Elektronik Uygulamalar(EN)

Superconducting Electronics

1. Superconducting Radiation Detectors

1.1. TES

Superconducting transition edge sensor (TES) bolometers sense small temperature changes that occur when photons are absorbed and converted to heat. Any bolometric sensor employs three basic components: an obserber of incident energy, a thermometer for measuring this energy, and a thermal link to base temperature to dissipate the absorbed energy and cool the detector. TES detectors are attractive to the scientific community for a variety of reasons. Among their striking attributes are an unprecedented high detection efficiency customizable to wavelengths from the milimeter regime to gamma rays, and a theoretical negligible bacground dark count level.
AIBU group has experience on thin film development, characterization and testing. At the same time ETU SEL and Ege SSL groups has experience on bolometer development, fabrication, characterization and testing. With a close collaboration between AIBU, ETU SEL and Ege SSL, the group aims to develop large scale TES systems together with read-out electronics.

2.2. Superconducting Tunnel Junctions (STJs)

Superconducting Tunnel Junctions (STJs) are the devices consisting of real superconducting tunnel junctions (Josephson junctions) used for detecting low energy photons with the resolution of a single photon. STJs can detect a low energy single photons by using the cooper-pair breaking and tunneling current pulses mechanisms. Because of their ultra-sensitive detection ability, STJ detectors can be used for optoelectronic applications where ultra high resolution is needed.

Ege SSL group has experience on STJ development based on their active collaboration with US group in CA. So, after establishing the cleanroom, this group will start collaborating the ETU SEL on development of ultrasensitive single photon detectors with STJs and TES.

2. Superconducting Integrated Circuits

Superconducting integrated circuits (Rapid single flux quantum circuits – RSFQ) are a kind of digital and analog electronics technology for signal switching based on the quantum effects of superconducting materials and main circuit elements are Josephson Junctions and Inductors. Logic definition is based on the existence of the magnetic flux quanta in the Josephson junction – Inductor loops. Since RSFQ circuits are based on the superconductivity, cryogenic environment is needed for their operation. The basic advantages of RSFQ over their CMOS equivalents are the switching ability up to THz frequencies and much lower power consumption. Furthermore, RSFQ technology is compatible with standard CMOS design and production systems so that the integration of RSFQ circuits with CMOS and optical based circuits is possible. As of today, three commercially available foundry services (SRL in Japan, Hypres in USA, and IPHT-Jena in Europe) are available for integrated circuit fabrication.

It is believed that RSFQ technology is an excellent candidate for petaflops-scale computers since conventional silicon-based technology is getting to its limits. It should be noted that, RSFQ circuits should not be considered as a candidate to replace the semiconductor based systems. It mainly is considered as a complementary technology where the semiconductor technology is inadequate. In the recent decade, the developments in the design and production area enabled the demonstration of RSFQ technology in potential real life applications such as microprocessors, network routers, software defined radio systems by several groups so far.

ETU SEL is working in this field where the group is now focussed on developing optimized libraries, designing and testing superconducting integrated circuits such as Analog to digital converters and arithmetic logic units. There are close collaborations between Japanese and European RSFQ groups. At the moment, current mass is sufficient to fulfill the development of a new optimizer and circuit analyzer tool as well as a simple ALU to test the developed tool.

With a close collaboration with Ege SSL the group aims to develop a sensor system with RSFQ read-out.

After the cleanroom for device process as explained in Section 9 has been established and fully functionalized, starting with some simple circuits, research on fabrication of integrated circuits are also planned.

3. Magnetic Field Sensors (SQUIDs)

Superconducting QUantum Interference Devices (SQUIDs) are the most sensitive magnetic field sensors that have been developed so far. SQUID, basically, consists of superconducting loop interrupted by one or more (depending on the operation mode; RF, DC or digital) Josephson junctions and can detect magnetic flux variations down to flux quantum. Since the SQUID sensor detects incoming flux and produces periodic voltage signal with period of flux quanta, it is commonly known as flux-to-voltage transducer. This ultra-sensitive nature of SQUIDs makes them the most powerful and indeed the most desired sensor elements in circuits and systems specifically designed to sense any physical quantity that can be converted into magnetic field signals such as position, current, voltage etc. SQUID and STJ based systems are currently being developed (and some of them have already been commercialized) for various applications such as underwater (submarine) detection, mine detection, food analysis, DNA research, low field MRI, MCG, scanning SQUID microscope, NDE etc. They are also being used as current amplifiers in the cryogenic dark matter search, X-ray absorption spectroscopy and low energy single photon detection experiments by integrating them to ultra-sensitive superconducting radiation detectors (TES and STJs).

