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Conversion of BaF₂ to BaO for in-situ growth of Y-Ba-Cu-O thin films Gao, Yuan 1990

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C O N V E R S I O N O F B A F 2 T O B A O F O R IN-SITU G R O W T H O F Y - B A - C U - O T H I N F I L M S By Yuan Gao B. Sc. (Physics) University of Science and Technology of China A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES PHYSICS We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA January 1990 © Yuan Gao, 1990 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of Physics The University of British Columbia Vancouver, Canada D a t e Feb. 1 2 , 1990 DE-6 (2/88) Abstract The YiBa2Cu30x superconducting thin films were made by both sequential electron-beam evaporation and co-evaporation of Cu, BaF 2 and Y, followed with high temperature post-deposition annealing. The possibility of making YiBa 2Cu30 x superconducting film in situ with BaF2 as the Ba source was investigated by studying the conditions of in situ conversion of BaF 2 to BaO. The BaO concentration in the film as function of the substrate temperature was estimated with an equilibrium thermodynamic model and found to agree with the experimental results. A brief experimental and theoretical exploration of BaCl 2 also showed that BaCl 2 does not have any advantage over BaF 2 as the Ba source for in situ growth of Y-Ba-Cu-0 films from the growth temperature point of view. 11 Table of Contents Abstract ii List of Tables v List of Figures vi Acknowledgement vii 1 Introduction 1 1.1 High Te Superconductivity and High Te Superconductor Films 1 1.2 Y-Ba-Cu-0 Superconductor Film 3 1.3 Outline of This Thesis 5 2 Apparatus 7 2.1 Vacuum Evaporation System Description 7 2.2 Pressure Measurement 10 2.2.1 Pressure inside the Helical Resonator 13 2.2.2 Pressure in Front of Substrates 13 2.3 Hot Stage and the Temperature Measurement 14 2.4 Deposition Rate and Thickness Monitoring 15 3 Preparation of YiBa 2 Cu 3 O x Films 18 3.1 Electron-Beam Evaporation 18 3.2 Substrate Preparation 18 iii 3.3 Film Deposition 19 3.3.1 Sequential Deposition 19 3.3.2 Co-deposition 19 3.4 Post-Deposition Annealing 20 4 Analysis of Prepared YiBa 2 Cu 3 O x Films 21 4.1 EDX Composition Analysis 21 4.2 Resistance Measurement of the Films 22 4.3 Scanning Electron Micrograph 23 5 In-Situ Conversion of BaF2 and BaCl2 to BaO 26 5.1 Film Preparation . . 26 5.2 Thermodynamic Analysis 26 5.2.1 Conversion of BaF 2 26 5.2.2 Conversion of BaCl 2 35 5.3 XRD and EDX Analysis 35 5.3.1 XRD Analysis of BaF 2 Films 35 5.3.2 EDX Analysis of BaCl 2 Films 42 5.3.3 Discussion 42 6 Conclusions 44 Bibliography 46 iv List of Tables 5.1 AH° and AS° values for some selected materials[30,31] v List of Figures 1.1 The structures of Y-Ba-Cu-0 superconductor [35] 4 2.2 Vacuum Evaporation System 8 .2.3 Substrate Holding Block 9 2.4 Top view of the evaporators showing the configuration of the evaporators and the substrate holding block 11 2.5 Plasma Discharge Unit 12 4.6 Four-Point-Probe 22 4.7 Resistance vs. Temperature Curve of Y iBa 2 Cu30 x Codeposited Film #890511. Tc(onset) = 87K Tc(midpoint) = 68K T C (R = 0) = 53K 24 4.8 S E M Picture of Codeposited Film #890511 25 5.9 The equilibrium curves of the BaF 2 —> BaO reaction in the C B a o — T space. 33 5.10 CeaO - T equilibrium curves for PH 2O = 1 X 10 - 4 Torr and different deposi- . tion rates 34 5.11 The equilibrium curves of the BaCl 2 —• BaO reaction in the CBao—T space. 36 5.12 X-ray Diffraction Pattern of B a F 2 Powders 37 5.13 X-ray Diffraction Pattern of BaO Powders 38 5.14 X R D Patterns of the Transition from BaF 2 to BaO 40 5.15 BaO Concentration as a Function of Temperature 41 vi Acknowledgement I would like to thank my supervisors, Dr. Tom Tiedje and Dr. Richard Cline. It is their constant support and help that made this work possible. Especially, many ideas in this work came from the discussions with Dr. T. Tiedje. Thanks also to Norman Osborne for his kindly help in resistance and X R D measurements. vii Chapter 1 Introduction 1.1 High Tc Superconductivity and High Tc Superconductor Films Superconductivity was first discovered by H. K. Onnes in 1911 when he cooled mercury down to below 4.2K and found that the DC electrical resistance of the metal sharply droped to zero [1]. A number of materials were later found to exhibit superconductivity with transition temperature from near 4K to slightly above 20K during the following 70 years. These materials were also found to exhibit the total exclusion of magnetic field in the superconducting state. This is called the Meissner effect. However, the applied magnetic field must not exceed the critical field H c , or the superconducting state will be lost and the magnetic field will penetrate perfectly into the sample. Some superconduc-tors were found to exhibt the Meissner effect below the lower critical field H c l and lose superconductivity above the higher critical field H c 2 but exhibit a mixed state between H c i and H C 2 . In this mixed state, the applied magnetic field partially penetrates into the sample and is confined in vortex lines. These vortex lines are believed to be "pinned" by defects and form an array (flux lattice) so that the superconductivity is only destroyed in these local regions. These superconductors are called type II superconductors[2]. The microscopic nature of superconductivity was explained in 1957, 46 years after the dis-covery of superconductivity, by Bardeen,Cooper and Schrieffer (BCS) [2,3] based on the assumption that two electrons bind through the exchange of a phonon to form a Cooper pair at the temperature below the superconducting transition temperature T c . 