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Aspects of Bi₂Sr₂Ca₂Cu₃O on silicon bolometer fabrication and operation Zindler, Ryan Walter 1992

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ASPECTS OFBi2rCau3O-ON-SILICONBOLOMETER FABRICATION AND OPERATIONbyRYAN WALTER ZINDLERB.A.Sc. University of British Columbia, 1990A THESIS SUBMITTED IN PARTIAL FULFILLMENT OFTHE REQUIREMENTS FOR THE DEGREE OFMASTER OF APPLIED SCIENCEinTHE FACULTY OF GRADUATE STUDIES(Engineering Physics)We accept this thesis as conformingto the required standardTHE UNIVERSITY OF BRITISH COLUMBIASeptember 1992© RyatiWalter Zindler, 1992In presenting this thesis in partial fulfilment of the requirements for an advanceddegree at the University of British Columbia, I agree that the Library shall make itfreely available for reference and study. I further agree that permission for extensivecopying of this thesis for scholarly purposes may be granted by the head of mydepartment or by his or her representatives. It is understood that copying orpublication of this thesis for financial gain shall not be allowed without my wiittenpermission.Department of SICThe University of British ColumbiaVancouver, CanadaDate 5e. /cLZDE-6 (2/88)AbstractPreliminary work to the production of aBi2SrCau3Osuperconducting bolometer wasdone. An analysis of the noise performance of Bi2Sr2Ca2Cu3Oy thin films was done withparticular attention to the character and magnitude of 1/f noise in the material.Deposition of Bi2Sr2Ca2Cu3Oyonto a silicon substrate was attempted through differentmethods of producing a barrier between theBi2SrCau3Oand the chemically corruptingsilicon. The buffer layers would not endure the high temperature processing required to produceBi2SrCau3Ofilms without cracking or peeling.A mixed buffer layer scheme was used in order to minimize the thermal stresses betweenthe different layers. In situ processing ofBi2SrCau3Owas attempted in order to eliminatethe high temperature annealing step of the Bi2Sr2CaCuOy. Finally, epitaxial buffer layers weregrown in order to strengthen the buffer layer against the thermal stresses.11Table of ContentsAbstract.iiList of Tables VList of Figures vi1. INTRODUCTION 1Bolometers 1Need For Silicon Substrates 2Applications 5Thesis Objective 52. BOLOMETER SYSTEM THEORY AND DESIGN 6Introduction 6Theory,,,, 6Bolometer System Design 83. ELECTRICAL NOISE IN Bi-Sr-Ca-Cu-O 13Introduction 13Theory of 1/fNoise 13Noise measurement 161/fNoise in BSCCO Material 17Implications on Bolometer Performance 224. X-RAY DIFFRACTION ANALYSIS 24Introduction 24Crystallography 24Basic X-ray Diffraction Theory 255. PREPARATION OF Bi2Sr2Ca2Cu3-Oy 28Introduction 28Sputtering 29Ex-Situ Preparation of Bi2Sr2Ca2Cu3Oy 31111Theory of In-situ BSCCO Preparation 32Experimental Results and Discussion 33Oxygen Content Control 48Surface Treatment 516. MIXED BUFFER LAYERS 56Introduction 56Linear Thermal Expansion Model 57Results and Discussion 587. EPITAXIAL BUFFER LAYERS 60Introduction 60Need for Stabilization of Zirconia 60Hydrogen Termination of Silicon 61Experimental Procedures 62Results and Discussion 638. CONCLUSION 65Summary of Results 65Noise Properties of BSCCO 65In-Situ Preparation of Bi2Sr2Ca2Cu3Oy 65Buffer Layers on Silicon 65Recommendations for further work 66Conclusion 67BIBLIOGRAPHY 68ivList of Tables5.1 Deposition parameters for in-situ processed BSCCO films 355.2 X-ray rocking curve results 556.1 Shear force calculations for non-mixed buffer layer deposited at 20 °C 576.1 Shear force calculations for mixed buffer layer deposited at 20 °C 586.1 Shear force calculations for mixed buffer layer deposited at 450 °C 597.1 Contamination measurements on hydrogen terminated silicon 62VList of Figures1.1 Bolometer membrane structure 32.1 Bolometer system schematic 82.2 Four point probe 92.3 Temperature controller circuit schematic 103.1 Noise measurement system schematic 173.2 Noise spectral density vs. temperature for 0.45 mA driving current 183.3 Noise spectral density vs. temperature for 2.7 mA driving current 193.4 Noise vs. Frequency for 0.45 mA driving current 203.5 Noise vs. Frequency for 2.7 mA driving current 214.1 Definition of (m1 m2 m3) crystal plane 244.2 Bragg angle demonstration for (1 0 0) crystal diffraction 254.3 Diffraction by the (3 1 0) crystal plane 264.4 Goniometer schematic 265.1 Magnetron sputtering schematic 305.2 Composition ofBSCCO film vs. deposition temperature 325.3 Substrate heater schematic 345.4 Data on in-situ processed batch #1 375.5 X-ray scan for in-situ processed batch #2 385.6 X-ray scan for in-situ processed batch #3 385.7 X-ray scan for in-situ processed batch #4 395.8 Data on in-situ processed batch #5 395.9 Data on in-situ processed batch #6 405.10 X-ray scan for in-situ processed batch #7 415.11 X-ray scan for in-situ processed batch #8 42vi5.12 X-ray scan for in-situ processed batch #9 425.13 X-ray scan for in-situ processed batch #10 435.14 Data on in-situ processed batch #11 435.15 X-ray scan for in-situ processed batch #12 445.16 X-ray scan for in-situ processed batch #13 455.17 Data on in-situ processed batch #14 455.18 X-ray scan for in-situ processed batch #15 465.19 Data on in-situ processed batch #16 475.20 X-ray scan for in-situ processed batch #17 485.21 Resistance transition for sample of batch #6 after oxygen treatment 495.22 Resistance transition for sample of batch #6 after oxygen treatment 495.23 Resistance transition for sample of batch #6 after oxygen treatment 505.24 Resistance transition for sample of batch #6 after oxygen treatment 515.25 X-ray pole figures for samples of batch #13 535.26 Resistance transition for samples ofbatch #13 546.1 Mixed buffer layer structure 567.1 Dimensions of yttrium stabilized zirconia sputterring target 617.2 X-ray scan of YSZ sample 64vii11. INTRODUCTIONBolometersA bolometer is a device that detects radiation via the change in resistance which arises fromthe change in temperature from the energy gained by the absorption of the radiation. For use in abolometer, a material must have a resistance value that is temperature dependent.Superconductors are very well suited for use in bolometers because of their high temperaturecoefficient of resistance (dRJdT) in the transition region, which results in high sensitivity. Inparticular, high temperature superconductors (HIS) are suitable because of the greatly reducedcost of operating at temperatures above the boiling point of liquid nitrogen (77K).The resulting device (bolometer) is capable of detecting incident radiation of anywavelength, as long as the absorbed power is enough to overcome background noise.Specifically, the detection of long wavelength infrared radiation is a major application forbolometers, since long wavelength infrared cannot be detected easily by other methods.The ultimate limitation on the performance of an infrared detector is that of the backgroundlevels of infrared radiation. However, a practical limitation on bolometer performance is electricalnoise, and in particular, current dependent electrical noise with a 1/f spectrum is a problem whenusing a HTS.To achieve good bolometer sensitivity, it is important to have a small thermal mass of the“active” material (ie. material whose resistance is being measured). The small thermal mass isnecessary so that a given amount of absorbed energy will result in a maximum change intemperature, and thus a large resistance change. Also, a small thermal mass is desirable tominimize the time constant of the device.It is thus desirable to have the superconducting material in the form of a thin film. Thereare many possible techniques for producing superconductors in thin film form. The method used2for film deposition in this research was magnetron sputtering, because it provides a goodcompromise between cost, film quality, and deposition rates, and is thus easily adaptable toindustrial settings.’For this project, Bi-Sr-Ca-Cu-O (BSCCO) material was chosen as the superconductor,because one of its crystal phases has a high transition temperature (—S 107 K), and because of itsstability in air. The more commonly usedY1Ba2Cu3-O(YBCO) material has a lower transition(90 K) and reacts with moisture in air.Need For Silicon SubstratesAs mentioned, one of the main goals when designing a bolometer is to minimize the thermalmass of the active region in order to increase the sensitivity and shorten the time constant of thedevice. In addition to the requirement that the HTS be a thin film, it is also necessary to usemicro machining techniques to achieve the desired minimal dimensions. Existing micromachiningtechniques are applicable to only few materials, including silicon, and it is thus crucial to use sucha material for substrate material on which to deposit the superconducting film. It is also desirableto use silicon substrates so that any electronics for control or interfacing to the bolometers may befabricated on the same substrate (monolithically) with the bolometer.A problem arises when depositing the superconductor onto silicon. When BSCCO (orYBCO) is deposited on silicon, its superconducting properties are destroyed, due to silicondiffusion into the BSCCO crystal, as well as the copper (from the BSCCO) diffusion into thesilicon.2 This interdiffusion is enhanced by the fact that there is a strong reaction between copperand silicon producing CuSi.In order to use a Si substrate, it is thus necessary to deposit a buffer layer between thesilicon and the BSCCO film in order to form a barrier to the diffusion mentioned above. This isnot easy because the buffer layer must be chemically compatible with the superconductor3(preferably with matched crystal spacing), it must form an adequate barrier to the diffusionmentioned above, and it must be able to withstand the high temperatures (eg. 865 K) for the longtimes required to anneal the BSCCO to form the 107 K transition crystal phase. Some suitablecandidates for buffer materials are zirconia (Zr02), or magnesia (MgO).The basic design idea is to deposit a ‘microbridge” of HTS onto a membrane of silicon.The arrangement is depicted in figure 1.1.Figure 1.1: Membrane structure, plan view (a), and cross section (b).