratio amounts to 30%, which is sufficient for most practical applications.

Detailed measurements of FFO emission (V.P. Koshelets, et all., "Optimization of the Phase-Locked Flux-Flow Oscillator for the Submm Integrated Receiver", "IEEE Trans, on Appl. Supercond. ", vol. 15, pp. 964-967, 2005) revealed that Josephson radiation results at frequencies above 1/3 of the energy gap (or about 450 GHz for Nb-AlO _x -Nb junctions) in a raise of photon assistant tunneling and, therefore, in a substantial increase of damping in the distributed junction. The resonance mode is then replaced by that of viscous flow of vortices. This transition manifests itself in the voltage-current characteristic by well-defined feature at certain threshold voltage (frequency).

Changes in the nature of processes taking place in the junction also significantly affect the line width of the FFO radiation. At frequencies slightly above the threshold (about 500 GHz for Nb-AlOx-Nb junctions) and at the current density of about 10 kA/cm ² , which is the optimum value for submillimeter SIS mixers, the FFO line width is about 10-30 MHz, which makes the PLL frequency control mode with a high-quality spectrum unattainable at these frequencies. However, it is this range that is especially important for many practical

applications such as Earth atmosphere monitoring from high-altitude aerostats. The above described widening of the oscillation line is a substantial drawback of FFO devices based on Nb-AlOx-Nb junctions. Besides, at frequencies below 450 GHz, continuous frequency tuning of such FFO is impossible due to the presence of Fiske resonance steps. All these reasons make a local oscillator for an integrated receiver based on Nb-AlOx-Nb junctions inoperable in the frequency range of 350 to 550 GHz, which is of an immense importance for practical applications.

DISCLOSURE OF INVENTION

The present invention aims at providing a cryogenic local oscillator (CLO) for a submillimeter integrated receiver with a phase-locked loop (PLL) frequency control system featuring a substantially wider operating frequency range. The cryogenic local oscillator according to the present invention comprises a distributed tunnel junction and a harmonic mixer integrated with it, both implemented based on tunnel junctions in the form of a multilayer film structure. The improvement lies in the fact that all tunnel elements of the CLO are implemented in the form of a three-layer Nb-AlN-NbN film structure. The length of the distributed junction is selected in the range of 300 to 550 μm, which allows enabling a continous frequency tuning; matching elements connecting the distributed tunnel junction are implemented so as to allow matching the abovementioned elements in the frequency range of 350 to 700 GHz.

The technical effect of the invention consists of a substantial widening of the operating frequency range of CLO devices designed for integrated receiving systems. An operating frequency range of a local oscillator is the frequency range wherein the autonomous line width of the oscillator radiation is below 10 MHz; this enables obtaining a PLL frequency control mode with a spectral ratio above 30%.

The fundamental feature of the disclosed solution lies in the use of a new type of Nb-AlN-NbN tunnel junction with the energy gap value up to 3.7 mV, which, in principle, allows raising the operational frequency of FFO units based on such junctions up to 900 GHz.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be disclosed in detail in the following description provided below with references to the accompanying drawings. In the drawings:

Fig. 1 shows a diagram of a cryogenic local oscillator included in a submillimeter integrated receiver of the invention;

Fig. 2 shows a microphotograph of the integrated structure with the cryogenic local oscillator;

Fig. 3 shows voltage-currency characteristics of the Nb-AlN-NbN-Nb FFO as measured in different magnetic fields;

Fig. 4 shows spectra of the Nb-AlN-NbN FFO in autonomous mode and in PLL frequency control mode; and

Fig. 5 shows the frequency dependency of the FFO line width.

BEST MODE FOR CARRYING OUT THE INVENTION

The submillimeter integrated receiver with a PLL frequency control system (Fig. 1) comprises: a FFO 1 operating in the range of 100 to 700 GHz, a harmonic SIS mixer (HM) 2, matching structures 3 connecting the FFO to the HM, and a receiving unit 4 consisting of a superconducting planar antenna and a superconducting mixer based on SIS tunnel junction. The integrated receiver is located in a cryostat at the liquid helium temperature (4.2 ⁰ K).

The FFO 1 is an adjustable, voltage-controlled oscillator operating in the frequency range of 100 to 700 GHz designed for pumping the superconducting mixer included in the receiving unit 4 of the receiver. The FFO 1 operates in unidirectional flow of magnetic flux quanta mode (Nagatsuma, et al., "Flux-flow type Josephson oscillator for mm and submm wave region", J Appl. Phys., vol. 54, p. 3302, 1983). Length of the oscillator junction (about 500 μm) is many times larger than the depth of the field penetration into the tunnel barrier, which is why a junction of this type is called a distributed one.