1 Chapter 1. Introduction 2 The slow progress of studying superconductivity was changed, however, in April 1986 by the discovery by Bednorz and Muller that Ba doped La2Cu04 becomes supercon-ducting at a temperature of 35K [4]. Since then, this area has drawn a lot of attention and great effort has been made. Tl-Ba-Ca-Cu-0 has been found to have the highest T c of 120K [5] so far and Y-Ba-Cu -0 film can have a critical current density as high as 6 x 10 6 A/cm 2 at 77K [6]. All the superconductors found in the past three years with T c above 35K are called "high T c superconductors". They are all found to be type II super-conductors so they can maintain the superconducting state with relatively high magnetic field up to the higher critical field HC2. Also,, the conventional BCS theory is facing challenges from the results of the newly found high T c superconductors. For example, T c is proportional to the energy gap according to the BCS theory. Thus, the newly found high T c materials should have higher energy gaps. But this is not confirmed by the experiments. Questions about such topics as the nature of the charge carriers in the superconducting state and the type of pair coupling, are still controversial [7,8]. The high T c superconducting mechanism is still not certain. Various mechanisms have been proposed. For example, Baskaran, Zou and Anderson proposed a mechanism based on the Resonating Valence Bonds concept with holons (unoccupied sites) carrying charges as bosons and spinons (unpaired spins in the sea of valence bonds) carrying spins as fermions so that Bose condensation of holons in doped samples leads to the superconductivity[33,34]. More experimental and theoretical effort is needed to explain the high T c superconductivity mechanism. High quality thin film materials are very suitable for a variety of experiments in studying the mechanisms responsible for the high T c , such as, optical transmission and tunneling experiments[16]. In addition, great effort has been made to apply high temperature superconductors in practical use, especially, in applications related to electronics, such as, SQUID detectors, faster electronic interconnects, passive electromagnetic filtering devices, e£c.[l]. Chapter 1. Introduction 3 1.2 Y-Ba-Cu-O Superconductor F i l m YiBa2Cu306.g was discovered by Chu e£a/[9] in 1987 to have a critical temperature around 90K, which is well above the boiling point of liquid nitrogen (77K). Cu-0 planes in the YiBajCuaOe.o. structure are found to be responsible for superconductivity at high temperatures [10]. Figure 1.1 shows the crystal structure of Y-Ba-Cu-0 superconductor as determined by neutron diffraction[35]. Several techniques have been used to make superconducting YiBa2Cu306.9 films, such as, evaporation [11,15], sputtering [12], laser ablation [13], etc.. YiBa2Cu306.9 superconductors, both bulk materials and thin films, were found to be sensitive to moisture and decompose to CuO,Ba(OH) 2 and Y2BaCu05 in water and lose superconductivity. Those films made with BaF 2 as the Ba source were found to de-compose less readily [14,15]. By using coevaporation of Y,Cu and BaF 2, YiBa2Cu306.g superconducting films made by Mankiewich et al were found not only insensitive to mois-ture but also having high critical current densities (10 6A/cm 2 at 8IK,about 2 orders of magnitude higher than the best results obtained from bulk material) which is believed to be due to better aligned grains in the films [15]. Also, BaF2 is very easy to evapo-rate both with an electron-beam source or a thermal evaporator. In contrast, element metal Ba reacts actively with O2, H 2 O and C O 2 in the ambient resulting in inconsistent evaporation behavior [15,18]. As-deposited films made with absence of O2 are normally amorphous and insulating. Thus, post-deposition annealing is normally required for these as-deposited films. For films made by evaporation with BaF2 as the Ba source, typically a two-step annealing process is needed: a high temperature anneal at about 850 °C in the presence of oxygen and water vapour to convert the fluoride to oxide, and a low temperature anneal at about 400 °C - 700 °C in pure oxygen to form the superconducting phase, YiBa2Cu3U6.9 Chapter 1. Introduction 4 (a) (b) (a) The structure of the tetragonal parent phase, (b) The structure of the orthorhombic ordered phase. Figure 1.1: The structures of Y-Ba-Cu-0 superconductor [35]. Chapter 1. Introduction 5 [17,18]. For films made from BaO or Ba, only pure oxygen is needed during the annealing procedure and the temperature ranging from 300 °C to 700 °C is needed to crystallise the amorphous as-deposited film and form the right superconducting phase [18]. The technology of in situ growth of superconducting Y-Ba-Cu-0 films by coevapo-ration without annealing has also been developed [19,20]. One normally accomplishes this by heating up the substrate to about 600 °C and introducing activated oxygen (such a s ozone) into the deposition chamber during the deposition procedure [19,20]. In situ growth has not yet been achieved with BaF 2 as the Ba source material. There are obvious advantages to the in situ method. The post-deposition annealing step is undesirable for the fabrication of various layered devices, including planar tun-neling junctions where success depends upon the formation of clean surfaces and abrupt interfaces [20]. The in situ method also offers the advantage of low temperature fabrica-tion. Superconducting thin film devices can be grown at a temperature compatible with semiconductor processing. It also gives a good prospect of attaining a highly-perfect, clean, and uniform film surface suitable for surface-sensitive measurements such as elec-tron tunneling [21]. With the advantage of making Y-Ba-Cu-0 superconducting film from BaF 2 in mind, it is very useful to investigate the possibily of fabricating epitaxial Y-Ba-Cu-0 supercon-ducting film in situ with BaF 2 as a source. 1.3 Outline of This Thesis In this project, the Y i B a 2 C u 3 O x superconducting films were made by sequential electron-beam evaporation of Cu, BaF 2 and Y and also by coevaporation of Cu,Y ( by electron-beam ) and BaF 2 (by thermal evaporator) followed with high temperature post-deposition annealing. The possibility of making Y-Ba-Cu-0 superconducting film in situ with BaF 2 Chapter 1. Introduction 6 as the Ba source was investigated. The conditions for in situ conversion of BaF 2 to BaO were studied theoretically in the thermodynamic equilibrium limit and were found to be consistent with the experimental results. The conditions of in situ conversion of BaCl2 to BaO was also studied. Chapter 2 shows the apparatus used in this project. The preparation method of YiBa 2Cu30 x superconductor films is shown in Chapter 3 and the analysis of those films is shown in Chapter 4. In Chapter 5, the conditions for in situ conversion of BaF 2 and BaCl 2 to BaO are studied theoretically and compared with the experimental results. Chapter 2 Apparatus 2.1 Vacuum Evaporation System Description The evaporation system used in this project consists of a vacuum chamber, two electron-beam guns, one thermal evaporator, a heated substrate holding unit, plasma discharge unit and some attendant instrumentation for pressure, temperature and film deposition