Acciones dirigidas a la preparación de las maestras

An experim ental set-up for PLD of thin film s designed and used is illustrated in figure 5.1. A deposition cell, different ports, vacuum gauges, rotary pum p, diffusion pum p and optical com ponents w ere used in the construction o f PLD system. N early all the experim ents w ere perform ed using Q-switched N d:Y A G laser (Quantel, m odel Y G 571C-10) w ith 4ns duration (FW H M ), 532nm wave length and an energy o f IJoule that could be reduced by the help o f neutral density filters. The repetition rate could be set to lOHz, 5Hz, 2H z or single pulse (controlled manually). N orm ally, the laser beam w as directed to a vacuum cham ber via a series o f mirrors and a lens (f=15cm) at an angle o f 30° w ith the norm al to the surface o f a slow ly rotating target (5 Rev./m in). T he laser energy at the target was

m easured using a M olectron, J50 jo u le meter. Targets, 13mm in diam eter, w ere

synthesised using ceram ic technique for ablation-deposition purpose. The distance

betw een the target and substrate could be varied by m oving the target back and forth.

U sually, prio r to deposition the cham ber w as flooded w ith oxygen, evacuated to a pressure o f Ix lO '^ m b a r by only a rotary pum p but som e time it could be further pum ped dow n to Ix lO ’^m bar by a diffusion pump. To obtain partial pressure o f any gas during deposition,

a needle-valve w as used near the substrate. Superconducting thin P b -S r-(Y ,C a)-C u -0

layers w ere grow n on single crystal, M gO (001) or S i ( l l l ) substrates. The choice o f the substrate w as m ade on the ground o f i) chem ical stability above 700°C ii) availability on econom ical basis iii) or minimal lattice m ism atch.

N d ;Y A G L A S E R (4 iise c., 5 3 2 n m ) Rotating Target M gO Substrate 103 Electrical & ' Therm ocouple — Feedthrough Sihcon Nitride Heater

G as out/ Pum piag System

F igure 5.1. Schematic diagram o f the experimental set-up fo r PLD o f Pb-Sr-(YCa )-CuO thin film s.

Initially, a home made substrate heater was used for in-situ deposition. It was prepared using a 3mm thick plate made of high density alum ina ceramic and was delicately m achined with 1.0mm deep grooves on one surface. The heating w ire (FeCrA lY ) was inserted in the grooves and covered with cem ent (mixture o f water, sodium silicate or silica glass as a binder and fine alumina powder). FeCrA lY has the property that once fired its surface oxidises without affecting the inner core of the wire. This heater can reach up to 800°C in oxygen ambient. A C hrom el- alumel therm ocouple wire was spot welded at one end and connected to the nearest position of the substrate. Therefore, the growth tem peratures of the films are referred to in this thesis, are of the heater and not exactly of the substrate. A fter several cycles of heating, the home made heater was degrading, realised by an

increase in its electrical resistance. Later, a com mercial silicon-nitride substrate heater (R = 5 6 n ), with longer life the home-made one, and which could be heated up to 950°C at lOOV even under 0.5m bar oxygen pressure, were used by connecting directly to step- dow n variac.

5.2 T h e m u lti-ta rg e t h o ld e r

A m ulti-target holder was designed and manufactured out o f stainless steel at UCL, E lectronic and Electrical Engineering workshop to deposit in-situ multilayers o f different m aterials. It can accom m odate up to five different targets and could be adjusted one by one m anually in front of laser beam without breaking vacuum . A shield was placed in front o f the m ulti-target holder so that it can protect the other targets from contam ination during the ablation of the one rotating target. A schem atic diagram of the m ulti-target holder is shown in figure 5.2.

F igure 5.2. Schematic diagram o f the m ulti-target holder.

5.3 T h e p ro cessin g fu rn a c e

The bulk material processing or ex-situ annealing of the deposited thin layers was carried out in a quartz tube furnace controlled by a program m er to run different required annealing procedures. After home made winding and using insulation paste o f alum ina cem ent, the furnace was calibrated for temperature w ith in the quartz tube o f 6 cm in diam eter and 100 cm in length. The temperature of the sam ples was m onitored by a C hrom el-alum el therm ocouple wire placed near the samples. A uniform tem perature was found only with in the length of 4cm at the centre of the tube. All the heat

105

treatm ents w ere perform ed in the range o f uniform tem perature by keeping the sam ples at the m id o f the furnace on platinum foil.

