完美的基板晶格匹配度對高溫超導薄膜的不良影響-晶格缺陷,雜相析出及超導性質的作用

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研究生：	廖延宗 Yang Chung Liao
論文名稱：	完美的基板晶格匹配度對高溫超導薄膜的不良影響-晶格缺陷,雜相析出及超導性質的作用 The adverse effect of nearly perfect lattice matching substrates on high-temperature-superconductor thin film - the interplay between dislocations, precipitation and superconductivity
指導教授：	齊正中 Cheng Chung Chi
口試委員:
學位類別：	博士 Doctor
系所名稱：	理學院 - 物理學系 Department of Physics
論文出版年：	2001
畢業學年度：	89
語文別：	英文
論文頁數：	104
中文關鍵詞：	基板晶格應變、非組分物質析出、晶格缺陷、超導薄膜、尖塔狀的析出物、晶界耦合、約瑟生結的網絡模型
外文關鍵詞：	Strain effect, precipitation of off-stoichiometric phases, dislocations, superconducting film, tower-like precipitates, intergranular coupling, network model of Josephson junctions
相關次數：	點閱：3 下載：0
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自從釔鋇銅氧化物(或大部分稀土元素取代釔的系統)被發現以來, 基板晶格應變對薄膜成長模式的效應已有相當的了解. 但一般文獻於應變對非組分物質析出的影響並無著墨. 作者透過一系列的實驗闡述晶格缺陷(如為舒解應變而產生的錯排)與非組分物質析出間的關係. 在嘗試了許多的成長條件後,我們發現EuBa2Cu3O7-δ和SmBa2Cu3O7-δ薄膜成長在完美晶格匹配的基板(鈦酸鍶)具有弱晶界耦合而相同的膜長在晶格匹配較差的基板其晶界耦合卻強了許多. 從這些膜的析出物密度及晶界耦合隨膜厚的演進,我們發現錯排及析出物有很密切的關係. 在基板晶格匹配較差的系統,超過臨界厚度所產生的錯排會吸引非組分物質因而生成尖塔狀的析出物. 這些析出物會更進一步地吸引晶界上的非組分物質. 因此這些物質不會均勻地分布在晶界上, 晶界耦合便好. 相反地,沒有那些一開始就引發的錯排的吸引,非組分物質會均勻地分布在晶界上. 這些物質會於晶粒形成超導後阻礙超導電流造成弱晶界耦合. 約瑟生結的網絡模型可以解釋我們所觀察到的現象. 這個弱晶界耦合的特性可能應用在製作約瑟生結的元件.

Strain effect on the growth mode of high-temperature-superconductor (HTSC) (RBa2Cu3O7-d, R= Y and most rare earth elements) thin films has been investigated thoroughly since the discovery of this material. However the relationship between strain and the precipitation of off-stoichiometric phases has not been fully studied. Through a series of experiments, the author elucidates the interplay between the dislocations induced from the release of strain and the off-stoichiometric phases segregated out from the single crystal grains. Despite of many attempts to optimize the deposition conditions, we have found that there is always a substantial resistive tail below the bulk Tc of EuBa2Cu3O7-d and SmBa2Cu3O7-d films deposited on nearly perfect lattice matched SrTiO3 (STO) substrates. This is in contrast to the same films deposited on YSZ, LaAlO3 (LAO) and NdGaO3 (NGO) substrates under similar conditions. The films on YSZ, LaAlO3 and NdGaO3 substrates have a sharp resistive transition without any tails although the lattice match is poorer. The evolution of the precipitate density and intergranular coupling with increasing film thickness has been investigated for the films deposited on both nearly perfect matched and less matched substrates. It reveals that dislocations induced from the release of strain affect the precipitation of off-stoichiometric phases. We have developed a self-consistent scenario to explain this phenomenon. In this scenario, dislocations attract the off-stoichiometric phases and thus lead to tower-like precipitates. These tower-like precipitates further segregate the potential impurities out from the grain boundaries. Therefore the intergranular coupling of RBCO film deposited on the less lattice matched substrate is getting better with the increasing thickness. On the other hand, in the film grown on the nearly perfect matched substrate, the off-stoichiometric materials homogeneously coat each columnar grain in absence of these tower-like precipitates. A network model of Josephson junctions, which is composed of well-oriented (along all three axes) crystalline grains coated with non-superconducting materials of thickness of a few nm, can explain our experimental observation. This is a novel granular system different from the conventional one where the grains are misoriented with each other. This adverse effect for the transition temperature and intergranular coupling may find application in the fabrication of Josephson junctions.