rate/thickness measurements (See Figure 2.2). The vacuum system used is a commercial Varian/NRC 3117 Vacuum System, which has a 191 — inch water cooled metal bell jar sitting on a | — inch thick steel flange. The vacuum chamber is evacuated by a 6-inch diffusion pump with a Welch Duo Seal mechanical pump as foreline pump. Pressure is measured by a Varian ion gauge as shown in Figure 2.2. The lowest, or the base pressure, that has been achieved in this system, was 3 x 10 - 7 Torr. There are two electron-beam evaporation sources with crucible capacity of 6cc each mounted inside the chamber. One is an Airco Temescal Model STIH270-1 4-hearth electron-beam source which has a four-crucible turret to provide the means to evaporate ..four different materials sequentially. At the time that one crucible is at the evaporation position, the other three crucibles are isolated from stray beams and falling particles. The design also prevents cross-contamination of evaporants between crucibles. The second electron gun is an Airco Temescal Model SFIH-270 electron-beam source which has only one fixed hearth. Both electron-beam sources are water cooled and the power is supplied 7 Chapter 2. Apparatus Figure 2.2: Vacuum Evaporation System Chapter 2. Apparatus 9 P T T Thermocouple Wire * « lj Copper Block H S c r e w s Subs t ra tes Stainless Steel Substrate Holder Figure 2.3: Substrate Holding Block by an Airco Temescal CV-8 power supply. The thermal evaporator is a home-made tantalum boat with 7cc capacity, constructed from 0.127mm thick tantalum sheet. Substrates were mounted by a stainless steel holder onto a copper block with four 150W cartridge heaters imbedded in it. A piece of stainless steel sheet covers the block as a radiation shield. The temperature is measured by a type-J (Iron-Constantan) ther-mocouple wire attached at the back of the copper block (See Figure 2.3). Substrates were 34cm above the centre of the three evaporators. The configuration of the evaporators Chapter 2. Apparatus 10 and the substrate holder is shown in Figure 2.4. There is also a 50W plasma discharge unit pointing to the substrates from 10cm away, fed from a Granville-Philip leak valve. This enables activated gas to be introduced during the deposition procedure (see Figure 2.2). The plasma discharge is produced by means of a Helical resonator with the resonant frequency of 50 MHz (see Figure 2.5). The gas flows through the coil and is ionized by the electric field generated by the rf power and then ejected from the 1.5mm hole at the front end of the quartz tube. The three quartz thickness/rate monitors, positioned 33cm directly above each evap-oration source, moniter the deposition rate from each evaporator and provide the rate feedback to the evaporation sources. Three 3 inch long 1 inch in diameter stainless steel tubes are attached to each monitor sensor head as shown in Figure 2.2, and prevent the three evaporators from interfering with each other's thickness/rate readings. The measurement principle and details will be discussed in Section 2.4. 2.2 Pressure Measurement The pressure of the system was monitored by a Varian ion gauge below the gate valve of the vacuum chamber (See Figure 2.2). The background pressure was about 2 x 10 - 6 Torr during most runs though the lowest pressure ever measured was 3 x 10 - 7 Torr. During the deposition process, 0 2 or H2O vapour was introduced through the leak valve and the plasma discharge unit. Because this was the only major inlet gas source in the vacuum chamber during the process, the pressure inside the tube of the helical resonator and the pressure in front of the substrates can be estimated from the pressure measured by the ion gauge and from the geometry of the vacuum system. Chapter 2. Apparatus 11 Substrate Holding Block (34cm above the 3 evaporators) I I t Thermal Evaporator Figure 2.4: Top view of the evaporators showing the configuration of the evaporators and the substrate holding block Chapter 2. Apparatus 12 1^ - " I.D. < • 1.5 mm Figure 2.5: Plasma Discharge Uni t Chapter 2. Apparatus 13 2.2.1 Pressure inside the Helical Resonator The gas throughput at the aperture at the front end of the helical resonater tube should be the same as the throughput of the diffusion pump. Therefore [22], C1(P1 - P2) = S2P2 (2.1) Where C\ is the air flow conductance of the aperture, S2 is the pumping speed of the diffusion pump, Pi is the pressure inside the resonator tube and P2 is the chamber pressure measured by the ion gauge. The conductance of an aperture in a flat sheet is known to be [22], Cx(liter I sec) = U.6A(cm2) . (2.2) where A is the area of the aperture which is 1.77 x 10~2cm2. The pumping speed S2 is provided by the manufacturer as 2400 litre/sec in the pressure range of 10"4Torr -10"8Torr. Thus, Px is related to P2 as, Pi = 1.17 x 10 4P 2 (2.3) In most depositon runs, P2 was controlled at 1.5 x 10 - 5Torr. Therefore, the pressure inside the resonator tube P2 can be estimated as 1.8 x 10 - 1Torr. 2.2.2 Pressure in Front of Substrates One can estimate the pressure in front of the substrates by assuming that the flux in-tensity of the molecules coming out of the helical resonator tube has a cos20 angular distribution. Thus, the effective area covered by this flux in front of the substrates is, Chapter 2. Apparatus 14 A e f f = / cos ;20r 2dO = 2wv2 I cos20sin0d0 Jo (2.4) where r=10 cm is the distance between the helical resonator tube and the substrates. From Eq(2.2), the conductance of the aperture with area Aeff can thus be estimated to be 2400 litre/sec. The conductance of the 7"-diameter and 20"-long tube connecting the pump and the vacuum chamber is 830 litre/sec [32]. Therefore, the total conduc-tance between the substrates and the pump is 600 litre/sec. By using the relation of C(P — P 2 ) = S2P2 shown in Eq(2.1), the pressure P in front of the substrates is esti-mated to be 1 x 10 - 4Torr. 2.3 Hot Stage and the Temperature Measurement The substrate hot stage consists of a 2" x 2" x 0.5" copper block with four 150W cartridge heaters (Watlow #E2A56) uniformily imbedded in it. Four substrates can be mounted by a stainless steel holder tightly screwed on the copper block so that there is tight contact between substrates and the copper block. A piece of stainless steel sheet covering the whole block is used as a radiaton shield to increase the heating capability. The temperature is measured by a Iron-Constantan thermocouple tightly screwed on the back of the copper block (See Figure 