(a)(b)The micromachining technique required to create this structure is the use of KOH(Potassium Hydroxide) chemical etchant. KOH has the property that it etches siliconpreferentially in a specific crystal direction, and that it does not etch boron doped Si (at least notquickly).4The existing method of producing BSCCO films with a 107 K transition involves a “post-annealing” step, where the film is baked at —865 K for several hours after the sputtering iscompleted. The problem is that the buffer layer will not stand up to these high temperatures, andwhen subjected to them, the buffer layer will crack or even peel off. The resulting cracks allowthe interdiffusion of silicon and copper to occur, causing the destruction of superconductingproperties in the BSCCO.One possible solution to this problem is to use “in-situ” processing of the BSCCO. Withthis method, the BSCCO is sputtered onto a hot substrate, so that the crystallization occursduring the deposition. This process would not require the post annealing step, and would involvelower substrate temperatures than the post-annealing described above.Another possible solution to the problems is to minimize the thermal expansion forcesbetween the different film layers. This is done by inserting a mixed layer of Si-Zr-O between theZr02 and Si02 layers. The forces can be further minimized by altering the temperatures at whicheach of the layers is deposited.A third possible solution is to strengthen the Zr02 layer. When Zr02 (or BSCCO) issputtered onto a substrate, it will be in polycrystalline form. It is felt that the cracks in the bufferlayer that form as a result of annealing form along the boundaries of the crystal grains. Byattaining an epitaxial (single crystal) film, these grain boundaries will not exist, and the resultingfilm will likely be more resistant to cracking.Another problem is that the membrane structure is extremely fragile, and will likely notsurvive the high temperatures. The solution here is to deposit and anneal the buffer and BSCCObefore etching the silicon. However, the problem with this is that the BSCCO is destroyed by theetchant (wax & epoxy covers were used to protect the top side from the etchant, but were foundto be ineffective at protecting the superconductor).The solution to both the buffer layer and etching problems has not yet been found. Thethree different possible solutions to the buffer layer problem were the subject of study, and theyare prescnted in later chapters of this work.5ApplicationsBolometers are widely used as detectors of far-infrared radiation, X-rays and particles.3 Itwas the intention of this project to build a bolometer to detect infrared radiation. Infrared (W)detection has recently become more important for applications such as spectral earth mapping,environmental contamination control, meteorology, volcanism study, agricultural research,resource exploration, planetary and space research, and star mapping.3 For these applications,detectors capable of detecting the far JR (wavelengths> 100 jim) need to be developed. Mostcompeting methods of JR detection cannot detect beyond 15 jim, while none of the competingmethods are capable of detecting wavelengths greater than 240 jim. A superconductingbolometer has been shown to feasibly detect up to 1000 jim.3While bolometers have been built with low temperature superconductors, the use of HTSmaterials will greatly reduce their cost of construction and operation. Infrared imaging of longwavelengths becomes possible and practical with the creation of a monolithically fabricated two-dimensional array of bolometers with its interface electronics.Thesis ObjectiveIt was the objective of this thesis work to construct and characterize a working bolometerusing Bi-Sr-Ca-Cu-O superconductor on a silicon substrate. The ultimate objective is to producea marketable device for imaging of far infrared light. Firstly, the suitability of the Bi-Sr-Ca-Cu-Osuperconductor for use in bolometers must be investigated, with specific reference to the electricalnoise properties of the material. Secondly, there are several technical obstacles to producing thedevice. These obstacles were to be addressed.62. BOLOMETER SYSTEM THEORY AND DESIGNIntroductionIn this chapter, the detail of the usage of a bolometric device is dealt with. Also, themethods of achieving optimal performance, as well as the methods of evaluating that performanceare discussed.Theory4’56789The basic figure of merit of a bolometer is its specific detectivity -- D*, given by:rjAAf (2.1)vNwhere AD is the area of the sensitive element, zf is the measurement bandwidth, VN is the noisevoltage, and r is the responsivity of the system, where:r—---- (22)- 1,, G(1+a2r)1 2where V0 is the output signal voltage, is the incident radiation power,‘b is the bias current,R is the resistance, r is the optical absorptance, G is the thermal conductance from the sensitiveelement to the heat sink, Ci is the angular frequency of the incident radiation, and 3 is thetemperature coefficient of resistance:(2.3)RdTt is the time constant of the system, and is given by: t=CIG where C is the thermal capacity of thesensitive element.As was mentioned in the introduction, micromachining is essential to producing acompetitive device, because of the influence of the thermal capacity C. Because of the influence7of the active area in the equations above, it is most advantageous to have a thin membranestructure. A membrane structure will also provide a small thermal conductance which also*slightly enhances DAnother goal in designing bolometers is to limit the electrical noise. The figure of merit forthe noise performance of the system is the noise equivalent power (NEP) which is the power ofincident radiation required to create a signal to noise ratio of 1.I1EP= rcJ(2.4)There are four different sources of noise in a bolometer system in terms of which the NEP may beexpressed:NEP = -4kBTG(f) + 4kBTCR + -- + 4kBTNR (2.5)77 Ir jrThe first term in the square root represents phonon shot noise (shot noise is noise due toquantization effects: electrical shot noise is negligible unless very small currents are used), thesecond term represents Johnson noise, the third term represents the 1/f noise of thesuperconductor, whose magnitude can only be determined empirically, and the fourth termrepresents amplifier noise with an effective noise temperature TN. In an ideal system, only thefirst two terms will be significant, because they are the only ones that cannot be reduced.The dependence of the detectivity D* on the NEP is:(2.6)NEPValues for D* in the range of iO10 cm’IHzJW have been achieved with low temperaturesuperconductors, and values of 108 cmsJHz/W have been achieved with YBCO material.’08Bolometer System DesignThe basic elements of a bolometer system are shown in figure 2.1:0Radiation SourceThe bolometric sensor is a conventional four-point probe resistance measurement of thesuperconducting film. A four point probe is as shown in figure 2.2, with a constant current sourceand a low (ideally zero) current voltage probe. The advantage of using a four-point probe is thatcontact resistances do not corrupt the resistance signal, since no current flows in the voltagesensing leads.Resistance SignalTemperature SignalLiquid Nitrogen DewarMechanical ChopperOutput SignalFigure 2.1: Overall bolometer system9outThe current sources used were a combination of a 9 V battery and a resistor giving currentsbetween 0.4 mA and 3 mA.Controlling the steady state temperature of the bolometric element is a crucial part of theoperation of a bolometer system. Here, the liquid nitrogen (LN) is used to cool the sample, whilea resistive coil is used to electrically heat the sample.A normal diode was used to sense the temperature of the sample. By running a constantforward bias current through the diode, it was possible to determine the temperature by readingthe voltage. The diode was determined to have linear voltage vs. temperature characteristicsbetween room temperature and LN temperature, and was thus calibrated by measuring the voltageat these two temperatures and then interpolating the other points.A basic proportional control feedback system was implemented to electronically control thesample temperature. The amount of power applied to the heater is proportional to the differencebetween the set point temperature and the measured temperature. The advantages of this schemeare simple implementation, and reliability.” The disadvantages are that there is a “steady stateerror” in the temperature meaning that the temperature will never reach its exact setpoint, and thatthe steady state temperature is slow to be reached. The more complex proportional-integralderivative (PD) controllers can achieve quicker response times and smaller steady state errors,but are difficult to build and work with since they must be delicately tuned to any system theycontrol, and are vulnerable to effects such as “integrator windup” which arises when the heatercannot heat quickly enough.Figure 2.2: the 4-point probe10In order to modulate the power delivered to the heater, a pulse-width-modulation (PWM)scheme was used. PWM is a scheme where the power delivered is either fully on or fully off,rather than just directly modulating current in the DC sense. A fixed frequency oscillator is used,and in order to apply more power to the heater, the system merely spends a larger portion of acycle in the fully on state. If the controller dictates, the PWM system may be on or off full-time.The advantage of this scheme is that it allows much more efficient power delivery to the heater.If a power transistor is in a “half-on” state, then the same amount of power applied to the heater(or load) will be dissipated in the transistor (up to more than 5 W in this case). This will result ina waste of power, and in the overheating and eventual destruction of the power heater(30 ohms)DiodeV inAs can be deduced from the circuit, the oscillation frequency is approximately 80 kHz.This frequency was chosen to be high enough not to cause electrical interference with bolometricmeasurement (around 100 Hz) but low enough for the power transistor to operate. In figure 2.3,the lower branch of the schematic is an oscillator, which produces a triangular wave form, and theupper branch is a feedback circuit. They merge into a comparator which in turn drives the base of1M1M1M324KFigure 2.3: temperature controller circuit11the power transistor. The resulting system could keep the temperature of the system stable to ±0.5 K.The amplifier used was a Princeton Applied Research (PAR) model 113 low noiseamplifier. The PAR 113 is fortunately optimized for use at around 100 Hz, but it is alsooptimized for an input source resistance of a few Mi). Given that the resistance of a typicalB SCCO film near its transition (- 107 K) is a few ), the resulting noise created