In a cryogenic oscillator based on distributed Josephson junction, Josephson vortices move under the effect of the applied magnetic field and the carrier current. Each of these vortices contains a magnetic field quantum φ _o = h/2e . Magnetic field can be created in the

FFO using either an external coil or a control line (CL) with a current IQ _L - According to Josephson relation, f = (2π/φ ₀ )V , (1) which means that a Josephson junction biased at voltage V generates electromagnetic oscillations at frequency / (about 483.6 GHz/mV). φ _o is a magnetic flux quantum (2 10 ^"

¹⁵ Wb). Velocity and density of fluxons, and consequently, radiation power and frequency, can be adjusted by changing the bias current and/or the magnetic field.

Fig. 2 shows a microphotograph of the integrated structure with a local oscillator based on Nb-AlN-NbN that was used to determine the FFO characteristics. All main elements can be seen in the photograph: the long Josephson junction (1), the harmonic mixer (2), the matching structure (3), and the receiving SIS mixer with a planar antenna (4).

The device microcircuit is a three-layer film structure manufactured in a single vacuum cycle. The lower electrode made of niobium is covered by a thin (5 to 7 ran thick) aluminum layer using the magnetron deposition technique. This layer is then nitridated in a pure-nitrogen plasma; obtaining the required thickness of the tunnel barrier is possible by varying the discharge power and the nitridation time. Similarly to the case of thermal oxidation, the remaining thin aluminum layer is superconducting due to the effect of its proximity to the niobium, since the coherent length in aluminum is much larger than the layer thickness. Next, the upper electrode made of niobium nitride is sputtered over to the thickness of 100 to 150 run.

Techniques for manufacturing high-quality Nb-AlN-NbN junctions have previously been disclosed in P. N. Dmitriev, et al., "High Quality Nb-based Integrated Circuits for High Frequency and Digital Applications", "IEEE Trans, on Appl. Supercond. ", vol. 13, No 2, pp. 107-110, June 2003. Our subsequent research demonstrated that combining an AlN tunnel barrier with a NbN upper electrode allows obtaining high-quality SIS junctions even at high-density tunneling currents. This is important for developing terahertz-range mixers. The same technique was applied to implement complex integrated microchips where a single crystal contains both a FFO and a SIS mixer.

Results of studies of Nb-AlN-NbN are disclosed in detail below. Our research confirmed the applicability of these junctions as elements of a superconductive integrated receiver operating in the frequency range of 350 to 700 GHz. Fig. 3 shows a family of voltage-current characteristics of the FFO, where each curve corresponds to a specific value of the magnetic field, which is set by applying current to a dedicated control line (from 10 milliamp for the first, leftmost, curve up to 80 milliamp for the last curve in the right- hand part of the plot). The FFO used in this sample was a long Josephson junction with the following parameters: length L = 400 μm; width W = 14 μm; critical current density about 7 kA/cm ² . The SIS junction pumping current induced by the FFO is determined by the maximum power applied by the FFO to the harmonic SIS mixer, which, in its turn, is

determined by the matching quality between the oscillator and the mixer unit. Our studies also revealed that the FFO provides more than enough power for SIS mixer pumping in a test structure with low losses in the matching circuit that was set up for the frequency range of 350 to 700 GHz.

Although FFO devices based on Nb-AlN-NbN junctions feature characteristics similar to those of conventional niobium-based FFO units, there also exist some important differences (see Fig. 3). The first singularity seen in the plot (at about 1 mV) is caused by the singularity corresponding to the difference of energy gaps AmN- ^m - The second singularity is observed at -1.2 mV (600 GHz) where the density of the curves rises: this is the limiting voltage of Josephson pumping. Above this voltage, internal damping grows in the FFO due to self-pumping. Self-pumping (absorption of FFO-emitted microwave radiation by quasi-particles in the long junction channel) significantly alters the FFO properties under the voltage V « V _JSC = \ζ Vg . When the voltage V > V _JSC is reached, differential resistances of the FFO grow significantly higher, which results in widening of the FFO line in this range. This, in its turn, makes phase synchronization of the FFO difficult or impossible. In a completely niobium-based FFO, the value of V _JSC corresponds to the voltage of -0.94 mV (450 GHz). Therefore, using junctions containing NbN allows implementing the frequency range of 450 to 500 GHz, which presents difficulties for Nb junctions.