2.3). The power of the heaters is provided by a variable autotransformer and about 250 W power was used to achieve 850 °C temperature reading. The heaters' lives were limited to a few hours time by running heaters at the tem-perature above 850 °C with power exceeding 300 W. Therefore, the highest temperature Chapter 2. Apparatus 15 that the hot stage was operated at was limited to the temperature slightly above 850 °C. The temperature at the surface of the substrate is lower than the temperature mea-sured at the back of the copper block due to the finite thermal conductivity of the substrate, the radiation of the substrate and the thermal contact between the substrate and the block. A lower limit to the temperature difference could be estimated by a simple model assuming perfect contact between the substrate and the copper block [23], Where T\ and Ti are the temperatures of the copper block and at the surface of the substrate respectively, k is the thermal conductivity of the substrate, A r is the thickness of the substrate, e is the tolal emissivity at the surface of the substate and a is the Stefan-Boltzmann constant. The substrates used in this project have been 0.5mm thick magnesium oxide single crystal. The thermal conductivity at 800 °C was found to be lOWatts/m- K [24]. And the total emissivity is not greater than 0.43 [25]. Thus, the temperature drop at the surface of the MgO substrate due to the radiation is estimated as about 2 °C at 800 °C. The temperature drop due to possible imperfect contact is unknown. 2.4 Deposition Rate and Thickness Monitoring The success of making superconducting films largely depends on stoichiometric composi-tion of the film. The stoichiometric composition of a film is determined by the deposition rate of each evaporant, in the case of coevaporation, or the thickness of each film layer, in the case of sequential evaporation. Three Inficon quartz crystal thickness monitors were used in this project. The three sensor heads were 33cm directly above each evaporation source to monitor the deposition eaT* (2.5) Chapter 2. Apparatus 16 rate and thickness of each individual evaporant and provide the feedback to the evapora-tors. Three 3-inch-long 1-inch-diameter stainless steel tubes were attached to each sensor head so that evaporant molecules could only go straight to the sensor from the evapo-ration source directly below it. However, molecules from other two evaporation sources could still arrive at the sensor not directly above them by collisons between evaporants. Test runs showed that this effect gives an error of less than 1% with the pressure of 1 x 10~6Torr - 1 x 10~5Torr (mean free path 5m - 50m) and with deposition rates lower than 15A/s. The deposited film thickness is determined by the monitor from the resonant frequency shift of the loaded quartz-crystal [26], Tj = -^-tan-i{Ztan[7rif%fc)}} h L Jq where Tj = film thickness fq = resonant frequency of unplated crystal fc = resonant frequency of loaded crystal C — a constant depending on densities of quartz and of film Z = acoustic impedance ratio depending on the shear moduli of deposited film and quartz crystal The quartz crystals used in this project all have / , = 6MHz and the monitor refers 100% of crystal life to a 1 MHz frequency shift from 6 MHz. The Z-ratio of some materials used in this project were not available. In those cases, the Z-ratio was set equal to 1. With the life of the crystal of thick),.'.ss monitor kept below 15% in those occassions, the (2.6) Chapter 2. Apparatus 17 error introduced from setting Z = 1 is less than 3%. One can easily see this from the Taylor expansion of Tj wi th respect to Jq Jq Jq Jq ^Jq Jq Jq Chapter 3 Preparation of YiBa2Cu30 x Films > 3.1 Electron-Beam Evaporation The most involved technique in the film preparation of this work is electron-beam evap-oration (see §3.2). In the electron-beam evaporation process,vaporization occurs at the surface of the evaporant by electron bombardment heating. A stream of electrons is emitted from a hot tungsten filament and accelerated by 5 to 10 kV electric field. The electron beam is confined longitudinally by a permanent magnet and swept laterally by an electro-magnet. The accelerating voltage used in this work was normally between 6 kV and 8 kV and the emission current was between 0.04 A to 0.06 A. 3.2 Substrate Preparation Single crystal MgO (100) substrates were used in this work to promote epitaxial growth of Y 1 B a 2 C u 3 O x films. Lattice mismatch between MgO substrate and Y i B a 2 C u 3 0 6.9 is only 8% [27]. Although single crystal SrTi0 3 (100), which gives 1% mismatch, would be a better choice, MgO (100) was chosen due to the relative costs. Substrates were about 10mm x 3mm x 0.5mm in dimension and cleaned with trichlorethylene, acetone and methanol prior to deposition. 18 Chapter 3. Preparation of Y i B a 2 C u a O x Films 19 3.3 Film Deposition Two major different methods were employed in the deposition of YiBa2Cu30 x films in this work. One is a sequential deposition technique in which a multilayer film is deposited by means of a four-hearth-rotating-turret electron beam source. The second is a co-deposition technique in which two electron beam sources and one thermal evaporator are used to deposit all materials simultaneously. 3.3.1 Sequential Deposition In the sequential deposition, the sequence Cu — BaF 2 — Y was deposited in 15 layers (5 periods) with a total thickness at about 1.5 — 2/im. The deposition of Cu and BaF 2 was carried out in a vacuum of about 1 x 10~6Torr while deposition of the Y was done at 1 x 10~5Torr with 0 2 leaked in. The deposition rates of Cu and Y were kept at 5A/s and 1.5A/s respectively and the deposition rate of BaF 2 was between 5A/s and 20A /s . 