by the amplifieraccording to the manufacture?s specifications is around 30 to 40 dB. This is intolerable, and itwas therefore necessary to introduce a transformer to aid the impedance matching of theamplifier.A PAR AM-i low noise transformer was inserted in the circuit for this purpose, with a100:1 turns ratio. Even though the transformer itself adds noise to the system, the resultingreduction of amplifier noise makes its use advantageous. A 5 Q source resistance at 100 Hz willhave about 6 dB of noise created by the transformer, but the resulting 50 K2 resistance seen bythe amplifier will result in only 0.5 dB of noise created by the amplifier. The resultant noisecreation is far lower than the 30 dB created by the amplifier by itself. The transformer wascapacitively coupled to the four-point probe circuit, because its low resistance to DC wouldcorrupt the resistance signal, and a DC signal through the transformer will magnetize the core.It was found that the connection between the transformer and the four-point probe wasextremely sensitive to 60 Hz pickup noise since this is the only unamplified signal in the system.Consequently, connections to the transformer were done in differential mode, such that each ofthe two voltage leads was shielded. Further, the two leads were made as short as possible(around 20 cm) to minimize this pickup. Because of the very large amplification required, it wasfound that even these short shielded lines produced pickup noise, but by simply twisting orbending these wires to an unpredictable position, the pickup noise could be effectively minimized.The output signal of the transformer was found not to be so crucial, because the voltage signalhad already been amplified by 100 due to the turns ratio of the transformer.12The final signal comes out of the lock-in analyzer. A lock-in analyzer takes 2 inputs: areference sin wave and the signal. It responds with two outputs which are simply the componentsof the signal that are in and out of phase with the reference signal. By putting a low pass filter onthe output (<1 Hz), the lock in analyzer essentially behaves similarly to a bandpass filter at thereference frequency. The EG&G Princeton Applied Research model 5204 lock in analyzer usedhere could produce its own reference signal, which would control the mechanical chopper as well.To summarize, the operation of the system is as follows: The lock-in analyzer produces areference signal that controls a mechanical light chopper. The mechanical chopper modulates theradiation signal to the sensitive element of the bolometer, resulting in a small resistance change atthis frequency. This resistance signal is amplified and fed into the lock-in analyzer. Thecomponents of this signal that are in and out of phase with the reference signal are then the outputof the overall system representing the amount of radiation detected.133. ELECTRICAL NOISE iN Bi-Sr-Ca-Cu-OIntroductionAs stated in chapter 2, the noise characteristics of the superconductor are crucial to theuseftilness of the device. In particular, 1/f noise is of interest because it is a reducible form ofnoise. The magnitude and characteristics of the 1/f noise in BSCCO material were thusinvestigated, in order to make comparisons to competing superconducting materials.Theory of 1/fNoise1/f noise (or flicker noise) is noise with a roughly 1/f spectrum. This is to say that the noisepower per decade is roughly constant. 1/f noise is present in nature, in many unrelatedcircumstances. Even freeway traffic, the loudness of a piece of classical music, and flow rates ofrivers exhibit fluctuations with a 1/f spectrum.’2 There is no unifying principle for systemsexhibiting 1/f noise, but for particular systems, sources can often be identified.When considering the 1/f noise of a simple resistor, there are many things that have beenfound to influence its magnitude. The material used, the construction, and especially the endcapconnections influence the magnitude of i/f noise. While Johnson (or Nyquist) noise and shotnoise is generally well understood, there are as yet no explanations as to the microscopic sourceof i/f noise in a simple resistor.13 There have been several proposed models of the 1/f noisemechanism, but at best the models have been applicable to only a few specific materials.The original and simplest model to predict 1/f noise magnitude results in the followingequation:S(f)=y (3.1)14where Si,, is the noise spectral density, Nc is the number of charge carriers in the sample, andand are constant with c1, f3O, and y an empirically derived number where y2x1O3. Theflaw in this model is the assumption that y is a constant. While there have been many roomtemperature measurements of both metals and semiconductors which produce a value for y closeto 2x103, the model accounts for no temperature dependence, and no surface effects, both ofwhich are known to exist. However, embodied in equation (3.1) Is the voltage (or current)dependence of the noise. This is to say that 1/f noise (unlike Johnson noise) does not exist in theabsence of a driving current. The inverse proportionality to Nc is also accurately represented inequation (3.1).As stated, there is no unifying theory, and no single model for the mechanisms of 1/f noise.The current mode of thinking is that there are multiple mechanisms for 1/f noise, even in a simpleresistor. These multiple sources are additive, each with their own temperature dependence andfrequency dependence. So in a certain temperature range, one source of 1/f noise may bedominant, while in another temperature range, another source will be dominant.There is, however, one known source of 1/f noise which is of interest to bolometer work.This is 1/f noise due to thermal fluctuations. Thermal fluctuation 1/f noise arises simply becauseof temperature variations of the resistive material. The temperature variations cause resistancechanges, which in turn causes a voltage fluctuation (assuming a constant current).There has been some extensive development of the thermal fluctuation model,’4 and anoverview of the results follows. We consider a canonical ensemble, and define the sampletemperature as a fluctuating variable through the relation: AECvAT. As stated, temperaturefluctuations lead to resistance fluctuations through the temperature coefficient of resistance(dRIctF) of the sample, resulting in voltage fluctuations as follows:<All2 >= V2 R2 <AR2>DC (3.2)= VR(dRIdT) <AE2>But for a canonical ensemble:15<E2>=kBTCV (3.3)and if we define 13 as in equation (2.3) f3=1/R(dR/Jf):<AV2 >= CI32kBTc1 (3.4)Now writing the Langevin diffusion equation for the local temperature T(x,t):=DV2T+C1V.F (3.5)here, D is the thermal diffusivity, and F is an uncorrelated random driving term. The spectrumresulting from the above equations is:= J< T(t + )T(t)> cos(an)dr (3.6)It was found that for a three dimensional sample with dimensions ii x 12 x 13, where 11>12>13, fourfrequency ranges can be identified, separated by the three frequencies coj=D/21i2. The result isthat for:ST(o)ccof3’2ST(o)ocw112 ()(02> W> ST(a)oc(const-In0)S(w)ccconst.One may notice that the in the above results there is no region where there is a 1/fspectrum. There are a number of methods to mathematically introduce a 1/f region, all of whichare ad hoc. However, the results shown in the next section are in agreement with the derivationgiven above.Thermal fluctuation 1/f noise is of interest to bolometer operation because of the hightemperature coefficient of resistance (dR/dT) of a superconductor in the transition region. Thishigh value which is crucial for successful bolometer operation also gives rise to thermalfluctuation 1/f noise. While thermal fluctuation 1/f noise has been shown to be dominant in thetransition region of low temperature superconductors, there is some disagreement as to whetherthey dominate the 1/f noise in HTS.’5’6716Noise measurementSamples ofBi2SrCau3-O were RF magnetron sputter deposited on 1.3 mm thick,(100) single crystal MgO substrates, and post-deposition annealed as described in theintroduction, such that the high T material was well oriented but not epitaxial (single crystal).Silver pads were sputtered on the film for four point probe resistance measurements. The silvercontacts were then treated at 550°C for 30 mm. in 0.067 atm of 02. Wires were attached to thesilver pads with the use of silver paint. The current sources used for the four point probes were9V batteries with a series resistance that was large compared to the sample resistance (<10 ohms).The noise signal was then capacitively coupled to a Princeton Applied Research AM-i low noisetransformer in order to achieve a better impedance match between the sample and the PAR 113amplifier as described in chapter 2. The output of the amplifier was fed into a Hewlett Packard35 82A spectrum analyzer. The temperature was controlled by the system described in chapter 2(figure 2.3) so that the temperature was stable to ±0.5 K. The system is shown pictorially infigure 3.1.171/fNoise in BSCCO MaterialThe basic figure for measuring noise is the noise power spectral density (Sn). It is definedas follows:V2 (3.8)VBwhere V is the rms noise voltage in the bandwidth B, and Vdc is the dc applied voltage. Thebandwidth of the measurements given here was 3 Hz in all cases. The Vd term in the definitionis present in order to normalize the noise measurements with respect to the driving current. It isincluded here, because it is the current dependent noise sources that are of interest.Measurements were taken by programming the spectrum analyzer to average the rms noiseNoise Si TemperatureControllerTemperature SignalLiquid Nitrogen DewarSpectrumAnalyzerFigure 3 1: Noise measurement system.18voltages of 100 samples. The non-current dependent noise sources (Johnson and amplifier noise)were measured by removing the current source from the circuit, and measuring the noise in thesame manner. The non-current dependent noise was then subtracted from the total noisemeasurement, resulting in only the current dependent noise remaining. The current sources usedwere 0.45 mA and 2.7 mA, and as a result, shot noise is insignificant.