Currently, many research groups and companies in the world are actively working on the SQUIDs and developing SQUID based systems for either scientific or commercial purposes. Although most of those groups are mainly working on low-Tc (Nb based) SQUIDs, there are several research groups developing HTS (YBCO) based SQUIDs and systems. In Turkey, on the other hand, there is only one research group developing HTS YBCO based DC-SQUID sensors and system development technologies. Two research groups from Ege University (Ege SSL) and TOBB ETU (ETU SEL) are in active collaboration for developing the HTS SQUID systems for real life applications. However, there is no any group working on the Nb based process yet. SQUID fabrication process details are described in Section 9 (Superconducting Electronics Device Process).

It might be considered that the critical mass in Turkey to keep the work going on SQUID development with both HTS and LTS thin films (or more generally, on small scale superconducting electronics) is to grow the number of researchers (including MSc and PhD students) up to 20% of the total superconductivity community. Currently, we are far below this percent, but the recent activities (discussions, conferences, workshops etc.) showed that the young researchers would like to start or switch-on working in the field of small scale superconductivity. The main challenge on this might be setting up the cleanroom, fabrication and test facilities.

Mainly 2-year plan from the beginning has to be scheduled by focusing on a full functional cleanroom establishment and process system installation in a specific manner. This laboratory has to be convenient for both HTS and LTS device fabrication and testing. After establishing this lab, current groups will be more than ready to develop SQUID and related superconducting electronic devices. Following the device development, and of course know-how accumulation in two-year period, SQUID based systems explained above could be developed and commercialized in 5-year plan.

4. Intrinsic Josephson Junctions – THz source

4.1. Mesa Structures (L. Özyüzer)
4.2. Whiskers

Application of high temperature (HTc) superconducting materials to the technological fields is very important for technological advances in the future. Different research centers, institutes and also industrial organizations are intensively studying on this subject and significant improvements have also been provided. By considering the fact that some HTc superconductors with highly anisotropic property, such as Bi2Sr2CaCu2O8, (BiPb)2Sr2CaCu2O8, Tl2Ba2Ca2Cu3Ox, showed intrinsic Josephson effect (IJE), small-scale devices are fabricated for micro/nano-electronic industry.

For the nano/micro technological applications of superconductors, high quality and single phase materials are required. Single-crystal HTc superconducting whiskers fit these requirements due to their perfect structural, electrical and magnetic properties. They are sufficiently flexible for handling and small vibrations. Recently, BSCCO whiskers are used successfully in some device applications, such as Josephson junctions and terahertz frequency generators/oscillators using Josephson plasma properties. They have also significant potential to use as SQUID or microwave radiation detectors and high speed switching gates.

Inonu University, Superconductivity Research Group is studying to fabricate the superconducting whiskers, to prepare intrinsic Josephson Junctions and to apply the technological fields. However, the number of researchers studying on this field is not sufficient compared to the number of researchers studying on IJJ on the world. We have scheduled to establish new laboratories such as cleanroom and to install new devices in 2-year plan. We believe that once the clean room and electronics applications laboratory is established, the researchers in Turkey working in the field will be able develop new IJJ applications.

5. Quantum Computing

Practical implementation of quantum computation algorithms requires development of a special kind of hardware, which can broadly be described as a controllable many-body quantum network. Superconducting devices such as superconducting quantum interferometers, quantum boxes, and superconducting single electron transistors are suitable candidates for qubit applications. In contrast to microscopic qubits such as ions and spins, decoherence effects is minimized in superconducting based qubits. In addition, control mechanism of superconducting qubits is much better than their counterparts. Currently, some qubits are experimentally realized.

