Kazuya Kikunaga

Fabrication of (Cu, C)Ba2CuOy superconducting thin film by RF magnetron sputtering

We report the superconducting properties of (Cu, C)Ba2CuOy thin films deposited by RF magnetron sputtering from targets with a nominal composition of Ba2Cu1.5Oy. The thin films were grown in mixed Ar (5~40 mTorr), O2 (0~1.2 mTorr) and CO2 (0~1.8 mTorr) atmosphere. The epitaxial c-axis oriented thin films with c-axis length in the range between 7.96 A and 8.82 A were obtained on SrTiO3(100) substrates, but superconductivity was observed only for the films which have the c-axis length with 8.01 A and 8.32 A. By optimizing deposition conditions, the excellent (Cu, C)Ba2CuOy thin film with Tc = 62K and superconducting transition width ΔTc = 1.5 K was successfully obtained.

Study of Band Alignment at CBD-CdS/Cu(In 1- x Ga x )Se 2 ( x = 0.2 - 1.0) Interfaces by Photoemission and Inverse Photoemission Spectroscopy

Dependence of band alignments at interfaces between CdS by chemical bath deposition and Cu(In 1-x Ga x )Se 2 by conventional 3-stage co-evaporation on Ga substitution ratio x from 0.2 to 1.0 has been systematically studied by means of photoemission spectroscopy (PES) and inverse photoemission spectroscopy (IPES). For the specimens of the In-rich CIGS, conduction band minimum (CBM) by CIGS was lower than that of CdS. Conduction band offset of them was positive about +0.3 ~ +0.4 eV. Almost flat conduction band alignment was realized at x = 0.4 ~ 0.5. On the other hand, at the interfaces over the Ga-rich CIGS, CBM of CIGS was higher than that of CdS, and CBO became negative. The present study reveals that the decrease of CBO with a rise of x presents over the wide rage of x , which results in the sign change of CBO around 0.4 ~ 0.45. In the Ga-rich interfaces, the minimum of band gap energy, which corresponded to energy spacing between CBM of CdS and valence band maximum of CIGS, was almost identical against the change of band gap energy of CIGS. Additionally, local accumulation of oxygen related impurities was observed at the Ga-rich samples, which might cause the local rise of band edges in central region of the interface.

Deposition of superconducting (Cu, C)–Ba–O films by pulsed laser deposition at moderate temperature

Superconducting (Cu, C)?Ba?O thin films have been epitaxially grown on (100) SrTiO3 at a low growth temperature of 500?600??C by pulsed laser deposition. The dependences of their crystallinity and transport properties on preparation conditions have been investigated in order to clarify the dominant parameters for carbon incorporation and the emergence of superconductivity. It has been revealed that the CO3 content in the films increases with increasing both the parameters of partial pressure of CO2 during film growth and those of growth rate and enhancement of superconducting properties. The present study has also revealed that the structural and superconducting properties of the (Cu, C)?Ba?O films are seriously deteriorated by the irradiation of energetic particles during deposition. Suppression of the radiation damage is another key for a high and uniform superconducting transition. By these optimizations, a superconducting onset temperature above 50?K and a zero-resistance temperature above 40?K have been realized.

Characterization of electronic structure of superconducting (Cu, C)-Ba-O films by in situ photoemission spectroscopy

Change of electronic structure of superconducting (Cu, C)-1201 thin films grown by pulsed laser deposition with superconducting critical temperature Tc has been characterized by means of in situ photoemission spectroscopy. The finite spectral weight at the Fermi level in the valence band of (Cu, C)-1201 superconducting thin films and a rigid-band-like approach of the main peak of the valence band toward the Fermi level with a rise of Tc has been observed, which reveals hole-doping-induced superconductivity of this system. The obtained Ba 4d and Cu 2p signals with an increase of Tc suggest excess oxygen should be introduced around Ba ions in the early stage of the emergence of superconductivity. Then, they should be preferentially introduced around Cu ions. The changes of core signals with Tc and comparisons of the obtained data with infinite-layer and other cuprate superconductor compounds suggest that the (Cu, C)–O charge reservoir in this system is in the heavily hole-doped state, similar to that of the Cu–O chain in YBa2Cu3O7−δ.