(mis) = ..J2qIB (3.9)Here, q is the electronic charge (1.60x109)so that for 0.45 mA and a 3 Hz bandwidth, the shotnoise current would be only 2x101’A. The result is that the only current dependent noise that issignificant is 1/f noise. It is thus that 1/f noise can be easily measured and distinguished from theother sources of noise present.The noise power spectral density measurements Sn are shown in figures 3.2 & 3.3.10—1110—1210—’101410-15UI10—16101700C105 108 111 114 117 120Temperature (K)Figure 3.2: Normalized noise spectral density S vs. T for a dc current of 0.45 mA at 100Hz.Good correlation between S and j32 can be easily seen for the lower temperatures, and is strongevidence that the source of the 1/f noise at these temperatures is thermal fluctuations.194, were measured at 100 Hz, and normalized with respect to the dc voltage across the twoinner points of the four point probe (as in eqn. 3.8). The resistance of the sample is also shown inthe figures in order to show the breadth of the transition region. In figure 3.2, measurementswere taken with a 0.45 mA current source; and in figure 3.3, a 2.7 mA source. 100 Hz waschosen as the measurement frequency, because the transformer and amplifier combinationintroduced noise at lower frequencies.From the two figures, it can be seen that there is very good correlation between f32 and thenormalized Sn at lower temperatures, where f3 is the temperature coefficient of resistance asdefined in equation 2.3. The [32 term was calculated from the resistance data shown in thefigures, and has been multiplied by a constant in order to demonstrate the proportionality betweenS and p2. This good correlation indicates that the source of the 1/f noise is in fact thermalfluctuations, due to the relation in equation (3.4). The poor correlation between S and f32 athigher temperatures is attributed to another, as yet undetermined, source of 1/f noise which isrelatively independent of temperature, and becomes the dominant noise source as the thermal101310—1410-15u:10—170.0105 106 107 108 109 110 111 112 113 114 115Temperature (K)Figure 3.3: Normalized noise spectral density S vs. T for the same conditions as figure 3.2,except that the current is 2.7 mA. Again, there is good correlation between S and 82 at lowertemperatures.0>10-00020fluctuation noise decreases at the higher temperatures. It is inferred that below about 109 K the1/f noise is dominated by thermal fluctuations. These results are very similar to those reported byBlack et. al.’° forY1Ba2Cu3-Omaterial.Also supporting the thermal fluctuation model is a plot of the frequency dependence of thenoise. Figures 3.4 & 3.5 show the relationship between the noise level and frequency at twodifferent temperatures with input currents of 0.45 mA and 2.7 mA respectively.:10—11 liii I III I3 4 5678 2 3 4 5678102 10f(Hz)Figure 3.4: Noise voltage vs. frequency at two different temperatures for a dc current of 0.45 mAand a bandwidth of 3 Hz. The change in the frequency dependence (a) at the differenttemperatures is evidence that a different noise mechanism is present.From these plots, different a’s can be clearly seen, where:snac:E (3.10)The values for a given in the figures were determined using a least squares fit to the noise data.’8The change in a with temperature is strong evidence for a different noise mechanism. The a’s ofT1071Kxr0.480 T=llOKa=1.1300021about 0.5 at 107 K are the same as that predicted by the thermal fluctuation theory describedearlier, and in equation (3.7).2T=lO7Ka=O.540 T=llOKa=O.792 3 4 567 2 3 4 567102 1Of(Hz)Figure 3.5: Noise voltage vs. frequency for the same conditions as figure 3.4, except that the dccurrent is 2.7 mA. Again, a change in a is seen indicating a different noise mechanism.Given a film thickness on the order of a micron, and a sample width of about 1 mm,because the a observed (for T < 108K) was 0.5 for all frequencies between 1 and 1000 Hz, athermal diffitsivity in the range i0 > D > 10-8 is implied. The value for a of —0.5 is also inagreement with the range of 0.53-0.57 found by Black et. a1. forY1Ba2Cu3-Omaterial. Thisis another indication that thermal fluctuations are responsible for the noise increase, since thefrequency dependence of other noise mechanisms would likely be material dependent. The valueof ct0.5 is seen for temperatures ranging from about 108 K down to well below the zeroresistance transition of the superconductor. At 110 K, while the noise power Sn already showsgood correlation to f32 (see figure 3.2), the other sources of 1/f noise are not yet overpowered bythe thermal fluctuations, and thus the observed a at 110 K is greater than 0.5. One may also note00108432109022that the data in figures 3.2 & 3.3 was measured at 100 Hz, where the effect of the change in a issmall compared to higher frequencies.By comparing the figures, it can be seen that the thermal fluctuation 1/f noise is roughlylinearly dependent on the input current. For temperatures less than about 108 K, Sn is roughlyequal for both 0.45 mA and 2.7 mA (figures 3.2 & 3.3). Also, the values of V at 107 K areroughly proportional to the input current. However, the other undetermined sources of 1/f noiseare strongly current dependent. For temperatures above 109 K the effect of the current on S isstrong. Since the source of this 1/f noise is not known, it is difficult to speculate about a modelfor the current dependence, but the noise voltage V seems to be roughly proportional to i2,implying that Sn is roughly linear with current for this source of 1/f noise.In summary,Bi2SrCau3-Omaterial shows remarkably similar noise properties to thosereported forY1Ba2Cu3-O systems. There is fairly strong evidence that the source of the 1/fnoise in the lower part of the transition region is thermal fluctuations, due to the J32 dependenceof the noise, and the a value of0.5 which is consistent with both the findings of Black et. al.,1°and the thermal fluctuation noise theory of Voss and Clarke.14Implications on Bolometer PerformanceFor bolometric response, the sensitivity AR is proportional to clR/dT; but, the 1/f noisepower S is proportional (AR/R)2. As a result, one would want to bias the temperature of abolometer in the higher temperature portion of the transition region, where there is still a highsensitivity of the bolometer (dR/dT large), but the noise magnitude is still relatively small (ARIRstill small). As seen in Figs. 3.2 & 3.3, the midpoint of the transition has the high dR/dI’, but the1/f noise has not yet increased to excessive levels as a result of thermal fluctuations.23The implications of this are that thermal fluctuation 1/f noise need not interfere withbolometer operation. The low levels of noise achieved here for a 0.45 mA source should be lowenough for useful operation of a HTS bolometer.244. X-RAY DIFFRACTION ANALYSISIntroductionIn this chapter, some background information will be given on the primary method foranalysis of the HTS materials and the buffer layers. By examining x-ray difl1action (XRD)spectra, we can determine which crystal phases are present, the quality of crystallization, and theorientation of the crystal. The device used for the analysis in this work was a Rigaku powderdiffractometer.CrystallographyBefore going into the details of the theory of X-ray diffraction analysis, it is important tohave a basic understanding of the terms of crystallography. The most crucial concept to the workhere is the means of defining the crystal planes. Firstly, we define the “basis” to be the set ofatoms that replicates itself at every lattice point. We then name the vectors a, b, and c to be thevectors from one basis to another in each of the 3 cartesian coordinates. We then define thecrystal plane (m1 m2 m3) to be that as shown in figure 4.1.Cb—cin31-—am1a25figure 4.1: definition of (m1 m2 m3) crystal planeWhen considering crystals, there are frequently many symmetries. Consequently, when werefer to a crystal plane, we refer to all such planes that are equivalent within the crystal. Forexample, for a simple cubic crystal, the (1 0 0) plane is equivalent to both the (0 1 0) and (0 0 1)planes, so we refer to all of them collectively as the (1 0 0) plane of the crystal.Basic X-ray Diffiaction TheoryWhen monochromatic (of single wavelength) x-rays are directed towards a crystalstructure, these waves are diffiacted according to the Bragg Law:n2=2dsmO (4.1)This law is derived from the basic principles of constructive and destructive interference, and is asshown in figure 4.2. 2 is the wavelength of the incident x-rays, n is any integer, d is the latticespacing, and 0 is the angle of the incident x-rays.o 0o 0o 0Lattice PointFigure 4.2: X-ray diffraction (XRD) by the (1 0 0) plane of a crystal lattice,demonstrating the Bragg angle as in equation (4.1).e0 00, 00 0 000 00 026The result of this is that for values of 0 which satisfy equation (4.1) there will be a peak in theintensity of x-rays detected. Figure 4.2 depicts diffraction by the (1 0 0) (assuming a cubiclattice), and shown in figure 4.3 is a 2-dimensional representation of diffraction by the (3 1 0)plane (satisfying the same Bragg condition).7/o a a /o /o/ /0 0 0 0 0(3 1 0) planefigure 4.3: diffraction by the (3 1 0) planeThe diffractometer uses what is called a goniometer to perform scans. A goniometerschematic is depicted in figure 4.4.detector//X—raysource A—— samplefigure 4.4: Goniometer schematic27The goniometer consists of two rotating axes: the 0-axis, and the v-axis as shown in the figure.The only motion of the detector is rotation along the v-axis, and the only motion of the sample isits rotation about the 9-axis.The most basic scanning mode is called a 0/20 scan, and as the name implies, it is simplyscanning the 8-axis and v-axis together so that v=20 (The v-axis is actually referred to as the 20-axis). This type of scan provides a lot of information about the crystal, and is the primary methodof determining which of the BSCCO crystal phases are present in a sample. The angles at whichintensity peaks occur correspond to the spacing of the (0 0 L) planes (parallel to the substrate), orc-axis spacing, and from this, the crystal phase can be determined by comparing it to establishednorms. However, when performing a scan in this manner, we see only the peaks corresponding tothe spacing of the (0 0 L) planes.In order to get information about the spacing of other planes, we use what is called an Xray pole scan. For a pole scan we must rotate the sample about the axis normal to its surface (thec-axis), and move the 0 and v axes independently. To this end, an attachment was built for thediffractometer to rotate the sample about its normal. Since sputter deposited BSCCO is not aperfect crystal, this method is crucial for determining the quality of alignment of the crystal grainsin the ab-plane. In order to do this, an ‘off-axis”, or non (0 0 L) peak