Josephson Junctions and Quantum Computing (JJQC) Group is planning to work on the following research aspects for the coming 2 and 5 years:

• The first one is related with numerical modeling and simulation of different types of superconducting qubits such as
• Josephson junction charge qubits
• Josephson junction phase qubits
• Flux qubits
• Coupled charge and phase qubits
• Possible new type of superconducting qubits
• The second one is related to qubit operations and decoherence effects.
• Decoherence of qubit systems
• Qubit readout methods
• Superconducting qubit coupling
• The third one is related with the possible realization of the superconducting qubits in corporation with experimental groups in Turkey.
• Development of Josephson junction fabrication and ultra-low temperature measurements
• Experimental realization of superconducting qubits and entanglement effects

For the first two years, the research will concentrate on modeling and simulation. The research group (i.e., Ankara-Turgut Özal University group) is currently working on this aspect. For the five years, on the other hand, the research progress will include other stages such as designing, fabricating, testing, and implementation. For this purpose, we are planning to use some research facilities and experiences of ETU-SEL, EGE-SSL, and IYTE-THz groups. For the realization of this topic, at least 10 researches are required.

6. Dynamics of Josephson junction(s)

Josephson junction devices have been used in many applications such as ultrahigh sensitive detectors and superconducting quantum interference devices. From this point, investigation of the dynamics of Josephson junction has remained interesting for fifty years. It has been investigated that the Josephson junctions based on high-Tc superconductors and other type superconducting structures such as superconductor-anti-ferromagnetic superconductor structures and superconducting superlattice structures reveals anharmonic current-phase relations with negative and positive sign of the amplitude of second harmonics.

Josephson Junctions and Quantum Computing (JJQC) Group is planning to work on the following possible research topics for the coming 2 and 5 years:
• Calculation of IV characterizations of different type of junctions
• Influence of shunted effects and many junctions on Dynamics
• Chaotic phenomena in Josephson circuits and application in communication networks

For the coming two years, the research will concentrate on modeling and simulation of dynamics and average characteristics of different Josephson systems. The research group (i.e., Ankara-Turgut Özal University group) is actively working on this field. For the five years, on the other hand, the research progress will be extended to cover design, fabrication, testing, and implementation. For the realization of this research, we are planning to use some research facilities and experiences of ETU-SEL and EGE-SSL groups. As a human resource, at least 8 researches are required or this case.

7. Superconducting Electronic Device Process

Superconducting electronics devices such as SQUIDs, TES, STJs and RSFQ are required rather complex fabrication and test facilities especially for the Nb based devices. A full functional Class-100 cleanroom and process systems have to be installed in a specific process line in order to get high quality tunnel junctions, clean interfaces and thus high-yield on the wafer-scale productions. The fabrication process of these low-Tc devices, technically, is similar to the CMOS process, but, these devices are required a specific lab environment because of the high sensitivity of the superconducting thin film materials and tunnel junction interfaces to the cross contamination. Therefore, although there are a few cleanrooms around in Turkey for CMOS or semiconducting materials process, we need to establish a specific cleanroom for the superconducting device fabrication. Furthermore, these devices are required different deposition systems from the CMOS devices.

In the cleanroom, the following equipments are planned to be established:

1. UHV Thin film deposition system (Multitarget DC/RF Magnetron Sputter with Load-lock)
2. Transparent thin film thickness measurement system
3. Reactive Ion Etch (RIE) system with end point detector
4. Tabletop electron microscope
5. Wafer scriber
6. Closed cycle dilution refrigerator (10 mK)
7. Mask aligner
8. Bright/Dark field optical microscope
9. Wet bench stations
10. Photoresist spinner-develop-clean-etch system

Currently Ege SSL group is working on the YBCO SQUID and bolometer fabrication. In addition, they have Nb based device fabrication experience since this group is in active collaborations with the Berkeley Microlab and SFSU groups in USA. ETU SEL group has experience of design and modeling of superconducting sensors and circuits and has close collaboration with Nagoya University, Japan as well as European groups. In addition, JJQC group has a considerable experience in theoretical modeling of Josephson Junctions. Therefore, after establishing the cleanroom, with increasing collaboration among these groups, initial devices and circuits will come to life.