In-situ characterization of transport properties of superconducting (Cu, C)-1201 films

Transport properties of (Cu, C)-1201 thin films have been characterized by in-situ four probe method without breaking vacuum, subsequent to their growth by pulsed laser deposition, in order to clarify intrinsic transport properties. Owing to the in-situ measurements, degradation of contact resistance and normal state conductivity were successfully suppressed. Obtained results reveal the positive correlation between Tc and normal state conductivity δ at 290 K, and that between Tc and temperature coefficient of resistivity (TCR) at 290 K. The films exhibit Tc(ρ=0) > 20 K on the cases of δ[290 K] > 4 x 102 S/cm and TCR > 1.5 x 10-3 K-1. The high Tc with low conductivity of (Cu, C)-1201 films indicates the presence of extrinsic defects such as grain boundaries. The absence of saturation of Tc with an increase of TCR indicates doping level of the (Cu, C)-1201 films in this study should be in between under-doped to optimally-doped states. They suggest there would be some room for further increases of Tc.

Synthesis of superconducting (Cu, C)-Ba-O films on SrCuO2 buffer by pulsed laser deposition

(Cu, C)-1201 films have been grown on SrCuO2 buffer layer over SrTiO3 (100) by pulse laser deposition. Effects of the insertion of SrCuO2 buffer on growth mode of the 1201 phase has been investigated by comparing in-situ RHEED pattern of the films growth on SrCuO2 / SrTiO3, SrTiO3 and NdGaO3. (Cu, Q-1201 films directly grown on SrTiO3 and NdGaO3 exhibit diffusive streak patterns in the beginning of the film growth. On the other hand, the films on the buffer show sharp streaks in the beginning from the film growth on buffer, a-axis parameters of the films on SrTiO3 and NdGaO3 estimated by RHEED patterns drastically expanded from 3.90 ± 0.06 A and 3.93 ± 0.10 A at 10 nm thick to 4.07 ± 0.05 A and 3.99 ± 0.03 A at 40 nm thick, respectively. The films on 10 nm thick buffer gradually expanded from 3.95 ± 0.05 A at 10 nm thick to 4.01 ± 0.05 A at 40 nm thick. The films on 10 nm and 20 nm thick buffer layers showed the rises of superconducting onset temperature Tc-onset to 72 K and 62 K, respectively, whereas Tc-onset of the films directly grown on SrTiO3 was about 50 K. Due to the growth mode without misfit dislocations in the beginning of the film growth, the SrCuO2 buffer could strain the 1201 phase. These results indicated that the interfacial strain should be important Tc for (Cu, Q-1201 films, and superlattice based on it.

Conversion from transparent antiferromagnet KNiF3 to transparent ferrimagnets

We succeeded in performing a magnetic conversion from transparent antiferromagnet KNiF3 to transparent ferrimagnets, with a spontaneous ferromagnetic moment by a percolation method. Considering Coulomb repulsion, the origin of the spontaneous ferromagnetic moment is explained by an inhomogeneous antiferromagnetic spin arrangement, even if nonmagnetic ions are dispersed homogenously over KNiF3. The ferromagnetic transition temperature was obtained to be 24–26K. The transmittance at 600–1000nm was excellent. This compound shows an alternative that can be used for an optical isolator that uses commercial semiconductor lasers, and can widely be used in next generation optical information networks.

In-situ Characterization of As-grown Surface of CIGS Films

In-situ characterization of composition, electronic structure and their depth profiles of surface of Cu(In 1-x Ga x )Se 2 (CIGS) film grown by three stage co-evaporation has been carried out by means of photoemission and inverse photoemission spectroscopy (PES/IPES), for the purpose of investigating the formation mechanism of the CIGS-side wide band-gap region adjacent to CBD-CdS/CIGS interface in cell structure. Sample-transportation in vacuum below 1 x 10 -8 Torr yielded almost contamination-free feature of the CIGS surface. The as-transferred surface of Cu0.93(In 0.65 Ga 0.35 )Se 2 grown at the identical condition for the high performance solar cell exhibited seriously Cu and Ga deficient composition. Chemical formula of this region was inbetween Cu : (In+Ga): Se = 1 : 3 : 5 and 1 : 5 : 8. In-situ UPS/IPES measurements CIGS showed that the as-grown surface region of the CIGS already had expanded band gap energy up to 1.4 eV and n-type character. A gradual decrease of band energy and a rise of valence band maximum as a function of depth from the original surface were observed. These results have revealed that the surface of CIGS by the three stage method already has the wide band gap, which might originate in so-called Cu-vacancy ordered phase.