is found such that 28W.The sample is then rotated about its normal axis. For a perfectly oriented crystal, the peak shouldimmediately disappear when the sample is rotated, and reappear at 90° intervals (assumingsymmetry in the a and b crystal directions- otherwise 1800 intervals).The last analysis method is what is referred to as a rocking curve. For this, all that needs tobe done is to find a peak on the 0/28 scan, fix the v-axis, and scan the 8-axis. From this, we seethe width of the peak in the 0-axis, and have an estimate of the quality of alignment of the crystalgrains in the c-axis.285. PREPARATION OFBi222çQ1IntroductionBSCCO has several superconducting crystal phases. They all have a chemical equation ofthe form: Bi2Sr2Ca1Cu-O where n=1 ..5. The n=4 and n=5 phases are not very stable, andare difficult to deposit in the thin film form. The n=1 (2201) and n=2 (2212) phases can be easilyformed, but have low transition temperatures (80 K for n=2). The phase n=3 (2223) is thecrystal phase of choice, and is the one with zero resistance at - 07 K.One of the crucial steps in the existing process of BSCCO thin film preparation is that ofpost-annealing. As previously stated, The temperatures of —865 °C required for this step toproperly crystallize the superconductor are destructive to the buffer layers, and thus to theBSCCO itself through interdiffusion with the silicon substrate. By depositing the BSCCO onto ahot substrate, this step can be avoided. Since crystallization of the BSCCO will occur as it isbeing deposited, the temperatures required for this are significantly lower. When thecrystallization occurs while the particles are being deposited, the re-ordering requires far lessenergy, since it all occurs on the surface.The experiments investigating in-situ preparation were done using magnesium oxide (MgO)for the substrate material, MgO was chosen since it is known to be compatible with BSCCO, andtherefore the effectiveness of a buffer layer is not a variable in the process. It may also benoteworthy that MgO was the substrate material used in the original experiments on thepreparation of BSCCO thin films. Further, a study of the effect of the surface quality of the MgOon the in-situ preparation ofBSCCO was performed.29SputteringAll film depositions in this work were done using magnetron sputtering. A briefbackground to the theory of magnetron sputtering is thus included here.By definition, sputtering occurs when material is ejected from the surface of a solid due tomomentum transfer from a bombarding particle. Sputter deposition occurs when the ejectedmaterial is collected on a substrate, forming a film. The different sputtering processes differ onlyin the means by which bombarding particles are generated, and they all require a vacuum chamber.The sputtering system used here generates bombarding particles by what is called the glowdischarge method. Glow discharge bombardment uses a low pressure gas plasma (usually argon)for the bombarding particles. Positive ions are accelerated towards the negatively charged targetmaterial as shown in figure 1.1, and eject (sputter) the target material, as well as secondaryelectrons. These secondary electrons then produce additional ions when they collide with thesputtering gas atoms. The sputtering process is therefore self-sustaining.30TargetVacuum ChamberFigure 5.1: Magnetron sputtering schematic.Positive ions are attracted to the negative target, so that ions striking the target eject targetmaterial to coat the substrate, and secondary electrons which generate more ions. The magnetarrangement enhances ion generation close to the target.For magnetron sputtering, a set of magnets is inserted behind the target material as shownin figure 5.1. These magnets serve to enhance the plasma (ion) generation, and help confine theplasma close to the target. Secondary electrons ejected from the target are forced to move in aspiral manner due to the Lorentz force:F=q(E+vxB) (1.1)The Lorenz force on the electrons serves to confine them close to the target.Unfortunately, when sputtering insulating material, a dc voltage applied to the target willnot maintain the plasma, because a charge will build up on the surface of the target counteractingSubstrate•Ejected Target Material® Bombarding Ion0 Secondary electron+31the effect of the applied voltage. Instead, an if field is applied to the target. Because of theasymmetry of the chamber acting as a capacitor, a negative bias voltage is formed at the target.The sputtering then continues as with dc sputtering, except at a generally lower deposition rate.Because the HTS materials are relatively poor conductors at room temperature, if sputtering isrequired to deposit HTS films.Ex-Situ Preparation ofBi2i223QvIn order to prepare thin films of the n=3 phase of BSCCO (2223), the material must bedeposited onto a suitable substrate by sputtering or some other method. First off, the target mustbe prepared. The targets were fabricated by mixing the appropriate ratios of Bi203, SrCO3,CaCO3, CuO, and PbO powders in order to obtain Pbl.3Bi2Sr2Ca2Cu3-Oy for the targetcomposition. These were then fired in air at about 780 °C for 12 hours, cooled, and thenmechanically reground in an agate mortar. The firing and regrinding was repeated at 790°C and795 °C. The mixtures were then pressed into disks at 1.4 kbar and sintered at 825 °C for 12hours. These repeated firings and sintering were required to form strong, homogeneous, andwell-reacted targets. The carbon is driven off by this process, but the lead remains. The presenceof lead in the target was found to encourage the preferential crystallization of the n=3 phase. 1After the sputter deposition of this target material, the substrate must then be annealed atabout 865 °C (close to the melting point) in an oxygen atmosphere. These temperatures must bemaintained for about 14 hours in order for the crystallization to complete. The lead which isoriginally in the target is eventually driven off by this “post-annealing” process, so that almost nolead will remain in the final material. The result is a superconductor with a superconductivityonset temperature (TCon) of about 110 K and a zero resistance temperature (Tü) of about 107K.320 100 200 300 400 500 600 700 800Substrate Temperature (Celsius)Figure 5.2: Composition of the BSCCO film as a function of substrate temperature.1 The valuesare normalized so that Sr, Ca, and Cu add up to their room temperature total.Theory of In-situ BSCCO PreparationThe difficulty in in-situ preparation of BSCCO is that of getting the correct composition ofthe different elements, while attaining a high enough temperature for the proper crystallization tooccur. The most serious problem involved is that as the temperature of the substrate rises, theconcentration of bismuth in the film drops dramatically. This can be seen in the graph in figure5.2.5040j30o 20C00100BismuthStrontiumCalcium-H-CopperIt is felt that this loss ofbismuth is due to preferential resputtering of the bismuth. Resputtering iswhen negative ions impact on the substrate, and eject the previously deposited material.Apparently, as the temperature of the substrate rises above about 350 °C, bismuth is particularlyvulnerable to this, significantly more so than the other elements ofBSCCO.33There are ways, however, to minimize the loss of bismuth in the film. It is felt that it isoxygen ions which are responsible for the resputtering effect. The bismuth content can thus beincreased by varying the oxygen pressure, the total system pressure, the target to substratespacing, and the substrate bias.’9 By increasing the total pressure or the substrate target distance,the likelihood of collisions increases, and thus the average energy of particles impacting on thesubstrate is reduced, reducing the resputtering effects. Putting a negative bias on the substratewill also reduce resputtering, since incident oxygen ions will be negatively charged.There is another factor in the in-situ preparation of BSCCO, which is that of the oxygencontent of the film. It is important to set the partial pressure of oxygen during sputtering toachieve the proper stoichiometry in the film. Further, while the substrate is cooling after thedeposition, some oxygen will escape. It is thus necessary to maintain a high oxygen pressureduring the cooling of the sample, and cool the sample slowly.Experimental Results and DiscussionThe experimental arrangement is the same as that described above, with the exception thata substrate heater was added. The substrate is heated by 4 projector bulbs powered by a simpledc power supply, and is depicted in figure 5.3.34Figure 5.3: Substrate heater.The samples were attached to the heater via silver paint. Silver paint provides goodthermal contact, and a relatively simple procedure for fixing and removing the samples. Thesample temperature was measured by the use of two thermocouples. One thermocouple wasembedded into the back side of the heater block, while the second was “glued” to one of thesamples with silver paint. There were some problems with the reliability of the temperaturesgiven by the thermocouples. It was felt that a shorting out of the thermocouple wires wasresponsible for unreliable readings.One problem which arose was that the stainless steel heater block would outgas whenheated. It was found that areas of the samples which were close to the edge were chemicallycorrupted by this outgassing. The solution to this problem was to cover the entire surface of theheater block with plates of alumina. These alumina plates were also held in place by silver paint,and successfully protected the samples from the outgassed material.Table 5.1 presents a summary of the different parameters that went into the depositions ofin-situ BSCCO films.Stainless Steel ThermocoupleInsulating shield35Batch Heater power Temperature Oxygen Pressure Total Pressure1 420 W 718 °C 220 mTorr 220 mTorr2 370W 695°C 250mTorr 25OmTorr3 430W 645°C 25OmTorr 250mTorr4 420W 676°C 250 mTorr 250 mTorr5 350W 645°C 25OmTorr 250mTorr6 360W 610°C 250 mTorr 250 mTorr7 350 W 670 °C 250 mTorr 250 mTorr8 450W 715°C lOOmTorr 25OmTorr9 460W 755°C 2SOmTorr 2SOmTorr10 370W 670°C 250 mTorr 250 mTorr11 360W 600°C 250 mTorr 250 mTorr12 290W 680°C 250 mTorr 250 mTorr13 355W 735°C 2SOmTorr 2SOmTorr14 380W 750°C 2SOmTorr 2SOmTorr15 380W 740°C 2SOmTorr 300mTorr16 370W 730°C 2SOmTorr 300mTorr17 385W 750°C 250 mTorr 300 mTorrTable 5.1: deposition parameters for in-situ prepared B SCCO samples.The detailed procedure