3.3.2 Co-deposition YxBa 2 Cu30 x films were also made with simultaneous evaporation of Cu, BaF 2 and Y. Cu and Y were evaporated by electron-beam sources #1 and #2 respectively (see Figure 2.2) and BaF 2 was evaporated by the thermal evaporator with a current between 160A -180A. The deposition was done in 1.5 x 10 _ 5Torr 0 2 with a background pressure of 1 x 10 - 6Torr. The deposition rates of the three evaporants were repeateadly adjusted against the E D X (Energy-Dispersive X-ray) composition analysis results of the films to achieve the right stoichiometry (See §4.1). As-deposited films were about 1/zm thick and deposition rates ranged from 1.5A/s to 5A/s. Chapter 3. Preparation of Y i B a 2 C u s O x Films 20 3.4 Post-Deposition Annealing High temperature post-deposition annealing of films is needed for two reasons: first, to remove fluorine from the film, second, to crystallise and oxygenate the amorphous as-deposited film to form the YiBa2Cu306.9 superconducting phase. The as-deposited samples were put in a quartz tube furnace on a quartz boat with t h e film side up. The furnace was heated up to 850 °C at the ramp rate of 5 °C/min and then dwelled for an hour at this temperature. During this period, wet 02 was introduced into the furnace tube by bubbling 0 2 through distilled water. The water was heated to about 60 °C - 70 °C to increase the water vapour pressure. The total air flow rate (02 + H20 vapour) was about 1 cc/second. Then, the flow of wet 0 2 was replaced by the flow of dry 0 2 at the same flow rate. The temperature was kept at 850 °C for another hour and then declined at the rate of 2 °C/min to 300 °C. The samples were taken out of the furnace and cooled to room temperature at this time. Chapter 4 Analysis of Prepared YiBa2Cu30 x Films 4.1 EDX Composition Analysis The compositional information of the prepared films were provided by the E D X (Energy-Dispersive X-ray) analysis. A Hitachi S-570 Scanning Electron Microscope equipped with a Kevex 8000 E D X system was used. The characteristic X-rays generated by an electron beam striking the atoms of the sample were detected and matched to the standard. The standard was a | " - diameter yg" thick Y 1 B a 2 C u 3 0 6.g pellet numbered 88 Jul. 2, 23 provided by U B C High T c Superconductivity Group. The EDX system was not able to detect elements lighter than Na (atomic number smaller than 11). Therefore, the oxygen content of the films was not determined. For compositional analysis of each film, data was collected and random error calculated for several points (normally 4) on the sample. The size of the analysis spot on the sample was 0.3mm x 0.4mm. The E D X analysis results were used after each film making process to adjust the thickness of the individual layers, in sequential deposition, or the deposition rate of the individual evaporants, in the co-deposition case, to appoach the correct composition. The ratio of atomic composition of the best sequential deposited film is determined to be (with order of Y:Ba:Cu) 1.0:2.1:3.1 with a random error of 10%. In the best co-deposited film, the ratio was 1.0:1.9:3.1 with a random error of 4%. 21 Chapter 4. Analysis of Prepared YiBa2Cu3 0 X Films 22 | 2 mm | 2 mm | 2 mm Substrate Film Figure 4.6: Four-Point-Probe 4.2 R e s i s t a n c e M e a s u r e m e n t of t he F i l m s One of the most important characteristics of superconducting thin films is the sharp transition of the electrical resistance to zero at some well-defined temperature. Thus, measuring the resistance as a function of temperature of the films, is necessary. The four-point-probe in the Th in F i l m Technology Lab (Parsons Lab) was used for this mea-surement. As shown in Figure 4.6, four contact leads were stuck on the sample wi th silver paint. The leads were colinear and 2mm apart. The resistance was measured by passing a current between the outer two contacts and measuring the voltage drop between the inner two. Because very litt le current flows in the voltage sense leads, the contribution of the contact resistance to the resistance measurement is negligible. The sample was mounted on a copper block inside the bottom end of a stainless steel Chapter 4. Analysis of Prepared Y i B a 2 C u s O x Films 23 tube with the bottom end wall made of copper to enhance the temperature uniformity. The temperature of the sample was monitored by a silicon diode which was imbedded in the copper block. The diode was calibrated against a platinum resistor from 25K to 325K. The overall uncertainty is believed to be ± 0 . 5 K [28]. The measurement was taken with the probe being inserted into a liquid helium dewar and the probe tube being evacuated and backfilled with helium gas. Data was collected with a microcomputer controlled UBC Electronics Shop Model 87-033-5 system which has a resolution of 0.3mfl [28]. The resistance vs. temperature curve of the best co-deposited YiBa2Cu30x film is shown in Figure 4.7. The current of 0.1 mA through the sample was used in the measurement. The low T C (R = 0) and the broad transition range is believed to be due to a stoi-chiometry that is different from the ideal "1:2:3" ratio (see §4.1). To achieve narrow, sharp transition (within few Kelvins), one has to have the stoichiometry within about 0.1% of the ideal ratio [27]. For this purpose, an electron microprobe, which has the same mechanism as E D X but uses individual elements as standards instead of the compound and has higher resolution, should be used to determine and guide the film composition. 4.3 Scanning Electron Micrograph Scanning electron micrographs (SEM) were taken for the best co-deposited YiBa2Cu30x film. As shown in Figure 4.8, it had the texture of local oriented needles along (001) plane of the substrate. Good connectivity between long grains is also shown in the picture. Chapter 4. Analysis of Prepared YiBa2Cu30 x Films Resistance (Ohm) 24 Temperature (K) Figure 4.7: Resistance vs. Temperature Curve of Y!Ba2Cu30x Codeposited Film #890511. Tc(onset) = 87K Tc(midpoint) = 68K TC(R = 0) = 53K Chapter 4. Analysis of Prepared Y i B a 2 C u 3 0 x Films 25 Figure 4.8: S E M Picture of Codeposited F i lm #890511 Chapter 5 In-Situ Conversion of BaF2 and BaCb to BaO One of the main obstacles in the fabrication of superconducting Y 1 Ba 2 Cu 3 O x films in situ with BaF 2 as the Ba source is the undesired fluorine in the material. One would like to convert BaF 2 to BaO during the deposition process. Therefore, studying the conditions of in situ conversion of BaF 2 to BaO is very important. BaCl 2 was studied as an alternative of BaF 2 in this chapter. 