Continuous frequency tuning in the range below 600 GHz (1.2 mV) is possible for a Nb-AlN-NbN FFO 300 to 550 μm in length, although the damping is not sufficient to fully suppress the resonance structure of Fiske steps. In short junctions featuring a small wave damping factor α, distances between Fiske steps may be so long that FFO frequency tuning would only be possible in certain 'allowed' frequency ranges. For a 400 μm-long Nb-AlN- NbN junction, the damping factor is sufficiently high, Fiske steps manifest a noticeable slope, and the distance between them are short (see Fig. 3). This allows setting any FFO frequency in Fiske mode for 300 to 550 μm-long junctions, but for each frequency, bias current values must lie within a certain range. In this case, there exist forbidden current values, in contrast to forbidden voltage values for niobium FFO devices. This allows developing an automatic working point control system for a cryogenic local oscillator based on a Nb-AlN-NbN junction.

FFO radiation line width was measured using specifically designed microchips, wherein the FFO, a harmonic SIS mixer and high-frequency matching structures were

integrated. The experimental setup included a SIS junction mixer about 1 μm ² in area that had a tuning structure designed to compensate its capacity at the operating frequency, a three-step impedance transformer, and a distributed Josephson junction (DJJ) as the unit under test. Parameters of all high-frequency (microwave) elements were optimized for the frequency range of 350 to 700 GHz. The FFO signal was mixed in the SIS junction with the m ^th harmonic of the reference oscillator f _re f, which produced the intermediate-frequency signal f _1F = ±(f _FF0 -mf _ref ) .

Analysis of Nb-AlN-NbN FFO radiation spectra revealed that the autonomous emission line is of Lorentzian shape (the dashed line in Fig. 4), similarly to that of Nb- AlO _x - Nb junctions. Preliminary studies performed for junctions of different topologies allow assuming that all dependencies of emission line width on FFO topology and parameters established earlier for all-Nb junctions are also valid for Nb-AlN-NbN junctions.

No additional noise was found in the system with a Nb-AlN-NbN FFO, which allowed synchronizing the FFO throughout the frequency range. In particular, the line width amounted to 1.7 MHz at the frequency of 605 GHz (see Fig. 4), which allows synchronizing up to 92% of the FFO power and providing a low noise level of about -90 dB/Hz. At frequencies of about 500 GHz (which present problems for Nb-AlO _x -Nb junctions) emission line width amounted to about 1 MHz.

Fig. 5 illustrates the frequency dependency of the Nb-AlN-NbN FFO radiation line width. It can be seen that the line width does not exceed 3 MHz in the frequency range of 350 to 600 GHz. Once again, it should be noted that due to overlapping of Fiske steps, setting any frequency and continuous frequency tuning are possible in limited ranges. Several stars corresponding to the same frequency mean that the working point can be selected depending on the displacement current (while scattering of line width values is not too large, and all provided values are practically useable). Each star corresponds to an allowed value of displacement current, as described above. Although the FFO tuning at Fiske steps is somewhat difficult, the applied effort is compensated by obtaining the autonomous line width below 3 MHz throughout the frequency range of 350 to 600 GHz.

Thus, an integrated superconductive local oscillator according to the present invention developed based on Nb-AlN-NbN tunnel junctions and featuring energy gap voltage of 3.7 mV and small leakage currents (R _j IR _n = 30 ) enables generation in the frequency range of 350 to 700 GHz. Using an upper electrode made of niobium nitride does not produce any additional noise. The line width is below 1 MHz at the frequency of

500 GHz, which allows phase-locking above 90% of the FFO power and obtaining a low level of phase noise (about -90 dB/Hz).

The fundamental feature of the disclosed solution lies in the use of a new type of Nb-AlN-NbN tunnel junction with the energy gap value up to 3.7 mV, which, in principle, allows raising the operational frequency of FFO units based on such junctions up to 900 GHz.

INDUSTRIAL APPLIABILITY

The device of the invention can be implemented according to the above description using conventional materials and techniques employed in cryoelectronics.

It will be apparent to those skilled in the art that although the present invention has been described in terms of an exemplary embodiment, modification and changes may be made to the disclosed embodiment without departing from the essence of the invention.