5.4.1 DC resistivity measurements

A ll the electrical and m agnetic characterisations were perform ed using an A PD cryostat in Im perial College, at Physics department. The DC electrical characterisation records an electrical resistance versus tem perature for a sample. This is com m only perform ed by attaching four leads at the periphery o f the sam ple and cooling dow n to the tem perature w here the resistance o f the sample becom es zero. The V an der Pauw technique [1] facilitates the com parison o f different samples o f different shapes by using the four point contact configurations into geom etry independent resistivity m easurem ents. The m ethod is valid for a flat sample, homogeneous and isotropic, uniform in thickness o f any arbitrary shape if the contacts are sufficiently small, located at the circum ference o f the sam ple as shown in figure 5.3. Between two points, a dc current. I, flows that m ust be below the critical current, Ig o f the sample. The other two leads m easure the voltage across the sample. The reversal o f the current direction serves to elim inate any dc offset voltages, for exam ple, therm o-electric effects in the leads and the contacts due to tem perature inhom ogeneities and connections could result in errors in the order o f few m icro volts. To m inim ise this error, it is necessary to m easure voltage response, AY o f the film w ith or w ithout current flowing through it.

B

F igure 5.3. A sample o f arbitrary shape with fo u r po ints contacts on its periphery.

Hence, a dc current is applied through the sam ple in forw ard (+) and reverse (-) direction; the corresponding voltages V+ and V_ are m easured and the sam ple resistance is then obtained as

R = ( V + - V _ )/21. (5.1)

F or the determ ination o f the zero resistance, a voltage criterion is selected, w hich depends on the noise level in the set-up, but is typically lp,V /cm (i.e. per cm o f distance betw een the voltage leads). If the m easured value falls short o f this lim it, then a zero voltage drop and no electrical resistance, R is registered in com puter program m e, Lotus 123 spread sheet.

5.4.2 Contact problem

C onnecting the leads onto the thin film sam ples by using silver paste causes deterioration o f these contacts upon cooling, during several therm al cycles and

disconnection usually occurs while cooling below 80K. The technique o f

annealing the contacts prepared by silver past at low tem perature (400°C) in air or oxygen is not applicable as in the case o f Y -Ba-Cu-O or B i-Sr-C a-C u-O film s due to serious oxidation problem with Pb-2213 phase. A nnealing o f contacts m ade by silver paint in nitrogen deteriorated the superconducting properties and usually the contacts becam e insulating. Evaporating indium pads onto the film for connection purpose is tim e consum ing and lengthy process. A nother trick was used to prepare connections. The film at the corners was slightly rem oved and then silver paint was applied on the substrate and joined w ith film. The connecting w ires w ere attached to the part o f the silver on the substrate instead o f the films.

5.5.1 Diamagnetic measurements

The experim ental set-up for the diam agnetic transition (M eissner effect) was also

used at Im perial College, London. It is a m odified apparatus o f a design by

X eninkos and Lem berger [1]. The sample is laid on top o f a flat spiral coil that forms part o f a resonant LC circuit, operating at about IM H z. D uring diam agnetic transition, a superconducting sample induces changes in the inductance o f the coil, that shift the dc voltage versus frequency curve. Due to diam agnetic transition o f superconducting sample, a sudden change is induced in the inductance o f the coil.

107

T hese variations, typically several percentages, are detected by the system. The m agnetic flux lines, O, emanating perpendicularly to the coil are equal to the product o f inductance, L, and current. I, flowing through the coil:

0 = L I (5.2)

W hen the magnetic lines of induction are excluded from the interior o f the superconducting sample on the top of the coil, they are deform ed, as depicted in figure 5.4. As a result of this deformation, 0 decreases lightly w hich in turn leads to a slight change in the inductance L.