for depositing these films is as follows. First, the samples were“g1ued to the heater block with silver paint. The remaining surface of the heater block was thencovered with pieces of alumina that were also attached to the heater with silver paint. A smallthermocouple wire was then attached to the top surface of one of the samples, again with silverpaint. After the silver was allowed to dry, the vacuum chamber was pumped down at the sametime the heater was turned on. The heater power had to be turned up gradually in order to avoid36cracking of the samples or alumina due to uneven heating and thermal expansion. When thetemperature of the system was stabilized, the sputtering gas (oxygen and argon) was introducedwith a constant flow so that the chamber pressure was also constant. The target was then “presputtered” for about 30 minutes with a shutter covering the substrate. The purpose of presputtering is to remove any impurities that may be on the surface of the target. After presputtering, the shutter was opened. Deposition was continued for about 2 hours. Thetemperature of the substrate was maintained by tweaking the heater power up or down asrequired. After the depositions were completed, the oxygen flow valve was opened completely,and the pump and heater were turned offX-ray diffraction analysis in the 9/20 mode was performed on all of the samples produced.By comparing the angles where intensity peaks occur to known standards, we can determinewhich crystal phases are present. Shown in figures 5.4 through 5.20 are the X-ray scans for the“best” sample in each of the 17 batches. In all of the figures, the vertical lines represent theknown locations of intensity peaks for each of the BSCCO crystal phases present. The mostindicative peaks are the ones at less than 10°. The 2223 phase has a peak at about 4°, the 2212phase has one at slightly more than 5°, and the 2201 phase has one at about 6.50. If the samplelooked as if it contained some of the 2223 crystal phase, a resistance vs. temperature plot wastaken, with the results also shown in the figures.37C:5Li.v)CU)Figure 5 .4a: X-ray scan for batch #1 showing mostly 2212 crystallization. The lines represent theknown peak locations for the 2212 crystal.1 40120100E. 80U)UC60U)0::402020 30 40 50 60 70 80 90 100 110 120Temperature (K)25 30 35 40Angle (degrees)Figure 5.4b: Resistance vs. Temperature curve for batch #1.U)>U)a)CU)C:5>U)Cci,C0 5 10 15 20 25 30 35 40 45 50 55 60 65Angle (degrees)Figure 5.5: X-ray scan batch #2 showing mostly 2212 crystallization.38Jj0 5 10 15 20 25 30 35 40 45 50 55 60 6Angle (degrees)Figure 5.6: X-ray scan for batch #3 showing no identifiable crystallization. The two sets of linesshow the known peak locations for 2212 and 2201 peaks.C:3>“3CC))C:3>-C),C0)CFigure 5.7: X-ray scan for batch #4 showing 2201 crystallizationwith a small proportion of 2212 crystallization.39Figure 5.8a: X-ray scan for batch #5 showing a mixture of 2212 and 2223 crystallization. Thelines show the known peak locations for 2223 and 2212 crystallization.10 15 20 25 30 35 40 45 50 55 60 65Angle (degrees)25 30 35 40Angle (degrees)4020 -18-16-14-12-C-);20 30 40 50 60 70 80 90 100 110 120 130Temperature (K)Figure 5. 8b: Resistance vs. temperature for batch #5.U)a>Cl,Cci,Angle (degrees)Figure 5.9a: X-ray scan for batch #6 showing almost pure 2223 crystallization.Of particular interest from among these batches is batch 6. As seen on the X-ray scan infigure 5.9, there was success in crystallizing one of the samples in the 2223 phase. However, thesuperconducting transition is only at about 50 K, instead of the 107 K temperature which thiscrystal is known to be superconducting. This has been attributed to the existence of poorly41connected grains of the 2223 crystal. The overall resistance is thus dependent on the material thatfills the ‘gaps’ between the 2223 grains.201816141220 30 40 50 60 70 80 90 100 110 120 130Temperature (K)Figure 5.9b: Resistance vs. temperature for batch #6.U):5>-Ca)C45 5Angle (degrees)Figure 5.10: X-ray scan for batch #7 showing 2212 crystallization.42(I,:5>-I),Cb 5Angle (degrees)Figure 5.11: X-ray scan for batch #8 showing 2201 crystallization.U)C:5i:i>CIi)1b12O 5 3 65Angle (degrees)Figure 5.12: X-ray scan for batch #9 showing 2201 crystallization.C£5>-LI)Cci,CLI)CD£5>-.U)Ca,CFigure 5.13: X-ray scan for batch #10 showing 2212 crystallization.Figure 5.14a: X-ray scan for batch #11 showing a mixture of 2223 and 2212 crystallization.430 5 10 15 20 25 30 35 40 45 50 55 60 65Angle (degrees)10 15 20 25 30 35 40 45 50 55 60 65Angle (degrees)35 -30 -25 -q)ci10550 100 150 200 250Temperature (K)Figure 5. 14b: Resistance vs. temperature for batch #11.440 300DC>-C,)CwC5 10 15 20 25 30 35 40 45 50 55 60 65Angle (degrees)Figure 5.15: X-ray scan for batch #12 showing no identifiable crystallization.C,,C:5>-0,SnCSnC:5C>-SnCSnC0 5 10 15 20 25 30 35 40 45 50 55 60 65Angle (degrees)Figure 5. 17a: X-ray scan for batch #14 showing 2212 crystallization.4515 20 25 30 35 40 45 50 55 60 65Angle (degrees)Figure 5.16: X-ray scan for batch #13 showing a mixture of 2212 and 2223 crystallization.J461 .61 .41 . 100 150 200Figure 5. 17b: Resistance vs. temperature for batch #14.Batches 15 through 17 used a bismuth rich target, with composition(Pb2)Bi4SrCau3Oy. The extra bismuth should allow an increase in deposition temperaturewithout destroying the stoichiometry of the resulting film. The results were not at allencouraging, and the X-ray scans for these batches are seen in figures 5.18 through 5.20.U)C>-.U)Ca)C6525 30 35 40Angie (degrees)Figure 5.18: X-ray scan for batch #15 which used a bismuth rich target, showing 2212crystallization.C,,>C,)CC876C.,CCU,(I,0)O320100470 5 10 15 20 25 30 35 40 45 50 55 60 65Angle (degrees)Figure 5. 19a: X-ray scan for batch #16 which used a bismuth rich target, showing 2201crystallization.30 40 50 60 70 80 90Temperoure (K)Figure 5.1 9b: Resistance vs. temperature for batch #16.48Some post-deposition treatment was attempted on this particular sample in order to solvethis problem, and hopefully increase the transition temperature. Manipulation of the oxygencontent of the film was attempted by baking the samples at about 500 °C in various atmospheres.500 °C is not hot enough to affect the crystal form of the film, but is hot enough to drive off, orabsorb oxygen (depending on the atmosphere). This temperature would likely also be withstoodby the buffer layer structure necessary for using silicon substrates.As expected, these procedures had little effect on the X-ray scan results of the film, but hada significant effect on the transition temperature. Initially, the sample was heated up to 600 °C in760 mlorr of 02 for 30 minutes. The resulting transition is seen in figure 5.21.U)CI>-U)CCOxygen Content Control0 5 10 15 20 25 30 35 40 45 50 55 60 65Angle (degrees)Figure 5.20: X-ray scan for batch #17 which used a bismuth rich target, showing 2201crystallization.495045403530252015105020 30 40 50 60 70 80 90 100 110 120 130Temperature (K)Figure 5.21: Resistance transition for sample of batch #6 after being baked at 600 °C in 760mTorr of 02 for 30 minutes.As seen, the zero resistance temperature dropped by over 20 °C compared to its original level. Itis felt that this is due to the depletkm of oxygen in the film due t the kw oxygen partial pressureduring baking. The sample was then baked at 500 °C in flowing 02, in an open-ended tubefurnace, for 30 minutes. The resulting transition is shown in figure 5.22.403530252015105020 30 40 50 60 70 80 90 100 110 120 130Figure 5.22: Resistance transition for sample of batch #6 after being baked at 500 °C in flowing02, in an open-ended tube furnace, for 30 minutes.50As seen, this restored the transition temperature to near its original level. This same sample wasthen baked again, at 500 °C in flowing 02 with both ends of the tube furnace capped (with a smallhole on one end). The resulting film shows slight improvement, with the results seen in figure5.23.403530252015105020 30- 40 50 50 70 50- 90 100- 1-10 120 130Figure 5.23: Resistance transition for sample of batch #6 after being baked at 500 °C in 1 atm 02for 1 hour.Again, the sample was baked in the same furnace arrangement, but this time, argon was alsoflowing so that approximately 0.9 atm of oxygen was present. It was thought that perhaps therecould be an excess of oxygen content in the film, and that the introduction of argon (to lower the02 partial pressure) would help to balance the oxygen content. Again, the resulting transition isshown in figure 5.24.514035302502015105040 50 60 70 80Temperature (K)Figure 5.24: Resistance transition for sample of batch #6 after baking at 500 °C in 0.9 atm of 02.Finally, the sample was brought up to 800 °C at 20 ° per minute, and then immediately allowed tocool to room temperature, in pure flowing 02 as above. The result is that the 2223 crystallizationwas broken down, and the sample was essentially destroyed.The oxygen content of the film does in fact affect the transition temperature of thesuperconductor. However, the oxygen content does not seem to be responsible for the extremelylow transition of the 2223 crystals formed in-situ. Altering the oxygen content only increased thetransition by a few degrees at best.Surface TreatmentIt was felt that one way to encourage the connectivity of the 2223 crystal grains was toencourage alignment of these grains in the plane of the substrate surface. One controllable factorin determining alignment is that of the surface character of the MgO. It has been previously foundthat the growth of BSCCO occurs via an “island mechanism”.20’That is to say that the materialtends to crystallize onto existing “islands” of the film. There is further evidence that these islands2 90 100 110 120 13052tend to nucleate at the edges of steps on the MgO surface. Therefore, by preparing a substratesurface with a high density of step edges, it is hoped that the superconducting properties of thefilm can be improved, especially the transition temperature.In order to form steps in the surface of MgO substrates, all that is necessary is to anneal thebare MgO at 1100 - 1200 °C for 12 - 24 hours.2° Mechanically polished substrates were cut intothree pieces, and two of the three underwent the annealing process. The smooth and roughenedsamples were then placed symmetrically onto the heater block as before, so that they shouldundergo identical deposition parameters in case of a temperature gradient in the heater block.In order to measure the alignment of