5.1 Film Preparation BaF 2 was evaporated from a tantalum boat onto a MgO (100) substrate. H 2 0 vapour was introduced during the deposition. The H 2 0 background pressure was 1.5 x 10~5Torr (see §2.2). The deposition rate was controlled at l.OA/s throughout the process and the substrate was heated to different temperatures to study the effect of temperature on the conversion. The films were 0.5/xm thick. BaCl 2 was evaporated under the same conditions as BaF 2 except for a two-hour preheat to outgas water from BaCl 2 • 2H 20 which is hydrated in its normal form. 5.2 Thermodynamic Analysis 5.2.1 Conversion of BaF2 The overall reaction taking place during the film deposition process is: B a F 2 ( s ) + H 2 0 ( g ) = B a O ( s ) - r 2 H F ( g ) 26 Chapter 5. In-Situ Conversion of BaF2 and BaCl2 to BaO 27 The direction of the reaction is determined by the concentrations of reactants and products, the temperature, and the pressures of the gases. The equilibrium concentra-tions can be obtained by minimizing the total free energy. The equilibrium appoximation is valid when the deposition rate of BaF 2 is much slower than the reaction rates in both directions (infinite reaction rate). The total Gibbs free energy of the system can be written as where ^ hj is the total enthalpy and ^ Sj is the total entropy of the system summed j j over all the reactants and products. We assume there are N sites on the surface of the film and each site can only be occupied by BaF 2 or by BaO. Therefore, where CeaO is the BaO concentration and h; denotes the enthalpy of the ith material. Because H 2 0 vapour was constantly supplied during the deposition process, N' is a independant constant. The total entropy can similarly be written as, ^ Sj = N C B a O S B a O + N(l - C B a 0 ) s B a F 2 + 2 N C B a O S H F + [N' - N C B a o ] s H 2 0 + Ss i te (5.10) where SBao> s B a F 2 > s H F and SH 2O stand for entropies of BaO, B a F 2 , HF and H 2 0 molecules. Ssite is the entropy due to the occupation of the surface sites by BaF 2 or BaO. Therefore, (5.8) £ hj = . N C B a o h B a O + N(l - C B a o ) h B a F 2 + 2 N C B a 0 h H F + [N' - NC B a o]h H 2 o (5.9) S8ite = kln( N! ) (5.11) nBao'nBaF2! Chapter 5. In-Situ Conversion of B&F2 and BaC l2 to BaO 28 where n B a 0 = C B a 0 N (5.12) n B a F 2 = (1 - C B a 0 ) N " (5.13) are the numbers of BaO and BaF 2 molecules occupying the N surface sites respectively; k is the Boltzmann constant. Minimizing the total free energy with respect to CB ao> o n e has, dGtoui = 0 (5.14) d C B a o Note that hup and hn2o a r e dependent on the partial pressures PHF a n d PH 2O respectively, h HF = KF + kTln(|S5L) = h H F + k T l n ^ F (5.15) * a t m h H 2 o = h^2o + k T l n ( ^ ) = h H 2 0 + k T b ^ o (5.16) P where hj\ F and h H 2 0 are standard enthalpies at 1 atm. V = —— denotes the dimension-* atm less pressure normalized by P A T M . Since the H F pressure is proportional to the rate at which BaF 2 is converted to BaO, P H F OC 2 N C B a 0 (5.17) PH 2O OC (N' - N C B a 0 ) (5.18) then we have, dh HF kT d C B a o C B a o dh H 2 o NkT (5.19) d C B a 0 N' - N C B a o ( 5 ' 2 0 ) Putting Eq(5.8),Eq(5.9),Eq(5.10),Eq(5.11) and Eq(5.14) together and using the ap-poximation InN! ~ N(lnN — 1), we obtain, Chapter 5. In-Situ Conversion of BaF2 and BaC l2 to BaO 29 where kT + Ah - TAs + Tkln( C b * ° ) = 0 (5.21) 1 — ^ B aO Ah = hB ao + 2 h H F - h B aF 2 - hH jo (5.22) As = S B a Q + 2SHF - SBaFj - SH2O (5.23) so that, ' B a O / A s _ i \ A h e^  k ll e kT / A s -I \ A h 1 + e^  k L) e kT / • A S _ i \ A H eS R L> e RT 1 + e / A S _ -I \ A H ^ R ' e RT where (5.24) A H = N A A h (5.25) AS = N A A s (5.26) NA = Avogadro constant. (5.27) A H can be expressed as [29,19], AH = A H 0 + R T l n ( ^ ^ ) (5.28) "PH2O where VHF and PH 2O are partial pressures of HF and H 2 0 normalized by P a t m and A H 0 = A H ° B a O + 2A H ? H F - A H ° B a F 2 - A H ° H 2 o (5.29) AS = A S f ° B a 0 + 2 A S F ° H F - A S f ° B a F 2 - A S f ° H 2 0 . (5.30) AH° and AS° are standard enthalpy and entropy of formation at 25 ° C and 1 atm which can be found in various handbooks [30,31]. AH° and AS° for the relevant materials are shown in Table 5.1. Thus, we have, Chapter 5. In-Situ Conversion of BaF2 and B a C b to BaO 30 <p / A S i \ AH° 1 + T ^ e 1 - R ^ e RT From the above information, the equilibrium curves for different values of the Vu2o/"^HF ratio in CeaO—T space can be obtained (see Figure 5.9). The experimental value of ^ H J O / ^ H F m front of the film during the deposition process, can be estimated as dis-cussed below. Because of the logarithmic dependence of the conversion temperature on ^ H J O / ^ H F f° r fixed CsaO) precise measurements of the ratio are not necessary. The partial pressure of H2O in front of the film is estimated to be 1 x 10_4Torr in §2.2.2. The upper limit of the partial pressure of the reaction product HF just in front of the film can be estimated as follows: The total number of HF molecules in the chamber should be equal to the HF production rate times the residence time: N = Nt (5.32) The residence time is determined by: t = ^ - (5.33) where Seff is the effective pumping speed, V is the volume of the chamber. Since N PV = - R T , S i SefT is determined by the pumping speed of the pump S and the conductance of the pipe connecting the pump and the chamber C [32], (5.35) Chapter 5. In-Situ Conversion of BaF 2 and BaCl 2 to BaO 31 A H " (kcal/mol) AS? (cal/K • mol) BaF 2 (s) -288.5 23.03 BaO (s) -131.000 [31] 17.225 [31] H 2 0 (g) -57.796 45.104 HF (g) -64.8 41.508 BaCl 2 (s) -205.2 29.56 HCl (g) -22.062 44.646 H 2 (g) 0.076 34.343 Al (s) 0 6.77 AICI3 (s) -168.3 26.45 Table 5.1: AH° and AS° values for some selected materials[30,31]. Chapter 5. In-Situ Conversion of B a F 2 and BaC l2 to BaO 32 The pipe is 7 inches in diameter and 20 inches long, so that, C = 830 litre/s [32]. The pumping speed S is 2400 litre/s (see page. 13). Thus, Sefr = 6201itre/s. From the fact that the BaF 2 was deposited at a rate of lA/s (which is about | monolayer/s) and the area size of a molecule is about 10A 2 , the flux <j> of HF molecules coming out from the reaction can be estimated as follows: . . 