Ken’ichi Kuroda

Buried transverse-junction stripe laser for optoelectronic-integrated circuits

A buried transverse‐junction stripe (TJS) laser, a suitable laser diode for integration, has been fabricated using molecular‐beam epitaxial growth. The fundamental characteristics of the laser were as good as those of a conventional TJS laser. We have also demonstrated a fabrication process for the laser that is compatible with that for metal‐semiconductor field‐effect transistors (MESFETs) to be monolithically integrated. The characteristics of the MESFETs fabricated on substrates processed for the laser were the same as those fabricated on virgin substrates when such processed substrates were etched 2 μm deep.

Hysteretic Josephson Junction Behavior of Ba1-xKxBiO3 Grain Boundary Junctions Using SrTiO3 Bicrystal Substrates

We have fabricated Ba1-x Kx BiO3 artificial grain boundary junctions with low resistance on a SrTiO3 bicrystal substrate. These junctions show hysteretic junction behavior with a clear superconducting energy gap structure and Josephson current. The area resistance was low; it ranged from 5.2×10-8 to 2.0×10-5 Ω cm2. The I cR n product of the 20-µm-wide junction was 3.5 mV at 4.2 K. The gap voltage increased as the heater temperature increased and reached about 8 mV. A weak magnetic field applied normal to the substrate induced a Fraunhofer-type pattern.

DC superconducting quantum interference devices with BiSrCaCuO bicrystal grain boundary junctions at 77 K

We have developed dc superconducting quantum interference devices (SQUIDs) with grain boundary junctions of a BiSrCaCuO (BSCCO) thin film on a SrTiO3 (100) bicrystal substrate with a misorientation angle of 36.8°. The T c and J c of the BSCCO film were 93 K and 3×105 A/cm2 at 77 K, respectively. These SQUIDs operated at 77 K. The I-V characteristics of the SQUIDs were of resistively shunted junction (RSJ) type. A SQUID with 20-µ m-wide junctions showed Fraunhofer-like flux-voltage characteristics. The I0R n product and the modulation voltage of a SQUID with 10-µ m-wide junctions were 23 µ V and 6 µ V at 77 K, respectively. The flux noise measured in a flux-locked loop mode was 2.0×10-5Φ0/ Hz0.5 in the white noise region.

Ba1-xKxBiO3 Grain Boundary Junctions on a MgO Bicrystal Substrate.

Ba1-x Kx BiO3 (BKBO) grain boundary junctions were fabricated on a MgO bicrystal substrate with a misorientation angle of 24° using a BaBiO3 (BBO) buffer layer. These junctions showed a superconductor-insulator-superconductor (SIS) behavior with a gap structure and Josephson current. The I cR n product of a 12-µ m-wide junction was about 2.8 mV and the critical current density was 1300 A/cm2 at 4.2 K. The characteristic of the magnetic field dependence of the critical current of a 12-µ m-wide junction was close to an ideal Fraunhofer pattern. The barrier height and barrier thickness were estimated to be about 220 meV and 1.7 nm, respectively. Furthermore, we found that the reduced barrier thickness was about 0.4 nm by measuring the Fiske mode junction resonance.

BiSrCaCuO Bicrystal Grain Boundary Junction SQUIDs Operating at 77K

We have developed dc superconducting quantum interference devices (SQUIDs) with bicrystal grain boundary junctions of BiSrCaCuO thin film on SrTiO3 (100) bicrystal substrates with a misorientation angle of 36.8°. The Tc and Je of the BSCCO film were 93 K and 3×105 A/cm2 at 77 K, respectively. The I-V characteristics of the SQUIDs were of resistively shunted junction (RSJ) type. The SQUID with 20-µm wide junctions showed a Fraunhofer-like flux-voltage characteristics. These SQUIDs operated at 77 K. The IoRn product and the modulation voltage of the SQUID with 10-µm wide junctions were 17 µV and 6 µV at 77 K, respectively. The flux noise as measured in a flux locked loop mode was 2.3×10−5 Φ0/Hz0.5 in the white noise region.

Bi-Sr-Ca-Cu-O Thin Films Grown by Laser Ablation

Superconducting Bi-Sr-Ca-Cu-O thin films were grown on SrTiO3(100) substrates in oxygen atmosphere by laser ablation using an ArF excimer laser and Bi2Sr2Ca2Cu3O x composition sintered targets. X-ray diffraction patterns showed that the films consist of the mixture of 2223 and 2212 phases, whereas inductively coupled plasma (ICP) data indicated their compositions are almost 2223 ratio, especially that deposited in higher oxygen pressure. The highest critical temperture (Tc0) of the films is 70 K.

Fabrication of BaKBiO Single Crystal Films on (100) MgO Substrates

Superconducting BaKBiO films are deposited on (100) MgO substrates which have been pre-annealed in O2 atmosphere of 600 mTorr. Thermal desorption spectroscopy of the MgO substrate is measured to determine the suitable temperatures of the pre-annealing. We find that the pre-annealing temperature of 640 °C is high enough to desorp the contaminations on the MgO surface. MgO substrates pre-annealed at 640 °C for 2 hours provide the BaKBiO single crystal films. The as-grown BaKBiO film shows the critical temperature of 19 K and the room-temperature resistivity of 0.4 m Ω · cm.

Fabrication and Cross-sectional TEM Observation of BiSrCaCuO Step-edge Josephson Junctions

Microstructure of BiSrCaCuO step-edge Josephson junctions using SrTiO3(100) and MgO(100) substrates has been investigated with cross-sectional TEM. The film on the SrTiO3(100) substrate had c-axis preferred orientation normal to the (100) planes of SrTiO3, and the c-axis of the film using the MgO(100) substrate was normal to the substrate surface even on the slope of the steps. Artificial grain boundaries were introduced in the film using the MgO substrate only. The difference between the microstructures on the SrTiO3 and MgO can be explained with the lattice mismatch between the BiSrCaCuO and the substrate materials. Both films exhibited RSJ like I-V characteristics and the response to the external magnetic field, but the critical current density of the junctions using the MgO substrate was larger than that on the SrTiO3 substrate.

100 GHz Band Planar-Type SIS Mixer Using BaKBiO Grain Boundary Junctions

BaKBiO (BKBO) grain boundary junctions were fabricated on a MgO bicrystal substrate with a misorientation angle of 24°. These junctions showed a superconductor-insulator-superconductor (SIS) behavior with a wide gap voltage (2Δ ≈ 8 mV). A 100 GHz band planar-type SIS mixer consisting of microwave integrated circuit elements such as a power splitter, directional couplers, matching circuits, band rejection filters and resistors (Ta2N) was fabricated.

SIS Tunnel Junctions Using BaKBiO Grain Boundary

Ba1-xKxBiO3(BKBO) grain boundary junctions were fabricated on SrTiO3(100) bicrystal substrates by RF magnetron sputtering. Hysteretic current-voltage (I-V) characteristics with a clear energy gap and a Josephson current were observed. Typical values of the area resistance, the IcRn product and the gap voltage of a 20µm wide junction were 2×10−6 Ω•cm2, 3.5mV and 7.5mV at 4.2K, respectively. Also, Fraunhofer patterns were clearly observed.