Study of Band Alignment at the Interface between CBD-CdS and CIGS grown by H 2 O-introduced co-evaporation

For understanding the origin of the improvements of properties in the CIGS-based cells, of which the CIGS absorber has been fabricated by H 2 O-introduced co-evaporation [CIGS-H 2 O], band alignment at the interfaces between chemical bath deposited CdS and CIGS-H 2 O with Ga substitution ratio ~ 40 % has been studied by photoemission and inverse photoemission spectroscopy. The CdS layer over the CIGS-H 2 O showed an identical electronic structure with that of CdS on the conventionally grown CIGS; band gap energy of 2.3 ~ 2.4 and a location of conduction band minimum (CBM) and valence band maximum (VBM) relative to Fermi level were + 0.75 eV and -1.6 ~ -1.7 eV, respectively. In the interface region, decreases of CBM and a rise of VBM were observed. Total amount of the decrease of CBM over the interface was 0.2 ~ 0.3 eV. XPS measurements of the core-level signals over the interface showed a small upward bend bending of 0.1 ~ 0.2 eV. Consequently, the conduction band offset (CBO) and valence bad offset (VBO) at the CBD-interface over the CIGS-H 2 O (Ga~40%) are about +0.1, and 0.9 ~ 1.1 eV, respectively. This positive CBO is contrast with a slightly negative CBO at the interface between CBD-CdS/conventionally grown CIGS with Ga ~ 40 % measured previously. These results indicate that the H 2 O introduction is effective to extend the upper limit of the Ga substitution ratio where the Type-I conduction band alignment is maintained. The observed band alignments are consistent with the rise of V oc and efficiency in the CIGS-H 2 O based cells.

Sputter Synthesis of c-axis YBCO Films with Excellent Surface Smoothness and Fabrication of Sandwich type Junctions with Interface Engineered Barrier

Flat surface is essential to base electrodes of sandwich type Josephson junctions. In the present study, c-axis YBa2Cu3O7 (c-YBCO) films with excellent surface smoothness were fabricated by off-axis sputtering. For the flat surfaces, key parameters in the sputtering process were growth temperature, atmosphere conditions and surface nature of substrates. The correlation between the sputtering conditions and the characteristic of the c-YBCO films reveal the following phenomena; i) The films deposited at low temperature of 660 °C showed a low zero-resistance temperature because of a poor connection between the c-axis crystallites. Surfaces of these low temperature grown films involve large and rectangular shaped a-axis grains, thereby the peak-to-valley (PV) amplitude of the surface was beyond 80 nm. The experiments indicate that the growth of the grains should be caused by low surface diffusion of adatoms at the low growth temperature. ii) Contrarily, at high Ts above 770 °C, a serious deviation of film composition from the stoichiometry took places, which also promoted an outgrowth of a-axis grains. Consequently, the high temperature grown films had seriously rough surfaces (PV amplitude > 25 nm). iii) The growth at the optimised Ts of 765 °C led a remarkable reduction of surface roughness (root-mean-square (RMS) of the roughness < 1.8 nm) without any degradation of superconducting properties. The achieved surface morphology is classified as one of the smoothest surfaces of the YBCO film grow by sputtering. iv) A density of the a-axis outgrowth was related to the surface defects of the substrate. The usage of SrTiO3 (100) plane consists of atomically flat and wide terraces and unit-cell high steps resulted in almost outgrowth-face surface with an excellent smoothness (PV amplitude < 10 nm, RMS of the roughness < 1.0 nm).