grains, X-ray pole scans and rocking curves wereperformed on each of the samples in a batch (see chapter 4). The 9/20 scans looked very similarfor each of the three samples of a batch, and are not very useful in assessing the grain alignment.Batch 13 (see table 5.1) showed the best results for this particular set of experiments, and so theresults for that batch will be presented here.The BSCCO crystal is symmetrical in the a and b axes. The crystal forms preferentiallywith its c-axis perpendicular to the substrate, so that an X-ray pole scan of a perfect BSCCOcrystal should show peaks at 90° intervals. The results of the pole scans are shown in figure 5.25.Samples 2 & 3 are the roughened samples, where sample 2 was mounted in the centre of theheater, and samples 1 and 3 were mounted symmetrically around it (sample 1 was polished).53Sample #1+Sample #2Sample #3a-C.Q)Ia÷1E*_ — —= +* * + * + +0 10 20 30 40 50 60Sample Angle (degrees)Figure 5.25: X-ray scan showing the in-plane alignment of the 2223 crystal. Zero degrees wasdefined to be the angle at which the maximum intensity was found. The data for samples 1 & 2show preferential misalignment of 45° and a slight preference for 22.5° misalignment. Sample 1data shows only slight preferential misalignment angles, but shows a relatively large intensity peakfor ALL sample angles, indicating poorer crystal grain alignment.A resistance vs. temperature scan was then done for each of the three samples. The resultsare shown in figure 5.26.547.0sample 16.3 sample 2sample 35.6__________4.9 //C)4.2C)//C) 41 47 53 59 65 71 77 83 89 95Temperature (K)Figure 5.26a: Resistance transition for three samples of batch #13.0.050— sample 1/ /0,045 sample 2— — —- sample 30.040 I//0.03501 /0,030o/j/0.025//(/2 /0.020 / /C)0.015 / / /0.010 10.000 — —! —: - - I0.005--32 33 34 35 36 37 38 39 40 41 42Temperature (K)Figure 5.26b: Blow up of transition to zero resistance for samples of batch #13.55Although the surface treatment was moderately successful at producing films with better in-planealignment, as seen in figure 5.25, in-plane crystal grain alignment makes only a slight difference tothe transition temperature of the superconductor.X-ray rocking analysis was also performed on each of the 3 samples. The results weresomewhat surprising, and are shown in table 5.2.Sample #1 Sample #2 Sample #3Peak width 1.00 1.3° 1.2°(FWHM)Table 5.2: X-ray rocking curve results showingpeak widths in the 0-axis for the peak at 20=39°.The results are surprising because the samples with the best alignment in the ab-plane (and bestzero resistance temperatures) show the poorest alignment of the c-axis. However, the poorer caxis alignment can be intuitively attributed to the step edges on the surface.The transition temperatures found for in-situ prepared BSCCO films here compare wellwith those found in the literature.22’3 456 However, because the transition is still well below77 K (liquid nitrogen temperature), the films are not useful for an economical device. Somesignificant improvement to the process is still required in order to raise the transition temperatureto more useful levels.566. MIXED BUFFER LAYERSIntroductionAs previously mentioned, buffer layers which are required to protect the BSCCO filmsfrom the chemically incompatible silicon substrates do not survive the high temperature and longduration annealing process required to crystallize theBi2SrCau3O(2223) phase. Cracksformed in the buffer layer, allowing the interdiffusion of silicon and copper that results in thedestruction of the superconducting properties ofBSCCO.One possible approach to solve this problem is to minimize the thermal stresses in the layerstructure. This can be done by inserting a layer of mixed Si02 and Zr02 between ‘pure’ layers ofeach of these materials, so that the structure looks as in figure 6.1.BSCCOZ r02Si—Zr—USi02SiliconFigure 6.1: lVlixed buffer layer structure.The insertion of the mixed layer of Zr-Si-O allows for a smoother transition of the thermalexpansion coefficient of the films, and will thus hopefully render the structure more capable ofwithstanding the high temperatures.The mixed layer is deposited by sputtering both a zirconium and silicon targetsimultaneously (in an oxygen/argon atmosphere), and moving the substrate back and forth so thatit is in front of both of these targets alternately. Since the time required to deposit a monolayer ofatoms is on the order of seconds, the resulting film should be a relatively uniform mixture.57By varying the thickness and deposition temperatures of each of the layers in the film, oneshould be able to arrive at an optimal situation for minimizing the thermal stresses and shearforces on the buffer layer components.Linear Thermal Expansion ModelIn order to mathematically derive the optimal deposition parameters, many simplifyingassumptions were made. These assumptions are: (1) a linear (non-temperature dependent)thermal expansion coefficient, (2) a linear elastic modulus, (3) perfect film adhesion, and (4) arigid substrate such that all layers have equal length distortion. The resulting equations resultfrom these assumptions:(6.1)L “ Ed(6.2)Here, cLn is thermal expansion coefficient of the nth layer, E is the elastic modulus of the nthlayer, d is the thickness of the nth layer, and Fn is the force per unit width of the “beam’ on thenth layer. Simultaneous solutions to these equations were calculated numerically. The interfacialshear forces (Sn) were then calculated such that:S—S,1=F (6.3)InterfacialThickness c E Deposition ShearLayer: (iim) ‘xl 0-6) (ON/rn2) Temp. Force at(°C) 800 °C(N/rn)BSCCO 1.0 11 200 201862Zr02 0.65 7.5 185 20265458Si02 (sputtered) 0.35 43 67 203458Si02 (thermal) 0.5 43 67 11003050Si 350 2.6 166 N/ATable 6.1: Shear force calculation resultsfor no mixed layer, and buffer layer deposition at 20 °CResults and DiscussionThe best results for BSCCO crystallization on this structure were obtained by the followingparameters: 0.5 p.m of thermally grown Si02, then 0.2 p.m Si02, 0.3 p.m Si-Zr-0, and 0.5 p.mZr02, all sputtered onto the substrate at about 450 °C. The BSCCO was then sputtered onto theroom temperature substrate to about 1 p.m.27 Postdeposition annealing was performed via threetemperature ramps at 5 °Clmin to 730 °C for 2 h, 760 °C for 1 h, 785 °C for 2 h (all in open air),and a final anneal at 600 °C for 10 mm. in 0.3 Torr 02. The furnace was allowed to cool to 500C following each high temperature soak.InterfacialThickness c Deposition ShearLayer: (p.m) x1 o6) (GN/m2) Temp. Force at(°C) 800 °C(N/rn)BSCCO 1.0 11 200 201862Zr02 0.5 7.5 185 202470Zr-Si-0 0.3 25.3 126 203244Si02 (sputtered) 0.2 43 67 203703Si02 (thermal) 0.5 43 67 11003295Si 350 2.6 166 N/ATable 6.2: Shear force calculation resultsfor a mixed buffer layer deposited at room temperature59InterfacialSThickness E Deposition hear ForceLayer: (tim) (xlO6) (GN/m2) Temp. at 800 °C(°C) (N/rn)BSCCO 1.0 11 200 201865Zr02 0.5 7.5 185 4502177Zr-Si-O 0.3 25.3 126 4502540Si02 (sputtered) 0.2 43 67 4502751Si02 (thermal) 0.5 43 67 11002344Si 350 2.6 166 N/ATable 6.3: Shear force calculation results for a mixed buffer layer deposited at 450 °CUnfortunately, even the best of these buffer layers did not adequately protect the BSCCOfilm above 800 °C. The X-ray scans of these films showed predominantly 2212 crystallization,with some signs of the 2201 phase present. Additional evidence to the presence of these crystalphases is that the resistance vs. temperature scans showed a transition starting at 86 K and a tailon the curve so that zero resistance is not achieved until about 45 K.If the buffer layer structure is made thinner, the shear forces are reduced, and the bufferlayer can then survive the high temperatures. However, if the layers are too thin, the bufferbecomes ineffective at stopping the interdiffusion of copper and silicon. If the buffer layers aremade thicker, the shear forces are higher, and the buffer cracks or peels resulting again in theinterdiffusion of copper and silicon through the cracks.Although some improvements can be made to the effectiveness and robustness of the bufferstructure, these improvements are not enough to achieve effective crystallization of the 2223phase. There was no success in depositing Bi2Sr2Ca2Cu3Oy onto silicon by these methodsalone.607. EPITAXIAL BUFFER LAYERSIntroductionAnother approach to strengthening the buffer structure is to have the Zr02 buffer layergrown epitaxially. Epitaxial simply means that the thin film is a single crystal. Normally,sputtered films are polycrystalline. That is that the films are made up of microscopic crystal grainsthat are not generally ordered. The cracks which form in the buffer layer as in chapter 6, willalmost certainly form on the boundaries of these crystal grains. Since there are no grainboundaries in an epitaxial film, it is felt that the buffer layer will be much more resilient to thethermal stresses, and allow the annealing ofBi2SrCau3Oon a silicon substrate. Cubiczirconia is well suited for epitaxial growth onto silicon, because its lattice constant is closelymatched to that of silicon.28Need for Stabilization of ZirconiaZirconia, like BSCCO, has more than one crystal phase. This presents a major problem forgrowing epitaxial films, because there will be 2 or more different crystals being formed.However, by adding small amounts of elements such as yttrium or calcium, the cubic zirconiacrystal phase can be stabilized. Ytrria stabilized zirconia (YSZ) films with a composition ofapproximately Y015Zr085O2provide the required stabilization of the cubic zirconia. Theyttrium content needs to be high enough to stabilize the crystallization, but low enough so as notto change the lattice constant and thermal expansion coefficient too drastically.For this purpose, a special sputtering target was fabricated. This target is merely a disk ofZr metal, with a strip of Y metal through the middle of it as in figure. 