High-Jc MgB2 Josephson junctions with operating temperature up to 40 K

Abstract

Sandwich-type MgB 2 / MgO/ MgB 2 Josephson junctions with Au or MgB 2 interconnection were fabricated using hybrid physical-chemical vapor deposited MgB 2 thin films and RF-magnetron-sputtered MgO barrier. The junctions show properties similar to those in high-J c Nb junctions with J c up to 275 kA/ cm 2 at 4 K, which remains nonzero up to 40 K. Critical current modulations by applied magnetic field and constant voltage steps under microwave radiation were observed. Combined with the larger energy gaps in MgB 2 than in Nb, the junctions presented here allow simple MgB 2 digital circuits to work over 20 K or with a clock speed above 1 THz. © 2010 American Institute of Physics. ͓doi:10.1063/1.3298366͔Superconducting integrated circuits using Josephson junctions have higher speed and lower power consumption than semiconductor circuits.1-3 However, the only mature technology uses Nb ͑critical temperature T c = 9.25 K͒, which requires a low operating temperature of about 4 K. Superconducting circuits using the 40 K superconductor MgB 2 can operate above 20 K ͑Ref. 4͒ cooled by more efficient, more compact, and less costly cryocoolers than those used at 4 K. 5Such a breakthrough requires fabrication technologies for MgB 2 Josephson junctions with high critical current density J c and multilayers consisting of superconducting, insulating and resistive films. There have been reports of Josephson junctions with one MgB 2 and one low-T c superconductor electrodes, 6-8 which cannot operate above the lower T c . All-MgB 2 weak-link junctions 9-13 are not ideal for integrated circuits. All-MgB 2 sandwich-type Josephson junctions have been reported, 14-16 some with near-ideal Fraunhofer patterns, however, the J c values are very low and drop quickly to zero at 20-25 K. In this letter, we report MgB 2 / MgO/ MgB 2 sandwich-type Josephson junctions with high J c up to 275 kA/ cm 2 at 4 K, which remains nonzero up to 40 K. The ultimate speed limit of Josephson junction-based rapid single flux quantum ͑RSFQ͒ circuits is proportional to the product I c R n , where I c is the critical current and R n the resistance of the junction in the normal state.1 To eliminate hysteresis in I − V characteristics, tunnel junctions in RSFQ circuits are usually critically damped by an external shunt resistor, and the maximum circuit speed is approximately proportional to J c 1/2 . 17 The highest operating speed for Nb circuit, 770 GHz, has been demonstrated by an RSFQ toggle flip-flop ͑TFF͒ frequency divider with J c greater than 60 kA/ cm 2 . 18 The energy gap in MgB 2 ͑ϳ2.2 meV for the smaller of the two gaps 19 ͒ is larger than that of Nb ͑ϳ1.5 meV͒, implying a limiting clock speed for MgB 2 digital circuits of above 1 THz. 2,18The structure of the MgB 2 / MgO/ MgB 2 sandwich-type Josephson junctions were illustrated in Fig. 1͑a͒. A 100 nmthick MgB 2 film was first grown on a SiC͑0001͒ substrate by hybrid physical-chemical vapor deposition ͑HPCVD͒ 20 at 710°C under conditions similar to those described in Ref. 8. The root-mean-square roughness...

Fabrication of sandwich-type MgB2/Boron/MgB2 Josephson junctions with rapid annealing method

“…All junctions agree with a linear temperature dependence of an SeIeS junction, supporting that the boron film plays a tunneling barrier in our junctions. Junction #1 has a highest critical current of 25.3 mA at 5 K and remains non-zero near 25 K, which is comparable to the best results achieved thus far [10]. Fig.…”

“…These junctions lacked reproducibility and pair current tunneling could not be observed. Subsequently, Chen et al prepared MgB 2 /MgO/MgB 2 junctions with high J C up to 275 kA/cm 2 at 4 K, which remained nonzero up to 40 K [9,10]. Nevertheless, the ultrathin MgO barrier (0.8e2 nm) lacked reproducibility using hybrid physical-chemical vapor deposited MgB 2 thin films.…”

“…Junctions using superconducting MgB 2 can work above 20 K cooled by more cost-efficient cryocoolers than Nb junctions, which is the only mature technology for Josephson junctions. Give that the potential advantages of all MgB 2 junctions, with high quality tunnel barriers, to obtain high critical current density J C and multilayers consisting of superconducting, insulating and resistive films, have been established, the primary requirement is an essential fabrication technology [2]. However, fundamental challenges exist under practical conditions, because disorder, nonstoichiometries and contamination occurs during the hightemperature growth of the top MgB 2 electrode (600e850 C) on the oxide or nitride barrier [3,4].…”

Momentum-dependent multiple gaps in magnesium diboride probed by electron tunnelling spectroscopy

“…3a). These are properties of clean superconductor junctions that must be confronted when considering their applications 27 .…”

Large scale two-dimensional arrays of magnesium diboride superconducting quantum interference devices

“…Flat steps occur in I-V for currents near 480 and 640 lA that we attribute to local heating effects like those described by Skocpol et al 22 These "hot spots" are common in MgB 2 Josephson junctions with high current densities. 23 Figure 3(b) shows I-V measured at 10.3 K modulated by an ac magnetic field. The modulation amplitude of the voltage as a function of current is shown in the inset.…”