1 . . molecules (p ~ 3 m o n ° ^ a y e r / s e c x — i o^2— ~ 6.7 x 1018molecules/m2sec. (5.36) where factor 2 represents the fact that each BaF 2 molecule generates 2 HF molecules. Since the reaction takes place all over the hot area facing the deposition, which is the area of the bottom side of the heating block (2inch x 2inch, or 5.1cm x 5.1cm), the production rate of HF is, N = 4> x (5.1cm x 5.1cm) ~ 1.7 x 101 6 molecules/sec (5.37) Thus, the partial pressure PHF is estimated to be 9 x 10 _ 7Torr. It follows that the ratio VH2O/'PHF 1 S ahout 1 x 101 1. The solid line in Figure 5.9 shows the equilibrium CBSO—T curve with VfyofPuF = 1 x 101 1 which represents the experimental conditions in the film preparation. Since PHF is proportional to the BaF 2 deposition rate (see Page 32), changing the deposition rate will shift the C B a o—T curve. Figure 5.10 shows the C B a o—T curves with PH 2O = 1 X 10~4Torr and the deposition rates being O.lA/s, lA/s and lOA/s. The rate of O.lA/s is probably too slow for practical film growth. Chapter 5. In-Situ Conversion of BaF2 and BaC l2 to BaO 33 BaO 600 650 700 750 800 850 900 950 1000 Figure 5.9: The equil ibrium curves of the B a F 2 B a O reaction in the CaaO—T space. Chapter 5. In-Situ Conversion of B a F 2 and BaC l2 to BaO 34 BaO 6 0 0 6 5 0 7 0 0 7 5 0 8 0 0 8 5 0 9 0 0 9 5 0 1000 Figure 5.10: CB BO—T equilibrium curves for P H 2 o = 1 x 10 4Torr and different deposition rates. Chapter 5. In-Situ Conversion of BaF2 and BaC l2 to BaO 35 5.2.2 Conversion of B a C l 2 For BaCl 2 conversion, the overall reaction can be written as: B a C l a ( B ) + H 2 0 ( g r B a O ( 8 ) + 2 H C l ( g ) Following procedure similar to § 5.2.1, we find, ^ - e ( T T _ 1 ) e _TtT-Cfiao = r ™ Q „ AHo (5.38) 1 + ^ 2 £ e ( i r - 1 ) e"rer •p2 Here A S = A S ? B a o + 2AS f ° H c i - A S f ° B a C l 2 - A S ? H 2 O (5-39) A H 0 = A H ? B A 0 + 2 A H f ° H C 1 - A H F ° B A C L 2 - A H ° H 2 o (5-40) Similarly, the equilibrium curves for different values of the ratio PHao/^HCi m C B a o —T space can also be drawn (see Figure 5.11). The solid line shows the CBaO—T curve with 'PHZO/^HCI = 3 x 1 ° U - Comparing Figure 5.9 and Figure 5.11, one can find that BaClo requires a higher temperature to convert than BaF 2 does under the same conditions. Therefore, BaCl 2 is less favourable than BaF 2 as the Ba source for in situ growth of Y ! B a 2 C u 3 O x films. 5.3 XRD and EDX Analysis 5.3.1 XRD Analysis of BaF 2 Films Because B a F 2 and BaO have different crystal structures, they exhibit different X-ray diffraction (XRD) patterns. As shown in Figure 5.12 and Figure 5.13, B a F 2 has a very simple pattern while BaO has a relatively complex pattern due to the existance of many hydrated forms of BaO. BaF 2 films made under different temperatures were studied by Chapter 5. In-Situ Conversion of B a F 2 and B a C l 2 to BaO 36 600 650 700 750 800 850 900 950 1000 Figure 5.11: The equilibrium curves of the BaCl 2 BaO reaction in the C B B O - T space. Chapter 5. In-Situ Conversion of B a F 2 and B a C l 2 to BaO 37 15 20 25 30 35 40 45 50 55 60 Figure 5.12: X-ray Diffraction Pattern of B a F 2 Powders Chapter 5. In-Situ Conversion of B a F j and B a C l 2 to BaO 38 Figure 5.13: X-ray Diffraction Pattern of B a O Powders Chapter 5. In-Situ Conversion of BaF2 and B a C l 2 to BaO 39 X-ray diffraction. Figure 5.14 shows the transition from BaF 2 to BaO over the tem-perature range 775 °C — 850 °C. Although the peak intensities of X R D pattern from powder diffractometer are not useful for a quantitative comparison between B a F 2 and BaO concentrations due to the strong orientation dependence of the intensities, they can still provide a qualitative picture of the conversion. Figure 5.15 shows the BaO concen-tration in the films against the temperature at which the film was deposited. The BaO concentration is defined as, C fBaOmax BaO = J —J -* BaOmax ' BaF^max where iBaOmax a n < i iBaFimax a r e the highest peak intensities of BaO and B a F 2 in the film X R D patterns respectively. As shown in Figure 5.15, the experimental data are shifted to higher temperature from the theoretical prediction. However, we believe this difference in temperature is within the experimental error in the temperature determination. The surface temperature of the substrate is expected to be lower than the measured tempera-ture of the heating block due to imperfect contact between the substrate and the heating block (see §2.3) and radiative cooling of the substrate. The anomalous behaviour at 805 °C to 820 °C suggests that the random variations in the temperature of the substrate measurement are around 15 °C . BaF 2 films were also made under the same conditions as above, but with H 2 0 vapour replaced by 1) H 2 0 vapour + 0 2 , 2) 0 2 plasma, and 3) H 2 0 plasma. No substantial improvement of BaO conversion were found. This is not a surprising result, if the surface is already in thermodynamic equilibrium with the partial pressure of H 2 0 , in the absence of the plasma. The plasma is expected to facilitate the achievement of equilibrium by breaking down activation barriers. The equilibrium concentration does not depend on the path, or the reaction rates. As shown above, it has already achieved the equilibrium with H 2 0 vapour. Chapter 5. In-Situ Conversion of B a F 2 and B a C l 2 to BaO (in) (200) 40 v o I CO CD CO Ba$ Powder B a K j BaKj Deposited at 775 C B a K j B a K , I Deposlted at 790 C B a K j Deposited at 805 C e B a F 2 f Deposited at 820 C A Deposited a t 8 3 5 ° c e Deposited at 850 C f BaO Powder 15 20 25 30 35 38 29 ( ° ) Figure 5.14: X R D Patterns of the Transition from B a F 2 to BaO Chapter 5. In-Situ Conversion of BaF2 and B a C l 2 to BaO A Experimental BaO • Theoretical 1 0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 0.1 H 0 i i • • 1 • • • • 1 • • • • 1 • ' • * 1 • ' u ^ . ' 1 1 1 j j j . / i i i | i i " i i i i i i i | i £r&\ i i i i | i i i i | i i i i j i i i i 600 650 700 750 800 850 900 950 1000 Figure 5.15: B a O Concentration as a Function of Temperature Chapter 5. In-Situ Conversion of BaF2 and BaC l2 to BaO 42 5.3.2 E D X Analysis of B a C l 2 Films In Situ conversion of BaCl 2 to BaO was studied as an alternative to BaF 2 conversion. Be-cause both BaCl 2 and BaO are hygroscopic, they both give complex X R D patterns which are not easily distinguishable between each other. On the other hand, E D X provides a very effective way to detect elements heavier than Na such as Cl (see §4.1). Therefore, the E D X technique was used to analyze the conversion of BaCl 2 to BaO. One BaCl 2 film was made under the same conditions as those of the BaF 2 analysed in the last section at substrate temperature of 740 °C. This sample was tested by E D X with BaCl 2 • 2H 2 0 powder as the standard. The atomic ratio between Ba and Cl of the film was found to be 1:1.6 with a random error of 5%. This result of losing Cl in the film indicates that 20% of the BaCl 2 has been converted to BaO in situ. The temperature requirement is similar to that of B a F 2 conversion which is consistent with the prediction made in §5.2.2. 