7617 Zr Y Zr( o.i cm5.0cmFigure 7.1: Dimensions of YSZ target.By sputtering in an oxygen atmosphere of appropriate partial pressure, YSZ films of the correctstoichiometry can be formed.Hydrogen Termination of SiliconAnother obstacle to the epitaxial growth of YSZ on silicon is that of the surface oxide onthe silicon (Si02). If the silicon surface is not clean of oxide, YSZ films will not growepitaxially,28 presumably because of the change in lattice constant. Removal of the oxide byhydrofluoric acid (HF) is easy enough, except that on exposure to air, an oxide layer is instantlyformed on a bare silicon surface. Since there is no mechanism in the lab for etching the Si02 andthen commencing sputtering without breaking the vacuum in between (ie. exposing it to air), thesilicon must be treated so that oxidation is inhibited.By terminating the entire surface of the silicon with hydrogen, the hydrogen providespassivation of the silicon surface against recontamination or oxidation. By leaving nounterminated silicon bonds on the surface, the hydrogen prevents contamination, but readilyallows replacement by deposited elements such as YSZ.30Hydrogen termination of silicon is achieved by mixing the HF etchant with water andethanol. The HF serves to etch off the native oxide on the surface, while the ethanol and water62serve to terminate the bare silicon with hydrogen. For this experiment, substrates were dippedinto a 1:1:10 ratio mixture of HF, water, and ethanol respectively, and ultrasonically agitated for 5to 10 minutes.Samples treated by such a mixture can survive brief exposures to room air without drasticcontamination. Table 7.1 shows some quantitative measurements of contamination of substratesthat underwent a similar procedure.Carbon Oxygen Fluorinecontamination contamination contaminationTreatment (monolayers) (monolayers) (monolayers)immediately after etch 0.025 ± 0.005 0.005 ± 0.002 0.010 ± 0.002dip in water 0.15 0.022 0.003latmN2forl0min. 0.035 0.008 0.0081 atm air for 1 mm. 0.064 0.010 0.012Table 7.1 Contamination measurements on hydrogen terminated silicon.30As can be seen from the table, even after exposure to air for 1 mm., less than 10% of thesurface is touched by contaminants. By treating the silicon in this manner, the surface can be keptrelatively free of oxide while the substrate is transferred into the vacuum chamber for sputtering.Experimental ProceduresHighly polished silicon with (111) crystal orientation was used for substrates for theseexperiments. The substrates underwent the etching procedure described above, and wereimmediately mounted onto the heater block. The samples were held onto the heater block bysmall bolts and washers, with silver paint applied to the back side of the sample in order to obtaingood thermal contact to the heater. The vacuum chamber was then immediately pumped down,63so that the samples would spend no more than about 3-4 minutes in room air after being etched inthe HF/water/ethanol mixture.The samples needed to be heated to about 750 - 800 °C by the same substrate heaterdescribed in chapter 5. These high temperatures pose a problem, because the heater blockassembly begins to outgas significantly at these temperatures, and a good vacuum is crucial toepitaxial growth of YSZ. In order to achieve the high vacuums required, the system needed to bepumped for several hours, while the substrate was hot.After a pressure of about 8*10-6 Torr was achieved with a hot substrate, argon wasintroduced for presputtering. The introduction of oxygen to the system (necessary for reactivesputtering of YSZ) was delayed so as to minimize the amounts of Si02 on the surface of thesubstrates. After presputtering was completed (ie. the target was adequately “cleaned”), about 3seconds of sputtering was done in the absence of oxygen. This was calculated to be about enoughto deposit one monolayer of metal, and was done to avoid silicon oxidation as oxygen wasintroduced for the reactive sputtering of YSZ. About 0.4 mTorr of oxygen was introduced, andDC sputtering of the Y/Zr target was begun immediately.After the completion of sputtering, there was no special treatment of the samples. Theheater was shut off, and air was introduced into the system when the temperature dropped belowabout 300 °C.Results and DiscussionAn X-ray (0/20) scan of one of the samples is shown in figure 7.2.64C,)C>-()Cci,C40Sample Angle (degrees)Figure 7.2: X-ray scan of YSZ sample. The peak at 28.4° is the silicon (111) peak, and is due tothe substrate. The vertical lines show the location of the YSZ peaks for a cubic crystal, with theindices shown.This figure is evidence that the YSZ film is indeed epitaxial. The absence of YSZ peaks with anindex other than (x00) shows that there is good crystal alignment in the c-axis. Since this is acubic crystal, alignment for the c-axis should be the same as that for the a and b-axes. There was,however, no off-axis peak found for the film to verify this assumption by an X-ray pole scan.It is curious that the YSZ is of (100) orientation, while the silicon substrate is of (111)orientation. It might be expected that YSZ of(1 11) orientation would grow onto (111) silicon.The effectiveness of this YSZ as a BSCCO buffer layer has not yet been tested. Since it isdeposited at 750-800°C, it is unlikely to crack during post-annealing of BSCCO. However,cooling the film to room temperatures may have caused micro cracks that will allow thedestructive interdiffusion of silicon and copper.There is a further question about the compatibility of yttrium with BSCCO. It is possiblethat yttrium will degrade the properties of the 2223 crystal. If this is the case, calcium may beused as a stabilizer for the cubic phase of zirconia. Since calcium is a constituent of BSCCO, it isunlikely to have detrimental effects. However, the fabrication of a target for sputtering calciumstabilized zirconia will be more difficult than for YSZ.(iii)Si (1 110)00)(311)50(400)658. CONCLUSIONSummary of ResultsNoise Properties ofBSCCOThe noise properties of aBi2SrCau3film were investigated with the purposeof determining the applicability of the material for bolometer use. The noise magnituderesults of S=10-15 FJ1 in the transition region at 100 Hz compare well with thosereported in the literature for YiBa2Cu3Oy. From the electrical noise standpoint then,Bi2SrCau3Ois a suitable material for use in a bolometer.In-Situ Preparation ofAccording to X-ray diffraction analysis, BSCCO of the 2223 crystal phase wasprepared in-situ (as deposited). The best transition temperature of about 55 K is similar tothose reported in the literature for in-situ BSCCO. However, 55 K is too low for useffiluse as a bolometer, and thus in-situ processing is not at present practical for crystallizingpractical BSCCO onto silicon.Buffer Layers on SiliconAn attempt was made to make the buffer layer more robust by the insertion of amixed layer of Si-Zr-O. The insertion of this layer reduced the thermal stresses, but wasstill not enough of a reduction to allow annealing of BSCCO without cracking of thebuffer. By making the layers thicker, the stresses are increased, but by making the layersthinner, copper and silicon will difihise through the buffer layers.66The second method of making the buffer layers more robust is to use epitaxial filmsofyttrium stabilized zirconia. Epitaxial YSZ was successfully grown, but has not yet beentested for its effectiveness as a buffer layer.Recommendations for further workThere are various ways that the noise performance of BSCCO material may be improved.Firstly, the driving current for the four point probe can be optimized for maximum sensitivity.Secondly, the quality of crystallization of the BSCCO can be improved. Specifically, thermalfluctuation 1/f noise can be reduced by increasing the thermal capacity. This, however, willreduce the sensitivity of a bolometer, so an optimization process may be required.The effectiveness of epitaxial YSZ buffer layers for BSCCO on silicon has not yet beeninvestigated. Whether or not the epitaxial nature of the film provides enough resilience to endurethe thermal stresses involved is not known.There is also the possibility that the yttrium stabilizing element may react chemically withBSCCO. If this is the case, calcium may be used as a stabilizing element since calcium does serveto stabilize zirconia,3’and is an element of BSCCO. If this needs to be done, a suitable methodfor sputtering calcium stabilized zirconia must be developed. This might be done by pressing amixture of calcium oxide and zirconia powders into a sputtering target.Some further study is needed to provide a method for the fabrication of the membranestructure. It is doubtful that a membrane structure will survive any annealing process, andtherefore, a way to protect the BSCCO from the KOH etchant must be found.A method of microscopically patterning the BSCCO films must be developed, sinceconventional photoresist methods will likely destroy the superconducting properties. Onepossible way of doing this would be to selectively destroy the buffer layer before depositing theBSCCO, so that only the areas where a superconductor is desired are protected from the silicon.67ConclusionAlthough the goal of producing a BSCCO bolometer on silicon was not met, some keyelements to meeting this goal were investigated. No other reports of successful deposition ofBi2SrCa2Cu3-Oyonto a silicon substrate were found in the literature.68BIBLIOGRAPHY1S.K. Dew, M.A.Sc. Thesis, University of British Columbia, 1989.2H. Nakajima, S. Yamaguchi, K. Iwasaki, H. Morita, H. Fujimori and Y. Fujino: Appi. Phys.Lett. 53, 1437 (1988).3K.J. Stahl: Photonics Spectra 23, 95 (1989)4P.L. Richards, J. Clarke, R. Leoni, Ph. Lerch, S. Verghese, M.R. Beasley, T.H. Geballe, R.H.Hammond, P. Rosenthall, and S.R. Spielman. Appl. Phys. Lett. 4, 283 (1989).S. Verghese, P.L. Richards, K. Char and S.A. Sachtjen: Proceedings of SPIE - The InternationalSociety for Optical Engineering 1292, 137 (1990).6B.E. 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Leon: “Linear Algegra with Applications”, MacIVlillan Pub. Comp. (1986).‘9R.T. Kampwirth, J.M. Grace, D.J. Miller, D.B. McDonald, K.E. Gray, M. Reiten, M.Ascolese,and H. Latvakoski: IEEE Trans. on Magnetics, 27, 1219 (1991).20B.H. Moeckly, D.K. Lathrop, S.E. Russek, R.A. Buhnnan, M.G. Norton, and C.B. Carter:IEEE Trans. on Magnetics, 27, 1017 (1991).21M.G. Norton, L.A. Tietz, S.R. Summerfelt, and C.B. Carter: Appi. Phys. Lett. , 2348(1989).22R.J. Lin and P.T. Wu: IEEE Trans. Mag., 27, 1560 (1991).23 Hakuraku, S. Higo, D. Miyagi, and T. Ogushi: Jap. J. Appi. Phys, 29, L600 (1990).24K. Kuroda, K.Kojima, M. Tanioku, K.Yokoyama and K. Hamanaka: Jap. J. Appl. Phys, 29,L291 (1990).25TP. Thorpe, M.S. Osofsky, E.F. Skelton, S.B. Qadri, and C.R. Gossett: Physica C, 162-164,645 (1989).26K. Nakamura, J. Sato, K. Ogawa: Jap. J. Appl. Phys, , L77 (1990).27R Parsons, F. Orfino, N. Osborne, D. Grigg, and R. Zindler: J. Vac. Sci. Technol. B,.Q, 697(1992).28QX. Su, L. Li, Y.Y. Zhao, Y.Z. Zhang, and P. 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