I have vacuum-deposited c-axis and (110) oriented $PrBa_2(Cu_{0.8}A_{l0.2})_3O_7$ thin films on (001) $LaAlO_3$ and (110) $SrTiO_3$ substrates by rf-sputtering and Pulsed Laser Deposition (PLD). The initial growth stage in PLD process was studied by Atomic Force Microscopy (AFM) method. The oxygen partial pressure and thickness effect on thin film surface and property also has been studied. The surface smoothness and crystallinity decreased as thin film thickness increases. Thickness and stoichiometry distribution of the thin films were studied by Rutherford Backscattering Spectrometry (RBS) measurement. From the temperature dependence of electrical resistivities for both the target and the films, we found that the electrical conduction was mainly through the variable range phonon-assisted hopping mechanism. This suggests that Al-doping in this material has given rise to extensive impurity states in the band gap, which, in turn, localized the charge carriers, especially at lower temperatures. The resistivity of the doped $PrBa_2Cu_3O_7$ (PBCO) is much higher than the pure PBCO, ranged from ~1 Ω -cm at room temperature to about five orders of magnitudes higher at ~30 K. The electronic structure of the samples was studied through measuring the optical absorption coefficients vs. photon energy from which the band gap and defect state energies were inferred. Soft x-ray emission spectra obtained for pure and Al-doped PBCO thin films show shift in Cu Lα peak position and change in Cu Lα / $L_β$ intensity ratio upon doping. This indicates the change of Cu-O bonding due to the partial Al substitution. In addition, Raman spectra from the Al-doped PBCO thin films exhibit an additional peak near the apical oxygen peak, a result suggestive of the broken symmetry of the Cu-O chains caused by the partial replacement of Cu-atoms with the Al on the Cu-O chains. C-axis oriented YBCO films are very common but not well suited for the fabrication of sandwich-type superconductor-normal conductor-superconductor (SNS) or SIS (I as insulator) Josephson tunnel junctions due to the short coherence length along the c-axis (ξc = 3-5Å). Along the planes, on the other hand, the coherence length is much larger $(ξc \approx; 15Å)$, it is hence advantageous to grow both (110) oriented YBCO and Al-doped PBCO thin films in a SIS junction tri-layer structure. High-quality Al-doped PBCO thin films with (110) orientation were fabricated by PLD method on (110) STO substrate. The in-plane c-axis alignment of Al-doped PBCO has been confirmed by X-ray pole figures at x= 20˚ and 35˚. The Raman spectra at different temperatures show Al substitution on the Cu-O chains. The Resistivity-Temperature measurements show the anisotropic properties in the c-axis and ab plane of the thin films. Surface smoothness is essential to use YBCO and related oxide epitaxy thin films in SIS Josephson junction applications. This necessitates not only high quality but also atomic scale smooth superconducting and insulating films. Such thin film can not simply obtained from the thin film deposition process by PLD because the generation of particulates during deposition. Ar(3000) ion cluster beam was used to smooth Al-doped PBCO thin films with different thickness and surface roughness. The surface roughness of the films decreased significantly by ion cluster beam bombardment with different dosage. The damaged layer can be removed by annealing in oxygen ambient. All these results suggest Al-doped PBCO is a prominent insulating material for YBCO superconductor in SIS Josephson junction and other electronic applications.
본 연구에서는 rf-sputtering 및 Pulsed Laser Deposition (PLD) 방법으로 c축 및 (110) 방향으로 배열된 $PrBa_2(Cu_{0.8}A_{l0.2})_3O_7$ 박막을 (001) $LaAlO_3$ 과 (110) $SrTiO_3$ 기판에 증착하였다. PLD 증착시 박막의 초기 성장과정을 Atomic Force Microscopy (AFM) 으로 관측하였으며 산소분압과 박막두께가 박막의 표면 및 특성에 미치는 영향도 연구하였다. 박막의 두께 및 조성의 분포를 Rutherford Backscattering Spectrometry (RBS)로 측정하였다. 측정한 타겟과 박막의 전기전도도-온도의 의존관계로부터 전기전도 메카니즘은 3-D variable range phonon-assisted hopping 임을 제시하였다. 이는 Al의 도핑으로 인하여 재료의 밴드갭 사이에 많은 impurity state가 생성, 즉 charge carriers가 저온에서 localized 됨을 설명한다. Al이 도핑된 $PrBa_2Cu_3O_7$ (PBCO)의 전기전도도는 상온에서의 ∼1 Ω-cm부터 30 K에서는 만배정도로 증가함으로써 순수한 PBCO의 전기전도도보다 훨씬 높았다. 박막시편의 optical absorption coefficients vs. photon energy를 측정함으로써 광학 밴드갭도 확인하였다. 순수한 PBCO와 Al이 도핑된 PBCO 박막의 soft x-ray emission spectra에서 Cu Lα peak 이 shift 하고 Cu Lα/Lβ intensity ratio가 변화함을 측정하였다. 이는 Al의 도핑으로 인하여 PBCO에서 Cu-O bonding이 변화함을 설명한다. Al이 도핑된 PBCO 박막의 라만 스펙트럼을 측정한 결과 apical oxygen peak의 근처에 additional peak이 나타났다. 이는 Cu-O chains에서의 broken symmetry를 설명하며 Al이 부분적으로 Cu-O chain 위치에 대체된다는 것을 확인하였다. c-축으로 배향된 YBCO 박막은 아주 보편적이지만 c-축에서의 coherence length(ξc = 3∼5)가 작기 때문에 sandwich-type의 superconductor-normal conductor-superconductor (SNS) 또는 SIS (I as insulator) 죠셉슨 터넬 접합에는 적합하지 않다. 반면에 planes 방향에서의 coherence length (ξab = 15)는 c-축 방향에서보다 훨씬 크기 때문에 (110) 방향으로 배열된 YBCO 와 Al이 도핑된 PBCO 박막은 SIS 접합에 유리하다. Al이 도핑된 (110) 박막을 PLD 방법으로 (110) STO 위에 증착하였으며 배향성은 X-ray pole figures로 확인하였다. (110) 박막의 Raman spectra도 역시 Al이 부분적으로 Cu-O chains 위치를 대체하였음을 증명한다. (110) 박막의 전기전도도-온도를 측정함으로써 c-축과 ab plane 방향에서 anisotropic 특성을 확인하였다. 박막의 표면 smoothness는 YBCO 및 관련된 절연체 박막재료를 SIS 죠셉슨 접합에 응용함에 있어서 관건적인 문제이다. 이러한 표면이 smooth한 박막은 단순한 PLD 방법으로 증착하기 어려운데 그 원인은 PLD 증착시에 particulates가 생성되기 때문이다. 본 연구에서는 Ar(3000) 이온 cluster빔으로 박막표면을 smoothing시켰으며 그 효과는 아주 현저하였다. 박막표면의 damaged layer는 annealing으로 회복시킬 수 있다. 이러한 결과로부터 Al이 도핑된 PBCO는 YBCO 초전도체를 SIS 죠셉슨 접합에 응용함에 있어서 아주 유망한 절연체 재료임을 확인하였다.