5.3.3 Discussion As shown in §5.3.1, it was found that the substrate must be heated to 800 °C in order for the BaF 2 to convert to BaO in situ during the film deposition process. An attempt was made to grow a co-deposited Y-Ba-Cu-0 film at this temperature. The resulting film was transparent even though the thickness monitors gave the final thickness of the film as 1 fim (thickness monitors' crystals were water cooled to make the resonant frequency consistent). This temperature was found to be too high for the growth of Y-Ba-Cu-0 films presumeably because the deposited copper re-evaporates at these high temperatures. However, the growth temperature can be reduced by reducing the H F partial pressure (see §5.2.1). Adding an HF getter (such as quartz wool) to the film growth environment and reducing the heated area exposed to B a F 2 would help reduce the HF partial pressure. An experimental investigation of this might be worthwhile. Chapter 5. In-Situ Conversion o / B a F 2 and B a C b to BaO 43 From the growth temperature point of view, BaCl2 does not offer any advantages over B a F 2 as a source for B a in the in situ growth of YiBa2Cu30x films. Also, B a C l 2 is hygroscopic, and the long preheating necessary to outgas water is an undesireable complication. , Chapter 6 Conclusions The YjI^CusOx superconducting films have been reproducibly made by both sequential evaporation and co-evaporation of Cu, B a F 2 and Y, followed by high temperature post-deposition annealing. The best film achieved the onset critical temperature of 87K and zero resistance critical temperature of 53K. The low T C (R = 0) and the broad transition range is believed to be due to stoichiometry different from the ideal "1:2:3" ratio. Thus^ more accurate control over the film stoichiometry is needed. The electron microprobe, instead of EDX, might be the choice. The possibility of making Y 1 B a 2 C u 3 O x superconducting film in situ with B a F 2 as the Ba source has also been investigated by studying the conditions for in situ conversion of BaF 2 to BaO. The BaO concentration in the film as function of the substrate temperature was provided by the equilibrium thermodynamic analysis. The experimental results showed agreement with the equilibrium model within the experimental measurement error. It was also found that producing a plasma of the gas reactant had no substantial effect on the conversion. This is believed to be due to the equilibrium nature of the conversion reaction at the given conditions. Under the current apparatus conditions, the substrate temperature was found to have to be as high as 800 °C to achieve the in situ conversion of BaF 2 to BaO. However, this temperature can be reduced by reducing the HF partial pressure in the reaction scene. Adding an HF getter (such as quartz wool) to the deposition chamber and reducing the heated area exposed to B a F 2 are suggested. It might be also be interesting to see the plasma effect at the lower substrate temperature 44 Chapter 6. Conclusions 45 if the above mentioned suggestions are practiced. A brief exploration of BaCl2 showed that BaCl2 does not have any advantage over B a F 2 as the Ba source for in situ growth of Y-Ba-Cu-0 films from the growth temperature point of view. In summary, a possible route of low temperature in situ fabrication of Y-Ba-Cu-0 superconductor film using electron-beam evaporation technique with BaF2 as the Ba source was investigated and one of the key obstacles yet to be overcome for in situ growth is keeping the HF partial pressure low. Bibliography [1] A . M . Wolsky, R.F. Giese, E . J . Daniels, Scientific American, 260(2), 61, (1989) [2] N.W. Ashcroft, N.D. Merrnin, Solid State Physics, Holt, Rinehart and Winston, Philadelphia, 1976, pp. 726-753 [3] J . Bardeen, L.N. Cooper, and J.R. Schrieffer, Phys. Rev., 108, 1175 (1957) [4] J .G. Bednorz and K . A . Muller, Phys., B64, 189 (1986) [5] Z.Z. Sheng, A . M . Herman, Nature, 332, 58 (1988) [6] R.K. Singh, L. Ganapathi, P. Tiwari and J. Narayan, Appl. Phys. Lett., 55(22), 2351 (1989) [7] W.A. Little, Science, 242, 1390 (1988) [8] R. Pool, Science, 243, 741 (1989) [9] C W . Chu, P.H. Hor, R.L. Meng, L. Gao, Z.T. Huang, Y . K . Wang, Phys. Rev. Lett., 58, 405, (1987) [10] A.W. Sleight, Science, 242, 1519 (1988) [11] D.K. Lathrop, S.E. Russek, R.A. Burhman, Appl. Phys. Lett., 51, 1554, (1987) [12] S.J. Lee, E.D. Rippert, B.Y. Jin, S.N. Song, S.J. Hwu, K. Poeppelmeier, J.B. Ket-terson, Appl. Phys. Lett., 51, 1194, (1987) 46 Bibliography 47 [13] D. Dijkkamp, T. Venkatesan, X.D. Wu, S.A. Shaheen, N. Jisrawi, Y . H . Min-Lee, W.L. McLean, M . Croft, Appl. Phys. Lett., 51, 619, (1987) [14] M.F. Yan, R.L. Barns, H.M. O'Bryan Jr., P.K. Gallagher, R.C. Sherwood, S. Jin, Appl. Phys. Lett., 51, 532, (1987) [15] P.M. Mankiewich, J .H. Scofield, W.J . Skocpol, R.E. Howard, A . H . Dayem, E . Good, Appl. Phys. Lett.,51, 1753, (1987) [16] T . H . Geballe, J. Appl. Phys. 63(8), 4003, (1988) [17] P.R. Browssard, J.H. Claassen, S.A. Wolf, IEEE Transactions on Magnetics 25(2), 2356,(1989) [18] S.W. Chan, B .G. Bagley, L .H. Greene, M . Giroud, W.L. Feldmann, K.R. Jenkin II, B.J. Wilkins, Appl. Phys. Lett., 53(15), 1443, (1988) [19] N.Missert, R. Hammond, J .E. Mooij, V. Matijasevic, P. Rosenthal, T . H . Geballe, A. Kapitulnik, M.R. Beasley, IEEE Transactions on Magnetics, 25(2), 2418,, (1989) [20] D.D. Berkley, B.R. Johnson, N. Anaud, K . M . Berkley, L . E . Conroy, A . M . Goldman, J. Maps, K. Mauersberger, M . L . Mecartney, J . Morton, M. Tuominen, Y - J . Zhang, Appl. Phys. Lett., 53(20), 1973, (1988) [21] J . Kwo, M . Hong, D.J. Trevor, R .M. Fleming, A . E . White, R.C. Farrow, A.R. Kor-tan, K . T . Short, Appl. Phys. Lett.,53(26), 2683, (1988) [22] J.F. O'Hanlon, A User's Guide to Vacuum Technology, John Wiley & Sons, New York, 1980, pp. 22-42 [23] A.J . Chapman, Heat Transfer, Macmillan Publishing Company, New York, 1984, pp. 37, pp. 374 Bibliography 48 [24] American Institute of Physics Handbook 3rd Edition, McGraw-Hill Book Company, New York, 1972, pp. 4-159 [25] Handbook of Chemistry and Physics, 69th Edition, C R C Press Inc., 1988, pp. E-395 [26] X T C Thin Film Thickness and Rate Monitor Technical Manual, Inficon Leybold - Heraeus Company, 1987, pp. 4-6 [27] D. Pavuna, J.H. James, High Temperature Superconductivity, Tata Energy Research Institute, New Delhi, 1989, pp. 90, pp. 87 [28] S.K. Dew,Preparation and Characterization of High Temperature Super-conducting Thin Films, University of British Columbia, 1989, pp. 30 [29] M.K. Kemp Physical Chemistry, Marcel Dekker Inc., New York, 1979, pp. 202, pp. 600 [30] Handbook of Chemistry and Physics, 70th Edition, C R C Press Inc., 1989, pp. D51-69 [31] M.W. Chase, J.L. Curnutt, H. Prophet, R.A. McDonald and A.N. 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