Abstract     i
Acknowledgement      iii

Figure list  vii

Table list   ix

1  Introduction      1

   (1.A) Physics of thin film growth 2

     (1.A.1) Features evolved with thickness 2

      (1.A.2) Precipitates formation 5

      (1.A.3) The role of dislocations and strain in HTSC    7

   (1.B) The granularity of HTSC     10

2  Thin film fabrication     14

   (2.A) Target preparation  14

   (2.B) Thin film deposition by PLD 15

      (2.B.1) Principle      15

      (2.B.2) Parameter optimization 16

   (2.C) Deposition process  20

      (2.C.1) Growing procedure      20

      (2.C.2) Substrates     21

3  Characterization of physical properties   23

   (3.A) Structure   23

      (3.A.1) X-ray diffraction (XRD)        23

      (3.A.2) Secondary ion mass spectroscopy (SIMS) 27

   (3.B) Surface properties  29

      (3.B.1) Optical microscopy     29

      (3.B.2) Scanning electron microscopy   29

      (3.B.3) Auger electron spectroscopy    35

      (3.B.4) Atomic force microscopy        41

   (3.C) Superconductive properties  53

      (3.C.1) Resistive property     53

      (3.C.2) Magnetic property      58

4 Strain-driven effect on the formation of precipitates      68

   (4.A) Recovery of TC, R=0 of RBCO film on STO deposited

        by the bi-layer process      68

   (4.B) The evolution with thickness        78

      (4.B.1) AFM results    78

      (4.B.2) XRD patterns   87

      (4.B.3) The intergranular coupling evolved with thickness      88

5  Conclusion        96

References   101

Figure list

(1)  Static equilibrium of interface tension 3

(2)  Thin-film-growth modes  3

(3)  Pseudoternary phase diagram of YO1.5-BaO-CuO    6

(4.a) TC in different deposition conditions  18

(4.b, c) Deposition processes        19

(5)  XRD patterns of EuBCO on NGO and STO    24

(6)  XRD patterns of EuBCO on LAO and YSZ    25

(7)  XRD pattern of clay     26

(8)  SIMS of one-layer EuBCO on STO  28

(9)  OM pictures of SmBCO on the four substrates     30

(10) The emission of secondary electron in different geometry        31

(11) SEM pictures of 180nm EuBCO film on LAO and STO 32

(12) SEM pictures of 60nm EuBCO film on LAO and STO  33

(13) The emission of Auger electron  37

(14) Auger electron spectrum for 180nm EuBCO film on LAO     38

(15) Auger electron spectrum for 60nm EuBCO film on LAO      39

(16) Microanalysis of Auger electron spectroscopy    40

(17) The set-up of AFM       42

(18) AFM pictures of 180nm EuBCO film on LAO and STO 43

(19) AFM pictures of 180nm EuBCO film on LAO and STO 44

(20) The line scan of Fig.19 (b)     45

(21) AFM pictures of 180nm EuBCO film on LAO and STO 46

(22) AFM pictures of 180nm EuBCO film on LAO and STO 47

(23) The line scan of Fig.22 (a)     48

(24) The line scan of Fig.22 (b)     49

(25) AFM pictures and their corresponded contour plots       50

(26) AFM pictures of 180nm EuBCO film on STO 51

(27) Normalized Rab(T) on different substrates       56

(28) Rab(T) on different substrates  57

(29) Normalized m(T) on different substrates 62

(30) Low field magnetic hysteresis   63

(31) M(T) in different magnetic field of SmBCO on STO        64

(32) M(T) in different magnetic field of SmBCO on NGO        65

(33) The modeling M(T) curve in different magnetic field     67

(34) Normalized Rab(T) of films by using different processes and

    substrates       71

(35) The XRD patterns of the films on STO by using the

    two different processes  72

(36) The normalized m(T) on different samples        74

(37) SIMS of films deposited by the bi-layer process 75

(38) AFM pictures of samples undergone the bi-layer process  76

(39) SEM/EDX analysis of the four samples    77

(40) AFM pictures of bare substrates 79

(41) AFM pictures of films on STO with different thickness   80

(42) AFM pictures of films on STO with different thickness   81

(43) AFM picture of 24nm film on STO         82

(44) AFM pictures of films on LAO with different thickness   83

(45) AFM pictures of films on LAO with different thickness   84

(46) AFM pictures of 24nm films on NGO and LAO       85

(47) The rocking curves of (005) of films on STO and LAO     90

(48) The evolution of TC and saturation of intergranular coupling    91

(49) The evolution with thickness of normalized m(T) curve of

    EuBCO on STO in the magnetic field of 1 Gauss    92

(50) The evolution with thickness of normalized m(T) curve of

    EuBCO on STO in the magnetic field of 10 Gauss   93

(51) The evolution with thickness of normalized m(T) curve of

    EuBCO on LAO in the magnetic field of 1 Gauss    94

(52) The evolution with thickness of normalized m(T) curve of

    EuBCO on LAO in the magnetic field of 10 Gauss   95

(53) The cartoon of evolution with thickness for RBCO on the

    less lattice matching substrate  99

(54) The cartoon of evolution with thickness for RBCO on the

    nearly perfect lattice matching substrate        100

Table list

(1)  Lattice constants and thermal expansion constants of different

  substrates at 300K 22

(2)  Lattice mismatch of SmBCO with the four substrates……….22

(3)  Δ2θand c-axis length of EuBCO c-axis-oriented films…….26

(4)  The parameters used in the calculation of M(T)…………..…66

(5)   Δ2θand c-axis length of EuBCO films by using two different

   processes

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